1. art and entertainment
2. visual art and design
3. design

Final Report
Development of Design Criteria for
Stormwater Treatment Areas (STAs)
in the Northern Lake Okeechobee
Watershed
Prepared for
South Florida Water Management District
October 2009
CERTIFICATION
I hereby certify, as a Professional Engineer in the State of Florida, that the information in
this document was assembled under my direct personal charge. This report is not
intended or represented to be suitable for reuse by the South Florida Water Management
District or others without specific verification or adaptation by the Engineer. This
certification is made in accordance with the provisions of the Laws and Rules of the
Florida Board of Professional Engineers under Chapter 61G15-29, Florida
Administrative Code.
Christopher H. Keller, P.E.
Florida P.E. No. 54040
Wetland Solutions, Inc.
2809 NW 161 Ct.
Gainesville, FL 32609
Certificate of Authorization No. 28785
Date:
(Reproductions are not valid unless signed, dated, and embossed with Engineer’s seal.)
WETLAND SOLUTIONS, INC.
Development of Design Criteria for Stormwater Treatment Areas (STAS) in the Northern Lake Okeechobee Watershed
Contents
Contents.......................................................................................................................................... i
Executive Summary............................................................................................................... ES-1
Introduction...................................................................................................................................1
Analytical Approach ....................................................................................................................4
Variables ............................................................................................................................4
Equations ...........................................................................................................................5
Modeling Tools .................................................................................................................5
P-k-C* Model........................................................................................................5
DMSTA2 ...............................................................................................................6
Aspect Ratio.................................................................................................................................11
DMSTA2 Simulation Results ........................................................................................14
Cost Impacts....................................................................................................................19
Recommendations ..........................................................................................................19
Wetland Area...............................................................................................................................21
DMSTA2 Simulation Results ........................................................................................22
Cost Impacts....................................................................................................................24
Recommendations ..........................................................................................................24
Inflow Concentration and Mass Loading Rate .....................................................................25
DMSTA2 Simulation Results ........................................................................................25
Cost Impacts....................................................................................................................29
Recommendations ..........................................................................................................29
Water Depth.................................................................................................................................30
DMSTA2 Simulation Results ........................................................................................30
Cost Impacts....................................................................................................................31
Recommendations ..........................................................................................................31
Hydraulic Loading Rate ............................................................................................................32
DMSTA2 Simulation Results ........................................................................................32
Cost Impacts....................................................................................................................32
Recommendations ..........................................................................................................32
Hydraulic Residence Time .......................................................................................................34
DMSTA2 Simulation Results ........................................................................................34
Recommendations ..........................................................................................................35
Cell Compartmentalization ......................................................................................................36
DMSTA2 Simulation Results ........................................................................................36
Cost Impacts....................................................................................................................36
Recommendations ..........................................................................................................38
Deep Zones ..................................................................................................................................39
Sediment Accretion Rate and System Life Expectancy .......................................................41
Sediment and Phosphorus Accretion Processes ........................................................41
Sediment and Phosphorus Accretion Rates................................................................45
Sediment and Phosphorus Accretion Rates in Natural Wetlands..............45
Sediment and Phosphorus Accretion Rates in Treatment Wetlands .........47
Options for Management of Long-Term Sediment Accretion .................................51
WETLAND SOLUTIONS, INC.
Development of Design Criteria for Stormwater Treatment Areas (STAS) in the Northern Lake Okeechobee Watershed
i
Allow for Sediment Accretion in System Design..........................................52
Mechanical Removal .........................................................................................52
Drawdown/Consolidation ..............................................................................53
Drawdown/Burning.........................................................................................53
Summary..........................................................................................................................53
Levee Height Considerations ...................................................................................................56
DCM-2 Summary............................................................................................................56
High and Significant Hazard Potential ..........................................................56
Low Hazard Potential.......................................................................................57
STA Freeboard Design Examples....................................................................57
Wildlife Habitat and Public Use Features .............................................................................58
Vegetation........................................................................................................................59
Wildlife.............................................................................................................................60
Public Use ........................................................................................................................61
Plant Community Considerations...........................................................................................63
Description of the Target STA Wetland Plant Communities ...................................63
Emergent Plant Community ............................................................................65
Submerged Aquatic Plant Community ..........................................................70
Hydrologic Optima and Tolerance Ranges for Target STA Plant Communities ..71
STA Plant Community Studies........................................................................71
Analysis of STA Hydrologic Data...................................................................73
Development of a Preliminary STA Plant Community Assessment Tool 73
Summary and Recommendations ................................................................................76
STA Construction Costs ............................................................................................................78
STA Cost Effectiveness .............................................................................................................81
Cost Basis.........................................................................................................................81
Land.....................................................................................................................81
Levee Construction ...........................................................................................81
Deep Zone/Canal Construction......................................................................81
Cell Grading .......................................................................................................82
Water Control Structures..................................................................................82
Clearing and Grubbing.....................................................................................82
Pump Stations ....................................................................................................82
Operations and Maintenance...........................................................................82
Adjustment to Present Worth Costs ...............................................................82
Prototype STA Designs..................................................................................................82
Cost-Effectiveness vs. Aspect Ratio .............................................................................83
Cost-Effectiveness vs. Area ...........................................................................................84
Cost-Effectiveness vs. Inflow Phosphorus Concentration........................................85
Cost-Effectiveness vs. Phosphorus Mass Loading Rate............................................86
Cost-Effectiveness vs. Mean Depth..............................................................................87
Cost-Effectiveness vs. Hydraulic Loading Rate .........................................................88
Cost-Effectiveness vs. Hydraulic Residence Time.....................................................89
Cost-Effectiveness vs. Compartmentalization............................................................90
References ....................................................................................................................................92
Appendix A
Appendix B
WETLAND SOLUTIONS, INC.
Development of Design Criteria for Stormwater Treatment Areas (STAS) in the Northern Lake Okeechobee Watershed
ii
Executive Summary
The purpose of this document is to demonstrate, through Dynamic Model for
Stormwater Treatment Areas Version 2 (DMSTA2) modeling, the effects of various
design criteria on the cost-effectiveness of phosphorus load reduction for STAs that
could be constructed in the Northern Lake Okeechobee Watershed. Review of
fundamental design equations and interpretation of the modeling results demonstrate
that many of the design variables are related and adjustments to one force responses in
others. These responses may have positive or negative impacts on phosphorus removal.
None of the DMSTA2 calibration data sets, including those located north of Lake
Okeechobee, were for systems designed specifically for load rather than concentration
reduction. Therefore, some of the model scenarios push DMSTA2 beyond its calibration
limits for some design parameters (flow per unit width and mean depth) in order to
explore how load reduction systems might perform. DMSTA2 should not be used for
final design without the user’s full awareness of its strengths, limitations, and calibration
boundaries. Efforts to further expand the DMSTA2 calibrations should be considered
prior to standardizing its use for load reduction projects.
Preliminary design criteria recommendations that can be drawn from the analyses
summarized in this report include the following:
•
Aspect ratios in the range of 1:1 to 3:1 are appropriate as general design
guidance. However, it is strongly recommended that STA designers focus more
on maximizing hydraulic efficiency by promoting even flow distribution rather
than targeting a specific aspect ratio. In all likelihood, an STA design will be
forced to conform to the geometry of the available site and sub-divisions (cells)
may be required based on topographic constraints.
•
Because land costs can be significant, the minimum area required to
conservatively meet project objectives should be determined for each specific
STA application. A full cost-benefit analysis will be required during the
development of project alternatives, but it is clear that for any set of operating
conditions, there is a break point in the area versus performance curve at which
cost-effectiveness declines.
•
The modeling results show that for a fixed area, there is a benefit to increasing
the mass loading rate. Because the source water phosphorus concentration can
not be increased to achieve higher mass loading rates, the flow must be increased
instead. Regardless of system scale, modeled load reduction rapidly increases as
the HLR approaches about 5 to 7 cm/d. At these HLRs, for any given inflow
concentration, mass removal rates are estimated to be effectively maximized. As
a secondary conclusion, these results imply that if a choice is to be made between
two potential projects with equal available area, priority should be given to the
site with the highest inflow phosphorus concentrations.
WETLAND SOLUTIONS, INC.
Development of Design Criteria for Stormwater Treatment Areas (STAS) in the Northern Lake Okeechobee Watershed
ES-1
•
At present, and unless future calibration enhancements to the DMSTA2 show
otherwise, there is no strong basis for the District to modify the current design
control depth of approximately 40 cm. Site-specific detailed design calculations
will be required to show that a particular STA will operate within a depth range
that maximizes inundation of the wetland area without exceeding plant
community tolerances. Plant community data indicate that emergent marshes
dominate when average depths range from about 30 to 50 cm.
•
Considering the depth issues that can occur with increasing flow and HLR, and
the range of potential STA sizes that could be constructed in the watershed, a
long-term average HLR of about 6 cm/d is suggested as an upper limit for
projects designed for maximum load reduction.
•
An STA should be designed so that the presumed HLR and water depth regime
are reasonable. If these conditions are met, there is no need to attempt to achieve
a particular nominal HRT.
•
As general design guidance, it is recommended that each STA project should
have a minimum of 2 parallel treatment trains with serial cell construction as
required based on topographic and hydraulic constraints. It should be noted,
though, that the construction of serial cells guarantees a minimum system-wide
N of at least 2.
•
Deep zones are effective for initial flow distribution and outlet collection.
Internal deep zones may also improve performance, particularly when low
outflow concentrations are required, as long as the fraction of deep zone area to
total area is constrained.
•
Sediment accretion is a normal and important process in treatment wetlands that
provides a long-term, stable repository for nutrients and other pollutants of
concern. Typical long-term accretion rates that can be used in treatment wetland
design range from 5 to 10 mm/yr, and at these rates, the effects on system life are
expected to be minimal (about 30 to 60 years of system life per foot of levee
freeboard).
•
Design approaches for levee height requirements are well described in DCM-2.
For planning purposes, levee heights of about 7 feet should be sufficient for STAs
constructed north of the Lake.
•
STAs, even when not designed with species-specific wildlife requirements in
mind, provide valuable, high-quality habitat and offer tremendous opportunities
for public recreation and education.
•
Emergent plant communities typically dominated by cattail but also with
relatively high diversity of subdominant emergent plant species provide the
overall preferred plant community option for STAs north of Lake Okeechobee
due to their high carbon production and tested resilience to fluctuating and
continuous water levels.
WETLAND SOLUTIONS, INC.
Development of Design Criteria for Stormwater Treatment Areas (STAS) in the Northern Lake Okeechobee Watershed
ES-2
•
Model results indicate that cost-effective phosphorus removal can be achieved
between HLRs of 4 to 8 cm/d, regardless of STA scale. However, within this
range, some simulations exceeded DMSTA2 calibration boundaries for flow per
unit width (26 – 210 m2/d), mean depth (35 – 76 cm), and HLR (1.1 – 6.5 cm/d).
By prioritizing projects to sub-watersheds with the highest inflow
concentrations, cost-effectiveness can be further maximized. Based on the
analyses presented in this document, the range of cost-effective phosphorus
removal is about 75 to 125 $/kg (50-yr present worth basis).
To aid future planning and preliminary STA sizing efforts for projects in the Northern
Lake Okeechobee Watershed, a series of tables are provided in Appendix A. These
tables provide estimates of phosphorus load reduction as a function of treatment area
(100 to 20,000 acres), hydraulic loading rate (3 to 8 cm/d), and inflow phosphorus
concentration (100 to 800 ppb). The values in these tables have been constrained to
ranges that were determined to maximize cost-effectiveness. All model runs assumed
steady-state conditions (constant flow and constant inflow concentration) and the
tabulated results likely over-estimate the removal that would be estimated using a
highly dynamic (variable flow and variable inflow concentration) input data set. Load
reduction estimates between steady-state and dynamic simulations may differ by as
much as 20 to 40 percent (WSI 2003). These tables are intended to facilitate preliminary
sizing efforts. Project design will still require the use of site-specific information and
more complex analytical procedures.
WETLAND SOLUTIONS, INC.
Development of Design Criteria for Stormwater Treatment Areas (STAS) in the Northern Lake Okeechobee Watershed
ES-3
Introduction
The Taylor Creek and Nubbin Slough Stormwater Treatment Areas (STAs) are the
prototype STAs being implemented north of Lake Okeechobee. Both STAs are fully
constructed but only Taylor Creek is fully operational. Operation has not been initiated
at the Nubbin Slough STA due to structural issues with the pump station. The Taylor
Creek and Nubbin Slough STAs were estimated to remove a long-term average of 2.08
and 5.14 metric tons of phosphorus per year, respectively. Likewise, the Lakeside Ranch
facility is nearing final design and is estimated to provide about 21 metric tons of
phosphorus (P) removal per year. These estimates were developed using earlier versions
of the Everglades STA design model, and may be optimistic for soils and loading rates
included in the estimates.
Simple input/output analysis found in the literature provides some general guidance
relative to P loading rates and effluent P concentrations but they can not be accurately
applied to a particular wetland without due consideration to site-specific conditions.
Environmental conditions including soil properties, vegetation types, previous land
uses, surface water total P concentrations, rainfall patterns, and hydraulic loads are
significantly different north of the lake and may reasonably be expected to result in
different development and performance of STAs constructed and operated in the Lake
Okeechobee watershed. The implementation of the prototype STAs is important for
demonstrating the effectiveness of the STA technology in areas north of the lake. Site
specific information obtained from operation of these prototype treatment wetlands will
help to improve design and operational guidance as additional STAs are planned in the
watershed.
The South Florida Water Management District’s (SFWMD or District) primary goal of
the STAs north of the lake is to maximize the long-term mass removal of total
phosphorus (TP) and to minimize operational costs per pound of TP removed. Unlike
the STAs south of the lake, the STAs in the Lake Okeechobee watershed are not
mandated to achieve a target outflow TP concentration. As such, the Okeechobee STAs
offer greater flexibility in terms of design and performance goals or desired levels of
treatment.
Although there is no “cookbook” for successful implementation of STAs anywhere, it is
assumed that these facilities if properly designed, constructed, and operated can provide
predictable performance.
The primary goal of this effort is to develop design criteria and guidelines specific to
conditions north of the lake and to predict performance of future STAs in the Lake
Okeechobee watershed with greater reliability and certainty. These design standards
and guidelines are also intended to serve as a tool for making future land purchases in
the watershed deemed suitable for STAs.
A previous project document (Appendix B) was prepared to assess relevant site-specific
conditions in the 21 basins comprising the northern Lake Okeechobee watershed
WETLAND SOLUTIONS, INC.
Development of Design Criteria for Stormwater Treatment Areas (STAS) in the Northern Lake Okeechobee Watershed
1
(Exhibit 1). The goal of that effort was to identify local environmental conditions that
would be most conducive to the successful implementation of an STA and to allow
interested parties to identify specific areas in each of the 21 individual basins that would
be most appropriate for STA siting.
This document has been prepared to provide a quantitative assessment of the effects of
various STA design criteria on estimated TP mass load reduction and unit cost ($ per
kilogram) for TP removal. Specific STA design parameters that were quantitatively
evaluated include the following:
•
STA wetted area
•
TP inflow concentration and mass loading rates
•
Cell number and configuration
•
Water depth
•
Cell aspect ratio
•
Hydraulic loading rate (HLR)
•
Hydraulic residence time (HRT)
•
Volumetric efficiency (included as a component of the aspect ratio analysis)
These design parameters are interdependent and one can not be adjusted without
affecting the others to some extent. To the extent possible, these quantitative design
variables have been evaluated independently using the Dynamic Model for Stormwater
Treatment Areas Version 2 (DMSTA2; Walker and Kadlec 2008) and other tools when
appropriate. DMSTA2 has been calibrated for areas north of Lake Okeechobee.
Other design parameters that could not be directly evaluated with DMSTA2 were
assessed in a semi-quantitative or qualitative manner. Those parameters include the
following:
•
Deep zone sizing and locations
•
Sediment accretion rate and system life expectancy
•
Levee height considerations
•
Wildlife habitat and public use features
•
Plant selection
WETLAND SOLUTIONS, INC.
Development of Design Criteria for Stormwater Treatment Areas (STAS) in the Northern Lake Okeechobee Watershed
2
EXHIBIT 1
Northern Lake Okeechobee Watershed Drainage Basins
WETLAND SOLUTIONS, INC.
Development of Design Criteria for Stormwater Treatment Areas (STAS) in the Northern Lake Okeechobee Watershed
3
Analytical Approach
Treatment wetland (i.e., STA) design is not one-dimensional and multiple constraints
must be met in order to create a successful project. Owners and operators of wetland
systems frequently inquire as to the optimum value for any particular variable that is,
more-or-less, under the designer’s control. The reality of wetland design and behavior is
that all of the key design parameters are inter-related and any adjustment to one causes
a response in one or more of the others. In many cases, the reaction of one variable to the
manipulation of another is counteractive to water quality improvement processes. For
example, increasing aspect ratio to presumably improve hydraulic efficiency and
phosphorus removal effectiveness will, at some threshold inflow rate, increase frictional
losses to the point that impacts the wetland vegetation and actually decreases
phosphorus removal. At the other end of the spectrum, lowering hydraulic loading rates
to levels that minimize outflow phosphorus concentrations could starve the wetland for
water if evapotranspiration and seepage demands are not met.
An understanding of the relationships between various design parameters can be gained
by reviewing the common variables and equations used in the wetland design process.
Variables
A
=
wetted surface area
Ci
=
inflow concentration
Co
=
outflow concentration
C*
=
background concentration
Da
=
Damköhler number
h
=
mean water depth
k
=
first-order, area-based removal rate
L
=
mean length of wetland
N
=
number of tanks-in-series
Q
=
wetland inflow rate
q
=
hydraulic loading rate
τ
=
nominal hydraulic residence time
u
=
horizontal velocity
V
=
wetland volume
W
=
mean width of wetland
WETLAND SOLUTIONS, INC.
Development of Design Criteria for Stormwater Treatment Areas (STAS) in the Northern Lake Okeechobee Watershed
4
Equations
A = LW
EQ-1
V = Ah
EQ-2
q=
Q
A
EQ-3
τ=
V h
=
Q q
EQ-4
u=
Q
Wh
EQ-5
Da =
k kτ
=
q
h
EQ-6
Modeling Tools
Two modeling tools are used in the following sections to explore the relationships
between various design parameters and phosphorus removal performance. Each is
briefly summarized below.
P-k-C* Model
Kadlec and Wallace (2009) promote the P-k-C* model as the preferred tool for sizing
treatment wetlands and determining removal rate parameters from operational data.
This model is a variant on the previously-published tanks-in-series (TIS) formulation of
the first-order k-C* model (Kadlec and Knight 1996). The model equation is given below
and incorporates the following key principals:
•
Wetland removal processes are area-based and follow first-order kinetics;
•
For some parameters, internal cycling results in non-zero background
concentrations (C*);
•
Physical factors that influence the hydraulic efficiency of wetlands, including
topography, wetland geometry, vegetation density and spatial distribution, and
wind fetch lead to non-plug-flow conditions and should be included in
calculations; and
•
Factors that describe pollutant mixtures or contaminant “weathering” should
also be included in the model.
⎛ C o − C* ⎞ ⎛
k ⎞
⎜⎜
⎟ = ⎜⎜1 +
⎟⎟
* ⎟
⎝ C i − C ⎠ ⎝ Pq ⎠
−P
EQ-7
Where:
WETLAND SOLUTIONS, INC.
Development of Design Criteria for Stormwater Treatment Areas (STAS) in the Northern Lake Okeechobee Watershed
5
P
=
apparent number of tanks-in-series
In the earlier TIS model, the effects of hydraulic efficiency were described by the
parameter N, the number of TIS. In the updated model, N has been replaced by P and
combines the effects of hydraulic efficiency and pollutant mixtures or weathering such
that P < N (Kadlec and Wallace 2009).
DMSTA2
DMSTA Version 2 was developed to estimate the phosphorus removal performance of
shallow reservoirs and treatment wetlands and is the primary tool used for design of the
STAs. The DMSTA2 model was calibrated and tested against data from approximately
70 datasets derived from experimental platforms, field-scale test facilities, and full-scale
treatment wetlands located in Florida. These data represent a variety of spatial scales,
vegetation types, hydraulic regimes, and concentration regimes (up to 800 parts per
billion, ppb).
The model provides a flexible set of options for parameter selection, water balance
issues, water flows and internal hydraulics, and cell configurations. The DMSTA2 model
offers the following factors that are not included in previous generations of wetland
design tools:
•
Temporal Variations in Inflow Volume, Load, Rainfall, and ET
•
Hydraulic Compartments (Cells, Internal Levees for Flow
Redistribution)
•
Hydraulic Efficiency (Number of Stirred Tanks in Series)
•
Cell Aspect Ratio (Length/Width)
•
Water Level Regulation
•
Outflow Regulation (Discharge vs. Water Level)
•
Compartmentalization of Biological Communities
•
Dry-Out Frequency and Supplemental Water Needs
•
Bypass Frequency, Quantity, and Quality
•
Seepage Collection and Management
The phosphorus component of the DMSTA2 model structure is summarized in Exhibit
2.
WETLAND SOLUTIONS, INC.
Development of Design Criteria for Stormwater Treatment Areas (STAS) in the Northern Lake Okeechobee Watershed
6
EXHIBIT 2
DMSTA2 Phosphorus Cycling Model Construction (Walker and Kadlec 2008)
Where:
State Variables
M
Water Column P Storage
mg/m2
S
Temporary P Storage in Biota, etc.
mg/m2
Z
Water Column Mean Depth
m
Driving Variables
L
P Load, Including Atmospheric Deposition mg/m2/yr
Q
Outflow
m/yr
Parameter Values
Fz
Depth Multiplier for Gross Uptake
dimensionless
Fc
Concentration Multiplier
dimensionless
K1
Maximum Uptake Rate
m3/mg-yr
K2
Recycle Rate
m2/mg-yr
K3
Burial Rate
1/yr
WETLAND SOLUTIONS, INC.
Development of Design Criteria for Stormwater Treatment Areas (STAS) in the Northern Lake Okeechobee Watershed
7
Of note in the model construction is the inclusion of a depth multiplier (Fz). Exhibit 3
shows that for marsh systems, the term Fz effectively reduces the phosphorus uptake rate
at depths less than 40 centimeters (cm). In practical terms, Fz accounts for hydraulic
effects such as increased short circuiting at shallow depths caused by incomplete
inundation of the wetland. The depth multiplier is maximized between depths of 40 and
100 cm, but then has a negative effect on the uptake rate when the 30-day rolling average
depth ranges between 100 and 200 cm. If the STA depth exceeds 200 cm (30-day rolling
average), the net phosphorus settling rate is set to a value of 1 m/yr. The decline in
removal rate at depths exceeding 100 cm corresponds to observations of vegetation stress
and shearing in large-scale wetlands.
EXHIBIT 3
DMSTA2 Depth Effects on Phosphorus Uptake Rate (Walker and Kadlec 2008)
The concentration multiplier (Fc) reflects saturation of the phosphorus uptake rate at
high inflow concentrations. As the inflow concentration increases, the removal rate
effectively declines (see Exhibit 4).
WETLAND SOLUTIONS, INC.
Development of Design Criteria for Stormwater Treatment Areas (STAS) in the Northern Lake Okeechobee Watershed
8
EXHIBIT 4
DMSTA2 Concentration Saturation Effect (Walker and Kadlec 2008)
DMSTA2 has been calibrated for the following aquatic ecosystem types:
EMG_3 – Emergent marsh constructed on impacted soils. The median removal
rate constant for this calibration is k = 16.8 m/yr.
PEW_3 – Emergent marsh community in a pre-existing wetland area such as the
Water Conservation Areas. The median removal rate constant for this calibration
is k = 34.9 m/yr.
SAV_3 – Submersed Aquatic Vegetation community. The median removal rate
constant for this calibration is k = 52.5 m/yr.
PSTA_3 – Periphyton dominated system with sparse emergent macrophytes. The
median removal rate constant for this calibration is k = 23.6 m/yr.
RES_3 – Reservoir or lake dominated by open water. The median removal rate
constant for this calibration is k = 5.0 m/yr.
The median rate constants for the PEW_3, SAV_3, and PSTA_3 system types are higher
than that for the typical emergent marsh (EMG_3) in part because of relatively high
background calcium concentrations (>75 mg/L) that facilitate co-precipitation of
phosphorus with calcium carbonate.
Only the EMG_3 calibration was used for the analyses included in this document. The
model provides flag messages when results fall outside the ranges of the calibration data
sets for inflow or outflow concentration (19.5 to 800 ppb), flow per unit width (26 to 210
m2/d), mean water depth (35 to 76 cm), and frequency of dryout (0 to 9%).
WETLAND SOLUTIONS, INC.
Development of Design Criteria for Stormwater Treatment Areas (STAS) in the Northern Lake Okeechobee Watershed
9
None of the DMSTA2 calibration data sets, including those located north of Lake
Okeechobee, were for systems designed specifically for load rather than concentration
reduction. Therefore, some of the model scenarios will push DMSTA2 beyond its
calibration limits for some design parameters in order to fully investigate how load
reduction systems might be expected to behave. DMSTA2 should not be used for final
design without the user’s full awareness of its strengths, limitations, and calibration
boundaries.
WETLAND SOLUTIONS, INC.
Development of Design Criteria for Stormwater Treatment Areas (STAS) in the Northern Lake Okeechobee Watershed
10
Aspect Ratio
The length-to-width (aspect) ratio has been theorized to positively correlate with
pollutant removal efficiency (WPCF 1990). As the aspect ratio increases, the hydraulic
behavior of a pond or wetland is presumed to approach the ideal plug-flow case where
removal efficiency is maximized. However, tracer studies of operational wetlands with
wide ranges in scale do not fully support this theory because the effects of wind mixing,
vegetation density and spatial distribution, microtopography, and inlet/outlet location
and design can not be separated from the effects of aspect ratio alone (Kadlec and
Wallace 2009). Attention to inlet and outlet configuration, the use of spreader canals
(deep zones) or other features that promote effective flow distribution, and constructing
cells in series may be more likely to improve treatment performance than simply
increasing aspect ratio.
Although hydraulic performance may be improved, increasing the aspect ratio can have
negative consequences that counteract pollutant removal performance and decrease
cost-effectiveness:
•
For a given flow, increasing the aspect ratio increases horizontal velocities in the
wetland. Velocities may increase to the point that frictional head losses yield
water depths that will not support emergent vegetation communities.
•
Increasing the aspect ratio increases the perimeter levee requirements. For a
given area, increasing the aspect ratio from 1:1 to 5:1 increases the levee
perimeter by 34 percent. Increasing the aspect ratio from 1:1 to 10:1 increases the
levee perimeter by 74 percent.
Various researchers have modeled the theoretical effects of aspect ratio on ponds and, to
a lesser extent wetlands, using two-dimensional hydrodynamic models that simulate
flows and the transport of conservative tracers between the inlet and outlet (Thackson et
al. 1987, Persson et al. 1999, Jenkins and Greenway 2005). However, the models used in
these studies were not independently capable of evaluating velocity- or depth-related
effects on plant community stability and composition. Therefore, the parameter
definitions and results from these studies have been used to provide a quantitative basis
for evaluating the competing effects of aspect ratio using DMSTA2 by varying the
number of tanks-in-series (N) per cell. DMSTA2 accepts non-integer inputs for N up to a
maximum value of 10.
From Thackson et al. (1987):
η=
τm
τn
EQ-8
η
=
volumetric efficiency
τm
=
mean hydraulic residence time (determined from tracer)
WETLAND SOLUTIONS, INC.
Development of Design Criteria for Stormwater Treatment Areas (STAS) in the Northern Lake Okeechobee Watershed
11
τn
=
nominal hydraulic residence time (V/Q)
From Persson et al. (1999):
λ=
τp
τn
EQ-9
λ
=
hydraulic efficiency
τp
=
time to tracer peak
From Kadlec and Knight (1996):
N=
τm
EQ-10
τ m −τ p
By substitution of Equations 8 and 9 into Equation 10:
N=
η
EQ-11
η −λ
Jenkins and Greenway (2005) modeled unvegetated ponds (with point inlets and outlets)
with aspect ratios ranging from 0.357:1 to 35.7:1 and estimated values for the volumetric
(η) and hydraulic (λ) efficiencies. Exhibits 5 and 6 show the resulting data. The
following regression equations were developed from the study data:
η = 0.983(1 − e −0.307 L / W )
r2 = 0.99
EQ-12
λ = 0.445η 2 + 0.515η
r2 = 0.99
EQ-13
For any aspect ratio, Equations 12 and 13 can be combined with Equation 11 to estimate
N as a function of aspect ratio. Exhibit 7 shows a plot of the theoretical relationship
between aspect ratio and N. Curve fitting gives the following regression equation:
N=
20.907
1 + 9.15e −0.307 L / W
r2 = 0.99
EQ-14
As noted, this approach was based on modeling primarily conducted for unvegetated
ponds. Wetland vegetation will generally retard flow and increase mixing potentially
yielding higher N values than equally sized open ponds. Further, each modeled pond
had a single inlet and outlet. STA designers have routinely specified multiple inlet and
outlet structures, as well as distribution channels, to improve hydraulic performance.
Therefore, the N values used in the following DMSTA2 simulations are considered to be
conservative.
WETLAND SOLUTIONS, INC.
Development of Design Criteria for Stormwater Treatment Areas (STAS) in the Northern Lake Okeechobee Watershed
12
1.0
η (Effective Volume Ratio)
0.8
y = a(1 - e-bx)
a = 0.983
b = 0.307
r2 = 0.99
0.6
0.4
0.2
0.0
0
5
10
15
20
25
30
35
40
Aspect Ratio (L/W)
EXHIBIT 5
Estimated Effective Volume Ratio as a Function of Aspect Ratio (data from Jenkins and Greenway 2005)
1.0
y = 0.4451x2 + 0.5146x
r2 = 0.99
λ (Hydraulic Efficiency)
0.8
0.6
0.4
0.2
0.0
0.0
0.2
0.4
0.6
0.8
1.0
η (Effective Volume Ratio)
EXHIBIT 6
Estimated Hydraulic Efficiency as a Function of Effective Volume Ratio (data from Jenkins and Greenway 2005)
WETLAND SOLUTIONS, INC.
Development of Design Criteria for Stormwater Treatment Areas (STAS) in the Northern Lake Okeechobee Watershed
13
25
Tanks in Series (N)
20
y = a/(1 + be-cx)
a = 20.907
b = 9.150
c = 0.307
r2 = 0.99
15
10
5
0
0
5
10
15
20
25
30
Aspect Ratio (L/W)
EXHIBIT 7
Estimated Number of Tanks-in-Series as a Function of Aspect Ratio
DMSTA2 Simulation Results
An array of DMSTA2 simulations were run for three STA sizes (100; 1,000; and 10,000
acres) at hydraulic loading rates (HLRs) of 3, 6, 12, and 30 centimeters per day (cm/d).
These were steady-state simulations with a constant inflow TP concentration of 250 parts
per billion (ppb). Average STA water depths were calculated using the DMSTA2
algorithms with a coefficient value (a) of 0.7 and exponent (b) of 3.5. These are
approximate mid-range values based on the best calibration data sets (Walker and
Kadlec 2008). In all cases, the outflow control elevation was set at 40 cm, generally
consistent with SFWMD operational guidelines. Exhibit 8 shows the presumed values of
N used in the simulations based on the numerical approach described above and the
constraints of the DMSTA2.
WETLAND SOLUTIONS, INC.
Development of Design Criteria for Stormwater Treatment Areas (STAS) in the Northern Lake Okeechobee Watershed
14
EXHIBIT 8
Modeled Value of N as a Function of Aspect Ratio
Parameter
Aspect Ratio
0.5:1
1:1
1.5:1
2:1
2.5:1
3:1
5:1
7.5:1
10:1
20:1
1
2.4
2.7
3.1
3.5
4.0
4.5
7.0
10.9
14.7
20.5
2
2.4
2.7
3.1
3.5
4.0
4.5
7.0
10
10
10
N
N
1
N calculated from Equation 14
2
Revised N based on DMSTA2 maximum of N = 10
Exhibits 9, 10, and 11 show the estimated effects of aspect ratio on average STA water
depth, TP load reduction, and TP load reduction efficiency. Mean water depths increase
in response to increasing aspect ratio, increasing HLR, and increasing STA area. Two
factors create this response: first, for a given area, increasing the aspect ratio reduces the
STA width, increases the horizontal velocity, and increases vegetative friction losses;
second, for a given HLR, velocity increases with increasing area.
Because the DMSTA2 code includes a performance penalty when the 30-day average
depth exceeds 100 cm, estimated TP removal increases with increasing aspect ratio until
the combination of STA area and HLR cause the depth threshold to be met. At that
point, removal rates measured in kilograms per year (kg/yr) begin to decrease (Exhibit
10). For larger STAs operating at high sustained HLRs, TP removal performance appears
to rapidly decrease with increasing aspect ratio. Load reduction efficiency (Exhibit 11)
follows the same general trends as estimated mass load reduction, however the relative
magnitude of incremental change with increasing aspect ratio is not as dramatic. This is
because the load reduction values result from generally decreasing reductions in TP
concentration multiplied by increasing volumes of water.
WETLAND SOLUTIONS, INC.
Development of Design Criteria for Stormwater Treatment Areas (STAS) in the Northern Lake Okeechobee Watershed
15
120
100 ac STA
Mean Depth (cm)
100
80
60
40
20
0
0
2
4
6
8
10
12
14
16
18
20
12
14
16
18
20
12
14
16
18
20
Aspect Ratio (L:W)
160
1,000 ac STA
Mean Depth (cm)
140
120
100
80
60
40
20
0
0
2
4
6
8
10
Aspect Ratio (L:W)
250
10,000 ac STA
Mean Depth (cm)
200
150
100
50
0
0
2
4
6
8
10
Aspect Ratio (L:W)
3 cm/d
6 cm/d
12 cm/d
30 cm/d
EXHIBIT 9
Estimated Mean Water Depth as a Function of Aspect Ratio, STA Area, and Hydraulic Loading Rate
Depths calculated using DMSTA2 for constant flow conditions.
WETLAND SOLUTIONS, INC.
Development of Design Criteria for Stormwater Treatment Areas (STAS) in the Northern Lake Okeechobee Watershed
16
1,000
Load Removed (kg/yr)
900
800
700
600
500
400
300
200
100 ac STA
100
0
0
2
4
6
8
10
12
14
16
18
20
12
14
16
18
20
12
14
16
18
20
Aspect Ratio (L:W)
10,000
Load Removed (kg/yr)
9,000
8,000
7,000
6,000
5,000
4,000
3,000
1,000 ac STA
2,000
1,000
0
0
2
4
6
8
10
Aspect Ratio (L:W)
90,000
Load Removed (kg/yr)
80,000
70,000
60,000
50,000
40,000
30,000
10,000 ac STA
20,000
10,000
0
0
2
4
6
8
10
Aspect Ratio (L:W)
3 cm/d
6 cm/d
12 cm/d
30 cm/d
EXHIBIT 10
Estimated TP Load Reduction as a Function of Aspect Ratio, STA Area, and Hydraulic Loading Rate
Load reduction estimated using DMSTA2 with constant flow rate and constant inflow TP concentration of 250 ppb.
WETLAND SOLUTIONS, INC.
Development of Design Criteria for Stormwater Treatment Areas (STAS) in the Northern Lake Okeechobee Watershed
17
70%
Removal Efficiency (%)
100 ac STA
60%
50%
40%
30%
20%
10%
0%
0
2
4
6
8
10
12
14
16
18
20
12
14
16
18
20
12
14
16
18
20
Aspect Ratio (L:W)
70%
Removal Efficiency (%)
1,000 ac STA
60%
50%
40%
30%
20%
10%
0%
0
2
4
6
8
10
Aspect Ratio (L:W)
70%
Removal Efficiency (%)
10,000 ac STA
60%
50%
40%
30%
20%
10%
0%
0
2
4
6
8
10
Aspect Ratio (L:W)
3 cm/d
6 cm/d
12 cm/d
30 cm/d
EXHIBIT 11
Estimated TP Load Reduction Efficiency as a Function of Aspect Ratio, STA Area, and Hydraulic Loading Rate
Load reduction efficiency estimated using DMSTA2 with constant flow rate and constant inflow TP concentration of 250
ppb.
WETLAND SOLUTIONS, INC.
Development of Design Criteria for Stormwater Treatment Areas (STAS) in the Northern Lake Okeechobee Watershed
18
Cost Impacts
The effect of aspect ratio on capital cost can be approximately evaluated by examining
the relationship to STA perimeter length. Ignoring the possibility of more complex
geometries (circular, triangular and other polygonal shapes), and assuming that an STA
will be constructed as a rectangle, the minimum perimeter occurs for a 1:1 (square)
aspect ratio. Increases or decreases in aspect ratio elongate the polygon and increase
perimeter berm requirements. Exhibit 12 shows the relationship between aspect ratio
and perimeter length independent of area. For any size STA, increasing the aspect ratio
from 1:1 to 3:1 increases the perimeter length by 15%. All other variables being equal,
construction costs for embankments would follow the same general curve.
200%
Percent Increase in Perimeter
150%
100%
50%
0%
0.01
0.1
1
10
100
Aspect Ratio
EXHIBIT 12
Percent Increase in Perimeter for Aspect Ratios Less than or Greater than 1:1
Recommendations
Based on the DMSTA2 simulations described above, there appears to be little benefit in
constructing STA cells with aspect ratios exceeding 3:1. Embankment quantities and
costs could be expected to vary only by about 15% for aspect ratios ranging from 0.3:1 to
3:1. Kadlec and Wallace (2009) have recommended against very small aspect ratios
presumably because there are increasingly fewer opportunities to correct or make up for
short circuited flow when the overall flow length decreases. Thus aspect ratios in the
range of 1:1 to 3:1 seem appropriate as general design guidance. However, it is strongly
recommended that STA designers focus more on maximizing hydraulic efficiency by
WETLAND SOLUTIONS, INC.
Development of Design Criteria for Stormwater Treatment Areas (STAS) in the Northern Lake Okeechobee Watershed
19
promoting even flow distribution rather than targeting a specific aspect ratio. In all
likelihood, an STA design will be forced to conform to the geometry of the available site
and sub-divisions (cells) will be required based on topographic constraints.
WETLAND SOLUTIONS, INC.
Development of Design Criteria for Stormwater Treatment Areas (STAS) in the Northern Lake Okeechobee Watershed
20
Wetland Area
Water quality improvement processes in wetlands have been shown to follow first-order
kinetics with rates that are highly dependent on the effective wetland area (Kadlec and
Wallace 2009). This is because most of the processes occur within the biofilms attached
to plants and at the sediment interface. Wetland outflow concentration versus area
profiles plot as exponentially-declining curves that start at the inflow concentration (area
= 0) and approach an asymptotic value equal to the background concentration observed
in unimpacted systems. Exhibit 13 shows the general relationship between water quality
improvement (estimated outflow concentration and load reduction) and treatment area
for a fixed flow of 50 cfs, inflow phosphorus concentration (Ci) of 250 ppb, first-order
removal rate (k) of 10 m/yr, background concentration (C*) of 3 ppb, and N = 3. The
first-order, steady-state P-k-C* model (EQ-7) was used to generate these curves. The
reader is cautioned that the P-k-C* model does not include performance penalties for
high hydraulic loading rates and resulting high water depths. It is therefore possible to
produce results with the P-k-C* model that are well outside the range of calibration and
physically impractical. For this example and for areas less than 50 acres, the resulting
HLRs meet or exceed the peak daily average HLRs measured in the EAA STAs. It
should also be noted that the P-k-C* model is an estimator of long-term performance
and presumes that the input parameters, including HLR, are representative of average
rather than peak conditions. In spite of these limitations and caveats, the P-k-C* model
was used here for illustrative purposes.
As shown in Exhibit 13, performance increases dramatically as the area approaches
about 4,000 acres (for this example) and then levels off as area continues to increase. Full
capital costs need to be included to determine the “best” design area for this example. It
may be very cost effective to increase area from 1,000 to 2,000 acres because the
estimated load reduction increases by 39 percent and outflow concentrations improve
from 115 to 63 ppb. However it is likely that the cost of increasing from 4,000 to 5,000
acres is not justified by a 3-percent increase in load reduction and a slight improvement
in outflow concentration (26 to 19 ppb).
WETLAND SOLUTIONS, INC.
Development of Design Criteria for Stormwater Treatment Areas (STAS) in the Northern Lake Okeechobee Watershed
21
300
12
10
k=
Ci =
10 m/yr
250 ppb
*
3 ppb
50 cfs
3
C =
Q=
P=
200
8
150
6
100
4
50
2
0
0
1,000
2,000
3,000
4,000
5,000
6,000
7,000
8,000
9,000
Load Removed (Mt/yr)
Outflow Concentration (ppb)
250
0
10,000
Area (ac)
Concentration
Load
EXHIBIT 13
General Improvement in Water Quality as a Function of Treatment Area
DMSTA2 Simulation Results
There are different approaches that can be taken to select the wetland area for a given
project. The maximum area can be established based on the available land for
construction and the flow can be sized based on the area, or the designer can determine
the flow to be treated (or desired load to be removed) and acquire the area needed to
treat that flow and load.
DMSTA2 simulations were run to further evaluate the relationship between wetland
area and phosphorus removal performance. STA areas were varied from 100 to 10,000
acres. These were steady-state simulations with a constant flow of 100 cfs which resulted
in HLRs ranging from 0.6 to 60.5 cm/d. Three groups of simulations were run with
inflow TP concentrations of 200, 400, and 600 ppb, or inflow loads of 17.9, 35.8, and 53.7
Mtons/yr. Outflow control elevations were set at 40 cm. All configurations were
assumed to consist of single STA cells with an aspect ratio of 1.5:1 and a presumed N =
3. These values for aspect ratio and N are maintained through the remainder of the
simulations in this document.
Exhibit 14 shows the results of the DMSTA2 simulations for variable wetland areas.
These plots show that load reduction initially increases rapidly as area increases (and
HLR decreases) and then reaches a plateau where incremental area increases do not
result in proportional load reduction increases. The point of diminishing returns shifts to
WETLAND SOLUTIONS, INC.
Development of Design Criteria for Stormwater Treatment Areas (STAS) in the Northern Lake Okeechobee Watershed
22
the right (larger areas) as the inflow concentration increases. For the case of Ci = 200 ppb,
88 percent of the applied load was removed for an area of 6,000 acres, but expanding the
area by 66.7 percent (to 10,000 acres) only increased the estimated load reduction to 95
percent of the inflow value.
It is important to note that while only the area (length and width) were varied for each
inflow concentration set of simulations, the HLR was forced to vary based on the
assumption of a constant 100-cfs flow. In addition, there are corresponding effects on
mean water depth, horizontal velocity, and HRT. The overall caveat is that, similar to
the P-k-C* results, there are regions of these curves that are outside a reasonable design
envelope. While the DMSTA2 penalties for sustained high water depths were included
in these results, other than providing automated warning messages that indicate a
simulation is outside the range of the calibration data sets, the model does not “break”
based on pre-defined or user-defined hydraulic constraints.
Load Removed (kg/yr)
60,000
50,000
40,000
30,000
20,000
10,000
0
0
1,000
2,000
3,000
4,000
5,000
6,000
7,000
8,000
9,000
10,000
6000
7000
8000
9000
10000
Area (ac)
100%
Removal Efficiency (%)
90%
80%
70%
60%
50%
40%
30%
20%
10%
0%
0
1000
2000
3000
4000
5000
Area (ac)
TP = 200 ppb
TP = 400 ppb
TP = 600 ppb
EXHIBIT 14
General Improvement in Water Quality as a Function of Treatment Area
WETLAND SOLUTIONS, INC.
Development of Design Criteria for Stormwater Treatment Areas (STAS) in the Northern Lake Okeechobee Watershed
23
Cost Impacts
The effect of area on capital cost can be simply approximated as a linear function with
the application of a reasonable unit area land cost. For the purposes of this analysis, an
average land cost of $10,000 per acre was assumed. As shown in Exhibit 15, capital costs
for land increase dramatically as STA area is increased. Construction costs also increase
as the facility size increases. A more detailed cost benefit analysis is presented in another
section of this document.
100
90
80
Land Cost ($Millions)
70
60
50
40
30
20
10
0
0
1,000
2,000
3,000
4,000
5,000
6,000
7,000
8,000
9,000
10,000
Area (ac)
EXHIBIT 15
Land Purchase Costs as a Function of Total Area
Recommendations
Because land costs can be significant, the minimum area required to conservatively meet
project objectives should be determined for each specific STA application. A full costbenefit analysis will be required during the development of project alternatives, but it is
clear that for any set of operating conditions, there is a break point in the area versus
performance curve at which cost-effectiveness declines.
WETLAND SOLUTIONS, INC.
Development of Design Criteria for Stormwater Treatment Areas (STAS) in the Northern Lake Okeechobee Watershed
24
Inflow Concentration and Mass Loading Rate
In general, wetland nutrient removal rates are fairly well correlated with inflow
phosphorus concentration but are strongly correlated with inflow mass loading rate
(Kadlec and Knight 1996). The effects of inflow concentration and mass loading rate on
mass and concentration reduction were explored in the following ways:
•
Estimate performance at a constant inflow rate (100 cfs) and inflow TP
concentrations ranging from 100 to 800 ppb for a single-size STA. This approach
provides results that are a function of variable concentration and variable mass
loading rate.
•
Estimate performance for a single-sized STA at varying flow rates and varying
inflow concentrations.
DMSTA2 Simulation Results
DMSTA2 simulations were conducted for a 1,000-ac STA with control depths set to 40
cm. All configurations were assumed to consist of single STA cells with an aspect ratio
of 1.5:1 and a presumed N = 3. Exhibit 16 shows the results for DMSTA2 simulations
with constant flows and variable inflow phosphorus concentrations. These plots show
that as inflow concentration and mass loading rate increase, the total load removed also
increased, but efficiency declined. For this example, an 8X increase in inflow
concentration yielded approximately a 4X increase in load reduction with a 2X decrease
in efficiency. Estimated outflow concentrations were reduced by decreasing fractions as
the differential between the inflow concentration and assumed background
concentration (3 ppb) decreased.
Exhibit 17 shows the results of DMSTA2 simulations for a 1000-ac STA with varying
inflow rates and inflow concentrations. For each inflow concentration curve, flows were
varied from 15 to 300 cfs (HLR = 1 to 18 cm/d) to span a range of mass loading rates.
These results show that for a fixed area, more load can be removed, but at a decreasing
efficiency, by increasing the inflow rate (hydraulic loading rate). Load removal is also
increased with increasing inflow concentrations. For larger STAs (for example, 10,000
acres), the depth penalty becomes a concern at HLRs exceeding about 9 cm/d (Exhibit
18).
WETLAND SOLUTIONS, INC.
Development of Design Criteria for Stormwater Treatment Areas (STAS) in the Northern Lake Okeechobee Watershed
25
Inflow Mass Loading Rate (kg/ha/yr)
0
20
0
100
40
60
80
100
120
140
160
180
200
Load Removed (kg/yr)
16,000
14,000
12,000
10,000
8,000
6,000
4,000
2,000
0
200
300
400
500
600
700
800
900
Inflow P Concentration (ppb)
Removal Efficiency (%)
Inflow Mass Loading Rate (kg/ha/yr)
0
20
0
100
40
60
80
100
120
140
160
180
200
100%
90%
80%
70%
60%
50%
40%
30%
20%
10%
0%
200
300
400
500
600
700
800
900
600
700
800
900
Inflow P Concentration (ppb)
Outflow P Concentration (ppb)
700
600
500
400
300
200
100
0
0
100
200
300
400
500
Inflow P Concentration (ppb)
EXHIBIT 16
Estimated Effect of Inflow Concentration and Mass Loading Rate for a 1,000-ac STA at Constant Flow
WETLAND SOLUTIONS, INC.
Development of Design Criteria for Stormwater Treatment Areas (STAS) in the Northern Lake Okeechobee Watershed
26
Load Removed (kg/yr)
16,000
9 cm/d
1 cm/d
14,000
18 cm/d
12,000
10,000
8,000
6,000
4,000
2,000
0
0
100
200
300
400
500
600
Mass Loading Rate (kg/ha/yr)
100%
Removal Efficiency (%)
90%
1 cm/d
80%
70%
60%
50%
40%
30%
20%
9 cm/d
10%
18 cm/d
0%
0
100
200
300
400
500
600
Mass Loading Rate (kg/ha/yr)
Load Removal Rate (kg/ha/yr)
40
9 cm/d
1 cm/d
35
18 cm/d
30
25
20
15
10
5
0
0
100
200
300
400
500
600
Mass Loading Rate (kg/ha/yr)
100 ppb
200 ppb
400 ppb
600 ppb
800 ppb
EXHIBIT 17
Estimated Effect of Inflow Concentration and Mass Loading Rate for a 1000-ac STA at Variable Flow Rates
WETLAND SOLUTIONS, INC.
Development of Design Criteria for Stormwater Treatment Areas (STAS) in the Northern Lake Okeechobee Watershed
27
Load Removed (kg/yr)
160,000
9 cm/d
1 cm/d
140,000
18 cm/d
120,000
100,000
80,000
60,000
40,000
20,000
0
0
100
200
300
400
500
600
Mass Loading Rate (kg/ha/yr)
100%
Removal Efficiency (%)
90%
1 cm/d
80%
70%
60%
50%
40%
30%
20%
9 cm/d
10%
18 cm/d
0%
0
100
200
300
400
500
600
Mass Loading Rate (kg/ha/yr)
Load Removal Rate (kg/ha/yr)
40
9 cm/d
1 cm/d
35
18 cm/d
30
25
20
15
10
5
0
0
100
200
300
400
500
600
Mass Loading Rate (kg/ha/yr)
100 ppb
200 ppb
400 ppb
600 ppb
800 ppb
EXHIBIT 18
Estimated Effect of Inflow Concentration and Mass Loading Rate for a 10,000-ac STA at Variable Flow Rates
WETLAND SOLUTIONS, INC.
Development of Design Criteria for Stormwater Treatment Areas (STAS) in the Northern Lake Okeechobee Watershed
28
Cost Impacts
The modeling results summarized above indicate that total load reduction can be
increased by either increasing the volume of water applied to a particular area or by
increasing the size of the area so that lower outflow concentrations can be achieved. Cost
impacts would then be a function of land area and/or pump station capacity.
Recommendations
The modeling results show that for a fixed area, there is a benefit to increasing the mass
loading rate. Because the source water phosphorus concentration can not be increased to
achieve higher mass loading rates, the flow must be increased instead. Regardless of
system scale, modeled load reduction rapidly increases as the HLR approaches about 5
to 7 cm/d. At these HLRs, for any given inflow concentration, mass removal rates are
effectively maximized.
WETLAND SOLUTIONS, INC.
Development of Design Criteria for Stormwater Treatment Areas (STAS) in the Northern Lake Okeechobee Watershed
29
Water Depth
Because wetland performance has been shown to be area-based, rather than volumebased, increasing the operational water depth, and therefore the nominal HRT
(assuming flow is constant), does not necessarily result in lower outflow concentrations
or greater mass removal (Kadlec and Wallace 2009). Increasing depth decreases contact
between the applied water and the active surfaces in the wetland where removal
processes occur.
DMSTA2 Simulation Results
Exhibit 19 shows the results of DMSTA2 simulations that were run for a 1000-ac STA
with outflow control depths ranging from 10 to 250 cm and HLRs of 3, 6, and 12 cm/d
(50, 100, and 200 cfs). The presumption of vegetatively-controlled head loss used in the
DMSTA2 calculations resulted in effective mean depths that ranged from about 53 to 250
cm even when control depths were set lower. For a HLR of 3 cm/d, control depths of 10,
20, 30, and 40 cm each resulted in a mean operational depth of 53 cm. Mean operational
depths were equal to the control depths for specified control depths of 60, 80, 100, 125,
150, 175, 200, and 250 cm. Thus, on Exhibit 19, there are four data points at an x-axis
value of 53 cm. At a HLR of 6 cm/d, the transition between vegetatively-controlled
depth (i.e., friction) and outlet-controlled depth occurred at a mean depth of 64 cm. At a
HLR of 12 cm/d, the transition occurred at 78 cm.
The load reduction response to increasing operating depth follows the DMSTA2 depthdependent rate constant curve described earlier. For a constant flow or HLR, estimated
load reduction is maximized and stable for operational depths of up to 100 cm.
Performance then declines as the system presumably shifts from an emergent marsh
community to open water (i.e., a pond or reservoir).
WETLAND SOLUTIONS, INC.
Development of Design Criteria for Stormwater Treatment Areas (STAS) in the Northern Lake Okeechobee Watershed
30
9,000
Load Removed (kg/yr)
8,000
7,000
6,000
5,000
4,000
3,000
2,000
1,000
0
0
50
100
150
200
250
300
200
250
300
Mean Operating Depth (cm)
100%
Removal Efficiency (%)
90%
80%
70%
60%
50%
40%
30%
20%
10%
0%
0
50
100
150
Mean Operating Depth (cm)
50 cfs (3 cm/d)
100 cfs (6 cm/d)
200 cfs (12 cm/d)
EXHIBIT 19
Effect of Mean Operating Depth on Load Reduction for a 1000-ac STA
Cost Impacts
The costs for STA construction that relate to increasing the target operational depth are
associated with levee height considerations and perhaps a scale-up in structure
dimensions to accommodate a wider range of stage control.
Recommendations
At present, and unless future calibration enhancements to the DMSTA2 show otherwise,
there is no strong basis for the District to modify the current design control depth of
approximately 40 cm. Site-specific detailed design calculations will be required to show
that a particular STA will operate within a depth range that maximizes inundation of the
wetland area without exceeding plant community tolerances (see Plant Community
Considerations).
WETLAND SOLUTIONS, INC.
Development of Design Criteria for Stormwater Treatment Areas (STAS) in the Northern Lake Okeechobee Watershed
31
Hydraulic Loading Rate
Increasing the HLR for a given STA size must increase the flow, water depth, and mass
loading rate, while it decreases HRT and removal efficiency. Results from previous
sections showed that increasing the mass loading rate to a fixed area also increases mass
removal as long as average depths do not exceed 100 cm.
DMSTA2 Simulation Results
Exhibit 20 shows DMSTA2 results for a 1000-ac STA operating at HLRs ranging from
about 1 to 18 cm/d. These results show that load removal does increase with increasing
HLR but each curve (inflow concentrations of 200, 400, and 600 ppb) begins to level off
when the HLR exceeds about 6 cm/d. These results do not extend to high enough HLRs
to trigger depth concerns in the model. In the section discussing aspect ratios, Exhibit 10
showed that, for moderate aspect ratios (up to about 2:1), sustained HLRs between 6 and
12 cm/d did not degrade performance. Exhibit 10 also showed that the HLR threshold
decreased with increasing STA size.
Cost Impacts
The costs for STA construction that relate to increasing the design HLR are primarily
tied to pump station capacity but could also be associated with levee height
considerations and perhaps a scale-up in structure dimensions.
Recommendations
The District’s existing STAs in the EAA have been operated at long-term average HLRs
ranging from about 0.3 to 2.9 cm/d (SFWMD 2009). These projects are focused on
concentration reduction and thus must be operated at more conservative HLRs than
projects that have been and will be constructed in the NLO watershed. The only
operational STA in the NLO watershed is the Taylor Creek STA. Between June 2008 and
February 2009, it was operated at an average HLR of about 6 cm/d and has removed
about 30 percent of the applied phosphorus load (SFWMD unpublished data). An
attempt was made to model the observed performance of the Taylor Creek STA with
DMSTA2, but was unsuccessful because the system was operating under start-up
conditions. DMSTA2 was constructed to model long-term dynamic performance after
the development of a stable vegetative community.
Considering the depth issues that can occur with increasing flow and HLR, and the
range of potential STA sizes that could be constructed in the watershed, a long-term
average HLR of up to about 6 cm/d is recommended for projects designed for
maximum load reduction.
WETLAND SOLUTIONS, INC.
Development of Design Criteria for Stormwater Treatment Areas (STAS) in the Northern Lake Okeechobee Watershed
32
Load Removed (kg/yr)
14,000
12,000
10,000
8,000
6,000
4,000
2,000
0
0
2
4
6
8
10
12
14
16
18
20
14
16
18
20
Hydraulic Loading Rate (cm/d)
100%
Removal Efficiency (%)
90%
80%
70%
60%
50%
40%
30%
20%
10%
0%
0
2
4
6
8
10
12
Hydraulic Loading Rate (cm/d)
TP = 200 ppb
TP = 400 ppb
TP = 600 ppb
EXHIBIT 20
Effect of HLR on Load Reduction for a 1000-ac STA
WETLAND SOLUTIONS, INC.
Development of Design Criteria for Stormwater Treatment Areas (STAS) in the Northern Lake Okeechobee Watershed
33
Hydraulic Residence Time
Stormwater treatment design guidance documents positively-correlate increased load
reduction efficiency with an increase in nominal HRT (Harper and Baker 2007). Gross
load reduction, however, does not follow the same relationship.
The nominal HRT of an STA can be increased by decreasing the inflow rate or increasing
the volume (by increasing the area or depth). For a given area, decreasing the flow rate
or increasing the depth will decrease the total load reduction. Conversely, if flow is held
constant and area is allowed to increase (so that volume also increases) the total load
reduction will increase.
DMSTA2 Simulation Results
Results from the same model runs described in the previous section (Hydraulic Loading
Rate) were used to evaluate the effects of the nominal HRT on load reduction and load
removal efficiency (Exhibit 21). Because these simulations were run for a fixed STA size
(1,000 ac), HRT varied in response to the changing inflow HLR and its related effect on
operational depth. These results show that load removal was maximized at low HRTs
when inflow HLR and MLR was maximized. Conversely, removal efficiency increased
with increased HRT.
WETLAND SOLUTIONS, INC.
Development of Design Criteria for Stormwater Treatment Areas (STAS) in the Northern Lake Okeechobee Watershed
34
Load Removed (kg/yr)
14,000
12,000
10,000
8,000
6,000
4,000
2,000
0
0
5
10
15
20
25
30
35
40
45
50
35
40
45
50
Hydraulic Residence Time (d)
100%
Removal Efficiency (%)
90%
80%
70%
60%
50%
40%
30%
20%
10%
0%
0
5
10
15
20
25
30
Hydraulic Residence Time (d)
TP = 200 ppb
TP = 400 ppb
TP = 600 ppb
EXHIBIT 21
Effect of Nominal HRT on Load Reduction for a 1000-ac STA
Recommendations
An STA should be designed so that the presumed HLR and water depth regime are
reasonable. If these conditions are met, there is no need to attempt to achieve a
particular nominal HRT.
The mean HRT (tracer-determined) can be maximized by maximizing the hydraulic
efficiency of the system. For load reduction projects where there remains a substantial
differential between treated outflow concentrations and irreducible background
concentrations, the incremental benefit of improving the mean HRT is not significant. If
mass removal efficiencies are required to be extremely high (or outflow concentrations
are required to be near background), improvements in mean HRT can lead to significant
improvements in performance.
WETLAND SOLUTIONS, INC.
Development of Design Criteria for Stormwater Treatment Areas (STAS) in the Northern Lake Okeechobee Watershed
35
Cell Compartmentalization
The subdivision of an available area into multiple cells is based on the following
circumstances:
•
Existing topography may make it more cost-effective to terrace an area into
multiple cells-in-series than to grade a larger cell to a consistent elevation.
Terracing, however, may increase seepage rates between cells. The induced
seepage could increase operational costs such as pumping to maintain water in
the cells with higher ground elevations, or it could result in some cells
experiencing more frequent or longer dryouts.
•
The need for uninterrupted operations may require multiple parallel treatment
trains that allow for individual cells to be taken off line without shutting down
the entire facility.
•
If trying to achieve very low outflow concentrations or very high removal
efficiencies, the incremental performance improvement that can be gained by
constructing cells-in-series may offset the added cost of additional levees and
water control structures.
•
Compartmentalization may facilitate future conversions of some cells to a
different vegetative community (such as SAV) if higher removal rates can be
demonstrated.
DMSTA2 Simulation Results
DMSTA2 simulations were conducted for a hypothetical 1,000-ac site with an initial
aspect ratio of 1.5:1. For simplicity, it was assumed that 1,000-ac of effective area would
remain independent of how many cell subdivisions were made. Exhibit 22 shows the
modeled cell configurations. For an inflow rate of 100 cfs and an inflow concentration of
250 ppb, the simulation results (Exhibit 23) show little difference in total load reduction,
final outflow concentration, and removal efficiency. The primary reason for this
apparent lack of effect is that performance is not very sensitive to the N value if the
overall removal efficiency is not extremely high. Presuming that each cell has N=3
further dampens the benefit of serial compartmentalization in this case. Because each
cell would be designed with attention to inflow distribution and outflow collection
patterns, N was not varied as a function of the resulting aspect ratios. The sensitivity of
model estimates to the type and degree of compartmentalization increases as the target
outflow concentration decreases.
Cost Impacts
Compartmentalization increases costs in several ways. First, there is additional levee
construction required to sub-divide the project site. There is also a proportional increase
in the quantities associated with inflow and outflow distribution channel construction
WETLAND SOLUTIONS, INC.
Development of Design Criteria for Stormwater Treatment Areas (STAS) in the Northern Lake Okeechobee Watershed
36
and flow control structures, although for parallel cell configurations, the unit cost per
structure may decrease.
Q
Q/2
Q/2
Area = 0.5A
Width = 0.5W
L:W = 3:1
Area = 0.5A
Width = 0.5W
L:W = 3:1
Q/2
Q/2
Area = 0.5A
Width = W
L:W = 0.75:1
Area = 0.25A
Width = 0.5W
L:W = 1.5:1
Area = 0.25A
Width = 0.5W
L:W = 1.5:1
Area = 0.5A
Width = W
L:W = 0.75:1
Area = 0.25A
Width = 0.5W
L:W = 1.5:1
Area = 0.25A
Width = 0.5W
L:W = 1.5:1
a)
b)
Area = A
Width = W
L:W = 1.5:1
Q
c)
d)
EXHIBIT 22
Possible STA Cell Configurations for a 1,000-ac Site
WETLAND SOLUTIONS, INC.
Development of Design Criteria for Stormwater Treatment Areas (STAS) in the Northern Lake Okeechobee Watershed
37
EXHIBIT 23
Estimated Performance for Varying STA Cell Configurations for a 1,000-ac Site
Cell Configuration
Performance
Measure
Single Cell
2 Cells-in-Series
2 Parallel Cells
2 Parallel trains of
2 Cells-in-Series
Load Removed
(kg/yr)
7,618
7,763
7,618
7,763
Outflow P (ppb)
165
163
165
163
Removal Efficiency
34
35
34
35
Recommendations
As long as the entire available wetland area remains in operation, parallel cell
construction alone would not be expected to enhance overall system performance unless
the individual cell geometries were drastically improved (for example, increasing aspect
ratio from <<1:1 to 1:1). However, for reasons of increasing operational flexibility and in
order to be able to continue to treat water if the need arises to take a flow path offline, at
least two parallel trains should be considered during the design process.
Serial cell construction is very likely to be dictated by topographic constraints and the
need for terracing. There is no particular minimum or maximum area limitation for a
single cell that has been reported in the literature. Existing individual cell areas in the
District’s STAs range in size from about 60 to 3,500 acres. For proposed large STAs,
hydraulic profile calculations may also indicate that serial cell construction is needed to
constrain differential water depths between the inlet and outlet so that plant community
composition is not impacted. While designers would not intentionally lay out cells to
have poor hydraulic efficiencies, constructing serial cells guarantees a minimum systemwide N of at least 2.
As general design guidance, it is recommended that each STA project should have a
minimum of 2 parallel treatment trains (Exhibit 22b) with serial cell construction as
required based on topographic and hydraulic constraints.
WETLAND SOLUTIONS, INC.
Development of Design Criteria for Stormwater Treatment Areas (STAS) in the Northern Lake Okeechobee Watershed
38
Deep Zones
Deep zones are open water trenches that extend transversely to the direction of flow.
Deep zones must be excavated to a depth (typically greater than 3 feet) that precludes
colonization by emergent aquatic vegetation. Deep zones serve multiple purposes such
as facilitating the settling of suspended solids enhancing aeration of the water column,
allowing ultraviolet light to penetrate and increasing pathogen elimination, and
increasing habitat for fish and wildlife (Kadlec and Wallace 2009).
The direct effect of deep zones on water quality improvement (particularly for nutrients)
is uncertain. Some data indicate greater pollutant reductions with the presence of deep
zones (Knight et al. 1994), while other studies show no clear benefit (Kadlec 2007). One
reason that deep zones may not improve performance is that functional wetland area is
replaced with open water. Because performance is area-based, any reduction in effective
area can have a negative impact on pollutant removal. On the other hand, deep zones
have been promoted based on the presumption that they increase mixing, intercept
short-circuited flow, and redistribute flow across the width of the wetland (Kadlec and
Wallace 2009, Lightbody 2007).
Firm design criteria for deep zones are not found in the treatment wetlands literature.
Empirical data from systems with deep zones are confounded by effects of wind mixing,
variable hydraulic and mass loading rates, varying deep zone geometry, varying
vegetation type and density, and stochastic variability in performance. However, recent
laboratory and field studies (Lightbody et al. 2007, Lightbody et al. 2008), coupled with
the results of mathematical models (Lightbody et al. 2007, Lightbody et al. 2009, ASU
2002) provide a preliminary basis for developing deep zone design criteria.
The potential improvement in hydraulic performance provided by deep zones requires
the existence of preferential flow paths upstream from the deep zone. It is highly likely
that the majority of wetlands have slow and fast flow paths and the results from visual
tracer studies confirm this notion (CH2M HILL 2003, DB Environmental 2000, WSI
2009).
ASU (2002) found that recirculation within deep zones was dependent on the water
depth ratio between deep and vegetated areas and deep zone side slope. Width and
length of deep zone had little effect on recirculation. Field results showed that multiple,
narrow deep zone configurations produce more “plug flow-like” hydraulics, and the
number of deep zones was usually similar to the number of tanks in series.
Results from a bench-scale physical model showed that recirculating currents formed in
deep zones under the conditions encountered in full-scale wetlands (Lightbody et al.
2007). The results from a mathematical model showed that deep zones improved
simulated performance under some conditions.
Lightbody et al. (2009) built on their earlier work and developed a more complex
numerical model to further evaluate the effects of deep zones on wetland performance.
The results showed that deep zones offset the negative effect of short-circuiting by
WETLAND SOLUTIONS, INC.
Development of Design Criteria for Stormwater Treatment Areas (STAS) in the Northern Lake Okeechobee Watershed
39
mixing water from slow and fast flow paths and by reducing the probability that fast
flow paths align throughout the wetland. The modeling showed that at least one deep
zone within a large wetland was beneficial to wetland performance regardless of its
ability to remove pollutants. The size and number of deep zones that benefited
performance varied with wetland length, Damkohler number, and the fraction of flow
assigned to fast flow paths. The simulations and reported field data showed that deep
zones did not improve water quality performance when the deep zone fraction exceeded
0.36 (area of deep zone divided by total area). A threshold was reached where the
hydraulic improvements provided by deep zones were over-shadowed by the
conversion of wetland area (high removal rate) to open water area (lower removal rate).
The results can be interpreted to yield the following guidelines for deep zone design:
•
Most wetlands require a deep zone length in excess of 10 meters (m) to induce
recirculation (deep zone lengths of 5, 15, and 60 m were modeled),
•
The number of beneficial deep zones increases approximately linearly at a rate of
1 deep zone per 100 m of wetland length (for wetlands of up to 1,000 m in
length), and
•
In no case should the deep zone fraction exceed 0.36.
WETLAND SOLUTIONS, INC.
Development of Design Criteria for Stormwater Treatment Areas (STAS) in the Northern Lake Okeechobee Watershed
40
Sediment Accretion Rate and System Life
Expectancy
Phosphorus removal in STAs initially occurs via uptake into a growing “biomachine”
(the macrophytes, algae, microbial assemblages, and invertebrates in the wetland), and
sorption to sediments (Kadlec 1999). Both of these storage compartments can be
saturated, with the time required for saturation dependent upon multiple factors
including the HLR, inflow phosphorus concentration, substrate type and antecedent soil
phosphorus concentration, and biomass growth rate. Once these storages are filled, the
long-term removal mechanism is in the accretion of new sediments (Kadlec 1999). This
sustainable process is further described below.
The rate of sediment accretion is an important variable in the ageing process of
treatment wetlands. They may or may not reach a point at which the depth of accreted
sediments impacts the operation. Over time, the average ground surface elevation in the
wetland can be expected to increase. This will, in turn, require higher water surface
elevations to maintain design flows and depths. Increasing water surface elevations
could affect pump station operation, the flexibility of passive water control structures,
the need for additional levee freeboard, or even the composition and density of the
target vegetation community in the wetland.
This section summarizes sediment and phosphorus accretion data from operational
treatment wetlands and natural wetlands. The purpose of this section is to provide a
simplified but correct summary of the sediment accretion process, identify the range of
accretion rates that have been observed, relate these rates to existing and expected future
conditions in the STAs, and present design and management alternatives for
maximizing treatment wetland design life.
Sediment and Phosphorus Accretion Processes
Sediment accretion in wetlands occurs through a variety of physical and chemical
processes. Water movement is typically very slow in wetlands, which facilitates the
physical deposition of particles that have settling velocities greater than the horizontal
velocity through the wetland. These particles enter the wetland via surface runoff, in the
case of some natural wetlands, or in pumped or gravity inflows, in the case of treatment
wetlands. Physical settling can also occur through the collision of particles with plant
stems, the trapping of particles in biofilms attached to macrophytes and the
sediment/water interface, or any random process that moves particles to a surface
(Kadlec and Knight 1996).
In addition to particles that enter a wetland with surface water inflows, some particles
are deposited with direct rainfall or dryfall. Dryfall particulate matter includes windblown dust, ash, and pollen.
WETLAND SOLUTIONS, INC.
Development of Design Criteria for Stormwater Treatment Areas (STAS) in the Northern Lake Okeechobee Watershed
41
Wetlands directly generate sediments through processes such as the decomposition of
plant leaves and stems, algal growth and decomposition, and the death of wetland
invertebrates. These accreted sediments are often rich in organic matter and have the
properties of the organic peat formed in many natural palustrine wetlands.
Under certain water quality conditions, chemical reactions may also produce solids that
contribute to the sediment layer. The most common precipitates found in treatment
wetlands are iron oxyhydroxides, calcium carbonate, and divalent metal sulfides
(Kadlec and Knight 1996).
There are important differences between the effects of mineral versus organic sediment
deposition in wetlands. Bulk densities for mineral wetland soils generally range from 1.0
to 2.0 grams per cubic centimeter (g/cm3), while bulk densities for organic soils are
typically less than 0.3 g/cm3 (Mitsch and Gosselink 2000). For a given solids loading
rate, mineral deposits occupy a smaller volume than organic sediments. Mineral solids
contain more sand, silt, and clay particles than organic soils and generally do not
compact much without mechanical force. Organic sediments are, in the short-term, more
flocculent than mineral sediments, but are subject to gravitational compaction with time.
Organic sediments are also typically more degradable than mineral sediments. Under
certain conditions, organic sediments can be oxidized, a process which may increase the
bulk density and reduce their volume.
Another distinction is needed between flocculent materials above the sediment/water
interface and more consolidated deposits at the interface. Dense particles will typically
settle directly to the sediment surface, but lighter particles may form a layer of flocculent
material that compacts at a slower rate. This type of settling is analogous to the process
of flocculent settling that occurs in wastewater treatment clarifiers. Over time, the
flocculent layer will consolidate and become more or less indistinguishable from the
underlying native sediments. For the purposes of this document, net accretion is defined
as the long-term process that includes consolidation of flocculent materials, and gross
accretion is defined as the short-term deposition of flocculent materials. It should be
noted that it is difficult to collect sediment samples in a manner that allows for accurate
and discrete measurements of the flocculent and more consolidated fractions of the total
sediment deposits. It should also be noted that short-term field measurements of the
flocculent material zone are not equivalent to measurements of long-term net accretion.
Phosphorus occurs in a variety of soluble and insoluble complexes in organic and
inorganic forms in wetlands (Mitsch and Gosselink 2000). Phosphorus accretion
processes include settling of incoming particulate phosphorus, decomposition and
settling of biomass containing phosphorus, and precipitation of phosphorus with metal
cations. Co-precipitation of phosphorus with calcium carbonate is an additional
component of the accretion cycle in some south Florida STA wetlands.
Each of the processes described above occurs across the entire wetted surface area of the
wetland, but with variable magnitudes that are a function of the system age and loading
rate. Sediment generation and resuspension pathways associated with biomass growth
and decomposition are particularly influenced in this manner. During the start-up
phase, applied nutrients are rapidly incorporated into an expanding active biomass
compartment. As the system matures, and under relatively stable pollutant and
WETLAND SOLUTIONS, INC.
Development of Design Criteria for Stormwater Treatment Areas (STAS) in the Northern Lake Okeechobee Watershed
42
Water Column P Concentration
(mg/L)
hydraulic loading rates, a steady-state case (Exhibit 24) develops in which decreasing
gradients can be measured from the wetland inlet to outlet. These gradients exist for
measurements of biomass (live, dead, and total), water column pollutant concentration,
accreted sediment depth, and sediment pollutant concentration. In adequately-sized
wetlands operated past the startup phase (typically 1 to 5 years), the gradients level off
where all processes are in equilibrium. In Exhibit 24b, this is indicated by the occurrence
of an inflection point in the sediment profile, but water and sediment phosphorus
concentrations also follow a similar trend of decreasing to an equilibrium average
outflow concentration.
0.300
0.200
0.100
0.000
0
0.2
0.4
0.6
0.8
1
Fractional Distance from Inlet
(a)
Biomachine and Sedimentation Processes
Inlet
Outlet
200 ppb
40 ppb
Flocculent Sediments
Accreted Sediments
Original Substrate
(b)
EXHIBIT 24
Steady-state Development of Concentration and Sediment Gradients in Wetlands
If loads are increased, the magnitude of the biomachine processes can be expected to
increase near the inlet zone and for some distance downstream (Exhibit 25). This
increased biomachine results in an increasing net rate of sediment and nutrient
accumulation as indicated by larger downward arrows. The inflection point shown in
the hypothetical sediment thickness profile in Exhibit 25a will also move downstream.
WETLAND SOLUTIONS, INC.
Development of Design Criteria for Stormwater Treatment Areas (STAS) in the Northern Lake Okeechobee Watershed
43
Water Column P Concentration
(mg/L)
0.500
0.400
0.300
Time
0.200
0.100
0.000
0
0.2
0.4
0.6
0.8
1
Fractional Distance from Inlet
a)
Biomachine and Sedimentation Processes
Inlet
Outlet
400 ppb
40 ppb
Flocculent Sediments
Accreted Sediments
Original Substrate
b)
Time
Biomachine and Sedimentation Processes
Inlet
Outlet
400 ppb
60 ppb
Flocculent Sediments
Accreted Sediments
Original Substrate
c)
EXHIBIT 25
Effect of Increasing Inlet Concentration on Treatment Wetland Gradients
a) shows the water column P concentration shifting with time following a load increase. b) shows the initial response of
the wetland to the increased loading. The biomachine expands near the inlet, but the effects of the increase are not
evident throughout the wetland. c) shows the estimated steady-state condition with higher biomachine activity
throughout and greater sediment accretion rates.
WETLAND SOLUTIONS, INC.
Development of Design Criteria for Stormwater Treatment Areas (STAS) in the Northern Lake Okeechobee Watershed
44
Depending upon the loading rate, the wetland may still meet the same water quality
objectives as before, but the equilibrium point will move closer to the wetland outlet.
With continuing increases in loading rate the average outlet phosphorus concentration
will also increase, eventually resulting in an apparent breakthrough of phosphorus
because the wetland is too small for nutrient additions to be balanced by sustainable
removal mechanisms.
Decreasing loads have the opposite effect of increasing loads, with one important
difference (Exhibit 26). As the biomachine gradient effectively shrinks back toward the
inlet zone, and the water column phosphorus gradient also shifts backward, sediments
that previously were at equilibrium with the water column will release some
phosphorus until a new equilibrium is achieved. This phenomenon is denoted in the
schematic figures as an upward arrow proportional to the rate of net phosphorus
release. Some of this released phosphorus will be almost immediately re-incorporated in
growing biomass and redeposited as newly accreted sediments while the rest will travel
downstream in the wetland until it arrives at the point where the previous equilibrium
existed for that higher nutrient concentration. No additional net sediment nutrient
release will occur past that point. Under most conditions of declining pollutant inputs to
a treatment wetland, the water column phosphorus concentrations at the wetland outlet
will not be negatively impacted and will gradually decline in response to the new lower
inlet pollutant loading regime. A family of concentration vs. gradient curves is drawn in
Exhibit 26a to illustrate this temporal and spatial adjustment to reduced phosphorus
inlet concentrations.
The phenomena described above for phosphorus are generally applicable at differing
scales to all dissolved pollutants of concern in treatment wetlands, including the various
forms of nitrogen and a variety of trace metals and organics. These pollutants typically
follow first-order removal rates and have sediment/water column equilibria but have
differing rate constants and background values due to their differing chemistries and
biological affinities.
Sediment and Phosphorus Accretion Rates
The rate of sediment accretion, particularly if very rapid, is an important consideration
for treatment wetland design. Phosphorus accretion rates are also important as a
measure of the long-term, stable phosphorus removal capacity in treatment wetlands.
The following sections compare sediment and phosphorus accretion rates in natural and
engineered treatment wetlands.
Sediment and Phosphorus Accretion Rates in Natural Wetlands
Detailed sediment and phosphorus accretion rates have been measured in the Water
Conservation Areas (WCAs) that lie between Lake Okeechobee and Everglades National
Park. The WCAs are expansive natural marshes that are managed by the District for
water supply, flood control, and habitat preservation. A portion of WCA 2A has been
impacted by 30+ years of agricultural runoff discharges that have elevated the water
column nutrient concentrations above background conditions. Average sediment
accretion rates in WCA 2A ranged from 1.6 mm/yr in unimpacted areas to 4.0 mm/yr in
enriched areas (Craft and Richardson 1993). The highest value of 11.3 mm/yr was
WETLAND SOLUTIONS, INC.
Development of Design Criteria for Stormwater Treatment Areas (STAS) in the Northern Lake Okeechobee Watershed
45
Water Column P Concentration
(mg/L)
0.200
Time
0.100
0.000
0
0.2
0.4
0.6
0.8
1
Fractional Distance from Inlet
a)
Biomachine and Sedimentation Processes
Inlet
Outlet
100 ppb
40 ppb
0
Flocculent Sediments
Accreted Sediments
Original Substrate
b)
Time
Biomachine and Sedimentation Processes
Inlet
Outlet
100 ppb
20 ppb
Flocculent Sediments
Accreted Sediments
Original Substrate
c)
EXHIBIT 26
Effect of Decreasing Inlet Concentration on Treatment Wetland Gradients
a) shows the water column P concentration shifting with time following a load decrease. b) shows the initial response of
the wetland to the decreased loading. Sediment nutrient fluxes change in response to a lower water column P
concentration .c) shows the estimated steady-state condition with lower biomachine activity throughout and reduced
sediment accretion rates.
WETLAND SOLUTIONS, INC.
Development of Design Criteria for Stormwater Treatment Areas (STAS) in the Northern Lake Okeechobee Watershed
46
measured near an inflow structure that carried nutrient-enriched water (Reddy et al.
1993). Accretion rates in WCA 3A ranged from 2.0 to 3.2 mm/yr. In both areas, peat
accretion rates were highest in areas of the marsh with the longest hydroperiods.
Phosphorus accretion rates ranged from 0.08 to 0.23 g/m2/yr in the unenriched areas of
WCA 2A and WCA 3A to 0.46 g/m2/yr in the enriched zones of WCA 2A. The lowest
phosphorus accretion rates were measured in interior areas of the WCAs where
phosphorus loading rates were consistent with historical atmospheric inputs (0.06-0.08
g/m2/yr) while the highest rates were observed where agricultural phosphorus
loadings averaged 0.53 g/m2/yr (Craft and Richardson 1993).
The St. Johns River Water Management District (SJRWMD) manages several large
marshes in the Upper St. Johns River Basin (USJRB). Long-term (since 1900) sediment
accretion rates from ten marsh sites averaged 3.3 mm/yr, but rates since circa 1963
increased to 5.3 mm/yr as a result of hydrologic changes and increasing non-point
source pollutant loads (Brenner et al. 2001). Recent (>1970) phosphorus accretion rates
ranged from 0.08 to 0.38 g/m2/yr with higher rates near inflow control structures. Preimpact phosphorus accretion rates averaged 0.02 g/m2/yr.
Sediment and Phosphorus Accretion Rates in Treatment Wetlands
Sediment and phosphorus accretion have been measured in several operational
treatment wetlands (Exhibit 27), including the Everglades Agricultural Area STAs. It
should be noted that some of the data are from short-term studies in which the depths of
flocculent sediments were measured. In general, accretion rates are higher in treatment
wetlands than in natural wetlands and are a function of incoming particulate loading
rates and a variety of internal processes.
Short-term Studies (Flocculent Sediments)
In south Florida, short-term gross accretion data are available for mesocosms, field-scale
research cells, and full-scale STAs. Short-term sediment accretion rates were measured
in periphyton-based STA (PSTA) mesocosms (6-18 m2), 2,020-m2 test cells, and 20,200-m2
field-scale cells using sediment traps. Results from the 26 individual experimental
treatments ranged from 1.5 to 35 mm/yr and averaged 17.9 mm/yr (CH2M HILL 2003).
Average short-term gross sediment accretion rates were higher in the test cells (28.8
mm/yr) and field-scale cells (23.5 mm/yr) than in the smaller mesocosms (15.0 mm/yr).
Short-term phosphorus accretion rates in the PSTA cells ranged from 0.01 to 1.95
g/m2/yr and averaged 0.34 g/m2/yr across all platforms. Phosphorus accretion
increased with system scale with rates of 0.29 g/m2/yr for the 19 mesocosms, 0.42
g/m2/yr for the 3 test cells, and 0.52 g/m2/yr for the 4 field-scale cells.
The District recently collected sediment cores from STA-1W (formerly called the ENR),
STA-2, STA-5, and STA-6. The depths of flocculent material are reported in Exhibit 27.
These measurements likely over-estimate net sediment deposition rates, as it is difficult
to measure accumulation of flooded sediments with a coring device. Flocculent
sediment accretion rates in the STAs ranged from 12 to 24 mm/yr. These rates are 2 to 4
times greater than the longer-term net accretion rates for the ENR reported by Chimney
et al. (2000) that were based on depths over feldspar horizon markers (see below).
WETLAND SOLUTIONS, INC.
Development of Design Criteria for Stormwater Treatment Areas (STAS) in the Northern Lake Okeechobee Watershed
47
The highest short-term gross accretion rates reported in the treatment wetlands
literature are from the Lake Apopka Marsh Flow-Way Demonstration project. Coveney
et al. (2002) measured a median accretion rate of flocculent sediments of 137 mm/yr in
the first cell (74 ha) of a 217-ha marsh designed to remove nutrients from
hypereutrophic lake water. The corresponding phosphorus accumulation rate was 1.9
g/m2/yr. It should be noted that influent TSS concentrations to this prototype marsh
flow-way ranged from 35 to 190 milligrams per liter (mg/L), well above the typical
values (<10 mg/L) reported for the Everglades STAs (Goforth et al. 2004) and for many
treatment wetlands in Florida and the U.S. (<20 mg/L) reported by Kadlec and Knight
(1996).
EXHIBIT 27
Summary of Sediment and Phosphorus Accretion Rates and Phosphorus Loading Rates for Treatment Wetlands
Site
Sediment
Accretion
Rate mm/yr
P Accretion
2
Rate g/m /yr
P Loading
Rate
g/m2/yr
Source
Short-term (gross) accretion studies
PSTA Mesocosms
1.5 – 28.3
0.01 – 1.95
0.42 – 1.61
CH2M HILL 2003
PSTA Test Cells
24.4- 35.2
0.27 – 0.55
0.31 – 0.39
CH2M HILL 2003
PSTA Field Scale Cells
14.2 – 34.7
0.19 – 0.76
0.55 - 0.77
CH2M HILL 2003
STA-1W
18.4 – 22.1
--
--
SFWMD
STA-2
12.5 – 13.0
--
--
SFWMD
STA-5
15.8 – 23.8
--
--
SFWMD
STA-6
13.7 – 14.1
--
--
SFWMD
137
1.91
0.08
Coveney et al. 2002
5.6
0.44
0.47 – 29.6
Chimney et al. 2000
2.7 – 14.8
0.08 – 3.68
0.08 – 8.71
UF 2001
10
0.17
0.27
Apopka Marsh Flow-way
Long-term (net) accretion studies
Everglades Nutrient Removal
Project
Orlando Easterly Wetland
Houghton Lake
USEPA 1993; Kadlec
1997
Note: PSTA and STA data are known to represent highly flocculent material
WETLAND SOLUTIONS, INC.
Development of Design Criteria for Stormwater Treatment Areas (STAS) in the Northern Lake Okeechobee Watershed
48
Long-term Studies (Net Accretion)
Data from the first 6 years of operation of the Everglades Nutrient Removal Project
(ENR) indicate an average net sediment accretion rate of 5.6 mm/yr and a phosphorus
accretion rate of 0.44 g/m2/yr (Chimney et al. 2000). While these sediments were
reportedly still somewhat flocculent in nature, the data suggest that the sediment depths
measured in other operational STAs will decline with time as compaction processes
continue.
The University of Florida collected sediment data from the 486-hectare (ha) Orlando
Easterly Wetlands (UF 2001). Samples were collected from each of 17 cells and represent
accumulation over 13 years of operation. Most of the cells are shallow to deep marshes.
The terminal cells consist of a deep lake and a hardwood swamp forest. Sediment
accumulation averaged 5.5 mm/yr across the site (White et al. 2001a). Across all cells,
phosphorus accretion rates at the OEW ranged from 0.08 to 3.68 g/m2/yr and averaged
0.66 g/m2/yr. Phosphorus accretion rates did not follow a clear gradient through the
system although rates generally declined through the first four of five stages in the
treatment wetland. Approximately 68 percent of the stored phosphorus was in inorganic
forms (UF 2001).
White et al. (2001b) reported that there had been an accumulation of sediments in the
inflow region of the OEW that caused hydraulic short-circuiting and needed removal to
avoid impacts to long-term phosphorus removal. Exhibit 28 (Black and Wise 2003)
summarizes the inflow and outflow phosphorus concentration data from the OEW.
Within the available period-of-record data prior to the sediment removal project in
August 2002 to March 2003, there is no indication of a decrease in phosphorus removal
performance over time. Slight increases in outlet phosphorus concentration correspond
to periods of higher inflow concentration and not to any apparent ageing effect
discernible in the actual wetland outflow data. Longitudinal transect data show that the
water column equilibrium concentration has been consistently reached at a fractional
distance from the outlet of about 0.7 (Exhibit 29; Black and Wise 2003). This is in spite of
much increased mass loading rate to the wetland in recent years (Exhibit 30; Black and
Wise 2003). These data indicate that the OEW treatment wetland still had excess capacity
and that no sediment removal program was needed to sustain that capacity. Based on
preliminary results reported for OEW, pre- and post-sediment removal, it appears that
average outlet phosphorus concentration was slightly reduced as a result of this project
(0.077 mg/L pre-removal and 0.059 mg/L post-removal, 12-month averages). These
concentration averages are both within the normal range of historical data and are not
significantly different (α = 0.05).
The Houghton Lake marsh has been receiving wastewater effluent since the early 1970’s.
Organic soils historically accreted at a rate of about 2 to 4 mm/yr, as determined from
radio-carbon dating techniques (USEPA 1993; Kadlec 1997). The rate has increased to
about 10 mm/yr in the discharge area since effluent application began. Houghton Lake
is one of the longest-running surface flow treatment wetlands (30+ years), and to date,
has not required any sediment management activities to maintain stable phosphorus
removal rates (Kadlec 1997). The concentration and biomass gradients described above
have been a subject of detailed investigations at the Houghton Lake site (Kadlec 1997).
WETLAND SOLUTIONS, INC.
Development of Design Criteria for Stormwater Treatment Areas (STAS) in the Northern Lake Okeechobee Watershed
49
Monthly Average P Concentration (mg/L)
1.4
1.2
Sediment
Removal
1
0.8
0.6
0.4
0.2
Inflow
Ju
l- 0
2
Ju
l- 0
3
Ju
l- 0
0
Ju
l- 0
1
Ju
l- 9
8
Ju
l- 9
9
Ju
l- 9
6
Ju
l- 9
7
Ju
l- 9
4
Ju
l- 9
5
Ju
l- 9
2
Ju
l- 9
3
Ju
l- 9
0
Ju
l- 9
1
Ju
l- 8
8
Ju
l- 8
9
0
Outflow
EXHIBIT 28
Inflow and Outflow P Concentration Data from the Orlando Easterly Wetlands (adapted from Black and Wise 2003)
0.35
0.30
P Concentration (mg/L)
0.25
0.20
0.15
0.10
0.05
0.00
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
Fractional Distance from Inlet to Outlet
1993
1994
1995
1996
1997
1998
1999
2000
2001
2002
EXHIBIT 29
P Concentration Transect Data from the Orlando Easterly Wetlands (adapted from Black and Wise 2003)
WETLAND SOLUTIONS, INC.
Development of Design Criteria for Stormwater Treatment Areas (STAS) in the Northern Lake Okeechobee Watershed
50
2
Monthly Average P Mass Loading (g/m /yr)
6
5
Sediment
Removal
4
3
2
1
Inflow
Ju
l-0
2
Ju
l-0
3
Ju
l-0
0
Ju
l-0
1
Ju
l-9
8
Ju
l-9
9
Ju
l-9
6
Ju
l-9
7
Ju
l-9
4
Ju
l-9
5
Ju
l-9
2
Ju
l-9
3
Ju
l-9
0
Ju
l-9
1
Ju
l-8
8
Ju
l-8
9
0
Outflow
EXHIBIT 30
Inflow and Outflow P Mass Loading Data from the Orlando Easterly Wetlands (adapted from Black and Wise 2003)
Long-term net phosphorus removal data from about 250 treatment wetlands are
available in the North American Treatment Wetland Database v.2 (CH2M HILL 1998).
These data include calculated phosphorus retention rates with a median value of 4.28
g/m2/yr, an average value of 33.3 g/m2/yr, and a maximum value of 287 g/m2/yr.
These data illustrate the fact that much higher sustainable phosphorus removal is typical
of treatment wetlands that receive higher phosphorus loads (Kadlec 1999).
Options for Management of Long-Term Sediment Accretion
There are several options available for designing treatment wetlands to include
additional capacity for sediment accretion or managing sediments once they have
accreted. It is sound engineering practice to provide for sediment accretion in the design
of the treatment wetland.
STAs are lightly loaded compared to treatment wetlands that receive municipal
wastewater effluent, industrial effluents, livestock runoff, or in some cases, eutrophic
lake water or highly-turbid river water. Thus, the biomachine processes described above
can be expected to result in relatively low sediment accretion rates for the STAs.
In no known case has sediment removal been necessary to maintain phosphorus
removal in a treatment wetland dominated by emergent vegetation. Sediment removal
has occurred at both the OEW and Apopka sites, but primarily for hydraulic reasons.
Neither site was constructed with much levee freeboard, so increases in wetland bottom
elevation caused by sediment deposition impacted the physical operation of these
systems. As indicated in the previous section, sustainable phosphorus removal at OEW
was apparently not impacted by the sediment accretion.
WETLAND SOLUTIONS, INC.
Development of Design Criteria for Stormwater Treatment Areas (STAS) in the Northern Lake Okeechobee Watershed
51
In 2007, the District removed sediment from the western flow-way of STA-1W. The very
loose flocculent layer of soil and poor water quality were creating a condition that
would not allow the submerged aquatic vegetation (SAV) to grow; the top layer was
scraped and hauled offsite. Following rehydration, the SAV recolonized and water
quality performance improved (T. Piccone, personal communication, August 25, 2009).
The disadvantage of post-construction management techniques is that they require the
wetland, or individual wetland cells, to be taken off line for varying periods of time. For
a treatment wetland designed to meet a specific outflow concentration requirement, this
requires treatment area and cell redundancy and a significant cost increase for
construction. Once such a system has been serviced, there is likely to be additional time
needed to restore pre-maintenance biomass levels and achieve stable effluent quality.
This is typically not the most cost-effective design approach for new treatment wetlands.
Allow for Sediment Accretion in System Design
A passive approach for managing sediment accumulation in wetlands is to provide for
additional storage in the system. This can be accomplished either by increasing the levee
height and resulting freeboard beyond what is needed based on system hydraulics
and/or by excavating deep zones below the wetland grade.
Incremental increases in required levee height could be determined from the estimated
or measured accretion rate and the projected life span of the project. Because there is
usually a decreasing gradient of sediment depth from inlet to outlet, levee top elevations
may be sloped accordingly, as long as hydraulic constraints do not mandate otherwise.
Levee height increases can be made as needed at anytime during the life of a treatment
wetland project, deferring construction costs to the future to reduce total present worth
costs.
Mechanical Removal
The mechanical removal of sediments can be accomplished through suction dredging or
excavation using a clamshell or backhoe. Suction dredging can occur while the system is
flooded, but excavation equipment is best used when the system is dewatered and
vegetation is removed.
Suction dredging may temporarily impact water quality by suspending solids and
releasing nutrients. Mechanical sediment removal following a drawdown could also
impact water quality once the system is re-flooded. Wetland soils tend to release
nutrients upon initial flooding (Kadlec and Knight 1996), so operational strategies must
be employed that manage the discharge of water that may not meet permit limitations.
A disadvantage of these mechanical techniques is that they require the drying, hauling,
and disposal of the removed wet sediments. Stewart and Zivojnovich (2004) prepared a
conceptual cost estimate for removing sediments from a treatment wetland.
Assumptions included an accretion rate of 2.8 cm/yr and a residuals management
process that included excavation, dewatering, windrowing, loading, and hauling. The
estimated unit cost was $360 per cubic yard of finished product. They speculated that
the finished composted material could be sold for $5 per cubic yard, which would lower
the overall disposal cost.
WETLAND SOLUTIONS, INC.
Development of Design Criteria for Stormwater Treatment Areas (STAS) in the Northern Lake Okeechobee Watershed
52
Drawdown/Consolidation
Drawdown is a recognized tool in the lake management field for consolidating
flocculent sediments and oxidizing organic matter. Intentional treatment wetland
drawdowns are not often used because nutrients and metals that are bound in the
sediments can be mobilized upon re-flooding. Short-term nutrient releases have been
observed in the Everglades STAs that have experienced dry-outs caused by drought
conditions (Goforth et al. 2004). Nutrient spikes were also observed in the PSTA test cells
and field-scale cells after induced periods of dry-out (CH2M HILL 2003).
A drawdown was successful at the Lake Apopka Marsh Flow-Way for consolidating
sediments. Over the first 29-month period of operation, approximately 33 centimeters
(cm) of flocculent sediment accumulated in the first wetland cell. A drawdown was
conducted, and maximum consolidation of sediments was reached after 65 days, with
bulk densities increasing from about 5 to 36 g/L (Coveney et al. 2002).
Drawdown studies were conducted at the OEW to determine the relationship between
the duration of dry-out and the availability of labile phosphorus. The studies indicated
that drawdowns lasting 60 days or longer may reduce labile phosphorus concentrations
in accreted sediments (UF 2001).
The District attempted to use drawdown in the western flow-way of STA-1W but after
about 4-6 months following rehydration, the flocculent sediments became
unconsolidated and were easily resuspended in the water column, thereby preventing
SAV from re-establishing. The 2007 scraping effort described above was conducted after
the drawdown technique failed (T. Piccone, personal communication, August 25, 2009).
Drawdown/Burning
Drawdown followed by burning can also be conducted to reduce the depth of
accumulated sediments. A prescribed burning experiment was conducted in Cell 3 of
the OEW in 1994 that reduced cell biomass by 60 to 70 percent (UF 2001). An increase in
water column nutrient concentrations was observed following the gradual rehydration
of the cell, but the water was not discharged until concentrations declined to an
acceptable level (UF 2001).
A disadvantage of burning is the loss of vegetation, detritus, and microbial communities
that are responsible for many of the water quality enhancement processes that occur in
wetlands. Denitrification, for example may be limited due to decreased levels of
available organic carbon. Some period of decreased performance is anticipated
following burning.
Summary
While treatment wetlands and STAs in particular have been proven to provide longterm phosphorus removal over a wide range of loading rates, there are differing
opinions as to the need for maintenance, particularly sediment management, during the
design life of a wetland. To date, very few emergent-dominated wetlands have required
any sort of sediment management, and even those where sediments were removed, did
not exhibit degradation in overall performance that implied maintenance was necessary.
WETLAND SOLUTIONS, INC.
Development of Design Criteria for Stormwater Treatment Areas (STAS) in the Northern Lake Okeechobee Watershed
53
Based on the District’s experience, SAV-dominated systems may require sediment
management if resuspension degrades water clarity to the point that the vegetation can
not survive.
Long-term net sediment accretion rates in treatment wetlands ranged from 2.7 to 14.8
mm/yr, with a median value of 5.6 mm/yr. Typical long-term accretion rates that can be
used in treatment wetland design range from 5 to 10 mm/yr, and at these rates, the
effects on system life are expected to be minimal (about 30 to 60 years of system life per
foot of levee freeboard).
Options for managing sediments in treatment wetlands and STAs can be grouped into
two categories: design approaches and maintenance techniques. Treatment wetlands can
be designed with cost-effective sediment management in mind by including deep zones
and/or additional levee freeboard. Incremental increases in required levee height could
be determined from the estimated or measured accretion rate and the projected life span
of the project. Because there is usually a decreasing gradient of sediment depth from
inlet to outlet, levee top elevations may be sloped accordingly, as long as hydraulic
constraints do not mandate otherwise. Once a treatment wetland has reached its design
life (typically from 50 to 100 years) it can be rejuvenated for continuing use by adding
additional height to the original levees.
Though not often necessary or recommended, physical maintenance activities may be
implemented in treatment wetlands that include dredging, drawdowns, controlled
burns, and scraping and removal of plant materials. One significant disadvantage of
post-construction management techniques is that they require the wetland, or individual
wetland cells, to be taken off line for varying periods of time. Once a system has been
serviced, there is additional time needed to restore pre-maintenance biomass levels and
achieve stable effluent quality. These techniques also increase operational costs.
The following conclusions are offered:
•
Sediment accretion is a normal and important process in treatment wetlands that
provides a long-term, stable repository for nutrients and other pollutants of
concern.
•
Long-term net sediment accretion rates in treatment wetlands are reasonably
predictable based upon observations from existing systems.
•
Sediment accretion should be considered during the design of levees and deep
zones for treatment wetlands. For STAs constructed in the Lake Okeechobee
watershed design accretion rates between 0.5 and 1 cm per year are considered
reasonable.
•
Treatment wetland life can be extended cost-effectively by increasing existing
berm dimensions.
•
The potential impacts of dry-outs, dredging, and burning, such as water quality
degradation following rehydration and economic impacts from necessary
increases in treatment wetland area, mandate careful consideration before these
WETLAND SOLUTIONS, INC.
Development of Design Criteria for Stormwater Treatment Areas (STAS) in the Northern Lake Okeechobee Watershed
54
approaches are used in south Florida treatment wetlands, should sediment
management become necessary during the operational life.
WETLAND SOLUTIONS, INC.
Development of Design Criteria for Stormwater Treatment Areas (STAS) in the Northern Lake Okeechobee Watershed
55
Levee Height Considerations
The District and U.S. Army Corps of Engineers have developed design procedures to
determine necessary freeboard heights based on design storm events, wind-caused wave
effects, and the hazard classification potential of the impoundment. District Design
Criteria Memorandum 2 (DCM-2) recommends specific wind and precipitation criteria
for freeboard design for stormwater treatment areas (STAs) and reservoirs. DCM-1
covers hazard potential classification.
“High hazard” impoundments are those where failure or poor operation will probably
cause loss of human life. “Significant hazard” impoundments are those where failure
would cause high economic losses or loss of highly valued ecosystems. “Low hazard”
impoundments are those where failure is not likely to result in loss of life or large
economic impacts, or significant damage to receiving ecosystems.
DCM-2 Summary
Freeboard design approaches based on hazard classification are summarized below.
High and Significant Hazard Potential
Freeboard height is determined for each of the following cases of storm events and
antecedent conditions:
•
Case 1 - 100 year wind with the probable maximum precipitation.
•
Case 2 - 100 year precipitation in conjunction with Saffir Simpson Scale category
5 hurricane winds.
•
Case 3 - Probable maximum wind (PMW) at 200 mph with a normal full storage
level condition. Note that this case is not to be used as design criteria, only a
sensitivity measurement.
•
Case 4 - Wind and precipitation from a specific historic storm event.
For each case, wave height and runup are determined using the Shore Protection
Manual (SPM), Steady State Spectral Wave model (STWAVE), or the Automated Coastal
Engineering System (ACES). Wind setup for each case is estimated using the Zeider Zee
equation if the average water depth exceeds 16 feet, or the Bretschneider method
(Bretschneider 1966) for depths less than 16 feet.
The freeboard value obtained for each case is estimated as a function of the flood
surcharge (routed flood) depth, wind setup, wave height and runup, embankment
settlement, waves induced by potential landslides, and provisions for spillway or
discharge malfunction. The maximum freeboard value resulting from these scenarios
should be used in the design (SFWMD 2006).
WETLAND SOLUTIONS, INC.
Development of Design Criteria for Stormwater Treatment Areas (STAS) in the Northern Lake Okeechobee Watershed
56
Low Hazard Potential
Freeboard for low hazard areas is determined using the maximum of 3 feet or the
combined impact of wind setup and wave runup for a 60-mph 1-hr wind condition at
the maximum storage water level. This water level is determined by the 100-yr 24-hr
precipitation event. Designs in high and significant hazard areas may not determine
freeboard in this manner (SFWMD 2006).
STA Freeboard Design Examples
C-44 Storage Reservoir/STA Project - For internal and external STA embankments, an
encompassing design of 6-ft embankments was used. Freeboard was 2 feet above the
maximum water depth of 4 feet (USACE 2004a).
Taylor Creek STA - Levee freeboard was estimated using effects of a 10-yr 24-hr
precipitation event, wind shear surge, wave runup and backwater effects. Each factor
required 6 inches, 4 inches, 18 inches, and 8 inches of freeboard, respectively. Therefore
the total freeboard was determined to be 3 ft with side slopes of 1V:3H. This freeboard
was added to the levee crest design pool elevation (USACE 2009).
Lakeside Ranch STA – Levee freeboard was estimated as 3 feet with a total berm height
of 6.2 feet (CDM 2007).
WETLAND SOLUTIONS, INC.
Development of Design Criteria for Stormwater Treatment Areas (STAS) in the Northern Lake Okeechobee Watershed
57
Wildlife Habitat and Public Use Features
While improving water quality is the primary objective of the proposed STA projects
north of Lake Okeechobee, creating wildlife habitat is an ancillary outcome of any
constructed wetland project. The trend to create multi-purpose constructed wetlands has
helped to generate an expanding data base for wildlife use in constructed wetlands
receiving highly treated municipal and stormwater effluents.
Wilhelm et al. (1989) describe the planning and design of the City of Show Low, Arizona
treatment wetlands, one of the earliest intentional multi-use constructed treatment
wetlands in the U.S. This system was observed to have very high waterfowl and other
wildlife usage. This wetland was also designed to be user friendly for humans interested
in nature study and waterfowl hunting. Based on that early work a number of authors
have described the ancillary wildlife and human use benefits resulting from treatment
wetlands (Sather 1989; Freierabend 1989; Knight 1992, 1997).
The U.S. EPA conducted a pilot study of wildlife usage and habitat functions of
constructed water quality wetlands during the summer of 1992. The EPA used a
consistent rapid-assessment protocol at six constructed surface flow wetlands (including
two Florida wetlands—Orlando Easterly and Lakeland) to evaluate their habitat
structure and function and the possibility of environmental hazards (McAllister 1992,
1993a, 1993b). No detrimental effects to wildlife that colonize constructed water quality
wetlands were documented by that study. The U.S. EPA subsequently published a
detailed description of the water quality and wildlife habitat benefits of 17 constructed
wetlands for water quality polishing throughout the U.S. (USEPA 1993).
In 2001, at least 21 treatment wetlands in the U.S. listed wildlife habitat creation and/or
human use as principal goals (Knight et al. 2001). Since that time dozens of new systems
have been added, including a large number of examples in Florida (Exhibit 31). In
addition, hundreds of other wetlands have collected and reported quantitative data on
wildlife and/or human uses. The North American Treatment Wetland Database
(NADB) Version 2 (http://www.wetlandsolutionsinc.com/papers.html) is the principal
centralized repository for habitat data for water quality polishing wetlands. This section
summarizes the wildlife habitat structure and function expected to result from creating
constructed STA wetlands north of the Lake.
WETLAND SOLUTIONS, INC.
Development of Design Criteria for Stormwater Treatment Areas (STAS) in the Northern Lake Okeechobee Watershed
58
EXHIBIT 31
Representative Constructed Water Quality Wetlands in the NADB v. 2 that Include Wildlife Habitat and/or Human Use
as Principal Objectives
Wet Area (ha)
Source of
Wastewater
Arcata, CA
15.2
MUN
X
X
Beltway 8 (Harris County), TX
89.0
STW
X
X
Des Plaines, IL
10.1
OTH
X
X
DuPont (Victoria) TX
21.4
IND
X
X
Greenwood Urban Park (Orlando), FL
2.0
STW
X
X
Hayward, CA
58.7
MUN
X
X
Hemet/San Jacinto, CA
14.2
MUN
X
Hillsboro, OR
35.7
MUN
X
X
Incline Village, NV
173.3
MUN
X
X
Indian River County, FL
75.3
MUN
X
X
Iron Bridge (Orlando) FL
494.0
MUN
X
X
Mt.View Sanitary District, CA
37.0
MUN
X
X
Olentangy (Columbus), OH
2.0
OTH
X
X
Phinizy Swamp (Augusta), GA
162.0
MUN
X
X
Pinetop/Lakeside, AZ
51.0
MUN
X
X
Santa Rosa, CA
4.1
MUN
X
Show Low, AZ
54.2
MUN
X
X
Sweetwater (Tucson), AZ
7.0
MUN
X
X
Tres Rios, AZ
4.2
MUN
X
X
Wakodahatchee (Palm Beach County), FL
21.0
MUN
X
X
Site Name & Location
Wildlife Habitat Human Use
Source of Wastewater: MUN - municipal, STW - stormwater, IND - industrial, OTH – other
Vegetation
The constructed wetland environment is generally characterized by a high diversity and
abundance of plants. In many cases wetland plant communities include multiple vertical
strata ranging from groundcover species to shrubs and sub-canopy trees to canopy tree
species. Wetland plant diversity is important in determining wildlife diversity because
of the niches associated with differing vegetative structure, reproduction strategies,
flowering and seeding phenologies, gross productivity, and rates of decomposition
(Mitsch and Gosselink 2000). In addition to their diversity of species and growth
habitats, wetland plants are important for water quality polishing because the physical
and chemical structure they provide supports microbial populations (Kadlec and Knight
1996; Vymazal et al. 1998).
WETLAND SOLUTIONS, INC.
Development of Design Criteria for Stormwater Treatment Areas (STAS) in the Northern Lake Okeechobee Watershed
59
More than 593 macrophytic plant species have been reported from constructed
enhancement wetlands and 427 species from natural water quality enhancement
wetlands. Emergent herbaceous macrophytes account for 501 species in the constructed
wetlands in the NADB v.2 and 290 species in the natural wetlands. A significant variety
of tree and shrub species occurs in some constructed wetlands. Tree and shrub species
are well represented in natural wetlands with 88 species recorded. Vegetative diversity
can be designed into STA projects by establishing multiple habitats (Emergent Marsh,
Littoral Zones, Transitional/Deep Zones, and Tree Islands), by preserving existing
habitats (cypress domes and strands, mesic uplands), and by hydrologically restoring
existing prairie wetlands.
Wildlife
All major animal groups and trophic levels that occur in natural wetlands are
represented in constructed water quality wetlands. Further, population size and
diversity in enhancement wetlands are generally as high as or higher than in other
wetlands. Over 1,400 species of wildlife have been reported for constructed and natural
water quality enhancement wetlands in the NADB v. 2.0. These include more than 700
species of invertebrates, 78 species of fish, 21 species of amphibians, 31 species of
reptiles, 412 species of birds, and 40 species of mammals. Over 800 animal species have
been reported in constructed treatment wetlands alone. Because species lists have been
determined for only a small fraction of the wetland sites listed in NADB v. 2.0, and
because of the widely disparate methods and seasons of measurement, these species
totals underestimate the diversity that exists in water quality wetlands in North
America.
Total populations of mosquito larvae and pupae in water quality enhancement wetlands
are reported from a few projects. Average densities are similar in constructed and in
natural wetlands. Considerable research has been conducted on mosquito breeding in
enhancement wetlands in Florida and California (Knight et al. 2003). Mosquito
populations in these systems are quite low as long as larvivorous fish such as the
mosquitofish ( e.g., Gambusia holbrookii) are resident in the wetland system.
A total of 28 fish species were found in a study of a south Florida STA by Chimney and
Jordan (2008). A total of 29,000 fish were collected during this study, mostly small
species adapted to life in low-oxygen wetland waters such as the mosquitofish, various
killifish, and mollies (e.g., Lucania goodie, Heterandria formosa, and Poecilia latipinna). Five
exotic fish species were collected at this site. Estimated fish density averaged about 76.9
fish/m2 and 1.4 g dry weight/m2. The small fish listed above accounted for about 98% of
the fish density but only about 15% of the overall estimated fish dry weight biomass.
The study by Chimney and Jordan concluded that one reason for the success of wading
birds in the south Florida STAs might be because of the high standing crop of fish and
other aquatic prey organisms supported by these nutrient-enriched man-made
ecosystems.
Bird species counts and population densities vary between sites and even at a single
wetland site on a seasonal basis. Bird species have recently been documented in two of
the District’s STA constructed wetlands (Chimney and Gawlik 2007). A total of 139 bird
WETLAND SOLUTIONS, INC.
Development of Design Criteria for Stormwater Treatment Areas (STAS) in the Northern Lake Okeechobee Watershed
60
species were reported representing 39 families. Wading birds (Ciconiiformes) were
represented by 15 species, shorebirds (Charadriiformes) by 31 species, gallinules and
coots (Gruiformes) by 7 species, and ducks (Anseriformes) by 16 species. Passerines
(Passeriformes) were represented by 39 species. Sixteen of the observed bird species in
the STAs were federally listed as Endangered, Threatened, or of Special Concern. Large
seasonal population variations in the STA bird populations were reported to be due to
migrating birds and due to water depth fluctuations. It is likely that the STAs north of
Lake Okeechobee will need an Avian Protection Plan (APP) similar to those prepared for
the EAA STAs, as the nesting of migratory birds can impact STA operations (M.
Chimney, personal communication, August 25, 2009).
Construction of STAs north of Lake Okeechobee will provide highly productive aquatic
ecosystems that will attract and support higher wetland-dependent wildlife densities
than what currently occurs in existing pastures and disturbed lands. Creation of
multiple habitats, hydrologic restoration of dehydrated areas, and preservation of the
existing high quality habitats within the constructed STAs will further enhance wildlife
diversity and wildlife density.
There are no documented occurrences of detrimental effects to wildlife caused by the
pollutant-cleansing function of constructed wetlands. Eustrongylides ignotus is a
nematode that is parasitic in fish and is transferred to birds when they feed on fish
(Coyner et al, 2002). Mortality of young wading birds may be a consequence of food
transferal from adult to hatchling birds (Spalding et al. 1993). The occurrence of this
parasite was intensively studied in a variety of Florida constructed wetland habitats
(Coyner et al. 2002). Observed infection rates in fish in these environments (principally
mosquitofish) were found to be very low (about 0.6%). Higher infection rates appear to
be correlated with higher nutrient concentrations at wetland sites. However, no
evidence currently exists that shows that wading bird populations and productivity are
lower at constructed water quality enhancement wetlands due to parasitic infections. In
fact, population densities of wading birds are frequently higher at these sites than at
nearby natural but similar wetland sites.
Public Use
Water quality enhancement wetlands provide exceptional public use benefits with
regards to nature studies, exercise activities, education, and miscellaneous activities.
Public use is currently included in the Taylor Creek STA and in several of the EAA STAs
and can be an important aspect of new STAs north of the Lake. Public use facilities can
be phased in as funding allows.
Limited human use data from water quality enhancement wetland systems are included
in the NADB v.2. This database is the only comparative source of information
concerning the variety and intensity of human uses in wetlands constructed for water
quality enhancement.
The Arcata, California, constructed wetland reported an estimated 100,000 visitors per
year in 1992 (Benjamin 1993). This level of activity is sustained because the system is
located in a progressive, coastal California community near a trail system and park-like
setting. Data from Arcata summarized in NADB v. 2.0 indicate that from 27,000 to 64,000
WETLAND SOLUTIONS, INC.
Development of Design Criteria for Stormwater Treatment Areas (STAS) in the Northern Lake Okeechobee Watershed
61
human use-days per year (HUD/y) are devoted to general picnicking and relaxing.
These data may also be expressed on a unit area basis as a total of about 1,600 HUD per
hectare per year (HUD/ha/y) for the entire Arcata Marsh and Wildlife Sanctuary. At the
Show Low, Arizona, constructed water quality wetland, human use data are lumped for
all categories and averaged about 370 HUD/y or about 7 HUD/ha/yr. In 1996, the Iron
Bridge, Florida, constructed wetland reported an overall estimated human use of about
4,800 HUD/y or about 10 HUD/ha/y. These numbers reflect the wide variability in
human uses of water quality enhancement wetlands due to their diverse access issues.
Human use data from four water quality enhancement wetlands in Florida for 2008
were reported by their operations managers. The lowest reported rate of human use was
at the Indian River County constructed wetland (135 acres [55 ha]) with 700 to 800
HUD/y (13 to 15 HUD/ha/y). This system is open to the public for hiking and
birdwatching but there are no facilities to accommodate visitors other than a small
parking lot. The reported total human use at the 1,200 acres (486 ha) Orlando Easterly
Wetland in 2008 was about 15,000 HUD/y (31 HUD/ha/y). This facility has an
informative kiosk and open-air interpretive center and restrooms but is about 20 miles
from the closest metropolitan area. A new nature center was built in 2008 that is
expected to increase public use activities. The Wakodahatchee and Green Cay
constructed wetlands are located in Palm Beach County in south Florida, a highly
urbanized area. Wakodahatchee is about 50 acres (20 ha) in extent and includes a
spacious boardwalk, excellent signage, and a moderate sized parking lot. Estimated
human use at that wetland is 175,000 HUD/y (8,650 HUD/ha/y). Green Cay is perhaps
the most deluxe enhancement wetland in the world. It encompasses about 75 acres (30
ha) and includes a spacious air-conditioned nature center, over 2 miles of 10-foot-wide
boardwalks, an excellent landscape plan and interpretive signage, and the largest paved
parking lot at any U.S. constructed wetland. The estimated total human use at that
facility in 2008 was 290,000 HUD/y (9,550 HUD/ha/y).
The information summarized in the NADB v. 2.0 indicates that humans are using
constructed treatment wetlands for a variety of recreational and aesthetic purposes.
Specific design features such as adequate parking, elevated boardwalks, resting
facilities, and interpretive signage appear to increase the amount of human use activity
at a water quality enhancement wetland.
Very little information is available about how to best integrate human use with water
quality wetlands. Benjamin (1993) provides a useful summary of the issues related to
public perception and use of the most-visited water quality wetland in the United States,
the Arcata Marsh and Wildlife Sanctuary in California. That study concluded that the
Arcata Marsh is a great success in its role as a community open space and as a
recreational, ecological, and educational resource. Interviews with visitors identified
bird and wildlife viewing as the most popular public use activities at the marsh. The
second most popular human use activity focused on its aesthetic qualities, including
scenery, beauty, and open space. The most common response to the survey question
concerning what the public disliked about the Arcata Marsh was “nothing.” These
obvious benefits are being accomplished even as the Arcata Marsh meets its primary
goal of water quality protection.
WETLAND SOLUTIONS, INC.
Development of Design Criteria for Stormwater Treatment Areas (STAS) in the Northern Lake Okeechobee Watershed
62
Plant Community Considerations
Plant communities in existing treatment wetlands have been extensively described in a
number of available publications (Kadlec and Wallace 2009, Kadlec and Knight 1996,
Vymazal et al. 1998). In general, similar or identical plant types and species are utilized
in treatment wetlands worldwide. Specific plant species recommended for use in south
Florida, and particularly in treatment wetlands to be developed on sandy soils north of
Lake Okeechobee, are well known, both from experience with the EAA STAs south of
the Lake and from a variety of full-scale constructed wetlands in south and central
Florida north of the Lake (e.g., Taylor Creek STA, Wakodahatchee and Green Cay
Wetlands in Palm Beach County, Indian River County Wetland, Viera Wetland in
Brevard County, Titusville Blue Heron Wetland, and Orlando Easterly Wetland in
Orange County).
Plant community maintenance challenges faced by the designer and operator of STAs
north of the Lake include possible short circuiting of flows, dry-out for extended
periods, seasonally high hydraulic loading rates, and competition from less desirable
wetland and invasive plant species. The purpose of this section is to briefly summarize
the state of the knowledge concerning the selection, establishment, and maintenance of
desirable plant communities that will optimize phosphorus removal performance in
STA constructed wetland cells and minimize maintenance activities.
Description of the Target STA Wetland Plant Communities
Three general plant community types typically can be found in constructed treatment
wetlands and STAs:
•
Emergent macrophyte (EMG)
•
Submerged aquatic vegetation (SAV)
•
Floating aquatic vegetation (FA)
The dominant plant community in most constructed treatment wetlands is EMG (Kadlec
and Knight 1996). This plant community type has been found to be highly reliable for
effective nutrient removal in most applications and is clearly the first choice for new
STAs located north of Lake Okeechobee.
The use of SAV as a dominant plant community was first carefully evaluated for
enhanced phosphorus concentration reduction south of the Lake in the District’s EAA
STAs and now this plant community is being used extensively at those locations. One
large constructed wetland north of the Lake (Orlando Easterly Project) has implemented
conversion to SAV in some downstream cells although performance information from
that conversion has not yet been published.
FA plant communities have been used extensively in treatment ponds throughout the
southern U.S. and extensive data are available for ponds dominated by either water
WETLAND SOLUTIONS, INC.
Development of Design Criteria for Stormwater Treatment Areas (STAS) in the Northern Lake Okeechobee Watershed
63
hyacinth or duckweed. Published results generally indicate some effectiveness for these
species to take up nutrients; however, effective use of these plant species requires a high
level of maintenance (including plant harvesting and disposal) and sustainable nutrient
uptake rates for un-harvested FA systems are lower than for EMG and SAV plant
communities described above.
In most treatment wetlands a specific plant community is initially specified by the
designer, planted following the completion of site grading, and watered during
establishment with site water. While survival of planted species is not typically 100%,
these species can be established with care in relatively predictable plant assemblages or
monospecific stands. In the EAA STAs, plant community establishment has generally
relied on little to no planting and natural recruitment of adapted wetland plant species
following construction and site hydration.
Following initial plant establishment and startup, plant community composition often
varies from plan and follows a course dictated by the multiple environmental influences
of water depth and flooding duration, water quality, pre-existing seed bank in site soils,
weather, herbivorous insects and other fauna, and plant diseases. Over time, all plant
communities in treatment wetlands, unless they are rigorously maintained, tend to
deviate from the original planned assemblage. These shifts are not necessarily
detrimental to wetland water quality treatment performance. In light of minimizing
unproductive costs, maintenance activities in constructed should be limited to only as
much as necessary to maintain the desired wetland plant community type – not a
preordained list of “desirable” plant species.
Emergent and submerged aquatic plant communities have been identified as being most
desirable for STA performance south of Lake Okeechobee; however, their plant species
dominance has not been described in detail and their range of tolerance to the
fluctuating water regime actually experienced in operational STAs has not been fully
quantified. Detailed analysis of historical STA plant community and hydrological data is
ongoing (Mike Chimney, SFWMD personal communication). These efforts are
preliminary and specific water regime models for predicting plant species dominance in
the STAs are not currently available. A preliminary analysis of relevant treatment
wetland plant community and hydrological data is summarized below to provide
guidance on the most tolerant target plant species in STAs in south Florida to optimize
water quality performance.
The actual plant communities occurring in the existing STAs and in most other
treatment wetlands are much more complex than indicated by general terms such as
“emergent” or “submerged aquatic”. For this plant community evaluation it is assumed
that the best source of information on the nature and composition of the plant
communities that are likely to colonize STAs north of the Lake is existing information
for vegetation within the operational EAA STAs. It should be noted that floristic surveys
in the existing south Florida STAs are by no means exhaustive. STA 1-W has received
the greatest amount of plant species identification due to its longer existence, but none
of the other STAs have even had a single detailed floristic analysis. Where appropriate,
information from treatment wetland plant communities in other parts of south and
central Florida were also examined to fill in incomplete data from the south Florida
STAs.
WETLAND SOLUTIONS, INC.
Development of Design Criteria for Stormwater Treatment Areas (STAS) in the Northern Lake Okeechobee Watershed
64
A total of 131 plant species (not including algae) have been reported from the EAA STAs
(Mike Chimney, SFWMD unpublished data). Exhibit 32 provides an edited list of the
dominant plant species currently recorded from the existing south Florida STAs. This
list of 53 plant species was generated by only including those that have been identified
in at least two STAs and omits many of the most uncommon plant species. This list is
used as a starting point to characterize the diversity of plants that might be best adapted
for the proposed STAs north of the Lake. When possible all plant species in this list are
categorized into three groups based on their origin (native or exotic), their general
tolerance to flooding as indicated by the classification scheme developed by the U.S. Fish
and Wildlife Service (ranging from obligate [OBL] at the wet end of the hydrologic
spectrum, through facultative [FAC, FACW, and FACU] in the middle, and to upland
[UPL] at the driest end of the spectrum), and their growth habit (emergent, submerged,
floating, shrub = woody, or vines). While most of these species are herbaceous (soft
plant tissues) a few are woody (such as willow, primrose willow, wax myrtle, and
elderberry). This list primarily includes obligate and facultative wetland plant species
and does not include upland plant species that have been observed in the STAs under
highly unfavorable conditions of extended drought.
Emergent Plant Community
The majority of wetlands constructed for water quality improvement worldwide and in
the U.S. have targeted an emergent plant community (Kadlec and Knight 1996).
Dominant species used in these emergent wetland designs have included:
•
Typha spp. (cattails)
•
Phragmites communis (common reed)
•
Schoenoplectus (Scirpus) spp. (bulrush)
These particular plant species have been favored world wide in treatment wetlands for
two primary reasons: they are highly tolerant of continuous inundation (at least in the
root zone) and they are highly productive and produce a large amount of fixed carbon
that is essential for most of the water quality purification microbial processes that occur
in treatment wetlands. The published literature for treatment wetlands shows no
consistent preference for any single emergent plant species for phosphorus removal but
does indicate that wetlands with emergent plants are significantly more effective than
systems without plants (open water). An emergent plant community was favored in
early south Florida STA designs due to the proven track record of this plant community
type in dozens of constructed wetlands designed for phosphorus removal and due to its
occurrence in Water Conservation Area (WCA) 2A which was used as a data source for
initial STA process design (Kadlec and Newman 1992).
The Everglades Nutrient Removal (ENR) Project was the prototype for all later STA
designs and principally relied on natural recruitment by cattails (primarily Typha
domingensis). A variety of other emergent wetland plant species were purposely planted
in the ENR, but this practice was found to be cost-prohibitive and unnecessary for
meeting Phase 1 Everglades Construction Project phosphorus removal performance
goals. While a mono-culture of cattails appears to be highly effective for water quality
improvement, tolerant of a fairly wide range of water levels and hydraulic loading rates,
WETLAND SOLUTIONS, INC.
Development of Design Criteria for Stormwater Treatment Areas (STAS) in the Northern Lake Okeechobee Watershed
65
EXHIBIT 32
Common Name
Acrostichum danaeifolium
giant leather fern
Alternanthera philoxeroides
alligatorweed
X
X
Amaranthus australis
southern amaranth; southern waterhemp
X
X
Azolla caroliniana
Carolina mosquitofern
X
X
Bacopa caroliniana
lemon bacopa; blue waterhyssop
X
Bacopa monnieri
herb-of-grace; smooth waterhyssop
X
Ceratophyllum demersum
coontail
X
X
X
X
X
Chara sp.
muskgrass
X
X
X
X
X
Cladium jamaicense
Jamaica swamp sawgrass
X
X
X
Commelina sp.
dayflower
X
Cyperus esculentus
yellow nutgrass; chufa flatsedge
X
Cyperus sp.
sedge
X
Eichhornia crassipes
common water hyacinth
X
Eleocharis interstincta
knotted spikerush
Eleocharis sp.
spikerush
X
Equisetum sp.
horsetail; scouring rush
X
Eupatorium capillifolium
dogfennel
X
Hydrilla verticillata
waterthyme; hydrilla
X
X
Community type
Habitat
Origin
STA-6
X
STA-5
X
STA-3/4
STA-2
Species
STA-1W
STA-1E
Dominant Plants Occurring in the Everglades Agricultural Area STAs (Data Assembled By Mike Chimney, SFWMD, Unpublished Data)
NAT
OBL
EMG
EXO
OBL
EMG
X
NAT
OBL
EMG
X
NAT
OBL
FLT
X
NAT
OBL
EMG
X
NAT
OBL
EMG
NAT
OBL
SAV
X
NAT
OBL
SAV
X
NAT
OBL
EMG
X
-
FACW
EMG
X
EXO
FAC
EMG
X
-
FACW
EMG
X
EXO
OBL
FLT
NAT
OBL
EMG
NAT
OBL
EMG
X
-
FACW
EMG
X
NAT
FAC
EMG
X
EXO
OBL
SAV
X
X
X
X
X
X
WETLAND SOLUTIONS, INC.
Development of Design Criteria for Stormwater Treatment Areas (STAS) in the Northern Lake Okeechobee Watershed
X
X
X
X
X
X
X
66
EXHIBIT 32
Dominant Plants Occurring in the Everglades Agricultural Area STAs (Data Assembled By Mike Chimney, SFWMD, Unpublished Data)
Hydrocotyle sp.
marshpennywort
X
X
Ipomoea cordatotriloba
tievine
Lemna sp.
duckweed
Limnobium spongia
American spongeplant; frog's-bit
Ludwigia peruviana
Peruvian primrosewillow
X
X
Ludwigia repens
creeping primrosewillow; red ludwigia
X
X
Mikania scandens
climbing hempvine
Myrica cerifera
southern bayberry; wax myrtle
Najas guadalupensis
southern waternymph; southern naiad
Nuphar advena
spatterdock; yellow pondlily
Nymphaea odorata
American white waterlily; fragrant waterlily
Nymphoides aquatica
big floatingheart; banana lily
X
Panicum hemitomon
maidencane
X
X
Panicum repens
torpedograss
X
X
Panicum sp.
-
X
X
Phyla nodiflora
turkey tangle fogfruit; capeweed
X
X
Pistia stratiotes
water lettuce
X
X
Pluchea odorata
sweetscent
Polygonum sp.
smartweed; knotweed
Pontederia cordata
X
X
-
FACW
EMG
X
NAT
FACU
EMG
NAT
OBL
FLT
X
NAT
OBL
FLT
X
X
EXO
OBL
EMG
X
X
NAT
OBL
SAV
X
X
NAT
FACW
VIN
NAT
FAC
EMG
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
NAT
OBL
SAV
X
X
NAT
OBL
FLT
X
X
NAT
OBL
FLT
X
X
NAT
OBL
FLT
X
X
X
X
NAT
OBL
EMG
X
X
X
X
EXO
FACW
EMG
X
X
-
FACW
-
NAT
FAC
EMG
NAT
OBL
FLT
NAT
FACW
EMG
X
X
X
X
X
X
X
X
X
X
X
X
X
X
-
OBL
EMG
pickerelweed
X
X
X
X
X
NAT
OBL
EMG
Potamogeton sp.
pondweed
X
X
X
X
-
OBL
SAV
Sagittaria kurziana
springtape; strap-leaf sagittaria
X
NAT
OBL
SAV
WETLAND SOLUTIONS, INC.
Development of Design Criteria for Stormwater Treatment Areas (STAS) in the Northern Lake Okeechobee Watershed
X
67
EXHIBIT 32
Dominant Plants Occurring in the Everglades Agricultural Area STAs (Data Assembled By Mike Chimney, SFWMD, Unpublished Data)
Sagittaria lancifolia
bulltongue arrowhead; duck potato
Sagittaria latifolia
broadleaf arrowhead; duck potato
Sagittaria sp.
arrowhead
Salix caroliniana
carolina willow; coastalplain willow
Salvinia minima
X
X
X
X
X
X
X
X
X
X
NAT
OBL
EMG
X
NAT
OBL
EMG
X
-
OBL
-
X
X
X
X
X
NAT
OBL
EMG
water spangles; water fern
X
X
X
X
X
EXO
OBL
FLT
Sambucus nigra
American elder; elderberry
X
NAT
FACW
EMG
Sarcostemma clausum
white twinevine
X
NAT
FACW
VIN
Spirodela polyrhiza
common duckweed; giant duckweed
X
NAT
OBL
FLT
Typha domingensis
southern cattail
X
X
X
X
X
X
NAT
OBL
EMG
Typha sp.
cattail
X
X
X
X
X
X
-
OBL
EMG
Urochloa mutica
paragrass
X
X
EXO
FACW
EMG
Utricularia floridana
Florida yellow bladderwort
X
X
NAT
OBL
SAV
Utricularia sp.
bladderwort
X
X
X
X
X
NAT
OBL
SAV
Taxa Counts
26
47
37
39
22
X
X
X
Development of Design Criteria for Stormwater Treatment Areas (STAS) in the Northern Lake Okeechobee Watershed
X
X
X
WETLAND SOLUTIONS, INC.
X
26
68
and requires minimal maintenance, cattail mono-cultures are not typical in the STAs. A mix
of emergent and floating plant species is typical of all of the existing south Florida STA
EMG cells. Bulrush species have only been recorded in STA 1-W where they were purposely
planted and common reed has not been reported from any of the south Florida STAs. For
these reasons cattails are the primary species of choice in EMG STA cells north of Lake
Okeechobee.
Cattails dominated three of the original four cells in the ENR for more than five years
following construction and initial vegetation recruitment. Experience gained in the ENR and
in its later incarnation as STA 1-W indicated that cattails would continue to dominate the
wetland plant community as long as water depths were adequate but not too deep. Longterm periods with high hydraulic loading rates to STA 1-W resulted in prolonged water
depths greater than 2 feet in STA 1-W cells 1-3 and the gradual attrition of the dominant
cattail community until water levels were subsequently lowered. There is good evidence
from STA 1-W that an emergent wetland plant community will shift to an ecosystem
dominated by floating and/or submerged aquatic species when water depths consistently
exceed about 2 feet.
Colonization and disturbance history are also important in establishing an emergent
wetland plant community. Natural recruitment of cattails and other desirable wetland
emergent species is retarded in the presence of an existing upland plant community such as
exotic grasses or shrubs. With careful site preparation and water management during
construction, cattail recruitment can be optimized (GGI 2005). Once an emergent plant
community dominated by cattails is established it is fairly resilient to invasion by upland
plant species during droughts. However an EMG plant community with poor cover and
open un-vegetated areas is very susceptible to invasion by competitive upland plant species
following an extended drought.
The highest and therefore driest areas of the STAs are likely to colonize with woody plant
species such as willows and Brazilian pepper. These woody species will subsequently die
out when wetter conditions return, resulting in replacement by emergent herbaceous
species. Existing models of treatment wetland performance are not refined enough to
demonstrate that a woody emergent plant community colonized by young willows or
Brazilian pepper is any better or worse for phosphorus removal than a cattail marsh.
However, higher areas that are outside of the effective flow path will definitely be
ineffective for water quality improvement, and marginally wet areas that colonize with
willows and subsequently convert to cattails may instead result in nutrient releases
following re-flooding and subsequent die-off of the upland/woody plant community.
In summary, the target STA EMG wetland plant community is dominated by a high cover of
cattails. Based on the list presented in Exhibit 32 and from experience from other Florida
constructed treatment wetlands, other emergent plant species that could also contribute to
high primary productivity and plant cover in STA emergent zones include the following:
•
Eleocharis spp. (spikerush)
•
Cladium jamaicense (sawgrass)
•
Pontederia cordata (pickerelweed)
WETLAND SOLUTIONS, INC.
Development of Design Criteria for Stormwater Treatment Areas (STAS) in the Northern Lake Okeechobee Watershed
69
•
Panicum hemitomum (maidencane)
•
Sagittaria spp. (duck potato/arrowhead)
However, due to actual hydrological and water quality conditions, only the first two of
these species, spikerush and sawgrass, are likely to be candidates as suitable alternative
dominants in emergent STAs in south Florida.
Submerged Aquatic Plant Community
Following construction in 1994, Cell 4 of the ENR/STA 1-W was intentionally treated with
herbicides to encourage a non-emergent wetland plant community. The original intention
was to create an open water/mixed marsh/periphyton dominated system that was similar
to plant communities in WCA 2A and in the natural Everglades wetland mosaic that were
known to predominate in areas of low phosphorus concentrations. The actual result of
herbicide applications in Cell 4 was the creation of a deep-water wetland dominated by two
submerged aquatic plant species, Naja guadalupensis (southern naiad) and Ceratophyllum
demersum (coontail). After about five years of operation of Cell 4 with this SAV plant
community, phosphorus removal results were remarkable in this cell, both in terms of the
first-order area-based phosphorus removal rate constant and with respect to the lowest
achievable P concentration.
Concurrent research in a variety of experimental mesocosms helped to verify and refine
these full-scale results and subsequently led the District to an across-the-board program to
replace downstream cattail emergent cells in all of the STAs with deeper water SAV cells.
This re-engineering has had mixed success as various unexpected consequences of wetland
plant ecology have been experienced by the District. Hydrilla verticillata (hydrilla) has been
found to be highly competitive with the desired SAV species (southern naiad and coontail)
and is now tolerated, although not preferred, in SAV cells. Herbicide control of hydrilla has
not been effective in the STAs (Toth, pers. comm. 2008). All SAV plant communities were
found to be adversely affected by high water current velocities induced by excessive
hydraulic loading rates and by high winds during hurricanes. Use of transverse emergent
plant zones at frequent intervals across SAV-dominated STA cells has been adopted as a
reasonable method to counteract wind or current-induced wholesale movement of SAV.
SAV plant species are also relatively easily impacted by continuous shading due to highly
colored inflow waters and by floating aquatic plants such as Eichhornia crassipes (water
hyacinth) and Hydrocotlye spp. (pennywort). Perhaps the most significant challenge for
maintenance of SAV-dominated plant communities in south Florida STAs is drought
management. Most SAV species cannot withstand extensive periods of dryout and may be
totally replaced by open water conditions following a drought. Even after a short period of
dry down, most SAV plant communities require an extended period of several months to reestablish pre-drought plant biomass levels. Wholesale loss of SAV plant species during a
drought may require costly re-inoculation when adequate water inflows are re-established.
In summary, the most desirable SAV species in the STAs are southern naiad and coontail.
Other SAV species such as Potamogeton spp. (pondweed) and Sagittaria kurtziana (strap-leaf
sagittaria) that occur rarely and at low densities in the STAs are not considered to be viable
substitutes for the two species listed above. Other subdominant SAV plant species such as
Utricularia spp. (bladderwort) and the macroalga Chara spp. (muskgrass) are also found in
WETLAND SOLUTIONS, INC.
Development of Design Criteria for Stormwater Treatment Areas (STAS) in the Northern Lake Okeechobee Watershed
70
the south Florida SAV plant communities. Since these two species rarely achieve high
densities and do not prefer habitats with elevated phosphorus concentrations, it is not
considered likely that SAV cells dominated by them would provide acceptably high
phosphorus removal rates.
Hydrologic Optima and Tolerance Ranges for Target STA Plant
Communities
All wetland plants exhibit tolerance to a range of hydrologic conditions. The optimal
portion of this range can be considered the zone where a given plant species is able to
maximize its net primary production, resulting in the greatest amount of accumulated plant
tissue in a given growing season. The range of tolerance to flooding may be considered as
the portion of the water regime between low and high water conditions where the plant is
actually found in natural field conditions (under competitive stress from other plants and
due to grazing by wildlife). While many wetland plants in the absence of competing species
actually have their highest growth rates in saturated but unflooded conditions, in the
competitive environment that occurs in constructed wetlands, the optimal plant growth
may occur in deeper water due to exclusion of upland plant species that would otherwise
compete for sunlight and nutrients.
Plant tolerance to a range of hydrologic conditions can best be observed by looking for
zonation of plant communities over a vertical gradient. Ideal data sets are most likely to be
collected from bowl-shaped wetlands and lakes with long periods of hydrologic data
collection. In such an ideal study site plant communities typically follow a progression of
upland species at the highest elevations, through facultative and obligate wetland species
with distance down gradient. Plant community zonation in response to water depth
variation may be much more difficult to observe in relatively level wetlands where water
depth does not vary along a gradient but is more stochastic.
Currently, there is no single publication that summarizes plant gradient studies and
tolerance ranges in Florida or that synthesizes these data into a general model that can be
used to predict plant survival under a range of water regimes. Until such a comprehensive
synthesis is available, it is still advantageous to analyze relevant local data to develop
general hydrologic tolerance ranges for the target EMG and SAV wetland plant
communities.
STA Plant Community Studies
The District has recently compiled a summary of plant community data collected from the
south Florida STAs (Mike Chimney, SFWMD unpublished data). Data collection methods
and frequency in the STAs have varied over the approximate 15-year period-of-record for
these constructed wetlands. STA 1-W (former ENR) is the most intensively studied south
Florida STA and quantitative data were available for this analysis for the period from 2003
through 2007. In 2003 semi-quantitative estimates of plant dominance (percent cover by
species) were made in quadrats located in all cells in STA 1-W, 2, 5, and 6. In 2004 and 2005
all existing STAs were sampled using a revised methodology consisting of identification of
the single most dominant species as well as up to three sub-dominant species on multiple
transects located in each cell. A third method of semi-quantitative plant community analysis
WETLAND SOLUTIONS, INC.
Development of Design Criteria for Stormwater Treatment Areas (STAS) in the Northern Lake Okeechobee Watershed
71
was applied in 2006 and 2007 in select STAs (1-E, 1W, and 5). These data consist of general
cover categories by species and notes on the first and second dominant species at numerous
sampling sites in each STA cell. Portions of these data sets are summarized and analyzed
below.
In addition to the studies described above, aerial photography from the STAs was used to
estimate overall plant species/community percent cover and dominance in 1999 (STA 1-W),
2002 (STA 1-W), 2003 (STAs 1-W, 2, 5, and 6), 2005 (STAs 1-W, 1-E, 2, 3/4, 5, 6), and 2006
(STAs 1-W, 1-E, 2, 3/4, 5, 6). Exhibit 33 provides a summary of the overall plant community
estimates provided by these aerial photographs. Overall average cell water depths for the
prior or actual water year for each STA cell are also included in this table. This preliminary
analysis indicates that EMG plant communities are dominant in cells with mean water
depths in the range of 45 to 60 cm. SAV dominates in cells with an average water depth
range between 29 and 60 cm. It is clear that this analysis offers minimal predictive power for
STA plant community analysis.
EXHIBIT 33
Summary of STA Plant Community Percent Cover from Aerial Photo Interpretation
Average
Depth
(cm)
STA
Date
EMG
FA
SAV
SHRUB
OTHER
Water
Year
1-E
Feb-05
34.4
0.3
53.0
4.1
8.3
--
--
1-E
Mar-06
28.5
0.0
57.1
1.5
12.9
2006
29
1-W
Apr-99
50.0
6.6
37.4
6.0
0.0
1998
60
1-W
Jan-02
24.7
11.7
59.1
4.4
0.0
2001
54
1-W
Feb-05
21.5
1.8
68.8
4.2
3.6
2004
59
1-W
Mar-06
33.6
0.1
38.9
9.2
18.3
2005
60
2
Dec-03
65.3
0.2
34.5
--
0.1
2002
45
2
Feb-05
63.0
0.2
34.8
0.0
2.1
2004
45
2
Mar-06
60.6
0.1
37.5
0.1
1.8
2005
45
3/4
Feb-05
65.3
1.5
22.0
8.1
3.2
--
--
3/4
Mar-06
45.0
0.3
30.8
8.6
15.3
2006
52
5
Dec-03
46.0
3.6
45.4
5.0
0.0
2002
49
5
Feb-05
46.4
0.5
47.5
5.7
0.0
2004
59
5
Mar-06
46.5
1.1
43.6
1.8
7.0
2005
59
6
Dec-03
67.9
0.9
25.4
5.8
0.0
2002
48
6
Feb-05
71.4
0.4
18.6
9.5
0.1
2004
56
6
Mar-06
68.0
--
12.8
19.0
0.2
2005
56
Plant community data from Mike Chimney, SFWMD unpublished data
Average water depths by STA from SFER 2007
EMG - emergent herbaceous vegetation, FA - floating aquatic vegetation, SAV - submerged aquatic
vegetation, OTHER - includes open water, herbicide-treated areas, and uplands
WETLAND SOLUTIONS, INC.
Development of Design Criteria for Stormwater Treatment Areas (STAS) in the Northern Lake Okeechobee Watershed
72
Analysis of STA Hydrologic Data
Wetland hydroecology is generally considered to be dependent on three principal measures
of flooding: water depth, duration of flooding, and frequency of flood events. In fact there
are many additional environmental measures that also influence wetland hydroecology that
are not included in these three principal forcing functions. Examples of these additional
contributing factors include water quality, soil saturation, competing plant species, grazing
by herbivores or fire, and initial conditions. For most analyses, summarization of the depth
and duration of flooding is sufficient to develop estimates of plant tolerance to flooding.
This analysis is based primarily on those two measures. However, flooding frequency is
subjectively assessed in this analysis.
There are many methods for summarizing and reporting hydrological (water level) data for
wetlands. A hydrograph consists of a time series of water level measurements, typically
collected by use of a data-logging stage recorder. Since water depth is a function of both the
water surface elevation and the ground surface at each point within a wetland, a
hydrograph typically can only illustrate the average time-varying water depth within a
wetland cell.
Water level and water depth data can also be summarized by use of frequency graphs that
illustrate the frequency or probability of encountering all observed water levels or depths
over a given period-of-record. A table of “P values” or exceedance probability values is
often included with a probability of exceedance graph to summarize some of the key points
along the entire distribution of recorded water depth information for the subject wetland.
Analysis of P values is the primary quantitative method utilized in this technical
memorandum for plant community analysis.
Development of a Preliminary STA Plant Community Assessment Tool
STA plant community and water level data collected by the District were used to prepare
graphical relationships between flooding depth probabilities (P values) and dominance
(percent cover) by specific target plant communities. In most cases water level data collected
for the year prior to collection of the plant community data were used for correlation. This
procedure was adopted in light of the relatively rapid response of most of the STA plant
species to changing hydrology. Plant community dominance data collected in each STA cell
were used to provide a semi-quantitative estimate of percent cover by species and these
cover estimates were rolled up to provide an estimate of cover by each major plant growth
form or community type as previously summarized in Exhibit 33. Existing plant community
and hydrological data from STAs 1-W, 2, 5, and 6 were used for this analysis.
Exhibits 34 and 35 summarize the observed relationships between water depth probabilities
or percentiles (P values) and plant cover dominance for two target plant associations
utilized in the STAs (EMG and SAV). For example, the P-10 water depth is that depth that is
exceeded 90 percent of the time (10 percent of the water depths are less than that value)
while the P-90 is a deeper water depth (90 percent of the observed water depths are less
than that value). The correlations between water depth frequencies and percent cover
developed for this analysis do not provide information for specific plant species. If desired,
adequate data do exist to provide a more detailed analysis of flooding tolerance for
individual plant species.
WETLAND SOLUTIONS, INC.
Development of Design Criteria for Stormwater Treatment Areas (STAS) in the Northern Lake Okeechobee Watershed
73
100
90
-0.0668x
y = 221.39e
2
R = 0.3857
80
-0.0928x
y = 4295.6e
2
R = 0.6742
P-10
P-50
P-90
-0.0841x
y = 12965e
2
R = 0.7285
Percent Cover (%)
70
60
50
40
30
20
10
0
0
20
40
60
80
100
120
Water Depth (cm)
EXHIBIT 34
Water Depth Percentiles vs. Emergent Vegetation Percent Cover (STA-1W, STA-2, STA-5, STA-6)
WETLAND SOLUTIONS, INC.
Development of Design Criteria for Stormwater Treatment Areas (STAS) in the Northern Lake Okeechobee Watershed
74
100
0.121x
y = 0.0027e
2
R = 0.2432
90
P-10
P-50
P-90
80
Percent Cover (%)
70
60
50
0.0917x
y = 0.0026e
2
R = 0.1836
40
30
0.0613x
y = 0.3326e
2
R = 0.069
20
10
0
0
20
40
60
80
100
120
Water Depth (cm)
EXHIBIT 35
Water Depth Percentiles vs. Submerged Aquatic Vegetation Percent Cover (STA-1W, STA-2, STA-5, STA-6)
WETLAND SOLUTIONS, INC.
Development of Design Criteria for Stormwater Treatment Areas (STAS) in the Northern Lake Okeechobee Watershed
75
Results from this analysis indicate that there should be no difficulty promoting
emergent wetland plant communities in STA with median water depths in the range
from 30 to 50 cm and maximum depths less than about 70 cm. Within this range of water
depths, constructed wetland cells are expected to have a range of expected emergent
plant cover greater than 50 percent. These models indicate that SAV plant communities
are only likely to dominate when minimum (P-10) water depths are greater than about
40 cm and median (P-50) depths are at least 60 to 80 cm. However, the establishment and
maintenance of SAV systems has required additional measures, such as herbicide
application and inoculation (T. Piccone, personal communication, August 25, 2009).
When the STA target plant communities are estimated at less than 100 percent, it can be
assumed that the rest of the coverage will be made up of alternate, non-target plant
associations. For example, when the estimated EMG cover is less than 100 percent, the
remaining cover will likely include open water (no plant cover), periphyton, SAV, FAP,
and upland species. When SAV cover is estimated as less than 100 percent, the
remainder of the STA area will likely be dominated by a mixture of EMG, open water,
periphyton or algae, FA, and upland species.
Summary and Recommendations
The principal conclusions of this analysis are:
•
Emergent plant communities typically dominated by cattail but also with
relatively high diversity of subdominant emergent plant species provide the
overall preferred plant community option for STAs north of Lake Okeechobee
due to their high carbon production and tested resilience to fluctuating and
continuous water levels;
•
Periodic dry-out is possible in STAs north of the Lake due to the event-driven
hydrology of stormwater inflows. Lengthy dry outs may result in plant
community shifts away from herbaceous EMG to woody EMG plant species in
these cells. These plant communities will naturally shift back to dominance by
herbaceous species with little intervention;
•
Spraying of woody vegetation in STA cells that have been exposed to drought
conditions is not recommended due to the potential impact on normal cell
operations and overall phosphorus removal performance (although this has been
conducted in the EAA STAs);
•
Self-supporting submerged aquatic vegetation wetland plant communities are
considered to be less favorable for the STAs north of Lake unless supplemental
water is available. A detailed analysis of operational data from recently
converted SAV cells at the Orlando Easterly Wetland is recommended to provide
design guidance for this promising plant community in STAs north of Lake
Okeechobee.
•
With the addition of supplemental water in nearly every year a SAV-dominated
wetland plant community might be viable in downstream STA cells. Also, water
should be held in these cells near the end of flow events for as long as possible to
extend saturated soil conditions favorable for maintenance of SAV plant species.
WETLAND SOLUTIONS, INC.
Development of Design Criteria for Stormwater Treatment Areas (STAS) in the Northern Lake Okeechobee Watershed
76
However, extensive intervention will likely be required in addition to
supplemental water additions since the average water regime for these cells is
well within the normal preferred tolerance for competitive emergent plant
species such as cattails.
•
Dominance by sawgrass rather than or in addition to cattails should be
encouraged in STA cells due to the adaptation this species has to surviving
prolonged drought typical of the natural Everglades;
•
Quantitative plant community dominance and water regime data should be
collected by cell in all of the Lake Okeechobee STAs. Water level data should be
collected continuously while plant community data can be collected less
frequently (e.g., once each year at the end of the growing season).
WETLAND SOLUTIONS, INC.
Development of Design Criteria for Stormwater Treatment Areas (STAS) in the Northern Lake Okeechobee Watershed
77
STA Construction Costs
To date, two STAs (Taylor Creek and Nubbin Slough) have been constructed in the Lake
Okeechobee Watershed and a third is currently under construction (Lakeside Ranch). Six
STAs have been constructed in the EAA in various phases and others are constructed
(Ten Mile Creek) or planned (C-44 and C-43) in other watersheds. Cost data were
compiled from Basis of Design Reports (BODRs) and bid tabulations for Nubbin Slough
(USACE 2004b), C-44 (HDR 2006), STA-2 Cell 4 (Brown and Caldwell 2005), STA-5 Cell 3
(URS 2005a), STA-6 Section 2 (URS 2005b), and Lakeside Ranch (CDM 2007), and used to
estimate order-of-magnitude unit costs for key components of STA construction. The
purpose of this exercise was not to develop complete construction cost estimates, but
rather to provide a realistic basis for comparing STA cost-effectiveness as a function of
the design variables discussed in this document. It is recognized that these costs are
dated and have not been normalized (to 2009 dollars for example). It is also recognized
that BODR-level costs may vary significantly from the final engineer’s estimates or bid
tabulations.
Exhibit 36 summarizes the total estimated cost for major construction categories
including mobilization, clearing and grubbing, levee construction, canal construction,
grading, hydraulic structures, removal and demolition, pump stations, electrical and
instrumentation requirements, erosion and sediment control, vegetation management,
and miscellaneous costs. It should be noted that the estimates for expansion projects
(STA-2, STA-5, and STA-6) do not include all the same features and costs as newconstruction projects. The C-44 pump station cost is several times greater than necessary
for the STA component alone as it was sized to fill a 3,500-ac reservoir. Based on the
STAs identified above, and recognition of the differences between projects, the average
cost for wetland construction is approximately $15,300 per acre.
Exhibit 37 summarizes estimated unit costs for major construction components.
WETLAND SOLUTIONS, INC.
Development of Design Criteria for Stormwater Treatment Areas (STAS) in the Northern Lake Okeechobee Watershed
78
EXHIBIT 36
STA Construction Costs Summarized from Engineer’s Estimates at BODR1 Level of Completion
Category
Nubbin Slough
2
C-44
STA-2 Cell 4
Lakeside Ranch
STA-5 Cell 3
STA-6 Section 2
Mobilization/Personnel
---
$2,877,269
$946,279
$801,969
$250,000
$250,000
Clearing and Grubbing
$210,329
$5,487,291
$204,603
$2,440,246
$95,275
$90,125
Levee Construction
$938,288
$1,696,517
$1,563,641
$940,082
$2,450,000
$1,662,500
Canal Construction
$641,440
$18,305,390
$2,132,712
$2,063,422
$3,272,500
$2,555,738
Grading
$448,602
$147,828
$590,642
$13,862,380
$432,500
$390,250
$1,191,183
$4,725,620
$853,403
$6,659,066
$2,285,000
$3,525,000
Miscellaneous
$461,532
$2,259,293
$812,802
$3,507,088
$131,422
$2,386,500
Removal/Demolition
$571,344
$2,347,498
$188,813
$80,364
---
---
$2,630,275
$45,613,029
---
$4,761,131
$618,750
---
Electrical/Instrumentation
$654,440
$1,500,000
$768,500
$1,143,213
$1,120,950
$1,156,950
Erosion/Sediment Control
$1,116,335
$12,095,633
$82,283
$44,682
---
---
Hydraulic Structures
Pump Station
Vegetation Management
---
---
---
---
$533,240
$345,600
Construction Cost
---
$97,055,368
$8,143,678
$36,303,642
$11,189,637
$12,362,663
Engineering
---
---
---
---
$1,118,964
$1,236,266
Construction Management
---
---
---
---
$783,275
$865,386
Contractor Markups
---
$48,818,805
$3,257,873
---
---
---
Indirect/Additional Costs
---
---
---
$20,970,037
---
---
Subtotal
---
---
$11,401,551
$36,303,642
$13,091,875
$14,464,316
Contingency
---
---
$2,886,468
$13,707,230
$2,618,375
$2,892,863
Total
$8,863,768
$145,874,172
$14,288,019
$70,980,909
$15,710,250
$17,357,179
Acres
809
6,200
1,902
2,400
1,985
1,387
$10,956
$23,528
$7,512
$29,575
$7,914
$12,514
Cost per Acre
1
BODR – Basis of Design Report
2
Nubbin Slough costs represent lowest bidding price
WETLAND SOLUTIONS, INC.
Development of Design Criteria for Stormwater Treatment Areas (STAS) in the Northern Lake Okeechobee Watershed
79
EXHIBIT 37
Estimated STA Unit Costs Summarized from Engineer’s Estimates at BODR1 Level of Completion
Component
Nubbin Slough2
C-44
STA-2 Cell 4
Lakeside Ranch
STA-5 Cell 3
STA-6 Section
2
Average
Mobilization/Personnel
---
---
$2,877,26
ls
$946,27
ls
$801,96
ls
$250,000
ls
$250,000
ls
$1,025,103
ls
Clearing and Grubbing
$818
ac
---
---
$1,204
ac
$7.61
cy
$515
ac
$515
ac
$763
ac
Levee Construction
$3.38
cy
---
---
$0.92
cy
$1.55
cy
$3.50
cy
$3.50
cy
$2.57
cy
Canal Construction
$1.52
cy
---
---
$1.78
cy
$3.46
cy
$4.25
cy
$4.24
cy
$3.05
cy
Grading
$0.76
cy
---
---
$1.48
cy
$3.95
cy
$2.54
cy
$2.73
cy
$2.29
cy
Slide gates
---
---
---
---
$27,861
ea
$3,950
ea
---
---
---
---
$15,906
ea
Gated RCBs
---
---
---
---
---
---
---
---
$380,833
ea
$352,500
ea
$366,667
ea
Sluice gates
---
---
---
---
$73,313
ea
---
---
---
---
---
---
$73,313
ea
Gated control
---
---
$133,800
ea
---
---
---
---
---
---
---
---
---
---
Spillways
---
---
$107,247
ea
---
---
---
---
---
---
---
---
---
---
Vertical gates
---
---
$195,000
ea
---
---
---
---
---
---
---
---
---
---
Piping
$141.31
lf
---
---
$283
lf
$202
lf
---
---
---
---
$242.58
lf
Weir gates
$7,733
ea
$110,443
ea
---
---
---
---
---
---
---
---
$59,088.00
ea
Removal/Demolition
$15.75
cy
---
---
$4.01
cy
$80,364
ls
---
---
---
---
---
---
Pump Station
$26,303
cfs
$41,466
cfs
---
---
$13,009
cfs
$11,250
cfs
---
---
$23,007
cfs
Instrumentation
Erosion/sediment
Vegetation
---
---
---
---
---
---
$19,054
ls
---
---
---
---
$19,054
ac
$31.12
cy
$41
sy
$0.15
sy
$0.18
sy
---
---
---
---
$14
sy
---
---
---
---
---
---
---
---
$206.00
ac
$240.00
ac
$223.00
ac
1
BODR – Basis of Design Report
2
Nubbin Slough costs represent lowest bidding price
WETLAND SOLUTIONS, INC.
Development of Design Criteria for Stormwater Treatment Areas (STAS) in the Northern Lake Okeechobee Watershed
80
STA Cost Effectiveness
Previous sections of this document showed the estimated relationships between design
variables and phosphorus load reduction. While relatively small adjustments to some
variables resulted in large changes in estimated load reduction, costs must be included
in order to fully understand the impacts of selected design criteria. This section
combines the modeling results from previous sections with general cost data to estimate
overall cost per kilogram of phosphorus removal as each design variable is changed.
Cost Basis
The cost analysis is not intended to be complete, but includes line items for major
construction components that vary based on definable relationships to selected design
criteria. Unit cost assumptions are summarized below.
Land
Land acquisition was included and estimated at $10,000/ac.
Levee Construction
Perimeter and cell sub-dividing levees were assumed to have the following dimensions:
•
Top width of 14 feet
•
Height of 7 feet above the design wetland grade
•
Side slopes of 3:1
These dimensions result in a cross sectional area of 245 square feet (ft2) and a fill volume
of approximately 47,900 cubic yards (cy) per mile. Using a unit cost for compacted fill of
$3/cy resulted in a levee construction cost of about $143,700 per mile.
Deep Zone/Canal Construction
Internal deep zones and distribution/collection channels were assumed to have the
following dimensions:
•
Top width of 100 feet
•
Depth of 4 feet below the design wetland grade
•
Side slopes of 3:1
These dimensions result in a cross sectional area of 348 ft2 and a cut volume of
approximately 68,050 cy per mile. Using a unit cost for excavation of $3.50/cy resulted
in a construction cost of about $238,200 per mile.
WETLAND SOLUTIONS, INC.
Development of Design Criteria for Stormwater Treatment Areas (STAS) in the Northern Lake Okeechobee Watershed
81
Cell Grading
Cell grading requirements depend on existing site topography and the need to create
level surfaces. This analysis assumed that 1 foot of material would be moved across the
site. At a unit cost of $2.50/cy, grading costs were estimated as $4,030/ac.
Water Control Structures
Water control structures (inflow, outflow, and cell-to-cell) were assumed to have a unit
cost of $250,000 each.
Clearing and Grubbing
Clearing and grubbing was assigned a unit cost of $1,000/ac.
Pump Stations
Pump stations were assumed to have a unit cost of $17,000/cfs.
Operations and Maintenance
Operations and maintenance (O&M) costs were included and estimated to be $1,000 per
acre per year.
Adjustment to Present Worth Costs
Life-cycle costs were estimated as the sum of the total capital construction costs and the
present value of the O&M costs. Present worth O&M costs were estimated using a
discount rate of 6% and design life of 50 years (present value multiplier = 15.76).
Prototype STA Designs
With the exception of comparing results for varying wetland area, two generic STA
configurations were used for the cost-benefit analysis. Exhibit 38 shows general design
criteria for the two STA configurations and notes the exceptions needed to evaluate the
various modeling scenarios presented in this document. For example, to evaluate the
effect of aspect ratio, simulations were conducted for single-cell 1,000-ac and 10,000-ac
STAs with constant hydraulic loading rates of 6 cm/d and a constant inflow phosphorus
concentration of 250 ppb. Only aspect ratio was varied (range 0.5:1 to 20:1). STA
dimensions (and cost) varied in response to the fixed wetted area and changing aspect
ratio.
WETLAND SOLUTIONS, INC.
Development of Design Criteria for Stormwater Treatment Areas (STAS) in the Northern Lake Okeechobee Watershed
82
EXHIBIT 38
Prototype STA Design Criteria Ranges
Design Variable
STA-1
STA-2
Exceptions
Effective Treatment Area (ac)
1,000
10,000
Areas ranging from 100 to 10,000
ac used to evaluate effect of area
on cost-effectiveness
Site Aspect Ratio
1.5:1
1.5:1
Values of 0.5:1 to 20:1 were used
to evaluate effect of aspect ratio
Systems with single cells, 2
parallel cells, 2 series cells, and
2x2 parallel and series cells were
used to evaluate effect of
compartmentalization
Number of Cells
1
1
Flow Rate (cfs)
100
1,000
See HLR exception
Hydraulic Loading Rate (cm/d)
6
6
Loading rates were varied from 1
to 18 cm/d to evaluate effect of
increasing HLR (or flow) for fixed
areas
Outlet Control Depth (cm)
40
40
Values of 10 to 250 were used to
evaluate effect of operating depth
Deep Zone Fraction
0.15
0.15
Structures per Cell
4
4
Inflow Phosphorus
Concentration (ppb)
250
250
Values of 100 to 800 ppb were
used to evaluate the effect of
inflow concentration and mass
loading rate
Cost-Effectiveness vs. Aspect Ratio
Exhibit 39 shows the estimated effect of aspect ratio on the cost-effectiveness of
phosphorus removal. As noted above, inflow hydraulic loading rates and phosphorus
concentrations were unchanged between the 1,000-ac and 10,000-ac scenarios. For a
1000-ac STA, 50-year present worth costs are relatively unimpacted by the selected
aspect ratio (range of $109/kg to $111/kg). Costs are more affected as the STA area
increases. For the 10,000-ac case, present worth costs increased from about $104/kg to
$138/kg as a function of aspect ratio. This is because STA performance declined in
response to increased water depths and lower net removal rates for aspect ratios greater
than 3:1.
WETLAND SOLUTIONS, INC.
Development of Design Criteria for Stormwater Treatment Areas (STAS) in the Northern Lake Okeechobee Watershed
83
160
140
Cost of P Removal ($/kg)
120
100
80
60
40
20
0
0
2
4
6
8
10
12
14
16
18
20
Aspect Ratio
1,000-ac STA
10,000-ac STA
EXHIBIT 39
Effect of Aspect Ratio on Cost of Phosphorus Removal
Cost-Effectiveness vs. Area
Exhibit 40 shows the estimated effect of treatment area on the cost-effectiveness of
phosphorus removal. For these cases, the inflow rate was held constant (100 cfs). Two
inflow phosphorus concentrations were modeled (250 and 400 ppb). Cost-effectiveness
improved with increasing inflow phosphorus concentration. Regardless of inflow
concentration, cost-effectiveness was maximized for a treatment area of about 800 to
1,000-ac (HLR of 5 to 6 cm/d). If inflow rates were increased, the shape of the curves
would be expected to remain similar to those in Exhibit 40, but the minimum cost point
would shift to the right (larger area required).
WETLAND SOLUTIONS, INC.
Development of Design Criteria for Stormwater Treatment Areas (STAS) in the Northern Lake Okeechobee Watershed
84
400
350
Cost of P Removal ($/kg)
300
250
200
150
100
50
0
0
1,000
2,000
3,000
4,000
5,000
6,000
7,000
8,000
9,000
10,000
Wetted Area (ac)
250 ppb
400 ppb
EXHIBIT 40
Effect of Wetland Area on Cost of Phosphorus Removal
Cost-Effectiveness vs. Inflow Phosphorus Concentration
Exhibit 41 shows the estimated effect of inflow phosphorus concentration on the costeffectiveness of phosphorus removal for 1,000-ac and 10,000-ac STAs. Cost-effectiveness
improved substantially with increasing inflow phosphorus concentration and slightly
with increasing STA size. Cost-effectiveness was maximized at inflow concentrations
exceeding about 450 ppb.
WETLAND SOLUTIONS, INC.
Development of Design Criteria for Stormwater Treatment Areas (STAS) in the Northern Lake Okeechobee Watershed
85
250
Cost of P Removal ($/kg)
200
150
100
50
0
0
100
200
300
400
500
600
700
800
900
Inflow P Concentration (ppb)
1,000-ac STA
10,000-ac STA
EXHIBIT 41
Effect of Inflow Phosphorus Concentration on Cost of Phosphorus Removal
Cost-Effectiveness vs. Phosphorus Mass Loading Rate
Exhibit 42 shows the estimated effect of inflow phosphorus mass loading rate on the
cost-effectiveness of phosphorus removal for 1,000-ac and 10,000-ac STAs. Because mass
loading rate is simply the product of the flow rate and inflow concentration, these
results have the same relationship as those in Exhibit 41. Cost-effectiveness improved
substantially with increasing phosphorus mass loading rate and slightly with increasing
STA size.
WETLAND SOLUTIONS, INC.
Development of Design Criteria for Stormwater Treatment Areas (STAS) in the Northern Lake Okeechobee Watershed
86
250
Cost of P Removal ($/kg)
200
150
100
50
0
0
20
40
60
80
100
120
140
160
180
200
Phosphorus Mass Loading Rate (kg/ha/yr)
1,000-ac STA
10,000-ac STA
EXHIBIT 42
Effect of Phosphorus Mass Loading Rate on Cost of Phosphorus Removal
Cost-Effectiveness vs. Mean Depth
Exhibit 43 shows the estimated effect of mean operating depth on the cost-effectiveness
of phosphorus removal for 1,000-ac and 10,000-ac STAs. For these simulations, outlet
control depths were varied from 10 to 250 cm. Because STA depths are often controlled
by vegetative friction rather than outlet control elevation, the estimated mean operating
depths exceed the specified control depths for control depths less than 80 cm. Although
a wide range of control depths were evaluated, it is not recommended to deviate from
the ad hoc District standard setting of 40 cm. The relationship between cost-effectiveness
and operating depth follows the DMSTA2 depth versus removal rate curve (Exhibit 3).
Cost-effectiveness was constant and maximized for operating depths less than 100 cm
and degraded rapidly as depth increased above 100 cm.
WETLAND SOLUTIONS, INC.
Development of Design Criteria for Stormwater Treatment Areas (STAS) in the Northern Lake Okeechobee Watershed
87
2,000
1,800
Cost of P Removal ($/kg)
1,600
1,400
1,200
1,000
800
600
400
200
0
0
50
100
150
200
250
300
Mean Operating Depth (cm)
1,000-ac STA
10,000-ac STA
EXHIBIT 43
Effect of Mean Operating Depth on Cost of Phosphorus Removal
Cost-Effectiveness vs. Hydraulic Loading Rate
Exhibit 44 shows the estimated effect of HLR on the cost-effectiveness of phosphorus
removal for 1,000-ac and 10,000-ac STAs. There was very little difference in costeffectiveness as a function of scale for HLRs less than 9 cm/d. Cost-effectiveness was
relatively stable (about $100/kg to $110/kg) between HLRs of 5 and 11 cm/d. These
results tend to confirm the recommendation presented earlier in this document that
STAs in the Northern Lake Okeechobee Watershed (where the focus is load reduction)
may be operated at higher sustained HLRs than those in the EAA (where the focus is
minimizing outflow concentration).
WETLAND SOLUTIONS, INC.
Development of Design Criteria for Stormwater Treatment Areas (STAS) in the Northern Lake Okeechobee Watershed
88
300
Cost of P Removal ($/kg)
250
200
150
100
50
0
0
2
4
6
8
10
12
14
16
18
20
Hydraulic Loading Rate (cm/d)
1,000-ac STA
10,000-ac STA
EXHIBIT 44
Effect of Hydraulic Loading Rate (or flow) on Cost of Phosphorus Removal
Cost-Effectiveness vs. Hydraulic Residence Time
Exhibit 45 shows the estimated effect of nominal HRT on the cost-effectiveness of
phosphorus removal for 1,000-ac and 10,000-ac STAs. As previously indicated, it is not
necessary to design for a specific nominal HRT. If an STA is designed so that other
parameters are constrained to a reasonable range, HRT will, by default, fall within a
reasonable range as well. For these simulations, cost-effectiveness was maximized at
nominal HRTs of about 7 days for a 1,000-ac STA and 11 days for a 10,000-ac STA.
WETLAND SOLUTIONS, INC.
Development of Design Criteria for Stormwater Treatment Areas (STAS) in the Northern Lake Okeechobee Watershed
89
300
Cost of P Removal ($/kg)
250
200
150
100
50
0
0
10
20
30
40
50
60
70
Hydraulic Residence Time (d)
1,000-ac STA
10,000-ac STA
EXHIBIT 45
Effect of Nominal Hydraulic Residence Time on Cost of Phosphorus Removal
Cost-Effectiveness vs. Compartmentalization
Exhibit 46 shows the estimated effect of compartmentalization on the cost-effectiveness
of phosphorus removal for 1,000-ac and 10,000-ac STAs. For the 1,000-ac example STA,
the single-cell configuration was the most cost-effective ($110/kg). Estimated mass load
reduction did not increase enough to overcome the added cost of additional levee
construction and control structures. A slight improvement in cost-effectiveness was
estimated for the 10,000-ac STA for the cells-in-series case. In spite of the slight increases
in cost per kilogram, it would still be advisable to construct parallel cells to increase
operational flexibility.
WETLAND SOLUTIONS, INC.
Development of Design Criteria for Stormwater Treatment Areas (STAS) in the Northern Lake Okeechobee Watershed
90
125
120
Cost of P Removal ($/kg)
115
110
105
100
95
90
Single Cell
2 Cells-in-Series
2 Parallel Cells
2x2 Parallel and Series
Compartmentalization
1,000-ac STA
10,000-ac STA
EXHIBIT 46
Effect of Compartmentalization on Cost of Phosphorus Removal
WETLAND SOLUTIONS, INC.
Development of Design Criteria for Stormwater Treatment Areas (STAS) in the Northern Lake Okeechobee Watershed
91
References
Arizona State University (ASU). 2002. Evaluation of flow fields in wetlands using
physical models. U. S. Bureau of Reclamation, Denver, CO, USA.
Benjamin, T.S. 1993. Alternative Wastewater Treatment Methods as Community
Resources: The Arcata Marsh and Beyond. Master of Landscape Architecture Thesis,
University of California at Berkeley.
Black, C.A., and W.R. Wise. 2003. Evaluation of past and potential phosphorus uptake at
the Orlando Easterly Wetland. Ecological Engineering, 20, pp. 277-290.
Brenner, M., C.L. Schelske, and L.W. Keenan. 2001. Historical rates of sediment and
nutrient accumulation in marshes of the Upper St. Johns River Basin, Florida, USA.
Journal of Paleolimnology 26: 241-257.
Bretschneider, C. L. 1966. “Engineering Aspects of Hurricane Surge”, Chapter 5 in
Estuary and Coastline Hydrodynamics, Arthur T. Ippen, Ph. D. McGraw-Hill Book
Company.
Brown and Caldwell. 2005. Basis of Design Report – Stormwater Treatment Area 2 Cell 4
Expansion Project. Prepared for the South Florida Water Management District.
CDM. 2007. Lake Okeechobee Fast Track Project Basis of Design Report – Lakeside
Ranch STA. Prepared for the South Florida Water Management District.
CH2M HILL. 1998. Treatment Wetland Habitat and Wildlife Use Assessment Project.
Prepared for U.S. Environmental Protection Agency Office of Wastewater Management,
U.S. Bureau of Reclamation, and City of Phoenix, Arizona.
CH2M HILL. 2003. PSTA Research and Demonstration Project Phase 1, 2, and 3
Summary Report. Prepared for the South Florida Water Management District.
Chimney, M.J. and D.E. Gawlik. 2007. The Avifauna of Constructed Treatment Wetlands
in South Florida Used for Everglades Restoration. Florida Field Naturalist 35(1): 9-21.
Chimney, M.J. and F. Jordan. 2008. The Fish Assemblage of a Constructed Treatment
Wetland in South Florida. Florida Scientist 71(3): 246-264.
Chimney, M.J., M. Nungesser, J. Newman, K. Pietro, G. Germain, T. Lynch, G. Goforth,
and M.Z. Moustafa. 2000. Chapter 6: Stormwater Treatment Areas – Status of Research
and Monitoring to Optimize Effectiveness of Nutrient Removal and Annual Report on
Operational Compliance. In G. Redfield, ed., Everglades Consolidated Report, South
Florida Water Management District, West Palm Beach, FL.
Coveney, M.F., D. L. Stites, E.F. Lowe, L.E. Battoe, and R. Conrow. 2002. Nutrient
removal from eutrophic lake water by wetland filtration. Ecol. Eng. 19, 141–159.
WETLAND SOLUTIONS, INC.
Development of Design Criteria for Stormwater Treatment Areas (STAS) in the Northern Lake Okeechobee Watershed
92
Coyner, D.F., M.G. Spalding, and D.J. Forrester. 2002. Epizootiology of Eustrongylides
ignotus in Florida: distribution, density, and natural infections in intermediate hosts.
Journal of Wildlife Diseases, 38(3), 483-499.
Craft, C.B., and C.J. Richardson. 1993. Peat accretion and N, P, and organic C
accumulation in nutrient-enriched and unenriched Everglades peatlands. Ecological
Applications, Vol. 3(3), 446-458.
DB Environmental. 2000. Submerged Aquatic Vegetation/Limerock Treatment System
Technology for Removing Phosphorus from Everglades Agricultural Area Waters:
Follow-on Study, STA-1W Cell 4 Tracer Study Report. Prepared for the South Florida
Water Management District.
Freierabend, J.S. 1989. Wetlands: The Lifeblood of Wildlife Chapter 7, pp. 107-118. In
D.A. Hammer (ed.), Constructed Wetlands for Wastewater Treatment: Municipal,
Industrial, and Agricultural. Chelsea, Michigan: Lewis Publishers. 1989.
Goforth, G., K. Pietro, G. Germain, N. Iricanin, L. Fink, D. Rumbold, and R. Bearzotti.
2004. Chapter 4A: STA Performance and Compliance. In G. Redfield, ed., 2004
Everglades Consolidated Report, South Florida Water Management District, West Palm
Beach, FL.
Gary Goforth, Inc. (GGI) 2005. Summary of STA Vegetation Management Practices.
Prepared for the South Florida Water Management District. February 2005.
Harper, H.H. and D. Baker. 2007. Evaluation of Current Stormwater Design Criteria
within the State of Florida. Prepared for the Florida Department of Environmental
Protection.
HDR Engineering, Inc. 2006. C-44 Reservoir/STA Project Basis of Design Report.
Prepared for the South Florida Water Management District.
Jacksonville District U.S. Army Corps of Engineers (USACE). 2004a. Central and South
Florida Indian River Lagoon – South, Final Integrated Project Implementation Report
and Environmental Impact Statement.
Jacksonville District U.S. Army Corps of Engineers (USACE). 2004b. Abstract of Offers –
Construction, Lake Okeechobee Water Retention, Phosphorus Removal Project, Nubbin
Slough – New Palm Storm Water Treatment Areas, Okeechobee County, Florida.
Jacksonville District U.S. Army Corps of Engineers (USACE). 2009. Lake Okeechobee
Water Retention/Phosphorus Removal Project: Taylor Creek Stormwater Treatment
Area (STA) Water Control Plan.
Kadlec, R.H. 1997. An Autobiotic Wetland Phosphorus Model. Ecological Engineering,
Vol. 8, No. 2, pp. 145-172.
Kadlec, R.H. 1999. The limits of phosphorus removal in wetlands. Wetlands Ecology and
Management 7: 165-175.
Kadlec, R.H. 2007. The effects of deep zones on wetland nitrogen processing. Water
Science and Technology 56(3): 101 – 108.
WETLAND SOLUTIONS, INC.
Development of Design Criteria for Stormwater Treatment Areas (STAS) in the Northern Lake Okeechobee Watershed
93
Kadlec, R.H. and R.L. Knight. 1996. Treatment Wetlands. CRC Press/Lewis Publishers,
Boca Raton, FL. 893 pp.
Kadlec, R.H. and S. Newman 1992. Phosphorus removal in wetland treatment areas.
Principles and Data. Everglades Protection. Stormwater Treatment Area Design Support
Document. South Florida Water Management District.
Kadlec, R. H. and S.D. Wallace. 2009. Treatment Wetlands, 2nd ed. CRC Press, Boca
Raton, FL, USA, 662 pp.
Knight, R.L. 1992. Ancillary Benefits and Potential Problems With the Use of Wetlands
for Nonpoint Source Pollution Control. Ecological Engineering 1:97-113.
Knight, R.L. 1997. Wildlife Habitat and Public Use Benefits of Treatment Wetlands.
Water Science & Technology 35(5): 35-43.
Knight, R.L., R.A. Clarke, and R.K. Bastian. 2001. Surface Flow (SF) Treatment Wetlands
as a Habitat for Wildlife and Humans. Water Science and Technology 44(11-12): 27-37.
Knight, R.L., J. Hilleke, and S. Grayson. 1994. Design and performance of the Champion
pilot-constructed wetland treatment system. TAPPI Journal 77(5): 240-245.
Knight, R.L., W.E. Walton, G.F. O’Meara, W.K. Reisen, and R. Wass. 2003. Strategies for
Effective Mosquito Control in Constructed Treatment Wetlands. Ecological Engineering
21:211-232.
Jenkins, G.A. and M. Greenway. 2005. The hydraulic efficiency of fringing versus
banded vegetation in constructed wetlands. Ecological Engineering 25(1): 61-72.
Lightbody, A.F. 2007. The physical role of transverse deep zones in improving
constructed treatment wetland performance. Ph.D. Thesis, Massachusetts Institute of
Technology, Cambridge, MA, USA, 289 pp.
Lightbody, A. F., M.E. Avener, and H.M. Nepf. 2008. Observations of short-circuiting
flow paths within a free-surface wetland in Augusta, Georgia, USA. Limnol. Oceanogr.
53: 1040-1053.
Lightbody, A. F., H.M. Nepf, and J.S. Bays. 2009. Modeling the hydraulic effect of
transverse deep zones on theperformance of short-circuiting constructed treatment
wetlands. Ecological Engineering 35, 754-768.
Lightbody, A. F., H.M. Nepf, and J.S. Bays. 2007. Mixing in deep zones within
constructed treatment wetlands. Ecological Engineering 29, 209-220.
McAllister,L.S. 1993a. Habitat Quality Assessment of Two Wetland Treatment Systems
in the Arid West: A Pilot Study. U.S. Environmental Protection Agency. Environmental
Research Laboratory, Corvallis, Oregon. July 1993. EPA/600/R-93/117.
McAllister,L.S. 1993b. Habitat Quality Assessment of Two Wetland Treatment Systems
in Florida: A Pilot Study. U.S. Environmental Protection Agency. Environmental
Research Laboratory, Corvallis, Oregon. November 1993. EPA/600/R-93/222.
WETLAND SOLUTIONS, INC.
Development of Design Criteria for Stormwater Treatment Areas (STAS) in the Northern Lake Okeechobee Watershed
94
Mitsch, W.J. and J.G. Gosselink. 2000. Wetlands. Third Edition. Van Nostrand Reinhold,
New York. 920 pp.
Persson, J., N.L.G. Somes, and T.H.F. Wong. 1999. Hydraulics efficiency of constructed
wetlands and ponds. Water Science and Technology 40(3): 291-300.
Reddy, K.R., R.D. DeLaune, W.F. DeBusk, and M.S. Koch. 1993. Long-term nutrient
accumulation rates in the Everglades. Soil Sci. Soc. Am. J. 57: 1147-1155.
Sather, J.H. 1989. Ancillary Benefits of Wetlands Constructed Primarily for Wastewater
Treatment. Chapter 28a, pp. 353-358 in: D.A. Hammer (ed.) Constructed Wetlands for
Wastewater Treatment, Municipal, Industrial, and Agricultural. Lewis Publishers,
Chelsea, MI.
South Florida Water Management District. 2003. Unpublished STA sediment depth data
provided in spreadsheet format.
South Florida Water Management District (SFWMD). 2006. Design Criteria
Memorandum: DCM-2. Acceler8 Ref #: P 599.
South Florida Water Management District (SFWMD). 2007. South Florida Environmental
Report.
South Florida Water Management District (SFWMD). 2009. South Florida Environmental
Report.
Spalding, M.G., G.T. Bancroft, and D.J. Forrester. 1993.The Epizootiology of
Eustrongylidosis in Wading Birds (Ciconiiformes) in Florida. Journal of Wildlife
Diseases 29: 237-249.
Stewart, E.A., and M.J. Zivojnovich. 2004. Performance Assessment for a 0.50-1.0 MGD
Managed Aquatic Plant System (MAPS) located in Okeechobee, Florida with
Comparative Present Value Analysis with Passive Treatment Wetlands. Unpublished
paper dated April, 2004.
Thackson, E.L., F.D. Shields, and P.R. Schroeder. 1987. Residence time distributions of
shallow basins. Journal of Environmental Engineering (ASCE) 113(6): 1319-1332.
University of Florida (UF). 2001. Following Thirteen Years of Service, Evaluation of the
Existing Capacity of the Orlando Easterly Wetland Treatment System to Remove
Phosphorus from Wastewater, Final Report. Prepared for City of Orlando.
URS. 2005a. Draft Basis of Design Report – Stormwater Treatment Area 5 Flow-way 3.
Prepared for the South Florida Water Management District.
URS. 2005b. Draft Basis of Design Report – Stormwater Treatment Area 6 - Section 2 and
Modifications to Section 1. Prepared for the South Florida Water Management District.
U.S. Environmental Protection Agency (USEPA). 1993. Constructed Wetlands for
Wastewater Treatment and Wildlife Habitat – 17 Case Studies. EPA832-R-93-005, 174 pp.
Vymazal, J., H. Brix, P.F. Cooper, M.B. Green, and R. Haberl. 1998. Constructed
Wetlands for Wastewater Treatment In Europe. Backhuys Publishers, Leiden. 366 pp.
WETLAND SOLUTIONS, INC.
Development of Design Criteria for Stormwater Treatment Areas (STAS) in the Northern Lake Okeechobee Watershed
95
Walker ,W.W., and R.H. Kadlec. 2008. Dynamic Model for Stormwater Treatment Areas
Version 2. (available at http://www.wwwalker.net/dmsta) [Computer Program].
Water Pollution Control Federation. 1990. Natural Systems for Wastewater Treatment
(WPCF Manual of Practice FD-16). First Edition. Water Environment Federation,
Alexandria, Virginia.
Wetland Solutions, Inc. (WSI). 2003. Comparison of Wetland Design Models (DRAFT).
Prepared for Hydrologics Inc. and the South Florida Water Management District.
Wetland Solutions, Inc. (WSI). 2009. Performance Summary for the Columbus Water
Works Nitrate Removal Pilot Wetland Project. Prepared for Jordan, Jones, & Goulding,
Inc. and Columbus Water Works.
White, J.R., K.R. Reddy, and T.A. DeBusk. 2001a. Preliminary Report – phosphorus
removal capacity of the Orlando Easterly Wetland Treatment System. Prepared for Post
Buckley Shuh and Jernigan, Inc. Orlando, FL.
White, J.R., K.R. Reddy, and T.A. DeBusk. 2001b. Design of vegetation modifications and
pilot development of sediment management protocols for the City of Orlando’s Easterly
Wetlands Treatment System. A proposal for the City of Orlando, FL.
Wilhelm, M., S.R. Lawry, and D.D. Hardy. 1989. Creation and Management of Wetlands
Using Municipal Wastewater in Northern Arizona: A Status Report. Chapter 13a, pp.
179 185 in D.A. Hammer (ed.), Constructed Wetlands for Wastewater Treatment:
Municipal, Industrial, and Agricultural. Lewis Publishers, Chelsea, MI.
WETLAND SOLUTIONS, INC.
Development of Design Criteria for Stormwater Treatment Areas (STAS) in the Northern Lake Okeechobee Watershed
96
Appendix A
Preliminary STA Sizing Tables
WETLAND SOLUTIONS, INC.
Development of Design Criteria for Stormwater Treatment Areas (STAS) in the Northern Lake Okeechobee Watershed
Estimated Phosphorus Load Reduction for HLR = 3 cm/d
Flow
Area (ac)
(cfs)
(ac-ft/yr)
100
5
3,593
200
10
7,185
300
15
10,778
400
20
14,370
500
25
17,963
600
30
21,555
700
35
25,148
800
40
28,740
900
45
32,333
1,000
50
35,925
2,000
99
71,850
3,000
149
107,776
4,000
198
143,701
5,000
248
179,626
6,000
298
215,551
7,000
347
251,476
8,000
397
287,402
9,000
447
323,327
10,000
496
359,252
12,000
595
431,102
14,000
695
502,953
16,000
794
574,803
18,000
893
646,654
20,000
992
718,504
Mean Outflow Phosphorus (ppb)
Overall Load Reduction (%)
100
0.280
0.559
0.839
1.12
1.40
1.68
1.96
2.24
2.52
2.80
5.59
8.39
11.2
14.0
16.8
19.6
22.4
25.2
28.0
33.5
39.1
44.7
50.3
55.9
38
63
200
0.529
1.06
1.59
2.12
2.64
3.17
3.70
4.23
4.76
5.29
10.6
15.9
21.2
26.4
31.7
37.0
42.3
47.6
52.9
63.5
74.0
84.6
95.2
106
82
59
Phosphorus Load Removed (metric tons/yr)
Inflow Phosphorus (ppb)
300
400
500
600
0.736
0.905
1.04
1.15
1.47
1.81
2.09
2.31
2.21
2.72
3.13
3.46
2.94
3.62
4.17
4.62
3.68
4.53
5.21
5.77
4.41
5.43
6.26
6.93
5.15
6.34
7.30
8.08
5.89
7.24
8.34
9.24
6.62
8.15
9.38
10.4
7.36
9.05
10.4
11.5
14.7
18.1
20.9
23.1
22.1
27.2
31.3
34.6
29.4
36.2
41.7
46.2
36.8
45.3
52.1
57.7
44.1
54.3
62.6
69.3
51.5
63.4
73.0
80.8
58.9
72.4
83.4
92.4
66.2
81.5
93.8
104
73.6
90.5
104
115
88.3
109
125
139
103
127
146
162
118
145
167
185
132
163
188
208
147
181
209
231
135
198
267
342
55
51
47
43
Values in italic font indicate that the flow per unit width (Q/W) was outside the range of the calibration data sets.
Values in blue font indicate that the mean depth was outside the range of the calibration data sets.
Steady-state (constant flow, constant concentration) simulations using DMSTA2.
WETLAND SOLUTIONS, INC.
Development of Design Criteria for Stormwater Treatment Areas (STAS) in the Northern Lake Okeechobee Watershed
700
1.25
2.49
3.74
4.99
6.23
7.48
8.73
9.97
11.2
12.5
24.9
37.4
49.9
62.3
74.8
87.3
99.7
112
125
150
175
199
224
249
421
40
800
1.32
2.65
3.97
5.29
6.61
7.94
9.26
10.6
11.9
13.2
26.5
39.7
52.9
66.1
79.4
92.6
106
119
132
159
185
212
238
265
504
37
Estimated Phosphorus Load Reduction for HLR = 4 cm/d
Flow
Area (ac)
(cfs)
(ac-ft/yr)
100
7
4,790
200
13
9,580
300
20
14,370
400
26
19,160
500
33
23,950
600
40
28,740
700
46
33,530
800
53
38,320
900
60
43,110
1,000
66
47,900
2,000
132
95,801
3,000
198
143,701
4,000
265
191,601
5,000
331
239,501
6,000
397
287,402
7,000
463
335,302
8,000
529
383,202
9,000
595
431,102
10,000
662
479,003
12,000
794
574,803
14,000
926
670,604
16,000
1,059
766,404
18,000
1,191
862,205
20,000
1,323
958,005
Mean Outflow Phosphorus (ppb)
Overall Load Reduction (%)
100
0.316
0.633
0.949
1.27
1.58
1.90
2.21
2.53
2.85
3.16
6.33
9.49
12.7
15.8
19.0
22.1
25.3
28.5
31.6
38.0
44.3
50.6
57.0
63.3
47
53
200
0.584
1.17
1.75
2.34
2.92
3.50
4.09
4.67
5.26
5.84
11.7
17.5
23.4
29.2
35.0
40.9
46.7
52.6
58.4
70.1
81.8
93.5
105
117
101
49
Phosphorus Load Removed (metric tons/yr)
Inflow Phosphorus (ppb)
300
400
500
600
0.796
0.963
1.10
1.20
1.59
1.93
2.19
2.40
2.39
2.89
3.29
3.61
3.18
3.85
4.38
4.81
3.98
4.82
5.48
6.01
4.78
5.78
6.57
7.21
5.57
6.74
7.67
8.41
6.37
7.71
8.77
9.61
7.17
8.67
9.86
10.8
7.96
9.63
11.0
12.0
15.9
19.3
21.9
24.0
23.9
28.9
32.9
36.1
31.8
38.5
43.8
48.1
39.8
48.2
54.8
60.1
47.8
57.8
65.7
72.1
55.7
67.4
76.7
84.1
63.7
77.1
87.7
96.1
71.7
86.7
98.6
108
79.6
96.3
110
120
95.5
116
131
144
111
135
153
168
127
154
175
192
143
173
197
216
159
193
219
240
166
237
315
397
45
41
37
34
Values in italic font indicate that the flow per unit width (Q/W) was outside the range of the calibration data sets.
Values in blue font indicate that the mean depth was outside the range of the calibration data sets.
Steady-state (constant flow, constant concentration) simulations using DMSTA2.
WETLAND SOLUTIONS, INC.
Development of Design Criteria for Stormwater Treatment Areas (STAS) in the Northern Lake Okeechobee Watershed
700
1.29
2.58
3.86
5.15
6.44
7.73
9.01
10.3
11.6
12.9
25.8
38.6
51.5
64.4
77.3
90.1
103
116
129
155
180
206
232
258
483
31
800
1.36
2.72
4.07
5.43
6.79
8.15
9.51
10.9
12.2
13.6
27.2
40.7
54.3
67.9
81.5
95.1
109
122
136
163
190
217
244
272
571
29
Estimated Phosphorus Load Reduction for HLR = 5 cm/d
Flow
Area (ac)
(cfs)
(ac-ft/yr)
100
8
5,988
200
17
11,975
300
25
17,963
400
33
23,950
500
41
29,938
600
50
35,925
700
58
41,913
800
66
47,900
900
74
53,888
1,000
83
59,875
2,000
165
119,751
3,000
248
179,626
4,000
331
239,501
5,000
414
299,377
6,000
496
359,252
7,000
579
419,127
8,000
662
479,003
9,000
744
538,878
10,000
827
598,753
12,000
992
718,504
14,000
1,158
838,255
16,000
1,323
958,005
18,000
1,489
1,077,756
20,000
1,654
1,197,507
Mean Outflow Phosphorus (ppb)
Overall Load Reduction (%)
100
0.342
0.684
1.03
1.37
1.71
2.05
2.40
2.74
3.08
3.42
6.84
10.3
13.7
17.1
20.5
24.0
27.4
30.8
34.2
41.1
47.9
54.8
61.6
68.4
54
46
200
0.620
1.24
1.86
2.48
3.10
3.72
4.34
4.96
5.58
6.20
12.4
18.6
24.8
31.0
37.2
43.4
49.6
55.8
62.0
74.5
86.9
99.3
112
124
116
42
Phosphorus Load Removed (metric tons/yr)
Inflow Phosphorus (ppb)
300
400
500
600
0.834
0.998
1.13
1.23
1.67
2.00
2.25
2.46
2.50
3.00
3.38
3.69
3.34
3.99
4.51
4.92
4.17
4.99
5.63
6.14
5.00
5.99
6.76
7.37
5.84
6.99
7.89
8.60
6.67
7.99
9.02
9.83
7.51
8.99
10.1
11.1
8.34
9.98
11.3
12.3
16.7
20.0
22.5
24.6
25.0
30.0
33.8
36.9
33.4
39.9
45.1
49.2
41.7
49.9
56.3
61.4
50.0
59.9
67.6
73.7
58.4
69.9
78.9
86.0
66.7
79.9
90.2
98.3
75.1
89.9
101
111
83.4
99.8
113
123
100
120
135
147
117
140
158
172
133
160
180
197
150
180
203
221
167
200
225
246
187
265
347
433
38
34
31
28
Values in italic font indicate that the flow per unit width (Q/W) was outside the range of the calibration data sets.
Values in blue font indicate that the mean depth was outside the range of the calibration data sets.
Steady-state (constant flow, constant concentration) simulations using DMSTA2.
WETLAND SOLUTIONS, INC.
Development of Design Criteria for Stormwater Treatment Areas (STAS) in the Northern Lake Okeechobee Watershed
700
1.31
2.62
3.93
5.24
6.56
7.87
9.18
10.5
11.8
13.1
26.2
39.3
52.4
65.6
78.7
91.8
105
118
131
157
184
210
236
262
522
25
800
1.38
2.76
4.14
5.51
6.89
8.27
9.65
11.0
12.4
13.8
27.6
41.4
55.1
68.9
82.7
96.5
110
124
138
165
193
221
248
276
613
23
Estimated Phosphorus Load Reduction for HLR = 6 cm/d
Flow
Area (ac)
(cfs)
(ac-ft/yr)
100
10
7,185
200
20
14,370
300
30
21,555
400
40
28,740
500
50
35,925
600
60
43,110
700
69
50,295
800
79
57,480
900
89
64,665
1,000
99
71,850
2,000
198
143,701
3,000
298
215,551
4,000
397
287,402
5,000
496
359,252
6,000
595
431,102
7,000
695
502,953
8,000
794
574,803
9,000
893
646,654
10,000
992
718,504
12,000
1,191
862,205
14,000
1,389
1,005,906
16,000
1,588
1,149,606
18,000
1,786
1,293,307
20,000
1,985
1,437,008
Mean Outflow Phosphorus (ppb)
Overall Load Reduction (%)
100
0.362
0.724
1.09
1.45
1.81
2.17
2.53
2.89
3.26
3.62
7.24
10.9
14.5
18.1
21.7
25.3
28.9
32.6
36.2
43.4
50.6
57.9
65.1
72.4
59
41
200
0.647
1.29
1.94
2.59
3.23
3.88
4.53
5.17
5.82
6.47
12.9
19.4
25.9
32.3
38.8
45.3
51.7
58.2
64.7
77.6
90.5
103
116
129
127
36
Phosphorus Load Removed (metric tons/yr)
Inflow Phosphorus (ppb)
300
400
500
600
0.860
1.02
1.15
1.25
1.72
2.04
2.30
2.49
2.58
3.07
3.44
3.74
3.44
4.09
4.59
4.99
4.30
5.11
5.74
6.23
5.16
6.13
6.89
7.48
6.02
7.16
8.03
8.73
6.88
8.18
9.18
9.98
7.74
9.20
10.3
11.2
8.60
10.2
11.5
12.5
17.2
20.4
23.0
24.9
25.8
30.7
34.4
37.4
34.4
40.9
45.9
49.9
43.0
51.1
57.4
62.3
51.6
61.3
68.9
74.8
60.2
71.6
80.3
87.3
68.8
81.8
91.8
99.8
77.4
92.0
103
112
86.0
102
115
125
103
123
138
150
120
143
161
175
138
164
184
200
155
184
207
224
172
204
230
249
203
285
371
460
32
29
26
23
Values in italic font indicate that the flow per unit width (Q/W) was outside the range of the calibration data sets.
Values in blue font indicate that the mean depth was outside the range of the calibration data sets.
Steady-state (constant flow, constant concentration) simulations using DMSTA2.
WETLAND SOLUTIONS, INC.
Development of Design Criteria for Stormwater Treatment Areas (STAS) in the Northern Lake Okeechobee Watershed
700
1.33
2.65
3.98
5.31
6.63
7.96
9.29
10.6
11.9
13.3
26.5
39.8
53.1
66.3
79.6
92.9
106
119
133
159
186
212
239
265
551
21
800
1.39
2.78
4.18
5.57
6.96
8.35
9.74
11.1
12.5
13.9
27.8
41.8
55.7
69.6
83.5
97.4
111
125
139
167
195
223
251
278
643
20
Estimated Phosphorus Load Reduction for HLR = 7 cm/d
Flow
Area (ac)
(cfs)
(ac-ft/yr)
100
12
8,383
200
23
16,765
300
35
25,148
400
46
33,530
500
58
41,913
600
69
50,295
700
81
58,678
800
93
67,060
900
104
75,443
1,000
116
83,825
2,000
232
167,651
3,000
347
251,476
4,000
463
335,302
5,000
579
419,127
6,000
695
502,953
7,000
811
586,778
8,000
926
670,604
9,000
1,042
754,429
10,000
1,158
838,255
12,000
1,389
1,005,906
14,000
1,621
1,173,556
16,000
1,853
1,341,207
18,000
2,084
1,508,858
20,000
2,316
1,676,509
Mean Outflow Phosphorus (ppb)
Overall Load Reduction (%)
100
0.376
0.753
1.13
1.51
1.88
2.26
2.63
3.01
3.39
3.76
7.53
11.3
15.1
18.8
22.6
26.3
30.1
33.9
37.6
45.2
52.7
60.2
67.7
75.3
64
36
200
0.666
1.33
2.00
2.66
3.33
3.99
4.66
5.32
5.99
6.66
13.3
20.0
26.6
33.3
39.9
46.6
53.2
59.9
66.6
79.9
93.2
106
120
133
136
32
Phosphorus Load Removed (metric tons/yr)
Inflow Phosphorus (ppb)
300
400
500
600
0.879
1.04
1.16
1.26
1.76
2.08
2.32
2.52
2.64
3.12
3.49
3.78
3.51
4.16
4.65
5.04
4.39
5.19
5.81
6.30
5.27
6.23
6.97
7.56
6.15
7.27
8.14
8.81
7.03
8.31
9.30
10.1
7.91
9.35
10.5
11.3
8.79
10.4
11.6
12.6
17.6
20.8
23.2
25.2
26.4
31.2
34.9
37.8
35.1
41.6
46.5
50.4
43.9
51.9
58.1
63.0
52.7
62.3
69.7
75.6
61.5
72.7
81.4
88.1
70.3
83.1
93.0
101
79.1
93.5
105
113
87.9
104
116
126
105
125
139
151
123
145
163
176
141
166
186
201
158
187
209
227
176
208
232
252
215
300
388
478
28
25
22
20
Values in italic font indicate that the flow per unit width (Q/W) was outside the range of the calibration data sets.
Values in blue font indicate that the mean depth was outside the range of the calibration data sets.
Steady-state (constant flow, constant concentration) simulations using DMSTA2.
WETLAND SOLUTIONS, INC.
Development of Design Criteria for Stormwater Treatment Areas (STAS) in the Northern Lake Okeechobee Watershed
700
1.34
2.67
4.01
5.35
6.69
8.02
9.36
10.7
12.0
13.4
26.7
40.1
53.5
66.9
80.2
93.6
107
120
134
160
187
214
241
267
571
18
800
1.40
2.80
4.20
5.60
7.00
8.41
9.81
11.2
12.6
14.0
28.0
42.0
56.0
70.0
84.1
98.1
112
126
140
168
196
224
252
280
665
17
Estimated Phosphorus Load Reduction for HLR = 8 cm/d
Flow
Area (ac)
(cfs)
(ac-ft/yr)
100
13
9,580
200
26
19,160
300
40
28,740
400
53
38,320
500
66
47,900
600
79
57,480
700
93
67,060
800
106
76,640
900
119
86,220
1,000
132
95,801
2,000
265
191,601
3,000
397
287,402
4,000
529
383,202
5,000
662
479,003
6,000
794
574,803
7,000
926
670,604
8,000
1,059
766,404
9,000
1,191
862,205
10,000
1,323
958,005
12,000
1,588
1,149,606
14,000
1,853
1,341,207
16,000
2,117
1,532,808
18,000
2,382
1,724,409
20,000
2,647
1,916,010
Mean Outflow Phosphorus (ppb)
Overall Load Reduction (%)
100
0.388
0.776
1.16
1.55
1.94
2.33
2.72
3.10
3.49
3.88
7.76
11.6
15.5
19.4
23.3
27.2
31.0
34.9
38.8
46.6
54.3
62.1
69.8
77.1
67
33
200
0.680
1.36
2.04
2.72
3.40
4.08
4.76
5.44
6.12
6.80
13.6
20.4
27.2
34.0
40.8
47.6
54.4
61.2
68.0
81.6
95.2
109
122
135
143
29
Phosphorus Load Removed (metric tons/yr)
Inflow Phosphorus (ppb)
300
400
500
600
0.893
1.05
1.17
1.27
1.79
2.10
2.35
2.54
2.68
3.15
3.52
3.81
3.57
4.21
4.69
5.07
4.46
5.26
5.87
6.34
5.36
6.31
7.04
7.61
6.25
7.36
8.21
8.88
7.14
8.41
9.38
10.1
8.04
9.46
10.6
11.4
8.93
10.5
11.7
12.7
17.9
21.0
23.5
25.4
26.8
31.5
35.2
38.1
35.7
42.1
46.9
50.7
44.6
52.6
58.7
63.4
53.6
63.1
70.4
76.1
62.5
73.6
82.1
88.8
71.4
84.1
93.8
101
80.4
94.6
106
114
89.3
105
117
127
107
126
141
152
125
147
164
178
143
168
188
203
161
189
211
228
177
209
233
252
225
311
401
493
25
22
20
18
Values in italic font indicate that the flow per unit width (Q/W) was outside the range of the calibration data sets.
Values in blue font indicate that the mean depth was outside the range of the calibration data sets.
Steady-state (constant flow, constant concentration) simulations using DMSTA2.
WETLAND SOLUTIONS, INC.
Development of Design Criteria for Stormwater Treatment Areas (STAS) in the Northern Lake Okeechobee Watershed
700
1.35
2.69
4.04
5.38
6.73
8.07
9.42
10.8
12.1
13.5
26.9
40.4
53.8
67.3
80.7
94.2
108
121
135
161
188
215
242
267
587
16
800
1.41
2.82
4.22
5.63
7.04
8.45
9.85
11.3
12.7
14.1
28.2
42.2
56.3
70.4
84.5
98.5
113
127
141
169
197
225
253
279
681
15
Appendix B
Site Selection Criteria Memorandum
WETLAND SOLUTIONS, INC.
Development of Design Criteria for Stormwater Treatment Areas (STAS) in the Northern Lake Okeechobee Watershed
2809 NW 161 Court
Gainesville, FL 32609
(386) 462-1003
(386) 462-3196 fax
TECHNICAL MEMORANDUM NO. 1
Development of Design Criteria for Stormwater
Treatment Areas (STAs) in the Northern Lake
Okeechobee Watershed – Site Selection Criteria
TO:
Odi Villapando/SFWMD
File
Chris Keller/WSI
Bob Knight/WSI
June 1, 2009
COPIES:
FROM:
DATE:
Contents
Contents .............................................................................................................................1
Introduction.......................................................................................................................1
Land Use ............................................................................................................................4
General Land Availability Considerations .................................................................11
Hydrologic Setting .........................................................................................................11
Proximity to Water Source ...............................................................................11
Flows and Loads................................................................................................11
Soils...................................................................................................................................14
Topography .....................................................................................................................18
Environmental Considerations.....................................................................................19
Threatened and Endangered Species..............................................................19
Hazardous Wastes.............................................................................................29
Other Potential Constraints...........................................................................................29
Historic Significance .........................................................................................29
Proximity to Airports........................................................................................29
Utility Easements...............................................................................................33
References........................................................................................................................33
Introduction
The Taylor Creek and Nubbin Slough Stormwater Treatment Areas (STAs) are the
prototype STAs being implemented north of Lake Okeechobee. Both STAs are fully
constructed but only Taylor Creek is fully operational. Operation has not been initiated at
the Nubbin Slough STA due to structural issues with the pump station. The Taylor Creek
and Nubbin Slough STAs were estimated to remove a long-term average of 2.08 and 5.14
metric tons of phosphorus per year, respectively. Likewise, the Lakeside Ranch facility is
nearing final design and is estimated to provide about 21 metric tons of phosphorus (P)
removal per year. These estimates were developed using earlier versions of the Everglades
STA design model, and may be optimistic for soils and loading rates included in the
estimates.
1
WETLAND SOLUTIONS, INC.
Simple input/output analysis found in the literature provides some general guidance
relative to P loading rates and effluent P concentrations but they can not be accurately
applied to a particular wetland without due consideration to site-specific conditions.
Environmental conditions including soil properties, vegetation types, previous land uses,
surface water total P concentrations, rainfall patterns, and hydraulic loads are significantly
different north of the lake and may reasonably be expected to result in different
development and performance of STAs constructed and operated in the Lake Okeechobee
watershed. The implementation of the prototype STAs is important for demonstrating the
effectiveness of the STA technology in areas north of the lake. Site specific information
obtained from operation of these prototype treatment wetlands will help to improve design
and operational guidance as additional STAs are planned in the watershed.
The South Florida Water Management District’s (District) primary goal of the STAs north of
the lake is to maximize the long-term mass removal of total phosphorus (TP) and to
minimize operational costs per pound of TP removed. Unlike the STAs south of the lake, the
STAs in the Lake Okeechobee watershed are not mandated to achieve a target outflow TP
concentration. As such, the Okeechobee STAs offer greater flexibility in terms of design and
performance goals or desired levels of treatment.
Although there is no “cookbook” for successful implementation of STAs anywhere, it is
assumed that these facilities if properly designed, constructed, and operated can provide
predictable performance.
The primary goal of this project is to develop design criteria and guidelines specific to
conditions north of the lake and to predict performance of future STAs in the Lake
Okeechobee watershed with greater reliability and certainty. These design standards and
guidelines are also intended to serve as a tool for making future land purchases in the
watershed deemed suitable for STAs.
The purpose of this document is to assess relevant site-specific conditions in the 21 basins
comprising the northern Lake Okeechobee watershed (Exhibit 1). The goal of this
evaluation is to identify local environmental conditions that are most conducive to
successful implementation of an STA in the watershed. Development of these site selection
criteria should allow interested parties to identify specific areas in each of the 21 individual
basins that are most appropriate for STA siting.
2
WETLAND SOLUTIONS, INC.
EXHIBIT 1
Northern Lake Okeechobee Watershed Drainage Basins
3
WETLAND SOLUTIONS, INC.
Land Use
Historic, current, and to a lesser extent, future land use can be expected to be one of the
most dominant factors in selecting a site for successful STA implementation. For this
analysis, current land use data were considered to be more important than historic or future
land use data for the following reasons:
•
The potential impact of historic land uses can be considered in alternative ways by
reviewing legacy phosphorus concentration data and available databases for known
hazardous waste remediation sites and sites and structures of historical significance.
•
It is unlikely that land use changes at a particular site would trend towards being
more suitable for STA implementation than less suitable. For example, former
industrial sites are more likely to be redeveloped as commercial or even residential
land uses than to be converted to agricultural or open land uses.
•
Future estimated land use is of limited value for selecting a suitable site. If the
existing land use and per acre cost meets the District’s requirements, then assuming
all other site selection factors are also met, the site should be strongly considered
regardless of the estimated future condition.
Although the majority of the study area falls within the District’s boundaries, small portions
of several sub-basins encroach into the Southwest Florida and St. John’s River Water
Management District’s. The following land use GIS files were merged and then intersected
with the study area drainage basin boundaries (LOPP_SumBasins) to facilitate a detailed
review of current land use types:
•
SFWMD: 2004_05_LCLU_SFWMD_Geodatabase (http://www.sfwmd.gov)
•
SJRWMD: lu_sjrwmd_2004 (http://www.fgdl.org)
•
SWFWMD: lu_swfwmd_2006 (http://www.fgdl.org)
Exhibit 2 summarizes the land use for each of the 21 study basins based on Level 1 Florida
Land Use and Cover Classification System (FLUCCS) codes. The Level 1 codes are as
follows:
1000 - Urban and Built-up
2000 - Agriculture
3000 - Rangeland
4000 - Upland Forests
5000 - Water
6000 - Wetlands
7000 - Barren Land
8000 - Transportation, Communication, and Utilities
4
WETLAND SOLUTIONS, INC.
EXHIBIT 2
Summary of Level 1 Land Use Area (acres) by Drainage Basin
Level 1 FLUCCS Code (areas in acres)
Basin
1000
2000
3000
4000
5000
6000
7000
Basin Total
(ac)
8000
C-40 Basin (S-72)
60
35,886
291
441
382
6,464
435
6
43,964
C-41 Basin (S-71)
3,425
70,056
1,188
3,448
736
14,702
974
126
94,654
Fisheating Creek
2,418
155,404
13,111
44,243
1,522
71,745
231
693
289,366
L-48 Basin (S-127)
522
16,256
220
5
464
2,744
564
20,774
L-49 Basin (S-129)
158
10,217
3
67
282
1,063
302
12,093
L-59E
132
9,094
9
18
847
3,225
1,085
14,409
6
208
41
6,440
L-59W
6,184
L-60E
97
4,309
27
2
12
544
47
5,038
L-60W
19
2,933
34
93
21
105
67
3,271
L-61E
9
10,725
2
125
3,420
5
14,286
13,567
L-61W
7,914
162
1,323
40
4,008
120
54,964
135,634
28,911
46,618
61,139
60,413
521
3,947
392,147
Nicodemus Slough (Culv 5)
187
15,242
1,051
2,774
282
5,598
348
159
25,641
S-131 Basin
557
5,202
266
95
220
617
206
S-133 Basin
7,818
13,896
185
791
857
1,588
262
262
25,660
S-135 Basin
1,011
13,452
437
139
906
1,352
747
44
18,089
S-154 Basin
2,399
23,947
379
938
215
5,633
137
149
33,798
191,749
275,356
61,499
77,592
143,587
249,908
2,834
23,273
1,025,797
5,612
204,862
70,424
26,820
5,789
113,153
2,352
271
429,283
38
41,555
2,050
71
588
13,435
751
6,167
96,452
1,345
2,713
547
12,696
488
347
120,754
277,341
1,154,576
181,590
208,192
218,565
572,622
12,518
29,277
2,654,680
Lake Istokpoga (S-68)
S-65 (Lake Kissimmee)
S-65A,B,C,D,E
S-84 Basin (C-41A)
Taylor Creek/Nubbin Slough (S-191)
Project Area Total
5
7,164
58,488
WETLAND SOLUTIONS, INC.
Agricultural land uses dominate the drainage basins (43% of total area) with natural
wetlands as a sub-dominant category (22% of the total area). In individual drainage basins,
agricultural land uses range from 27 percent (S-65 Lake Kissimmee) to 96 percent (L-59W) of
the basin areas. Not surprisingly, water and wetland land uses comprise 38 percent and 31
percent of the S-65 (Lake Kissimmee) and S-68 (Lake Istokpoga) sub-basins, respectively.
Land use types that are potentially compatible with STA implementation were determined
by reviewing the Level 4 classifications and filtering out those that were considered
unsuitable. Unsuitable land uses include urban (Level 1000, except for 1900 [Open Land]),
existing water (Level 5000) and wetlands (Level 6000), barren land (Level 7000), and
transportation (Level 8000). Areas expected to have high legacy phosphorus (i.e. dairies)
were also screened out. Upland forested areas were considered to be less desirable because
land clearing costs would be higher than for more open land uses, but could be used if there
were insufficient acreage available in the preferred land use categories. Preferred land uses
for STA implementation were therefore limited to the following categories:
1900 – Open Land
2100 – Cropland and Pastureland
2110 – Improved Pastures
2120 – Unimproved Pastures
2140 – Row Crops
2150 – Field Crops
2156 – Sugar Cane
2420 – Sod Farms
2430 – Ornamentals
2600 – Other Open Lands – Rural
2610 – Fallow Cropland
3100 – Herbaceous Upland Nonforested
3200 – Shrub and Brushland
3210 – Palmetto Prairies
Secondary land uses that could be used alone or in combination with the preferred land
uses include the following:
1900 – Open Land
2130 – Woodland Pastures
2200 – Tree Crops
2210 – Citrus Groves
2220 – Fruit Orchards
6
WETLAND SOLUTIONS, INC.
2230 – Other Groves
2400 – Nurseries and Vineyards
3230 – Abandoned Groves
3300 – Mixed Upland Nonforested
4100 – Upland Coniferous Forests
4110 – Pine Flatwoods
4140 – Pine – Mesic Oak
4220 – Brazilian Pepper
4240 – Melaleuca
4270 – Live Oak
4271 – Oak – Cabbage Palm Forests
4280 – Cabbage Palm
4340 – Upland Mixed Coniferous/Hardwood
4370 – Australian Pine
4400 – Tree Plantations
4410 – Coniferous Pine
4420 – Hardwood Plantations
4430 – Forest Regeneration
Exhibit 3 summarizes the total available area within each drainage basin that fell within the
preferred land use categories. No attempt was made to screen potentially available land
uses based on a minimum polygon size. It should also be noted that individual polygons
may surround isolated wetlands or other features that would possibly be included in a
specific STA site’s footprint. Therefore, the area totals shown in Exhibit 3 are slightly lower
than the maximum available area based on preferred land use categories only. Exhibits 4
and 5 summarize the total land use areas by basin in the preferred, secondary, and exclusion
categories.
7
WETLAND SOLUTIONS, INC.
EXHIBIT 3
Summary of Level 4 Preferred Land Use Area (acres) by Drainage Basin
FLUCCS Code
Basin
1900
2100
C-40 Basin (S-72)
2110
2120
2140
2150
2156
18,357
1,773
257
158
7,310
28,962
5,711
111
365
8,623
80,840
43,799
62
523
14,458
567
15
228
L-49 Basin (S-129)
6,655
2,321
L-59E
7,744
620
L-59W
3,666
604
L-60E
1,821
506
L-60W
1,370
397
L-61E
5,423
3,068
L-61W
5,569
724
32,391
17,449
6,421
1,130
C-41 Basin (S-71)
357
Fisheating Creek
L-48 Basin (S-127)
Lake Istokpoga (S-68)
93
7,483
13,471
Nicodemus Slough
14
1,152
6,442
181
34
24
3
48%
15,576
75%
9,498
79%
8,560
59%
4,270
66%
12
15
2,354
47%
11
23
2,031
62%
8,491
59%
3
36
49
6,384
47%
1,752
21,037
5,100
107,720
27%
55
13,595
53%
183
4,824
67%
68
62
12,975
51%
174
156
12,015
66%
115
89
85
21,842
65%
48
13,305
13,195
28,210
252,800
25%
4
3,262
15,395
49,829
244,151
57%
21
1,256
173
621
32,978
56%
86
393
257
319
80,372
67%
376
23,128
57,690
88,749
1,051,544
40%
S-135 Basin
83
4,519
41
57
S-154 Basin
237
19,006
1,254
33
1,022
115,005
41,548
1,329
8,110
121,174
43,110
6,575
4,772
24,473
5,622
621
156
37
66,131
5,600
371
6,948
40
575,170
176,873
10,346
24,168
32,823
1,519
56
137,665
53
171
5,015
3
48%
1,465
854
163
4,481
45,380
239
33
20,795
352
64%
451
876
24,937
350
28,063
5,158
8,341
Grand Total
723
% of
Area
142
1,035
186
124
Grand
Total
2,306
S-133 Basin
Taylor Creek/Nubbin
Slough (S-191)
3210
704
90
S-84 Basin (C-41A)
3200
54
3
794
3100
208
230
153
S-65A,B,C,D,E
2610
7
2,846
7,324
2600
187
35
15,428
2430
496
S-131 Basin
S-65 (Lake Kissimmee)
2420
16
6,984
3,819
425
5,053
30
41
5,339
1,082
10,068
8
WETLAND SOLUTIONS, INC.
EXHIBIT 4
Summary of Primary, Secondary, and Excluded Land Use Areas (acres) by Drainage Basin
Land Use Grouping
Basin
Primary
Secondary
Excluded
Basin Total
(ac)
C-40 Basin (S-72)
28,063
8,555
7,347
43,964
C-41 Basin (S-71)
45,380
29,475
19,799
94,654
Fisheating Creek
137,665
74,174
77,527
289,366
L-48 Basin (S-127)
15,576
998
4,201
20,774
L-49 Basin (S-129)
9,498
789
1,805
12,093
L-59E
8,560
561
5,288
14,409
L-59W
4,270
1,914
256
6,440
L-60E
2,354
1,984
700
5,038
L-60W
2,031
1,028
212
3,271
L-61E
8,491
2,236
3,559
14,286
L-61W
6,384
3,015
4,168
13,567
107,720
103,317
181,110
392,147
13,595
5,472
6,573
25,641
S-131 Basin
4,824
489
1,851
7,164
S-133 Basin
12,975
2,765
9,919
25,660
S-135 Basin
12,015
2,021
4,052
18,089
S-154 Basin
21,842
2,419
9,537
33,798
S-65 (Lake Kissimmee)
252,800
169,175
603,822
1,025,797
S-65A,B,C,D,E
244,151
54,658
130,473
429,283
S-84 Basin (C-41A)
32,978
10,697
14,813
58,488
Taylor Creek/Nubbin Slough (S-191)
80,372
16,386
23,996
120,754
1,051,544
492,129
1,111,008
2,654,680
Lake Istokpoga (S-68)
Nicodemus Slough (Culv 5)
Grand Total
9
WETLAND SOLUTIONS, INC.
EXHIBIT 5
Summary of Primary, Secondary, and Excluded Land Use Areas
10
WETLAND SOLUTIONS, INC.
General Land Availability Considerations
In addition to having a suitable land use, regions considered for STA implementation
should also be reviewed for compliance with the following general constraints:
•
The most desirable areas would consist of large single-owner parcels to minimize the
number of parties involved in land purchase negotiations.
•
STA sites should be located at the downgradient end of a particular watershed in
order to maximize the availability of surface water flows and phosphorus loads.
Hydrologic Setting
Proximity to Water Source
Ideally, STAs would be sited on lands immediately adjacent to water bodies that deliver a
treatable quantity of phosphorus to Lake Okeechobee. Small farm ditches, unless directly
connected to a larger stream would not be expected to yield a sustainable supply of water to
operate an STA. Exhibit 6 shows the spatial relationship between primary and secondary
land use types and waterways likely to provide adequate flows and loads.
Flows and Loads
Exhibit 7 summarizes average annual flows and phosphorus concentrations and loads
(SFWMD et al. 2008). Some sub-basins are lumped together (Taylor Creek/Nubbin Slough
and Indian Prairie) and it is beyond the scope of this effort to conduct an analysis of
individual basin loads. These data do show that there is a significant opportunity to reduce
phosphorus loads, especially within the Lower Kissimmee, Taylor Creek/Nubbin Slough,
Indian Prairie, Fisheating Creek, and Nicodemus Slough basins.
11
WETLAND SOLUTIONS, INC.
EXHIBIT 6
Water Control Structure and Water Quality Stations within the Study Area
12
WETLAND SOLUTIONS, INC.
EXHIBIT 7
Summary of Average Annual Flows and Phosphorus Loads to Lake Okeechobee (1991 – 2005) by Watershed (SFWMD et al. 2008)
Basin
Area (ac)
Average Annual
Discharge (ac-ft)
Average Annual
P Load (mt)
Average Annual
P Concentration
(ppb)
1,021,674
954,204
91
78
429,283
378,836
77
166
Taylor Creek/Nubbin Slough
198,299
187,583
124
537
Lake Istokpoga (S-68)
392,147
299,656
23
63
Indian Prairie
294,147
249,175
89
289
Fisheating Creek and Nicodemus Slough (Culvert 5)
315,007
224,368
55
199
2,650,557
2,293,822
439
155
Upper Kissimmee (S-65)
Lower Kissimmee (S-65A, B, C, D, E)
1
2
Total
1
Taylor Creek/Nubbin Slough includes S-133, S-135, S-154, S-191
2
Indian Prairie includes L-59E, L-59W, C-40, C-41, C-41A, L-60E, L-60W, L-61E, S-61W, S-127, S-129, S-131
13
WETLAND SOLUTIONS, INC.
Soils
The District’s existing STAs, as well as other treatment wetlands throughout central and
southern Florida have been constructed on a variety of soil types ranging from mineral
sands to organic peat and muck. Native soil type undoubtedly has some impact on the
minimum achievable phosphorus concentrations in an STA and this has been shown at the
mesocosm, field, and full scales in the Everglades Agricultural Area. However, for the
Northern Lake Okeechobee watershed, the primary focus is on load reduction rather than
minimizing outflow concentrations to the protective levels required for discharge into the
Everglades National Park. General soil classification is therefore not considered to be as
important as the effects of historical and current land uses on stored soil phosphorus
concentrations.
Over the last 15 years, the District and its consultants have studied soil phosphorus
concentrations throughout the Northern Lake Okeechobee watershed. In 2007 and 2008, Soil
and Water Engineering Technology, Inc. (SWET) prepared a series of technical documents
summarizing available soil phosphorus concentration data, estimating and mapping soil
phosphorus loads that could be transported to Lake Okeechobee, and evaluating various
methods for remediating soil phosphorus loads at site-specific and regional scales. Of
primary interest in this recent work is the estimated spatial distribution and magnitude of
soil phosphorus loads. SWET found that the highest concentrations of soil phosphorus of
anthropogenic origins that could be transported (termed “legacy phosphorus”) occurred in
areas with intensive dairy operations where cattle waste was ineffectively treated. Exhibits
8 and 9 show the estimated legacy phosphorus content of soils in the “A” horizon (generally
the top 10 to 15 centimeters of the soil profile) and below the “A” horizon. The highest
legacy phosphorus was found in dairy intensive lactating pastures, dairy high intensity
holding pastures, abandoned intensive dairies, and isolated wetlands in dairy pastures. In
these areas, legacy phosphorus content was estimated to range from 841 kilograms per
hectare (kg/ha) to 6,230 kg/ha. Various other agricultural land uses (livestock operations
and crops) had estimated legacy phosphorus contents ranging from about 250 to 650 kg/ha.
All other studied land uses had legacy phosphorus contents less than 200 kg/ha. Below the
“A” horizon, legacy phosphorus was still high on dairy sites ranging from 760 to 2,660
kg/ha and was also estimated to be elevated in urban, industrial, and commercial areas
served by septic tanks (480 to 2,370 kg/ha).
Exhibit 10 shows the relationship between legacy phosphorus content (kg/ha) and soil bulk
density (g/cm3) for an assumed “A” horizon depth of 15 cm. For an organic soil with an
assumed bulk density of 0.2 g/cm3, a stored phosphorus value of 500 kg/ha equates to a
concentration of 1,667 mg/kg. For a mineral soil with a bulk density of 1.0 g/cm3, the same
stored phosphorus value of 500 kg/ha equates to a soil concentration of 333 mg/kg. For
comparison, soil phosphorus concentrations have been measured in the EAA STAs during
operational monitoring. Average concentration values ranged from about 125 to 700 mg/kg
with individual sample values exceeding 1,500 mg/kg and average bulk densities ranged
from 0.17 to 1.03 g/cm3 (SFWMD 2009). The District has recently contracted with the
University of Florida to conduct a detailed review of STA soil phosphorus and water quality
performance data, but at present, no clear relationship has been drawn between preconstruction soil concentrations and observed STA performance.
14
WETLAND SOLUTIONS, INC.
EXHIBIT 8
Legacy Phosphorus in the A Soil Horizon (SWET 2007)
15
WETLAND SOLUTIONS, INC.
EXHIBIT 9
Legacy Phosphorus Below the A Soil Horizon (SWET 2007)
16
WETLAND SOLUTIONS, INC.
EXHIBIT 10
Estimated Soil Phosphorus Concentration (mg/kg) as a Function of Legacy Phosphorus Content and Bulk Density
Legacy P Storage (kg/ha)
Bulk Density
3
(g/cm )
50
150
250
500
1000
0.10
333
1,000
1,667
3,333
6,667
0.15
222
667
1,111
2,222
4,444
0.20
167
500
833
1,667
3,333
0.25
133
400
667
1,333
2,667
0.30
111
333
556
1,111
2,222
0.35
95.2
286
476
952
1,905
0.40
83.3
250
417
833
1,667
0.45
74.1
222
370
741
1,481
0.50
66.7
200
333
667
1,333
0.55
60.6
182
303
606
1,212
0.60
55.6
167
278
556
1,111
0.65
51.3
154
256
513
1,026
0.70
47.6
143
238
476
952
0.75
44.4
133
222
444
889
0.80
41.7
125
208
417
833
0.85
39.2
118
196
392
784
0.90
37.0
111
185
370
741
0.95
35.1
105
175
351
702
1.00
33.3
100
167
333
667
1.05
31.7
95.2
159
317
635
1.10
30.3
90.9
152
303
606
1.15
29.0
87.0
145
290
580
1.20
27.8
83.3
139
278
556
1.25
26.7
80.0
133
267
533
1.30
25.6
76.9
128
256
513
1.35
24.7
74.1
123
247
494
1.40
23.8
71.4
119
238
476
1.45
23.0
69.0
115
230
460
1.50
22.2
66.7
111
222
444
Assumed soil horizon depth of 15 cm
17
WETLAND SOLUTIONS, INC.
For the purposes of initial screening of potential STA sites, and pending the results of the
University’s analyses, it is recommended that the District establish an interim “A” horizon
legacy phosphorus threshold of 500 kg/ha. The selection of this value would result in
estimated soil phosphorus concentrations that span the range of observed measurements in
the EAA STAs for likely ranges of soil bulk densities.
Topography
Topography plays a large role in first determining site suitability and then in dictating the
layout of STA cells within the available project boundaries. In general, it is most desirable to
locate an STA on a site with no appreciable change in elevation from one end or side to the
other or to grade the site so that cell bottoms are level. The operational issues that can arise
due to irregular topography are numerous and include the following:
•
Ground slopes that run parallel to the design flow path create a gradient of the water
depths that, at the extremes, are too shallow at the upstream end to efficiently
inundate the available treatment area and/or too deep at the downstream end to
support the target vegetation community. Such slopes also create problems during
initial plant establishment because an optimum depth can not be achieved across the
entire cell. In addition, longitudinal ground slopes may increase the likelihood of
creating preferential flow paths when the cell is initially filled and each time there is
a drought period that exposes the sediment surface at the upstream end.
•
Ground slopes that run transverse to the flow path likewise create a gradient of
water depths, but in this case the deeper side of the cell carries the majority of the
flow and results in hydraulic inefficiency.
•
In addition to the general topographic gradient on a site, other topographic features
such as remnant irrigation ditches and planting beds can further exacerbate
problems with the formation of preferential flow channels.
It is challenging to screen sites for suitable topography at the basin level because there is
naturally a wide range in elevations throughout each watershed. Converting elevation
changes to slopes can be accomplished using GIS software, but fairly complicated criteria
would need to be developed to determine acceptable maximum slopes as a function of land
use polygon or parcel size. It is therefore recommended that other factors (land use, parcel
ownership, proximity to water source, etc.) be used to short-list potential sites and that
topography then be reviewed to determine suitability on a site by site basis.
For individual STA cells, it is recommended that elevation changes be limited to no more
than 1.5 to 2 feet if level grading is cost prohibitive. Where site elevation changes exceed 2
feet, multiple cells-in-series are likely to be required. The economic tradeoffs between
terracing a sloped site into multiple cells versus level-grading a larger cell will be explored
in the companion document to this Technical Memorandum (Evaluation of STA Design
Parameters).
18
WETLAND SOLUTIONS, INC.
Environmental Considerations
Various environmental considerations also factor in to the selection of a site for STA
implementation. Significant issues could include the presence of existing wetlands, the
presence or potential presence of threatened or endangered (T&E) species, and the presence
of hazardous waste contamination sites. The magnitude of potential impacts to existing
wetlands can be determined during the land use screening phase, so this section focuses on
T&E species and hazardous materials.
Threatened and Endangered Species
There are a number of T&E animal and plant species that occur within the Northern Lake
Okeechobee watershed and the specific habitat requirements of each could eliminate a site
from consideration, necessitate careful construction sequencing, and/or create operational
and management issues that negatively impact the primary water quality enhancement
objectives of an STA project. When any of these species are documented or expected to
occur on a project site, consultation with the U.S. Fish and Wildlife Service (USFWS) will be
required during the permitting and design phase.
Fauna
Six animal T&E species have habitat ranges that overlap some or all of the 21-basin study
area. These species include the Everglades snail kite (Rostrhamus sociabilis plumbeus), redcockaded woodpecker (Picoides borealis), scrub jay (Aphelocoma coerulescens), crested caracara
(Caracara cheriway), bald eagle (Haliaeetus leucocephalus), and Florida panther (Felis concolor
coryi).
Everglades Snail Kite
Exhibit 11 shows the range of the Everglades snail kite relative to the study basins. The snail
kite can occur throughout the study area with the exception of the western portion of the
Fisheating Creek basin.
The snail kite is a wetland-dependent bird as its primary food source is the native apple
snail (Pomacea paludosa). Therefore, STA construction and operation is not inconsistent with
the habitat requirements for this species. Although consultation with the USFWS would be
necessary during project permitting, an STA project would not be expected to have an
adverse effect on snail kite populations.
Red-Cockaded Woodpecker
Exhibit 12 shows the habitat range for the red-cockaded woodpecker (RCW). The RCW may
be found in large portions of the Kissimmee River (S-65) and Fisheating Creek basins as well
as in smaller portions of the Lake Istokpoga and S-65A, B, C, D, E basins.
The RCW inhabits forested areas and prefers old-growth pines for roosting and nesting. As
forested land uses are not in the recommended primary category for initial site screening, it
is likely that RCW habitat would be a minimal constraint in most basins.
Scrub Jay
Exhibit 13 shows the range for the scrub jay. The scrub jay may inhabit all basins but a few
(L-60W, L-49, L-48, and S-135) around the northern and eastern shores of Lake Okeechobee.
19
WETLAND SOLUTIONS, INC.
EXHIBIT 11
Consultation Area for the Everglades Snail Kite
20
WETLAND SOLUTIONS, INC.
EXHIBIT 12
Consultation Area for the Red-Cockaded Woodpecker
21
WETLAND SOLUTIONS, INC.
EXHIBIT 13
Consultation Area for the Scrub Jay
22
WETLAND SOLUTIONS, INC.
The scrub jay requires open oak or sand pine flatwoods with minimal canopy coverage and
patches of sandy bare ground. While this land use type would be suitable for construction,
as it is easy to clear, scrub habitat is rare and often protected.
Crested Caracara
Exhibit 14 shows that the range for the crested caracara covers the entire study area with the
exception of the northernmost portion of the Kissimmee River (S-65) basin.
The preferred habitat for the caracara is unimproved or improved pasture with scattered
cabbage palms for nesting. Because these land uses are also desirable for STA construction,
it is very likely that a significant level of permitting and coordination will be required for
this species. However, caracaras are opportunistic feeders and use wetland areas for
foraging.
Bald Eagle
Exhibit 15 shows the locations of bald eagle nesting sites based on 2006 data. In 2007, the
bald eagle was removed from the Federal and State endangered and threatened species lists.
However, there are still rules in place to protect eagle nest sites, particularly during the
breeding season. These rules are copied below (FWC 2008):
•
No FWC Eagle Disturbance Permits will be issued for activities within 330 feet of an
active bald eagle nest during the nesting season, Oct. 1 to May 15, or whenever
eagles are present at the nest site.
•
Outside of the nesting season, an FWC Eagle Disturbance Permit can be used for
projects up to 100 feet from the nest. The FWC will not permit any activity within
100 feet of a nest any time of the year, except for nests built on artificial structures or
when similar scope may allow construction activities to occur closer than 100 feet.
•
An FWC Nest Removal Permit will be issued only in the case of a human and/or
eagle health or safety issue.
•
Any land-altering activity within 660 feet of an active or alternate bald eagle nest
that cannot be undertaken consistent with FWC eagle management activities may
require an FWC Eagle Permit. Activities beyond 660 feet do not ever require an FWC
Eagle Permit. When construction activities are planned inside the recommended
buffer zone of an active or alternate bald eagle nest, then issuance of the FWC Eagle
Permit will require conservation measures as follows:
o
For activities between 330 and 660 feet, one conservation measure is
sufficient.
o
For activities within 330 feet of a nest, two conservation measures should be
included with the application, and one of the two measures should be a
$35,000 contribution to the Bald Eagle Conservation Fund.
•
Permits will be issued if they further the management plan goal and objectives.
•
No permit will be required for activities that follow the FWC Eagle Management
Guidelines as described in the management plan.
23
WETLAND SOLUTIONS, INC.
EXHIBIT 14
Consultation Area for the Crested Caracara
24
WETLAND SOLUTIONS, INC.
EXHIBIT 15
Location of Bald Eagle Nests in the Study Area
25
WETLAND SOLUTIONS, INC.
•
For projects with a buffer between 330 and 660 feet from an eagle nest that do not
follow the guidelines and obtain a FWC Eagle Permit one conservation measure is
sufficient:
o
Contribute $35,000 to the Bald Eagle Conservation Fund to support bald
eagle monitoring and research.
o
Provide a financial assurance (such as a bond) in the amount of $50,000.
o
Grant a conservation easement over the 330-foot buffer zone of an active or
alternate bald eagle nest within the same or an adjacent county, or within the
same core nesting area. When the buffer is only partially owned by the
applicant, contribute an onsite easement over the portion of the 330-foot
buffer zone to which the applicant holds title.
o
Grant a conservation easement over suitable bald eagle nesting habitat (see
the next bullet below) onsite or offsite.
o
Propose an alternate conservation measure that advances the goal of the
management plan based upon the particular facts and circumstances
presented by the applicant.
o
For projects with a buffer of 330 feet or less, two conservation measures
should be included with the application, and one of the two measures should
be a $35,000 contribution.
o
Minimization measure may also be required for projects that are within 660
feet of an eagle’s nest. Some examples include (there are a total of nine):
ƒ
Implement the Bald Eagle Monitoring Guidelines (USFWS 2007) for
all site work or exterior construction activities. Avoid exterior
construction activities within 330 feet of the nest during the nesting
season.
ƒ
Create, enhance or expand the visual vegetative buffer between
construction activities and the nest by planting appropriate native
pines or hardwoods.
Florida Panther
Exhibit 16 shows the range of the Florida panther. The preferred land uses for STA
construction do not provide optimum panther habitat (pinelands, hardwood hammocks,
and mixed swamp forests) but likely provide travel corridors. STA construction would be
consistent with protecting land from development and providing areas where panthers can
roam with minimal human interference.
Flora
One specific plant species (Okeechobee gourd) and two groupings of plants (Lake Wales
Ridge and Southwest) may occur in parts of the study area (Exhibit 17). The Okeechobee
gourd (Cucurbita okeechobeensis) historically occurred in pond apple (Anona glabra) forests
that grew around the lake shore. Conversion of pond apple forests to agriculture and water-
26
WETLAND SOLUTIONS, INC.
EXHIBIT 16
Consultation Area for the Florida Panther
27
WETLAND SOLUTIONS, INC.
EXHIBIT 17
USFWS Consultation Areas for Threatened and Endangered Flora
28
WETLAND SOLUTIONS, INC.
level regulation in the lake have been the most significant impacts to this species (USFWS
1999).
Most of the Lake Wales Ridge plants are associated with dry scrub habitat and would not
likely occur in the areas most suitable for STA construction. The Southwest plants
consultation area impacts only the most western portion of the Fisheating Creek basin and is
too far upgradient to exclude likely STA sites.
Hazardous Wastes
Preliminary screening and risk assessment for the presence of hazardous waste materials on
a site can be done through the use of readily-available and frequently updated GIS data
layers that describe known spills or remediation projects and document the locations of
active hazardous waste generators (Exhibit 18).
As is customary with land acquisitions, the District would conduct a Phase I Environmental
Assessment to determine the presence and risk of chemical contamination on a potential
site. While the preferred land use types (i.e. pastures, groves, and other agriculture) offer
relatively low risks of encountering prohibitively high legacy phosphorus concentrations,
diesel fuel storage tanks and pesticide or herbicide tanks can be found on many agricultural
properties. In some cases, the soils around these tanks may require remediation prior to STA
construction and hydration.
Other Potential Constraints
Other potential constraints to the STA site-selection process include the presence of
historical or cultural resources, proximity to airports, and location of existing utilities. Each
is discussed briefly below.
Historic Significance
Exhibit 19 shows the locations of known historically-significant structures and areas that
have been surveyed for cultural and historical resources. The survey areas do not
necessarily indicate areas that must be precluded from land use modifications.
Historical structures and archaeological sites can not be disturbed without coordination
with the Division of Historical Resources and mitigation for impacts. Additional resource
surveys would need to be conducted on any site purchased for STA implementation.
Proximity to Airports
The Federal Aviation Administration (FAA) has issued Advisory Circular 150/5200-33B
(FAA 2007) which describes various wildlife hazards and attractants on or near airports
(Exhibit 20). The recommendations contained in the document apply only to public-use
airports that have received Federal grant-in-aid assistance. The document identifies
“artificial marshes” as wildlife attractants that may increase wildlife hazards (bird strikes).
The following recommendations are provided in the FAA circular:
•
Maintain a separation distance of 5,000 feet between the airport and attractant for
airports serving piston-powered aircraft.
29
WETLAND SOLUTIONS, INC.
EXHIBIT 18
Known Hazardous Waste Generators and Petroleum Contamination Sites
30
WETLAND SOLUTIONS, INC.
EXHIBIT 19
Historic Structures and Cultural Resource Study Areas
31
WETLAND SOLUTIONS, INC.
EXHIBIT 20
Airports in the Study Area
32
WETLAND SOLUTIONS, INC.
•
Maintain a separation distance of 10,000 feet between the airport and attractant for
airports serving turbine-powered aircraft.
•
Maintain a distance of 5 miles between the farthest edge of the airport’s air
operations area (AOA) and the wildlife attractant to protect approach, departure,
and circling airspace.
Utility Easements
Existing utilities may complicate the design of a particular STA site, but most of the EAA
STAs have been constructed around various utilities such as overhead power transmission
lines. STA cells can typically be laid out so that utilities are accessible from perimeter levees.
References
FWC. 2008. Fact Sheet – Bald eagle management plan. Florida Fish and Wildlife
Conservation Commission. October 16, 2008.
Soil and Water Engineering Technology, Inc. (SWET). 2007. Final Report, Task 2 Evaluation
of Existing Information, Technical Assistance in Review and Analysis of Existing Data for
Evaluation of Legacy Phosphorus in the Lake Okeechobee Watershed. Prepared for the
South Florida Water Management District. December 27, 2007.
SFWMD. 2009. 2009 South Florida Environmental Report. South Florida Water Management
District. West Palm Beach, FL.
SFWMD, FDEP, and FDACS. 2008. Lake Okeechobee Watershed Construction Project Phase
II Technical Plan. Prepared by the South Florida Water Management District, Florida
Department of Environmental Protection, and Florida Department of Agriculture and
Consumer Services. February 2008.
USFWS. 1999. South Florida Multi-Species Recovery Plan. United States Fish and Wildlife
Service.
33