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PUBLIC IMPROVEMENT DESIGN STANDARDS
FOR RIVER TERRACE
Stormwater Management
2015
Draft Date: 5/26/2015
City of Tigard
13125 SE Hall Blvd.
Tigard, OR 97223
Phone: 503-639-4171
Fax: 503-684-7297
Effective Date:
________, 2015
<Insert City’s signature block>
TABLE OF CONTENTS
1.
General ........................................................................................................................................................ 1
2.
Applicability ................................................................................................................................................ 1
3.
Definitions .................................................................................................................................................. 1
4.
Other Standards Apply ............................................................................................................................. 2
5.
Plan Review and Submittal Requirements ............................................................................................. 3
6.
Runoff Treatment and Control ............................................................................................................... 6
7.
Regional Facility Amenity Standards ....................................................................................................19
Appendices
Appendix A: River Terrace Standard Drawings and Example Drawings
Appendix B: Aesthetic Design Concepts Illustrations
A.
STORMWATER
1. General
These standards shall be used for the design, construction, installation, and testing of
stormwater system improvements in River Terrace. All stormwater system improvements
shall require approval from the City Engineer or designee and shall meet the requirements
set forth herein.
The River Terrace Community Plan provides the vision for River Terrace. The River Terrace
Stormwater Master Plan describes stormwater management strategies to help implement the
vision. These public improvement standards include specific technical and design
requirements that implement the vision and strategies of both plans.
2. Applicability
These standards shall apply to all new development and redevelopment in River Terrace.
Requirements differ according to Stormwater Strategy Area, as defined in the River Terrace
Stormwater Master Plan and shown in Figure 6.1.1, below.
3. Definitions
3.1. Definitions shall be in accordance with Clean Water Services’ Design and Construction
Standards 2007 (District Standards), or the most current version, except as modified
below.
A. Continuous Simulation Flow Model
A hydrologic modeling software that generates long-term runoff predictions for
performing flow duration and flood frequency analyses.
B.
Frequent Flow Storage Elevation
The elevation in a water quantity facility above which is expected to be inundated
only when the 2-year peak flow is exceeded under developed conditions.
C.
Flow Duration Analysis
A hydrologic analysis that estimates the percentage of time a flow rate is likely to
equal or exceed a specified value at a specified location.
D. Flood Frequency Analysis
A hydrologic analysis that predicts the values of discharge at a specified location
during the specified recurrence interval storm.
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Page 1
E. Interim Facility
A provisional stormwater facility serving a portion of a drainage basin before
installation of the regional stormwater facility designated to serve the entire
drainage basin as described in the River Terrace Stormwater Master Plan.
F.
Peak Flow
The greatest rate of flow passing a specified location.
G. Point of Compliance
The downstream-most discharge location (outfall) from a pond or other detention
facility where a flow duration analysis is performed.
H. Pre-Development Condition
Specified land cover conditions (e.g. forested, pasture) used in hydrologic modeling
of a site prior to development activity to establish baseline runoff results for
comparing with post-development runoff results.
I.
Regional Facility
A stormwater facility serving an entire drainage basin as described in the River
Terrace Stormwater Master Plan and providing water quantity control, with or without
water quality treatment.
J.
River Terrace
The area of the City of Tigard described in the River Terrace Community Plan.
K. T.R.U.S.T. Model
The Tualatin River Urban Stormwater Tool (T.R.U.S.T.) is hydrologic modeling
software that provides continuous simulation flow modeling and flood frequency
modeling.
4. Other Standards Apply
4.1. All public stormwater system improvements in River Terrace shall conform to the
below-listed sections of the City of Tigard’s Public Improvement Design Standards,
except where modified elsewhere in this document.
A.
Section A – General
B.
Section B – Providing for Future Development
C.
Section J – Design Modifications
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Page 2
D.
Section K – Construction Inspection
4.2. All public stormwater system improvements shall conform to the below-listed sections
of the District Standards except where modified elsewhere in this document.
A.
Chapter 1 – General Requirements and Administrative Provisions
B.
Chapter 2 – Administrative Procedures, except as modified in section 5, below.
C.
Chapter 3 – Sensitive Areas and Vegetated Corridors
D.
Chapter 5 – Conveyance Systems
E.
Chapter 6 – Erosion Prevention and Sediment Control
F.
Chapter 7 – General Construction Specifications for Conveyance Systems
G.
Chapter 8 – Technical Specifications for Conveyance Systems
H.
Appendix A – Planting Requirements, except as modified in section 6.2.F, below
I.
Appendix B – Standard Details, except as supplemented in Appendix A of these
standards
5. Plan Review and Submittal Requirements
Submittals are required for both land use review and engineering review.
5.1. Requirements for Land Use Approval
These requirements supplement, but do not replace, requirements for land use
approval in the Tigard Municipal Code.
A.
Submittal Minimum Requirements
1.
Landscape plans, reports, and maintenance instructions and planting plans,
reports, and specification covered by these rules shall be prepared by a
Landscape Architect registered in the State of Oregon.
2.
Engineering plans, calculations, and reports covered by these rules shall be
prepared by an Engineer registered in Oregon.
B.
The site plan shall show the locations, geometry, and sizes of stormwater facilities.
C.
For each water quantity and water quality facility, submit a plan meeting the
requirements of District Standards section 2.04.2.g.
D.
Submit a drainage report meeting the requirements of District Standards section
2.04.2.m, modified as follows:
1.
The downstream basin map is not required.
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Page 3
2.
Calculations for sizing of water quantity facilities shall include T.R.U.S.T.
model output demonstrating that the facility meets the flow duration standard
and the flood flow frequency standard.
(a) Model output shall be attached as an appendix to the drainage report, and
submitted electronically.
3.
Downstream analysis as defined in District Standards is not required.
4.
Maintenance plan for privately maintained facilities is not required as part of
the drainage report. (However, see section 5.2.E, Facility and Landscape
Operations and Maintenance Plan, below.)
E.
Preliminary Amenity Report and Landscape Plan
Submit a report and plans describing how the facility meets the Regional Facility
Amenity Standard. The report and plan shall include:
1.
A narrative description of features that meet the Regional Facility Amenity
Standard.
2.
A plan view at no smaller than 1”=20’ scale, showing the locations and
dimension of the following:
(a) Boundary of stormwater and amenity area(s).
(b) Contour line of the frequent flow storage elevation.
(c) Community amenity features.
(d) Existing topography and proposed grading with 1’ contours
(e) Cross sections of the facility, 2 minimum, one in each direction across the
length and width of the facility
3.
Plan and profile drawings for each amenity feature that is a structure or that
requires grading to construct (including walls, trails, overlooks, boardwalks,
sport courts, vertical features, etc.).
4.
Preliminary Landscape Plan drawings.
5.2. Requirements for Engineering Plan Approval
Engineering submittals shall be prepared in accordance with the District Standards,
section 2.04, Requirements for Engineering Plan Approval, except as modified below.
A.
Add a new sentence to the end of section 2.04.1.a. as follows:
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Page 4
1.
B.
Landscape plans, reports, and maintenance instructions and planting plans,
reports, and specifications covered by these rules shall be prepared by a
Landscape Architect registered in the State of Oregon.
The drainage report, section 2.04.2.m, shall be updated, if necessary, from the
preliminary report submitted at land use review and shall include the following
additional information:
1.
C.
Operation and Maintenance Plan for all public and private water quantity and
water quality facilities and conveyance systems.
Add a new section, 2.04.2.n, Geotechnical Report. The applicant shall submit a
geotechnical report prepared by a geotechnical engineer licensed in the State of
Oregon describing investigations and technical requirements pertinent to
excavation, fill, berms, walls, and infiltration.
1.
If infiltration is proposed, the geotechnical report shall include a soils report
prepared by a geotechnical engineer licensed in the State of Oregon, describing:
(a) Underlying soils on the site.
(b) The results of infiltration rate testing.
(c) The results of testing for hydraulic restriction layer (e.g. groundwater,
bedrock, impermeable soil).
(d) Groundwater assessment.
D.
Add a new section 2.04.2.o, Final Amenity Report and Landscape Plan. The
applicant shall submit a report and plans updating the Preliminary Amenity Report
and Landscape Plan. The final report and plan shall include the following
additional information:
1.
Specifications for each community amenity sign, furnishing, and play structure.
2.
Tabular plant counts.
3.
Irrigation plan.
E.
Add a new section, 2.04.2.p, Facility and Landscape Operations & Maintenance
Plan. The applicant shall submit a plan for operation and maintenance of each
stormwater facility, including plans for all components, including but not limited
to:
1.
Vegetation and landscaping in all areas of the facility.
2.
Structures, including catch basins, manholes, inlets, outlets, flow control
structures, walls, and any other structures.
3.
Amenity features, including furnishings, equipment, sport courts, trails, and any
other amenities.
Draft Date 5/26/2015
Page 5
5.3. Add a new section 2.04.2.q, Interim Facility Operations and Decommissioning Plan. If
an interim facility is proposed, the applicant shall submit a plan describing how the
interim facility will be maintained and decommissioned.
A.
B.
Interim Facility Operations and Decommissioning Plan, including:
1.
Facility location and description.
2.
Site ownership.
3.
Operations & Maintenance Plan per District and city standards.
4.
Party responsible for decommissioning.
5.
Proposed use of site after decommissioning.
Supporting legal documents as necessary to carry out the plan, including a
declaration of covenant running with the land, if appropriate, and appropriate
deeds and easements.
6. Runoff Treatment and Control
6.1. Runoff treatment and water quantity control facilities is applicable to development and
redevelopment sites in accordance with the River Terrace Stormwater Master Plan and
differs by geographic strategy area as shown in Figure 6.1.1, below.
A.
Proposed development and redevelopment projects in Strategy Area A shall
construct or connect to a combined regional water quality treatment and water
quantity detention facility to meet water quality and water quantity requirements.
B.
Proposed development and redevelopment projects in Strategy Area B shall
construct Low Impact Development Approaches to meet water quality
requirements and shall either preserve and enhance existing wetlands or connect to
a regional facility to meet water quantity requirements.
Refer to the River Terrace Stormwater Master Plan and coordinate with the City
Engineer for instructions on the use of LIDA and wetland preservation for water
quantity. Wetland enhancement requirements are not given in these standards.
Applicants responsible for enhancement of wetlands to meet water quantity
requirements should contact the City Engineer for instructions.
C.
Proposed development and redevelopment projects in Strategy Area C shall
construct street, site, and/or neighborhood scale Low Impact Development
Approaches to meet water quality requirements and shall construct or connect to
downstream conveyance improvements to meet water quantity requirements.
The strategy for downstream conveyance improvements has not been finalized;
Draft Date 5/26/2015
Page 6
therefore these standards do not contain requirements for downstream
improvements. Applicants should contact the City Engineer for instructions.
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Page 7
Figure 6.1.1: Strategy Areas
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Page 8
6.2. General Requirements for Water Quantity and Water Quality Facilities
General requirements for water quantity and water quality facilities shall be in
accordance with District Standards section 4.02 and the following additional standards:
A.
Regional facilities shall be above-ground vegetated facilities.
B.
Regional facilities shall offer community benefits in addition to stormwater
management in accordance with section 7, and as approved by the City Engineer.
C.
Vehicular access for maintenance of stormwater structures requiring a heavy
vehicle (e.g. vactor truck) maintenance, including inlets, outlets, and control
structures, shall be provided in accordance with District Standards section
4.02.4(b) – Standard Road Design for access to stormwater facilities.
D.
Maintenance access for walls, landscaping, and other amenity features may be
provided by approved paved trails.
E.
Minimum Facility Size.
1.
Regional facilities shall be designed with a minimum orifice size of 2 inches
diameter. Facility designs that do not achieve the applicable flow control
standard with a minimum 2 inch diameter orifice must be redesigned.
2.
If the applicant can demonstrate that the applicable flow control standard
cannot be achieved using a minimum 2 inch diameter orifice, then the City
Engineer may accept the facility as an interim facility until such time as the
regional facility is available. The interim facility shall satisfy conditions for an
interim facility in accordance with section 6.3.A, below.
F.
Requirements for vegetation (District Standards section 4.02.2) are replaced with
the following:
1.
2.
Vegetation for the following areas of a regional facility shall be in accordance
with District Standards Appendix A: Planting Requirements:
(a)
Water quality treatment area
(b)
Area(s) below the frequent flow storage elevation
(c)
Area(s) designed as a habitat amenity.
A Landscape Plan for areas above the frequent flow storage elevation shall be
proposed.
(a)
Vegetation and landscaping and shall be appropriate for the location, soil
conditions, light conditions, and purpose.
(b)
Permanent in-ground irrigation shall be provided.
Draft Date 5/26/2015
Page 9
3.
G.
H.
No invasive species shall be planted or permitted to remain within a facility,
including, but not limited to the following:
(a)
Himalayan blackberry (Rubus discolor)
(b)
Reed canarygrass (Phalaris arundinacea)
(c)
Teasel (Dipsacus fullonum)
(d)
English Ivy (Hedra helix)
(e)
Nightshade (Solanum sp.)
(f)
Clematis (Clematis ligusticifolia and C. vitabla)
(g)
Cattail (Typhus latifolia)
(h)
Thistle (Cirsium arvense and C. vulgare)
(i)
Scotch Broom (Cytisus scoparius)
(j)
Other invasive species as listed in the City of Tigard Urban Forestry Master
Plan
Requirements for fencing (District Standards section 4.02.3) are replaced with the
following:
1.
Use of fencing is discouraged unless required for safety or to meet building
code requirements, or for delineation purposes as described below.
2.
Delineation fencing is allowed to provide separation and demarcation between
mowed and un-mowed (natural) areas. Where used, delineation fencing shall be
split rail, wrought iron, or other durable fence of a rural character or decorative
design as approved by the City Engineer. Approved vegetation may
alternatively be used for demarcation purposes.
3.
Chain link fencing, if approved, shall be black vinyl coated.
4.
If a facility is fully fenced, the fence shall include a 12-foot wide lockable gate
at the maintenance access conforming to CWS Standard Drawing No. 740.
5.
If a facility contains a retaining wall, safety rail shall be provided in accordance
with building code regulations.
6.
If located in areas of public access and viewing, safety rail or fencing shall be
decorative.
Requirements for maintenance responsibility are in accordance with District
Standards section 4.02.5 except as modified below:
Draft Date 5/26/2015
Page 10
1.
Replace item (c) with the following: Regional facilities shall include the publicly
maintained water quality and quantity facilities, including areas designed as a
community amenity, in a tract dedicated to the City.
I.
Outlet pipes greater than 24 inches in diameter that extend through an
embankment shall include anti-seep collars in sufficient quantity to increase the
seepage length along the conduit by a minimum of 15%. An example schematic
drawing is included in Appendix A.
J.
Maximum Depth
1.
Maximum depth of a water quantity pool or basin shall not exceed 9 feet.
2.
Maximum depth of a combined water quality and water quantity pool or basin
that is outside of a jurisdictional wetland or wetland enhancement area shall
not exceed 9 feet.
3.
In consideration of wetland plant survival, the maximum depth of a combined
water quality and water quantity pool or basin that is within a jurisdictional
wetland or wetland enhancement area shall be as approved by the state and
federal permitting agencies.
K.
Infiltration facilities shall not be used, except as allowed in section 6.3.B.
L.
Walls
If used, walls shall conform to the following design and safety standards:
1.
No walls shall be located within the water quality treatment area of a regional
facility.
2.
Perimeter walls higher than 30 inches shall not surround more than 50% of a
regional facility.
3.
Walls that will be inundated or submerged during any design storm event shall
be cast in place with footings or other approved engineered solution.
4.
Walls shall be constructed of materials with a high quality visual character.
Examples include natural stone, large block retaining wall system such as
LOCK-BLOCK® or Redi-Rock, and cast in place walls constructed with
dimensional or patterned relief.
5.
Walls higher than 4 feet shall not be constructed of uniformly-sized concrete
blocks having a front face area of smaller than 400 square inches per block.
6.
Walls higher than 4 feet shall be screened with vegetation designed to provide
no horizontal gaps larger than 25 feet in length.
7.
Exceptions and alternate proposals for facility walls are subject to approval by
the City Engineer.
Draft Date 5/26/2015
Page 11
6.3. Exceptions
A.
Interim Facilities
1.
B.
An interim facility may be proposed and is subject to approval by the City
Engineer, in accordance with the following conditions:
(a)
The permanent infrastructure implementing the regional strategy as
described in the River Terrace Stormwater Master Plan will not be operational
in the basin before the proposed development will need to connect to it, as
determined by the City Engineer.
(b)
The interim facility shall meet the applicable technical requirements for
water quantity control and water quality treatment, as determined by the
City Engineer.
(c)
The interim facility shall not be subject to the amenity standard as
described in section 7.
(d)
The development shall not be relieved of the obligation to participate in
funding the permanent regional infrastructure identified in the River Terrace
Stormwater Master Plan for the applicable Strategy Area.
(e)
The applicant shall submit an Interim Facility Operations and
Decommissioning Plan and supporting legal documents as described in
section 5.3.
(f)
Removal of interim facilities shall be subject to City approval through a Site
Development Permit and/or an Erosion Control Permit.
Infiltration Facilities
1.
Infiltration facilities may be proposed, subject to approval by the City
Engineer, in accordance with the following conditions:
(a)
A geotechnical engineer licensed in the State of Oregon has investigated
the site and recommends infiltration.
(b)
The geotechnical engineer has provided a stamped geotechnical report.
(c)
The facility design conforms to criteria and recommendations listed in the
geotechnical report.
6.4. Water Quantity Control Requirements and Analyses
This section replaces District Standards, section 4.03.
Draft Date 5/26/2015
Page 12
A.
Mitigation Requirement for Water Quantity
1.
Each development and redevelopment shall construct or connect to a regional
water quantity control facility, as described for the site’s Strategy Area in the
River Terrace Stormwater Master Plan.
2.
A regional water quantity facility shall be sized to accept stormwater runoff
from development and roads in the upstream contributing basin as shown in
the River Terrace Stormwater Master Plan, and as approved by the City Engineer.
Developed condition assumptions shall include the expected future land use
and roads at full build-out condition as described in the River Terrace Community
Plan.
B.
Hydrologic Analysis and Design Criteria
1.
Water Quantity Design Standard. Stormwater discharges shall maintain the
duration of high flows at their pre-development levels for all flows greater than
one-half of the 2-year peak flow to the 10-year peak flow and maintain the 25year peak flow rate to the pre-development 25-year peak flow rate.
2.
Analyses Required. Water quantity facility design requires use of both flow
duration analysis and flood flow frequency analysis. Analyses shall be
completed and modeled using the continuous simulation hydrologic model,
Tualatin River Urban Stormwater Tool (T.R.U.S.T.).
For flood flow frequency analysis, the model uses the Bulletin #17B method
from the United States Geological Survey (USGS) to predict discharge at a
given recurrence interval. The flood flow frequency analysis is used to compute
the pre-development 2-year through 100-year flow frequency values and the
post-development runoff 2-year through 100-year flow frequency values from
the outlet of the proposed stormwater facility.
For the flow duration analysis, the model first performs flood flow frequency
analysis to determine the range of flows to use for the site. The model then
uses pond discharge rates to compare the pre-development and postdevelopment durations to allow the user to determine if the flow duration
discharge standard has been met.
3.
Pre-development Land Use Condition. Pre-development refers to the
conditions in River Terrace before development, as presented in Figure 6.4.1.
The majority of River Terrace was brought into the Urban Growth Boundary
(UGB) in 2002 when Metro Council adopted Ordinance No. 02-969B. The city
defines the pre-development land use conditions in River Terrace using 2002
aerial photography. Figure 6.4.1 is a map showing the designated pre-
Draft Date 5/26/2015
Page 13
development conditions. A GIS shapefile is also available from the city.
TABLE 6.4.1 - Pre-development Land Use Summary
Land Use
Area (aces)
Forested
126.1
Pasture
237.8
Lawn
111.3
Impervious
20.1
Wetland
22.9
TOTAL
518.2
4.
Point of Compliance. The Point of Compliance shall be established within
T.R.U.S.T. for each basin at the furthest downstream control structure.
Discharge durations and discharges shall be modeled at the Point of
Compliance.
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Page 14
Figure 6.4.1: Pre-development Land Use
Draft Date 5/26/2015
Page 15
C.
Hydraulic Design Criteria
1.
Overflow. A pond overflow shall provide for discharge of the 25-year peak
flow without overtopping the pond embankment or exceeding the capacity of
the emergency spillway.
2.
Emergency Spillway. An emergency spillway sized to pass the 100-year peak
flow shall be provided. The emergency spillways shall be located in existing
soils when feasible and armored with riprap or other approved erosion
protection extending to the toe of the embankment.
6.5. Water Quantity Facility Design Standards
This section replaces District Standards, section 4.04.
A.
Facility Design Criteria
1.
The facility may combine water quality and water quantity functions provided it
meets the criteria in section 6.5.C, below, and all other criteria for both
detention and water quality facilities.
2.
Side slopes shall be as follows:
(a)
Side slopes shall be 3H:1V or flatter.
(b)
Side slopes that need to be mowed shall be 4H:1V or flatter.
3.
Minimum freeboard shall be 1 foot from the 25-year design water surface
elevation.
4.
An approved outlet structure shall be provided for all flows. Approved designs
are shown in CWS Standard Drawing 270 and Appendix A of these standards.
(a) Certain situations may require use of multiple orifice plates to achieve the
desired outflow rates.
B.
Embankments in Water Quantity Facilities
1.
Pond embankments higher than 6 feet shall be designed by a geotechnical
engineer licensed in the State of Oregon.
2.
Embankments shall be no taller than 10 feet.
3.
Embankments shall have side slopes in accordance with section 6.5.A.
4.
The crest of the embankment shall have a minimum width of 10 feet or as
recommended by a geotechnical engineer.
Draft Date 5/26/2015
Page 16
5.
C.
For embankments that meet the thresholds for dam safety requirements under
Oregon Revised Statutes 540.350 through 540.400, the plans and specifications
for construction shall be prepared by a professional engineer licensed in the
State of Oregon, and the applicant shall obtain the appropriate authorization
from the Oregon Water Resources Department and approval of the City
Engineer.
Standards for Combined Water Quantity and Water Quality Facilities
1.
2.
Where a combined water quantity and water quality regional facility is required
in Strategy Area A, the designer may save space by combining the two
functions in the same facility. Options include:
(a)
Extended dry basin with additional detention storage for water quantity.
(b)
Constructed water quality wetland with additional detention storage for
water quantity.
(c)
Vegetated swale (or other flow-based water quality best management
practice) within detention storage for water quantity.
Design Criteria for Extended Dry Basin with Detention
District Standard design criteria for both Detention and Extended Dry Basin
shall apply, with the following modifications:
3.
(a)
A permanent pool not allowed.
(b)
The basin may be designed so that areas above the frequent flow storage
elevation can be used for community amenities, such as a sport court, in
drier months.
(c)
To determine orifice size, size the orifice separately for water quality and
water quantity. Select whichever orifice size is smaller. If necessary to
achieve the water quantity standard, water quality drawdown time for the
basin may be increased.
(d)
Provide a sump in the outlet structure.
Design Criteria for Constructed Water Quality Wetland with Detention
District Standard design criteria for both Detention and Constructed Water
Quality Wetland shall apply, with the following modifications:
(a) Permanent pool volume shall not be included in the required detention
volume.
(b) Do not over-excavate the second cell, or any subsequent cells; limit
sediment storage to the first cell. The purpose of this criterion is to allow
Draft Date 5/26/2015
Page 17
shallow-enough water in the second and subsequent cells for wetland plant
survival.
(c) Provide a sump in the outlet structure.
(d) To determine orifice size, size the orifice separately for water quality and
water quantity. Select whichever orifice size is smaller. If necessary to
achieve the water quantity standard, water quality drawdown time for the
basin may be increased.
(e) Limit the difference between the water quality design water surface and the
maximum water surface for 2-year runoff to 3 feet. Increase the size of the
constructed water quality wetland if the criteria cannot be met.
(f) Select plants that are adapted to water level fluctuations, such as Douglas’s
spiraea (spiraea douglasii) and small-fruited bulrush (Scirpus microcarpus).
(g) Amend soil or use topsoil within the top 12 inches of the facility to support
plant growth, as recommended by a certified soil scientist.
(h) Design the facility to not require mowing. If this criterion cannot be met,
design the facility to not require mowing more than twice annually.
4.
Design criteria for vegetated swale (or other flow-based water quality best
management practice) within detention storage for water quantity.
District Standard criteria for both Detention and the selected-flow based water
quality best management practice shall apply, with the following modification:
(a) Backwater from detention storage up to the 2-year peak flow shall not
enter the flow-based water quality facility.
6.6.
Water Quality Control Requirements
Regional facilities shall be designed in accordance with District Standards section 4.05,
with the following modification:
A.
Impervious area used in design shall include area from development and roads in
the upstream contributing basin as shown in the River Terrace Stormwater Master Plan,
and as approved by the City Engineer. Developed condition assumptions shall
include the expected future land use and roads at full build-out condition as
described in the River Terrace Community Plan.
B.
Proprietary Treatment Systems shall not be used.
Draft Date 5/26/2015
Page 18
6.7. Water Quality Facility Design Standards
A.
Regional water quality facilities shall be designed in accordance with District
Standards section 4.06 or may be designed using LIDA in accordance with section
6.7.B.
B.
LIDA facilities shall be designed in accordance with District Standards section 4.07
and the Low Impact Development Approaches Handbook (2009) except as
modified below:
1.
Only flow-through type facilities shall be used unless a geotechnical engineer
licensed in the State of Oregon has investigated the site and:
(a) Recommends infiltrating LIDAs;
(b) The geotechnical engineer has provided a stamped geotechnical report;
(c) The facility design conforms to criteria and recommendations listed in the
geotechnical report.
7. Regional Facility Amenity Standards
7.1. All public regional stormwater facilities in River Terrace shall offer community
benefits in addition to stormwater management.
A.
Purpose
The River Terrace Community Plan envisions regional stormwater facilities as an
integral component of meeting Goal 8: Parks, Recreation, Trails, and Open Space
and Goal 11: Public Facilities and Services. The plan states that “there are many
existing natural resource areas in River Terrace and several future regional
stormwater facilities that would benefit from co-location with parks, trails, and/or
open spaces” (River Terrace Community Plan, page 5-4). A recommended action
measure for stormwater management in River Terrace is “Develop stormwater
design standards and guidelines for regional stormwater facilities in River Terrace
that minimize their operational costs, guide their development as community
amenities and multi-functional facilities, and allow for need flexibility in
implementation” (River Terrace Community Plan, page 7-4).
Detention ponds and treatment facilities can be designed in a manner that both
meets engineering requirements and provides additional benefits to the
community. Stormwater facilities can serve as multi-use areas and wildlife habitat,
contribute to a sense of place, and provide educational opportunities through
visibly representing the hydrologic cycle or providing opportunities to interact with
water. Stormwater facilities should be designed for aesthetic interest and
integration with the lives of the surrounding community.
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B.
Approval Criteria
Regional facilities shall meet the Basic Amenity Design Criteria and shall include
one or more amenity features from at least one of the following categories:
Recreation, Habitat, and Education, as described in sections 7.1.D through F,
below. Designs shall also conform to Prohibitions as described in 7.1.G.
C.
D.
Basic Amenity Design Standards
1.
A facility shall be designed as an above-ground feature, except that control
structure manholes, pre-treatment manholes, and pipes may be buried.
2.
A facility shall be vegetated and be designed to appear natural.
3.
Provide public access to the facility. Examples of access include ensuring 25%
of the tract perimeter is open to the public or providing a trail within the tract.
4.
Provide at least one publicly accessible viewpoint that allows views into the
facility, or views of natural areas, or scenic views of the surrounding region.
5.
Provide at least one interpretive sign describing the facility’s purpose. Sign size,
content and materials shall be approved by the City Engineer.
6.
In each recreation area, if provided, and along public streets adjacent to the
facility, provide an approved dog waste bag dispenser, signage requiring pets to
be leashed, and an approved trash receptacle. See the current approved city list.
7.
Provide access control in the form of approved delineation fencing or
plantings to areas of the facility designed for water quality treatment, frequent
inundation, or habitat.
8.
All structures and furnishings that are installed in the facility shall be secured to
the ground and shall be constructed or manufactured to withstand periodic
inundation. See the current approved city list.
9.
Amenity designs shall conform to all applicable setbacks to Sensitive Lands
(Tigard Community Development Code Chapter 18.775) and any other
applicable setback set forth in the Tigard Municipal Code.
Recreation Amenity Standard
If the applicant elects to meet the Recreation Amenity standard, then the facility
shall provide recreation opportunities for the public in an area of the facility above
the frequent flow storage elevation.
The applicant shall include a passive element and an active element. The applicant
may propose a recreation concept or may select a minimum of one passive and one
active recreation element from the following lists:
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1.
2.
E.
Passive Recreation Elements
(a)
Park benches. See the approved city list.
(b)
Picnic tables. See the approved city list.
(c)
Open play space.
Active Recreation Elements
(a)
A sport court.
(b)
A pedestrian or multipurpose access trail to and around or along the
perimeter of the facility, designed to meet ADA requirements.
(c)
A nature-play area.
(d)
A circuit of outdoor exercise stations. Equipment shall be specifically
designed for outdoor use by an approved manufacturer. See the approved
city list.
(e)
Outdoor play structures. Playground products shall be certified by the
International Play Equipment Manufacturers Association and shall
conform to ASTMI Standard F1487 – Playground equipment and ASTM
F1292 – Play surface impact attenuation.
Habitat Amenity Standard
If the applicant elects to meet the Habitat Amenity standard, then the facility shall
include habitat for at least one species of animal native to northwest Oregon.
Applicants may propose a habitat concept or may select a habitat type from the
following list:
1.
2.
Bird habitat shall include the following elements:
(a)
A diversity of canopy covers for perching and nesting.
(b)
A diversity of native plant species that provide food, such as berries.
(c)
Demarcation or barriers discouraging human and canine access to portions
of the facility where birds may eat, perch, or nest.
(d)
Optional: provide approved bird boxes.
Amphibian habitat shall include the following elements:
(a)
A permanent pool with a shallow bench.
(b)
Boulders and logs or large wood along the shallow bench at the edge of the
permanent pool.
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F.
(c)
A diversity of emergent native vegetation on the bench.
(d)
A safe access route from the bench to dry areas of the facility or to
adjacent public land, such as a Vegetated Corridor or public park.
(e)
Optional: design the facility with multiple smaller basins that have different
water permanence regimes.
Education Amenity Standard
If the applicant elects to meet the Education Amenity standard, then the facility
shall include elements that provide information or learning opportunities about the
Tualatin River, stormwater, hydrology, or pollutants in stormwater.
The applicant may propose an education concept or may select an element from
the following list:
1.
A nature viewing platform with one or more interpretive signs and views of
areas with native vegetation, adjacent natural areas such as a Vegetated
Corridor or a public park, or regional scenic vistas.
2.
Simple signage or exhibits that use brief text and clear graphics to convey the
hydrologic and/or ecological functions of the facility. Examples include
connection of the area to the Tualatin River and/or larger Columbia River
watersheds or pollutant-reduction capabilities of the facility. A minimum of
three signs or exhibits visible from the street, parking, and/or path is required.
3.
Artwork or sculpture that conveys the hydrologic and/or ecological functions
of the facility and is designed for durability and permanence.
G.
H.
Prohibitions
1.
Community amenity features shall not impede or compromise the water quality
and water quantity functions of the facility.
2.
Community amenity features shall not be located within a water quality
treatment area. Recreation and education amenity features shall not be located
within areas of a detention basin below the frequent flow storage elevation.
3.
Facilities shall not contain off-leash dog parks.
4.
Facilities shall not contain community gardens.
Illustration of Aesthetic Design Concepts
Appendix B provides illustrations of select amenity design standards.
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