1. No category

Water Quality Design
Storm: What is the
first flush ?
Jon Hathaway, E.I.
Bill Hunt, PhD, P.E.
Eban Bean, E.I.
Jonathan T. Smith, P.E.
Stormwater
Engineering
What’s the First Flush?
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Wetland / Wet Pond Volumes
Cross Section: Definition of Storage Layers
Peak Mitigation
(or extra) Volume
Normal Pool
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First Flush
Volume
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Bypass Weir
First Flush Orifice
(Sets Normal Pool)
Emergency
Drawdown
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First Flush
Volume
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Suggested Definition of the
“First Flush”
z
z
Runoff containing 90% of annual pollution
Currently in NC
z
z
z
z
1.5” in CAMA counties
1.0” in other Phase I/II communities
In GA, this rainfall is 1.2 inches
Recent study in NC, shows variability
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Nine cities in NC
Brevard, Asheville,
Charlotte, Greensboro, Raleigh
Fayetteville, Wilmington, Greenville
Elizabeth City
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Data Analysis
z
z
z
30 years of rainfall (1974-2003)
15 min increments
9 cities in North Carolina
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Runoff hydrograph
0 .2
0 . 18
dishcarge (cfs)
0 . 16
0 . 14
0 . 12
0 .1
0 .0 8
0 .0 6
0 .0 4
0 .0 2
0
12/16/03
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12/17/03
12/18/03
12/19/03
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What about successive
storms?
0 .6
0 .5
Discharge
0 .4
0 .3
0 .2
0 .1
0
4 /3 0 /0 4
5 /1/0 4
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5 /2 /0 4
5 /3 /0 4
5 /4 /0 4
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What is a “storm”?
A period of rainfall in which individual rainfall
increments are not significantly separate
from one another chronologically:
Inter-event dry period < 6 hrs
Inter-event dry period > 6 hrs
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1 Storm
2 Storms
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30 years of storm data (Raleigh)
8
7
Depth (in)
6
5
4
3
2
1
0
0
500
10 0 0
15 0 0
2000
2500
3000
3500
Individual Events
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Long term Storm Data
Raleigh
Asheville
Storms
3045
3398
Max Storm (in)
7.5”
5.4”
# Annual Storms
104
113
Mean Storm Depth (in)
.41
.33
Annual depth (in)
42
37
Mean dry interval (days)
3.6
3.2
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Comparison
Location
Raleigh, NC
Annual Average Rainfall
(in)
42
Shreveport
~ 46
Baton Rouge
~ 61
New Orleans
~ 62
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Determining Capture Depth
8
7
Depth (in)
6
5
4
3
2
1
0
1
501
10 0 1
15 0 1
2001
2501
3001
Individual Events
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Determining 90% capture depth
8
7
D ep th (in )
6
5
4
10%
3
Capture Depth
2
90%
1
0
1
50 1
10 0 1
150 1
2001
2 50 1
3001
Individual Storm s
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Raleigh % capture
1.6 0
1.4 0
1.2 0
Depth (in)
1.0 0
0 .8 0
0 .6 0
0 .4 0
0 .2 0
0 .0 0
0
20
40
60
80
10 0
% Long term capture
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90% storms for NC cities
2 . 50
2 .2 4
1. 8 5
Depth (in)
2 .0 0
1. 6 0
1. 55
1. 50
1. 56
1. 59
1. 55
1. 4 4
1. 2 8
1. 0 0
0 . 50
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0 .0 0
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Sample 80% & 90% Storms
City/ Town
80% of
Rainfall <
90% of
Rainfall <
Raleigh (annual
precipitation ~42”)
0.97”
1.44”
Asheville ( annual
precipitation ~38”)
0.83”
1.28”
Wilmington (annual
precipitation ~56”)
1.40”
2.24”
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Impact of 90% storm on Pond
Sizing: Case Study
z
z
z
z
Raleigh (1.44”)
50 acres
Type C soil
Land Uses
z
z
Commercial
Residential ( 1/8, ¼, ½, 1 acre lots)
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Pond Footprint required
3 .0 0
1"
Pond Size (ac)
2 . 50
90%
2 .0 0
1. 50
1. 0 0
0 . 50
0 .0 0
1 acr e
1/ 2 acr e
1/ 4 acr e
1/ 8 acr e
C o mmer ci al
Land Use
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Summary
z
30 years of data from 9 cities analyzed
z
90% highly variable depending on location,
higher than 1”
z
Typical period between rain events ranges
from 3 to 4 days
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Questions?
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