Stormwater Stormwater Engineering Engineering Hydraulic & Hydrologic Hydraulic & Hydrologic Design of Stormwater Design of Stormwater Wetlands Wetlands Jon Hathaway, EI Extension Associate NCSU Bio. And Ag. Engineering
Stormwater Stormwater EngineeringEngineeringHydraulic & Hydrologic Hydraulic & Hydrologic
Design of Stormwater Design of Stormwater WetlandsWetlands
Jon Hathaway, EIExtension Associate
NCSU Bio. And Ag. Engineering
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6 Step Process6 Step Process1. Watershed Analysis (Runoff Volume
and Peak Flow)2. Determine H20 Storage Volume3. Calculate Surface Area4. Size Drawdown Orifice5. Size Overflow Weir6. Diagram Internal Features
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Additional TopicsAdditional Topics1. Perch or Excavate to Water Table?2. Bypass or Flow-Through?3. Forebay Design
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Watershed AnalysisWatershed Analysis(Determine Treatment Volume)(Determine Treatment Volume)
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Watershed AnalysisWatershed Analysis
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Determine VolumeDetermine Volume * Fact Finding *
Watershed AreaWatershed Land Use(s)Soil Type(s) within WatershedSoil Hydrologic Group
» Use Land Use Map & County Soil Survey » Verify soils data in the field
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Determine VolumeDetermine Volume * Calculate Runoff from Curve Number * (SCS – Curve Number Method)
P = PrecipitationCN = Curve NumberS = Storage Component
Determined from CNR/O = Runoff
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Determine VolumeDetermine Volume * Describing Land Use *
Curve Numbers - Relate Land Use and Soil Hydrologic GroupRange from 30 to 100Higher Number = More Runoff
» USDA-NRCS Tables
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Curve NumbersCurve Numbers
Land Use A B C D Paved Parking Lots; Roofs 98 98 98 98 Commercial & Bus. Distr. 89 92 94 95 Townhouses 77 85 90 92 Residential Lot (1/2 AC) 54 70 80 85 Residential Lot (1 AC) 51 68 79 84 Open Space: grass > 75% 39 61 74 80
Soil Group
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Curve NumbersCurve NumbersKeep Separate or Composite?
NCSU suggests Discrete Method for Urban Watersheds
Base Decision upon Soil Type Differences within watershed & Land Use
May Use Composite for Watersheds with low Imperviousness (< 10%)
Is Land Use Separation Distinct or Mixed
“Connected” Imperviousness
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Curve NumbersCurve NumbersIf Discrete:Calculate runoff from impervious areas separately – determine composite for rest of watershed and calculate runoff(Can calculate for each land use in watershed if desired)If Composite:
CNcomp = %Wa ∗ CNa + %Wb∗ CNb + ...
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Determine VolumeDetermine Volume
Decide Design Storm Calculate Runoff from Impervious and Pervious land uses Use Watershed Area to Find…
Total Runoff
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Determine VolumeDetermine Volume * Decide Design Storm *
Varies from 0.5” to 1.5”Retrofit versus New Development
Depends upon Built upon area, wetnessP = 1.0 in. “Old” Typical
Can be sized based on Water Quality Storm (earlier presentation)
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Determine VolumeDetermine Volume * Calculate Runoff from Curve Number * (SCS – Curve Number Method)
P = PrecipitationCN = Curve NumberS = Storage ComponentR/O = Runoff
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Determine VolumeDetermine Volume * Calculate Runoff from Curve Number *
S = 1000 ÷ CN – 10
R/O = (P - 0.2S)2 ÷ (P + 0.8S)
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Determine VolumeDetermine VolumeSimple Method ?
Imp% = Percent Imperviousness in WatershedP = Depth of Rainfall (in)RO = Depth of Runoff (in)V = Volume of Runoff (ft3)A = Area of Watershed (ft2)
V = ((Imp%*0.9) + 0.05) * P * A
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Determine VolumeDetermine Volume
0
10,000
20,000
30,000
40,000
50,000
60,000
70,000
1
Precipitation Depth (in)
Run
off V
olum
e (ft
3)
CompositeCN
Discrete
Simple
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Determine VolumeDetermine Volume * Example * Given:
Total Watershed Area = 35 AC20 AC is school (soil group B)
15 AC is fair condition grass (soil group C)
Find:Volume of Water to be Treated if P=1.5”
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Determine VolumeDetermine Volume * Example *Runoff Per Land Use/Soil Type:
School CN = 88S = 1000/88 - 10 = 1.36
Field CN = 79S = 1000/79 - 10 = 2.66
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Determine VolumeDetermine Volume * Example *Runoff Per Land Use/Soil Type:
School R/O =(1.5-0.2*1.36)2 ÷ (1.5+0.8*1.36)= 0.58 in
Field R/O =(1.5-0.2*2.66)2 ÷ (1.5+0.8*1.36)= 0.26 in
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Determine VolumeDetermine Volume * Example *Runoff Per Land Use/Soil Type:Volume of School R/O =0.58 in * 20 AC = 11.6 ac-in
Volume of Field R/O =0.26 in * 15 AC = 3.9 ac-in
Total R/O = Volume to Treat = 15.5 ac-inOr (1.3 ac-ft)
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Watershed AnalysisWatershed Analysis(Determine Peak Flow)(Determine Peak Flow)
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Peak Flow during Large StormsPeak Flow during Large StormsOn large systems or high risk, detailed modeling would be appropriate
Need to make sure that outlet can pass the peak flow from a large storm
Use Rational MethodMay not be accurate, be sure to include factor of safetyWidely used despite downfalls
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What is Peak Flow ?What is Peak Flow ?Largest amount of flow experienced during a given storm
0
0.02
0.04
0.06
0.08
0.1
0.12
0.14
0.16
0.18
0.2
12/16/03 12/17/03 12/18/03 12/19/03
dish
carg
e (c
fs)
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Rational MethodRational MethodBasic Equation:
Qp = C * I * A
Where: Qp = Peak flow (cfs)C = Rational Coefficient (composite)
I = Rainfall Intensity (in / hr)A = Watershed Area (acres)
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Rational CoefficientRational CoefficientSimilar to Curve Number – Variable with land useComposite for given watershedThe higher the rational coefficient the higher the peak flow
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Rainfall IntensityRainfall IntensityVaries depending on storm size Typically duration used is either 6 hour (safe estimate) or 24 hourSee Handout (State of NC Erosion and Sediment Control Planning and Design Manual)
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From Our Earlier ExampleFrom Our Earlier ExampleIntensity (Greensboro):
Choose 2 year event – 6 hour = ~0.36 in/hr
Rational Coefficient:School = 0.6Field = 0.15
Composite = (0.6 * 20 acres) + (0.15 * 15 acres) 35 acres
Composite = 0.41
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From Our Earlier ExampleFrom Our Earlier ExampleQp = C * I * A
Qp = 0.41 * 0.36 in/hr * 35 acres
Qp = ~ 5.2 cfs
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Calculate Surface AreaCalculate Surface Area
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Calculate Surface AreaCalculate Surface Area
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Calculate Surface AreaCalculate Surface AreaFunction of Volume and Allowable Depth
Capture Volume Previously Determined
Allowable Depth of Water Varies from 6”to 18”
* 12” suggested *
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Wetland VolumesWetland Volumes
Normal Pool
First Flush Volume
Peak Mitigation (or extra) Volume
Wetland Cross Section – Definition of Storage Depths
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Calculate Surface AreaCalculate Surface AreaS/A = Surface Area RequiredVolume = Total Volume CapturedDepth = Depth of water over normal pool (Depth of Storage Volume)
S/A = Volume ÷ Depth
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Calculate Surface AreaCalculate Surface Area * Earlier Example * Given:
Volume 15.5 ac-inDepth = 9 in
Find:Surface Area of Wetland
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Calculate Surface AreaCalculate Surface Area * Example * Find Surface Area
S/A = 15.5 ac-in ÷ 9 in S/A = 1.7 AC
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Calculate Surface AreaCalculate Surface AreaRemember !!
Account for some additional surface area if you are bypassing larger storms
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Size OutletSize Outlet
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Retaining StormwaterRetaining Stormwater How Long Can Water stay in
wetland (above normal pool)?• Plant Tolerance• Rainfall Event Frequency
– Mean time between events = 3 days
– 2-4 days reasonable
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Retaining StormwaterRetaining Stormwater Common Devices Used to Retain
Stormwater :Orifice (draw down)Weir (overflow)
(Flashboard Riser ?)
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Retaining StormwaterRetaining Stormwater Weir Equation
Q = Flow over WeirCw = Weir Coefficient (Commonly - 3.0)
L = Length of WeirH = Height of Water
upstream of weir
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Retaining StormwaterRetaining Stormwater * Devices Used to Retain Stormwater *
Weir Equation
Q = Cw * L * H 3/2
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Flow over WeirFlow over WeirH 2/3 H
Weir
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Retaining StormwaterRetaining Stormwater Orifice Equation Q = Flow through Orifice N = Number of Orifices (holes) Cd = Coefficient of Discharge (Commonly 0.6)
A = Area of Opening g = Gravity (32.2 ft/s2) H = Water Height over Orifice Centerline
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Retaining StormwaterRetaining Stormwater* Device Used to Retain Stormwater *Orifice Equation
Q = N * (Cd * A * (2*g*H)1/2)
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Retaining StormwaterRetaining Stormwater How Long to Empty Wetland of Treatment
Volume ?
Quick & Dirty Method:
1. Calculate Flow rate at 1/3 Full Height 2. Divide this Flow rate into Total Volume
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Retaining StormwaterRetaining Stormwater* Earlier Example *
Given:Wetland Storage (above Normal Pool) = 56,000 cubic feet (15.5 ac-in)Size of drawdown orifice = 5 inchesHeight of Water over orifice = 12 in
Find:Amount of Time to Release Storage
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Retaining StormwaterRetaining Stormwater Time to Release Storage: 1. Flow over weir when 1/3 full
Q = N * (Cd * A * (2*g*H)1/2)
2*g*H = 2 * 32.2 ft/s2 * 0.33 ft = 21.3
Q = 1 * (0.6 * 0.14 ft2 * (21.3)1/2) = 0.39 cfs
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Retaining StormwaterRetaining Stormwater Time to Release Treatment Volume: 2. Time to Release Water
Time = Vol / Discharge Rate Time = 56,000 cf / 0.39 cfs Time = 91,900 s = 40 hours
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Retaining StormwaterRetaining StormwaterBypass larger storms – What Size ??
Designer choice based on riskCost of failure
Safety of individuals living in areaMonetary cost
Must be able to pass peak flow for chosen stormWatershed Peak Flow = Peak Wetland Outflow
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Retaining StormwaterRetaining Stormwater* Earlier Example *
Overflow Weir Sizing
2 year – Peak Flow = 5.2 cfs
Will allow 6 inches over weir during event(Infrequent event, should not impact vegetation)
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Retaining StormwaterRetaining StormwaterWater will leave through:
drawdown orificeoverflow weir
The outflow from these two devices combined should be 5.2 cfs
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Retaining StormwaterRetaining StormwaterFlow through Orifice
Total depth over orifice is 18 inches
Q = N * (Cd * A * (2*g*H)1/2)
2*g*H = 2 * 32.2 ft/s2 * 0.67 ft = 43.1
Q = 1 * (0.6 * 0.14 ft2 * (43.1)1/2)Q = 0.55 cfs
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Retaining StormwaterRetaining StormwaterNeed to route 5.2 cfs - have 0.55 cfs routed
through orifice - Weir must handle 4.65 cfs
Q = Cw * L * H 3/2
Or
L = Q / (Cw * H3/2)
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Retaining StormwaterRetaining StormwaterL = Q / (Cw * H3/2)
L = 4.65 cfs ÷ (3.0 * (0.5 ft)3/2)
L = 4.38 ft (should include factor of safety)
L = 4.38 * 1.2 = 5.26
L = 5.3 ft
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Diagram Internal FeaturesDiagram Internal Features
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Stormwater Wetland FeaturesStormwater Wetland Features
Forebay
Outlet
Shallow Water
DeepPools
Upland “Shallow”Land
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Revised Revised PondingPonding DepthsDepths
Deep Pool
Shallow Water
Shallow Land
18-36” 0-12”1-6”
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How much of Each?How much of Each?
10-20%
40-60%
30-40%
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Perch or Excavate to Perch or Excavate to Water Table ?Water Table ?
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Perch or Excavate to W.T.?Perch or Excavate to W.T.?
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Perch or Excavate to W.T.?Perch or Excavate to W.T.?Determine Soil Type at W/L Site
What is Seasonally Low Water Table? > 4’ - You choose< 4’ - Excavate to 6” below W. T.
Highly dependent on Soil ConditionsDependent on Desired Cost of Construction
Clay Liner
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Perch or Excavate to W.T.?Perch or Excavate to W.T.?
Determine Soil Type at W/L SiteWhat is Permeability?
0.06 - 0.2 in/hr, Compact Necessary> 0.2 in/hr, Import Clay & Compact
Want K < 0.01 in/hrWant K < 0.01 in/hr
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Perch or Excavate to W.T.?Perch or Excavate to W.T.? * Summary *
If Seasonally Low Water Table at or near Surface -Excavate. If not, Perch.
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Perch or Excavate to W.T. ?Perch or Excavate to W.T. ?
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Bypass or FlowBypass or Flow--Through ?Through ?
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Water BypassWater Bypass
Flow Through Main Body?
Bypass Large Storms?
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Bypass or Not?Bypass or Not?If Actual Surface Area > 0.80 of Required Design Surface Area, you do NOT need to bypass flow
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Flow SplitterFlow Splitter
Bypass Weir
Normal Pool
2-Yr Storm Pool
Drawdown Orifice
“Emergency” Spillway
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What Size Storm?What Size Storm? It depends…
None. All storms through Wetland
“Extreme”Event. 10-25 YR
Storm
“Moderate”Event. 2-YR
Storm
All Storms > First Flush Event Bypass
Wetland
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ForebayForebay DesignDesign
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ForebayForebay DesignDesign
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ForebayForebay Design Design More Information on the Way !!!
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Questions ?Questions ?