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Stormwater Stormwater EngineeringEngineering
Bioretention DesignBioretention Design
Bill Hunt, PE, Ph.D. Extension Specialist &
Assistant Professor NCSU-BAEwww.bae.ncsu.eduwww.bae.ncsu.edu/stormwater/stormwater
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Bioretention DesignBioretention Design
Six Step Process1 Determine Volume to Treat2 Determine Surface Area Required3 Select Soil Type4 Decide Depth of Soil5 Size Underdrain Pipes6 Select Appropriate Overflow Bypass
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Treatment Volume: BackgroundTreatment Volume: Background
Collect Data:Watershed AreaWatershed Composition (rooftop, lawn, parking lot)If permeable areas: Soil Type & Group?
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Treatment Volume: BackgroundTreatment Volume: Background
Determine Curve Number:Function of Land Use and Soil GroupRange from mid 30’s to 98Higher # More RunoffDeveloped by USDA-NRCS
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Treatment Volume: Curve NumbersTreatment Volume: Curve Numbers
Soil GroupLand Use/ CoverA B C D
Parking Lot/ Rooftop 98 98 98 98Lawn, etc (grasscover 50-75%)
49 69 79 84
Lawn, etc (grasscover > 75%)
39 61 74 80
Woods in FairCondition
36 60 73 79
2
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Composite CN or Not?Composite CN or Not?
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Treatment Volume: Curve NumbersTreatment Volume: Curve Numbers
Composite?Not issue if only one land use/soil typeIf distinct regions Do NOT compositeIf watershed is well mixed Composite
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Treatment Volume: Curve NumbersTreatment Volume: Curve Numbers Composite?
Use following Equation:
CNCOMP = %W/SA • CNA + %W/SB • CNB
Where A & B are Land Use-Soil Type zones within Watershed (W/S)
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Treatment Volume: CalculationTreatment Volume: Calculation
Three Part Process:1 Decide Design Storm2 Determine Runoff from Each
Land Use/ Soil Type3 Multiply by Watershed Area
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Treatment Volume: CalculationTreatment Volume: Calculation
1 Decide Design Storm:Varies from 0.50” to 1.50”Dependent on...
frequency of rainfalldeveloped condition
1.00” Typically Used
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Treatment Volume: CalculationTreatment Volume: Calculation
2 Runoff Produced per Land Use/Soil Type:Use Curve Numbers (CN) Calculate Storage Volume on and within Soil (S)
S = 1000 ÷ CN - 10 where S in inches
3
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Treatment Volume: CalculationTreatment Volume: Calculation
2 Runoff Produced per Land Use/Soil Type:Use Storage Volume (S) and Design Storm (P) to Calculate Runoff (R/O)Employ SCS Equation
R/O = (P - 0.2S)2 ÷ (P + 0.8S)
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Treatment Volume: CalculationTreatment Volume: Calculation
3 Multiply by Watershed AreaVolume of Runoff to Treat
VolTREAT = A • R/O
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Treatment Volume: ExampleTreatment Volume: Example
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Treatment Volume: ExampleTreatment Volume: Example
Given: Total Watershed Area of 10,000 sf
2,000 sf of rooftop 8,000 sf of dense growth lawn
Find: Volume of Water To Treat from P=1.00”
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Treatment Volume: ExampleTreatment Volume: Example
Find Volume of Runoff from Rooftop
1 Determine Curve Number: 982 Find Soil & Surface Storage, S: S = 1000/ 98 - 10 S = 0.20
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Treatment Volume: ExampleTreatment Volume: Example
Find Volume of Runoff from Rooftop3 Find Runoff, R/O, from Design Storm P=1.00” S = 0.20”
R/O = (1.0 - 0.2 • 0.20)2
(1.0 + 0.8 • 0.20) R/O = 0.80 inches
4
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Treatment Volume: ExampleTreatment Volume: ExampleFind Volume of Runoff from Rooftop
4 Find Total Volume to Treat, VolTREAT
R/O = 0.80” Watershed Area = 2000 sf
VolTREAT = 2000 sf • 0.80 in VolTREAT = 1600 sf-in (133 cf)
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Treatment Volume: ExampleTreatment Volume: Example
Find Volume of Runoff from Lawn1 Determine Curve Number: 742 Find Soil & Surface Storage,
S = 3.51 in3 Runoff Amount, R/O = 0.02 in4 Treatment Volume, VolTREAT =
160 sf-in
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Treatment Volume: ExampleTreatment Volume: ExampleFind Total Volume from Roof & Lawn
VolTREAT (roof) = 1600 sf-in VolTREAT (lawn) = 160 sf-in
VolTREAT (total) = 1760 sf-in VolTREAT (total) = 147 cf
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Bioretention Surface AreaBioretention Surface Area
Factor of VolTREAT and Allowable DepthVolTREATpreviously determined
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Bioretention Surface AreaBioretention Surface Area
Allowable Water Depth range from 6”to 18”
PG Co, Maryland specifies 6”Author suggest 6-9” reasonable for most applications15-18” only if VERY SANDY application (e.g. in Sandhills)
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Bioretention Surface AreaBioretention Surface AreaDivide VolTREAT by Average Depth
S/A = VolTREAT ÷ D where S/A = surface area (sf) VolTREAT = volume stored in B-R (sf-in
or cf) D = Average Depth of Water (in or ft)
5
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BB--R Surface Area: ExampleR Surface Area: Example
Given: 1760 sf-in of water to be stored in Bio-Retention area
Normal Depth = 9”Find: Required Surface Area
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BB--R Surface Area: ExampleR Surface Area: Example
S/A = 1760 sf-in ÷ 9 in S/A = 200 sf
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Bioretention Soil: TypeBioretention Soil: Type
What is In-Situ Soil?Will site be compacted during construction?Alabama Study found Infiltration Rates in Sand Decrease 10 fold
If In-Situ Soil tighter than Sandy Loam ORSignificant Construction, then
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Bioretention Soil: TypeBioretention Soil: Type
Underdrains and Fill Soil Needed
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Bioretention Soil: TypeBioretention Soil: TypeSelecting Fill Soil Type
1 By Permeability:K range from 0.5” to 6” per hour1 to 2 in/hr (REC)
2 “Recipe” 85% - 88% Sand 8-12% Fines (Silt+Clay) 2-5% Organic Source
6
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Bioretention Soil: TypeBioretention Soil: Type
Selecting Fill Soil Type3 Mix of Known Fill Media
20-30% “Ball Field Mix”70-80% Medium Sand
In Phosphorus Sensitive Waters –choose low-medium P-Index (15 to 30 in NC)
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How Deep does the soil media need to be?
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Bioretention Soil: DepthBioretention Soil: Depth
Vegetation Depth (ft) Comments
Grass 1.5 - 2.0 Minimum
Shrubs/Trees 2.5 - 3.0 Minimum
Shrubs/Trees 3.5 - 4.0 Optimum
Shrubs/Trees > 4.0 Sufficient butExtra Cost
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Bioretention Soil: DepthBioretention Soil: Depth
Think about the PollutantTSS?Metals?Phosphorus?Nitrogen?Fecal Coliform?Temperature?
No Required DepthNo Required Depth
Soil Depth > 12 inchesSoil Depth > 12 inches
Soil Depth > 12 inchesSoil Depth > 12 inches
Soil Depth > 30 inchesSoil Depth > 30 inches
No Required DepthNo Required Depth
Soil Depth > 36 inchesSoil Depth > 36 inches
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+ 4 Hours
+18 Hours+18 Hours
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BR Water Table DepthBR Water Table DepthManual states 6’ is closest high w.t. can be to the surface? Is that too restrictive?Depends on Depth of Bioretention areaRecommend: No W.T. within 2 feet of bottom
B-R Area
7
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Bioretention Water TableBioretention Water Table
24”
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Water Flow Through BioretentionWater Flow Through Bioretention
Assume Device follows Darcy’s LawQ = K A ∆H/Lwhere K = Hydraulic Conductivity
A ≈ Surface Area of Bio-Retention∆H = Height of Water above Gravel Layer
L = Thickness of Soil
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DarcyDarcy’’s Law Applied to Bs Law Applied to B--RR
Soil - K (hydraulic conductivity) L ∆H
Drainage pipes with gravel envelope
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Bioretention Water TableBioretention Water Table
Soil Zone
Ponding Zone
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Calculating DrawdownCalculating Drawdown
1 Calculate Drawdown Rate from Darcy’s Law
2. Find Drawdown for PondingZone
3 Find Drawdown for Soil Zone
Soil
Pond
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Calculating DrawdownCalculating Drawdown
Find Drawdown RateUse Darcy’s Law
Assume ∆H ≈ LTherefore, ∆H/L ≈ 1
Q = (2.3E-5) • K • A • 1
units: Q (cfs), K (in/hr), A (sf)
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Calculating DrawdownCalculating Drawdown
Ponding Zone1 Find Treatment Volume2 Divide Treatment Volume
by Drawdown Rate
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Calculating DrawdownCalculating Drawdown
Soil Zone1 Choose Soil Porosity2 Find Treatment Volume3 Divide Treatment Volume
by Drawdown Rate
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Calculating Drawdown: ExampleCalculating Drawdown: Example
Given: A bioretention area is 200 sf. Bioretention has 4 feet deep layer of soil with K= 1 in/hr. Water allowed to pond 9 inches.
Find: Time to Draw water down 24”from surface once Bioretention is full.
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Calculating Drawdown: ExampleCalculating Drawdown: Example
1 Find Drawdown Rate
Q = 2.3E-5 • 1in/hr • 200sf
Q = 0.0046 cfs
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Calculating Drawdown: ExampleCalculating Drawdown: Example
2 Find Time to Drain Ponding Zone I. Determine Ponded Volume VP ≈ S/A • d VP ≈ 200sf • 0.75sf VP ≈ 150 cf
9
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Calculating Drawdown: ExampleCalculating Drawdown: Example2 Find Time to Drain Ponding Zone
II. Find Time to Remove PondedWater
TP = VP ÷ Q TP = 150 cf ÷ 0.0046 sec TP = 33,000 sec TP = 9 hours
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Calculating Drawdown: ExampleCalculating Drawdown: Example
3 Find Time to Drain Soil Zone I. Select Soil Drainable Porosity, n Range from 0.25 to 0.50
depending upon soil type and how loose it is
Assume Fill Soil Loose Choose n = 0.45
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Calculating Drawdown: ExampleCalculating Drawdown: Example
3 Find Time to Drain Soil Zone II. Determine Volume in Top 24” VS ≈ S/A • n • 2 VS ≈ 200sf • 0.45 • 2 VS ≈ 180 cf
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Calculating Drawdown: ExampleCalculating Drawdown: Example
3 Find Time to Drain Soil Zone III. Find Time to Remove Water
from top 24” of Soil TS = VS ÷ Q TS = 180 cf ÷ 0.0046 sec TS = 39,000 sec TS = 11 hours
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Calculating Drawdown: ExampleCalculating Drawdown: Example
4 Total Drawdown Time TP + TS = Total Time Total Time = 9 + 11 hours Total Time = 20 hours
This is within 48 hour window.
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Underdrain Pipe SelectionUnderdrain Pipe Selection
Soil Layer with Drawdown Rate, Q
Drainage pipes with gravel envelope
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Selecting Drawdown PipesSelecting Drawdown Pipes
As Factor of Safety: Design for pipes to remove 5-10X amount of water that flows thru SoilFind the Pipe Diameter, D, for a Type of Pipe (manning n)Use Form of Manning Equation:
D = 16 • (Q • n ÷ s 0.5) 3/8
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Selecting Drawdown PipesSelecting Drawdown PipesPipe Type andDiameter
Manning RoughnessCoefficient
4” Single WallCorrugated Plastic
0.014-0.015
4” Smooth WallPlastic
0.010-0.011
6” Single WallCorrugated Plastic
0.014-0.015
6” Smooth WallPlastic
0.010-0.011
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Selecting Drawdown PipesSelecting Drawdown Pipes
Find underdrain combination to handle water drawdown rateRedundancy of Pipes?Gravel Envelope at least 2” above top of drawdown pipe.
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Selecting Drawdown Pipes: Selecting Drawdown Pipes: ExampleExample
Given: Bioretention Area with area = 200sf and drawdown rate, Q, of 0.0046cfs
Find: Number & Diameter of Underdrain Pipes.
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Selecting Drawdown Pipes: Selecting Drawdown Pipes: ExampleExample
1 Flow Through Soil is QSOIL=0.0046 cfs
2 Apply Factor of Safety (10) to Flow QPIPE = 0.046 cfs3 Assume 4” or 6” Smooth-Walled
Plastic to be used. Manning Coeff: n = 0.011
11
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Selecting Drawdown Pipes: Selecting Drawdown Pipes: ExampleExample
4 Assume Internal Slope of Pipe s = 0.5%5 Insert Parameters into Manning
Equation. D = 16 • (0.05 cfs • 0.011 ÷ 0.0050.5) 3/8
D = 2.6 inches
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Selecting Drawdown Pipes: Selecting Drawdown Pipes: ExampleExample
6 Choose NXD combination to carry flow > Q2.6 inches.
Select one 4” corrugated plastic pipe.
7 Decide if Redundancy is needed. If so, two 4” pipes used.8 Gravel Envelope: 6” thickness
minimum (4” pipe + 2” cover).
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Converting Pipe Converting Pipe DiamDiam to # of 4to # of 4”” or or 66”” Pipe DiametersPipe Diameters
557.227.22
4410.1310.13446.666.66
339.119.11335.955.95
227.847.84225.135.13
# of 6# of 6””UnderdrainsUnderdrains
D < D < (inches)(inches)
# of 4# of 4””UnderdrainsUnderdrains
D < D < (inches)(inches)
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Sizing the Overflow WeirSizing the Overflow Weir
Set allowable Height of water OVER ponding height
Typically 2” (if parking lot median)BUT, could be higher (ex: 4”, 6”)
Dependant upon Design NeedsUse Weir Equation
Q = CW • L • H 3/2
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Sizing the Overflow WeirSizing the Overflow Weir
Adjusted Weir Equation L = Q ÷ (CW • H1.5)
where L= Length (ft) Q = Peak Flow from Design Storm
(cfs) CW = Weir Coefficient (set to 3) H = Height of Water above Weir (ft)
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Sizing the Overflow Weir: ExampleSizing the Overflow Weir: Example
Given: Runoff from 0.4 AC parking lot. 10-year, 24-hour storm outlet control; 2” Max HeightFind: Required size of Outflow
Parking Lot
Bioretention Area Outflow Weir
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Sizing the Overflow Weir: ExampleSizing the Overflow Weir: Example
1 Find Peak Flow QP(10)
C • I • A where QP = Peak Flow (cfs) C = Rational Runoff Coefficient I = Rainfall Intensity (in/hr) A = Watershed Area (acres)
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Sizing the Overflow Weir: ExampleSizing the Overflow Weir: Example
1 Find Peak Flow For Parking Lot, C = 0.95 RDU, 10-year, 24-hour I = 7.22 As given, A = 0.4 AC
QP = 3 cfs
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Sizing the Overflow Weir: ExampleSizing the Overflow Weir: Example
2 Find Weir Length Know: CW = 3; H = 0.17’; Q = 3 cfs Insert in: L = Q ÷ (CW • H1.5)
L = 14.25 ft
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Sizing the Overflow Weir: ExampleSizing the Overflow Weir: Example
3 Convert to Outlet SizeOutlet has 4 sides. Perimeter = Weir Length1 side square outlet = Weir Length ÷ 414.25’ ÷ 4 ≈ 3.5’ X 3.5’ OR
42” X 42” box
9”
42”
11”
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Surface Surface DrawdownDrawdown
13
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22--Yr Storm RoutingYr Storm Routing
Account for Surface DrawdownDepends on Fill Media TypeLoamy Sand (1.5 – 2.0 in/hr)Sandy Loam (0.5 – 1.0 in/hr)
Example of this found in Bioretention Model to be used this afternoon
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Bioretention Bioretention QQpp MitigationMitigation
- 5 .0 0
0 .0 0
5 .0 0
10 .0 0
15 .0 0
2 0 .0 0
2 5 .0 0
0 1 0 2 0 3 0 4 0 5 0 6 0 7 0 8 0
T i m e ( m i n u t e s )
Flow
(cfs
)
In f l o w ( P o s t D e v Q ) B R O u t f l o w p r e - d e v
1/3 Inflow Vol Stored