Bioretention Design · 2012. 11. 22. · 3 BAE Stormwater Engineering GroupBAE Stormwater Engineering Group “We Bring Engineering to Life” Treatment Volume: Calculation 2 Runoff

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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

“We Bring Engineering to Life”BAE Stormwater Engineering GroupBAE Stormwater Engineering Group


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


Treatment Volume: BackgroundTreatment Volume: Background

Collect Data:Watershed AreaWatershed Composition (rooftop, lawn, parking lot)If permeable areas: Soil Type & Group?


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


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

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Composite CN or Not?Composite CN or Not?


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


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)


Treatment Volume: CalculationTreatment Volume: Calculation

Three Part Process:1 Decide Design Storm2 Determine Runoff from Each

Land Use/ Soil Type3 Multiply by Watershed Area



1 Decide Design Storm:Varies from 0.50” to 1.50”Dependent on...

frequency of rainfalldeveloped condition

1.00” Typically Used



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

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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)



3 Multiply by Watershed AreaVolume of Runoff to Treat

VolTREAT = A • R/O


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”



Find Volume of Runoff from Rooftop

1 Determine Curve Number: 982 Find Soil & Surface Storage, S: S = 1000/ 98 - 10 S = 0.20



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

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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)



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


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


Bioretention Surface AreaBioretention Surface Area

Factor of VolTREAT and Allowable DepthVolTREATpreviously determined


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)


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)

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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


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



Underdrains and Fill Soil Needed


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

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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)


How Deep does the soil media need to be?


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


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



No Required DepthNo Required Depth



+ 4 Hours

+18 Hours+18 Hours


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

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Bioretention Water TableBioretention Water Table

24”


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


DarcyDarcy’’s Law Applied to Bs Law Applied to B--RR

Soil - K (hydraulic conductivity) L ∆H

Drainage pipes with gravel envelope


Bioretention Water TableBioretention Water Table

Soil Zone

Ponding Zone


Calculating DrawdownCalculating Drawdown

1 Calculate Drawdown Rate from Darcy’s Law

2. Find Drawdown for PondingZone

3 Find Drawdown for Soil Zone

Soil

Pond



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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Ponding Zone1 Find Treatment Volume2 Divide Treatment Volume

by Drawdown Rate



Soil Zone1 Choose Soil Porosity2 Find Treatment Volume3 Divide Treatment Volume

by Drawdown Rate


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.



1 Find Drawdown Rate

Q = 2.3E-5 • 1in/hr • 200sf

Q = 0.0046 cfs



2 Find Time to Drain Ponding Zone I. Determine Ponded Volume VP ≈ S/A • d VP ≈ 200sf • 0.75sf VP ≈ 150 cf

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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



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



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



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



4 Total Drawdown Time TP + TS = Total Time Total Time = 9 + 11 hours Total Time = 20 hours

This is within 48 hour window.


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


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


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.


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.



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

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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



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).


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)


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


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)


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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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)



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



2 Find Weir Length Know: CW = 3; H = 0.17’; Q = 3 cfs Insert in: L = Q ÷ (CW • H1.5)

L = 14.25 ft



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”


Surface Surface DrawdownDrawdown

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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


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

Bioretention Design · 2012. 11. 22. · 3 BAE Stormwater Engineering GroupBAE Stormwater Engineering Group “We Bring Engineering to Life” Treatment Volume: Calculation 2 Runoff

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