Stormwater Engineering Bioretention Design Bill Hunt, PE, Ph.D. Extension Specialist & Assistant Professor NCSU-BAE www.bae.ncsu.edu/stormwater Bioretention Design Six Step Process 1 Determine Volume to Treat 2 Determine Surface Area Required 3 Select Soil Type 4 Decide Depth of Soil 5 Size Underdrain Pipes 6 Select Appropriate Overflow Bypass Treatment Volume: Background Collect Data: Watershed Area Watershed Composition (rooftop, lawn, parking lot) If permeable areas: Soil Type & Group? Treatment Volume: Background Determine Curve Number: Function of Land Use and Soil Group Range from mid 30 s to 98 Higher # More Runoff Developed by USDA-NRCS Treatment Volume: Curve Numbers Land Use/ Cover Soil Group A B C D Parking Lot/ Rooftop 98 98 98 98 Lawn, etc (grass 49 69 79 84 cover 50-75%) Lawn, etc (grass 39 61 74 80 cover > 75%) Woods in Fair Condition 36 60 73 79 1
Composite CN or Not? Treatment Volume: Curve Numbers Composite? Not issue if only one land use/soil type If distinct regions Do NOT composite If watershed is well mixed Composite Treatment Volume: Curve Numbers Composite? Use following Equation: CN COMP = %W/S A CN A + %W/S B CN B Where A & B are Land Use-Soil Type zones within Watershed (W/S) Treatment Volume: Calculation Three Part Process: 1 Decide Design Storm 2 Determine Runoff from Each Land Use/ Soil Type 3 Multiply by Watershed Area Treatment Volume: Calculation 1 Decide Design Storm: Varies from 0.50 to 1.50 Dependent on... frequency of rainfall developed condition 1.00 Typically Used Treatment 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 2
Treatment 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 Treatment Volume: Calculation 3 Multiply by Watershed Area Volume of Runoff to Treat Vol TREAT = A R/O R/O = (P - 0.2S) 2 (P + 0.8S) Treatment Volume: Example Treatment 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 Treatment Volume: Example Find Volume of Runoff from Rooftop 1 Determine Curve Number: 98 2 Find Soil & Surface Storage, S: S = 1000/ 98-10 S = 0.20 Treatment Volume: Example Find Volume of Runoff from Rooftop 3 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 3
Treatment Volume: Example Find Volume of Runoff from Rooftop 4 Find Total Volume to Treat, Vol TREAT R/O = 0.80 Watershed Area = 2000 sf Vol TREAT = 2000 sf 0.80 in Vol TREAT = 1600 sf-in (133 cf) Treatment Volume: Example Find Volume of Runoff from Lawn 1 Determine Curve Number: 74 2 Find Soil & Surface Storage, S = 3.51 in 3 Runoff Amount, R/O = 0.02 in 4 Treatment Volume, Vol TREAT = 160 sf-in Treatment Volume: Example Find Total Volume from Roof & Lawn Vol TREAT (roof) = 1600 sf-in Vol TREAT (lawn) = 160 sf-in Vol TREAT (total) = 1760 sf-in Vol TREAT (total) = 147 cf Bioretention Surface Area Factor of Vol TREAT and Allowable Depth Vol TREAT previously determined Bioretention Surface Area Allowable Water Depth range from 6 to 18 PG Co, Maryland specifies 6 Author suggest 6-9 reasonable for most applications 15-18 only if VERY SANDY application (e.g. in Sandhills) Bioretention Surface Area Divide Vol TREAT by Average Depth S/A = Vol TREAT D where S/A = surface area (sf) Vol TREAT = volume stored in B-R (sf-in or cf) D = Average Depth of Water (in or ft) 4
B-R R Surface Area: Example Given: 1760 sf-in of water to be stored in Bio-Retention area Normal Depth = 9 Find: Required Surface Area B-R R Surface Area: Example S/A = 1760 sf-in 9 in S/A = 200 sf Bioretention 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 OR Significant Construction, then Bioretention Soil: Type Underdrains and Fill Soil Needed Bioretention Soil: Type Selecting Fill Soil Type 1 By Permeability: K range from 0.5 to 6 per hour 1 to 2 in/hr (REC) 2 Recipe 85% - 88% Sand 8-12% Fines (Silt+Clay) 2-5% Organic Source 5
Bioretention Soil: Type Selecting Fill Soil Type 3 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: 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 but Extra Cost Bioretention Soil: Depth Think about the Pollutant TSS? Metals? Phosphorus? Nitrogen? Fecal Coliform? Temperature? No Required Depth Soil Depth > 12 inches Soil Depth > 12 inches Soil Depth > 30 inches No Required Depth Soil Depth > 36 inches + 4 Hours +18 Hours BR Water Table Depth Manual states 6 is closest high w.t. can be to the surface? Is that too restrictive? Depends on Depth of Bioretention area Recommend: No W.T. within 2 feet of bottom B-R Area 6
Bioretention Water Table 24 Water Flow Through Bioretention Assume Device follows Darcy s Law Q = K A H/L where K = Hydraulic Conductivity A Surface Area of Bio-Retention H = Height of Water above Gravel Layer L = Thickness of Soil Darcy s s Law Applied to B-RB Bioretention Water Table Soil - K (hydraulic conductivity) L H Ponding Zone Soil Zone Drainage pipes with gravel envelope Calculating Drawdown Calculating Drawdown 1 Calculate Drawdown Rate from Darcy s Law 2. Find Drawdown for Ponding Zone 3 Find Drawdown for Soil Zone Pond Soil Find Drawdown Rate Use Darcy s Law Assume H L Therefore, H/L 1 Q = (2.3E-5) K A 1 units: Q (cfs), K (in/hr), A (sf) 7
Calculating Drawdown Calculating Drawdown Ponding Zone Soil Zone 1 Find Treatment Volume 1 Choose Soil Porosity 2 Divide Treatment Volume by Drawdown Rate 2 Find Treatment Volume 3 Divide Treatment Volume by Drawdown Rate 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 V P S/A d V P 200sf 0.75sf V P 150 cf 8
2 Find Time to Drain Ponding Zone II. Find Time to Remove Ponded Water T P = V P Q T P = 150 cf 0.0046 sec T P = 33,000 sec T P = 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 V S S/A n 2 V S 200sf 0.45 2 V S 180 cf 3 Find Time to Drain Soil Zone III. Find Time to Remove Water from top 24 of Soil T S = V S Q T S = 180 cf 0.0046 sec T S = 39,000 sec T S = 11 hours 4 Total Drawdown Time T P + T S = Total Time Total Time = 9 + 11 hours Total Time = 20 hours Underdrain Pipe Selection Soil Layer with Drawdown Rate, Q This is within 48 hour window. Drainage pipes with gravel envelope 9
Selecting Drawdown Pipes As Factor of Safety: Design for pipes to remove 5-10X amount of water that flows thru Soil Find 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 Pipes Pipe Type and Diameter 4 Single Wall Corrugated Plastic 4 Smooth Wall Plastic 6 Single Wall Corrugated Plastic 6 Smooth Wall Plastic Manning Roughness Coefficient 0.014-0.015 0.010-0.011 0.014-0.015 0.010-0.011 Selecting Drawdown Pipes Find underdrain combination to handle water drawdown rate Redundancy of Pipes? Gravel Envelope at least 2 above top of drawdown pipe. Selecting Drawdown Pipes: Example Given: Bioretention Area with area = 200sf and drawdown rate, Q, of 0.0046cfs Find: Number & Diameter of Underdrain Pipes. Selecting Drawdown Pipes: Example 1 Flow Through Soil is Q SOIL =0.0046 cfs 2 Apply Factor of Safety (10) to Flow Q PIPE = 0.046 cfs 3 Assume 4 or 6 Smooth-Walled Plastic to be used. Manning Coeff: n = 0.011 10
Selecting Drawdown Pipes: Example 4 Assume Internal Slope of Pipe s = 0.5% 5 Insert Parameters into Manning Equation. D = 16 (0.05 cfs 0.011 0.005 0.5 ) 3/8 D = 2.6 inches Selecting Drawdown Pipes: Example 6 Choose NXD combination to carry flow > Q 2.6 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 Diam to # of 4 4 or 6 Pipe Diameters Sizing the Overflow Weir D < (inches) 5.13 5.95 6.66 7.22 # of 4 4 Underdrains 2 3 4 5 D < (inches) 7.84 9.11 10.13 # of 6 6 Underdrains 2 3 4 Set allowable Height of water OVER ponding height Typically 2 (if parking lot median) BUT, could be higher (ex: 4, 6 ) Dependant upon Design Needs Use Weir Equation Q = C W L H 3/2 Sizing the Overflow Weir Adjusted Weir Equation L = Q (C W H 1.5 ) where L= Length (ft) Q = Peak Flow from Design Storm (cfs) C W = Weir Coefficient (set to 3) H = Height of Water above Weir (ft) Sizing the Overflow Weir: Example Given: Runoff from 0.4 AC parking lot. 10-year, 24-hour storm outlet control; 2 Max Height Find: Required size of Outflow Bioretention Area Outflow Weir Parking Lot 11
Sizing the Overflow Weir: Example 1 Find Peak Flow Q P (10) C I A where Q P = Peak Flow (cfs) C = Rational Runoff Coefficient I = Rainfall Intensity (in/hr) A = Watershed Area (acres) Sizing 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 Q P =3 cfs Sizing the Overflow Weir: Example 2 Find Weir Length Know: C W = 3; H = 0.17 ; Q = 3 cfs Insert in: L = Q (C W H 1.5 ) L = 14.25 ft Sizing the Overflow Weir: Example 3 Convert to Outlet Size Outlet has 4 sides. Perimeter = Weir Length 1 side square outlet = Weir Length 4 14.25 4 3.5 X 3.5 OR 42 X 42 box 42 11 9 Surface Drawdown 12
2-Yr Storm Routing Account for Surface Drawdown Depends on Fill Media Type Loamy 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 Q p Mitigation F l o w ( c f s ) 25.00 20.00 15.0 0 10.0 0 5.00 0.00-5.00 1/3 Inflow Vol Stored 0 1 0 2 0 3 0 4 0 5 0 6 0 7 0 8 0 Tim e (m inute s) In flo w (P o st D e v Q ) BR Outflow pre-dev 13