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Group Exercise: Bioretention Design
Bioretention Installation Training - Roger Williams Park, April 11-12, 2012 1
Bioretention Installation Training, Roger Williams
Park
Providence, RIGroup Exercise
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Group Exercise –Bioretention DesignApril 11-12, 2012
Site 3B – Carousel Parking Lot (1/2 of lot) – Bioretention
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Group Exercise: Bioretention Design
Bioretention Installation Training - Roger Williams Park, April 11-12, 2012 2
• Concept Plan developed from the field recon and GIS information
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Existing Conditions Plan
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Group Exercise: Bioretention Design
Bioretention Installation Training - Roger Williams Park, April 11-12, 2012 3
Site Information• Land Use: Parking lot for
Carousel, a major RWP attraction
• Drainage Area: 1.2 acres, 37% impervious (19,000 sf imp.)
• Soils: Test pit data provided
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p
Design Exercise
• Physical Constraints?• Determine required WQv
• Determine minimum bioretention surface area
• What type of pretreatment?• On-line or off-line?
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• What type of conveyence for inflow and outflow?
Group Exercise: Bioretention Design
Bioretention Installation Training - Roger Williams Park, April 11-12, 2012 4
Horsley Witten Group, Inc.Horsley Witten Group, Inc.Physical Constraints
•Compute WQv
WQv = [(1”) (I)] / 12
Water Quality Volume Calculation
Qv [( ) ( )]= [(1”)(19,000sf)] (1ft/12in)= ~1,583 cf
Min. WQv= [(0.2”)(DA)] / 12= [(0.2”)(1.2 ac)] (1ft/12in)= 0.02 ac-ft = ~871 cf
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Group Exercise: Bioretention Design
Bioretention Installation Training - Roger Williams Park, April 11-12, 2012 5
Bioretention Sizing Equation
Use sizing equation and values provided in Section 5.5.4:
A = (WQ ) (d ) / [(k) (h + d ) (t )]Af = (WQv) (df) / [(k) (hf + df) (tf)]
Af = surface area of filter bed (ft2)df = filter bed depth (ft) (2–4 ft, depending on site constraints)k = coef of permeability of filter media (1 ft/day)hf = ave ht of water above filter bed (ft) (half ponding depth)tf = design filter bed drain time (days) (2 days recom )
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tf design filter bed drain time (days) (2 days recom.)
Af = (1,583 ft3) (2’) / [(1’/day) (0.25’ + 2’) (2 days)] (With df = 2’, k = 1.0'/day, hf = 0.25’, tf = 2 days)
Af = ~704 sq ft
Bioretention: ~880 sf
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Group Exercise: Bioretention Design
Bioretention Installation Training - Roger Williams Park, April 11-12, 2012 6
Bioretention Cross-section
3” Mulch
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2’ Deep Bio Soil4”Deep Pea Gravel
12” Deep Gravel Trench
Pretreatment• 25% of Water Quality Volume – Sediment
Forebay = 0.25*(1,583 cf) = ~396 cf• Swale and Sediment Forebay combo• Swale and Sediment Forebay combo• Swale: 45 ft long, 9 in deep, 4 ft bottom
width, 15 ft top width = ~320 cf
Ave. 15’ Top WidthAve
Horsley Witten Group, Inc.Horsley Witten Group, Inc.Ave. 4’ Bottom Width
Ave. 9”
Depth
Group Exercise: Bioretention Design
Bioretention Installation Training - Roger Williams Park, April 11-12, 2012 7
• Sediment Forebay: 318 sf, 0.5 ft deep. Volume = ~160 cf
• Minimum SA Sizing Calculation in Section
Pretreatment
• Minimum SA Sizing Calculation in Section 6.4.1 of RI SW Manual:
As = 5,750 * QWhere:As = Minimum sedimentation surface area (sf)
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Q = discharge from drainage area = %WQv/86,400 sec
As = 5,750 * (0.25 * 1,583 cf/86,400 sec) = ~26 sf
* SA = 318 sf > 26 sf
Bioretention: ~880 sf
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Swale/Sediment Forebay: ~500 cf
Group Exercise: Bioretention Design
Bioretention Installation Training - Roger Williams Park, April 11-12, 2012 8
Check 75% WQv Storage• Show that 75% WQv is provided in the filter
bed, above the surface, and in forebay: 0.75*(1,583 cf) = 1,187 cf0.75 (1,583 cf) 1,187 cf
Total volume = Af*df*n + Af*h + Vpretreatment
Af = surface area of filter bed (ft2)df = filter bed depth (ft)n = filter media porosity (0.33)h = max ht of water above filter bed (ft)
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( )
(880 sf)*(2 ft)*(.33) + (880 sf)*(0.5 ft) + (320 cf+160 cf) = 1,500 cf > 1,187 cf
• Offline - Sidewalk Trench Drain• Sized to pass 100% of WQf during the 1.2-inch storm• 18” wide trench 6” deep When depths at inlet > 3”
2. Conveyance Design: Offline vs. Online Design?
• 18 wide trench, 6 deep. When depths at inlet > 3 , some flow will bypass bioretention.
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Group Exercise: Bioretention Design
Bioretention Installation Training - Roger Williams Park, April 11-12, 2012 9
Water Quality Peak Flow Calculation
Using the WQv, a corresponding modified CN is computed utilizing the following equation (Section 3.3.3.2):
CN 1000 / [10 + 5P +10Q 10(Q² + 1 25 Q*P)½]CN = 1000 / [10 + 5P +10Q - 10(Q² + 1.25 Q*P)½] Where:• P = rainfall, in inches (use 1.2 inches) • Q = runoff volume, in inches (equal to WQv ÷ area) • Q = (1,583 cf /43,560 sf/ac) (12 inches/ft) / (1.2 acres) = 0.363 inches
CN = 1000 / [10 + 5(1.2 in) +10 (0.363 in) - 10((0.363 in)² +
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CN 1000 / [10 5(1.2 in) 10 (0.363 in) 10((0.363 in) 1.25 (0.363 in)(1.2 in))½]
CN = 1000 / [10 + 6.0 + 3.63 – 8.26] = ~88
**Use this in either H/H model with a 1.2” precip event, or use TR-55 spreadsheet, to find WQf
Ia/P = (0 27)/1 2”= 0 225
qu = 645
csm/in
Ia for CN of 88 is 0.27 [Ia = (200/CN - 2)]
Exhibit 4-III Unit peak discharge (qu) for NRCS Type III rainfall distribution
Ia/P = (0.27)/1.2 = 0.225tc = 0.10 hours
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Group Exercise: Bioretention Design
Bioretention Installation Training - Roger Williams Park, April 11-12, 2012 10
Compute Peak Discharge (Qpeak)
Qpeak = qu * A * WQvWhere:• Qpeak = the peak discharge, in cfs Qpeak p g ,• qu = the unit peak discharge, in cfs/mi²/inch (645 csm/in) • A = drainage area, in square miles (0.0019 sq miles) • WQv = Water Quality Volume, in watershed inches (0.363 inches)
Qpeak = 645 * 0.0019 * 0.363 = ~0.45 cfs
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**Good rule of thumb is about 1.1 cfs/acre of imp. cover
Bioretention: ~880 sf
Flume inlet/ Sidewalk
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Sidewalk Trench Drain
Swale/Sediment Forebay: ~500 cf
Group Exercise: Bioretention Design
Bioretention Installation Training - Roger Williams Park, April 11-12, 2012 11
Planting Plan
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www.horsleywitten.com/biotraining
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