Bioretention Installation Training, Roger Williams ParkGroup Exercise: Bioretention Design...

Group Exercise: Bioretention Design

Bioretention Installation Training - Roger Williams Park, April 11-12, 2012 1

Bioretention Installation Training, Roger Williams

Park

Providence, RIGroup Exercise

Horsley Witten Group, Inc.Horsley Witten Group, Inc.

Group Exercise –Bioretention DesignApril 11-12, 2012

Site 3B – Carousel Parking Lot (1/2 of lot) – Bioretention




• Concept Plan developed from the field recon and GIS information


Existing Conditions Plan




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


p

Design Exercise

• Physical Constraints?• Determine required WQv

• Determine minimum bioretention surface area

• What type of pretreatment?• On-line or off-line?


• What type of conveyence for inflow and outflow?



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




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 )


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




Bioretention Cross-section

3” Mulch


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



• 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)


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



Swale/Sediment Forebay: ~500 cf



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)


( )

(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.




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 +


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




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


**Good rule of thumb is about 1.1 cfs/acre of imp. cover


Flume inlet/ Sidewalk


Sidewalk Trench Drain

Swale/Sediment Forebay: ~500 cf



Planting Plan


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Bioretention Installation Training, Roger Williams ParkGroup Exercise: Bioretention Design...

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