Designing Stormwater Ponds for Water Quality

Transcription

1 Count Designing Stormwater s for Water Quality Jay Dorsey ODNR-DSWC February 13, 2008 Designing Stormwater s for Water Quality Water Quality Volume Outlets Other Design Considerations The Purpose of Stormwater Regulations/Management Minimize impacts to receiving waters Offset or mitigate for the changed site hydrology and the loss of natural watershed services Traditional stormwater management approaches (e.g., Critical Storm Method) were aimed at matching postdevelopment to pre-development peak discharges for infrequent (extreme) storm events, resulting in large detention basins with large outlets Long-term Rainfall Characteristics Manmade Stormwater Management Systems Dayton, OH Rain Events ( ) Rain Depth (in) 1

2 Cumulative Occurrence Probability % Stormwater Management Regulation in Ohio Problems/concerns with peak discharge control methods (e.g., Critical Storm Method) Problem 1: Too little detention time for effective pollutant removal Problem 2: Peak discharge control methods allow smaller rainfall events to become channel eroding events Ohio Application of WQv Formula Water Quality Volume (WQv) Urban Runoff Quality Management, ASCE Manual of Practice No. 87, Amercan Society of Civil Engineers, Reston, VA, (1998). WQv = C * P * A / 12 Where: WQv = water quality volume (ac-ft) C = runoff coefficient P = 0.75 inch precipitation A = area draining to the BMP (acres) Why 0.75 Rainfall Depth? Clarification of WQv Dayton, OH Rain Events ( ) Rain Depth (in) Runoff Coefficient Drawdown Requirement 2

3 Runoff Coefficient Runoff Coefficient WQv Runoff Coefficient WQv Runoff Coefficient WQv Runoff Coefficent C C = runoff coefficient i = watershed imperviousness ratio (percent total imperviousness divided by 100) C 0.858i 3 C = runoff coefficient 0.78i i i = watershed imperviousness ratio (percent total imperviousness divided 100) Impervious Area (%) C 0.858i i i 0.04 Source: Urban Runoff Quality Management, ASCE, 1998, p Example WQv Formula Determine the Runoff Coefficient, C, for: 100 acre residential development, 0.5 acre lots, with 20% impervious area (i = 0.20) C = 0.17 WQv = C * P * A / 12 = (0.17*0.75*100)/12 = 1.06 ac-ft From Example 1, using Table 1 value of C = 0.8, WQv = C * P * A / 12 = (0.3*0.75*100)/12 = 1.88 ac-ft WQv = C * P * A / 12 Where: WQv = water quality volume (ac-ft) P = 0.75 inch precipitation A = area draining to the BMP (acres) C = runoff coefficient -???? Runoff Coefficient Comparison Runoff Coefficient Comparison Runoff Coefficent Comparison Calculated Published Impervious Area (%) CGP-C C Impervious Runoff Runoff Calc Publ Increase Area Coefficient Coefficient WQv WQv WQv SLU - Standard Land Use % C C acre-ft acre-ft % Urban Open Space Urban Parks Low Density Residential Med Density Res no alleys Duplex High Density Res no alleys Multi-Family Res no alleys Medium Industrial Office Park Strip Commercial

4 Runoff Coefficient Comparison C Determination Method Land Use % Impervious Runoff Coefficient Wqv Increase TSS TSS (ac-ft) in Wqv (lb/ac/yr) Reduction (%) (%) No Recommendation Use the formula, not the Table Urban Open Space 4.9 Calc Publ Duplex 39.1 No Calc C 0.858i i i 0.04 Publ No Medium Industrial 68.5 Calc Publ Adjustments to WQv Formula Wet s Adjustments to WQv Formula Structural BMPs (excl Wet s) OEPA-CGP p22 OEPA-CGP p22 Discharge Rate Sediment Settling Process How quickly do we release the WQv to meet our stormwater management goals? Inflow v w v s Permanent Pool/Sediment Storage Discharge 4

5 Volume (cu ft) Volume (cu ft) Sediment Settling Process Sediment Settling Process Inflow Inflow v w v w v s v s Discharge Permanent Pool/Sediment Storage Discharge v sand v silt v clay OEPA CGP Drawdown Requirements The OEPA CGP lists drain time (or drawdown) requirements for structural BMPs (CGP Table 2) Select an appropriate outlet to meet drawdown requirement for wet ponds and dry ponds Volume vs Drawdown Time Volume vs Drawdown Time 45,000 45,000 40,000 40,000 35,000 35,000 30,000 30,000 25, " Outlet 25, " Outlet 20,000 8" Outlet 20,000 8" Outlet 15,000 15,000 10,000 10,000 5,000 5, Drawdown Time (hr) Drawdown Time (hr) 5

6 Volume (cu ft) Volume (cu ft) Volume (cu ft) Volume (cu ft) Volume vs Drawdown Time Volume vs Drawdown Time 45,000 WQv The WQv orifice should be sized to release no more than one-half the WQv (0.5*WQv) in the first one-third of the target drawdown period (0.333*Td) 40,000 35,000 30,000 The drawdown curve should fall above and to the right of the ½ volume in 1/3 drawdown time target 0.5xWQv 25, " Outlet Target 20,000 8" Outlet 15,000 10,000 5, xTd Td Drawdown Time (hr) Drawdown Time (hr) WQv Outlet - Primary Considerations Performance Maintenance Water Quality Volume (WQv) Outlet For most detention pond designs, an orifice needs to be used to meet the drawdown requirements of the Water Quality Volume (WQv). WQv Outlet - Performance WQv Geometry (Surface Area, Depth) and Orifice Size Use an appropriately sized orifice 45,000 Volume vs Drawdown Time WQv = 40,000 cu ft, td = 24 hr 40, Ac MDR (1/4 ac lots) Volume vs Drawdown Time 40 Ac Multi-family 35, Ac Office Park 20 Ac Strip Commercial WQv 30,000 The WQv orifice should be sized to release no more than one-half the WQv (0.5*WQv) in the first one-third of the target drawdown period (0.333*Td) 25,000 20,000 A=1.8 Ac D=0.5 ft dia=12 in A=3.7 Ac D=0.25 ft dia=24 in 0.5xWQv 15,000 Target 10,000 A= 0.5Ac D=2 ft dia=4.5 in A=0.9 Ac D=1.0 ft dia=6 in 0.333xTd Td 5,000 A=0.2 Ac D=4 ft dia=3.5 in Drawdown Time (hr) Drawdown Time (hr) 6

7 TSS - % Reduction Orifice Diameter (in) % TSS Reduction by Area Sediment Settling Process Inflow v s v w Permanent Pool/Sediment Storage Discharge Average Area (Ac) TSS Reduction Orifice Size Multi-Stage Outlets Multi-Stage Outlets Most detention basins that include a Water Quality Volume (WQv) require separate outlets for the WQv and the peak discharge control. The exception is very shallow extended detention volumes in large surface area wet detention basins. Unprotected WQv Outlets Unprotected WQv Outlets 7

8 8

9 WQv Outlet - Maintenance WQv Outlet Reverse Slope Pipe Protect the orifice Protected orifice options: Reverse slope pipe Perforated tile/pipe with gravel filter WQv Outlet Perforated Riser/Gravel Filter 9

10 WQv Outlet Perforated Pipe/Gravel Filter 10

11 Other Design Considerations Discuss issues related to BMP selection Highlight other issues Health and safety Maintenance Performance WQv BMP Selection BMP Selection Drainage Area Drainage area Soil type Performance (Source area/pollutants? Local TMDL? Target pollutants? Runoff temperature?) State/Local Regulations Outlet Depth/High Water Table Soil type BMP Selection Soil Type HSG-A 1.2% HSG-B 18% HSG-C 61.2% HSG-D 19.5% Detention Basin Selection Wet pond (or wetland ED basin) Usually the best choice in Ohio given the predominance of C & D soils, water quality treatment performance, maintenance/ aesthetics. Dry ED basin May be a reasonable choice for smaller development sites (<20 acres), especially for HSG A&B soils. Many states have eliminated dry basins as an option because of concerns about performance, maintenance and mosquitos (from standing water). 11

12 Dry Basins? Other Design Considerations Health and Safety Discuss issues related to BMP selection Highlight other issues Health and safety Maintenance Performance Sideslopes Safety benches Inlets/outlets Mosquitos/West Nile virus Flood Routing Freeboard Emergency spillways Earthwork (embankments) Maintenance Sediment Forebays/Maintenance Access Sediment pre-treatment (filters and forebays) Maintenance access drains Inlets/outlets Dry basins Permanent Stormwater Maintenance Plan Responsible Management Entity (RME) 12

13 Drains Outlet Maintenance Poor outlet designs require constant attention to work as designed drains allow rapid draining of wet ponds (and dry ponds) to allow maintenance Dry Basins? Are Pilot Channels the Answer? How Big a Concern Is the Wet Spot? An Attractive Dry Basin? 13

14 Performance Pretreatment Opportunities Tailwater elevations/tailwater analysis Pretreatment/treatment trains Sediment forebays Dry basins forebays and micropools Flow path length Surface area Outlets that work Conversion from sed-pond to detention pond Pretreatment Opportunities? Pretreatment Opportunities? Sediment Forebays Micropools 14

15 Flow Path Length Sediment Settling Process Inflow v w v s Permanent Pool/Sediment Storage Discharge Flow Path Length Flow Path Length Outlets and Performance Having a functional Water Quality pond depends on functional outlets Detention as Sediment Sed basins must have appropriate outlet to drain dewatering volume in hours see RLD for guidance 15

16 Sediment to Detention Conversion References - Detention Basin Design - Rainwater and Land Development, Ohio DNR, Division of Soil & Water Conservation (2006). Design and Construction of Urban Stormwater Management Systems, ASCE Manual of Practice No. 77, Amercan Society of Civil Engineers, Reston, VA, (1992). Sediment basin to postconstruction basin outlets and conversion timing should be specified in SWPPP, checked during inspections Design of Detention Systems, J.N. Paine and A. Osman Akan, Chapter 7 in L.W. Mays (ed.), Stormwater Collection Systems Design Handbook, McGraw-Hill, New York (2001) s Planning, Design, Construction, USDA-NRCS Ag Handbook 590,