Extended Detention Basin Design

Extended Detention Basin Design 1 Extended Detention 2 Ohio Department of Transportation 1

Extended Detention Basin L&D Vol. 2 Section 1117.3 Provides quality and quantity treatment 3 Extended Detention Basin Treatment Processes 4 Ohio Department of Transportation 2

Design Process Treatment Goals Preliminary Det. Basin Sizing Siting Analysis Det. Basin Shape and Detailed Sizing Water Quality Outlet Primary Outlet Overflow Weir Other Considerations 5 Design Process Treatment Goals Preliminary Det. Basin Sizing Siting Analysis Det. Basin Shape and Detailed Sizing Water Quality Outlet Primary Outlet Overflow Weir Other Considerations 6 Ohio Department of Transportation 3

Project Example 7 Project Example Treatment Goals Project EDA = 47 ac A ix = 41.5 ac A in = 5.5 ac T% = [(A ix x 20)+(A in x 100)] / (A ix + A in ) T% = [(41.5 x 20)+(5.5 x 100)] / (41.5 + 5.5) T% = 29.4% 29.4% x 47 ac = 13.8 ac 8 Ohio Department of Transportation 4

Project Example Treatment Goals Project EDA = 47 ac >= 1 ac Need a post-construction BMP Some new development Need to treat 13.8 ac (29.4%) 5.5 acres of new impervious area in new R/W >1 ac of new impervious area in new permanent right-ofway Water quality and quantity treatment required 9 Design Process Treatment Goals Preliminary Det. Basin Sizing Siting Analysis Det. Basin Shape and Detailed Sizing Water Quality Outlet Primary Outlet Overflow Weir Other Considerations 10 Ohio Department of Transportation 5

Project Example Project EDA x T% = 13.8 acres WQ V = (0.75 in x 13.8 ac x 0.9) / 12 WQ V = 0.78 ac-ft Depth guess = 3 ft Det. basin area = 0.78 ac-ft 3 ft Det. basin area = 0.26 ac Order of magnitude space requirement 11 Preliminary Det. Basin Sizing ~0.26 acres Very preliminary Different shapes / depths Tributary area outside R/W May be split up into different basins 12 Ohio Department of Transportation 6

Design Process Treatment Goals Preliminary Det. Basin Sizing Siting Analysis Det. Basin Shape and Detailed Sizing Water Quality Outlet Primary Outlet Overflow Weir Other Considerations 13 Siting Analysis A lot of information needs Trial and error Identify fatal flaws Space Safety 14 Ohio Department of Transportation 7

Siting Analysis - Info R/W Limits Topography Streams / Ditches Drainage Patterns Tributary Areas Within R/W Impervious / Pervious Area Clear Zone Limits Undeveloped / Available Space Floodplain Wetlands and Riparian Buffers Vehicle Access 15 Siting Analysis - Considerations Space Enough R/W treatment Tributary area too large? Safety Environmental impacts Floodplain cut/fill R/W acquisition Vehicle access 16 Ohio Department of Transportation 8

Project Example Are there any open areas where at least 13.8 acres of R/W drain? Downstream of R/W drainage Upstream of floodplain / stream Outside of clear zone 17 Project Example 18 Ohio Department of Transportation 9

Project Example 2 sites Combined R/W tributary area = 13.9 ac 13.9 ac > 13.8 ac treatment requirement 19 Siting Analysis It can be difficult to find space. Intersections Interchanges R/W acquisition Underground detention 20 Ohio Department of Transportation 10

Design Process Treatment Goals Preliminary Det. Basin Sizing Siting Analysis Det. Basin Shape and Detailed Sizing Water Quality Outlet Primary Outlet Overflow Weir Other Considerations 21 Detention Basin Sizing Calculate WQ V based on tributary area WQ V must be between lowest outlet primary outlet (10-yr) 10% WQ V additional for forebay Dead storage 10% WQ V additional for micropool Dead storage 22 Ohio Department of Transportation 11

Detention Basin Sizing 4:1 side slopes (maximum) Vehicle Access Draw a Configuration that Fits the Available Space May Require Multiple Iterations 23 Project Example Total trib. area = 7.5 ac R/W trib. area = 7.2 ac R/W considered 100% imperv. 0.3 ac outside R/W 100% imperv. 24 Ohio Department of Transportation 12

Project Example - WQ V WQ V = (P*A*Cq)/12 Cq = 0.858i 3 0.78i 2 + 0.774i + 0.04 i = 7.5 ac / 7.5 ac = 1.00 Cq = 0.89 Per L&D Vol. 2, Section 1115.6.1, consider all area within existing right-of-way to be impervious with a runoff coefficient of 0.90. Cq = 0.90 WQV = (0.75 in x 7.5 ac x 0.90) / 12 WQV = 0.422 ac-ft 25 Project Example - WQ V 26 Ohio Department of Transportation 13

Stage / Storage Curve 27 Project Example - WQ V 28 Ohio Department of Transportation 14

Project Example - Forebay 29 Project Example - Forebay 30 Ohio Department of Transportation 15

Project Example - Forebay Forebay >= 10% WQ V WQ V = 0.422 ac-ft Forebay >= 0.042 ac-ft Depth = 795.5-793.5 = 2 ft Bottom area = 25 x 18 = 450 sf Top area = 41 x 34 = 1,394 sf Volume = (450 sf + 1,394 sf) / 2 x 2 ft Volume = 0.042 ac-ft 31 Project Example - Micropool 32 Ohio Department of Transportation 16

Project Example - Micropool 33 Project Example - Micropool Micropool >= 10% WQ V WQ V = 0.422 ac-ft Micropool >= 0.042 ac-ft Depth = 795.5-793.5 = 2 ft Bottom area = 25 x 20 = 500 sf Top area = 41 x 36 = 1,476 sf Volume = (500 sf + 1,476 sf) / 2 x 2 ft Volume = 0.045 ac-ft 34 Ohio Department of Transportation 17

Design Process Treatment Goals Preliminary Det. Basin Sizing Siting Analysis Det. Basin Shape and Detailed Sizing Water Quality Outlet Primary Outlet Overflow Weir Other Considerations 35 Water Quality Outlet Design Drain WQ V in 48 hours or more 50% of WQ V or less drained in 1/3 rd of the drain time 0.422 ac-ft * 50% = 0.211 ac-ft 48 hrs * 1/3 = 16 hrs Provide time for sedimentation 36 Ohio Department of Transportation 18

Water Quality Outlet Design Types of Discharges: Orifice Weir (Including Storm Grates) Pipe Composite Structures Orifice:. Weir: 37 Water Quality Outlet Design Orifices 38 Ohio Department of Transportation 19

Water Quality Outlet Design Or A Hydrograph drawdown curve is necessary Calculate a stage / discharge curve Use Pond Pack or HydroCAD Pond Pack is part of Bentley Hydraulics Suite 39 Project Example WQ Outlet 40 Ohio Department of Transportation 20

Software Inputs Structure Type (Stand Pipe, Orifice, Weir, Culvert, etc.) Invert Elevation Orifice Area / Diameter Number of Openings Orifice Coefficient Weir Coefficient Weir Length Tailwater Assumptions 41 Water Quality Outlet Design Drain WQ V in 48 hours or more 50% of WQ V or less drained in 1/3 rd of the drain time 0.422 ac-ft * 50% = 0.211 ac-ft 48 hrs * 1/3 = 16 hrs Provide time for sedimentation 42 Ohio Department of Transportation 21

Project Example WQ Outlet 43 Stage / Storage Curve 44 Ohio Department of Transportation 22

Project Example WQ Outlet 45 Design Process Treatment Goals Preliminary Det. Basin Sizing Siting Analysis Det. Basin Shape and Detailed Sizing Water Quality Outlet Primary Outlet Overflow Weir Other Considerations 46 Ohio Department of Transportation 23

Outlet Structure 3 parts of detention basin outlet Water quality outlet(s) Slowly draw down the WQ V Primary outlet(s) Convey the 10-yr design storm (L&D Vol. 2, 1117.3.3 Overflow weir Convey the 25-yr design storm (L&D Vol. 2, 1104.2.2 47 ODOT SCD WQ-1.1 48 Ohio Department of Transportation 24

ODOT SCD WQ-1.1 Water Quality Outlets Orifices Grate Primary Outlets Pipe 49 ODOT SCD WQ-1.1 50 Ohio Department of Transportation 25

ODOT Catch Basin No. 2-3 and No. 2-4 51 Project Example WQ Outlet No Cleaning above 799.00 52 Ohio Department of Transportation 26

Primary Outlet Design Size pipe discharging from catch basin for the 10-yr storm Set the catch basin grate elevation Confirm that the grate has the capacity to pass the 10-yr storm 53 Primary Outlet Design Size pipe discharging from catch basin for the 10-yr storm Set the catch basin grate elevation Confirm that the grate has the capacity to pass the 10-yr storm 54 Ohio Department of Transportation 27

Project Example WQ Outlet Discharge Pipe 55 Primary Outlet Pipe Sizing Rational Method: Q=CiA For post-construction, all R/W is imperv. For general conveyance sizing, use L&D Table 1101-2 for C value 56 Ohio Department of Transportation 28

Primary Outlet Pipe Sizing 1.5 ac pavement and paved shoulder C = 0.9 6.0 ac berms and slopes 4:1 or flatter C = 0.5 C weighted = (1.5*0.9 + 6.0*0.5) / 7.5 C weighted = 0.58 57 Primary Outlet Pipe Sizing Time of concentration (t c ) = 25 min Project Specific See design example for t c calculations Overland flow Shallow concentrated flow Channel flow Use t c with IDF curves to determine the intensity IDF curves in L&D Figure 1101-2 58 Ohio Department of Transportation 29

3.4 59 25 Primary Outlet Pipe Sizing Q = CiA C = 0.58 i = 3.4 in/hr A = 7.5 ac Q = 0.58 * 3.4 in/hr * 7.5 ac Q = 14.79 cfs 60 Ohio Department of Transportation 30

Primary Outlet Pipe Sizing Choose a pipe diameter that will convey 14.79 cfs Given: Conduit slope = 0.005 ft/ft Pipe roughness = 0.015 (L&D Vol. 2 Sec. 1104.4.5) No tailwater, free discharge 24-inch diameter pipe works 61 Project Example Primary Outlet Pipe 62 Ohio Department of Transportation 31

Primary Outlet Design Size pipe discharging from catch basin for the 10-yr storm Set the catch basin grate elevation Confirm that the grade has the capacity to pass the 10-yr storm 63 Primary Outlet Grate Elevation WQ V fills detention basin to 799.0 ft Below 799.0 ft, all runoff should pass through the WQ orifices Above 799.0 ft, flow rate not limited Set lowest primary inlet at 799.0 ft Lowest inlet is the top of the grate 64 Ohio Department of Transportation 32

Project Example Grate Elevation 65 Primary Outlet Design Size pipe discharging from catch basin for the 10-yr storm Set the catch basin grate elevation Confirm that the grate has the capacity to pass the 10-yr storm 66 Ohio Department of Transportation 33

Primary Grate and Window Capacity The capacity of the grate is dependent on the hydraulic head Check capacity while setting overflow weir elevation Overflow weir should be high enough such that the full 10-yr flow goes through the primary outlet, not the overflow weir 67 Design Process Treatment Goals Preliminary Det. Basin Sizing Siting Analysis Det. Basin Shape and Detailed Sizing Water Quality Outlet Primary Outlet Overflow Weir Other Considerations 68 Ohio Department of Transportation 34

Project Example Grate Elevation 69 Overflow Weir Elevation Set the overflow weir elevation so that the full 10-yr peak flow rate just goes fully through the primary outlet Flow rates over the 10-yr may bypass over the overflow weir Us L&D Figure 1102-1 Capacity of a Grate Catch Basin 70 Ohio Department of Transportation 35

1.25 ft 14.79 cfs 71 Overflow Weir Elevation Grate elevation = 799.0 ft Head needed to pass 14.79 cfs = 1.25 ft Overflow weir elevation: 799.0 ft + 1.25 ft = 800.25 ft 72 Ohio Department of Transportation 36

Project Example Overflow Weir 73 Overflow Weir Capacity Overflow weir sized to convey at least the 25-yr storm Determine 25-yr peak flow rate t c = 25 min 74 Ohio Department of Transportation 37

3.8 75 25 Project Example Overflow Weir 25-yr peak flow rate Q = CiA = 0.58 * 3.8 in/hr * 7.5 ac Q = 16.53 cfs Q = CLH 1.5 Q is based on length of weir and head above weir Head is based on overall basin elevation 76 Ohio Department of Transportation 38

Project Example Overflow Weir 25-yr peak flow = 16.53 cfs Assume top of basin is 0.5 ft higher than overflow weir Weir elevation = 800.25 ft Top of basin = 800.25 ft + 0.5 ft = 800.75 ft 77 Project Example Overflow Weir Q = CLH 1.5 Q = 16.53 cfs C = 3 (L&D Vol. 2, 1102.3.4) H = 800.75 ft 800.25 ft = 0.5 ft 16.53 cfs = 3 * L * 0.5 1.5 L = 15.6 ft 78 Ohio Department of Transportation 39

Overflow Weir Weir length and height can be adjusted based on project needs If top of basin = 801.25 Q = 16.53 cfs C = 3 H = (801.25 800.25) = 1 ft 16.53 cfs = 3 * L * 1 1.5 L = 5.51 ft 79 Project Example Overflow Weir 80 Ohio Department of Transportation 40

Overflow Weir Flow rates over the 25-yr storm may overtop the detention basin uncontrolled. Consider erosion / scour protection Consider catastrophic failure 81 Design Process Treatment Goals Preliminary Det. Basin Sizing Siting Analysis Det. Basin Shape and Detailed Sizing Water Quality Outlet Primary Outlet Overflow Weir Other Considerations 82 Ohio Department of Transportation 41

Other Det. Basin Considerations Velocity / Scour Anti-Seep Collars Safety Slope Stability and Item 670 Maintenance Access FEMA Floodplain ODNR Dam Permit? 83 Velocity / Scour Check the velocities at inlet and all outlets Include channel protection: Inlet to basin Main discharge pipe Overflow weir 84 Ohio Department of Transportation 42

Velocity / Scour 85 Anti-Seep Collars Reference: Dept. of Interior, 1982 86 Ohio Department of Transportation 43

Anti-Seep Collars ODOT Standard Drawing WQ-1.2 87 Anti-Seep Collars Saturated zone length along conduit (Ls) Ls = Y(Z+4)[1+S/(0.25-S)] Y = depth of water at 10-yr elevation Z = slope of embankment S = slope of conduit Ls increase ( Ls) = 0.15Ls 88 Ohio Department of Transportation 44

Anti-Seep Collars Project 2 ft in all directions from pipe Total projection: P = W D W = height or width of collar (ft) D = diameter of pipe (ft) Minimum of 2 collars per outlet conduit # of collars = Ls / P Collar spacing: 10 25 ft apart 89 Project Example Collar Design Berm Slope 4:1 10-yr Elev. 800.25 ft Pipe Dia. 24 Pipe Slope 0.005 ft/ft Saturated Zone Pipe Inv. 795.0 ft 90 Ohio Department of Transportation 45

Project Example Collar Design Saturated zone length along conduit (Ls) Ls = Y(Z+4)[1+S/(0.25-S)] Y = 800.25 795.0 = 5.25 ft Z = 4 S = 0.005 Ls = 5.25(4+4)[1+0.005/(0.25-0.005)] Ls = 42.86 ft Ls = 0.15Ls = 0.15 x 42.86 = 6.4 ft 91 Project Example Collar Design Total projection: P = W D W = (2 ft + 2 ft Dia. + 2 ft) = 6 P = 6 ft 2 ft = 4 ft Minimum of 2 collars per outlet conduit # of collars = Ls / P 6.4 ft / 4 ft = 1.6 Use 2 collars 92 Ohio Department of Transportation 46

Project Example Collar Design Collar spacing Min: 5 x vert. projection Min: 5 x 2 ft = 10 ft Max: 14 x vert. projection Max: 14 x 2ft = 28 ft Max: 25 ft Spacing: between 10 ft and 25 ft 93 Project Example Collar Design Berm Slope 4:1 10-yr Elev. 800.25 ft 10 ft Pipe Dia. 24 Pipe Slope 0.005 ft/ft Ls = 42.86 ft Pipe Inv. 795.0 ft 94 Ohio Department of Transportation 47

Project Example Collar Design 95 Safety Detention basins are safety risks Clear zone: grading and 12 ponding Drowning hazard Local requirements? Fencing, trash rack, aquatic bench Impacts to upstream or downstream flooding 96 Ohio Department of Transportation 48

Slope Stability Do not locate on uncompacted fill or steep slopes (2:1 or more). Vegetate sides of basin Item 670, Slope Erosion Protection Don t plant grass under water Forebay Micropool 97 Maintenance Access Regular maintenance necessary Flat bench wide enough for a truck Grass needs to be mowed Woody vegetation cut from berm Regular unclogging of WQ outlet Every 6 months 98 Ohio Department of Transportation 49

Maintenance Access 99 FEMA Floodplain No BMPs in FEMA Floodplain Floodplains have open space for a reason No fill in FEMA Floodplain Unless compensatory cut/fill analysis Confirm limits of det. basin berms outside floodplain 100 Ohio Department of Transportation 50

ODNR Dam Permit Only for big detention basins No permit needed if: <10 Berm and <50 ac-ft storage; or < 15 ac-ft storage; or < 6 Berm 101 Re-Run the Numbers! Many iterations After values are set, re-run the analyses and check 102 Ohio Department of Transportation 51

Questions? Jon Prier, P.E. jonathan.prier@dot.ohio.gov 614-644-1876 103 Ohio Department of Transportation 52