Tables: Volumetric Reduction Standards¶
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1. Table 1¶
| Land Cover | CN | Sheet Flow Length (ft) | Shallow Concentrated Flow Length (ft) | Mannings's n | Slope |
|---|---|---|---|---|---|
| Building | 98 | 100 | 268 | 0.011 | 1% |
| Parking Lot | 98 | 100 | 218 | 0.011 | 1% |
| Lawn | 74 | 50 | 218 | 0.15 | 1% |
Source context and notes
Source: HydroCAD® Hydrograph Graph and Tabular Report; HydroCAD is a registered trademark of HydroCAD Software Solutions LLC. Used with permission. [Figure: img-5.jpeg (original PDF, p. 9)](SOURCE.pdf#page=9) # Step 3: Hydrologic and hydraulic analysis for the post-construction condition The parameters of the hydrologic and hydraulic models for the buidling, parking lot and the lawn area are shown below: | Land Cover | CN | Sheet Flow Length (ft) | Shallow Concentrated Flow Length (ft) | Mannings's n | Slope | | --- | --- | --- | --- | --- | --- | | Building | 98 | 100 | 268 | 0.011 | 1% | | Parking Lot | 98 | 100 | 218 | 0.011 | 1% | | Lawn | 74 | 50 | 218 | 0.15 | 1% | The combined peak flow rate from those three sources before entering the proposed bioretention BMP is 3.817 cfs. A 7,600 sf underdrained bioretention BMP is designed to meet the peak flow rate and runoff hydrograph duration requirements of the volumetric reduction standards. The underdrained bioretention BMP is arranged as 47.5 feet wide and 160 feet long. The bioretention BMP is installed with a 150-foot, 4 inch perforated corrugated pipe as the underdrain. The 4 inch perforated pipe is buried within a 1-foot crushed stone bed. The sand layer and the soil bed are 6 inches and 24 inches, respectively, in accordance with the design criteria in Ch. 9.8 of the BMP Manual. A 40% void ratio is assigned to the crushed stone layer where runoff will be drained and provide a temporary storage during the hydraulic modeling. No void ratio is assigned to the sand layer and soil bed because the soil bed is supposed to remain moist to sustain the planting, which significantly reduces the temporary storage during the hydraulic modeling.2. Table 2¶
| Inflow Area = 65,340 sf, 83.33% Impervious, Inflow Depth = 0.87" for WQL event | ||||
|---|---|---|---|---|
| Inflow = 3.817 cfs @ 1.09 hrs, Volume= 4,761 cf | ||||
| Outflow = 0.235 cfs @ 1.81 hrs, Volume= 4,761 cf, Atten= 94%, Lag= 43.1 min | ||||
| Primary = 0.235 cfs @ 1.81 hrs, Volume= 4,761 cf | ||||
| Routing by Stor-Ind method, Time Span= 0.00-48.00 hrs, dt= 0.01 hrs / 2 | ||||
| Peak Elev= 103.60' @ 1.81 hrs Surf.Area= 7,600 sf Storage= 3,826 cf | ||||
| Plug-Flow detention time= 190.7 min calculated for 4,760 cf (100% of inflow) | ||||
| Center-of-Mass det. time= 190.8 min (259.1 - 68.3) | ||||
| Volume Invert Avail.Storage Storage Description | ||||
| #1 100.00' 33,440 cf Custom Stage Data (Prismatic) Listed below (Recalc) | ||||
| Elevation (feet) Surf.Area (sq-ft) Voids (%) Inc.Store (cubic-feet) Cum.Store (cubic-feet) | ||||
| 100.00 | 7,600 | 0.0 | 0 | 0 |
| 101.00 | 7,600 | 40.0 | 3,040 | 3,040 |
| 101.50 | 7,600 | 0.0 | 0 | 3,040 |
| 103.50 | 7,600 | 0.0 | 0 | 3,040 |
| 104.50 | 7,600 | 100.0 | 7,600 | 10,640 |
| 105.50 | 7,600 | 100.0 | 7,600 | 18,240 |
| 106.50 | 7,600 | 100.0 | 7,600 | 25,840 |
| 107.50 | 7,600 | 100.0 | 7,600 | 33,440 |
| Device Routing Invert Outlet Devices | ||||
| #1 Device 2 98.50' 2.0" Vert. Low Flow Control Device C= 0.600 | ||||
| #2 Primary 98.50' 8.0" Round Discharge Pipe | ||||
| L= 25.0' CPP, projecting, no headwall, Ke= 0.900 | ||||
| Inlet / Outlet Invert= 98.50' / 86.00' S= 0.5000' /' Cc= 0.900 | ||||
| n= 0.012 Corrugated PP, smooth interior, Flow Area= 0.35 sf | ||||
| #3 Device 2 104.13' 2.5" Vert. 1st Orifice above Soil Bed C= 0.600 | ||||
| #4 Device 2 105.50' 4.0" Vert. 2nd Orifice above Soil Bed C= 0.600 | ||||
| #5 Device 2 106.75' 20.0' long x 0.5' breadth Top Overflow Weir for 100 Year Storm | ||||
| Head (feet) 0.20 0.40 0.60 0.80 1.00 | ||||
| Coef. (English) 2.80 2.92 3.08 3.30 3.32 | ||||
| #6 Primary 107.10' 10.0' long x 5.0' breadth Spillway | ||||
| Head (feet) 0.20 0.40 0.60 0.80 1.00 1.20 1.40 1.60 1.80 2.00 | ||||
| 2.50 3.00 3.50 4.00 4.50 5.00 5.50 | ||||
| Coef. (English) 2.34 2.50 2.70 2.68 2.68 2.66 2.65 2.65 2.65 | ||||
| 2.65 2.67 2.66 2.68 2.70 2.74 2.79 2.88 | ||||
| Primary OutFlow Max=0.235 cfs @ 1.81 hrs HW=103.60' (Free Discharge) | ||||
| 2=Discharge Pipe (Passes 0.235 cfs of 2.898 cfs potential flow) | ||||
| 1=Low Flow Control Device (Orifice Controls 0.235 cfs @ 10.79 fps) | ||||
| 3=1st Orifice above Soil Bed (Controls 0.000 cfs) | ||||
| 4=2nd Orifice above Soil Bed (Controls 0.000 cfs) | ||||
| 5=Top Overflow Weir for 100 Year Storm (Controls 0.000 cfs) | ||||
| 6=Spillway (Controls 0.000 cfs) |
Source context and notes
# Volumetric Reduction Standards Source revision: January 2026. | Inflow Area = 65,340 sf, 83.33% Impervious, Inflow Depth = 0.87" for WQL event | | | | | | --- | --- | --- | --- | --- | | Inflow = 3.817 cfs @ 1.09 hrs, Volume= 4,761 cf | | | | | | Outflow = 0.235 cfs @ 1.81 hrs, Volume= 4,761 cf, Atten= 94%, Lag= 43.1 min | | | | | | Primary = 0.235 cfs @ 1.81 hrs, Volume= 4,761 cf | | | | | | Routing by Stor-Ind method, Time Span= 0.00-48.00 hrs, dt= 0.01 hrs / 2 | | | | | | Peak Elev= 103.60' @ 1.81 hrs Surf.Area= 7,600 sf Storage= 3,826 cf | | | | | | Plug-Flow detention time= 190.7 min calculated for 4,760 cf (100% of inflow) | | | | | | Center-of-Mass det. time= 190.8 min (259.1 - 68.3) | | | | | | Volume Invert Avail.Storage Storage Description | | | | | | #1 100.00' 33,440 cf Custom Stage Data (Prismatic) Listed below (Recalc) | | | | | | Elevation (feet) Surf.Area (sq-ft) Voids (%) Inc.Store (cubic-feet) Cum.Store (cubic-feet) | | | | | | 100.00 | 7,600 | 0.0 | 0 | 0 | | 101.00 | 7,600 | 40.0 | 3,040 | 3,040 | | 101.50 | 7,600 | 0.0 | 0 | 3,040 | | 103.50 | 7,600 | 0.0 | 0 | 3,040 | | 104.50 | 7,600 | 100.0 | 7,600 | 10,640 | | 105.50 | 7,600 | 100.0 | 7,600 | 18,240 | | 106.50 | 7,600 | 100.0 | 7,600 | 25,840 | | 107.50 | 7,600 | 100.0 | 7,600 | 33,440 | | Device Routing Invert Outlet Devices | | | | | | #1 Device 2 98.50' 2.0" Vert. Low Flow Control Device C= 0.600 | | | | | | #2 Primary 98.50' 8.0" Round Discharge Pipe | | | | | | | | | L= 25.0' CPP, projecting, no headwall, Ke= 0.900 | | | | | | Inlet / Outlet Invert= 98.50' / 86.00' S= 0.5000' /' Cc= 0.900 | | | | | | n= 0.012 Corrugated PP, smooth interior, Flow Area= 0.35 sf | | | #3 Device 2 104.13' 2.5" Vert. 1st Orifice above Soil Bed C= 0.600 | | | | | | #4 Device 2 105.50' 4.0" Vert. 2nd Orifice above Soil Bed C= 0.600 | | | | | | #5 Device 2 106.75' 20.0' long x 0.5' breadth Top Overflow Weir for 100 Year Storm | | | | | | | | | Head (feet) 0.20 0.40 0.60 0.80 1.00 | | | | | | Coef. (English) 2.80 2.92 3.08 3.30 3.32 | | | #6 Primary 107.10' 10.0' long x 5.0' breadth Spillway | | | | | | | | | Head (feet) 0.20 0.40 0.60 0.80 1.00 1.20 1.40 1.60 1.80 2.00 | | | | | | 2.50 3.00 3.50 4.00 4.50 5.00 5.50 | | | | | | Coef. (English) 2.34 2.50 2.70 2.68 2.68 2.66 2.65 2.65 2.65 | | | | | | 2.65 2.67 2.66 2.68 2.70 2.74 2.79 2.88 | | | Primary OutFlow Max=0.235 cfs @ 1.81 hrs HW=103.60' (Free Discharge) | | | | | | 2=Discharge Pipe (Passes 0.235 cfs of 2.898 cfs potential flow) | | | | | | 1=Low Flow Control Device (Orifice Controls 0.235 cfs @ 10.79 fps) | | | | | | 3=1st Orifice above Soil Bed (Controls 0.000 cfs) | | | | | | 4=2nd Orifice above Soil Bed (Controls 0.000 cfs) | | | | | | 5=Top Overflow Weir for 100 Year Storm (Controls 0.000 cfs) | | | | | | 6=Spillway (Controls 0.000 cfs) | | | | | Source: HydroCAD® Summary Report; HydroCAD is a registered trademark of HydroCAD Software Solutions LLC. Used with permission.3. Table 3¶
| Time (hours) | Inflow (cfs) | Storage (cubic-feet) | Elevation (feet) | Primary (cfs) |
|---|---|---|---|---|
| 0.00 | 0.000 | 0 | 100.00 | 0.000 |
| 0.50 | 0.120 | 32 | 100.01 | 0.018 |
| 1.00 | 3.340 | 1,111 | 100.37 | 0.140 |
| 1.50 | 0.508 | 3,645 | 103.58 | 0.235 |
| 2.00 | 0.132 | 3,766 | 103.60 | 0.235 |
| 2.50 | 0.000 | 3,372 | 103.54 | 0.234 |
| 3.00 | 0.000 | 2,978 | 100.98 | 0.163 |
| 3.50 | 0.000 | 2,688 | 100.88 | 0.159 |
| 4.00 | 0.000 | 2,404 | 100.79 | 0.156 |
| 4.50 | 0.000 | 2,126 | 100.70 | 0.153 |
| 5.00 | 0.000 | 1,854 | 100.61 | 0.150 |
| 5.50 | 0.000 | 1,588 | 100.52 | 0.146 |
| 6.00 | 0.000 | 1,328 | 100.44 | 0.143 |
| 6.50 | 0.000 | 1,073 | 100.35 | 0.140 |
| 7.00 | 0.000 | 825 | 100.27 | 0.136 |
| 7.50 | 0.000 | 582 | 100.19 | 0.133 |
| 8.00 | 0.000 | 345 | 100.11 | 0.130 |
| 8.50 | 0.000 | 138 | 100.05 | 0.078 |
| 9.00 | 0.000 | 50 | 100.02 | 0.028 |
| 9.50 | 0.000 | 18 | 100.01 | 0.010 |
| 10.00 | 0.000 | 7 | 100.00 | 0.004 |
| 10.50 | 0.000 | 2 | 100.00 | 0.001 |
| 11.00 | 0.000 | 1 | 100.00 | 0.000 |
| 11.50 | 0.000 | 0 | 100.00 | 0.000 |
Source context and notes
# Volumetric Reduction Standards Source revision: January 2026. [Figure: img-7.jpeg (original PDF, p. 12)](SOURCE.pdf#page=12) Source: HydroCAD® Hydrograph Graph and Tabular Report; HydroCAD is a registered trademark of HydroCAD Software Solutions LLC. Used with permission. | Time (hours) | Inflow (cfs) | Storage (cubic-feet) | Elevation (feet) | Primary (cfs) | | --- | --- | --- | --- | --- | | 0.00 | 0.000 | 0 | 100.00 | 0.000 | | 0.50 | 0.120 | 32 | 100.01 | 0.018 | | 1.00 | 3.340 | 1,111 | 100.37 | 0.140 | | 1.50 | 0.508 | 3,645 | 103.58 | 0.235 | | 2.00 | 0.132 | 3,766 | 103.60 | 0.235 | | 2.50 | 0.000 | 3,372 | 103.54 | 0.234 | | 3.00 | 0.000 | 2,978 | 100.98 | 0.163 | | 3.50 | 0.000 | 2,688 | 100.88 | 0.159 | | 4.00 | 0.000 | 2,404 | 100.79 | 0.156 | | 4.50 | 0.000 | 2,126 | 100.70 | 0.153 | | 5.00 | 0.000 | 1,854 | 100.61 | 0.150 | | 5.50 | 0.000 | 1,588 | 100.52 | 0.146 | | 6.00 | 0.000 | 1,328 | 100.44 | 0.143 | | 6.50 | 0.000 | 1,073 | 100.35 | 0.140 | | 7.00 | 0.000 | 825 | 100.27 | 0.136 | | 7.50 | 0.000 | 582 | 100.19 | 0.133 | | 8.00 | 0.000 | 345 | 100.11 | 0.130 | | 8.50 | 0.000 | 138 | 100.05 | 0.078 | | 9.00 | 0.000 | 50 | 100.02 | 0.028 | | 9.50 | 0.000 | 18 | 100.01 | 0.010 | | 10.00 | 0.000 | 7 | 100.00 | 0.004 | | 10.50 | 0.000 | 2 | 100.00 | 0.001 | | 11.00 | 0.000 | 1 | 100.00 | 0.000 | | 11.50 | 0.000 | 0 | 100.00 | 0.000 | The hydrologic and hydraulic model shows that the peak flow rate for the WQDS is 0.235 cfs. The hydrograph of the discharge starts at 0.5 hours and ends at 10.50 hours, which results in a runoff hydrograph duration of 10 hours. The hydrologic and hydraulic analysis demonstrates that the proposed underdrained bioretention BMP has a smaller peak flow rate and longer runoff hydrograph duration than the equivalent woods on HSG D soil. The volumetric reduction standards are met.Images on table pages¶
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