more than one HSC captures runoff from the same impervious area (e.g., street trees covering a roof top that is disconnected). IAtotal = total impervious area in drainage area provides a template for calculation of the combined effective of HSCs in the drainage area (expressed in inches reduction of the design capture storm depth). Example .: Hydrologic Source Control Calculation Form (Worksheet A) A Drainage area ID 2.1 Total drainage area 1.3 Total drainage area Impervious Area (IAtotal) Effect of individual HSCi per criteria in HSC BMP Fact Sheets (TGD Impervious Area Section 4.2) HSC Type/ Description/ Tributary to HSCi (dHSCi) HSC ID Reference Section (IAi) di × IAi Downspout Dispersion, 1:2 ratio A-1 (0.5) of rooftop to pervious area 0.25” 0.38 0.095 for 0.38 acres Street perennial canopy A-2 over 0.25 acres of impervious 0.05” 0.25 0.0125 area Downspout Infiltration, 10-15 A-3 ft storage per 1000 sf of roof for 0.15” 0.21 0.032 0.21 acres Residential Rain Barrels, four 55 gallon barrels per 1000 sf of roof A-4 0.18” 0.2 0.036 (4*55*50%=110 gal/1000 sf) for 0.2 Box 1: di × IAi = 0.175 ∑ Box 2: IAtotal = 1.3 [Box 1]/[Box 2]: dHSC = 0.135
more than one HSC captures runoff from the same impervious area (e.g., street trees covering a roof top that is disconnected). IAtotal = total impervious area in drainage area provides a template for calculation of the combined effective of HSCs in the drainage area (expressed in inches reduction of the design capture storm depth). Example .: Hydrologic Source Control Calculation Form (Worksheet A) Drainage area ID A Total drainage area 2.1 Total drainage area Impervious Area (IAtotal) 1.3 HSC ID HSC Type/ Description/ Reference Section Effect of individual HSCi per criteria in HSC BMP Fact Sheets (TGD Section 4.2) (dHSCi) Impervious Area Tributary to HSCi (IAi) di × IAi A-1 Downspout Dispersion, 1:2 ratio (0.5) of rooftop to pervious area for 0.38 acres 0.25” 0.38 0.095 A-2 Street perennial canopy over 0.25 acres of impervious area 0.05” 0.25 0.0125 A-3 Downspout Infiltration, 10-15 storage per 1000 sf of roof for 0.21 acres 0.15” 0.21 0.032 A-4 Residential Rain Barrels, four 55 gallon barrels per 1000 sf of roof (4*55*50%=110 gal/1000 sf) for 0.2 0.18” 0.2 0.036 Box 1: ∑ di × IAi = 0.175 Box 2: IAtotal = 1.3 [Box 1]/[Box 2]: dHSC = 0.135 ft