Summary of stormwater credits function

Summary of stormwater credits function illustrating how to calculate water quality volume, channel protection and overbank storm volumes for different Better Site Design credits.
Link to this table.

Stormwater credit	Adjusted water quality volume	Channel protection and overbank storms
Natural area conservation	Subtract CA from site IC when computing Vwq	Adjust CN for CA to woods in good condition
Site reforestation/prairie restoration	Subtract 1/2 RA from site IC when computing Vwq	Adjust CN for RA to woods or prairie in fair condition
Streams and shoreline buffers	Subtract ADB from site IC when computing Vwq	Adjust CN for ADB to woods in good condition
Surface impervious cover and disconnection	Subtract DIA from site IC when computing Vwq	Adjust CN for DIA to grass in good condition. Adjust Tc
Rooftop disconnection	Subtract DRA from site IC when computing Vwq	Adjust CN for DIA to grass in good condition. Adjust Tc
Grass channels	Subtract GA from site IC when computing Vwq	Adjust Tc

Note: Unless otherwise noted, all units below are measured in acres

CA = combined area of all natural areas conserved at site

RA – Total area of site reforestation or prairie restoration

ADB – Total area draining to buffer with appropriate flow path distance

DIA – Total area of surface impervious cover that can be effectively disconnected

DRA – Aggregate rooftop area that can be effectively disconnected

GA - total non-roadway area draining to swale (rooftop, yard and driveway)

CN – Runoff curve number for area (units: dimensionless) (see Ch.8 and App. B)

Tc – Time of Concentration (units: time)
V_wq – Water quality volume, as defined by relevant MPCA sizing rule

IC - Impervious area of site (acres)