Flood detention is detaining and releasing stormwater runoff from a site at rates not to exceed the allowable 2-year and 100-yr 24-hour events. What is an allowable release rate depends on the watershed to which water is discharged (see Table 4-5 of MSD Rules and Regulations). Detention performance is evaluated based on TR-55 methodology, "Urban Hydrology for Small Watersheds," by the Natural Resources Conservation Service (NRCS). A checklist of items the detention analysis needs to include is here.
One advantage of volume-reducing BMPs is that the runoff volume lost through evapotranspiration, infiltration, and use can be credited toward the flood detention storage requirement (reducing the cost of detention). MSD uses curve number (CN) modification to establish this credit.
With the NRCS curve number and the calculated precipitation depth retained on site (calculated in the MEP tool), designers can use the Modified CN spreadsheet tool to select modified CNs for the drainage area to the BMP. Where multiple BMPs are distributed in the watershed, and different modified curve numbers apply to each BMP's drainage area, then an area weighted average curve number is allowed.