Technology Matrix Example: Redevelopment 2-Ac Commercial

In this example, assume the site is a 2-acre site whose pre-construction impervious area is 70 percent, and existing soils are HSG D. The site is located in a “zero increase” watershed. The site zoning allows up to 75% impervious coverage. Land disturbance exceeds more than 1 acre, and water quality BMPs are required. 15-year 20-minute differential runoff calculations show the additional undetained runoff is <2 cfs. Flood detention is not required. Channel protection is not required because the site is < 5 acres, and the site qualifies for exemption from channel protection detention.

The developer’s engineer proposes to use porous pavement for the parking stalls, such that the remaining water quality volume can be stored in bioretention in the parking lot islands and along the site perimeter. Building downspouts discharge into perimeter bioretention.  The approach is accepted by MSD.