Technology Matrix Example: New Development Residential

In this example, assume the site is a 20-acre wooded site and located in a “release rate” watershed (per Table 4-5 of 2006 Final Rules and Regulations (COMPLETE DOCUMENT)). Investigation of existing site resources indicates that existing soils are HSG B. The site is zoned for 1/4-acre residential lots, and the developer plans to build single family homes on each. Land discturbance 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 and channel protection are also required.

Using the matrix, the developer’s engineer proposes to use bioretention distributed throughout the site to meet runoff volume reduction and water quality treatment requirements associated with the project’s streets, sidewalks, and driveways. The grading plan preserves a forested buffer strip between the lots, allowing each home’s downspouts to be discharged via sheet flow into the buffer strips. At the downstream end of the site, a stormwater (wet) pond is provided for channel protection, flood detention storage, and recreation. The size of the flood detention basin is reduced based on the volume reduction provided by the bioretention cells (modified curve number) and downspout disconnection (wooded curve number). The approach is accepted by MSD.