Technology Matrix Example: Rooftop Amended Soils Disconnection

In this example, assume the site is a 3 acre infill site with 12 single family homes proposed. The site sits on a ridge and stormwater runoff currently discharges in all directions. Mature trees are located on the perimeter of the site. Land disturbance exceeds more than 1 acre, and water quality BMPs are required, however flood detention and channel protection are not required because the differential runoff (15-yr 20-min storm) is <2 cfs.

The developer’s engineer proposes to construct a cul-de-sac street in the center of the site to serve the homesites which will back to the wooded perimeter. Using the matrix, the developer’s engineer proposes to use bioretention at the front of the site to meet runoff volume reduction and water quality treatment requirements associated with the project’s streets, sidewalks, and driveways and front portion of the roofs. Amended soils disconnection is selected to meet runoff volume reduction and water quality treatment requirements associated with the project’s rear portion of the roofs and patios. The amended soils disconnection fits well with the existing sheet flow conditions and allows the developer to retain the trees on the rear of the lots. The approach is accepted by MSD.