Disconnected Impervious

Disconnecting impervious surfaces is one of the most cost effective ways to reduce runoff rates, volumes and sediment. By routing runoff across pervious surfaces prior to being concentrated in swales or sewers, sediment can be trapped in the vegetation and water infiltrated into the soil. In order for some, or all, of a site's runoff to be considered disconnected, it must meet all of the following requirements.

1. The source area flow length does not exceed 75 feet,
2. The pervious area is covered with a self-sustaining vegetation in “good” condition and at a slope not exceeding 8%,
3. The pervious area flow length is at least as long as the contributing impervious area flow length and there can be no additional runoff flowing into the pervious area other than that from the source area.
4. The pervious area must receive runoff in a sheet flow manner across an impervious area with a pervious width at least as wide as the contributing impervious source area.

One- and two-family rooftop runoff may be assumed disconnected provided the runoff has a flow length of at least 20 feet over a pervious area in good condition.

The percent of disconnected impervious should be no greater than that in the appropriate WinSLAMM standard land use files, unless the percent disconnection is known at the time of plan development.

Wisconsin DNR - Modeling Post-Construction Storm Water Management Treatment (pgs 12-13)