Non-Rooftop Disconnection

Non-Rooftop Disconnection

This credit is applied to credit disconnection of other impervious surfaces by encouraging drainage to overland treatment such as swales or filter strips. The manual needs to clearly document criteria for receiving credit.

Non-Rooftop Disconnection: Summary
Goals	Encourage the use of overland flow or infiltration areas to treat rooftop runoff.
Stormwater Management Objectives	Water Quality/ Recharge	Subtract the total disconnected area from the site's impervious cover.
	Channel Protection, Flood Control	Adjust disconnected impervious area to reflect woods in good condition.

Example:

In the non-rooftop disconnection credit, disconnected areas are subtracted from the total site impervious cover, and assigned a curve number for woods in good condition. In order to receive the credit, disconnections must meet the following criteria:

The maximum contributing impervious flow path length shall be 75 feet.
Runoff cannot come from a designated hotspot.
The disconnection must drain continuously through a vegetated channel, swale, or filter strip to the property line or STP.
The length of the "disconnection" must be equal to or greater than the contributing length.
The entire vegetative "disconnection" shall be on a slope less than or equal to 3.0%.
The surface imperviousness area to any one discharge location cannot exceed 1,000 ft².
Disconnections discharging over relatively permeable soils (HSGs A and B) do not require geotechnical testing.
If the site has less impermeable soils (HSGs C and D), testing by a geotechnical engineer is needed to determine if a spreading device, such as a french drain, gravel trench or other temporary storage device is needed to compensate for poor infiltration capability.
Credit is documented at a concept plan stage and verified with the final grading plan and with an as-built certification, or as required by the appropriate review authority.

The water quality and recharge credit can be calculated with the following equation:

C = (A_D/A_I)WQ_vWhere:
C = Non-Rooftop Credit (ac-ft)
A_D = Disconnected Impervious Area (acres)
A = Total site area (acres)
A_I = Site Impervious Area (acres)
WQ_v = Original Water Quality Volume.

Both water quality and recharge volumes are then reduced by the credit (C). Quantity credit is achieved by assigning disconnected rooftops a curve number equal to forest in good condition. The example below illustrates how this credit would be applied.