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WWHM-SWMM Tech Specs:

Impervious Runoff Dispersion

The WWHM-SWMM Lateral Flow Basin elements includes state-of-the-science algorithms that correctly and accurately model the dispersion of impervious runoff on an adjacent pervious surface.

Impervious runoff dispersion (also known as impervious runoff disconnection and lateral flow dispersion) offers unique opportunities to reduce impervious surface runoff by allowing it to sheet flow across an adjacent pervious surface, such as a lawn area. Impervious runoff flowing across the pervious landscape with some of the impervious runoff infiltrating into the soil and some becoming evapotranspiration.

WWHM-SWMM uses two elements in series to model this lateral movement of water draining from the impervious area to the pervious. The Lateral Flow Impervious Area element represents the impervious surface area, whether it is a roof, parking lot, or road surface. The Lateral Flow Soil Basin represents the adjacent pervious area. The user selects the soil, vegetation, and land slope that is representative of this pervious area.

WWHM-SWMM computes the runoff and evaporation from the impervious surface area and then spreads the impervious runoff over the adjacent pervious area soil and vegetation using HSPF continuous simulation hydrology.

The reduced total runoff can then be routed to a downstream conveyance system or storage to be in compliance with NPDES MS4 stormwater hydromodification/ flow control and water quality requirements.