Modeling Impervious Area Disconnection with SWMM
Substantial reductions in runoff volume can be obtained with designs integrated to disconnect runoff, especially when combined with measures to improve soil infiltration. The effect of impervious area disconnection was modeled on an office and commercial complex with 67% impervious cover. The perimeter of the site was graded as a gently sloping biofiltration swale. Flush curbs provided sheet flow conditions to the shallow sides of the swales. Runoff from office building roofs flowed toward lawn inlets under sheet flow and runoff from the commercial building roof discharged directly into the bottom of the swales. These swales, with shallow side slopes and flat bottoms, were designed to provide as much wetted area as possible.
PCSWMM was used to compare three different scenarios:
- Full1: Full disconnection with a Ksat of 2.54 cm/h (1.0 in./h) (improved soils);
- Full¼: Compared to full disconnection with a Ksat of 0.64 cm/h (0.25 in./h) (typical soils found in turf management); and
- None1: No disconnection with a Ksat of 2.54 cm/h (1.0 in./h) (i.e. direct connection of roof leaders and parking lots to storm drains).
These effects of disconnection onto improved soils were substantial. Annual runoff volume was reduced by 86%, while the volume of flows exceeding a threshold of 3.50 L/s/ha (0.05 f3/s/acre) was reduced by 81%. Both the number and duration of exceedances were reduced by 92%. Disconnection also performed well in the unimproved soils. The annual runoff volume was reduced by 60%, while the volume of exceedance over threshold was reduced by 50%. The number of exceedances was reduced by 77%, while the duration was reduced by 71%.
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