Modeling Impervious Area Disconnection with SWMM

摘要

Given careful design and measures to improve soil infiltration, remarkable results can be obtained from disconnecting runoff, even with relatively small receiving areas. Improving soil infiltration with deep tillage and incorporation of compost is well documented in the agricultural literature. The combination of disconnection with improved soils has considerable promise as an effective Green Infrastructure control.The effects of impervious area disconnection were modeled on an office/commercial complex with 67% impervious cover. The perimeter of the site is graded as a gently sloping biofiltration swale. Flush curbs and bumper blocks provide sheet flow conditions to the shallow sides of the swales Runoff from office building roofs sheet flow toward shallow lawn inlets. These inlets discharge directly into the bottom of the swales, as does the runoff from the commercial building roof. Given shallow side slopes and flat bottoms, nearly the entire area of the swales is wetted, providing as much wetted area as possible.PCSWMM was used to compare three different scenarios: Full Disconnection at 1.0 in/hr. K_sat, compared to Full Disconnection at 0.25 in/hr. K_sat and No Disconnection at 1.0 in/hr. K_sat. The 1.0 in/hr. Ksat represents improved soils, while 0.25 in/hr. K_sat represents typical soils with turf management. The No Disconnection Alternative represents the typical direct manifold of roof leaders and storm drains. The swales were modeled as trapezoidal conduits placed inside the swale subcatchments to represent the time lag that occurs at low flow depths in biofiltration swales. The following results were obtained for the 2005 design year:Scenario: Nonel.O Full0.25 Fulll.OMaximum Outflow (cfs): 10.72 8.342 6.089Mean Outflow (cfs): 0.01391 0.005505 0.002007Total Outflow (ft3): 438,700 173,600 63,300The effects of disconnection onto improved soils are considerable. Annual runoff volume is reduced by 86%, while the volume of exceedance over 0.05 cfs/acre is reduced by 81%. Both the number and duration of exceedances are reduced by 92%. It is also instructive to see how disconnection can perform in the unimproved soils. As may be expected, the results are worse, but still remarkable. The annual runoff volume is reduced by 60%, while the volume of exceedance over 0.05 cfs/acre is reduced by 50%. The number of exceedances is reduced by 77%, while the duration is reduced by 71%.