Volume-based imperviousness for storm water designs

The concept of low impact development (LID) is to apply decentralized on-site runoff source control to preserve the watershed hydrologic and ecological functions. An integrated layout using bioretention and vegetated landscape can decrease the developed runoff volume and peak flow. One of the key factors in urban hydrology is the imperviousness in the watershed. The conventional approach is to weight the imperviousness by the subareas in the watershed. Obviously, the area-weighted method has become inadequate when coping with LID because LID takes the flow path into consideration. When an impervious area directly drains onto a pervious area, the area-weighted method fails to count for the additional soil infiltration loss. In this study, the conventional area-weighted method is modified with a paved area reduction factor that converts the area-weighted imperviousness to its effective imperviousness. A family of curves was also developed for engineering applications. When a cascading process is introduced to the runoff flow path, the paved area reduction factor can be determined using the impervious to pervious area ratio and the infiltration to rainfall intensity ratio.