Paved Area Reduction Factors under Temporally Varied Rainfall and Infiltration

Abstract Best management practices (BMPs) for storm water drainage systems are used to increase storm water quality and reduce the amount of storm water runoff that is generated from a developed watershed. Paved area reduction factors, referred to as PARFs, are developed in this study to determine the reduction of storm water runoff when storm water BMPs are incorporated into storm water designs. PARFs determine the amount of runoff reduction by evaluating the effective impervious values of a watershed compared to the impervious value that is commonly determined by weighting the impervious and pervious areas. This study shows that there is a difference between the effective imperviousness and area-weighted imperviousness of a watershed. It also investigates PARFs that are developed under temporally varied rainfall and infiltration parameters and presents how they can vary from PARFs that are derived using constant rainfall and infiltration rates. This study creates an effective line to relate site-effective imperviousness with the infiltration to rainfall index that was developed in previous studies. The effective line provides an easy application to determine site-effective imperviousness from area-weighted impervious values. This study also quantifies the incentives of storm water BMPs by defining the net present value when imperviousness is a basis for storm water fees. It is found that a cost savings in storm water fees can be used to encourage storm water BMPs, and that applying temporally varied rainfall and infiltration for computing PARFs can show an increase in cost savings over previous methods.