Paved area reduction factors for stormwater low impact and development and incentives

Paved Area Reduction Factors, herein referred to as PARFs, developed within this research are applied to quantify the benefits of stormwater best management practices and low impact development. A PARF can be used to provide stormwater incentives that will enhance stormwater quality and reduce stormwater quantity. A site effective impervious model is developed by applying overland flow concepts that couple continuity, non-linear reservoir routing, and Manning's open channel flow equations. Overland flow equations are used to quantify the routing of stormwater flow from impervious to pervious drainage areas. When flow is routed from impervious to pervious drainage areas additional infiltration and depression losses occur into the pervious zone, which create an effective impervious value that is less than the area weighted imperviousness of a watershed. The effective impervious values in this research incorporate both constant and temporally varied infiltration rates and rainfall distributions. Based on the effective impervious value a monetary correlation is derived that considers the net present and future value of a site effective impervious model when it is compared to the area weighted impervious model. The site effective impervious model was tested at a water quality research facility located at Parking Lot K on the Auraria campus of the University of Colorado Denver. Field tests include a hydraulic model that was calibrated with three years of rainfall and runoff data recorded at the testing facility. There is a strong correlation with the theoretical development of PARFs when they are compared to measured rainfall events and also when design storm distributions are applied to the calibrated field model. Four case studies presented in this research conclude that PARFs are accurate and applicable to other regions and storm distributions and can also be applied to different hydrologic procedures to estimate runoff from a developed watershed. Case studies with the calibrated hydrologic model showed an agreement within 20 percent of the theoretical predicted values. It was also shown that the theory developed in this research can be applied to the widely used Soils Conservation Service rainfall distributions and hydrologic modeling applications. The PARFs developed in this research can be applied by a local stormwater utility or engineer to provide an incentive for stormwater management practices.