Storm-Water Investment Strategy Evaluation Model for Impaired Urban Watersheds

Abstract

The stormwater investment strategy evaluation (StormWISE) model identifies the mix of characteristics that storm-water retrofit projects should have in an impaired watershed to maximize multiple water quality benefits subject to constraints on financial resources. The model can assist watershed managers faced with the daunting task of selecting cost-effective sites to implement best management practices (BMPs) and low-impact development (LID) technologies in response to watershed assessments that identify stream impairments. A methodology is provided to bridge the large gap between the general recommendations, typically found in watershed-level studies, and the ultimate site-specific decisions required at the land parcel level. A multiobjective optimization formulation is used to maximize water quality benefits expressed as functions of decision variables that set target levels for investment in projects according to two aggregated land parcel attributes, land use category, and watershed drainage zone, and according to the BMP/LID technologies deployed. Results take the form of constellations of Pareto-optimal solutions that guide the search for specific project sites and BMP/LID technologies to be implemented. This screening level analysis will usually be followed by identification of sites that possess high-priority attributes determined by the screening model for major investments for which the corresponding high-priority BMP/LID technologies can be deployed. If possible, candidate projects are subjected to further analysis through detailed watershed-level simulations. The theory underlying the model is presented, including development of water quality benefit functions and mathematical formulation of the optimization model as a piecewise linear program. A case study of the model’s application is included for the impaired Little Crum Creek watershed in suburban Philadelphia, where StormWISE has been used to develop an action plan for a partnership of four municipalities having storm sewers that discharge into the stream.