Multiobjective Optimization Combining BMP Technology and Land Preservation for Watershed-based Stormwater Management

Recent progress has been made developing decision-support models for optimal deployment of best management practices (BMP’s) in an urban watershed to achieve water quality goals. One example is the high-level screening model StormWISE, developed by the author (McGarity, 2006) that uses linear and nonlinear programming to narrow the search for optimal solutions to certain land use categories and drainage zones. Another example is the model SUSTAIN developed by USEPA and Tetra Tech (Lai, et al., 2006), which builds on the work of Yu, et al., 2002), that uses a detailed, computationally intensive simulation model driven by a genetic solver to select optimal BMP sites. However, a model that deals only with best management practice (BMP) site selections may fail to consider solutions that avoid future nonpoint pollutant loadings by preserving undeveloped land. This paper presents results of a recently completed research project in which water resource engineers partnered with experienced professionals at a land conservation trust to develop a multiobjective model for watershed management. The result is a revised version of StormWISE that can be used to identify optimal, cost-effective combinations of easements and similar land preservation tools for undeveloped sites along with low impact development (LID) and BMP technologies for developed sites. The goal is to achieve the watershed-wide limits on runoff volume and pollutant loads that are necessary to meet water quality goals as well as ecological benefits associated with habitat preservation and enhancement. A nonlinear programming formulation is presented for the extended StormWISE model that achieves desired levels of environmental benefits at minimum cost. Tradeoffs between different environmental benefits are generated by multiple runs of the model while varying the levels of each environmental benefit obtained. The model is solved using piecewise linearization of environmental benefit functions where each linear segment of represents a different option for reducing stormwater runoff volumes and pollutant loadings. The solutions space is comprised of optimal levels of expenditure for categories of BMP's by land use category and optimal land preservation expenditures by drainage zone. To demonstrate the usefulness of the model, results from its application to the Little Crum Creek watershed in suburban Philadelphia are presented. The model has been used to assist a watershed association and four municipalities to develop an action plan for restoration of water quality on this impaired stream. References Lai, F., J. Zhen, J. Riverson, and L. Shoemaker (2006). "SUSTAIN - An Evaluation and Cost-Optimization Tool for Placement of BMPs," ASCE World Environmental and Water Resource Congress 2006. McGarity, A.E. (2006). A Cost Minimization Model to Priortize Urban Catchments for Stormwater BMP Implementation Projects. American Water Resources Association National Meeting, Baltimore, MD, November, 2006. Yu, S., J. X. Zhen, and S.Y. Zhai, (2002). Development of Stormwater Best Management Practice Placement Strategy for the Virginia Department of Transportation. Final Contract Report, VTRC 04-CR9, Virginia Transportation Research Council.