Integrated Water Quality Improvement Approach with Economic Consideration Using a Decision Support System and Application for Water Quality Design Storm Development

As an effective stormwater management tool implementing the integrated, watershed-based approach, the Los Angeles County Flood Control District has been developing the Watershed Management Modeling System (WMMS) for all of the County's 3,000-square mile coastal watersheds. Details of the hydrologic factors in the watershed have been represented in terms of rainfall variability, impervious cover distribution; various land use types, and loading characteristics of major pollutants. It has been a cooperative effort with the United States Environmental Protection Agency who provided technical and financial support.Built on an extensive body of relevant studies in the Los Angeles region and the state-of-the-art optimization technique, the WMMS provides a unique framework where municipalities or watershed planners can evaluate alternative stormwater BMPs. This decision support system allows managers to evaluate the ability of various BMP scenarios to provide necessary flow volume and pollutant load reductions and optimizes the scenarios based on benefits and costs. The WMMS simulates hydrologic and multi-pollutant transport processes in a watershed while evaluating benefits and costs of different BMP options, to ultimately identify a combination of the most cost-effective BMP solutions to a specific management objective such as TMDL compliance.WMMS can be used for effective regional watershed management purposes, including (1) development of watershed based, long-term water quality Improvement master plan, (2) technical framework for regional water quality funding initiative, and (3) multi-pollutant TMDL compliance planning for regulatory compliance.Ongoing efforts for assessment of BMPs in Los Angeles County have repeatedly indicated a need to identify a water quality design storm. The need for the Water Quality Design Storm (WQDS) has been widely recognized among MS4 permittees as well as regulators. Identification of a WQDS can assist in identifying the threshold storm size treated that is economically reasonable for pollutant reduction.WMMS can serve as the enhanced platform for design storm development. The development of WQDS is dependent on three key factors: (1) storm sizes and associated treatment volume, (2) costs, and (3) compliance of water quality standards. Finally, the paper will propose the WQDS sizes that are spatially variable across the County's watersheds. The results are also presented as BMP sizing recommendations for TMDL regulatory compliance.