Optimal TMDL Development using a Watershed Scale Simulation-Optimization Approach: A Los Angeles County Case Study
Abstract:Several Total Maximum Daily Loads (TMDL) have been developed for multiple river segments for pollutants (including nutrients, metals, and bacteria) within the Los Angeles County regional watershed area, with urban runoff and stormwater as the primary source. Managing stormwater quality is complex because it involves many aspects that must be considered collectively to make informed decisions that are cost -effective and meaningful. To address these challenges, the Los Angeles County Flood Control District through a joint effort with United States Environmental Protection Agency has developed the Watershed management Modeling System (WMMS) that identifies cost effective water quality improvement projects through an integrated, watershed based approach. The WMMS encompasses the Los Angeles County's coastal watersheds of approximately 3,100 square miles, which is composed of more than 80 incorporated cities and unincorporated County areas.
Traditional TMDL developments typically rely on iterative trial-and-error modeling approaches for testing compliance. Considering the tremendous implementation costs within this highly urbanized context, a key objective of the WMMS was to develop TMDL implementation options that are both environmentally compliant and financially efficient. As a result, the model development included mapping climate and land cover variability, BMP treatment opportunities, and a range of viable options at a high degree of spatial resolution (2,655 subwatersheds), with water quality targets at 166 unique TMDL compliance points within the instream network. Because simulation-optimization problems at this scale have been an unsolvable problem for decades, this study applied a new algorithm named Nonlinearity-Interval Mapping Scheme (NIMS) developed during this study, which was used to optimize management under various risk tolerance levels for achieving TMDL compliance with minimum BMP implementation cost. The results of this study show a promising trend for future watershed scale optimal TMDL development and implementation plan formulation.
Document Type: Research Article
Publication date: 2011-01-01
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