Climate change impacts on urban stormwater best management practices

Abstract

Total maximum daily load (TMDL) studies determine the amount of contaminant(s) that can be discharged daily from point (waste load allocation - WLA) and nonpoint (load allocation - LA) sources including a margin of safety (MOS) and then layout the path for achieving these levels by reductions in loadings. This has caused environmental agencies to require best management practices (BMPs) for control of urban stormwater contributions. Design storms for volume-based and peak discharge BMPs are typically determined from historic precipitation and runoff records that do not adequately address the impacts of climate change. We examine a 10-year period of predicted flows in the Spokane River watershed under 2050 climate predictions to determine the amount of additional LA removal required to meet water quality goals. While the current TMDL proposes a 50% reduction of nonpoint loading, our results indicate this will not be adequate. The implication is that urban BMPs are currently inadequately designed to handle nonpoint pollution in areas projected to experience increased precipitation events. The problem is particularly acute for rain on snow events where BMP performance is already impaired.