The Limitations of Mitigation-Based Stormwater Management in the Pacific Northwest and the Potential of a Conservation Strategy based on Low-Impact Development Principles

摘要

Stream ecosystems in the Pacific Northwest (PNW) were found to benefit from the retention ofrnnative forest and wetland cover, as well as the minimization of impervious surfaces throughoutrnthe watershed. In addition, wide, continuous riparian buffers consisting of mature, native forestrnprovided additional assurance of high aquatic ecosystem integrity. In many cases, however,rnthese are necessary but not sufficient conditions for retaining natural ecosystem structure andrnfunction. These findings can help guide comprehensive watershed management and applicationrnof these non-structural practices in site planning and design. Field surveys of structural bestrnmanagement practices (BMPs) found that, even with a relatively high level of attention, arnminority of the developed watershed area is typically served by these BMPs. There are a varietyrnof reasons for this lack of uniform BMP coverage, including pre-BMP development, changes inrnBMP design criteria over time, and numerous variances allowed by local regulators. ThosernBMPs installed are capable of mitigating an even smaller share of urban impacts, primarilyrnbecause of inadequacies in current design standards. Even with these shortcomings, though,rnresults showed that structural BMPs help to sustain aquatic biological communities, especially atrnmoderate-high urbanization levels, where space limits non-structural options. In addition,rnstructural BMPs can be effective in water quality treatment of stormwater pollution "hot-spots"rnor in "ultra-urban" settings. The most promise appears to be in the use of low-impactrndevelopment (LID) concepts that incorporate conservation of native vegetation and soil,rnminimize imperviousness, and utilize integrated, on-site treatment of stormwater.