FOREST COVER, IMPERVIOUS‐SURFACE AREA, AND THE MITIGATION OF STORMWATER IMPACTS¹

TLDR

Urbanization in King County has prompted progressive structural and nonstructural measures to protect aquatic resources and salmon, yet costly retrofits only mitigate flooding or erosion and cannot restore predevelopment flow or habitat, illustrating that forest conversion also degrades aquatic systems even with low imperviousness and offering lessons for other urbanizing regions. The study aims to establish new management goals for watersheds where existing development limits ecosystem recovery, advocating integrated mitigation that combines impervious‑surface limits, forest retention, stormwater detention, riparian buffers, wetlands, and slope protection. The authors propose an integrated mitigation framework that incorporates impervious‑surface limits, forest‑retention policies, stormwater detention, riparian‑buffer maintenance, and protection of wetlands and unstable slopes. Detention ponds, even with increasingly restrictive designs, remain inadequate to prevent channel erosion.

Abstract

ABSTRACT: For 20 years, King County, Washington, has implemented progressively more demanding structural and nonstructural strategies in an attempt to protect aquatic resources and declining salmon populations from the cumulative effects of urbanization. This history holds lessons for planners, engineers, and resource managers throughout other urbanizing regions. Detention ponds, even with increasingly restrictive designs, have still proven inadequate to prevent channel erosion. Costly structural retrofits of urbanized watersheds can mitigate certain problems, such as flooding or erosion, but cannot restore the predevelopment flow regime or habitat conditions. Widespread conversion of forest to pasture or grass in rural areas, generally unregulated by most jurisdictions, degrades aquatic systems even when watershed imperviousness remains low. Preservation of aquatic resources in developing areas will require integrated mitigation, which must including impervious‐surface limits, forest‐retention policies, stormwater detention, riparian‐buffer maintenance, and protection of wetlands and unstable slopes. New management goals are needed for those watersheds whose existing development precludes significant ecosystem recovery; the same goals cannot be achieved in both developed and undeveloped watersheds.