Kitsap County Stormwater

Kitsap County, Washington

HWA was contracted by Kitsap County to perform a stormwater site selection and design study, with the objectives of improving stream flows by enhancing stormwater infiltration in selected areas. Based on prior work, ten main areas including 22 individual sites were identified in selected drainages with identified instream flow issues as candidate sites for stormwater infiltration. HWA further evaluated the selected sites for feasibility based on geologic, geographic, and functional criteria.

HWA also conducted site explorations at selected sites, including hand auger borings and laboratory soil testing to determine grain size distribution. HWA estimated infiltration rates at selected sites based on the grain size testing results. Based on these findings, HWA recommended retrofitting and/or maintenance of five existing stormwater facilities and the construction of two additional stormwater ponds. HWA also recommended more detailed on-site investigations for specific sites selected for new infiltration pond construction.

HWA's work was funded through a department of Ecology Water Storage Preconstruction grant program, with the objectives of enhancing stormwater infiltration in selected drainages. Enhancing stormwater infiltration will 1) increase dry season stream base flows, 2) increase aquifer recharge, ground water levels, and ground water supply, and 3) reduce wet-season runoff and stream peak flows. Potential benefits of achieving these objectives include 1) improving salmon habitat, 2) improving surface water (streams and Puget Sound) water quality, and 3) reducing downstream flooding. HWA completed the project within budget and schedule. Because the project entailed site screening and selection followed by design level work, we were able to meet the overall project objectives using a flexible approach based on evolving project requirements.