Snohomish County, WA Environmental Rules: Erosion Control (2026)

450 makes the applicant for any new development or redevelopment project responsible for preventing soil erosion and the discharge of sediment and pollutants into receiving waters, no matter the project's size. A SWPPP must be submitted as part of the stormwater site plan, with a narrative and drawings describing when and where erosion and sediment control best management practices (BMPs) will be installed, the performance expected, and the corrective actions if that performance is not met. The SWPPP must be implemented before initial soil disturbance and stay in effect through final stabilization of the site.

During the wet season, from October 1 through April 30, the county imposes seasonal work limitations on land-disturbing activity: work is allowed only if the applicant demonstrates to the Snohomish County Department of Public Works' satisfaction that turbid water will be kept from leaving the site, based on existing vegetative cover, slope, soil type, proximity to receiving waters, limits on the extent of disturbed area, and the proposed erosion and sediment control measures. The county's Drainage Manual, incorporated by reference, sets the specific BMP design standards used to meet these requirements.

If turbid water is found leaving a site because erosion and sediment controls are not being met, the department can order the contractor to maintain the BMPs already installed or add new ones. Three narrow activities are exempt from the wet-season limitation: sites where stormwater runoff achieves 100 percent on-site infiltration into approved, installed facilities; routine maintenance and necessary repair of existing erosion and sediment control BMPs; and routine maintenance of public facilities or utility structures that does not expose soil or strip vegetative cover.

The section was most recently amended by Ordinance 26-021 (June 17, 2026, effective July 1, 2026), following earlier amendments in 2010, 2016 and 2021 that reorganized the SWPPP element sections referenced in this part of the chapter.