Yakima, WA Environmental Rules: Grading & Drainage (2026)

YMC Chapter 7.83 implements the post-construction stormwater control portion of Yakima's Eastern Washington Phase II MS4 permit and the SWMMEW. The chapter sets a regulatory threshold for projects that develop or redevelop more than 5,000 square feet of new or replaced hard surface and that involve less than one acre of clearing/grading and are not part of a larger common plan — these projects must implement post-construction BMPs per the SWMMEW. New development that meets the regulatory threshold and results in 10,000 square feet or more of new impervious surface and discharges stormwater directly or through a conveyance system into surface water must construct stormwater flow-control facilities sized to the SWMMEW. All projects must apply source-control BMPs (oil/water separators, catch basin inserts, covered loading docks, etc.) selected, designed, and maintained per the SWMMEW or an approved equivalent. The director uses Appendix 1 of the Eastern Washington Phase II Municipal Stormwater Permit and the latest SWMMEW as the technical basis for design and maintenance decisions. Drainage studies submitted with development applications must show pre- and post-development runoff for the SWMMEW design storms, demonstrate safe on-site conveyance, and show that runoff to adjacent properties is not increased. Grading itself is reviewed under the building code (IBC Appendix J as adopted in YMC Title 11), with minimum positive drainage of 5% for the first 10 feet away from foundations (per IRC R401.3). Many Yakima projects rely on UIC drywells in lieu of surface discharge; drywells are regulated under WAC 173-218 and require Ecology rule authorization plus city approval. Contact: Surface Water Engineer at (509) 575-6077; Code Administration at (509) 575-6126.