Issaquah, WA Environmental Rules: Erosion Control (2026)

Section 16.26.050(C)(4) of the Issaquah Municipal Code applies to all land disturbing activities, and the Director applies it when deciding whether to issue a clearing, grading and stormwater permit. The TESC plan has to follow the best management practices in Minimum Requirement No. 2 of the adopted Stormwater Design Manual, which is the 2019 Washington State Department of Ecology Stormwater Management Manual for Western Washington as modified by the City of Issaquah 2022 Addendum.

The plan must show the construction sequence for all erosion control work, temporary and permanent, for the drier season and the wetter winter months. It must map the streams, wetlands, steep slopes and buffers, native growth protection easements, greenbelt and native vegetation areas that stay undisturbed, plus soil preservation areas and protection of existing trees and root systems. It must also provide a monitoring discharge point so turbidity in water leaving the site can be measured. Except on small sites as the Stormwater Design Manual defines them, the plan must be designed by, and carry the seal of, a civil engineer holding certified erosion and sediment control lead (CESCL) certification. Devices are installed before construction activities begin and maintained until all possibility for erosion has passed and permanent vegetation is established, as the Director determines.

The wet season runs October 1st through April 30th. A wet season TESC plan must reach the Development Services Department by September 1st for approval to start or continue clearing and grading. The Director weighs site conditions, the nature of the work, the contractor's previous performance, slope, soil type, hydrology and proximity to receiving waters. Where wet season clearing and grading is prohibited, building construction can continue as long as no additional clearing and grading is performed and effective erosion control stays in place. Disturbed areas not covered by permanent impervious surface must be revegetated within seven days of completing grading or clearing unless seasonal or weather conditions are unfavorable, and temporary measures stay in place until then. The City can require an updated TESC plan at any time during the permit term if the approved plan proves ineffective.

The same section caps artificially created slopes at three feet horizontal to one foot vertical (3H:1V), limits rockeries on uncontrolled fill to four feet, and requires engineered design for retaining walls over four feet. Clearing or grading less than 1,000 square feet where the existing zoning is single-family residential is exempt under IMC 16.26.040(B)(9).