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Stormwater Standards
Stormwater Manual
All new development and redevelopment will need to follow new stormwater standards from the 2011 City of Issaquah Addendum to the 2009 King County Surface Water Design Manual. This update include revised flow control, temporary erosion and sediment control (TESC), and Stormwater Low Impact Development (LID) requirements.

Stormwater Flow Control
The default standard is the Level 2 flow control standard, assuming a historic (forested) pre-development condition. However, these (and other) exemptions are available:

  • Sites under one acre have separate requirements, basically following the 1998 King County Manual.
  • Sites with certain drainages in the Central Issaquah Area can assume currrent conditons for "pre-developed."
  • Saturated soils can be assumed as the pre-developed condition for areas mapped as being historically saturated
  • Other exemptions are available.

Refer to the 2011 Addendum for more specific information.

Water Quality
The default design standard for water quality treatment is Senstive Lake Protection (phosphorus control). Other design standards, including Basic, Enhanced Basic, and Oil Control, may also apply depending on the proposed land use and whether infiltration is proposed.

The City will permit treatment facilities that are contained in any technical manual approved by Ecology (i.e., King County, Ecology and WSDOT), as well as new BMPs that meet or are being tested under Ecology's Emerging Stormwater Treatment Technologies program.

Temporary Erosion and Sediment Control (TESC) permit coverage is required for all land disturbing activities (with some exceptions). TESC requirements are addressed in the Public Works permit for the project, typically a clearing and grading permit. For smaller projects that don't trigger a clearing and grading permit, a separate Public Works permit for TESC will be required.

A 100 NTU turbidity discharge limit is effective year-round, until the project is permanently stabilized.

Plan submittal requirements are revised to follow the 2011 Addendum. Designated TESC supervisors shall be a Certified Erosion and Sediment Control Lead (CESCL).
Note: A separate Department of Ecology Construction Stormwater General Permit is required for sites having 1 acre or more of disturbed area.   

Stormwater Low Impact Development (LID)
Stormwater LID is required in areas of the City that are favorable for infiltration (e.g., outwash and alluvial soils). In those areas, stormwater infiltration or LID techniques must be used, unless soil testing data shows the soil is unsuitable. See Special Requirement #6 in the 2011 Addendum for details. The City's stormwater code also allows for deviations from standards to allow Low Impact Development designs, such as narrower street widths and use of pervious pavement.

Issaquah Contacts

Kerry Ritland, PE
Surface Water Manager