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Stormwater
ADOPTED STORMWATER MANUAL
The City of Issaquah currently adopts the 1998 King County Surface Water Design Manual. This will be in effect through February 14, 2010. Drainage review is conducted as part of the Public Works Permit.
Although the design standards of the 1998 manual will be in effect until February, 2010, to avoid confusion on the permit applicantion and review process applicants are encouraged to follow the permit submittal requirements contained in the 2009 Addendum (see below). This is because 2009 Addendum was written to specifically reference the City of Issaquah permit process. Design guidance and methods from the 2009 Addendum can also be used.
STORMWATER REQUIREMENTS EFFECTIVE IN FEBRURARY 2010
Effective February 15, 2010, permit applications will need to follow new stormwater standards from the 2009 City of Issaquah Addendum to the 2009 King County Surface Water Design Manual (2009 Addendum). This update include revised flow control, temporary erosion and sediment control (TESC), and Stormwater Low Impact Development (LID) requirements.
The 2009 Addendum can be downloaded from the link at the bottom of this page. The 2009 stormwater code that references the adopted manual can also be found there.
Stormwater Flow Control
Most projects must meet the Level 2 flow control standard assuming a historic (forested) pre-development condition. However, sites under 1.0 acre can follow the flow control standard from the 1998 King County manual to Issaquah. Other exemptions are also available. Please refer to Table 1.1.1.A in the 2009 Addendum (see link at bottom of this page).
Water Quality
Water quality treatment shall be Senstive Lake Protection (i.e., phosphorus control). The City will allow treatment BMPs that are contained any technical manual approved by the Ecology (i.e., King County, Ecology, and WSDOT), and also new BMPs that meet or are being tested under Ecology's Emerging Stormwater Treatment Technologies program.
TESC
See the TESC page for requirements on TESC. These requirements were updated in September, 2009 (see 2009 Stormwater Management Amendments link at the bottom of this page for specific code requirements).
Stormwater Low Impact Development (LID)
In the 2009 Addendumn new stormwater low impact development (LID) requirements will apply 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 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.
For more information on stormwater standards contact Kerry Ritland, PE, Surface Water Manager.
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