Stormwater Division

Stormwater Standards

All new development and redevelopment projects in the City of Snoqualmie must follow the stormwater standards from the 2018 City of Snoqualmie Addendum to the 2016 King County Surface Water Design Manual. 

This update includes terminology, code, and drainage review guidance, as well as revised water quality, flow control, temporary erosion and sediment control (TESC), and other requirements. Construction and development parties are responsible for determining whether a construction or industrial NOI is required from the Department of Ecology.

Water Quality

The Snoqualmie Addendum lists various amendments to the 2016 King County Surface Water Design Manual regarding water quality. 


  • Excludes Core Requirements and, Downstream Water Quality Problems Requiring Special Attention, and Downstream Water Quality Mitigations, respectively.
  • Modifies Basic Water Quality Treatment Areas to be consistent with the 2005 Ecology Stormwater Management Manual for Western Washington. (SWMMWW)
  • Notes that requirements for Sensitive Lake and Sphagnum Bog Water Quality Treatment Areas are applied on a case-by-case basis.

The City will accept all water quality treatment facility-types identified in the 2005 Ecology Stormwater Management Manual for Western Washington, though emerging technologies are considered on a case-by-case basis provided that the product has received a level of use designation from the Washington Ecology Technical Assessment Protocol Ecology (TAPE) program.

Stormwater Flow Control

Flow control amendments are listed that provide consistency with the 2005 Ecology Stormwater Management Manual for Western. For instance:

  • Thresholds are listed for Basic Flow Control Exemptions, Impervious Surface Exemption for Transportation Redevelopment Projects, and the cost exemption for Parcel Redevelopment Projects that are consistent with the 2005 SWMMWW.
  • For Area-Specific Flow Control Facilities, the Impervious Surface Percentage Exemption on page 1-38 is not allowed in order to maintain equivalency with the 2005 SWMMWW.
  • For Sizing Credits for use for Flow Control BMPs, various suitable alternative manuals are listed for compliance.
  • For Flow Control designs, the Addendum notes that the minimum allowable orifice diameter is increased from 0.25 to 0.5 for detention facilities, Control Structures and in applying design criteria.
  • Refer to the 2018 Addendum for more specific information.