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Western Washington Flow Control Exemption Investigation




WSDOT, working in consultation with the Washington State Department of Ecology (Ecology), explored whether additional large-river flow control exemptions were warranted.  A consultant team from Herrera Environmental and Northwest Hydraulic Consultants conducted this study using a three-phase approach.

Phase I, generated a position paper presenting the theory and justification for stormwater exemptions in large-river systems and synthesized the relevant literature supporting the development of a methodology to exempt river reaches from stormwater flow control based on stream order, cumulative changes in watershed vegetative cover, percent impervious area, stream gradient, and tidal influence.

Phase II developed the methodologies necessary to produce acceptable analysis for seeking those exemptions. Discharge of Stormwater to High Order Streams: Determining Exempt Reaches (pdf 1.9 mb) documents the methodologies and findings generated under Phase II of this study. The position paper developed under Phase I is also included in Appendix B of this report. A key conclusion of the study is that most river segments in western Washington draining more than 100 square miles would likely qualify as exempt reaches per the land-cover change criteria.

Phase III was initiated In response to Ecology's April 2004 letter (pdf 874 kb) documenting their position on the methods and recommendations of the report. WSDOT initiated Phase III to apply the methodology developed under Phase II in western Washington to define stream reaches that meet the flow control exemption land-cover change criteria. The assessment has been completed and the investigation affirms most all of the river reaches brought over from WSDOT's instructional Letter 4002.02. The investigation also revealed a number of new river reach exemptions.

The final task in the investigation involved the Sammamish River. While the Sammamish River failed to meet the land-cover change criteria, investigators explored whether it was be possible to justify an exemption for the lower portion of the river based on hydraulic, hydrologic, and sediment transport considerations. As explained in Ecology’s August 2005 letter (pdf 104 kb)granting the exemption, their decision to grant the exemption is base upon the results of a conservative hydrologic and hydraulic case study of the Sammamish basin (pdf 316 kb).

Findings from these investigations are reflected in the revised exempt surface waters list (pdf 55 kb) in WSDOT's Highway Runoff Manual as well as amendments to Ecology's Stormwater Management Manual for Western Washington.