Dry Creek Bridge Replacement Hydraulic & Geomorphic Design

WSE was retained to provide hydraulic and geomorphic engineering services for the Dry Creek Bridge Replacement Project in Kittitas County, Washington.   The bridge carries rural McManamy Road over Dry Creek just east of the city of Ellensburg.  The existing bridge is 30 feet long and the County would like to replace it with a 60-to 70-foot long pre-fabricated steel span.  In addition to the conventional hydraulic and scour design evaluation, WSE had to consider potential future geomorphic channel changes that may occur at the project site in response to WSDOT’s proposal to replace their SR97 crossing with a larger structure.  SR97 crosses Dry creek less than one mile upstream from McManamy Road.  SR97 has a significant impact on channel sediment deposition and stability and there is a concern that a larger crossing at SR97 could allow these stream related issues to move downstream to McManamy Road.

Role / Services:  WSE conducted a site inspection, met with County officials, and completed detailed hydrologic, hydraulic, scour/erosion, and geomorphic assessments to evaluate the performance of the proposed crossing and to determine if the proposed WSDOT SR97 changes would impact the McManamy Road crossing.

Outcome:  WSE demonstrated that both the 60- and 70-foot crossings would satisfy current WDFW habitat and WSDOT hydraulic design requirements.   Explanations also concluded that stream conditions were unlikely to change at the McManamy Road crossing in response to SR97 proposed changes.