Otak Provides Hydraulic Modeling and Design for Spurgeon Creek Fish Passage
September, 2020
Otak’s Vancouver office is working with Thurston County, Washington to finalize the construction plans for two new bridges on Spurgeon Creek to restore fish passage. The project is part of a $1,700,000 grant awarded to Thurston County to replace four culverts with an 80-foot pedestrian bridge on the Chehalis Western Trail and a 46-foot vehicular bridge on Latigo Street Southeast. The state of Washington is working on many projects to remove fish passage barriers on state, local, tribal, and private land that block salmon and steelhead access to prime spawning and rearing habitat.
In the project location, Spurgeon Creek flows under two parallel crossings, one roadway and one old railroad grade that was previously turned into a trail. The crossings bisect a large wetland complex, creating a challenge to maintain the backwatered wetlands while opening up the creek for fish passage. The project is looking to incorporate large wooden structures to create hydraulic diversity and enhance habitat within the project reach. 
Otak performed an alternatives analysis for the crossings in 2018 to apply for grant funds through the Salmon Recovery Funding Board. Upon receiving funding, design has been progressing since November 2019.
Thurston County is performing structural and roadway design in-house but hired Otak to provide wetland delineation, as well as hydraulic engineering and stream design services during the design phase. Nathan Dasler, Senior Water Resource Engineer took on the role of project manager and lead engineer for the hydraulics work. “Balancing the wetland impacts with opening up the creek for fish passage has required lots of coordination with the Washington Department of Fish and Wildlife, the Army Corps of Engineers, the County, tribes and the funding agency to make sure all agency and stakeholder needs were incorporated,” Nathan said.
To gauge the impacts of any new design, Nathan worked with Gary Wolf, senior hydraulic modeler in Otak’s Portland office, to create a two-dimensional hydraulic model utilizing SRH-2D. The model calculates how the creek will flow through the wide floodplains and new bridges and habitat features. Stream velocities and water surface elevations were utilized to design bridge scour mitigation, determine streambed sediment size and evaluate potential flooding impacts. Thurston County and Otak devised and implemented a water level monitoring plan to determine the backwater effects of the existing crossings. The gages will be left in place to provide future information on the effects of the project on water elevations upstream and downstream of the crossings.
Construction is slated to take place during the summer of 2021.