CHANGES IN SEDIMENT TRANSPORT IN RESPONSE TO WATER CROSSING STRUCTURES AND CLIMATE CHANGE IN THE CHEHALIS RIVER BASIN, WA

The Chehalis basin in southwest Washington State has approximately 2,400 documented culverts that are barriers to aquatic organism movement. Due to climate change, peak stream flows are projected to increase in the Chehalis Basin within the service life of most new culverts. Therefore, there is a need to understand the performance of stream simulation culverts under these predicted changes. This study expanded upon ongoing work investigating the performance of stream simulation culverts under a range of hydrologic and channel conditions throughout the Chehalis. We used Delft3D to simulate streambed responses upstream, within, and downstream of a hypothetical culvert channel based on general characteristics of Comfort Creek (bankfull width and flow of 18.5 ft and 120 cfs), a tributary to the Satsop River in the greater Chehalis basin. Our goals were to assess the streambed response to various stream characteristics and simulation culvert designs under current and future climate conditions. We used simulated flows representative of current conditions ranging from 13 (critical flow for D50 sediment size) to 187 (Q10) cfs. Several culvert scenarios were considered, including: no road, narrow culvert (0.8*BFW), and wide culvert (1.2*BFW). Based on projected modeled data (for the 2080s) from the University of Washington’s Climate Impacts Group, simulated future flows were approximately 20% larger than current flows across the annual hydrograph. Results of modeling scenarios are discussed.