HYDROLOGICAL INVESTIGATION OF STREAM RESTORATIONS WITHIN THE INTERSTATE 90 CORRIDOR, WASHINGTON

The importance of stream restoration in providing a healthy ecosystem is widely recognized; however, many restoration projects take place in ecosystems with some level of habitat connectivity already established. During recent phases of Washington State’s improvement and expansion of the Interstate 90 Highway corridor, the Washington State Department of Transportation (WSDOT) implemented environmental changes to facilitate habitat restoration and wildlife passage. Prior to the I-90 corridor expansion, Noble and Price Creeks passed under the highway though large culverts that did not allow aquatic species or wildlife passage below the highway. To increase habitat connectivity, in 2019 the stream channel crossings under the highway were expanded, and the size and shape of these creeks were engineered to mirror what would be seen in a natural environment. Since construction, erosion has resulted in some channel reaches experiencing vertical incision, widening and isolation of the channel from the large wood installations, while other channel reaches have remained stable. Areas of channel stability vs. erosion were determined by comparing channel surveys completed prior to the start of the I-90 expansion, WSDOT as-built surveys immediately after construction, and new surveys conducted as part of this project. HEC-RAS hydraulic modeling was used to calculate the depth and flow velocity at various discharges. Sediment size within the channel and banks were compared with the maps of stable vs. eroded reaches. Together these data provide information about the possible causes of the channel erosion, as well as the utility of the engineered channels to provide effective habitat and passage of native Bull Trout and Pacific Coastal Salamander populations. The cross-section surveys above and below the engineered portions of the creeks illustrate greater variation in stream channel bedforms and velocity than in the channels directly under the highway. The more closely a restored stream can mimic a natural flow pattern, the more successfully it can be used to facilitate ecosystem and habitat connectivity for aquatic and wildlife species.