Sediment Transport Modeling Along the Gravel-Sand Transition Zone of the Snohomish River, WA

Abstract:

Long term sediment trapping efficiency was modeled for 1000-2000 ft long analysis segments of the tidally influenced, diked Snohomish River, upstream, along, and downstream of the ~3.5 mile long gravel-sand transition zone. A depth-averaged 2-dimensional hydrodynamic model was calibrated to a recent flood with a ~25 year recurrence interval, and the output used to extract parameters used to estimate shear stress over a range of high flows. Shear stress was estimated using (i) a roughness relation and (ii) the uniform flow approximation. Riverbed grain size distributions were estimated using pebble counts upstream, and Ponar grab samples within and downstream of the transition zone. Four different gravel and sand transport equations (Engelund-Hansen, Ackers-White, Wilcock-Crowe, and Yang) were applied to estimate transport rates at each flow level analyzed. The resulting rates were integrated over a 50 year period to compute total load entering and exiting each analysis segment to evaluate long term trends in bed elevation profiles. Results were sensitive to the choice of shear stress and bedload transport estimator with most method-dependent variation in trends apparent for segments within the transition zone.