Sediment Dynamics and Coastal Response to Large-Scale Dam Removal: Elwha River, USA

Two dams on the Elwha River, Washington State, USA trapped over 25 million m3 of mud, sand, and gravel since the early 1900s and contributed to erosion of the delta protruding into the Strait of Juan de Fuca. The removal of the Elwha and Glines Canyon dams, initiated in September 2011, is providing an unprecedented opportunity to examine the geomorphic response of a coastal delta to massive changes in river sediment supply. Observations of beach topography and nearshore bathymetry of the Elwha River delta were collected once or twice a year prior to and during dam removal using RTK GPS units on backpacks and personal watercraft equipped with single-beam echosounders. Surface sediments were collected with a grab sampler at elevations between -9 and -1 m around the delta. In addition, a hydrodynamic and sediment transport model was developed and validated for the Elwha river delta. Numerical model simulations of the initial release of sediment following dam removal are used to understand the processes responsible for the observed deposition. A comparison of the surveys conducted prior to and during dam removal shows large changes in the morphology and sediment grain size of the river mouth and submarine delta. There was widespread sediment accumulation at elevations between -12 and +2 m with a net accumulation of roughly 2,500,000 m3. The primary area of deposition was adjacent to the river mouth and covered approximately 6.27 ha (627,000 m2) with an average thickness of 3.3 m and a maximum thickness of 10 m. The surface sediment in this depositional area is coarse (primarily sand and gravel) with medium to fine sand to the east and mud to the west. Model results show that spatial variability in bottom stress and strong residual flow around the delta contribute to this complex pattern of sediment dispersal and deposition