Impacts of physical processes and sediment sources on the grain-size distribution of the Elwha River delta

Abstract

Grain-size distribution, one of important parameters reflecting sediment sources, transportation, and deposition, helps people understand and predict shoreline and bathymetric changes. For this study, wave-orbital velocity and tidal bed shear stress were compared to grain-size distribution of the Elwha River delta in order to find correlations between physical processes and the seabed composition. Grain-size distributions of sediments from the river, the bluff, and the delta were also examined to determine sources of sediments from the delta. The sand-dominant region found at the west side of the river mouth and some local landward coarsening patterns suggested that grain-size distribution was under influences of waves. Though no strong correlation was found between tidal bed shear stress and grain-size distribution, other evidence was provided that suggest the impacts of tidal-induced currents on grain-size distribution. We found the poorly sorted region at the western offshore of the river mouth and the coarse-grains dominant regions near the river mouth; it is possibly due to focused and strong currents that causes high bed shear stress in those regions. Moreover, tidal-induced currents have a net northeastward flow which brings fine sediments toward the east. Therefore, we observed a region of fine sediments at the east of the river mouth. Last, our results suggested that the bluff in Freshwater Bay supplies the delta with coarse sediments and the river mainly provides fine sediments.