Beach morphology and grain-size changes over short timescales at the Elwha River Delta, WA
Simans, Kevin J.
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The Elwha River dam removal project is the largest such project carried out in the U.S., and offers a unique opportunity to study the effects of a sudden sediment flux from the river to the coastal environment. A huge amount of new littoral sediment has accumulated at the river mouth; however, it is unknown whether it will migrate eastward and accrete on the chronically eroding beaches, and on what timescales this will occur. In this study, grain-size data and beach elevation profiles were collected along three shore- perpendicular transects east of the river mouth in early March, and again for three consecutive days in mid-April, with the goal of observing short-term changes. Physical sediment samples were collected for grain-size analysis, CobbleCam autocorrelation was used to obtain mean grain size from digital photographs, and beach profiles were constructed for these periods of spring tides and large waves. Results indicate that the upper foreshore of the transect farthest from the river mouth (Line 198) lost up to 0.38 m of elevation between the two study periods, and the lower foreshore gained up to 0.41 m elevation. In addition, slope breaks on all transects migrated cross-shore, and foreshore sediments coarsened, both temporally (during the three-day April sampling period) and spatially (down the foreshore). These results demonstrate that daily changes can occur on the Elwha beaches. This study adds to our understanding of how new sediments will influence this region, and possibly of mixed grain-size beach dynamics in general.