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Observations of coarse sediment movements on the mixed beach of the Elwha Delta, Washington

January 1, 2011

Mixed beaches, with poorly sorted grains of multiple sizes, are a common and globally distributed shoreline type. Despite this, rates and mechanisms of sediment transport on mixed beaches are poorly understood. A series of tracer deployments using native clasts implanted with Radio Frequency Identifier (RFID) tags was used to develop a better understanding of sediment transport directions and magnitudes on the mixed grain-size beach of the Elwha River delta. Using tracer samples selected to match the distribution of the coarse fraction on the beach we find that all grain sizes, up to large cobbles (128–256 mm), were mobile under most measured wave conditions and move in relationship to the direction of the alongshore component of wave energy as estimated by incident breaking wave angles. In locations where the breaking wave is normal to the shoreline we find that tracers move in both alongshore directions with approximately equal frequency. In locations where breaking waves are oblique to the shoreline we find that alongshore transport is more unidirectional and tracers can approach average velocities of 100 m/day under winter wave conditions. We use the tracer cloud to estimate the beach active width, the mobile layer depth and sediment velocity. Our results suggest that, while sediment velocity increases under increased incident wave angles, the active layer depth and width decrease, reducing sediment flux at the site with the more oblique breaking waves. This result is contrary to what is suggested by traditional wave energy transport models of alongshore sediment transport.

Publication Year 2011
Title Observations of coarse sediment movements on the mixed beach of the Elwha Delta, Washington
DOI 10.1016/j.margeo.2011.02.012
Authors I.M. Miller, Jonathan Warrick, C. Morgan
Publication Type Article
Publication Subtype Journal Article
Series Title Marine Geology
Index ID 70034877
Record Source USGS Publications Warehouse
USGS Organization Pacific Coastal and Marine Science Center