The transport of coarse sediment (sand and gravel) in mixed grain-size coastal systems is poorly understood and difficult to predict without large uncertainties. Accelerating coastal development, sea-level rise, and other perturbations in coastal environments motivates an improved understanding of the dynamics of mixed grain-size systems. Understanding how various factors, including waves, water level and sediment supply, combine to change the morphology of shorelines over various timescales will permit improved management of coastal habitats and morphologic features.
The removal of two dams on the Elwha River, Washington State provides an opportunity to evaluate coastal sediment dynamics in the context of a known future perturbation, the re-establishment of a "natural" sediment flow regime, on a mixed grain-size coastal delta. Sediment transport patterns in the intertidal and nearshore sub-tidal regions around the Elwha River delta were characterized using a combination of direct field observations of sediment movement and analysis of changing topography. Results were interpreted in the context of wave and river flow observations to better understand how the mixed grain-size delta responds to forcing conditions in the dammed state.
Coarse clast samples on the mixed beach of the Elwha delta were found to move with average velocities of up to 100 m/d, with the magnitude of the velocity related to wave energy flux, and the direction of transport related to the direction of the alongshore component of the wave energy flux. Fractional transport of clasts by size was only observed during lower wave energies; at higher wave energies there was no clear relationship between transport and clast size. The cross-shore magnitude of alongshore intertidal sediment transport was highest between 1.0 and 2.0 m above Mean Lower Low Water, which coincides approximately with the zone of swash impact on this beach. Depths of bed disturbance by waves averaged 22% of the wave height and always exceeded the magnitude of vertical profile change over single tidal cycles. Volumetric transport rates were found to vary between 1.1 and 173.1 m³/d under significant waves with heights between 0.1 and 2.1 m, suggesting gross sediment transport rates of ∼4000 m³/m/yr.
Patterns of morphologic change suggest that beach sediment divergence accounts for only a small fraction of the total volume lost from the delta on an annually-averaged basis. Net accretion of the Elwha River mouth averaged ∼15,000 m³/yr between 2007 and 2011, even as the intertidal beach to the east of the river lost ∼5,000 m³/yr over the same time period. Net volumetric river mouth change is closely linked to river flow. Our results suggest that, across the entire delta, ∼31,000 m³/yr of mixed sediment is exported from the sub-tidal zone, probably due to wave-driven sediment transport in the shallow sub-tidal zone.
|School:||University of California, Santa Cruz|
|School Location:||United States -- California|
|Source:||DAI-B 73/06, Dissertation Abstracts International|
|Subjects:||Geomorphology, Marine Geology, Sedimentary Geology|
|Keywords:||Alongshore transport, Dammed Elwha River Delta, Mixed beach morphodynamics, Sediment flux, Shoreline|
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