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Sediment Dynamics and Morphodynamics of River-Sea Sediment Dispersal Systems through Space and Time: A Source-to-Sink Perspective
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River Delta Response to a Massive Increase in Sediment Supply: The Influences of Sediment Supply, Waves and Tidal Currents (Invited)
Tuesday 8th @ 1330-1350, Conference Room 1
Jonathan Warrick* , U.S. Geological Survey
Presenter Email: jwarrick@usgs.gov |
| The geomorphic responses of depositional landforms, such as river deltas, to changes in sediment supply are a fundamental interest of geoscience research. Yet, field-based observations of deltas before, during and after changes in sediment supply are unusual owing to the challenges of forecasting these events and collecting pertinent time-series data. Here we provide field-based observations of the geomorphic response of a coastal delta to a massive increase in sediment supply to better understand delta morphodynamics and the controls of sediment supply, waves and tidal currents on sediment transport and morphology. In response to the 2011�014 removal of two dams on the Elwha River, Washington, USA, river sediment supply increased ~100-fold, resulting in ~19 Mt of sediment discharge to the coast during the first 5 yr following the removal project. Using semiannual and annual topographic and bathymetric surveys, aerial photographs collected approximately monthly, and multi-year oceanographic time-series of waves and currents, we document how the increases in supply changed the sediment budget of the delta from net erosional to net depositional. Additionally, we document the alteration of the coastal geomorphology in response to this supply, including the emplacement of recently discharged sediment in river mouth bars, and the subsequent across- and along-shore movement of this sediment around the delta as waves and currents acted upon it. These data suggest that the morphology of the intertidal portion of the delta, which consisted of mixed sand and gravel sediment, was dominated by fluvial sediment supply and subsequent reworking by waves. Sediment dispersed farther offshore was subjected to strong, spatially variable tidal currents capable of transporting sand, pebbles, and cobbles with kelp holdfasts. This suggests that different processes can dominate the sediment transport regime and morphology of a delta depending on location and sediment grain size.
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