HR: 16:15h
AN: H34C-02    [Abstracts]
TI: Modeling Shoreline Erosion and Channel Migration in a Highly Dynamic Bay Entrance: Willapa Bay, WA
AU: * Gelfenbaum, G
EM: ggelfenbaum@usgs.gov
AF: USGS, 345 Middlefield Rd, MS999, Menlo Park, CA 94025 United States
AU: Lesser, G
EM: glesser@usgs.gov
AF: Delft Hydraulics, 345 Middlefield Rd, MS999, Menlo Park, CA 94025 United States
AU: Kaminsky, G
EM: gkam461@ecy.wa.gov
AF: WA Dept of Ecology, PO Box 47690, Olympia, WA 98504 United States
AU: Landerman, L
EM: llanderman@usgs.gov
AF: USGS, 345 Middlefield Rd, MS999, Menlo Park, CA 94025 United States
AB: Shoreline erosion rates of up to 50 m/yr are well documented for the northern shoreline of Willapa Bay, a large estuary on the Washington coast. Despite this knowledge, there is not a good understanding of the large-scale morphodynamics of the bay entrance. Historical shoreline change calculated from T-sheets dating back to 1871 and aerial photographs from 1942 to 2002 show a decreasing trend in erosion rates. Historical bathymetric maps document migration of the 25-m deep main channel that parallels the shoreline erosion history during that same time period. To better understand the processes responsible for these decreasing channel migration and shoreline erosion rates and to help predict future erosion we developed a process-based morphological model of the bay and adjacent coast. A process-based numerical model of flow, waves, sediment transport, and bed-level updating (Delft3D) is used to understand and predict the morphological response of the estuary entrance to tidal and storm forcing. In order to conduct long-term morphological simulations (up to 5 yrs long), we employed a number of input reduction techniques, including calculating a representative tide, discretizing the wave climate into 19 classes, and employing a variable morphological factor to scale-up from a processes-based time scale to the morphological response. Model validation with field data occurs at two time scales. Model results are compared with measurements of water levels, currents, waves, and suspended-sediment concentration collected over 3 months from November 2002 to January 2003. In addition, net erosion and sedimentation from multi-year hindcasts are compared with measured bathymetric changes. Numerical simulations suggest a net export of sediment through the deep channel and a net import across the shallow entrance shoal. Wave breaking over the shoal induces a net circulation that drives this residual sediment transport. Residual transport patterns are also strongly controlled by the initial bathymetry. During the 1940s northeasterly net sediment transport across the entrance shoal forces the northward migration of the main channel and a high rate of shoreline erosion. By the 1990s the ebb delta is elongated to the north and the net sediment transport is more northwesterly, reducing the transport of sediment into the main channel and easing the northward migration of the channel.
DE: 4546 Nearshore processes
DE: 3022 Marine sediments--processes and transport
DE: 1815 Erosion and sedimentation
SC: Hydrology [H]
MN: 2004 AGU Fall Meeting