HR: 13:40h
AN: H33L-01 INVITED    [Abstracts]
TI: Feedback Limiting the Coastal Response to Irregularities in Shelf Bathymetry
AU: * List, J H
EM: jlist@usgs.gov
AF: U.S. Geological Survey, 384 Woods Hole Rd., Woods Hole, MA 02543, United States
AU: Benedet, L
EM: lbenedet@coastalplanning.net
AF: Coastal Planning and Engineering, 2481 NW Boca Raton Boulevard, Boca Raton, FL 33431, United States
AB: Observations and engineering studies have shown that non-uniform inner shelf bathymetry can influence longshore sediment transport gradients and create patterns of shoreline change. One classic example is from Grand Isle, Louisiana, where two offshore borrow pits caused two zones of shoreline accretion landward of the pits. In addition to anthropogenic cases, many natural situations exist in which irregularities in coastal planform are thought to result from offshore shoals or depressions. Recent studies using the hydrodynamic model Delft3D have successfully simulated the observed nearshore erosion and accretion patterns landward of an inner shelf borrow pit. An analysis of the momentum balance in a steady-state simulation has demonstrated that both alongshore pressure gradients (due to alongshore variations in wave setup) and radiation stress gradients (terms relevant to alongshore forcing) are important for forcing the initial pattern of nearshore sedimentation in response to the borrow pit. The response of the coast to non-uniform inner shelf bathymetry appears to be limited, however, because observed shoreline undulations are often rather subtle. (An exception may exist in the case of a very high angle wave climate.) Therefore, feedbacks in processes must exist such that growth of the shoreline salient itself modifies the transport processes in a way that limits further growth (assuming the perturbation in inner shelf bathymetry itself remains unchanged). Examination of the Delft3D momentum balance for an inner shelf pit test case demonstrates that after a certain degree of morphologic development the forcing associated with the well-known shoreline smoothing process (a.k.a., diffusion) counteracts the forcing associated with the inner shelf pit, producing a negative feedback which arrests further growth of the shoreline salient. These results provide insights into the physical processes that control shoreline changes behind inner shelf bathymetric anomalies (i.e. man-made dredge pits and natural bathymetric features) and are therefore relevant to the understanding and prediction of shoreline change on many coasts.
DE: 3020 Littoral processes
SC: Hydrology [H]
MN: 2007 Fall Meeting