HR: 0800h
AN: OS21B-1229    [Abstracts]
TI: Bathymetric Changes Shaped by Longshore Currents on a Natural Beach
AU: * Reilly, W L
EM: reilly@coastal.ufl.edu
AF: University of Florida, Department of Civil and Coastal Engineering, Gainesville, FL 32611-6590 United States
AU: Slinn, D
EM: slinn@coastal.ufl.edu
AF: University of Florida, Department of Civil and Coastal Engineering, Gainesville, FL 32611-6590 United States
AU: Plant, N
EM: nplant@nrlssc.navy.mil
AF: Naval Research Laboratory, Code 7440.3 Building 1005, Stennis Space Center, MS 39529 United States
AB: The goal of the project is to simulate beach morphology on time scales of hours to days. Our approach is to develop finite difference solutions from a coupled modeling system consisting of existing nearshore circulation, wave, and sediment flux models. We initialize the model with bathymetry from a dense data set north of the pier at the Field Research Facility (FRF) in Duck, NC. We integrate the model system forward in time and compare the results of the hind-cast of the beach evolution with the field observations. The model domain extends 1000 meters in the alongshore direction and 500 meters in the cross-shore direction with 5 meter grid spacing. The bathymetry is interpolated and filtered from CRAB transects. A second-degree exponential smoothing method is used to return the cross-shore beach profile near the edges of the modeled domain back to the mean alongshore profile, because the circulation model implements periodic boundary conditions in the alongshore direction. The offshore wave height and direction are taken from the 8-meter bipod at the FRF and input to the wave-model, SWAN (Spectral Wave Nearshore), with a Gaussian-shaped frequency spectrum and a directional spreading of 5 degrees. A constant depth induced wave breaking parameter of 0.73 is used. The resulting calculated wave induced force per unit surface area (gradient of the radiation stress) output from SWAN is used to drive the currents in the circulation model. The circulation model is based on the free-surface non-linear shallow water equations and uses the fourth order compact scheme to calculate spatial derivatives and a third order Adams-Bashforth time discretization scheme. Free slip, symmetry boundary conditions are applied at both the shoreline and offshore boundaries. The time averaged sediment flux is calculated at each location after one hour of circulation. The sediment flux model is based on the approach of Bagnold and includes approximations for both bed-load and suspended load. The bathymetry is then updated by computing the divergence of the time averaged sediment fluxes. The process is then repeated using the updated bathymetry in both SWAN and the circulation model. The cycle continues for a simulation of 10 hours. The results of bathymetric change vary for different time-dependent wave conditions and initial bathymetric profiles. Typical results indicate that for wave heights on the order of one meter, shoreline advancement and sandbar evolution is observed on the order of tens of centimeters.
DE: 4255 Numerical modeling
DE: 4532 General circulation
DE: 4546 Nearshore processes
DE: 4558 Sediment transport
SC: Ocean Sciences [OS]
MN: 2004 AGU Fall Meeting