Search

Alongshore currents and nearshore dynamics generated on plane and barred beaches are successfully simulated using a shallow water model, ADCIRC, whose traditional applications have been focused on shelf-scale processes. ADCIRC-2DDI is a two-dimensional, finite-element-based coastal circulation model. Wave fields obtained from REF/DIFI, a monochromatic wave model that simulates refraction, diffraction, and wave breaking, are used to compute surface wave radiation stress gradients that force the hydrodynamic model. Processes such as advection, lateral diffusion/dispersion, and nonlinear bottom stress are examined at high resolution (^5 m) in order to determine their relative influence on the computed nearshore circulation. Unsteady circulation patterns are observed in our plane beach simulations for decreased values of the nonlinear bottom friction and/or lateral mixing coefficients. The structure and dynamics of these unsteady circulation patterns are very similar to shear waves described by Bowen and Holman (1989). Model-data comparisons made using data from Leadbetter, California (1980) and the 1990 DELILAH experiment at Duck, North Carolina, indicate that alongshore currents forced by wave radiation stress gradients are well simulated by the ADCIRC-2DDI model., The original document contains color images. All DTIC reproductions will be in black and white. Prepared in cooperation with Sverdrup Technologies, Inc., Stennis Space Center, MS.

The cross-shore structure of the mean longshore current on a barred beach is investigated with observations from the Duck94 field experiment. Maxima of the hourly-averaged longshore current are most frequently located either slightly inshore of the bar crest or near the shoreline. At low tide the longshore current maxima are located closer to the bar crest, and the current is stronger and narrower than at high tide. The tidal cross-shore displacement of the longshore current maximum is qualitatively consistent with the observed radiation stress, S(xy), although the maximum currents are displaced shoreward of the maximum S(xy) gradients at both high and low tide. This spatial lag suggests that some mechanism (such as wave rollers) delays the transfer of momentum from waves to the mean flow. Bathymetric longshore inhomogeneities may also affect the cross-shore structure of the longshore current., Published in Proceedings of the International Coastal Engineering Conference (25th), held in Orlando, FL on 2-6 Sep 1996. The original document contains color images.

Previous investigations of longshore currents have included simplifying assumptions and restriction (such as a planar beach, a steady and depth uniform flow, spatially-variant bed shear stress and turbulent momentum exchange, and the exclusion of surface wind stress. These assumptions are quantitatively investigated by calculating the relative importance of each term in the longshore momentum balance with an emphasis on the relative importance of wind forcing across the barred nearshore. Wind and wave forcing of longshore currents across a barred beach are examined using both a numerical model and field measurements. A local momentum balance was measured at various locations across the surf zone during the SUPERDUCK experiment held at the USACE CERC field Research Facility, Duck, N.C. in October 1986. A moveable sled was instrumented with pressure, current, and wind sensors to measure the various terms in the longshore momentum equation. Stability-dependent atmospheric drag coefficients for the surf zone are determined from wind stress measurements acquired just beyond the surf zone and wind speed measurements acquired from an anemometer atop the 9 m sled mast. Breaking waves were visually identified and electronically marked on the data tapes.

A model forecasting wave height and longshore current distribution inside the surf zone is applied to an extensive set of laboratory and field data for testing and modification. The models are tested on planar beaches as well as barred beaches for a variety of wave conditions. The wave transformation model is based on solving the energy flux equation using a bore dissipation mechanism and describing the random wave heights with the Rayleigh distribution. The two model parameters, B and gamma, where B describes the amount of foam of a breaking wave and gamma is the proportionality constant which relates the rms wave height to the water depth, are combined into a single parameter BG. BG is shown to be a function of deep water surf similarity parameter. Applied to the present data sets, the rms error of the measured wave height and the model predicted wave height was usually less than 9% and ranged from 1.5% to 15.7% with a mean of 5.3% and a standard deviation of 3.1% for the whole 74 data sets. The wave transformation model is highly robust in describing the wave height distribution in the surf zone. The longshore current model is based on solving the steady state, alongshore momentum balance for straight and parallel contours using the radiation stress concept. The model requires specifying the bed shear stress and turbulent mixing coefficients. Applied to the present data sets the rms error between the measured and modeled longshore current values ranged from 4.5% to 55.5% with a mean of 24.6%. Turbulent mixing is not required for planar beaches, but is required for barred beaches to describe the longshore currents inside the surf zone. Theses.