Improvement and Validation of the Navy Longshore Current Model

The Navy Standard Surf Model, SURF 3.0, is known to have shortcomings in longshore current prediction. Currents are often found to be too weak over most of the surf zone and too high in very shallow water. Both bottom friction and horizontal eddy diffusion formulations in SURF 3.0 are carefully examined and modified. Instead of using a constant friction throughout the surf zone, a new bottom friction function is developed based on water depth and surf zone width. Data acquired during the 1990 Duck Experiment on Low-frequency and Incident-band Longshore and Across-shore Hydrodynamics (DELILAH) and the 1994 DUCK94 experiment are used to derive the optimal empirical constants for the bottom friction and eddy diffusion functions. The implementation of a new bottom-friction function and eddy viscosity coefficient significantly improves the magnitude and location of the maximum current, and greatly reduces the unrealistic high current near the water edge. The new model also produces lower root- mean-square errors. The general accuracy of resulting longshore current estimates is further verified using the 1980 data set from National Sediment Transport Experiment (NSTS), conducted at Leadbetter Beach, Santa Barbara, California. The improved model constitutes significant progress in longshore current prediction and will be used to upgrade the SURF 3.0 model.