Model predictions of nearshore processes near complex bathymetry

Abstract

Waves undergo significant transformation over complex bathymetry, and the resulting nearshore wave conditions can be sensitive to small changes in the offshore wave forcing. A potential consequence of this transformation sensitivity is large uncertainties in modeled nearshore waves owing to the amplification of the error in the deep water spectra used as initial conditions. In preparation for the upcoming Nearshore Canyon Wave Experiment in La Jolla, CA, a boundary condition sensitivity analysis was performed over the region's submarine canyon bathymetry using the SWAN wave model. The sensitivity analysis included varying the offshore spectrum discretization (frequency and directional bandwidths), the peak period and direction of the spectra, and the frequency and directional spreads. In each case, the magnitude of the spectral variations was governed by expected uncertainties when initializing a nearshore model with a) typical buoy data for the area, and b) global WAM model hindcasts or forecasts. In addition, data from the Torrey Pines Outer Buoy (located 12 km offshore) from the first week of November 2001 were used to initialize the model, and the maximum change seen in the domain over the course of the week were compared to those derived from the sensitivity analysis. The nearshore locations that showed the largest change in wave height over time were also the areas most sensitive to boundary condition errors, and correspond to areas of wave focusing. Errors in the estimation of the peak offshore wave direction were found to have the greatest impact on the accuracy of the nearshore wave predictions. The coarse directional resolution (15 degrees) of deep water spectra provided by the present generation of operational global models is shown to be a significant source of error when handcasting or forecasting nearshore waves over complex bathymetry.