MEASUREMENTS OF THE EVOLUTION OF OCEAN WAVE SPECTRA DUE TO BOTTOM FRICTION

Data from a field experiment in which seven wave measurement instruments were placed in a Linear transect across the Great Australian Bight are analyzed to determine the spectral decay which can be attributed to bottom friction. Since other processes such as atmospheric input, nonlinear interactions, ''whitecap'' dissipation, refraction, and shoaling will also be active, the spectral wave model WAM is used as an analysis tool, explicitly taking these additional processes into account in the analysis. The data show marked spectral decay across the shelf. Analysis reveals that the bottom friction coefficient C-f is not constant as often assumed in operational wave models. The bottom friction coefficient varies approximately inversely with the wave-induced bed velocity, consistent with the predictions of previous laboratory data and eddy viscosity models. It is possible that percolation may also be important in the region, but the available data do not allow the relative contributions of bottom friction and percolation to be determined.