Ocean surface wave transformation over a sandy sea bed

Abstract

Projecting naval forces in littoral areas requires a thorough understanding of the environmental processes that take place in those areas, especially ocean wave evolution, and the associated surf and wave-driven currents. The transformation of wave spectra in coastal environments is predicted with numerical models that include the effects of refraction, nonlinear wave-wave interactions, and parameterizations of wave breaking and bottom friction. This thesis presents a comparison between a new field data set and model predictions of wave transformation in shallow water. An array of 16 wave-measuring instruments was deployed outside the surf zone on the sandy sea bed of Martha's Vineyard's inner continental shelf in the fall of 2007. Data from these instruments are analyzed and the performance of the spectral wave prediction model SWAN (Simulating WAves Nearshore) is tested against the observations. The observations generally show gradual wave decay towards the shore with a reduction of as much as 15% of the incident wave height across only 4 km of continental shelf. Wave height variability is observed in both the crossshore and the alongshore directions, suggesting that the effect of bottom processes on wave energy is twodimensional. Comparisons of these observations with SWAN model predictions show that both bottom friction and refraction play a dominant role in the wave energy transformation outside the surf zone. Overall, the spectral wave decay is handled well by SWAN with any of the bottom friction parameterizations activated, including the widely used JONSWAP (Hasselmann et al., 1973) empirical parameterization. Deactivating bottom friction in SWAN yields a slight overprediction of nearshore wave heights.