Observations of wave dissipation over muddy sea beds.
Sheremet, A. and G.W. Stone
Measurements collected at two coastal observing stations lovated on the inner shelf are used to investigate the effects of heterogenous sediments on the propagation and dissipation of wave fields associated with cold fronts passing over the North Gulf of Mexico. The stations used are CSI 3, fronting Atchafalaya Bay, located in cohesive (muddy) sedimentary environments, and CSI 5, fronting Terrebonne Bay, located in a sandy inner shelf environment. Despite almost identical atmospheric forcing at the two sites, the wave field response observed in the middy environment.
At CSI 3 (muddy), storm waves have mean periods about 1 s shorter, on average, than at CSI 5. Significant wave heights over 2.0 m are common at CSI 5 , while at CSI 3 rarely exceed 1.5 m. Spectral analysis shows that two different wave systems dominate at two locations, one composed primarily of short frequency waves (sea) and the other of relatively long waves (swell). Figure 2a-b shows a typical spectral evoution sequence recorded over six days in February 2001. Wind velocity and significant wave height are given for reference in Figure 2c-d. The sea and swell spectral bands are seperated here by the (arbitrary) frequency value of 0.2 Hz. Swell dominates at CSI 5, while being essentially absent at CSI 3 (order of magnitude smaller in variance than at CSI 5). The strong attenuation of swell in the muddy environment is consistent with a strong interaction between long waves and the soft muddy bottom and has been observed before, although no comparisons with sandy environments have been reported. Rather than propagating across the shelf, seas are locally forced by wind and do not penetrate deep enough to interact strongly with the bottom. One would expect similar energy levels in the high frequency band, but observations show that although wave systems of comparable energy develop at both lovations, the waves dissipate almost completely as soon as the wind drops. Sea attentuation could be a result of increased viscosity due to sediment resuspension. Preliminary numerical simulations based on SWAN (Boij et al. 1999) produce similar wave spectra at the two sites, indicating that alternative dissipation mechanisms (eg. refractive scattering, depth limited breaking, described well by the model), do not explain the observations. Ongoing work focuses on spatial structure and evolution of the muddy bed during storm, using both observation and numerical simulations, and the development of a mathematical formulation for the wave- bottom interaction in cohesive sedimentary environments.