The Influence of Fine-Grained Sediment on Wave Propagation, West Louisiana Coast

By

SHEREMET, A., STONE, G.W., BENTLEY, S.J., WALKER, N.D., ROUSE, L.J., JR., WELSH, S.,

ROBERTS, H.H., INOUE, M., HUH, O.K., and HSU, S.A.

Coastal Studies Institute and Department of Oceanography and Coastal Sciences, Louisiana State University, Howe-Russell Geoscience Complex, Baton Rouge, LA

Wave fields associated with cold fronts passing through the Northern Gulf of Mexico are investigated using measurements collected at two WAVCIS observation stations: CSI 3, fronting Atchafalaya Bay, located in a cohesive (muddy) sedimentary environment, and CSI 5, seaward of Terrebonne Bay, located in a sandy environment. The two sites are otherwise exposed to similar ocean and atmospheric conditions. Despite almost identical atmospheric forcing, the wave field response observed in the muddy environment differs markedly from that of the sandy environment. At CSI 3 (muddy) storm waves have mean periods approximately 1 sec shorter, on average, than at CSI 5. Significant wave heights over 2.0 m are common at CSI 5, while at CSI 3 waves rarely exceed 1.5 m in height. Spectral analysis shows that storms are dominated by two types of wave systems. Locally forced, high frequency wave systems of comparable energy develop at both locations, but are quickly dampened in the muddy environment. Lower frequency waves that develop during the storm and propagate across the shelf are consistently less energetic at CSI 3 by about an order of magnitude. Strong attenuation of low frequencies is likely due to wave interaction with the soft muddy bottom; high frequency wave attenuation could be a result of increased viscosity due to sediment resuspension. Preliminary numerical simulations based on SWAN (Booij et al., 1999) produce similar wave spectra at the two sites, indicating that alternative dissipation mechanisms (e. g. 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 storms, using both observations and numerical simulations, and the development of a mathematical formulation for the wave-bottom interaction in cohesive sedimentary environments.

Booij, N., R.C. Ris, and L.H. Holthuijsen, 1999: A third generation wave model for coastal regions: Part I: model description and validation, JGR, V104, 7649-7666.