Long-Term Use of Louisiana Offshore Sand Resources: Implications for Unique Sediment Dynamics Over Intermittent Sandy and Muddy Seabed on Ship Shoal for Short-Term Geomorphological Changes, South-Central Louisiana

Muddy coasts of Louisiana have provided unique hydrodynamic features associated with cohesive sediments and various storms. Ship Shoal, a Holocene transgressive shoal, has been considered a potential sand resource for restoration of rapidly eroding Louisiana barrier islands and beaches and various studies regarding the geology and physical oceanography have been conducted since the 1980s.

Our collaborative study, initiated in 2005, has found that the shoal has provided a valuable habitat for benthic organisms and unique sediment dynamics, both of which have not previously been recognized in the scientific literature. Results from our physical oceanography study show that sediment dispersal from the Atchafalaya River shifts from prevailing west to southeast during post-frontal phases, which results in deposition of a thin layer of fluid mud and consequent sediment heterogeneity on the shoal. Results obtained from the bottom boundary layer arrays deployed in spring 2006 allowed for the identification of up to 30 centimeters of fluid mud; this fluid mud layer strongly interacted with storm waves and currents. While, results obtained from the arrays deployed in winter and spring 2008 show bottom sediments on the shoal comprising of non-cohesive materials. This provides an excellent comparison of hydrodynamics and sediment transport compared to those that occurred during a period in 2006 when the shoal bottom was composed of cohesive sediments.

The above findings suggest that Ship Shoal appears to have recurring sandy and muddy bottoms given the balance between storm-induced sediment reworking and fluvially-derived sediment supply. Our results also suggest that the blanket of fluid mud on the shoal may be patchy and may not remain in place long enough to become permanently consolidated mud, given the frequency of winter storms and dispersal shifts. However, sediment dynamics over the shoal likely contribute to short-term shoal geomorphologic changes and shoal physical and biological environments. >