[1] Near-bottom fluid-mud layers were observed during two experiments conducted on the muddy Atchafalaya inner shelf (subaqueous clinoform), Louisiana, United States. On the face of the subaqueous delta (4-7 m water depth, first experiment) fluid-mud layers are produced by seafloor liquefaction and resuspension forced by swells associated with cold front passages, and supported by near-bed wave-induced turbulence. The layers are episodic (lifetime of 9-12 h), form prior to significant postfrontal settling of sediment in the overlying water column, and flow seaward (downslope) at about 5 cm/s. Farther westward on the delta front (finer grain size, with negligible sand or coarse silt content, second experiment), similar wave-supported fluid-mud layers are observed to last longer (>2 days), show weaker alongshore (westward) flow of about 1-3 cm/s. The results suggest a sequence of near-bed sediment transport processes, triggered by frontal swell activity (bed liquefaction, resuspension and advection, modulated by the bathymetric characteristics of the clinoform) that contribute to the formation of clinoform stratigraphy of muddy subaqueous deltas.

[1] Observations of wave and sediment processes collected at two locations on the Atchafalaya inner shelf show that wave dissipation in shallow, muddy environments is strongly coupled to bed-sediment reworking by waves. During an energetic wave event (2 m significant wave height in 5 m water depth), acoustic backscatter records suggest that sediment in the surficial bed layer evolves from consolidated mud through liquefaction, fluid mud formation, and hindered settling to gelled, under-consolidated mud. Net swell dissipation increases steadily during the storm from negligible prestorm values, consistent with bed softening, but shows no correlation with detectable fluid mud layers. Remarkably, the maximum dissipation rate occurs poststorm, when no fluid mud layers are present. In the waning stage of the storm, the contribution of different wave-forcing processes to wave dissipation is analyzed using an inverse modeling approach based on a nonlinear three-wave interaction model. Although wave-mud interaction dominates dissipative processes, nonlinear three-wave interactions control the shape of the frequency distribution of the dissipation rate. In the wake of the storm, the viscosity values predicted by the inverse modeling converge toward measured values characteristic for gelled mud in a trend that is consistent with a fluid mud entering dewatering and consolidation stages.

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