Observations on cohesive bed reworking by waves: Atchafalaya Shelf, Louisiana

Abstract

Wave, current, acoustic backscatter and suspended sediment concentration measurements (both single‐point and vertical profiles estimated by conversion of acoustic backscatter data) are used to investigate wave‐current‐cohesive sediment interaction on the muddy Atchafalaya inner shelf. During an energetic storm, we propose that bed state follows a cycle of dilation due to fluidization, erosion, deposition with fluid mud formation and consolidation. A one‐dimensional‐vertical cohesive sediment transport model is calibrated using current and concentration profiles to estimate the physical parameters that could not be measured directly, e.g., bottom stresses. Estimated bed position and computed bottom stresses suggest that the critical erosion threshold is in the range of 0.3 Pa to 0.5 Pa. The study site is impacted by a sediment‐laden fresh water plume coming from the Atchafalaya River mouth. Bed density evolution during the storm is estimated from vertical sediment exchange between the water column and the bed excluding the duration of passage of a sediment‐carrying water front. The values are in the range of 1,030 kg/m3 to 1,200 kg/m3and indicate that the bed density increases during the erosion phase and decreases during deposition. At the end of the storm, it shows a steady increasing trend during hindered settling and exceeds the space‐filling value during consolidation. Both the critical erosion shear stress and bed density values are consistent with the results of laboratory tests on samples from the experimental site.