Interaction of bottom turbulence and cohesive sediment on the muddy Atchafalaya shelf, Louisiana, USA

Interaction of near-bed wave-induced turbulence and cohesive sediments in muddy environments is studied based on field observations and a bottom boundary layer model. Wave, current, and sediment observations were collected with a suite of acoustic and optical instrumentation at approximately 5-m depth on the muddy Atchafalaya clinoform, Louisiana, USA. Low wave-bias estimates of near-bed Reynolds stresses are obtained by a method that is based on differencing and filtering of velocities from two sensors. The event that is focused on in this study is characterized by moderate waves with high steepness, currents with speeds sometimes reaching 30 cm/s near bed, and Reynolds stresses and suspended sediment concentrations reaching to their maximum values throughout the experiment (0.4 Pa, 3 g/L). In general, Reynolds stresses are found to be correlated with short-wave near-bed accelerations and suspended sediment concentration, as previously observed on sandy beaches, where accelerations have been associated with bed fluidization and sediment transport.;A detailed numerical analysis of the observations is performed with a one-dimensional bottom boundary layer model for small scale turbulence and sediment transport processes on cohesive beds. The model accounts for the coupling between the fluid and the cohesive sediment phases, and uses a floc size that is constant in time and space. A representative floc size is selected for the experiment site, based on two independent sources that show consistency. Direct estimates of size distribution of suspended sediments in the vicinity of the experiment site show a remarkably stable floc mode peak under varying wave and turbulence conditions. Indirect estimates of equilibrium floc size are obtained through calculations of an analytical flocculation model that uses observation-based parameters. With a floc size input based on the observations, the model reproduces currents and suspended sediment concentrations accurately; modeled Reynolds stresses match the low wave-bias estimates, with better agreement for cases of stronger currents and smaller wave-orbital velocities. The numerical simulations suggest that sediment-induced stratification effects are the same order of magnitude as turbulent dissipation, and thus play a significant role in the turbulent kinetic energy (TKE) balance within the tidal boundary layer. However, inside the wave boundary layer, the ratio of stratification to shear-induced turbulence production (i.e., gradient Richardson number) decreases significantly; shear-induced turbulence production and turbulent dissipation dominate the TKE balance. For these observations, model results show that the vertical structures of currents and Reynolds stresses are relatively insensitive to the exact floc size.;Future efforts should include analysis of wider range of conditions (especially events with higher near-bed concentrations), and comparison of model results with a more detailed vertical structure of suspended sediment concentration.