Gail C. Kineke

Structure and dynamics of fluid muds on the Amazon Continental Shelf

Thick layers of fluid mud occur in strong tidal flows over the inner portion of the Amazon continental shelf in regions of strong salinity fronts associated with the plume discharged from the Amazon River. Detailed shipboard profile measurements obtained in this region during A Multidisciplinary Amazon Shelf Sediment Study (AMASSEDS) provide an unprecedented opportunity to examine the structure and dynamics of fluid muds under natural conditions. The analysis focuses on flows in which the motion is fully turbulent and suspended sediment dominates the stratification. Under these conditions a comparison of measurements and one-dimensional model calculations indicates that vertical transport is controlled by suppression of turbulent mixing at gradient Richardson numbers near 1/4. This constraint produces a distinctive vertical structure and leads to an upper bound on the total amount of suspended sediment that may be carried in a turbulent suspension by a tidal flow.

Observations of currents and water properties in the Amazon Frontal Zone

Measurements of currents and water properties were obtained at various locations in the frontal zone of the Amazon River outflow plume during four cruises in 1989–1991 as part of A Multidisciplinary Amazon Shelf Sediment Study. A salinity front resembling a salt wedge is found approximately 150 km seaward of the river mouth. It continues northwestward more than 400 km along the shelf between the 10- and 20-m isobaths. Tidal currents, which are oriented principally in the cross-shelf direction, reach 200 cm s−1 during spring tides and drop to less than 80 cm s−1 during neaps in the vicinity of the river mouth. The nontidal flow is strongly sheared, with near-surface speeds of up to 100 cm s−1 and weak near-bottom velocities. The direction of the near-surface flow is generally northwestward except near the river mouth, where river discharge leads to a significant offshore component of flow. The strong velocity shears are stabilized by strong stratification by salinity and by highly concentrated suspended sediment (or fluid mud) near the seabed. Variations in stratification and frontal position are due principally to spring-neap variations in tidal mixing and seasonal variations in runoff.

On modeling boundary layer and gravity‐driven fluid mud transport

[1]xa0A model for fine sediment transport processes in the fluvial and coastal environment is proposed based on a simplified two-phase formulation. This simplified approach for fine-sediment transport is essentially single-phased; however, it retains several critical mechanisms originated from the complete two-phase formulation. By incorporating closures of carrier fluid turbulence and turbulent suspension and rheology of the sediment, the model is first tested with laboratory measurements of flow velocity and concentration for fine-sand transport driven by steady currents. Next, particle diameter and density are prescribed in the model according to fractal dimensions of typical flocs for fluid mud observed on the continental shelf. The model is able to predict the dynamics of tidal-driven lutocline behavior observed on the Amazon shelf and wave-supported gravity-driven fluid mud transport measured at the Po prodelta. Model results also indicate strong interplay between flow turbulence and fluid-mud concentration, suggesting that the mechanism of sediment-induced stratification on damping the carrier fluid turbulence is crucial in determining the fluid mud behavior. Results are encouraging for incorporation in the future of more comprehensive descriptions on floc dynamics, mud-bed consolidation and various mud rheology closures.

Mechanisms of surface wave energy dissipation over a high‐concentration sediment suspension

Field observations from the spring of 2008 on the Louisiana shelf were used to elucidate the mechanisms of wave energy dissipation over a muddy seafloor. After a period of high discharge from the Atchafalaya River, acoustic measurements showed the presence of 20 cm thick mobile fluid-mud layers during and after wave events. While total wave energy dissipation (D) was greatest during the high energy periods, these periods had relatively low normalized attenuation rates (κu2009=u2009Dissipation/Energy Flux). During declining wave-energy conditions, as the fluid-mud layer settled, the attenuation process became more efficient with high κ and low D. The transition from high D and low κ to high κ and low D was caused by a transition from turbulent to laminar flow in the fluid-mud layer as measured by a Pulse-coherent Doppler profiler. Measurements of the oscillatory boundary layer velocity profile in the fluid-mud layer during laminar flow reveal a very thick wave boundary layer with curvature filling the entire fluid-mud layer, suggesting a kinematic viscosity 2–3 orders of magnitude greater than that of clear water. This high viscosity is also consistent with a high wave-attenuation rates measured by across-shelf energy flux differences. The transition to turbulence was forced by instabilities on the lutocline, with wavelengths consistent with the dispersion relation for this two-layer system. The measurements also provide new insight into the dynamics of wave-supported turbidity flows during the transition from a laminar to turbulent fluid-mud layer.