AN EXPERIMENTAL STUDY OF HIGH-DENSITY SUSPENSIONS UNDER WAVES

Several recent field and analytical studies have illustrated the importance of highly concentrated fine-grained sediment suspensions in estuarine and continental shelf environments (e.g., fluid mud). These High Density Suspensions (HDS) may constitute a significant fraction of shelf sediment transport when either a mean current or gravitational forcing is present and thus may strongly influence clinoform development. Previous studies hypothesized that the formation of HDS is genetically linked to the turbulence in the wave boundary layer on storm-dominated fine-grained shelves. While several detailed experimental and field studies have analyzed turbulent wave boundary layers over a solid or sandy boundary, very little data exists on turbulent wave boundary layers over easily mobilized fine-grained (silt and clay) sediment.

Responding to the need for laboratory data, we have constructed an oscillating U-tube. We performed experiments investigating turbulent wave boundary layers and their interaction with a fine-grained (D50=20 mm) sediment bed over a range of orbital velocities (20-60 cm/s) and periods (3-8 s). For each experiment, the vertical structure of the wave boundary layer was measured using an Acoustic Doppler Velocimeter (ADV), while profiles of sediment concentration and grain size were obtained using siphons. These experiments revealed the formation of HDS with large sediment concentrations (10 – 100 g/l). These highly concentrated layers were somewhat larger than, but on the order of, the clear-water boundary layer thickness. HDS were found to significantly alter the velocity and turbulent structure of the wave boundary layer, but not in a manner consistent with current analytical models. Despite the high sediment concentrations, the wave boundary layer remained as energetic as in the clear-water experiments. The bed coarsened beneath HDS, which often was coincident with the formation of ripples even though the original sediment mixture had a mean grain size of only 20 mm.