Coupled Dynamics of Interfacial Waves and Bedforms in Fluid Muds over Erodible Seabeds in Oscillatory Flows

Abstract:

Field investigations of the damping of ocean surface waves over muddy seabeds indicate waves on the interface between the thin layer of fluid mud and the overlying much thicker column of clear water, which are accompanied by bedforms on the relatively solid seabed beneath the fluid mud. The observed wavelengths of the interfacial waves and bedforms are inconsistent with existing theories and scalings. A linear analysis, based on the viscous layer-averaged long-wave equations for the fluid mud and the Meyer-Peter-Mueller equation for sediment transport within the underlying seabed, indicates instability to interfacial waves and bedforms with spatial and temporal scales consistent with the observations. In the model solution, the bedforms force interfacial waves, which in turn create velocities and stresses that produce a net transport of sediment within the seabed toward the bedform crests, thus causing bedform growth and enhancing the forcing of the interfacial waves. The observations and the model results suggest that interfacial waves formed by the mud-seabed instability might create mixing that helps control the thickness, density and viscosity of the mud layer, thus influencing the transport of mud and the damping of surface waves.