Dynamic response of fluid mud in a submarine trench to water waves

摘要

The dynamic response of fluid mud in a submarine rectangular trench to time-periodic surface waves propagating over the trench has been studied experimentally and theoretically. It was shown that for a particular trench geometry relative to the characteristic length scale of the surface waves, the fluid mud in the trench may be excited in a mode of resonant oscillation. Resonance within the trench resulted in large strain rates in the fluid mud compared to off resonance conditions, and thus reduced scale effects in the experiments. Detailed experiments were conducted to study the dynamics of fluid mud under wave action and its relationship with the fluid mud properties. A mathematical model was developed to predict the wave-induced motion of the fluid mud in the trench. The model employed small amplitude wave theory and treated the viscosity of fluid mud as a function of strain rate. The objective of the study was to determine how motions of sediment-laden fluids under water waves differ from that of a clear-water fluid, knowledge of which is important for description of near bottom kinematics in waters with "ill-defined" bottoms. It was found that the Carreau model described the viscosity of fluid mud well and shear rate and time of shearing had only minor effects on the fluid mud viscosity at small strain rates. Experimental measurements showed that the fluid mud in the trench was gradually eroded by wave action, but changes in density distribution were confined to a layer near the clear-water/fluid-mud interface. The theoretical model predicted the movement of fluid mud in the trench qualitatively. The results of this study indicated that fluid mud would behave as a Newtonian fluid at small strain rates.