| An important mud seabed parameter in hydrodynamic models of wave-mud interaction is the thickness of the oscillating fluid mud layer in which most wave energy is dissipated. Despite its significance, modelers conventionally define this thickness empirically by tuning the model to the value which produces the measured wave attenuation, thus limiting the model's predictive ability.;To address this issue, a laboratory study has been conducted on seabed sediment from the outer topset of the Atchafalaya River Delta in the Gulf of Mexico in order to develop a method to estimate the thickness of the oscillating fluid mud layer. This development relies on a rheological analysis of mud response to oscillatory shear stress representative of surface wave forcing.;Relevant properties of the mud, including its settling and self-weight consolidation characteristics, are determined from laboratory tests. The mud is further tested using a controlled-stress/controlled-strain rheometer and a flow equation is derived based on the measurements. The flow equation is then used to develop a set of model equations for determination of profiles within the mud of the horizontal velocity, the rate of strain, the bed shear stress and the viscous layer thickness, as well as the total wave energy dissipation rate. The model equations are validated with extensive flume tests involving the damping of monochromatic water waves over carefully prepared depositional mud beds.;At high shear stresses, the viscosity is found to vary by more than two orders of magnitude and the bed is found to behave simultaneously as an elasticoviscous, viscoelastic or elastic material depending on the depth within the bed. To what thickness the bed will be dominantly viscous depends primarily on the density of the bed and to a lesser degree on the wave characteristics. Under depositional conditions, the density of the upper layer of mud is found to be about 1,200 kg/m3. At this density, the mud readily transitions to a viscous state under even weak oscillatory shear stress. However, as the bed density increases with depth, mud shear strength increases nonlinearly and higher shear stresses are required to achieve this transition.;Due to the thixotropic property of this mud, the stress history plays a key role in how the mud responds to shear stress. The thickness of the dominantly viscous mud layer is greater when the preceding shear stress condition on the mud bed is higher than the imposed stress, in contrast with the case when the preceding stress is lower than the imposed stress.;A procedure has been developed to estimate the thickness of the dominantly viscous mud layer given water depth, wave properties and the mud rheological flow curve. Flume data are shown to be in good agreement with the results derived from this procedure, which also appears to show promise in field applications. Further investigations are required to examine effects of normal stress in mud, non-Gaussian wave fields and mud mass transport within the thickness of the viscous layer. |
