An experimental and numerical investigation on wave‐mud interactions

Wave attenuation over a mud (kaolinite) layer is investigated via laboratory experiments and numerical modeling. The rheological behavior of kaolinite exhibits hybrid properties of a Bingham and pseudoplastic fluid. Moreover, the measured time-dependent velocity profiles in the mud layer reveal that the shear rate under wave loading is highly phase dependent. The measured shear rate and rheological data allow us to back-calculate the time-dependent viscosity of the mud layer under various wave loadings, which is also shown to fluctuate up to 1 order of magnitude during one wave period. However, the resulting time-dependent bottom stress is shown to only fluctuate within 25% of its mean. The back-calculated wave-averaged bottom stress is well correlated with the wave damping rate in the intermediate-wave energy condition. The commonly adopted constant viscosity assumption is then evaluated via linear and nonlinear wave-mud interaction models. When driving the models with measured wave-averaged mud viscosity (forward modeling), the wave damping rate is generally overpredicted under the low wave energy condition. On the other hand, when a constant viscosity is chosen to match the observed wave damping rate (inverse modeling), the predicted velocity profiles in the mud layer are not satisfactory and the corresponding viscosity is lower than the measured value. These discrepancies are less pronounced when waves become more energetic. Differences between the linear and nonlinear model results become significant under low-energy conditions, suggesting an amplification of wave nonlinearity due to non-Newtonian rheology. In general, the constant viscosity assumption for modeling wave-mud interaction is only appropriate for more energetic wave conditions.