The interactions between free surface waves and layers of cohesive sediments including wave height attenuation and mud movement are of great importance in coastal and marine engineering. In this study, the results from a new analytical model were compared with those from literature experimental works and analytical models in terms of wave height dissipation rate and mud velocity. It was found that the new model provided good agreements in the case of coexisting waves and currents, while the literature model of Ng (explained in ^{Section 2} of the text) —assuming the mud layer as a highly viscous layer with high shear rates—matched well with the experimental data for high viscosity (mud viscosity, ν_m = O [0.01 m²/s]). In addition, it was found that the new model is able to successfully simulate particles velocity in the presence of co-current.

In this study, laboratory experiments are conducted to assess the effects of a set of novel ring-type collars and prevalent spiral threading around a circular pier on scouring in a clear-water flow regime in non-cohesive bed materials. Also, numerical simulations are conducted to reveal the flow field in the scour hole around a pier with spiral threading. Various angles and spacings of the ring collars are considered to find the best configuration. Moreover, four test scenarios of spiral threading with different elevation levels are considered to investigate the optimum upper bound of spiral threading elevation. Results show that the ring collars (Angle = 45˚, relative spacing = 4/3 of pier diameter) are the best configuration and reduce the scour depth by 29% according to the scour depth around a plain pier. This configuration of ring collars reduces maximum scour depth three times better than the best configuration of single spiral threading (suggested by previous research). Furthermore, the ring collars reduce the scour rate while the spiral threading has no considerable effect on the scour rate. The spiral threading under the initial bed level plays a major role in reducing local scour.