A multi-layered viscous fluid model is presented to investigate the wave field and mass transport velocity of mud layers under the action of surface waves. Essentially, the model treats the fluid system as Newtonian and the equivalent viscosity is proposed to introduce the nature of a Bingham fluid into mud layers.
　In the model, viscosities of mud layers are estimated by the iteration procedures and it is required to give only the total depth of mud layers which are in motion. The validity of the theory is checked by comparison with the experimental results.
　Experiments were conducted in a 11m long, 0.4m wide and 0.5m high soil-wave tank. Kaolinite clay and natural mud which was sampled at Ariake Bay were used as bed materials.
　The damping factors and wave lengths of surface waves, pore pressures and mud particle trajectories were obtained from the experiments. These results almost coincide with the theoretical values, especially when the mud depth is shallow. The theoretical mass transport velocities are obtained for the multi-layered viscous
fluid model.
　When the rate of water content of mud layer is uniform, the damping factor of the surface waves decreases as the wave height increases, because the equivalent viscosity decreases.