42A soft muddy bottom can interact strongly with waves propagating in the water above it. Measurements near the Mississippi River Delta by the SWAMP program demonstrated that this interaction can cause very large attenuation of the water waves. We have developed the SWIM computer program to model this process. SWIM models the soil as a layered anelastic medium with complex shear and bulk moduli. The equations of motion are solved by the propagator matrix technique and the attenuation is given by the imaginary part of the resulting wave number. Since the soil properties are a strong function of shear strain and the equations are linear, we use an equivalent linearization method based on the rms response of the system to the wave spectrum.
Soil properties near the Delta are far from uniform. Soil boring data indicate, for example, that the shear strength in deformational features is considerably less than that in undisturbed regions. Using soil boring data as well as SWAMP measurements of bottom heave in an undisturbed region, we have found a set of soil properties which enables SWIM to consistently match the observed wave attenuation. The soil motion and attenuation at the measurement site are calculated to be much less than in the mud flow lobes and gullies farther offshore. In winter storms, most of the calculated attenuation takes place in the gullies in water depths less than 50 m. On the other hand, much of the attenuation in hurricanes occurs in mudflow lobes in water depths up to 100 m.