Liquefaction and erosion of mud due to waves and current

Abstract

A research project was carried out at the Delft University of Technology in order to study the interaction between waves as weil as a current and a muddy bed. For this purpose several experiments were made on two artificial clays, namely China Clay and Westwald Clay. The results of the experiments on China Clay were reported by De Wit (1994). In the present report only flume experiments on Westwald Clay are discussed. In the experiments made special attention was paid to the liquefaction mechanism and the influence of liquefaction on the wave damping. Mineralogical analysis and rheological measurements showed that Westwald Clay is a more cohesive sediment than the China Clay used in the foregoing experiments. However, because of the unfavourable consolidation characteristics of this sediment, the average bed concentrations in the experiments were roughly half the value of the concentrations encountered in the China Clay beds. The experiments in the wave/current flume showed, among other things, that a layer of fluid mud was generated when the first waves had reached the test section. The waves were damped when a layer of fluid mud was generated. The damping was only little influenced by a current. Furthermore, it was observed that the fluid mud was transported very easily by a current. As the Westwald Clay sticked firmly to the glass sidewalls it must be concluded that in general observations and pressure measurements made at a transparent sidewall of a set-up are not representative of the actual physical processes away from the sidewalls. Only measurements carried out far from a wall give a quantitative description of the processes inside the bed. Pore-pressure measurements showed a transient decrease, possibly caused by the break down of the aggregate structure, succeeded by a build-up of an excess pore pressure so as to compensate for the decreased effective stress. Subsequently, the excess pore-pressure decreased gradually with time. The measured velocity amplitudes in the fluid mud agreed well with the calculated results using a modified version of Gade's model (1958). However, the measured wave damping was underestimated by the calculated wave damping by approximately 15 per cent.