Liquefaction and erosion of mud due to waves and current

Abstract

A research project was started at the Delft University of Technology in order to study the interaction between waves as well as a current and a muddy bed. For this purpose several experiments were made on artificial clays. In the present report only flume experiments on China Clay are discussed. In the experiments made special attention was paid to the liquefaction mechanism, the turbulence structure over a liquefied bed and the influence of liquefaction on the wave damping. The experimental results showed, among other things, that a layer of fluid mud was generated as soon as the wave height exceeded a certain threshold value. This value increases with the consolidation period. Pressure induced shear stresses in the bed calculated under the assumption of China Clay being a poro-elastic material, show that these stresses play an important role in the liquefaction process of mud. The waves were significantly damped as soon as 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 and hardly any mud was entrained into the water layer during this process. The velocity measurements showed that the turbulence intensities decreased in a stationary current when a layer of fluid mud was present, which result corresponds with visual observations made when dye was injected into the flow. The observations and pressure measurements usually made at the 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 gradual build-up of an excess pore pressure so as to compensate for the vanishing effective stress. The wave damping and velocity amplitudes in the fluid mud which were determined during the experiments, correspond wen with the calculated results using a modified version of Gade's model (1958).