Effect of air release height on the formation of sediment thermals in water

Abstract

In this study, the underwater behaviour of sediment thermals (i.e. sediment clouds in the self-preserving phase), formed by releasing dry sediments instantaneously from various heights above water, was investigated experimentally. Measurements show that as the air release height increases, the initial acceleration phase of the sediment cloud in water is shortened, while the growth rate during its self-preserving phase remains constant. Therefore, the effect of air release height can be accounted for by utilizing a new concept of simultaneous virtual distance and time origins, which are found to be related to an impact densimetric Froude number (defined as the ratio between the impact velocity of the sediment cluster at the water surface and the characteristic velocity of the sediment thermal generated by buoyancy excess). Based on the measurement results, a semi-empirical model is utilized to predict the growth and penetration behaviours of the sediment thermal. Predictions by the model with the virtual origins correction match well with the experimental data.