To elucidate fundamental parts in gravity mass flows, we have been focusing on how the ambient fluid influences the behaviour of debris flows. In order to achieve this we have performed a set of novel laboratory experiments consisting of a suite of tests varying the ambient environment from air to water, and the sediment composition from granular to viscoplastic material.
Field observations have shown that subaqueous mass flows seem more mobile than their subaerial counterparts, exemplified by the very long runout of subaqueous slides on slopes gentler than 1°. In our experiments we have been using a well equipped flume system at St. Anthony Falls Laboratory (U. of Minnesota), and in particular the use of high speed cameras and PIV techniques have made it possible to accurately study the internal flow characteristics of subaqueous as well as subaerial debris flows. The experiments document that subaerial flows behave according to well known principles, whilst subaqueous debris flows show a more complex behaviour. Low-clay content material demonstrates sediment break-up and turbidity current generation, while more clay-rich flows show a downslope acceleration of the frontal parts due to hydroplaning, leading to a stretching of the debris flow body.
The experiments also include detailed rheological measurements of the various slurry compositions, illustrating that the behaviour of subaerial debris flows are predictable based on their rheological properties. Due to the effect of the ambient fluid as explained, the behaviour of subaqueous mass flows seems less dependent on initial rheological properties.
We see this rather fundamental approach as an attempt to build a bridge between the traditionally separate fields within mass movements and to try to make it possible to transfer the right type of knowledge from subaerial deposits, flow observations and modelling to the subaqueous field and vice versa.
