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Triggering of submarine granular avalanches

Tuesday 6th January 2009 - 12:45 to 13:00
INI Seminar Room 1
Under the sea, granular avalanches and landslides exist, in which the interstitial fluid plays a major role. A good description of such events requires understanding the coupling between a granular flow and a fluid. The goal of this work is to study this coupling in the case of the initiation of immersed granular avalanches down rough inclined planes. More precisely, we study how the initial volume fraction of a granular layer strongly influences how it starts to flow when suddenly inclined. The experiment consists in preparing a uniform layer of glass beads (160µm in diameter) in a long box full of liquid by creating a suspension and by letting the particles sediment. The initial volume fraction of the layer can be precisely controlled by imposing successive taps on the box. The box is then suddenly inclined from horizontal. We then measure the time evolution of the free surface velocity of the layer of the pore pressure below the layer and of the volume fraction in the central part of the box. The avalanches can be classified in tow categories. In the loose cases a rapid acceleration first takes place and a transient velocity higher than the final velocity can be observed. In the dense case, a first stage characterised by a very low velocity is observed, before the velocity increases and reaches the steady state value. The higher the volume fraction, the longer is the triggering time. The qualitative explanation of the phenomenon is given by the coupling between the granular skeleton and the pore pressure. When the avalanche starts, the granular medium starts to deform, which induces a variation of volume fraction (a dilatation if the sample is initially dense, a contraction otherwise). This change in volume fraction causes a variation of the pore pressure, which in turn changes the stresses that apply on the granular skeleton. Parallel to the experiments, a theoretical model is developed. We have used the depth averaged approach developed by Pitman and Lee for thin two phase flows. By introducing in the model a relevant granular rheology, which takes into account the variation of volume fraction observed during the initial deformation, a quantitative agreement with the experimental observations is obtained.
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University of Cambridge Research Councils UK
    Clay Mathematics Institute London Mathematical Society NM Rothschild and Sons