Experimental Insights on the Propagation of Fine‐Grained Geophysical Flows Entering Water

Granular flows that propagate down a mountainside, then reach the sea, a lake or a river and finally, travel underwater, is a common event on the Earth's surface. To help the description of such events, laboratory experiments on gas‐fluidized granular flows entering water are performed, analyzed, and compared to those propagating in air. The originality of this study lies in the fluidization process, which improves the laboratory modeling of geophysical flows by taking their high mobility into account. Qualitatively, the presence of the water body promotes the generation of a granular jet over the water surface, a leading and largest wave, and a particle‐driven gravity current underwater. Hydrodynamic forces mainly play a dissipative role by slowing and reducing the spreading of the granular mass underwater, but a low amount of grains are still transported by the turbulent fluid as a gravity current far away. The temporal evolution of the granular jet and the particle‐driven gravity current are well described by ballistic motion theory and scaling laws of homogeneous gravity currents, respectively. Most currents propagate with a constant flow‐front velocity along the horizontal bottom, which is controlled by the flow height depending on the water depth. In contrast, the bulk volume concentration of particles in the current is estimated to be nearly constant, interpreted as a critical concentration above which the excess of particles cannot be maintained by the turbulent fluid. This experimental study highlights the complexity of the dynamics and deposits of granular masses when they encounter a water body. Plain Language Summary Geophysical granular flows driven by gravity occur frequently on the Earth's surface, as a result of climatic, tectonic or volcanic events. When they occur near the sea, a lake or a river, the granular flows may enter water, generate tsunamis, and propagate underwater. This study presents laboratory experiments on gas‐fluidized granular flows entering water, which are performed in a 7 m‐long channel and recorded by high‐speed cameras. The fluidization process ensures dynamic similarity for modeling of highly mobile geophysical flows composed of fine materials, which are predisposed to reach the coast and generate turbidity currents underwater. The main contributions of this study are summarized as follows: (i) we show that fine‐grained flows entering a water body generate both a granular jet over the water surface and a particle‐dr