Debris flows and lahars are dense masses of water and sediment which are common phenomena in mountainous and volcanic regions, respectively. Where these flows debouch into water bodies they can trigger impulse waves (tsunamis) and form subaqueous deposits. Such deposits are important indicators for areas at risk from debris flows, lahars, and tsunamis and form archives of past environmental conditions. Correctly interpreting this archive, however, depends on our understanding of the sedimentology and architecture of the deposits. While subaerial debris-flow deposits have been extensively studied, there is a comparative lack of understanding of the deposits of subaerial debris flows that debouch into a water body. We experimentally investigate the similarities and contrasts between subaerial and subaqueous debris-flow deposits for flows of various magnitudes and compositions initiated in a subaerial environment. We show that flows depositing on a subaqueous plane generally have a deposit area similar to flows forming in a subaerial setting. Deposits forming on a subaqueous plane, however, are typically shorter and wider with similar thickness, as a result of interactions between the flow and the reservoir water body. Both in subaerial and subaqueous environments the deposits form coarse-grained lateral levees and frontal snout margins. However, where the levees are able to laterally confine the subaerial flows leading to deposits with constant to tapering width, the subaqueous deposits widen with distance offshore because of flow fluidization. Moreover, the frontal snout is often very dispersed, a sharp frontal margin is absent, and many isolated particles are deposited in front of the main deposit margin as a result of interactions between the debris flow and the reservoir water body. These results largely agree with observations of subaqueous pyroclastic-flow deposits. The similarity in area of subaerial and subaqueous deposits suggests that we can apply empirical relations based on subaerial flows to estimate the inundation area and flow volume of subaerial–subaqueous flows.

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