Autogenic Processes on Supercritical Submarine Fans
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Submarine fans are large landforms typically built on the continental slope and abyssal plain. They are a amalgamation of depositional lobes emplaced over time through avulsion cycles. These type of intermediate scale processes, particularly related to the hydraulic and sediment transport properties, are poorly understood and understudied. This research experimentally explores supercritical submarine avulsion cycles primarily from a hydraulic perspective. To do so, a new methodology was developed that is capable of measuring the layer-averaged hydraulic variables of developing density currents. This methodology was applied to a series of submarine fan experiments to quantify hydraulic and sediment transport properties. The ability to calculate these properties is imperative toward understanding the intrinsic processes that give rise to autogenic avulsion. Fans developed over time through repeated avulsion consisting of channel incision and basinward extension, cessation of channel extension and mouth bar formation, bar aggradation and hydraulic jump initiation, and upstream propagation of the channel-to-lobe transition. The transition from erosion or bypass in the channels to deposition in an expanded-flow region downstream of the channels was most related to deceleration rather than dilution, and a choked-flow condition appeared to cause hydraulic jump initiation. Each avulsion cycle was responsible for an associated lobe deposit. Since hydraulic jumps were common during avulsion cycles, they were used to predict the maximum thickness of the lobe deposits as a function of the upstream flow depth and Froude number. The classic sequent-depth ratio equation was roughly representative of the data trend from the submarine fan experiments. The lobes emplaced by discrete avulsion cycles stacked up over time to form the overall fan. Though each cycle contained elements of both basinward extension and upstream backfilling, the fans showed net progradation at a long-term rate that can be representatively modeled using a mass balance approach based on sediment supply and equilibrium fan slope. Lastly, the properties of these subaqueous avulsion cycles were compared to similar cycles from alluvial fan experiments. Both systems have supercritical-to-subcritical transition at the channel-to-lobe transition region and detain substantial sediment there. Overall, these two systems appear mechanistically similar.