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Turbidity Currents That Co-Evolve With Channels Over Lengths as Much as 1000 km: How Can They Do it?


Here we consider the puzzle of long-runout turbidity currents and the channels they create. It is well known, through direct evidence of the flows or from the morphology that they create, that turbidity currents can run out over 1000 km in the ocean. The currents do so without dissipating themselves via the excess entrainment of ambient water. Existing layer-averaged formulations are, however, unable to capture this behavior. Here we use the formalism of a “Turbidity Current with a Roof” to show that the turbidity current partitions itself into two layers. The lower “driving layer” approaches an asymptotic behavior with invariant flow thickness, velocity profile and suspended sediment concentration profile. The upper “rarified layer” continues to entrain ambient water indefinitely, but the concentration in that layer becomes ever more dilute, and the layer ultimately has little interaction with bed morphology. This partition likely allows the driving layer to run out long distances while maintaining coherence, and to follow morphology of its own creation such as leveed subaqueous channels.