A TEST OF INITIATION OF SUBMARINE LEVEED CHANNELS BY DEPOSITION ALONE

Leveed submarine channels play a critical role in the transfer of sediment from the upper continental slopes to intraslope basins and ultimately deeper marine settings. Despite a reasonable understanding of how these channels grow once established, how such channels are initiated on previously unchannelized portions of the seafloor remains poorly understood. We conducted a series of experiments to test whether leveed channels can start by deposition on a planar rigid bed. We systematically varied the current density and outlet velocity to explore the relative influence of inertia and excess density on the depositional dynamics of currents entering a basin and undergoing abrupt unconfinement. Under flow conditions ranging from supercritical to subcritical (bulk Richardson numbers of 0.02 to 1.2) our experiments failed to produce deposits resembling or exhibiting the potential to evolve into levees needed to create a self-formed channel. In the absence of excess density, a submerged sediment-laden flow produced sharp-crested lateral deposits bounding the margins of the flow for approximately a distance of two outlet widths down-basin. These lateral deposits terminated in a centerline deposit that greatly exceeded marginal deposits in thickness. As excess density increased relative to the outlet velocity, the rate of lateral spreading of the flow increased relative to the downstream propagation of the density current, transitioning from a narrow flow aligned with the channel outlet to a broad radially expanding flow. Coincident with these changes in flow dynamics, the bounding lateral deposits extended for shorter distances, had lower, more poorly defined crests that were increasingly wider in separation than the initial outlet, and progressively became more oblong rather than linear. Based on our results and a review of previous experimental and numerical models, we suggest that initiation of leveed channels from sediment-laden density currents traversing non-erodible beds is unlikely. Partial confinement of these currents appears to be necessary to establish the hydrodynamic conditions needed for sediment deposition along the margins of a density current that ultimately may create confining levees. We suggest that erosion into a previously unchannelized substrate is the most likely source of this partial confinement.