The effects of topography on sedimentation from particle-laden turbulent density currents

We present results from a series of laboratory experiments that illustrate the influence of changes in channel topography (depth or width) on the sedimentation patterns produced by steady, particle-laden currents. On a planar surface in a channel with constant width, such currents created deposits that thinned exponentially with distance. As long as a current was not blocked to produce a bore, topographic features consisting of constrictions, ridges, and sudden openings caused no significant deviation from exponential deposit thinning or discontinuity in thickness, even when there was a transition in flow regime caused by the topographic feature. Changes in the channel width did affect the distance over which the deposit thinned, with the deposits associated with wider parts of the channel thinning more rapidly with distance. Zn contrast, if the topographic change was large enough to partially reflect the flow, producing an upstream-propagating bore, then the deposit did not thin exponentially. The results are consistent with a model in which the current is assumed to be turbulent and well mixed. In this case, sedimentation occurs at a rate proportional to the channel width, the settling speed of the particles, and their concentration within the current, and inversely proportional to the current discharge. Such a model predicts that as a flow passes through a topographic control that does not produce a bore, the deposit continues to thin exponentially, even if the flow undergoes a transition from the subcritical to the supercritical regime. The results suggest that in natural systems changes in current speed and flow regime do not in themselves produce changes in deposit thickness or gradient in thickness unless the flow is partially blocked and a fraction of the flow is reflected upstream.