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 sedi mentation patterns produced by steady, particle-laden currents. On a planar surface in a channel with constant width, such currents created deposits t hat thinned exponentially with distance. As long as a current was not block ed to produce a bore, topographic features consisting of constrictions, rid ges, and sudden openings caused no significant deviation from exponential d eposit thinning or discontinuity in thickness, even when there was a transi tion in flow regime caused by the topographic feature. Changes in the chann el width did affect the distance over which the deposit thinned, with the d eposits associated with wider parts of the channel thinning more rapidly wi th distance. Zn contrast, if the topographic change was large enough to par tially reflect the flow, producing an upstream-propagating bore, then the d eposit 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 cas e, sedimentation occurs at a rate proportional to the channel width, the se ttling 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 underg oes a transition from the subcritical to the supercritical regime. The resu lts suggest that in natural systems changes in current speed and flow regim e 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 i s reflected upstream.