Bed Topography and Sand Transport Responses to a Step Change in Discharge and Water Depth

Abstract

At base-flow conditions, ephemeral streams with mobile beds may inherit bed topography caused by previous higher flow events, resulting in bed topography that is not in equilibrium with flow conditions. Major flow events can generate large bedforms that result in hydrodynamics and sediment transport that are not in equilibrium with subsequent lower flow conditions. To simulate a large storm runoff–driven stream flow followed by lower base-flow conditions, changes in sand bed topography and sand transport caused by a simultaneous reduction in discharge and flow depth in a laboratory flume with an equilibrium sand bed and steady, approximately uniform, flow were investigated. Bed topography was evaluated using second-order structure functions, semivariograms, and statistical analyses. Sediment transport responded rapidly to a near-instantaneous negative step in mean flow depth and discharge. The relative contributions of reduced bedform height and celerity to the decreased transport rate are quantified, and bedform celerity was the primary contributor to reductions in transport rate. To evaluate the effect of initial bed topography on equilibrium topography, a flat bed initial condition was subjected to the same flow as was applied after the negative step. The equilibrium bed topography for the flat bed initial condition was similar to the topography that was formed after the negative step. Semivariogram analysis provided a reliable estimate of mean bedform height without the need for detecting individual bedforms. Total sediment load following a negative step in discharge and flow depth followed an exponential decay model that resulted from the combined exponential decay of bedform celerity and bedform height.