Bed topography and sand transport responses to a step change in discharge and water depth

Authors: Wren, Daniel; Langendoen, Eddy; Kuhnle, Roger

Submitted to: Journal of Hydraulic Engineering
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 3/7/2016
Publication Date: 6/13/2016
Publication URL: http://handle.nal.usda.gov/10113/62672
Citation: Wren, D.G., Langendoen, E.J., Kuhnle, R.A. 2016. Bed topography and sand transport responses to a step change in discharge and water depth. Journal of Hydraulic Engineering. 10.1061/(ASCE)HY.1943-7900.0001172.

Interpretive Summary: Models that are used for the prediction of sediment transport in streams assume that stream conditions do not change in time and that the bed of the stream contains bed forms that are appropriate for the existing flow conditions. This is often not the case, since large rain storms can generate high flows that create larger bed forms. Smaller flows that occur after the large event will inherit bed topography caused by the large event. The work described here is geared towards addressing how sand transport and bed topography change when lower flows are applied to a bed that was generated with a higher flow. This work is relevant to streams that are near areas of agricultural production, since these small streams often only have flow when rainfall events generate runoff that enters the stream. It was found that a common mathematical express, exponential decay, can be used to predict the rate of decrease in sand transport and bed form height. This information can be used to increase the accuracy of models used for sediment transport in small streams.

Technical Abstract: Ephemeral streams with sand and gravel beds may inherit bed topography caused by previous flow events, resulting in bed topography that is not in equilibrium with flow conditions, complicating the modeling of flow and sediment transport. Major flow events, resulting from rainfall with high intensity or long duration, can generate large bedforms that remain in place during subsequent flows from smaller runoff events, resulting in hydrodynamics and sediment transport that are not in equilibrium with flow conditions. Changes in sand bed configuration and sediment transport following a reduction in both discharge and depth are the topic of the work described here. It was found that sediment transport responded rapidly to a near-instantaneous negative step in depth and discharge, and the relative contributions of reduced bed form height and celerity are quantified. Similar flow conditions were arrived at for a flat bad initial condition and a negative step to the same flow conditions. Semivariogram analysis was shown to provide a reliable estimate of mean bed form height without the need for detecting individual bed forms. A basic exponential decay model for total load following a negative step, based on declines in bed form height and celerity, is proposed.