Changes in sediment transport and bed topography in response to step-up flows in a laboratory flume

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

Submitted to: Journal of Hydraulic Engineering
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 11/12/2020
Publication Date: 2/5/2021
Citation: Wren, D.G., Langendoen, E.J., Kuhnle, R.A. 2021. Changes in sediment transport and bed topography in response to step-up flows in a laboratory flume. Journal of Hydraulic Engineering. 147(4):5. https://doi.org/10.1061/(ASCE)HY.1943-7900.0001874.
DOI: https://doi.org/10.1061/(ASCE)HY.1943-7900.0001874

Interpretive Summary: Sediment transport in sand-bedded streams that normally have very little water flowing in them often respond quickly to runoff during rainfall events, which results in streams that undergo rapid changes in flow rate. In these cases, bed forms on the stream bed do not match what would be expected for the flow rate, and this makes it difficult to predict how much sediment will be transported in the stream. In order to better predict sand transport in rapidly increasing flow conditions and to understand what occurs in this situation, a series of laboratory experiments was undertaken. Sand bed forms responded much more rapidly to increasing flows than to decreasing flows, and sand transport rate stabilized even more quickly, in advance of the bed forms. This result indicates that it may not be necessary to take the adjustment time into account for sand transport modeling in rapidly increasing flow situations. This information will be used to improve the prediction of sand transport and the timing of bed adjustment to water flow, which aids in flood forecasting since the bed configuration also affects water depth.

Technical Abstract: It can be difficult to predict transport in sand-bedded ephemeral streams, due to rapidly changing conditions that may not be sustained for long enough to reach equilibrium. This situation often leads to the inheritance of bed topography from previous flows, which may result in under- or over-predictions of transport rate due to bed forms and flow resistance that are not in equilibrium with the flow conditions. In order to address this difficulty, a series of lab experiments was used to examine changes in sediment transport rate and bed topography after rapid increases in flow depth and discharge. It was found that sand transport rates reached equilibrium conditions in as little as 10-15 minutes after the step-up in flow rate and depth. Transport rate equilibrium was likely reached in advance of bed topography, although bed topography also responded quickly to step-up flows, and the response was more rapid than in previous experiments with decreases in flow rate and depth.