Structural changes of a mobile gravel bed surface for increasing flow intensity

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

Submitted to: Journal of Hydraulic Engineering
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 8/2/2019
Publication Date: 12/13/2019
Citation: Kuhnle, R.A., Wren, D.G., Langendoen, E.J. 2019. Structural changes of a mobile gravel bed surface for increasing flow intensity. Journal of Hydraulic Engineering. 146(2): 04019065. 10.1061/(ASCE)HY.1943-7900.0001699.

Interpretive Summary: Knowledge of the rate of gravel movement in streams is important for a number of reasons. The rate of movement and the erosion and deposition of coarse sediment are a major determinant as to whether a channel and its banks are stable or actively eroding. In agricultural watersheds, several studies have shown that channel erosion is often the main process by which soil is eroded and removed from the watershed. Thus accurate knowledge of rates of gravel transport in streams are important pieces of information needed by watershed managers to attain stable channels in agricultural watersheds to preserve the soil and the productivity of crop lands. Experiments were conducted in a laboratory flume at the National Sedimentation Laboratory to understand the processes controlling the transport of gravel in model river channels. It was found that the structure of the gravel was an important determinant of the rate and size of gravel sediment eroded and transported. Structure must be taken into account in order to predict accurate rates of gravel. This information will allow watershed managers to calculate improved estimates of gravel sediment erosion and transport and therefore allow more sustainable use of agricultural lands.

Technical Abstract: The structure of a mobile bed in a laboratory channel composed of sand and gravel (D50= 8 mm) was characterized over a series of experiments with steady flows from just above the initiation of motion of the bed material to flows in which most bed-material grain sizes were in motion. In each experiment, sediment transport rates were observed to initially be greater than the long-term mean rates, and fluctuations in transport rate decreased in period as bed shear stresses were increased. The bed surface median grain size increased with bed shear stress, while the sand fraction of the bed material organized into longitudinally extended patches, or corridors, that persisted as flow and transport rates were increased. The presence of the sand corridors was reflected by changes in the probability density function of the bed-surface elevation standard deviation evaluated at the grain-scale. The formation and organization of these corridors may have a strong influence on sand and gravel transport in channels with mixed sand and gravel bed material.