Influence of Sand Supply and Grain Size on Equilibrium Upper Regime Bedforms

Notwithstanding the large number of studies on bedforms such as dunes and antidunes, predicting equilibrium bedform type and geometry for a given flow regime, sediment supply and caliber remains an open problem. Here, we present results from laboratory experiments specifically designed to study how upper regime bedform type and geometry vary with sediment supply and caliber. Experiments were performed in a sediment feed flume with flow rates varying between 5 and 30 l/s and sand supply rates varying between 0.6 and 20 kg/min. We used both uniform and non-uniform sands with geometric mean diameters varying between 0.22 and 0.87 mm. Analysis of our data and data available in the literature reveals that the ratio of total (bedload plus suspension) volume transport rate of sediment to water discharge Q(s)/Q(w) plays a prime control on upper regime equilibrium beds. Equilibrium bedforms transition from washed out dunes (lower regime) to downstream migrating antidunes (upper regime) for Q(s)/Q(w) between 0.0003 and 0.0007. For values of Q(s)/Q(w) greater than 0.0015, the bedform length increases with Q(s)/Q(w). At these high values of Q(s)/Q(w), equilibrium in fine sand is characterized by upstream migrating antidunes, cyclic steps, and significant suspended load. In experiments with coarse sand, equilibrium is characterized by plane bed with bedload transport in sheet flow mode. Standing waves form at the transition between downstream migrating antidunes and upstream migrating bedforms.Plain Language Summary Bedforms are sediment bumps that form and move at the bottom of channels transporting water and sediment. Bump size and direction of movement depend on flow and sediment properties. Although bedforms have been observed and studied for a long time, predicting type and shape remains to be open problems. Here, we present results from laboratory experiments designed to study bedforms made of sand that form under fast flows. Our results show that, in these conditions, bedform type and size depend on the ratio between volumes of transported sediment and water and the size of sand grains. In addition, our results suggest that if sand is transported in a thin layer near the bed, bedforms tend to move in the direction of the flow, while bedforms tend to move in the direction opposite to the flow if some sand is suspended in the water column.

Automated summary

Despite numerous studies, predicting the equilibrium type and geometry of bedforms in response to different flow regimes, sediment supplies, and grain sizes remains an unresolved challenge. Laboratory experiments using varying flow rates, sediment supplies, and grain sizes reveal that the ratio of sediment transport volume to water discharge plays a significant role in controlling upper regime bedforms. The transition from washed out dunes to downstream migrating antidunes occurs when the ratio is between 0.0003 and 0.0007. Bedform length increases with higher ratios above 0.0015, accompanied by upstream migrating antidunes, cyclic steps, and suspended load in fine sand. Coarse sand experiments result in plane beds with sheet flow transport and standing waves at the transition zone.