Response of Grain-Size Distribution Characteristics of a Gravel Bar to Topographic and Hydraulic Conditions: A Case Study from the Upper Yangtze River in China

The characteristics of grain-size distribution on the surface of gravel bars have important implications for riverbed development and fish spawning. In this study, an unmanned aerial vehicle was used to sample 204 sites on the surface of a gravel bar in the upper reaches of the Yangtze River, China. Photogrammetry was used to generate digital elevation model data for the bar. Based on the calculation results of a two-dimensional river hydrodynamic model, clear distribution maps of the flow and grain-size fields were obtained. In addition, the correlation between the hydrodynamic and grain-size indicators was discussed, and a relational equation between the Froude number and median grain size was derived. Finally, the effect of topographic changes on the grain-size distribution was analyzed. The results showed that the grain size at the head of the gravel bar was larger than that at the tail, influenced by the scouring of the flow; coarsening of the gravel at the edge of the bar bend was also evident. A positive correlation was found between \({{D}_{{50}}}\) , flow velocity, and the Froude number. The sorting coefficient, \({{S}_{0}}\) , exhibited a negative correlation with the flow velocity and Froude number. A positive correlation was found between grain size variability indicator \({{C}_{{v}}}\) and topographic variability indicator \({{C}_{h}}\) . The variability in the grain-size distribution was the highest near areas with more drastic topographic variations. The bar surfaces exhibited a pattern of coarse-grained tops and fine-grained pools. The results of this study contribute to further understanding the geomorphology of gravel bars and may help investigate the spawning sites of organisms on the bar surface sensitive to bottom particle size.