Flow resistance in the channel-bar landscape of large alluvial rivers

Accurate approaches for estimating flow resistance in large alluvial rivers are fundamental for simulating discharge, sediment transport, and flood routing. However, methods for estimating riverbed resistance and additional resistance in the channel-bar landscapes remain poorly investigated. In this study, we used in situ river bathymetry, sediment, and hydraulic data from the Shashi Reach in the Yangtze River to develop a semi-empirical approach for calculating flow resistance. Our method quantitatively separates flow resistance into riverbed resistance and additional resistance and shows high accuracy in terms of deviation ratio (∼20%), root-mean-square error (∼0.008), and geometric standard deviation (∼3). Additional resistance plays a dominant role under low-flow conditions but a secondary role under high flows, primarily due to the reduction in momentum exchange in channel-bar regions as discharge increases. Riverbed resistance first decreases and then increases, which might be attributed to bedform changes in the lower and transitional flow regimes as flow velocity increases. Overall, our findings further the understanding of dynamic changes in flow resistance in the channel-bar landscapes of large river systems and have important implications for riverine ecology and flood management.