Patterns of Alluviation in Mixed Bedrock‐Alluvial Channels: 1. Numerical Model.

Mixed bedrock‐alluvial rivers can exhibit partial alluvial cover, which plays an important role in controlling bedrock erosion rates and landscape evolution. However, numerical morphodynamic models are generally unable to predict quasi‐steady persistent patches of depositional alluvial features under conditions where the sediment supply is less than the sediment transport capacity of the channel. Hence, we present a new two‐dimensional depth‐averaged morphodynamic model that can be applied to both fully alluvial and mixed bedrock‐alluvial channels, and we use the model to gain insight into the mechanisms responsible for the development of sediment patches and patterns of bedrock alluviation. The model computes hydrodynamics, sediment transport, and bed evolution using a roughness partitioning that accounts for differential roughness of sediment and bedrock, roughness due to sediment transport, and form drag. The model successfully replicates observations of bar development and migration from a fully alluvial flume experiment, and it models persistent sediment patches observed in a mixed bedrock‐alluvial flume experiment. Numerical experiments in which the form drag, sediment transport roughness, and bedform stress correction were neglected did not successfully reproduce the observed persistent sediment cover in the mixed bedrock‐alluvial case, suggesting that accounting for these different roughness components is critical to successfully model sediment dynamics in bedrock channels. Plain Language Summary: Some rivers have patches of sediment intermittently distributed among areas of bare bedrock. These sediment patches can influence flow conditions and patterns of bedrock erosion in these channels. Flume experiments and field studies have shown that the amount of sediment cover and the pattern of sediment patches in mixed bedrock‐alluvial rivers can sometimes stabilize over time, but numerical models have been largely unable to replicate those observations. Here, we present a new numerical model for mixed bedrock‐alluvial rivers that explicitly accounts for how the bedrock and sediment affect the flow. We show that the model can successfully simulate observations from flume experiments conducted with and without exposed bedrock, and it is able to simulate persistent partial sediment cover. These conditions develop because of the way the bedrock and sediment interact with the flow, which we demonstrate through simulations where some of those feedback are "turned off," resulting in completely exposed bedrock without sediment cover. Key Points: A new model is developed to predict the dynamics of alluvial patterns in mixed bedrock‐alluvial channelsModel‐predicted alluvial patterns are similar to those observed in fully alluvial and semi‐alluvial flume experimentsAccounting for differential roughness and shear stress correction are important for model prediction of the degree of exposed bedrock [ABSTRACT FROM AUTHOR]