Spatial and temporal dynamics of alluvial cover in mixed bedrock-alluvial fluvial systems

Abstract

Mixed bedrock-alluvial rivers, characterised by spatially variable sediment cover over exposed bedrock, control landscape evolution and are important aquatic habitats. However, the mechanisms controlling alluvial cover variability remain poorly understood, particularly in post-glacial landscapes where glacial inheritance interacts with contemporary fluvial processes. This thesis investigates three objectives: (1) examining how channel and sediment properties correlate with alluvial cover, (2) identifying main drivers influencing alluvial cover at the network scale, and (3) exploring how sediment pulses influence cover dynamics. The study combines empirical analyses and numerical modelling within the River Carron catchment in the Scottish Highlands. Field survey across 92 reaches showed that channel slope and bankfull width were the strongest predictors of alluvial cover distribution. Post-glacial inheritance created high spatial variability in grain size and disrupted typical downstream patterns. Contemporary floods rework sediments in most reaches (88% during 50-year floods), though 12% remained immobile even during extreme events. To address network-scale alluvial cover dynamics, the NetworkSedimentTransporter model was modified to track alluvial cover changes across river networks. The modelling showed that controlling factors vary with sediment supply conditions. Under supply-limited conditions, channel slope primarily controlled spatial distribution. Under transport-limited conditions, sediment supply and grain size became more influential. Even at high sediment supply rates, complete alluviation was not achieved. Sediment pulse simulations demonstrated that pulse dynamics depend on pulse characteristics and pre-existing cover conditions. Pulses introduced at a bedrock site exhibited faster downstream transport, while pulses at an alluvial site showed greater local storage. These findings demonstrate that channel slope is the primary alluvial cover control, though its influence varies with sediment supply regime. This study provides important understanding for river management and habitat conservation.