Abstract
Abstract. The cover effect in fluvial bedrock erosion is a major control on bedrock channel morphology and long-term channel dynamics. Here, we suggest a probabilistic framework for the description of the cover effect that can be applied to field, laboratory, and modelling data and thus allows the comparison of results from different sources. The framework describes the formation of sediment cover as a function of the probability of sediment being deposited on already alluviated areas of the bed. We define benchmark cases and suggest physical interpretations of deviations from these benchmarks. Furthermore, we develop a reach-scale model for sediment transfer in a bedrock channel and use it to clarify the relations between the sediment mass residing on the bed, the exposed bedrock fraction, and the transport stage. We derive system timescales and investigate cover response to cyclic perturbations. The model predicts that bedrock channels can achieve grade in steady state by adjusting bed cover. Thus, bedrock channels have at least two characteristic timescales of response. Over short timescales, the degree of bed cover is adjusted such that the supplied sediment load can just be transported, while over long timescales, channel morphology evolves such that the bedrock incision rate matches the tectonic uplift or base-level lowering rate.
Highlights
Bedrock channels are shaped by erosion caused by countless impacts of the sediment particles they carry along their bed (Beer and Turowski, 2015; Cook et al, 2013; Sklar and Dietrich, 2004)
Sediment particles play a key role in this erosion process; they provide the tools for erosion and determine where bedrock is exposed such that it can be worn away by impacting particles (Gilbert, 1877; Sklar and Dietrich, 2004)
P should be controlled by local topography, grain size and shape, hydraulic forcing, and the amount of sediment already residing on the bed
Summary
Bedrock channels are shaped by erosion caused by countless impacts of the sediment particles they carry along their bed (Beer and Turowski, 2015; Cook et al, 2013; Sklar and Dietrich, 2004). Due to a decrease in flow velocity, an increase in surface roughness, and differing grain geometry, the likelihood of deposition is higher over bed sections covered by alluvium compared to smooth, bare bedrock sections (Hodge et al, 2011). Using a model formulation similar to that of Nelson and Seminara (2011), Inoue et al (2016) reproduced bar formation and sediment dynamics in bedrock channels All of these studies used slightly different approaches and mathematical formulations to describe alluvial cover, making a direct comparison difficult. It has been somewhat difficult to compare and discuss the different cover functions obtained from theoretical considerations, numerical models, and experiments, since a unifying framework and clear benchmark cases have been missing We propose such a framework and develop type cases linked to physical considerations of the flow hydraulics and sediment erosion and deposition. As part of this model framework, we investigate the response time of a channel to a change in sediment input, which we illustrate using data from a natural channel
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