[1] Channel incision into bedrock plays a critical role in mountain landscape evolution. A bed that is completely alluviated cannot undergo incision. As a result, incision driven by the collision of bedload grains and bedrock requires transport at below-capacity conditions (for example, where the bed is partially covered by alluvium). In a mechanistic model of bedrock incision, it is important to have a formula describing the relationship between the bedload transport rate and the degree to which the bed is covered by sediment. The purpose of this work is to understand the mechanisms underlying the degree of bed exposure in bedrock rivers by means of an experimental study. A number of experiments with various hydraulic and morphological bed conditions were performed in order to characterize this model. Here we focus on planar bedrock beds with some roughness. The results suggest that the sediment supply, channel slope, hydraulic bed roughness, the degree of areal coverage, and thickness of antecedent gravel in the channel, as well as the presence of boulders in the channel are major factors controlling bedrock exposure. For sufficiently low ratios of sediment supply to transport capacity, it was found that bedrock roughness can play a role in determining the degree of bedrock exposure. For higher sediment ratios, on the other hand, the composite roughness associated with grain roughness, bars, and/or antidunes dominates, so that the underlying bedrock roughness no longer affects the degree of exposure of the bed. For lower bed slopes (i.e., less than 0.015, based on our experimental setting), bedrock exposure decreases more or less linearly with increasing values of the ratio of sediment supply rate to capacity rate. For higher bed slopes (i.e., more than 0.015), there is a range of lower values of the ratio of sediment supply rate to capacity rate where a bedrock surface becomes fully exposed without any alluvial deposit. For a given bedrock roughness, this range expands solely with increasing slope regardless of shear stress. Within the upper range of values of the ratio of sediment supply to capacity rate a linear relationship between the degree of bedrock exposure and this supply to capacity ratio still prevails. The addition of model boulders (can be viewed as very high hydraulic bed roughness) to the channel has been found to suppress the overexposure of bedrock, and so restore a linear relation between the degree of exposure and the ratio of sediment supply rate to capacity rate. Formulations for estimating bedrock exposure as a function of sediment supply to capacity ratio under different river channel characteristics are proposed. A linear relation between the degree of alluvial cover and the ratio of sediment supply rate to capacity rate is a previously proposed model. The present study expands the result for other cases including runaway alluviation. Some landscape evolution models assume an abrupt shift between fully exposed bedrock and complete alluviation, whereas others assume the linear relation....

[1] River delta complexes are built in part through repeated river-channel avulsions, which often occur about a persistent spatial node creating delta lobes that form a fan-like morphology. Predicting the location of avulsions is poorly understood, but it is essential for wetland restoration, hazard mitigation, reservoir characterization, and delta morphodynamics. Following previous work, we show that the upstream distance from the river mouth where avulsions occur is coincident with the backwater length, i.e., the upstream extent of river flow that is affected by hydrodynamic processes in the receiving basin. To explain this observation we formulate a fluvial morphodynamic model that is coupled to an offshore spreading river plume and subject it to a range of river discharges. Results show that avulsion is less likely in the downstream portion of the backwater zone because, during high-flow events, the water surface is drawn down near the river mouth to match that of the offshore plume, resulting in river-bed scour and a reduced likelihood of overbank flow. Furthermore, during low-discharge events, flow deceleration near the upstream extent of backwater causes enhanced deposition locally and a reduced channel-fill timescale there. Both mechanisms favor preferential avulsion in the upstream part of the backwater zone. These dynamics are fundamentally due to variable river discharges and a coupled offshore river plume, with implications for predicting delta response to climate and sea level change, and fluvio-deltaic stratigraphy.

[1] Many important insights into the dynamic coupling among climate, erosion, and tectonics in mountain areas have derived from several numerical models of the past few decades which include descriptions of bedrock incision. However, many questions regarding incision processes and morphology of bedrock streams still remain unanswered. A more mechanistically based incision model is needed as a component to study landscape evolution. Major bedrock incision processes include (among other mechanisms) abrasion by bed load, plucking, and macroabrasion (a process of fracturing of the bedrock into pluckable sizes mediated by particle impacts). The purpose of this paper is to develop a physically based model of bedrock incision that includes all three processes mentioned above. To build the model, we start by developing a theory of abrasion, plucking, and macroabrasion mechanisms. We then incorporate hydrology, the evaluation of boundary shear stress, capacity transport, an entrainment relation for pluckable particles, a routing model linking in-stream sediment and hillslopes, a formulation for alluvial channel coverage, a channel width relation, Hack's law, and Exner equation into the model so that we can simulate the evolution of bedrock channels. The model successfully simulates various features of bed elevation profiles of natural bedrock rivers under a variety of input or boundary conditions. The results also illustrate that knickpoints found in bedrock rivers may be autogenic in addition to being driven by base level fall and lithologic changes. This supports the concept that bedrock incision by knickpoint migration may be an integral part of normal incision processes. The model is expected to improve the current understanding of the linkage among physically meaningful input parameters, the physics of incision process, and morphological changes in bedrock streams.Citation: Chatanantavet, P., and G. Parker (2009), Physically based modeling of bedrock incision by abrasion, plucking, and macroabrasion,