Experimental study of bedrock channel alluviation under varied sediment supply and hydraulic conditions

Abstract

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. The present study shows that both formulations are valid under different settings of river characteristics.