The effect of wet-dry weathering on the rate of bedrock river channel erosion by saltating gravel

Previous work has shown that the bedrock erosion rate E because of collisions of saltating bedload can be expressed by E = βq_b(1-P_c), where q_b is the sediment transport rate, P_c is the extent of alluvial cover, and β is the abrasion coefficient. However, the dependence of the abrasion coefficient on the physical characteristics of the bedrock material is poorly known, and in particular, the effects of wet-dry weathering on the saltation-abrasion bedrock incision has not been specifically characterized. Observation suggests that the typical wet-dry cycling of exposed bedrock in river beds gives rise to cracks and voids that are likely to alter the incision rate of the material when subjected to impacts of moving sediment. In this study, flume experiments are performed to develop an understanding of how wet-dry cycling affects the rock tensile strength and the bedrock erosion rate. To represent the physical effects of weathering, boring cores taken from natural bedrock channel are exposed to artificial wet-dry cycles. The experimental results suggest the following: (1) the abrasion coefficient for fresh bedrock is estimated by β = 1.0 × 10^− 4σ_T^− 2(d/k_sb)^0.5, where σ_T is the tensile strength, d is the diameter of colliding gravel, and k_sb is the hydraulic roughness height of bedrock; (2) the tensile strength of the bedrock decreases exponentially as a result of repeated wet-dry cycles, σ_T/σ_T0 = exp (-C_TNW_a0/σ_T0), where σ_T0 is the initial tensile strength, W_a0 is the initial normalized rate of water absorption., N is the number of wet-dry cycles, and C_T is a constant; (3) the erosion rate of fresh bedrock depends on the inverse of the square of tensile strength, but the erosion rate of weathered bedrock depends on the − 1.5 power of tensile strength.