Experimental quantification of bedrock abrasion under oscillatory flow

Abstract

Abstract Although wave-driven abrasion of submarine bedrock affects the evolution of rocky coasts and reefs globally, the dependence of the abrasion rate on wave forcing and sediment availability remains poorly quantified. We performed experiments in which an artificial substrate was abraded by varying amounts of coarse-grained sediment subjected to oscillatory flows. In these experiments, the bedrock incision rate scaled by the square of bedrock tensile strength (I, m yr–1 MPa2) varied with mean root-mean-square (rms) velocity (<urms>, m s–1) according to a power law, I = 1.0<urms>4.2 (angle brackets indicate time-averaging over an entire experiment). Additionally, the relationship between sediment load and bedrock incision rate demonstrates tools and cover effects similar to abrasion in fluvial environments, such that incision is fastest at intermediate sediment loads. However, because oscillatory flows accumulate sediment into bedforms, the increased bedrock exposure reduces the efficiency of the cover effect for high sediment loads relative to unidirectional flow. Our results provide an empirical model that can be used to predict bedrock incision rates in nearshore environments based on wave forcing.