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Experimental Quantification of Bedrock Abrasion Under Oscillatory Flow

Abrasion is a powerful erosion force that shapes our landscapes. While stream-driven abrasion has been extensively studied in the lab, wave-driven abrasion has not, partly due to the difficulty of recreating waves in the lab. With two BlueRobotics T200 thrusters and Basic ESC controllers, the authors created a U-Tube that pushed sediment back and forth with the same force as a moderate wave in shallow water. The authors were then able to quantify how abrasion rate changes with different wave characteristics. Additionally, they found that abrasion rate changes in an interesting way with the amount of sediment lying around. With this research, the authors are now able to predict erosion rates on coral reefs and rocky coastlines more accurately.

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 (, m s–1) according to a power law, I = 1.04.2 (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.

Author: Bramante, J. F.; Perron, J. T.; Ashton, A. D.; Donnelly, J. P.

Journal: Geology

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