Controls on the geometry of potholes in bedrock channels

Abstract

AbstractPotholes (circular depressions carved into bedrock) are the dominant roughness elements in many bedrock channels. Here we show, using data from previous studies and new data from the Smith River, Oregon, that pothole depths increase in proportion to both the mean pothole radius (such that the most common pothole depth‐to‐radius ratio is 2) and the diameter of the largest clasts episodically stored in potholes. We present a theory for these observations based on computational fluid dynamics and sediment transport modeling of vortices in cylindrical cavities of different shapes and sizes. We show that the shear stress at the bottom of a pothole (which controls the rate of pothole growth) is maximized for potholes with a depth‐to‐radius ratio of approximately 1 and decreases nonlinearly with increasing depth‐to‐radius ratio such that potholes with depth‐to‐radius ratios larger than 3 are uncommon. Our model provides a mechanistic explanation for pothole shapes and sizes.