The influence of rock properties on bedrock and sediment erodibility in rivers

Abstract

Erosion of landscapes depends on the physical and chemical attack on rock and the resistance to breakdown by bedrock and sediment. Physical erodibility of bedrock has been shown experimentally to scale with the square of tensile strength, but this correlation has not yet been extended to sediment or field predictions of breakdown. This work combines experimental studies and fieldwork to investigate the role of rock properties in bedrock erodibility and sediment comminution from fluvial transport, using lithologies spanning the variation of rock properties found in the field. The laboratory studies use rock property measurements to investigate variations in experimentally measured bedrock erosion rates and sediment abrasion rates. Scanning electron microscopy is used to investigate textures resultant from different erosional processes and the evolution of rock surface textures through sediment abrasion. In the field where lithologic contacts create discrete upstream source areas of distinct rock types, the evolution of sediment size distributions due to particle breakdown downstream of the contacts can be scrutinized. Field results are compared with laboratory tensile strength measurements and tumbling abrasion rates to distinguish in the field between sorting and comminution and to estimate field size reduction due to breakdown. This lab work shows the erodibility of bedrock and sediment varies systematically with tensile strength and mineral grain size, not elasticity. These field and lab results may be useful in developing theory for predicting sediment comminution rates and to help establish an empirical scaling relationship between the relative and absolute erodibility of bedrock and sediment.