Field, experimental and numerical investigations into the mechanisms and drivers of lateral erosion in bedrock channels

Abstract

The process of lateral erosion in bedrock channels is poorly understood. This thesis sheds light on the mechanisms of lateral bedrock erosion as well as the larger scale drivers of lateral erosion. Contained in this thesis are three distinct studies: a field-based study that investigates the drivers of lateral planation of strath surfaces; an experimental study that seeks to identify specific mechanisms of lateral erosion; and a numerical modeling study that seeks to corroborate the findings of the experimental study and permit exploration of parameter space. The field study (Chapter 1) concludes that lateral planation of strath surfaces in the dominant channel erosion process during periods of elevated sediment supply. This study further concludes that the elevated sediment supply conditions were driven by changes in climate. The experimental study (Chapter 2) concludes that the deflection of saltating bedload particles by fixed roughness elements into the wall is an effective mechanism of erosion. In addition, the experimental study identifies a minimum roughness threshold that must be crossed before significant lateral erosion can occur. Finally, the experimental study suggests that once the roughness threshold is crossed (i.e. moving from a smooth bed devoid of roughness elements to a bed with roughness elements), further increases in bed roughness do not produce ever increasing rates of lateral erosion. Rather, an erosion rate plateau is reached shortly after the roughness threshold is crossed. The numerical modeling study (Chapter 3) corroborates the findings of the experimental study and demonstrates, from first principles, that lateral erosion by deflected bed load particles is an effective mechanism of lateral bedrock erosion. In addition, this study identifies an important trade-off between increased deflection surface area from larger roughness elements and the increased form drag associated with the larger roughness elements. Here, the increase in the number of particle deflections is offset by the decrease in particle kinetic energy on impact, resulting in the erosion rate plateau observed in the physical experiments.