Feedbacks between dissolution, abrasion, and bed roughness : a flume investigation of carbonate bedrock incision

Traditionally, the chemical dissolution and erosion of bedrock in eroding river channels has been viewed as minor to negligible in comparison to mechanical weathering processes, such as impact wear from transported bedload sediment. However, for relatively soluble rocks, such as carbonates and evaporites, dissolution could be a significant contributor to erosion. The motivation for this project is to understand controls on the relative importance of bedrock dissolution and abrasion under different lithological and hydrological conditions. I hypothesize that physical and chemical erosion will cause subsequent increases in bed roughness that will enhance dissolution. In addition, I hypothesize that dissolution will result in a more spatially uniform distribution of erosion scales as compared to abrasion because chemical dissolution is the product of water running over the entire bed whereas abrasion is the end result of sediment particles impacting the bed in localized areas. Through laboratory experiments, we actively eroded a flume bed made of plaster of paris (gypsum, CaSO4·2H2O) with water and very fine gravel to observe the processes of dissolution and abrasion. Plaster of paris was used as a proxy for carbonate rock in these experiments due to its high solubility relative to carbonates and ability to be easily cast into a suitable size and shape. High resolution measurements of topography were made with a triangulating laser and 3-D scanner to quantify changes in the bed form as physical and chemical erosion occurred. The spatial and temporal evolution of erosion rates and surface roughness were quantified from topography. Electrical conductivity was measured throughout the experiments to infer rates of gypsum dissolution and link water chemistry with changes in physical bed topography. I find that the spatial distributions of erosion are dependent on whether the bed topography has evolved due to dissolution or abrasion. The erosion distributions for both dissolution and abrasion are positively skewed with the most of the changes in bed topography being in contained areas of high erosion. However, across experiments, the erosion distributions for dissolution are closer to normal than the distributions for abrasion.Geological Science