Assessing the influence of grain size distribution on fluvial bedrock erosion

Abstract:

Fluvial incision into bedrock sets the pace of landscape evolution and modulates the flux of sediment to downstream basins. Existing theory and experiments exploring bedrock incision via abrasion often use unimodal or single grain size distributions, and we lack data on how the presence of wide grain size distributions, as is common in many rivers, impacts bedrock erosion rates. Here, we present new results from over 100 abrasion mill experiments investigating the influence of multiple grain sizes on bedrock incision rates in a controlled setting. We explored the difference in volume of eroded bedrock for experiments using single grain size distributions in isolation, relative to the erosion that occurred using mixtures of these sediment sizes. Our results show that erosion from the sediment mixture was equal to the sum of erosion from all the single grain size experiments in isolation for conditions of sediment supply far below transport capacity. In contrast, as sediment supply approached transport capacity, the sediment mixture developed an armor layer that limited erosion earlier than in the single grain-size experiments, causing the mixtures to produce less total erosion than the combined single grain size experiments. These results provide critical, and currently lacking data that can be used to verify predictions of bedrock erosion from grain size mixtures using a combination of single-grain-size incision models and multiple-grain-size transport models.