Weathering of Cohesive Mud Aggregates by Bedload Transport

Bed erosion, biological processes, slope failures, and dredging can produce dense mud aggregates (clasts, bed aggregates, fecal pellets, mud pebbles). Due to their size and density, these bed aggregates are more likely to transport as bedload than flocs or primary silt and clay particles, which transport primarily in suspension. The differences in transport mode between bed aggregates and flocs influences morphological response, biophysical feedbacks, management of marine transportation and flood management infrastructure. Mud aggregates decrease in size with transport (similar to cobble and gravel but at much faster rates). While gravel abrasion relates to mineral hardness, mineralogical composition, and structure, little is known of the factors that influence mud aggregate abrasion. The present study examines the physical aggregate characteristics and transport processes that contribute to mud aggregate weathering. Laboratory testing was performed, exposing mud aggregates of varying clay mineralogy, organic content, and density to the stresses of bedload transport. Bedload transport was generated by two methods: flow and transport in a rectangular flume and simulated by tumbling in a rotating drum. Aggregate breakup rates were determined from image and gravimetric analyses. Aggregate weathering occurred by two primary modes: abrasion and fracturing. Fracturing was observed for low plasticity aggregates (low water content) and recently formed, angular aggregates. Higher plasticity aggregates weathered by surface abrasion, reducing in size and mass by exponential decay.