Physical Factors that Influence Muddy Bed Aggregate Production During Erosion

Past scientific literature has shown that mud aggregates have frequently been observed in both the geologic record and modern depositional environments. Aggregation state significantly influences the size, density and transport characteristics of fine sediment. Understanding sediment transport and deposition processes in the nation’s navigable waterways is a primary mission for the US Army Corps of Engineers (USACE). Currently, bed aggregates associated with cohesive sediment erosion are largely absent from numerical models used to predict sediment transport. Instead, transport models frequently rely on sediment properties obtained from analysis of disaggregated bed samples, which may not best represent the physical state of material mobilized from cohesive beds.

Recently, the USACE has been conducting studies to evaluate the impact muddy bed aggregates have on sediment management issues within the waterways it manages. Identifying physical properties of sediment that may influence the production of bed aggregates is an important aspect of this research. To address this issue, sediment from a variety of harbors and channels managed by the USACE were tested with a newly developed camera system to evaluate the aggregation state of eroded particles. Results showed that bed aggregates were consistently produced in sediments across a wide range bed density, organic content, water content, plasticity index, and sand content. In mud dominant sediments, median aggregate size was approximately 40 to 400x larger than the median size of the disaggregated bed material. Additionally, the most consistent predictor for the presence of these mud clasts was found to be sand content in the sediment bed.