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Importance Of Muddy Bed Aggregate Processes In Cohesive Sediment Dynamics Associated With Sediment Management Projects
Perkey, David
Perkey, David
Abstract
The erosion and transport processes of fine sediment is largely impacted by the aggregation state. Understanding fine sediment transport processes is a key component to managing the nation’s navigation channels, ports, and reservoirs. To improve its ability to apply management strategies related to fine sediments, the USACE has undertaken research that focusses on the aggregation state of fine sediment. Of particular interest is the ability to expand the use of fine-grained sediment in projects that seek to beneficially use dredge material. In this study, a newly developed camera system was used to evaluate the aggregation state of eroded sediment from a variety of locations across the country. A field-based study examined cores collected in the tidal James River, VA and showed that bed sediments were composed mostly of mud, but that erosion predominately occurred in the form of aggregates with median sizes 10s-100sx larger than the disaggregated sediment. A numerical simulation demonstrated that mud aggregates were predicted to transport in incipient suspension or bedload, while disaggregated fines were predominately maintained in full suspension. This difference in transport mode has significant implication for sediment management and dredge material placement decisions within the system. Laboratory tests were performed on materials from numerous sediment management project locations. Aggregate clasts were produced across a wide range of shear stresses (0.1 – 8 Pa) and bed densities (1.2-2.0 g/cm3), demonstrating their presence across high and low energy conditions and poorly and well consolidated beds. Macro-aggregate (>250 µm) production was highly variable, ranging from 3%-72% of the eroded sediment mass. Physical properties commonly associated with cohesive behavior were evaluated to determine if they could be used as predictors for bed aggregate production and size. While physical properties of sediment such as water content, bulk density, and the solid volume fraction of the mud matrix were found to correlate with the erosion of bed aggregates, clay content was the most consistent property for predicting bed aggregate size and production. Additionally, durability testing indicated that bed aggregates in this study were robust enough to withstand tumbling on the order of 100s of meters and that their break-up rate followed Sternberg’s Law for determining bedload transport distances. Therefore, routine physical property testing of sediment allows for the estimation of macro-aggregate abundance and size immediately following erosion and the prediction of their bedload transport distances prior to break-up. These methods offer critical information needed for engineers and scientists to include aggregate processes into the numerical models and other decision-making tools used in the management of projects involving cohesive sediments.
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2022-01-01
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Virginia Institute of Marine Science
DOI
https://dx.doi.org/10.25773/v5-7q4t-rj65