Date of Award
Master of Science (MS)
Abdul A Khan
Earl J Hayter
Nigel B Kaye
Recent research has found that dredged material placed in rivers and estuaries tends to erode as aggregated particles as opposed to individual particles. These aggregated particles, or mud aggregates, are then observed to undergo abrasion during bedload transport. Testing of these mud aggregates in an aggregate tumbler and a flume suggested that the aggregates could only travel a few kilometers before losing over approximately 90% of their effective diameter due to abrasion (Perkey et al., 2019). Current sediment transport models do not simulate the process of abrasion.
An aggregate abrasion routine was derived from existing research and then added to a one-dimensional (1-D) sediment transport model developed during this research and to an existing three-dimensional (3-D) sediment transport model. The abrasion routine was developed to simulate the abrasion of mud aggregates that were being transported as bedload. Instead of changing the diameter of the aggregate as it is transported and undergoes abrasion, abrasion was simulated by transferring mass from aggregates moving as bedload to the next smallest aggregate size class as well as to a 20 µm aggregate size class representing the byproduct of abrasion. As the 1-D and 3-D sediment transport models use a Eulerian grid, transferring mass between size classes allowed the abrasion routine to be used in a Eulerian grid, as opposed to using a Lagrangian time frame where the diameter of each individual aggregate would decrease while being transported as bedload.
Using the 1-D sediment transport model, the simulations involving the abrasion routine increased the total mass of suspended load of the mud aggregates by 0.5% to 1% and decreased the total bedload mass of the aggregates by 0.25% to 0.5% over an hourlong simulation in a closed system.
Using a 3-D sediment transport model of the James River, the inclusion of the abrasion routine to the simulation resulted in the bedload concentrations of the two largest mud aggregate size classes, analyzed at 1.2 km and 2.2 km away from the center of area of the placement site, to be less than 5% of the bedload concentrations given by the model simulation without abrasion when compared at each time step during a 15-day simulation. This indicated that the mud aggregates in bedload were losing over 95% of their mass within the first few kilometers of transport due to abrasion, which was expected based on prior research by Perkey et al. (2019).
In the portions of the navigation channel that were analyzed, all within 11 km of the placement site, the inclusion of the abrasion routine to the simulation resulted in approximately a 55% to 75% decrease in total mass of mud aggregates in the sediment bed compared to the simulation without abrasion when using a 15-day, 3-D sediment transport model of the James River. This was due to the abrasion routine transferring mass from larger mud aggregates in bedload to smaller mud aggregates, primarily in suspension, that were less likely to deposit.
Hollingsworth, Jonathan C., "Modeling the Abrasion and Transport of Mud Aggregates in the James River, Virginia" (2021). All Theses. 3498.