How does the deposition and resuspension process alter the size and resuspension rate of mud flocs?

Cohesive mud flocs in coastal waterways can go through repeated cycles of deposition and erosion before being sequestered in a final deposit. Unlike sand and gravel, mud flocs have the potential to change size and density during their time on the bed. For example, it is conceivable that flocs could increase in density or aggregate with other flocs while on the bed. However, what happens to flocs while on the bed, and how any changes during this time might influence their resuspension characteristics, is poorly understood. Here we use laboratory experiments to quantify the influence of sediment and water column properties on resuspension characteristics of mud flocs. Specifically, we examine how concentration at the time of deposition, the time for which the deposited material is on the bed, and the turbulence levels in the water column while the flocs are on the bed all impact the resuspension characteristics. The resuspension characteristics we quantify are: (1) the difference between the floc size population before and after resuspension, (2) the rate of resuspension, and (3) the difference between the concentration at the time of deposition and at full resuspension. We found that resuspended flocs revert to the size population at the time of deposition within a few minutes. This occurs regardless of the time on the bed between deposition and resuspension. The results also show that under low turbulence levels, flocs moving around on the bed increase in size relative to their suspended counterparts, and that such flocs can be more difficult to resuspend. Furthermore, the time that the flocs are on the bed (6 hours to 12 days) has little impact on the resuspension rates unless the concentration in the water column at the time of deposition is high. For the higher concentration cases, the bed began to compact and become more resistant to erosion. Nevertheless, the mud eroded from these beds had floc characteristics that are consistent with the local mixing conditions.