Floc sizes and resuspension rates from fresh deposits: Influences of suspended sediment concentration, turbulence, and deposition time

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. Such changes could influence the rate of resuspension or the size characteristics of resuspended flocs relative to those that do not spend time on the bed. 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 length of 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 nature of the flocs on the bed and their 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 full resuspension. The experiments show that flocs grow in size while on the bed, but that resuspended flocs quickly revert to size populations that are in equilibrium with the local water column conditions. 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 (0.25-12 days) has little impact on the resuspension rates unless sediment concentration in the water column at the time of deposition is above a critical value for the onset of a space and load-bearing network. For concentrations above this critical value, freshly deposited mud begins to consolidate and become more resistant to erosion with time.