Experimental measurement of flocculation and settling behavior in fine sediments from the Three Gorges Reservoir, China

Abstract

Flocculation of cohesive sediments is widely observed in natural aquatic-sedimentary environments, such as river estuaries and nearshore coastal waters, where salinity, suspended sediment concentration and flow turbulence cause fine sediment particles to aggregate into larger flocs, resulting in intensified sedimentation. In contrast, there is currently very little evidence linking enhanced sedimentation, leading to significant storage loss within large freshwater reservoirs, specifically to fine sediment flocculation processes. Massive fine sediment volumes have been deposited in the Three Gorges Reservoir (TGR), China, which has been impounded over the past two decades. It has not yet been established what role, if any, fine sediment flocculation processes play a role in the amplified sedimentation rates that have been observed. The current study aims to test the hypothesis that fine sediment flocculation processes, which occur under prevalent hydrodynamic (i.e., turbulent shear rate G ≈ 4.67–227.39 s⁻¹) and sedimentary (i.e., sediment concentration C ≈ 0.02–2.00 kg/m³) conditions in the TGR, play important roles in sedimentation rates during reservoir impoundment. A laboratory-based, experimental study was conducted within a new, bespoke Couette flow system designed specifically to mimic the environmental shear flow conditions experienced during impoundment. Sediment samples obtained directly from the TGR bed deposits were tested within this facility to ascertain their flocculation and settling behavior under controlled shear flows and sediment concentrations. A comprehensive analysis combining the sediment composition, measured aggregate/floc size distributions and their corresponding settling behavior collectively pointed to the occurrence of fine sediment flocculation within the TGR. By disentangling the individual influences of flow velocity and sediment concentration on floc size, the combined influence of the flocculation parameters C/G^1/2 on floc size is fully revealed through experimental evidence, with a new empirical combined parameter, C^0.44/G^0.47, identified as being more appropriate for characterizing the variability in floc size under varying flow‒sediment conditions. The majority of flocs measured in the tests have a mean settling velocity nearly five times greater than that of the majority of primary fine sediment particles (i.e., with diameter D ≤ 29 μm) that comprise the sediment grain size distribution found in the TGR impoundment. This finding provides new evidence for the potential for flocculation-enhanced sedimentation rates occurring within the TGR and other similar large freshwater reservoirs, as dam construction disrupts the natural sediment continuity within the river system and inhibits the downstream transport of fine sediments.