Experimental study on the influence of cohesive sediment flocculation on the rheological properties of fluid mud

Fluid mud, widely distributed in coastal zones, estuaries, and reservoirs, significantly impacts regional ecosystems, environmental stability, and maritime navigation. The rheological properties of fluid mud, which reflect its capacity to resist deformation, flow, and structural changes, vary depending on its formation mechanisms. To quantify the effects of sediment flocculation on fluid mud density and rheological behavior, large-scale tank experiments were conducted to simulate cohesive sediment flocculation and deposition. Bed deposits were collected and subjected to rheometric testing. The results demonstrate that fluid mud density increases as the aggregate size decreases and the fractal dimension of aggregates increases in turbulent water. Both yield stress and viscosity of fluid mud exhibit power-law growth patterns as the aggregate size decreases and the fractal dimension increases. Specifically, a reduction in aggregate size by 0.02 mm and an increase in the fractal dimension by 0.03 correlate with an approximate yield stress increase of 0.3 Pa. Yield stress and viscosity also follow power-law relationships with fluid mud density, while the flow index decreases as density increases. When the fluid mud density remains below 1,110 kg/m³, yield stress and viscosity values show minimal divergence between flocculated and non-flocculated samples. However, beyond this critical density threshold, non-flocculated fluid mud exhibits abrupt accelerations in yield stress and viscosity growth rates, whereas flocculated fluid mud displays slower growth rates and higher flow indices, indicating enhanced fluidity. The disparity in yield stress and viscosity between the two types of fluid mud becomes increasingly pronounced with rising density. These macro-scale rheological differences arise from micro-scale particle interactions, particularly variations in the strength and quantity of contact points between sediment particles. Flocculation modifies particle configurations and interparticle forces, thereby altering macroscopic rheological responses. Analysis of rheological curves reveals that non-flocculated fluid mud under shear loading undergoes three distinct phases: solid-like, solid-liquid transition, and liquid-dominated phases. In the solid-like phase, shear stress exhibits approximately linear growth with increasing shear rate, while the growth rate gradually diminishes. The shear stress reaches its first local maximum at shear rates of 0.34-0.68 /s. The system then transitions into the solid-liquid phase, where shear stress initially decreases from the local maximum to a local minimum, followed by a nonlinear increase. The rheological complexity in this stage reflects the dynamic microstructural evolution of the fluid mud. Upon shear application, the internal structure of the fluid mud is disrupted, decomposing into primary sediment particles or smaller aggregates, thereby reducing cohesive interactions. As the shear rate increases further, the microstructure undergoes self-adjustment and reconfiguration, forming a new structural arrangement. A distinct inflection point emerges on the rheological curve at shear rates of 5.08-7.29 /s, marking the transition to the liquid-dominated stage. In this phase, the shear stress growth rate decelerates significantly, and the mud sample transitions into bulk fluid motion. In contrast, flocculated fluid mud, characterized by lower structural strength and higher porosity, lacks the solid-liquid transition phase due to its reduced structural integrity. Upon reaching a critical stress threshold, flocculated mud experiences abrupt yielding, transitioning directly into the liquid phase. Based on experimental data and mechanistic interpretations, two rheological models were developed by adapting the Bingham and power-law frameworks to explicitly incorporate the effects of flocculation. For flocculation-deposited fluid mud, the Bingham model approximates the linear stress-strain relationship in the liquid phase, while the power-law model more accurately captures the rapid stress growth at low shear rates. This study provides a foundational framework for numerical modeling of fluid mud transport dynamics, offering critical insights to address ecological, environmental, and navigational challenges associated with fluid mud accumulation. The established models enable precise simulation of flocculation-dependent rheological behavior, supporting targeted management strategies in sediment-laden aquatic systems.