Salt-wedge intrusion-retreat cycle induced sediment floc dynamics in bottom boundary layer (BBL) of a micro-tidal estuary

Abstract

Suspended sediment floc dynamics, particularly in the bottom boundary layer (BBL), has long been recognized as a complex and crucial factor influencing flow hydrodynamic and bed morphodynamics in tidal estuarine regions. Periodic salt-water intrusion in micro-tidal estuaries plays a crucial role due to its direct impacts on ecosystem, biological habitats, and human activities. To gain deeper understanding of the interactions between sediment flocculation and salt-water dynamics in estuaries, a continuous 115-h in-situ field survey onboard was conducted in a micro-tidal estuary, specifically the Huangmaohai estuary in China. This survey involved the measurements of suspended sediment concentrations (SSCs), floc properties, tidal current, turbulence and salinity in the BBL, utilizing state-of-art instrumentation such as the LISST-200X and ADV. Principal component analysis (PCA) was employed to examine the characteristics of sediment flocculation and analyze the influences of dynamic and structural factors on flocculation. The results reveal that, with the movement of the salt-wedge, two distinct stages of sediment flocculation process can be observed. Stage 1 occurs when the water column is vertically stratified, and a salt-wedge forms. Despite a reduction in SSC, this stage is characterized by a flocculation-dominated regime, in which large-sized, low density and low fractal dimension flocs settle to the bed. The presence of a stable salt-wedge enhances the flocculation by facilitating the formation of large-sized flocs with low density, low fractal dimension and high settling velocity while minimizing the disruptive effects of turbulence in the BBL. Statistical analysis reveals that saline water stratification contributes 19.76% to flocculation efficiency during this stage. Stage 2 occurs when the water column undergoes vertical mixing, causing the salt-wedge to dissipate. The regime transitions to a de-flocculation-dominated phase, leading to the resuspension of fine particles from the bed surface due to intense turbulence shear in the BBL. Consequently, flow shear becomes the key factor controlling factor for flocculation in the BBL with a contributing rate of 18.82%, statistically. A comprehensive conceptual model is presented to elucidate the intricate interactions between salt-wedge intrusion-retreat cycles and sediment flocculation within the BBL of a micro-tidal estuary.