Turbulence and fine sediment dynamics in a benthic boundary layer in Fujian Coastal Sea, SE China

Abstract

A short-term (∼ 25 h) intensive field observation focused on flow, turbulence, suspended sediment concentration, and floc/particle size distributions was conducted in a coastal benthic boundary layer to understand flocculation and erosion/deposition processes of fine sediment. Observations showed suspended particles were mainly flocculated into the size range of microflocs (50-200 μm). Macroflocs (> 230 μm) occurred at slack tides and SPM (suspended particulate matter) concentrations decreased as they settled faster. Twofold intra-tidal variations in floc size and settling velocity were observed with a larger medium floc size of 140 μm and a settling velocity of 1.3 mm/s occurring at slack tides than peak flood/ebb tides with their corresponding values of 70 μm and 0.7 mm/s. This variability can be ascribed to the turbulence modulation in that low turbulence enhances aggregation to form large flocs but they turn to break down by high turbulence. The inverse relationship was found between floc size and turbulence shear rate, SPM concentration, and their product. SPM concentration covaried with bed stress and the low critical erosion stress (∼ 0.02 Pa) indicates that the cohesive bed was primarily eroded as aggregates which formed a fluff layer above the consolidated bed. The fine sediment dynamics behave as a loop from resuspension, through to breakup and re-aggregation, and to settling in response to intro-tidal turbulence variability. This looped behavior is vital for understanding the sequestration of organic carbon and pollutants in fine sediments, awaiting more detailed and longer-term field observations and the assemblage into numerical models.