Influence of sediment sources, water mass, and physical processes on the dynamics of flocs at a location between the mouth of a river and the head of a submarine canyon

Abstract

A field experiment was conducted in the summer of 2011 to obtain detailed process–response data to understand the dynamics of suspended sediment off the Gaoping River mouth near the head of the Gaoping Submarine Canyon in Southern Taiwan. An instrumented tetrapod and a collocated floating platform were used to record the temperature, salinity, current and wave fields. Additionally, the volume concentration (VC) and mass concentration (SSC) of suspended sediments were recorded. Temperature and salinity signals over a tidal cycle distinguished water masses originating from the river plume and the canyon, which alternately dominated the surface dynamics of suspended sediment at the study area. Terrestrial–sourced suspended sediments aggregated into larger flocs, characterized by high VC, SSC, and low effective density values. Conversely, marine–sourced suspended sediments generally had smaller grain sizes, characterized by lower VC, SSC, and high effective densities. In the surface water, the effective density of suspended sediments decreased exponentially with size. In the bottom water, values of VC and SSC were higher under energetic wave conditions and lower under nonenergetic wave conditions, whereas mean size of suspended sediments showed an opposite trend. The hydrodynamics also affected the sediment–water interface, which was reflected in the grain size composition of surficial sediments. The silt and clay content on the seabed was lower under energetic wave conditions and higher under nonenergetic conditions. According to process–responses characteristics, suspended sediments in the bottom water were associated with two regimes: smaller mean size with lower effective densities formed under energetic wave conditions and larger mean size with higher effective densities formed under nonenergetic wave conditions. In the former regime, waves–induced turbulence dominated suspended sediment dynamics by entraining seafloor sediments into the bottom water and aggregating them into microflocs (<~125 μm) and macroflocs (> ~ 125 μm). However, the size distribution of suspended sediment showed a bimodal distribution, and the larger peak is close to the Kolmogorov microscale (around 250 μm). It suggests that strong turbulence associated with significant sediment suspension and flocculation also resulted in disaggregation of macroflocs. As the wave energy dissipated, most of suspended sediments including all fine ones were removed from the bottom water, via flocculation and deposition. Under nonenergetic wave conditions, suspended sediments in the bottom water were contributed from upcanyon water and river plumes, controlled by semidiurnal tidal currents. The size distributions of suspended sediment under nonenergetic conditions were mostly unimodal, with peaks around 100–300 μm, less than the Kolmogorov microscales of turbulence flow (350 μm). Therefore, it could be concluded that turbulence strength plays a key role in flocculation of suspended sediment in the bottom water.