2019 CSDMS meeting-091

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The influence of bio-physical processes on sediment motion over tidal flat

Ya Ping WANG, Nanjing University Nanjing , China. ypwang@nju.edu.cn
Jiasheng LI, Nanjing University NanJing , China. lijiasheng1984@126.com

Robust estimate of the critical shear stresses and flocculation is a prerequisite for analyzing the sediment dynamics of tidal flats. To determine the impact of clams (Meretrix meretrix Linnaeus) on critical shear stress and flocculation, in situ measurements were made on both a bare flat and a flat inhabited by abundant clams. on the Jiangsu Coast, China. Near-bed in situ floc size, grain size distribution of suspended particles in sea water, suspended sediment concentration (SSC), salinity, and currents were measured for three consecutive semidiurnal tidal cycles simultaneously at the two stations. Based on the observational data, mean floc size measured by LISST-100x was found to be more than three times the size of dispersed suspended sediment in water samples, suggesting remarkable flocculation processes were occurring. Correlation analysis indicated that the flocculation and break-up process in the study area appeared to be controlled by the variations in SSC and turbulent shear. Negative relationships were found between SSC, turbulent shear and floc size for both stations, but for a given SSC, floc size was usually larger at the clam aquaculture site. The comparison between the two sites shows that the near-bed in situ floc size in the aquaculture mudflat (mean 88 μm) was 70% larger than that in the bare tidal flat (mean 51 μm), suggesting significant biological modulation on the flocculation processes, as the hydrodynamics were very similar between the two sites. The content of extracellular polymeric substances was obviously higher in the sediment layer below the surface seabed at aquaculture site. Further, we find that the sediments on the flat inhabited by clams were more erodible and had a lower critical shear stress for erosion (0.13 N m-2) due to the physical and biological activities of clams. The results further show that the critical shear stress for deposition on the flat with M. meretrix (0.13 N m-2) was 30% greater than that in the bare flat (0.10 N m-2). Our results suggest that changes to the critical shear stress for erosion and deposition caused by the activity of clams can alter the sediment dynamics and geomorphologic processes of flats, as well as abundant filter feeders alter floc properties and enhance flocculation by excretion of exopolymer particles.