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Resuspension and physical properties of natural cohesive sediment in estuaries.
Komai, Katsuaki1, Hibino, Tadashi1, 1 Depertment of Social and Environmental Engineering, Higashi-Hiroshima, Hiroshima, Japan
ABSTRACT- In semi-enclosed eutrophied estuaries, a fluid mud layer is observed above the seabed, especially from summer to autumn. Since nitrogen and phosphorus absorb on the sediment particles of fluid mud in high concentrations, the mechanisms of resuspension of cohesive sediment and fluid mud movement with disturbance by wave and tidal current are important to coastal environments. Because the resuspension of cohesive sediment depends on physical and chemical properties of sediments and bed shear stress by disturbance of wave and tidal currents, the generalization of the resuspension mechanism is difficult determine. The purpose of this study is to clarify the physical properties (water content, wet density and vane shear strength) of natural cohesive sediment and the actual resuspension state with long-term observation in semi-enclosed estuary and hydraulic model experiments. Natural cohesive mud layers and fluid mud layers were sampled with a bottom mud sampler without disturbing the sediment layers in Hiroshima Bay. The vertical profile of vane shear strength was measured. At the observation point, time series of current speed and turbidity above the seabed were observed continuously for one month when the fluid mud layer was generated. The actual resuspension state under natural disturbance by wave and current was confirmed. The resuspension of natural cohesive sediment by wave or currents was investigated in a wave channel and a circular channel with sampled sediments and salt water. In the experiment, the physical properties of the sediment and bed shear stress by water flow were confirmed, and suspended sediment concentration was measured. From integration of suspended sediment concentration, the amount of resuspension was estimated.
Key words: resuspension, physical properties, cohesive sediment, bed shear stress