Physical and sedimentary processes on the tidal flat of central Jiangsu Coast, China: Headland induced tidal eddies and benthic fluid mud layers

Abstract

An 11-tidal-cycle record time series of current, wave, suspended-sediment, and bed-level characteristics was analyzed to identify physical and sedimentary processes on the tidal flat of Jiangsu Coast, China. A tripod observation system was placed near the transition between mid and upper tidal flat south of a newly constructed harbor for hydrodynamic and sediment dynamic measurements from 27 Apr 2013 to 3 May 2013. The observations confirm the stable longshore northward (ebb direction) current s with residual velocities ~59.2° anti-clockwise from the offshore direction. This phenomenon can be attributed to the effects of headland (the Harbor) induced tidal eddies based on comparable frictional length scale and the headland length scale. Benthic fluid mud layers occurred in 2 of 11 tidal cycles, with the conditions of strong waves during the flood phase. The fine sediment was resuspended by the waves and currents from the lower area, transported upward and concentrated at the observation station, resulting in the formation of a fluid mud layer with thickness of ~15cm and SSC of ~8kg/m3 at 10cmasb. Once formed, the fluid mud layer dramatically modified the flow structure, showing a large reduction of current speed from 20cmasb to 10cmasb, when the gradient Richardson number was around the critical value of 0.25, inferring that sufficient turbulence from waves and currents exists to maintain fluid mud suspension. The fluid mud processes appear to occur episodically and may play an important role of sediment dynamics on the tidal flat.