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 ~15 cm and SSC of ~8 kg/m³ at 10 cm asb. Once formed, the fluid mud layer dramatically modified the flow structure, showing a large reduction of current speed from 20 cm asb to 10 cm asb, 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.

@en

A key problem in the decadal morphodynamic modelling of tidal inlet system is on the evolution of the inlet channels, especially the unrealistic channel incision. The present study attempts to couple two processes, including the bedform roughness and sediment grain-size sorting, in the two-dimensional, depth-averaged process- based model to solve the problem. Based on the comparison of the modelling results and relevant field observations in the Dutch Wadden Sea, it is suggested that bedform roughness height predictor is a useful tool for determining spatial and temporal heterogeneous bed frictions.Within tidal channels, bedformroughness is dominated by dune roughness. In order to represent “the memory of old bedforms”, a relaxation time method was implemented. The corresponding large relaxation time (2 M2 tidal periods) smooths the tidal variations of dune roughness, and makes it respond to hydrodynamic changes on a longer time scale.When decadal morphological modelling is carried out, the model performance can be significantly improved by introducing either bedform (dune) roughness predictor or sediment sorting processes. The mechanisms are twofold. First, in the deep channels where flow velocities are high, large dunes develop and cause large bed drag coefficients. Second, coarser bed sediments within the channel area induced by sorting prevent the channel incision. If both effects are coupled in the morphodynamic modelling, the inlet channel incision is further controlled, which is not just a result of the linear superposition of both effects, because sorting can further promote the development of dunes within the deep channels. @en