Numerical Simulation On The Geomorphodynamics Of Tidal Channel-sand Ridge-tidal Flat System In The Southern Yellow Sea

The radial sand ridges system located on the seafloor of southern Yellow Sea,which is a unique large geomorphological system on the continental shelf.The radial patterns of the tidal current field and sand ridge-tidal channel shape have been discovered in between 1960's and 1980's.The change of geomorphology,the hydrodynamic and sediment dynamic features in this sand ridges system have received more and more attention and been under widely research since then.Moreover,the tidal channel,sand ridge and coastal intertidal flat form a complex tidal flat in terms of elevation along the onshore-offshore cross-sections,which is different with the previous study on the coastal intertidal flat with the monotone elevation variation.In this paper,the 2DH Delft3D hydrodynamic and sediment dynamic models were applied to simulate the tidal current and suspended sediment concentration fields,for the further calculation of residual current and net sediment transport in this region over the time scale of months.The geomorphological change of the whole radial sand ridges system was simulated between 1979 and 2011.The sediment distribution feature and mechanism for the topographic evolution were further analyzed.The results showed that:(1)The tidal current was quite strong in the study area.The current speeds ranged from 0.5 to 2.0 m/s during the spring tide,which increased landward and had the spatial pattern with stronger velocity in the trough than the shallow shoal.The tidal variation of suspended sediment concentration(SSC)was related with the current velocity,which was resulted from the dominating effect of re-suspension in this region.The calculated SSC field had higher value on the seaside sand ridges than the adjacent troughs,while the SSC maintained high value over 0.4 kg/m3 in the inner tidal channel over the tidal cycle,even in the low slack tide with smallest current speed.In addition to radial sand ridges area,the abandoned Yellow River Mouth and the east to Lengjiasha Sand Ridge also exhibited large SSC value.(2)Over the time scale of neap-spring tides,the Euler residual current dominated in the Lagrange residual current in the study area.The direction of Euler residual current was mainly controlled by topography and had a tendency to follow the isobaths outside the sand ridges.The north of Abandoned Yellow River Mouth,the part between northern radial sand ridges systems and Abandoned Yellow River Mouth developed an anticlockwise and a clockwise residual circulation respectively.A stream of residual currents valued pointed to the southeast along 50m isobaths outside the sand ridges.A clockwise residual circulation generally developed around the outer sand ridges.The water was net transported landward in nearshore tidal channels of southern radial sand ridge system,while it was net transported seaward in the northern channels.There was a steam of strong southeast Stokes residual current flowing from Abandoned Yellow River Mouth into the Xiyang Tidal Channel,with the current speed up to 0.05?0.1m/s,which might be major component of Subei coastal currents.(3)The transport patterns of bedload and suspended load sediment were quite similar with the pattern of residual current,and the latter was 2?3 orders of magnitude larger than the former.The suspended sediment was net transport around the outer sand ridges as the residual current.The transport rate generally ranged from 0.2?0.4 kg/s/m.Also,sediments were net transported in the southeast direction from Abandoned Yellow River Mouth into the southern radial sand ridge system,in the rate between 0.2 and 0.5kg/s/m.Over the time scale of decades,the bedload and suspended sediments were net transported from south to north in the center radial sand ridges by advection and diffusion.The possible net transport direction in the outer radial sand ridges might be to the south.(4)Between 1979 and 2011,the main tidal channel experienced drastic scouring and became straighter.The small underwater sand ridges merged,connected and elongated to the linear shape in the plane.The accumulated region mainly appeared in the inner sand ridges and the southern slope of outer sand ridges.The above landward combination and migration of sand ridges can be regarded as the geomorphological manifestation of the spatial gradient of net sediment transport.The fine component of sediments were proved to accumulate mainly in three areas,namely north of radial sand ridges system,east of Yangtze River Mouth and coastal intertidal flat,while the coarser component tended to accreted near the sand ridges.(5)There were also differences between the evolution processes of two complex tidal channel-sand ridge-intertidal flat systems.The merging mainland type,represented by Jianggang tidal flat develop into a complex onshore-offshore cross-sections with non monotonous changes.The offshore aggradation sand ridge tidal flat,which was represented by Dafeng-Xiyang-Dongsha,generally accumulated upward near the mean water level,when the tidal channel nearby experienced drastic erosion.