Sedimentary Dynamic Process And Formation And Migration Of Sand Waves In Taiwan Shoal

Seafloor sand waves affect the roughness of the seafloor,hydrodynamic and sediment transporting conditions.Their movement may change the geomorphology and marine environmental conditions of the area.With the development of the marine utilization,sand waves become more important to human activities.The formation and migration of sand waves would damage the offshore facilities,such as waterways,ports,offshore drilling platform,oil pipelines and submarine cables.Therefore,sand wave is still one of the most studied area in marine geology by now.Understanding the sedimentary dynamic process and migration tendency of sand waves contributes to eliminating the potential hazards,as well as improving the local economic development,protecting environment and even consolidating the national security.The study area of this thesis is seafloor sand waves in Taiwan Shoal.The data was collected in June and July 2012.Sedimentary dynamic data and seafloor sediment samples were collected.The research issues were as follows:? ADCP hydrodynamic data in fix anchor and topographic transects were used to analyze the hydrodynamic characteristics of the seafloor sand waves.? Data by tripod-equipped facilities were used to analyze the tidal currents and waves.And combined with the seabed sediment samplings,the sedimentary transport characteristics were analyzed.?Numerical model(the Delft3D),based on field data,were applied to analyze the tide,tidal currents,residual currents,suspended sediment transport and bed load sediment transport characteristics and predicted the sand wave evolution in different time scale.?Delft3D simulation and the field collected data,such as hydrodynamic data,sampling data and sedimentary dynamic data,were combined to build ideal experiments to discuss the formation process and morphological evolution of the sand waves.Results were as follows:(1)Taiwan Shoal were dominated by rectilinear flow.The flood duration was almost the same with ebb duration,which is the feature of regular semi-diurnal tides.Flood velocity was stronger than the ebb by 0.5 m·a-1 in west zone A,while they were the same in east zone B.The velocity differed greatly in profile,maximum velocity occurred in flood and ebb slack.This is the typical characteristic of standing wave.(2)Harmonic analysis showed Taiwan Shoal was mainly dominated by M2 and S2 constituent.M2 tidal wave were advancing wave in south part of Taiwan Strait,whereas rotary in north edge of Taiwan Island.The two tidal wave met near the Taiwan shoal,forming standing wave.The amplitude of tidal wave increased from two ends to the middle of the strait,and the maximum amplitude can be as much as 2.0 m.However,with the sharply drop in the south,the amplitude decreased to 0.2 m.The transport pattern of M2 and S2 is similar.And compared to M2,the amplitude of S2 is smaller,which can only reach 0.8 m.(3)The wave generated bottom shear stress magnitude was 10-3?10-1 N·m-2,and can do little in transporting the sediment on normal conditions.Bottom shear stress generated by currents,as much as 4.51 N·m-2,was much bigger than wave or the threshold shear stress.Sediments can easily be transported thereby.The suspended sediment concentration and transport rate were 6.50×10-3?1.62×10-2 kg·m-3 and 3.93×10-3?7.54×10-3×10-3 kg·m-1s-1 respectively.Whereas,the bed load transport rate reached 0.01?0.15 kg·m-1s-1.Thus,bed load transports played a dominating role in the particular area.(4)The simulation of geomorphological evolution in 30 years showed that the evolution trend of Taiwan Shoal was that it would be deeper in the center and wider in the boundary.The east and west side of the shoal eroded severer than the central part.And it showed a deposition trend in the boundary line.(5)On the condition of ideal flat terrain and uniform slope,an array of tidal ridge,which is nearly parallel to the tidal current,would come into being in Taiwan Shoal.Certain slope contributed to the form of tidal ridge.In case of residual sand wave,the sand accumulation body in the shoal area is 5m under the trend level of the model.(6)The sand waves in zone A was asymmetric,with the migration speed of 14 m·a-1 southwest theoretically.The shape of zone B sand waves were symmetrical and moves little towards west.And the simulation showed that the sand waves may depress in the future.To improve the simulation accuracy,several methods can be carried out.Remote sensing method can help in getting more accurate bathymetry data.To refine the model needs more elaborate grid and combing the wave and wind force in simulation.Moreover,extreme conditions data can be collected and combine with normal condition data to get comprehensive sedimentary transport characteristic.