Characteristics,Migration And Dynamic Mechanism Of Multi-Scale Sand Waves In The Taiwan Banks

The submarine sand waves are formed by the interaction between water column and sedimentary process,and are widely distributed,diverse in type and complex in origin.So far,submarine sand wave research is still hot focusing on main scientific problems such as formation mechanisms and applicants for hazard protection mechanism.Submarine sand waves are also an important type of submarine geological hazards.Their growth,deformation and migration can cause the rapid change in seabed surface,which is a serious geological hazard for sea-related engineering facilities,such as submarine oil and gas pipelines,submarine cables,oil platforms,sea-crossing bridges and offshore wind farms.On the topography of sand ridges,multi-scale sand waves with wavelengths of 10-1-103 m are superimposed in the Taiwan Banks,to form a typical continental geomorphic system of coexisting sand ridges and multi-scale sand waves.It is a typical sample for the studying modern submarine morphodynamic systems.Meanwhile,giant sand waves with wave height of over 10 m are unique in the China Sea and rare in the world.Their migration and formation mechanisms have not been solved.Based on multi-year high-resolution multi-beam bathymetric data,bottom-moored and shipborne observations obtained in the past 7 years,this thesis systematically studied the characteristics,migration and dynamic mechanism of the multi-scale sand wave system in the Taiwan Banks.The main scientific understandings are as follows:(1)The multi-scale sand wave system in the Taiwan Banks is mainly composed of primary giant sand waves(wave height of 6-18 m,wavelength of 400-1200 m),secondary sand waves(wave height of 0.5-3.0 m,wavelength of 25-100 m)and sand ripples(wavelength<0.6 m).As the main body of the multi-scale sand wave system,the primary sand wave height is much larger than other sand wave fields in the world under the same water-depth environment.According to sand wave shape,the primary giant sand waves are further divided into four types of trochoidal,trochoidal-progressive,bimodal,and progressive sand waves.Based on the analysis and statistics of 798 groups of sand waves,the correlation expression of sand wave height and wavelength in the study area is obtained for the first time:H=0.0625L0.8152.(2)The self-improved spatial cross-correlation analysis results show that the migration of the primary giant sand waves in the direction perpendicular to their crests is weak,but movements along their crests and accumulation-erosion changes in the vertical are significant.The superimposed secondary sand waves migrate perpendicular to their crests in the meter-scale during observations,with the migration characteristics of "northward in the north of the banks and southward in the south of the banks".Combined with hydrological observations,it was first found that the presence of the primary giant sand waves weakens the flow velocity component perpendicular to their crests and changes the bottom current direction,causing the sediment to transport along the giant sand wave crests,eventually leading to immobile primary giant sand waves during observations.The interaction of tidal residual currents and the background topography of the sand ridges in the south of the banks,determines the migration direction differentiation of the secondary sand waves in the north and in the south of the study area.(3)The dominated tidal current in the NE-SW direction in the northwest of the banks controls the bimodal-type primary sand waves to develop in the NW-SE direction,while the alternating effect of flood-dominated and ebb-dominated semi-diurnal tides forms the bimodal-shape.In the west of the banks,the dominated tidal current in the N-S direction controls the trochoidal-type and the trochoidal-progressive-type primary sand waves to develop mainly in the W-E direction,while the consistent effect of evenly matched flood and ebb tides forms the trochoidal-shape.(4)The simulated formation process of sand waves in the Taiwan Banks in three stages of the Holocene low sea level stage,the Holocene high sea level stage,and the present sea level stage indicate:(?)the primary giant sand waves is formed during the Holocene sea level rise;(?)during the sea level highstand,the primary giant sand waves are covered by newly-deposited beach sands and transformed by the gradually formed modern hydrodynamic environment;(?)when the sea level drops to the present elevation,the primary giant sand waves are retained,and the secondary sand waves and sand ripples are superimposed to form sand wave system that are compatible with the present hydrodynamic environment.