Hydrodynamic Characteristics Of Sand Waves In The Taiwan Shoal

Submarine sand wave affects the roughness of seafloor, flow conditions and transporting of sediments. With development of the marine and utilization of marine resources, had found that the migration of sand waves would damage the port, waterways and submarine cables, offshore drilling, oil and gas pipelines and other offshore facilities. Therefore, understanding the submarine sand wave features and particularity would help us to reduce these potential hazards, while improving the understanding of seabed sediment transport, regional ocean economic development, disaster mitigation, scientific research, environmental protection, and even to provide a scientific basis for national security.The object of this paper is Taiwan shoal submarine sand waves, most of bathymetric data based on national public projects,"the Pearl River Estuary, Taiwan shoal topography monitoring and management support systems" subtasks-Seafloor topography and time sequence variation detection research project of Taiwan shoal demonstration area and the National Natural Science Foundation of China. And typical shallow stratigraphic data, information and hydrodynamic surface sediment data in demonstration areas of Taiwan shoal had been got. Analysis temporal distribution characteristics typical demonstration in central Taiwan Shoal, got the sand wave position, height, wavelength, defined the sand waves classification model. Contact the regional geological structure, sediment types, hydrodynamic characteristics, and historical information with pre-human studies to explore dynamic characteristics of the submarine sand wave in Taiwan shoal.The mainly research issues area as follows:(1)Submarine digital terrain model (DTM):for research submarine sand wave characteristics, all of this starting from the actual measuring bathymetric data, then build a digital terrain model of the study area with t multibeam bathymetric data. All the features of the region topography changes had been shown directly in using the digital terrain model, so we can get the submarine sand wave characteristics.(2)Main sand wave characteristics and sand wave trending analysis:Based on the submarine topography DTM, and relying on advanced software mapping techniques, such as deep drawing area topographic maps and related profiles. According to changed contours and depth data, sand waves on the main stage of the study area had been divided some parts. The sand wave height, wavelength, position and other temporal and spatial distribution would different in different part, drown the main sand wave crest line trending to capture the main sand wave distribution in study area.(3) Type classification system and type of secondary sand wave built:Based on the fine seabed topography DTM, The secondary sand wave that developed between main sand waves had been divided to refining seabed classification. Analyze the different distributions of secondary sand waves, proposed different developmental models of secondary sand wave in Taiwan shoal, and analyze the characteristics of various types.(4) Dynamic characteristics of the submarine sand waves:a combination study that obtained seafloor sediment sampling and ADCP hydrodynamic data in fix anchor, analyzed the dynamic characteristics of the seabed sand wave with these data.(5) Causes of the sand wave:According to research areas related information (eg depth factor, ocean dynamic factors, marine sediments and other related factors), to analyze the response mechanism in the study area seafloor topography and sand waves between each of these factors, understand its causes and regular pattern.Surveyed the east and west sides sectional study area of the Taiwan Shoal, the west of study area is the central focus of the whole area covered by the survey area subdivision detailed investigation and detailed investigation of the area to repeat the survey (before and after the typhoon and1-year intervals), Monitoring Profiles crossed the Taiwan Shoal SE-NW (typhoons before and1-year intervals), received a more detailed profiles data of Taiwan Shoal and regional cross-time, single event-scale, and combined with the hydrodynamic characteristics of the study area, sediment data and access to information and the shallow stratigraphic section data. Described east and west sides sand wave crest trends, distribution and partition morphological characteristics of the Taiwan Shoal, based on detailed investigation of the whole area covered by the data further subdivided into secondary sand wave and developmental patterns. After repeated investigation and monitoring of cross-sectional area measuring3times in2years, and discussed the variation of sand wave and its regional influence in impacted with a single typhoon and one-year time scale. Analyzed the obtained data and historical data, summarize the causes of sand wave of Taiwan Shoal, discussed dynamic in special power environmental (typhoon) and conventional power environmental (normal), proposed that the environmental impact of Taiwan shoal special power and conventional power built sandwave regional development and different modes.Conclusions were summarized as follow:(1) Submarine sand waves are mainly linear distribution on Taiwan shoal, according to the development of submarine sand waves that can be divided into three zones:the eastern part of Taiwan Shoal sand waves crest NE-SW trending zone, central sand wave crest E-W trending zone and the estern part crest SE-NW trending zone. The west study area located in central of Taiwan Shoal that is the area sand waves trending changing, and the east of the study area located in sand ridge topography developing area, they all have typical characteristics about sand waves, topography and morphology in Taiwan shoal.The terrain of west of the study area are mainly dominated by sand waves topography, and the topography are stably, have strong regularity. The main sand wave and secondary sand wave development in it, main sand wave crest run nearly E-W, SE-NW distributed to linear. Shallow stratigraphic section show that the crest and trough area are development in a parallel distribution, stratigraphic sequence are well, and the mutual structure of submarine sand wave is relatively stable. Secondary sand wave developed in trough area of Taiwan shoal that between two main sand waves with linear type and meniscus type, and they have different crest morphology in different location. Where the secondary sand wave developed that relative to its sides of the main sand wave position. So that the secondary sand wave can be divided into three types:the parallel type, the skew type and the divergent type. The whole west study area show that the dense sand waves develop in the northern part of west study area, but sparse distribution in southern. According to spatial relationships of sand wave and sand wave trending, the whole west study area can be divided into two sand wave special zones:1) the northern dense sand waves with parallel type and twigs like type zone;2) the southern various isolated sand waves in sparsely distributed in parallel zone. Statistics found that about85%of a single main sand wave’s northern slope is greater than its southern slope.87%of E-W trending parallel-type main sand wave’s (bimodal distribution of main sand wave) slope proportion (northern slope is divided southern slope) in northern slope is greater than the slope proportion in southern slope, and where the percent of main sand wave(with trending parallel-type and its value of slope proportion is less than1) is about77%of a total sand waves. So this area is concentration area with geomorphology of main sand wave’s northern slope is greater than its southern slopeThe east study area locate in sand ridges topography zone of southeast Taiwan Shoal, the main sand wave develop on the undulating topography that accompanied by sand ridges, main sand waves of east study area is similar to the west study area, the main sand wave crest line of east study area is very clearly, there are obvious regional characteristics of distribution and trending in differences location. The whole west study area show that the dense sand waves develop in the northern part of west study area, but sparse distribution in southern. According to spatial relationships of sand wave and sand wave trending, the whole west study area can be divided into two sand wave special zones:1) the southern dense sand waves zone;2) the sparse northern sand waves zone. And the sparse northern sand waves zone can be divided into three sub-districts by tidal ridges system in three ridges two troughs structure. Statistics found that about95%of a single main sand wave’s northern slope is greater than its southern slope.(2) Repeat area had been surveyed three times in two years west of the study area show that the distribution of submarine main sand wave is stably and the secondary sand wave’s skew type and patterns type did not change too. Characteristics sectional comparison shows that sand wave’s (main sand wave and secondary sand wave) height, wavelength, two slope gradient change a less.Main sand wave height eroded in typhoon and silting back in the later, it shows high-low-high process characteristics; the displacement of the peak position and prompting wavelength subtle changes in sand waves highly variable in this process; and back silting process is accompanied by the peak height of the trough erosion, making ripples two slope gradient increases, the peak sharpened. Mainly characterized shows that submarine sand wave morphology remained relatively stable throughout the study area, not change significantly. Comparison with before and after the2012typhoon monitoring section, the whole monitoring section has severe erosion and a small amount of siltation in typhoon, it is divided into parts by typhoon center, the east part of monitoring section erosion is greater the other side. And one year after the typhoon comparison found that the overall monitoring section except for a small amount of Taiwan bank erosion edge stuff, the rest mostly is deposition area. Typhoon center is bounded to the east side area deposition point is greater than the amount of deposition on the west side area, just the opposite result with typhoon.(3) Taiwan shoal depth study area suitable for development5-25m of the main stage sandwaves provides hosting space. The trending of main sand wave in east side of the study area and the west side of the study area is mainly affected by the rules of mutual coupling effects of M2semidiurnal tide tide oval axle length direction.Fix anchoring measured the underlying is SN direction as mainstream, the maximum current is96cm/s, the average velocity is about44cm/s. Calculating M2ellipse tide absolute value is0.3, so the current is eciprocating current in submarine sand waves area, provide appropriate conditions for the formation of linear hydrodynamic sand wave primary study area. Coupled with fine surface sediments with the average size, sorting coefficient, skewness of Taiwan shoal study area, and it is supplied from Taiwan Strait west coast of Guangdong, Fujian and Taiwan Strait east coast has a large number of rivers. With a large number of these rivers annually sediment supplying that constitutes a basic source of surface sediments, and a wealth sandy sediments of local Taiwan shoal as sediment source provide a well sediment source for sand wave development.(4)Special hydrodynamic environment affected by the typhoon events, the Taiwan Shoal effect of larger erosion. From the monitoring section, the east part of monitoring section erosion is greater the other side in special environment and study area silt up after special environment. The monitoring section shows that siltation is located at the back current position and sand wave crest. Comparing monitoring section1and monitoring section2found that the east erosion is larger than the west in monitoring section divided by typhoon center, and monitoring section1of the west region showed a gradual decrease from outside to the center of the typhoon siltation characteristics. In paper, raised two patterns (erosion and siltation) of the environmental impact of specific dynamic mode in typhoon and conventional dynamic mode after typhoon:1) Taiwan shoal in the typhoon environmental impact of specific dynamic mode, Typhoon cyclone intensified tidal current velocity of Taiwan shoal, broken the original sand wave homeostasis, the sediment discretely transported at high current speed, the whole Taiwan shoal erosion. The dominant current control the two sides erosion between typhoon center, they are in the same direction, they would have lager erosion, if opposite, they would have less erosion.2) Back silting after typhoon environmental impact of a conventional hydrodynamic mode:the sediment would be back that discretely transported at high current speed, amount equal to the amount of silt erosion, large amount of silt erosion needs large and small needs of small amount of erosion siltation, and the sediments would automatically transport to format the homeostasis, when small erosion area adjacent to large erosion zone, so it would be a new erosion hotspots. The sand wave process the environmental impact of typhoon specific dynamic erosion (peaks and troughs erosion) and conventional dynamic restoration (sand wave crest remodeling), when back silt deposition amount can’t meet the peak amount of remodeling, the entire system would cut trough to push sand wave peaks of self-regulation in order to maintain the dynamic response of sand wave and current. All of this confirms sediments in Taiwan shoal that compose previous residue deposited and modern sedimentary characteristics, and the modern process is affected by current and storm. According to the profile crest high-low-high changing, summarizes the sand waves erosion-deposition process in Taiwan shoal, and concluded the pattern about sand wave migration.