| Submarine dune is a typical kind of flow-transverse rhythmic bedform,developed in fluvial,intertidal,and marine environments.The study on its morphology and evolution mechanism can not only improve the understanding of concerned sedimentary dynamics,facies and environment,but also help to effectively avoid the hidden dangers to submarine pipelines,harbors and navigation channels.So far,the formation and evolution of submarine dunes have not been well understood,especially the morphology and origin of low-angle dunes(LADs).Quantitative calculation of morphological parameters is the basis for the research on the morphology of submarine dunes.With the rapid development and widespread use of multibeam echo-sounder,high resolution water depth data is now available,which makes the study on the fine bedforms possible.As to the quantitative study on bedforms based on the high resolution bathymetric data,it is urgent to establish an accurate and efficient automatic analysis method.The offshore area along Jiangsu coast with the development of sand ridge field has the distinguished characteristics with rapid variation of the water depth and strong tidal dynamics.The study area is chosen in the central part of the sand ridge field to cover part of a sand ridge and a tidal trough..Multibeam echo-sounder exploration and seafloor surface sediment collection were conducted during January of 2017.In this study,an automatic quantitative analysis method for the morphology of submarine dunes was established.Applying this method,the morphologic parameters of submarine dunes developed in the study area were calculated.The spatial differentiation of the morphologic parameters was analyzed.Comparing with the water depth,sediment grain size and regional hydrodynamics,the main controlling factor of the LADs were discussed.The feasibility of the spectrum analysis methods was verified by using the ideal data of bedform superimposition model,but several problems were found in the calculation of the bathymetric DEM data.The DEM data was preprocessed and the extraction method of the target wavenumber of two-dimensional Fourier transform was optimized.It improves the accuracy of calculating the regional dominant orientation and wavelength of submarine dunes.The best method of different bedform separation is wavelet transform with data preprocessing.The morphological parameters of submarine dunes were calculated based on profile data filtered high frequency data noises.It has the advantage of calculating morphological parameters based on the original observation data,and at the same time it reduces the influence of data noise and small-scale superposition bedforms.It also avoids the deviation of submarine dune steepness and symmetry calculation results based on the target bedform profile.Previous researches on bedform automatic analysis methods were mostly classified by methods,however,different methods have absolute advantages at different stages.The methods with optimal performance of each phase were grouped into a set of frameworks for quantitative analysis of submarine dunes.This enables the automatic analysis of the quantitative calculation of the regional bedform morphological patterns and individual bedform parameters.Two-dimensional linear submarine dunes are generally developed in the study area.Along the east-west direction extending sand ridge,the morphology of the submarine dunes show the similarities.Three representative plots in the study area are selected at the northern slope,middle ridge and southern slope respectively to analyze the differences of the morphological characteristics of submarine dunes.Based on the statistics of total of 13,234 submarine dunes extracted in the three plots,all of these submarine dunes are LADs with the average leeside slope angle of 3.75°,and the maximum of 9.09°.The largest LADs are formed on the northern slope of the sand ridge,with the average wavelength of 37.36 m and average wave height of 0.63 m.Meanwhile,the LADs on the southern slope are the smallest,with the average wavelength and height of 15.61 m and 0.36 m.The morphology of LADs show the regional characteristics,and its spatial distribution has the close relationship with the topography of the ridge and trough.The sizes of LADs at the northern and southern slopes of the sand ridge are different obviously,and their moving direction are opposite as well.The common variation trend could be seen between wavelengths and wave heights of LADs at the northern slop.The steepness is positively correlated with the leeside slope angle,and they have the consistent change from north to south.The relationship between morphology of LADs with water depth,sediment grain size and tidal current velocity were analyzed progressively.It is obviously that the main controlling factor for the morphology of LADs at the different position of the sand ridge is also different.Comparing with Md50,the composition of sediment grain size had more significant effect.There are totally sand material covered the shallow area along the sand ridge with the water depth less than 8 m.The morphology of LADs at the northern slope is dominated by water depth and tidal current velocity,that is,the smaller the water depth and the lower the velocity,the smaller the size of LADs.On the contrary,in the area with water depth more than 8 m at the northern slope,sediment composition becomes the main controlling factor.The presence of silt components leads to a decrease in the size of LADs,and following the appearance of cohesive particles,LADs disappear.The symmetry of LADs is revered at the southern and northern slopes,which verifies the sand ridge residual circulation theory.Differences in the size of LADs at the northern and southern slopes are caused by the differences in the residual currents and the slope gradients.The leeside slope angles of LADs are mainly controlled by the steepness of LADs.In the shallow area,it decreases following the reduction of water depth.In relatively deep water area,its decrease is controlled by the fine particle composition of the sediments. |
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