| Sand waves and sand ridges are widely distributed on the tidal shelf.As the sand waves are not stable,their vertical growth or lateral migration could be a potential threat for the submarine engineering facilities,which are also important for the management of navigation and coastal environment.Therefore,it is of great significance to explore the morphology,distribution,and migration of sand waves and sand ridges.A large number of sand waves and sand ridges exist in the southeast of Beibu Gulf.Here,a three dimensional mathematical model with high resolution(up to 200m)is established to simulate flows and bottom shear forces in Beibu Gulf.Based on the multi-beam,side scan sonar,sub-bottom profile data and surface sediment,we analyze the morphology,distribution,and migration of sand waves in the southeast of Beibu Gulf.These results are compared with the modelled flows and shear forces to validate the results of numerical simulation.By analyzing the relationship between the simulated flows and the topography,we found that:(1)Flows near the bottom are reversing in the tidal cycle,whose direction is mostly northeast or southward,and the flow velocity is between 20 cm/s and 90cm/s.(2)Flow magnitude and distribution are mainly affected by the sand ridges,consistent with the distribution and migration of sand waves.Such flows can also affect the evolution of the sand ridges.(3)The distribution of average shear stress on the seabed is closely related to the characters of terrain and sediment.The low stress region corresponds to fine sediment such as clayey silt,while the high stress area corresponds to gravel sand and silt gravel and other coarse sediment areas.(4)The direction of shear stress on the seabed is in agreement with the measured sand migration direction.(5)Some symmetrical sand waves have beddings with opposite inclinations,which are possibly caused by the various hydrodynamics in different periods during the sea level changes. |
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