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The Numerical Study Of The Sandy Beach Evolution And Local Scour Around Seawall Under The Action Of Tsunami

Posted on:2022-10-01Degree:DoctorType:Dissertation
Country:ChinaCandidate:F FanFull Text:PDF
GTID:1520307034460634Subject:Hydraulic engineering
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As a kind of destructive nature disaster in ocean,the tsunami is famous for its highly destructive.It will cause huge economic losses and casualties to human beings every time it appears.The destruction of tsunami to the valuable beach resources is immeasurable.It will cause large area and large scale erosion to the nearshore beach when the tsunami appears.Meanwhile,tsunami can also cause scour on the foundation of offshore protective structures,resulting in structural instability and damagement.The above two kinds of damage can be summarized as the coastal sediment movement caused by tsunami.Therefore,it is of great significance to study the characteristics of coastal sediment movement and sediment movement around hydraulic structures under the action of tsunami.This paper carries on the secondary development based on the open source mathematical model of fluid computing Open Foam.A set of high-precision numerical models for beach evolution and local scour is established.The model is used to analyze the hydrodynamic characteristics of tsunami,the evolution characteristics of beach under the action of similar isolated tsunami wave,and the development process of local scour behind the vertical dike and the related influencing factors.This study has certain guiding significance for tsunami disaster protection,design of vertical seawall and beach restoration.The main research results are summarized as follows:(1)The sediment movement model of bed load and suspended load is coupled to the open source computational fluid dynamics Open Foam hydrodynamic model in the actual time as a result the beach evolution model and local scour mathematical model are constructed.In order to reduce the attenuation of wave height caused by overestimation of wave energy dissipation in the process of wave propagation simulation,a buoyancy force term is added to the turbulence model to effectively reduce the wave attenuation coefficient.The VOF method is used to track the change of the liquid level;the moving grid is used to capture the change of the bed surface;the finite area method(FAM)is used to realize the conversion between the three-dimensional and two-dimensional computational grid data;the sand sliding model is introduced to smooth the abnormal bed topography to ensure the accuracy of the model simulation results.(2)A series of physical model tests are used to verify the self-developed beach evolution and local scour model.In this paper,the shallow water deformation of waves is simulated by non breaking and breaking wave propagation models respectively;the shear stress calculation results are compared and analyzed by using the shock flow friction velocity test;the solution results of convection diffusion equation in the suspended sediment model are verified through the suspended sediment movement and start-up test;the local scour mathematical model is improved by gate jet test and submarine pipeline scouring test In addition,the beach evolution model and local scour mathematical model are verified by the beach evolution test under solitary wave and vertical dike scouring test.The calculation results of the mathematical model are in good agreement with the physical test results,which prove the accuracy of the beach evolution model and the local scour mathematical model.(3)Based on the verified mathematical model of beach evolution,the evolution process of pure beach topography under the action of tsunami is simulated,and the distribution of hydrodynamic characteristics of solitary wave is obtained.The change of wave surface and flow field in the propagation process of solitary wave is analyzed in detail.The simulation results show that the evolution of Beach mainly occurs in the process of solitary wave falling,which is consistent with the physical model test results It is found that the size(depth and width)of sand bar and sand pit increase with the increase of wave numbers.When the action wave numbers reach about 20,the sand bar and scour pit basically reach a stable state.At the same time,the distribution characteristics of suspended sediment concentration in the process of tsunami propagation are studied by using the suspended sediment convection diffusion equation.It is the first time to analyze the weight of the suspended sediment movement in the evolution of the beach.Results show that the suspended load plays a dominant role in the evolution of beach,and the main contribution to the terrain evolution is about 70%.(4)The local scour around the vertical sea wall under the action of Tsunami is simulated and analyzed through the verified local scour mathematical model.The hydrodynamic characteristics of the interaction between waves and vertical breakwaters and the development process of scour pits are analyzed in detail.It is found that the scour pit behind the vertical breakwater is mainly caused by the impact of overtopping water on the bottom.The main influencing factors are analyzed by changing wave height,still water depth,water height of vertical breakwater and slope gradient in front of the breakwater.The results show that: there is a direct linear relationship between the location and size of scour pit and wave height,water depth and water height of dike body;the influence of slope gradient in front of dike on the shape and size of scour pit is not obvious when other conditions are the same.A dimensionless analysis of the maximum scour depth is carried out by using regression fitting method based on the results of the numerical model.The influence of wave propagation from deep water to shallow water deformation in front of the breakwater is considered in the relevant formula,which is verified by existing physical model tests.The fitting degree is reasonable,which can provide some guidance for later engineering design.
Keywords/Search Tags:Tsunami wave, Sandy beach, beach evolution, upright retaining walls scouring
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