| High-energy outburst flood was caused by the outburst of large barrier lakes formed by landslides,debris flows or glaciers blocking river channels.With the discovery of high-energy outburst flood evidence in many places,this kind of extreme flood event gradually came into people’s view.High energy outburst flood has a large flow,high velocity and strong hydrodynamic conditions,which can strongly shape topography and landform,and also have a profound impact on global climate change,global hydrological cycle,surface processes and regional geomorphic evolution.In this paper,based on the deposition evidence of high-energy outburst flood in the middle reaches of Yarlung Zangbo River(Jiacha Gorge-Pai Village),HEC-RAS and DHI-Mike model were used to reconstruct the scale of high-energy outburst flood in the middle reaches of Yarlung Zangbo River based on Bernoulli’s principle and Navier-Stokes equations.Combined with field characteristics and physical and chemical indicators such as grain size,the reconstruction results were analyzed and verified,and the potential causes of high-energy outburst flood in the study area were briefly analyzed.The preliminary conclusions are as follows:(1)Twenty large sedimentary sand bars representing ancient high-energy flood sedimentary landforms were discovered in the 150 km-long reach of the middle reaches of the Brahmaputra(Jiacha Gorge-Pai Village),mainly including eddy bar,point bar,expansion bar and pendent bar.The flood peak water level of the ancient high-energy flood was reconstructed by the top elevation method,and the flood peak water level was between62 and 200 m.(2)In the middle reaches of the Yarlung Zangbo River(Jiacha Gorge-Pai Village),237 XS-Cutlines were created on average,the roughness coefficient of the main Channel is0.035,and that of the Gravel bars is 0.05.The contraction and expansion coefficients were0.1 and 0.3,respectively,and the energy gradient was 0.1%.The HEC-RAS model and hec-Georas module under Arc GIS platform were used to reconstruct the ancient high-energy outburst flood in the middle reaches of Yarlung Zangbo River on a scale,and the flood peak discharge was between between 0.2×10~6～2.2×10~6 m~3/s and the maximum submerged water depth is 366 m.(3)Mesh Generator software was used to create irregular triangular terrain mesh files for the study area(.mesh),and created MIKE21 FM HD model in DHI-Mike software.Relevant parameters were set for the model,including calculation of step number,step length,eddy viscosity coefficient,Manning coefficient,dry/wet boundary,etc.,to complete the model construction.The MIKE21 FM HD model was used to conduct large-scale reconstruction simulation of the ancient high-energy outburst flood in the middle reaches of the Yarlung Zangbo River and carry out numerical simulation study of river flood.The peak discharge of the ancient high-energy outburst flood is between 0.2×10~6～2.0×10~6 m~3/s.(4)HEC-RAS and MIKE21 FM HD model and their calculation results were compared from multiple perspectives and methods.Two methods were used in the middle reaches of the Yarlung Zangbo River with the same flow state and similar calculation results.But there were still some differences:1)HEC-RAS model is based on backwater curve method,and has many requirements on model parameters,such as evenly selecting XS-Cutlines,considering the manning roughness coefficient of river channel and floodplain separately,and considering the shrinkage and expansion factors of river channel.The DHI-MIKE model is based on the finite volume method,which requires relatively few model parameters,and takes the comprehensive roughness coefficient for the river channel;2)The results of the sensitivity test of the Manning roughness coefficient shows that when the amplitude of±35%is given,the DHI-MIKE model The variation range of the calculated results was smaller than the variation range of the HEC-RAS model.Indicated Manning roughness coefficient has little effect on the DHI-MIKE model.By comparison,it was found that the DHI-MIKE model can reduce the interference of human factors on the calculation results and increase the reliability of the reconstruction results. |
