Experimental Study On Movement Characteristics Of Water And Sediment In Cave Swallet Stream In The Karst Region

Karst landforms are widely distributed in China,especially in Southwest China.In recent years,with the continuous advancement of water conservancy project construction in Southwest China,the problem of sediment and water flow in karst area has gradually emerged.At present,there is no engineering experience in the study of the characteristics of sediment erosion and deposition in the reservoir area,and there is no relevant results that can be directly used for prediction.Therefore,it is of great theoretical and engineering significance to study the mechanism of sediment transport in karst area,which can provide scientific and technological support for hydropower development in Southwest China.In this paper,the methods of data collection,field investigation and generalized flume test are used to carry out the research.Firstly,the related research progress of the karst cave is summarized.Secondly,the characteristics of water and sediment in the natural karst cave reach are analyzed.Then,the clear water flume test of the river reach is carried out.The flow in different states is observed under six test conditions of backwater or not under different discharge the structure,water level change,Reynolds stress distribution and turbulence intensity change.Finally,the experiment of sediment erosion and deposition is carried out to observe the law of sediment erosion and deposition.The main conclusions are as follows:(1)Through field investigation,it is found that the swallet stream is generally open flow,which is the same as the surface river.At the peak of the flood,the flood may be blocked by the backwater of the tunnel,which has a certain effect of flood detention.After the downstream reservoir is built and operated,the outlet of the swallet stream tunnel will be silted up,but it is unlikely to be completely silted up.(2)Through the flume flow characteristic test,it is found that the flow in the swallet stream section is unstable,and there are different length transition sections at the inlet and outlet.Compared with the upstream open flow section,the flow pattern changes greatly,the water level is lower,the velocity is slightly higher,and the difference between the sections is obvious.After backwater,the flow pattern of the swallet stream section slows down,and the flow difference of each section tends to be smaller.(3)Through the flume flow characteristic test,the velocity distribution,Reynolds stress,and three-phase turbulence intensity distribution of the swallet stream section are observed and analyzed.In the range of relative height 0.2-0.8,the velocity distribution in the swallet stream section accords with the parabolic law.In the numerical value of Reynolds stress,the transverse component is larger than the longitudinal component,and there is almost no vertical component;the vertical distribution of each component is roughly triangular;the turbulence intensity is similar to the change of Reynolds stress.After backwater,the overall law remains unchanged.(4)Through the sedimentation test,the observation found that the upstream and downstream are strip-like siltation,the swallet stream inflow port generally has no siltation,the exit return area silt,the hole sporadic siltation on the central axis.After backwater the siltation has a tendency to move forward.(5)Through the analysis of the sediment gradation,it is found that the grain size of the sediment tends to be large in the upper and small in the lower;the grain size in the tunnel is between the upper and lower reaches.After backwater,the grain size distribution of each section is gradually narrowed.