Study On The Process And Flow Law Of Sand Bed Erosion By Fixed Pipeline Jet

According to the statistics in 2019,the total reservoir capacity in China is about 898.3billion m~3,and the reservoir capacity loss reaches 34 billion m~3 compared with 2013,of which sediment siltation is the most important reason.Due to the shortage of water resources in northern reservoirs,the dredging capacity of mechanical dredging methods such as empty reservoir dredging is limited and costly,while the spatial and temporal conditions and power conditions required for shaping heterogeneous heavy flow dredging using joint scheduling of multiple reservoirs are strict,which is difficult to achieve in most reservoirs.For this reason,this paper proposes the dredging method of artificial heterogeneous flow dredging by setting up a fixed pipe at the bottom of the reservoir funnel area and periodically jet eroding the siltation sediment.The physical process of fixed pipe jet erosion of sand bed,the erosion law of pit body and the subsequent movement law of muddy water heterogeneous flow are investigated through flume model experiments and theoretical research,and field dredging tests are carried out in Wang Yao reservoir.The main research work and conclusions of this paper are as follows:Firstly,a physical model study of sand bed erosion by buried pipe jet was designed and carried out to investigate the erosion process and the sand discharge effect of heterogeneous flow under different jet factors and sand bed boundary conditions,and to analyze the change of the morphological dimension of the wash pit under different test variables and the characteristics of heterogeneous flow movement.The results show that:the development process of the pits is divided into three stages,the change of erosion depth gradually slows down with time,the two-dimensional morphology of the pits is similar,the upstream and downstream slopes are asymmetrical,and the overall development is downstream;there is a certain negative angle and depth of burial between the jet stream and the sand bed,and the erosion efficiency is higher when the slope of the sand bed is smaller and each nozzle column erodes to form independent pits;the sediment is initiated by the jet stream erosion and forms muddy water under the action of water coiling and suction.The heterogeneous flow,whose measured concentration is much larger than the force of sand held by the water,achieves the long-distance transport of sand-bearing water with higher concentration,and the heterogeneous flow is measured to have high dredging efficiency and certain feasibility.Secondly,based on the model test,the theoretical simulation analysis of the movement law of the heterogeneous flow formed by the bottom turbid water jet in deep water environment is carried out by using the shallow water flow theory.After the contact with the ambient water,the rapid mixing,water turbulence intensifies,the turbulence energy increases rapidly,and then gradually decreases and depletes.Finally,the dredging vessel was designed and modified according to the results of flume test and theoretical analysis,as well as the full investigation of the siltation situation of Wang Yao reservoir,and the test of jet erosion shaping heterogeneous heavy flow dredging was completed.The field test results show that:the erosion range and depth of fixed jet is limited due to the reduction of turbulence intensity and organic matter in the siltation sediment at the bottom of the area with hydrophytes,but the size of erosion pit is easy to be maintained;the measured concentration of surface turbid water during erosion is about 10kg/m3,which is much higher than that measured in the flume model test;the turbid water heterogeneous flow can transport the sediment to the front of the dam during the operation.This proves that it has practical application value.