Scour Characteristics And Optimal Design Of Permeable Gabion Groins

Karakoram Highway is the only inland traffic artery connecting China and Pakistan.Since its completion,the highway has suffered many floods,resulting in traffic disruption.In order to reduce the flood disaster of highway,a large number of gabion groins are used as revetment structure to protect the safety of subgrade.However,as a peripheral building,the gabion groins are damaged by scour and gradually fail.Therefore,it is of great significance to optimize the design of gabion groins for improving the service life of gabion groins.It’s also important for the prevention and control of flood disaster of mountain roads to explore the flow structure as well as scouring & silting mechanism of gabion groins.In this paper,the field investigation of permeable gabion groins along Karakoram Highway is carried out,and the damage situation and damage reasons of gabion groins in use are clarified;FLOW-3D software is used to simulate the scour of gabion spur dike.Through controlling variables,the influence of the changes of porosity,length,width and angle of gabion spur dike on the flow structure and scour and deposition law is explored.On this basis,the optimal design of gabion spur dike considering water permeability effect is proposed.The main research results are as follows.(1)The flow structure of permeable gabion spur dikes is analyzed: due to the existence of spur dikes’ pores,the flow in front of the spur dikes can be divided into two forms: flow through the spur dikes and flow around the head of the spur dikes;For the velocity distribution behind the permeable gabion spur dike,it can be divided into three regions: the region of slowly increasing velocity,the region of rapidly increasing velocity and the region of relatively stable velocity.(2)The scouring and silting mechanism of permeable gabion groins is described: the scouring and silting of gabion groins can be divided into three stages: initial stage,development stage and balance stage.When the porosity is 0.15,the sedimentation height reaches the maximum,and when the porosity is greater than 0.25,the sedimentation height behind the spur dike is less than that of the impervious spur dike.(3)The scouring law of permeable gabion groins is studied: with the increase of porosity,the maximum scouring depth of riverbed tends to be far away from the dam head first and then gradually close to it.When the porosity is 0.1,it is farthest away from the dam head,and when the porosity is 0.25,it returns to the dam head again;With the increase of porosity,the scour depth at the dam head increases first and then decreases.When the porosity is 0.3,the scour depth at the dam head reaches the maximum.The empirical relationship between the ratio of the scour depth at the head of permeable gabion spur dike to that at the head of impervious spur dike and the porosity is obtained.(4)The design parameters of permeable gabion groins are suggested: The porosity ranges from 0.1 to 0.2;The dam length should be selected to ensure that the river width narrowing ratio is within the range of 0.2 ～ 0.25;It is suggested that the width to length ratio of spur dike should be about 0.3;The permeable groins should be cantilevered,and the cantilevered angle should be45° ～ 60°.