Study On The Influence Of The Layout Of Soft Mattress Ballast On The Flow Structure And The Effect Of Siltation Promotion

The riverbed in the middle reaches of the Yangtze River is thin and easy to be scoured and deformed,which results in frequent evolution of the channel beach.In order to ensure the stability of the channel,soft mattress bank protection and bottom protection are often used to protect the river bed in the channel regulation project.Soft mattress have the characteristics of filtration,isolation,scouring prevention,deformation and integrity.However,when the flow rate is high,the discharges are still prone to instability and damage.Therefore,the structure of riprap and pervious frame is often used to ballast the soft mattress to increase the stability of the soft mattress.Reasonable layout of ballast can effectively promote sediment deposition by changing the flow structure near the bottom,and increase the stability of soft mattress under the condition of reducing the volume of ballast block.In this paper,the influence of the layout of soft mattress ballast on the vertical distribution of flow structure and sediment content,and its siltation promoting effect are studied by flume test.The main contents and results of this paper are as follows:Aiming at the bottom layout of the Luofuzhou Channel Regulation Project,the layout ideas of orthogonal grids,parallelogram grids and strip-like three kinds of ballast,four kinds of grid spacing and three kinds of boundary width of ballast are proposed.The flow structure distribution,vertical distribution of sediment content and siltation of different ballast are measured by large-scale generalized flume test.The variation of flow velocity and turbulence intensity as well as the effect of sediment deposition were studied,and the layout of soft mattress ballast was optimized step by step.Compared with natural conditions,the velocity of the ballast grid in the upper region increases,while the velocity in the bottom region decreases.With the increase of the grid spacing and the width of the ballast frame,The velocity in the upper region decreases,,and the velocity in the bottom region increases.The variation range of vertical velocity decreases and the distribution is more uniform.Compared with natural conditions,the vertical turbulence intensity increases greatly under different configurations of pressure carriers.There are two peaks in the upper and bottom regions of the ballast grid,and the turbulence intensity is the smallest locally near the top of the pressure carrier.With the increase of the mesh spacing and the width of the ballast frame,the turbulence intensity and the variation range decrease.Longitudinal and transverse turbulence intensities are greater than vertical turbulence intensities on each vertical line.Compared with natural conditions,the sediment concentration in the upper region of the ballast grid increases,the sediment concentration in the middle region decreases,and the sediment concentration in the bottom region decreases slightly.With the increase of water depth,the sediment concentration increases first,then decreases,and then increases slowly.The vertical variation range of sediment concentration increases with the decrease of grid spacing and the width of ballast frame.Under different flow conditions,the change law of flow structure and the effect of sediment silting are also changed.When the experimental sediment concentration is increased,the flow structure are basically the same,and the sediment deposition increases greatly.The vertical distribution trend of sediment concentration is similar,but the variation range is slightly increased.When the experimental water depth is decreased,The velocity of flow and the intensity of turbulence increase greatly.the sediment deposition decreases greatly,and the vertical distribution of the sediment concentration has no trend of stratification change.The sediment concentration increases with the increase of water depth,and the sediment concentration is the largest at the bottom of the flume.Comparing comprehensively,the configurations of ballast arranged in orthogonal grids with three times the width of boundary frames and 15 meters width of boundary frames.It has the best effect on sediment deposition promotion.According to the engineering quantity,the effect of sediment deposition promotion and the difficulty of construction,it is recommended that the orthogonal grid arrangement be adopted in the project.The spacing of the grid is three times the width of the boundary frame and the width of the boundary frame is 15 meters.