Experimental Study On The Layout Of Permeable Spur Dike In The Bend Of Spillway Chute

A bend section with a smaller centerline radius sometimes exists in a spillway chute due to the geology,topography and overall engineering layout on site in dam engineering case and the flow conditions in the bend usually becomes relatively sophisticated based on this condition.Many engineering treatments have been applied in engineering practice to improve the flow conditions,such as slopy ridges,guide walls and so forth.This paper aims to improve the complex flow conditions in the bend by the installation of permeable spur dikes at the concave bank of the bend in terms of the diversion principle of the slopy ridge and the spur dike in a rectangular channel.There are mainly two advantages concerning the permeable spur dike,one is the diversion of dike which decreases the water surface difference between the concave bank and convex bank of the bend,and the other is the permeability of the spur dike which not only help to degrade the property of water blockage and the hydrodynamic pressure exerting on the permeable spur dike relatively to the solid spur dike,but also strength the collision dissipation of the flow behind the dike.These advantages make the permeable spur dike in a state of safe and reliable operation.This paper preliminarily determined the most desirable installing angles of the permeable spur dikes at the concave bank of the bend(1/4,1/2 and 3/4 of the concave bank)by hydraulic experiments,then conducted experiments of total 13 schemes with different installing locations(1/4,1/2 and 3/4 of the concave bank),different installing angles(45°,60°,75°)and different installing numbers(1,2,3)of the permeable spur dike.Combining the theoretical and statistical analysis,this paper studied the water depth and flow velocity in the bend with various installation patterns of the permeable spur dike,the backwater condition in front of the spur dike and the distribution characteristics of hydrodynamic pressure of the permeable spur dike and bottom plate of the bend according to the experiment results.The results are summed up as follows:(1)The water surface effect in the bend with the installation of a permeable spur dike can be significantly improved compared with the case without a permeable spur dike in the bend.The most appropriate installing angle of the permeable spur dike at the 1/4,1/2 and 3/4 of the concave bank of the bend are 60°,75 °,75°,respectively.When the spur dikes with different numbers are installed at various locations at the most appropriate installing angle,the water improving effect with three dikes is better than that with two dikes.In addition,the formula for calculating the maximum cross-section water-surface difference is obtained using the theoretical analysis method on the basis of the experiment results.(2)The range of backwater caused by a single permeable spur dike varies with the change of installing patterns of the dike in the bend,and the maximum backwater height occurs at the cross-section where the spur dike meets the concave bank of the bend.The formula for calculating the maximum backwater height in front of a single permeable spur dike was obtained based on the principle of momentum conservation,and the formula demonstrates that the maximum backwater height is influenced by those factors of the geometric parameters of the permeable spur dike and the bend,the layout of permeable spur dike in the bend,and the inflow discharge.(3)The longitudinal flow velocity in the bend with the installation of permeable spur dikes exhibits reduction to different degrees,particularly the flow velocity in the region of permeable spur dike.The minimum longitudinal flow velocity occurs behind the tip of the first permeable spur dike and the maximum occurs at the concave bank of the outlet of the bend for the same water depth.The permeable spur dikes alter the direction of partial transverse flow velocity in the center line of the bend,and the magnitude of the transverse velocity increases at the bottom and decreases at the upper.Nevertheless,the permeable spur dikes overall cut down the transverse circulation and improve the water surface evenness in the bend.However,both top of the permeable and impermeable spur dike have small negative pressure for smaller discharge in the bend.(4)The time-average hydrodynamic pressure is greater than the hydrostatic pressure at the upstream face of the permeable spur dike while behaves inversely at the downstream face of the dike.The difference between the time-average hydrodynamic pressure and the hydrostatic pressure at the upstream face of the permeable spur dike is greater than that at the downstream face of the dike at the same location of the permeable spur dike.With the case of the same water depth,the closer to the sidewall of the bend,the higher time-average hydrodynamic pressure at the face of the dike,inversely,the closer to the center of the bend,the lower the time-average hydrodynamic pressure.In addition,the time-average hydrodynamic pressure at the upstream face is always greater than that at the downstream face at the same location of the permeable spur dike.