Study On Discharge Capacity And Hydraulic Characteristics Of Flaring Gate Piers Of Surface Spillway In High Arch Dam

High arch dams located in river canyon have the characteristics of high water head and large discharge.The energy dissipation protection of flood discharge has become one of the key technical problems.The trajectory jet with energy dissipation in plunge pool is generally accepted in flood discharge through dam.The energy is dissipated by spray and aeration of water nappe and turbulence in plunge pool.With development of high dam construction,a series of new energy dissipators of jet flow have been proposed and applied in projects effectively.Applying flaring gate piers to surface spillway in high arch dams could enhance diffusion of water nappe and improve energy dissipation.The discharge capacity and hydraulic characteristics of flaring gate piers on surface spillway in high arch dam are studied on the basis of the Dongzhuang water conservancy project.The main studies are as follows:1.Effects of flaring gate piers with different configuration on discharge capacity of surface spillway in high arch dam and relationship between the discharge capacity and Froude number along the surface spillway are studied by numerical simulation.The results show that the discharge capacity of surface spillway decreases with increase of deflection angle and decrease of plunge angle,contraction ratio and the horizontal distance between starting flare and weir crest.A comprehensive influence coefficient k is proposed to reflect form parameters of flaring gate piers and water head over weir.When k is greater than or equal to 2,flaring gate piers have no effect on discharge capacity of surface spillway.When k is less than 2,the effect on discharge capacity is gradually enhanced with k decreasing.There is a knee point on the curve of Froude number along the surface spillway at the upstream of starting flare.The bigger Froude number at the knee point is,the less effect of flaring gate piers on discharge capacity of surface spillway.2.Characteristics of hydrodynamic pressure in plunge pool are studied by model test when trajectory type flood discharge through high arch dam is accepted with flaring gate piers on surface spillway.The peak value of mean pressure occurs in the impacting area of surface spillway jets on the bottom and slopes of plunge pool.The pressure gradient is relatively large on both sides of peak.The largest impact pressure is 35.20 m under the condition of check water level.There is a peak of fluctuating pressure in the impacting area.Amplitude characteristic,correlation characteristic and spectrum characteristic are related to the shape and placement of water jets.3.Two kinds of optimization scheme with and without impact of jets in the air are proposed to lower the impact pressure.Extension of sidewalls could enhance longitudinal diffusion of surface spillway water nappe.When the contraction section of flaring gate piers extends to 26 m,the decline of the largest impact pressure on the bottom of plunge pool under the condition of design water level and check water level is 63.10% and 56.82% of the original scheme.Decrease of deflection angle on the exit of surface spillway could enhance horizontal diffusion of water nappe to get jets to impact in the air.When deflection angle is 25°,the decline of the largest impact pressure under the condition of design water level and check water level is 41.60% and 87.36% of the original scheme.