Study On The Hydraulic Problems Of Flood Discharge And Energy Dissipation Of NYERERE Saddle Dam

The storage capacity of the NYERERERE project is 30.6 billion m³ at the average storage level,and the design discharge flow under PMF condition is 47755 m³/s.The discharge flood will be discharged through the two flood discharge systems of the main dam surface,the middle hole and the open spillway of the auxiliary dam.The discharge flow under the design condition of the auxiliary dam spillway is 18260 m³/s.The auxiliary dam spillway comprises of a 99m excavation spillway and a 1638m roller-compacted concrete spillway.The energy dissipation type is underflow energy dissipation,which forms the hydraulic problems of flood discharge and energy dissipation of super long overflow width and large discharge auxiliary dam spillway.It is also a significant and complex scientific and technological problem in extensive reservoir capacity engineering.To ensure the safe and stable operation of the project,the actual flow capacity of the low-head spillway on the weir,the complex hydraulic characteristics of the stilling basin behind the super-long overflow dam under the dual action of energy dissipation and lateral confluence,and the rationality of the distribution of hydraulic elements in the river channel behind the dam need to be fully demonstrated.Based on the hydraulic model test of the NYERERE open auxiliary dam spillway project and the numerical simulation method,this paper analyzes and discusses the related hydraulic problems,optimizes and adjusts the shape of spillway energy dissipater,and conducts in-depth research on the distribution law of corresponding hydraulic characteristics.The main conclusions are as follows:（1）The model test results show that the test flow value may be less than the design value due to side shrinkage and scale influence.The numerical simulation can avoid the above effect of the model,and the calculated value of the flow is greater than the design value,indicating that the actual flow capacity of the auxiliary dam spillway meets the design requirements.（2）The flow coefficient comparison results of different test schemes show that the numerical simulation results are closest to the actual value of discharge capacity.At the same time,through the comparative analysis of the simulated values and the flow coefficients of each test scheme,the flow influence coefficient formulas of each influencing factor on the relative design head（H₀/H_d）are fitted,which are the flow influence coefficient formula of circular side pier side shrinkage and the flow influence coefficient formula of circular side pier side shrinkage and cement mortar dam surface.The determination coefficients are 0.976 and0.998,respectively.Finally,the flow coefficient of the two test schemes is compared.The formula of the influence coefficient of the roughness of the cement mortar dam surface on the flow is obtained,and the determination coefficient is 0.990.（3）The physical model test results of the original design scheme and the numerical simulation results show that the flow velocity of the overflow dam to the stilling pool is high,and the maximum impact velocity is about 17～18 m/s,which affects the lateral confluence flow pattern.There is a remote-driven hydraulic jump in the overall model test.The numerical simulation results show that most of the left,middle and right discharges in the downstream river channel converge within X=300～400m behind the dam,and the average velocity of the mainstream after confluence is greater than 10m/s.（4）The numerical simulation and physical test results of different cross-section schemes show that the step height of 1.5m can maximize the energy dissipation rate along the dam surface.The overall model test found that the length of the stilling basin was shortened,and the confluence flow was reduced.Part of the flow directly entered the river channel,which reduced the?flow velocity increase caused by the lateral slope of the stilling basin,greatly improved the flow pattern of the river channel behind the stilling basin and formed the recommended shape of the project.（5）It is recommended that the flow pattern of the dam surface is smooth,the energy dissipation is sufficient,and the mainstream flow velocity of the river near the dam area is low,which meets the actual needs of the project.After the water flow passes through the step and the downstream stilling basin,the maximum average flow velocity is reduced by nearly 18.78%compared with the original design scheme in the X≤300m near the dam area.The test value of the mainstream time-averaged flow velocity in the test range is reduced.The maximum reduction of the measuring point is 28.94%,and the average decline is 11.52%.The research results have been adopted by the engineering design,and construction has begun,which also provides a reference for the?creation of similar engineering problems.