| River divertion during construction is the one of important tache in the project of hydropower engieering. Overflow cofferdam and iversion tunnel are used offen to diverse the flow especialy where or the river discharge changing widely.There are many rivers in mountainous area in our country, so this kind of cofferdam is used relatively frequently in River divertion during construction. While overflow cofferdam is used, the cofferdam and dam will face an evosion problem by the flow passed over it .. And if the scour is too serious, it will be difficult to protect main project from destroy and cause economical loss.Therefore to dissipate the unwanted energy and to make the cofferdam and dam can move safe against scour caused by flow is one of conmmon problems faced by this kind of hydraulic engineers.Since the boundary condition of river divertion during construction project is very complex,so physical model techneics are often used to checkout the design. In this paper a date from some real projects are used to analyse the facors that will affect the energy dissipation and erosion prevention . such as flood discharge across the cofferdam,crest elevation, downriver cofferdam, roughness coefficient and the gradient of the surface of dam,and so on.In this paper , The program based on hydraulics is checkout by the Tankeng waterpower Project in Zhejiang.The results prove that this program can be used to design any project where there is no obvious side shrinking phenomena.The programe used optimization validate whether the design on energy dissipation and erosion prevention of cofferdam and dam is in reason,and it can campare the results from different projects to sellect the best one.(1) Calculate the stilling basins' dimension.(2) Validate whether the crest elevation can meet the require of energy dissipation and erosion prevention during the beginning period of flood discharge.(3) calculate the altitude of downriver cofferdam , and make it meet therequire of energy dissipation and erosion prevention.(4) calculate some average velocity of flow through some sections,including contracted section in stilling basins behind upriver cofferdam,if the hydraulic jump happen in lower reaches of the river.the program will calculate the average velocity of flow of contracted section and the length of the apron.(5) According to the calculated average section velocity.we can decide the type of the revetment.(6) When the scour hole depth of the dam head and trail parts is calculated inthe paper.we can validate whether the revetment material is strong enough to resist the degradation of the flood flow.This paper presents an optimized computer program to analyze different factors that will affect for energy dissipation and erosion preventation which is especially important in design of cofferdams. The graphic in this paper also show the manner and degree of the effect that caused by these factors.(1) It was found from the obtained results that increasing of flux will led to decrease in Froude No. of the following water at the shrinking section which will reduce rate of disspated energy, this will make the protection of dam surface more difficult.(2) The changing in dam gradient affects the energy dissipation and erosion prevention. To use the athwart gradient protection which takes the end of dam as the datum mark will reduce the digging depth of the stilling basins is reducing, and increase the energy dissipation rate . The average velocity of flow passing over the dam is also decreased, so the protect of the dam surface is less difficult.(3)The roughness of the dam body has a little effect on the energy dissipation and erosion prevention.(4)Enhancing the downriver cofferdam will make the lower reaches of a river dissipate more energy.However, it will increase erosion and the protection will be more difficult.(5)Lowing down the dam body will reduce the digging deep of thestilling basin, decrease the velocity of flow on the dam body and cut the difficulties of... |
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