Research On Hydraulic Characteristics Of New Shape Overflow Dam And Energy Dissipation

The overflow gravity dam is the main dam type in the dam construction, and it is the dam type which adopted in most projects currently with many advantages like safe and reliable, simple in design and construction technique, adapt to geography and geological conditions well, and easy to solve the construction diversion and perpetual flood discharge problems. The design in profile is a main part of the design in overflow dam, and its task is to choose a suitable profile shape and contour size with a small volume, high discharge capacity and convenience in construction on the basis of safety, economy and strength requirements by means of analysis and calculation. Thus, the size of overflow dam profile is directly related to the quantity of the whole engineering, which is directly related to the amount of investment. So, finding out a optimal overflow profile by optimal design is a key issue in the design of overflow dam.In this paper, by the fundamental hydraulic computation and combining with a WES profile overflow dam, the writer firstly obtains the hydraulic elements of the downstream stilling basin among overflow dams with both general and new shape under the case of energy dissipation by hydraulic jump and forming the stilling basin by lowering the elevation of apron. Then the writer takes hydraulic model tests on both the unoptimized overflow dam and the optimized one. Finally, based on model test results, the size and the shape of stilling basin are adjusted and optimized, and analysis and comparison of hydraulic model test results of three optimum proposals is also made. According to theoretical calculation and model test rusults, the main conclusion of this paper are as follows:(1) The calculated results indicate that the length and depth of the stilling basin decreases remarkably of the overflow dam with new shape, which saves the engineering investment a lot by greatly reducing the concrete dosage used in the dam body and the excavation volume of the stilling basin.(2) The experimental results of overflow dam with general shape coincide with the theoretical calculation results preferably, which provides the rationality of both the depth and length of the stilling pool by hydraulic computation in the second chapter of this paper.(3) The experimental results of the new shape overflow dam have some difference with the theoretical calculation results, which mainly represents in the appearance that there is no hydraulic jump under certain design head. Because the water flow is jeted from the tail ridge to the downstream riverway, so that the hydraulic jump appears in farther downstream, which leads to large amplitude of pulsating pressure in underwater bed and serious fluctuations of the watersurface. Besides, there is no free nappe in the fall ridge of the dam toe, and the water flow converges with the water in stilling pool.(4) The first optimum proposal of the stilling basin has raised the tail ridge's elevation of the stilling pool, though hydraulic jump happens in the stilling pool, the flow regime offside the tail ridge is still free overflow, which results in a far-drive hydraulic jump in farther downstream and serious fluctuations of the watersurface, and these factors all lead to inadequate energy dissipation.(5) The second optimum proposal of the stilling basin has lowered the bottom elevation of the stilling pool. A hydraulic jump is formed in the stilling pool and a secondary water jump with high submergence degree is still generated offside the tail ridge. Besides, the amplitude and root-mean-square of fluctuation pressure in underwater bed and actual measurements of flow rate of each cross section in downstream riverway is smaller than the first optimum proposal, so the effect of energy dissipation is better than the first one.(6) The third optimum proposal of the stilling basin has lowered the baseboard elevation of the upstream stilling pool, forming a fall slot near the dam toe and the stilling basin becomes stepped, which improves the flow regime of both the stilling basin and downstream riverway and eliminates the secondary water jump offside the tail ridge of stilling basin. Besides, the amplitude and root-mean-square of fluctuation pressure in underwater bed is smaller than the second optimum proposal, also scouring of the river bed and side banks would decrease a lot, thus the effect of energy dissipation is better.