Download Operation and Safety of Dams and more Lecture notes Design in PDF only on Docsity! Operation and Safety of Dams …………………………...…………………………..Dr. Ammar H. Kamel 1 Stilling Basins: A stilling basin is a basin-like structure in which all or a part of the energy is dissipated. The positioning of a hydraulic jump on an unobstructed horizontal surface is very sensitive to the close match of sequent depths. If the downstream depth matches the sequent depth y2, the hydraulic jump will occur as desired on the apron. If the downstream depth is less than y2, y3<y2, the jump will occur downstream from the apron (a swept-out jump), and the river will become exposed to high scouring velocities. If the downstream depth is greater than y2, y3>y2, the jump will be submerged. Although a submerged jump is preferable to a swept-out jump, much of the initial kinetic energy remains in the form of a submerged jet, which alone can result in considerable scour. A carefully designed stilling basin will not only improve the dissipation characteristics of a hydraulic jump, it will shorten its length and stabilize the position of the jump so that it is not sensitive to fluctuations in tailwater levels. In a stilling basin, the kinetic energy causes turbulence and it is ultimately lost as heat energy. The stilling basins commonly used for spillways are of the hydraulic jump type, in which dissipation of energy is accomplished by a hydraulic jump. A hydraulic jump can be stabilized in stilling basin by using appurtenances (or accessories such as chute blocks, basin blocks and end sill. Types of Stilling Basin: Because stilling basin block arrangements are difficult to design analytically, their design must be based on experimental methods. Standard designs have been developed through both observations of existing installations and a systematic series of model studies. Four types of stilling basins are developed by the U.S. Bureau of Reclamation are explained in the following: Type I (1.7<Fr1<2.5) It is a rectangular stilling basin with a horizontal bottom, no chutes, no baffles or sills, which include a classical hydraulic jump. Because of high costs that come from the basin length is large as well as the hydraulic jump is sensitive to downstream level variation and effects on safety, it is not recommended. Stilling Basin Type I Type II (Fr1˃ 4.5; V1 ˃ 18 m/s) The Type II basin is designed for use on high dams, earth dam spillways for Froude numbers greater than 4.5. The chute blocks and end sill help reduce the basin length by 33%. The design includes blocks and dentate end sill. The end sill has a stabilizing effect and dissipative reason. The length of stilling basin calculates from the following equations: )]5.4(055.00.4[ 12 FryLII for 4.5 < Fr1 <10 235.4 yLII for Fr1 ˃ 10 Operation and Safety of Dams …………………………...…………………………..Dr. Ammar H. Kamel 2 Type II Stilling Basin Type III (Fr1˃ 4.5; V1 < 18 m/s) The Type III basin reduces the length by 45-60 % with the addition of chute blocks, baffle piers, and an end sill. This structure is also used for Froude numbers greater than 4.5, but its use is restricted to small spillways where the upstream velocity is less than 15-18 m/sec. It was developed for gravity dam, earth dam spillways. With the inclusion of baffles, the inflow velocities are restricted to avoid cavitation damage to the concrete surface and reduce the impact force to the blocks. The length of stilling basin can also calculate from the following equation: )]5.4(073.04.2[ 12 FryLIII for 4.5 < Fr1 <10 28.2 yLIII for Fr1 ˃ 10 For the Type III stilling basin, the dimensions h3(y3) and h4(y4) are given by, )]0.4(056.025.1[ )]0.4(164.03.1[ 114 113 Fryh Fryh
