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Energy Dissipation Near the Bed Downstream End-Sill

Author(s): Hani M. Noshi

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Keywords: Stilling Basin Design; Recirculation Zone; Length of Recirculation Zone; Energy Dissipation; Riprap Stability; Hydraulic Jump; Radial Gate; Maximum VelocityNear the Bed

Abstract: Stilling basins are often used for energy dissipation in conjunction with spillways, outlet works and regulating structures. The effect of the end-sill dimensions on the efficiency of energy dissipation is presented for a wide range of upstream jet flow intensity conditions. The study presents an estimate for the near-bed maximum velocity values downstream stilling basins with and without an end-sill. These values are useful for determining the size of bed protection material. The high turbulence intensity in the recirculation zone downstream the end-sill results in an unstable hydraulic conditions. The maximum expected near-bed velocity values in the recirculation zone is essential for the selection of size of bed protection material. A good estimate of the length of this recirculation zone is therefore considered a very useful design tool, especially for large projects where a long concrete slab or very large bed protection material can be extremely costly. The results of experimental work are presented for stilling basins downstream radial gates for a wide range of flow conditions in the case of submerged hydraulic jump. The results of velocity measurements demonstrate the large improvement on energy dissipation near the bed by the introduction of the end-sill. The results illustrate also the relatively high velocity values and turbulence intensity existing downstream the end-sill in the recirculation zone for a wide range of gate openings, flow intensity conditions, and end-sill heights. It is found that using an end-sill height of about 0. 15 the tail water depth presents a sufficient height that can improve the energy dissipation, for the range of flow conditions investigated. It is also found that increasing the end-sill height than this sufficient height does not improve the energy dissipation any further. The length of this recirculation zone downstream the end-sill is determined for different end-sill heights and flow intensity conditions. The length is found to be only dependent on the tail water depth values which was not expected. The length of recirculation zone is found to be always about 2. 30 times the tail water depth in the range of end-sill heights and flow conditions investigated. The information presented in this study is thought to be a very useful design tool to best optimize the end-sill dimensions to achieve better hydraulic conditions downstream stilling basins. The study present an estimate for the values of maximum near-bed velocity values necessary to determine the size of bed protection material downstream the stilling basin.


Year: 1999

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