Author(s): M. E. Meshkati Shahmirzadi; Tetsuya Sumi; S. A. Kantoush
Linked Author(s): Tetsuya Sumi
Keywords: Flood mitigation dam; Stilling basin; Abrupt drop; Sudden enlargement; Forced hydraulic jump; Slit-type end-sill
Abstract: In this paper, an ecofriendly concept of stilling basin (SB) is introduced; called In-ground Stilling Basin (ISB), which can be optionally equipped with a slit type (non-continuous) end-sill at its downstream end. ISB is a non-prismatic SB with a sudden enlargement in transversal cross section and an abrupt drop in the bed at its upstream end, and is accessorized by a positive step at its downstream end, so that it creates a drop box below FMD’s outlet; a geometry similar to an elongated pool below the bottom outlet. Particular interest during our study was to solve the problem of fish and sediment passages disruption in conventional stilling basin design. Thus, extended series of experiments were carried out to obtain the optimum ISB geometry (length and depth) and optimum end-sill geometry (height and width), so that be acceptable in terms of hydraulic, economic and environmental aspects. To achieve this purpose, the experiments were conducted under various hydraulic conditions such as: stilling basin length (L=125 and 75cm), step depth (s=5 and 15cm), end-sill height (h e =0,4, 8 and 12cm) and end-sill width (b e =50,40 and 30cm). Observation of the experiments showed that the forced hydraulic jump within ISB has a very complex 3D nature and highly influenced by geometry of ISB as well as end-sill. In this respect, a higher end-sill can effectively reduce the magnitude of velocity along the centerline of SB but it may be an obstacle for fish and sediment passage. However, considering two free spaces at the lateral side of end-sill shows almost equal function for velocity reduction to that of fully end-sill, providing additional positive solution for smooth fish and sediment passing.