Author(s): Nishank Agrawal; Ellora Padhi; Gopal Das Singhal

Linked Author(s): Gopal Das Singhal, Ellora Padhi

Keywords: Stilling basin Ogee spillway Negative step Dunal bed End sill

Abstract: Stilling basins are crucial hydraulic structures designed to manage and dissipate the high energy of water exiting outlets such as spillways, sluice gates, and pipe outlets. Effective energy dissipation is critical to prevent downstream erosion, which can compromise structural stability and nearby riverbanks. Dissipation is typically achieved through hydraulic jumps, where supercritical flow transitions to subcritical flow. In traditional stilling basin designs, elements like chute blocks, baffle blocks, and end sills induce hydraulic jumps to facilitate energy loss. However, the efficiency of energy dissipation relies heavily on specific design features within the basin. This study proposes an innovative hybrid stilling basin design downstream of an ogee spillway, replacing conventional elements with a rough dunal bed, an end sill, and a negative step. The negative step, created by lowering the bed elevation, with end sill forms a pool that generates uplift pressure, while the rough bed surface enhances energy dissipation over a shorter distance, potentially improving overall efficiency. The hybrid design concept integrates findings from previous research. Li (2012) and Sun et al. (2005) identified that excessive bottom velocities in stilling basins could lead to structural slab damage, a problem mitigated by incorporating a negative step to reduce velocities and enhance energy dissipation. Similarly, end sills have been shown to stabilize hydraulic jumps, reduce downstream velocities, and further improve dissipation. Shukry (1957) and Hager and Li (1992) research emphasized the role of end sill placement, influenced by the approaching Froude number and tailwater depth, in controlling flow patterns and stabilizing hydraulic jumps. Moreover, Agrawal and Padhi (2024) found that bed roughness enhances energy dissipation by reducing sequent depth and jump length due to increased shear stress. Despite extensive studies on individual design elements, the combined effects of rough bed surfaces, negative steps, and end sills on hydraulic jump behaviour and energy dissipation remain underexplored. To address this gap, this study examines flow characteristics, including sequent depth, roller length, and energy dissipation, for a hybrid basin design under discharge conditions of 11 and 15 l s1. Experiments were conducted in a 5.5 m long, 0.3 m wide, and 0.5 m deep rectangular open channel flume, using a Flowmeter in the supply line for discharge measurement, pointer gauges with ±1 mm accuracy for water depth monitoring, and a 6 mm diameter pitot static tube for velocity measurement along the channel centerline at 5 mm intervals. Three stilling basin models, shown in Fig. 1, were tested downstream of an ogee spillway with a height of 20 cm and a width of 25.78 cm, positioned on a 10 cm thick bed. The width of each basin model matched the channel width (30 cm), while the length of the basins was maintained at 1 m. a) Model 1: Smooth Bed Stilling Basin – A conventional smooth bed used as a baseline model for comparison. b) Model 2: Dunal Bed Stilling Basin – Featuring a 22 mm high amplitude and 68 mm wavelength triangulated dunal bed profile, designed to control flow patterns and support hydraulic jump formation. c) Model 3: Hybrid Design – Combining a rough dunal bed, a 50 mm high end sill, and a 50 mm high negative step, designed to maximize energy dissipation. Fig. 1. Definition sketch of different model of stilling basin downstream of ogee spillway (a) Model 1, (b) Model 2, and (c) Model 3 The results indicate that the hybrid model achieves superior energy dissipation, a lower sequent depth, and a slightly longer roller length in free jump conditions compared to the other models. As illustrated in Fig. 2, which presents the velocity profiles for each model at a distance of 0.5 m from the spillway toe, where Model 3 (hybrid design) shows reduced velocity and depth relative to the other configurations. These findings highlight the effectiveness of the hybrid model in enhancing energy dissipation and provide valuable insights for optimizing stilling basin designs in practical applications. This model holds significant potential for hydraulic structures such as ogee spillways, chutes, and pipe outlets. Fig. 2. Velocity profile of different model at x = 0.5 m from spillway toe Keywords: Stilling basin, Ogee spillway, Negative step, Dunal bed, End sill

DOI:

Year: 2025