Comparison of Hydraulic Behavior of Flow over Stepped Spillway and Chute Spillway

Stepped spillways are convenient and economical options for high dams compared to other types of spillways because of their special abilities such as construction procedure compatibility with the Roller Compacted Concrete Dams (RCC Dam) technology, the ability of self-aeration of flow and reduction the stilling basin dimensions at the downstream dam toe due to significant energy dissipation. Depending on the discharge rate, step height and slope, three flow regimes can be identified on stepped spillways: nappe, transition, and skimming flow. Nappe flow occurs for relatively low discharge rates and lower slopes. Transition flow with a range of intermediate flow rates has a chaotic flow motion with intense splashing. Skimming flow occurs for relatively higher discharge rates and steeper slopes, is characterized by a flow over the pseudo-bottom formed by small vortices between steps. On a stepped spillway, the steps act as macro roughness elements, contributing to enhanced energy dissipation and significant aeration. In a skimming flow, the upstream flow motion is nonaerated, and the free surface appears smooth and glossy up to the inception point of free-surface aeration. In this developing flow region, a turbulent boundary layer grows until the outer edge of the boundary layer interacts with the free surface and air entrainment takes place. In recent decades, much research has been done on various flow regimes over the stepped spillway, the way they dissipate energy and the impact of the geometry of the steps on flow structure. In this research, hydraulic performance of skimming flow over Zhaveh stepped spillway has been studied and qualitative and quantitative comparison of flow between stepped and chute spillways has been presented.

Methodology:

 A 2D numerical models of spillway has prepared using FLUENT 6.3.26 software, k-e RNG turbulence model and Mixture multiphase flow method to simulate and calculate the hydraulic characteristics of the energy dissipation of spillways. The softwares to create the spillways geometry and mesh were SOLIDWORKS and GAMBIT respectively. The meshes in the tank section were quadrilateral and due to irregular geometry and the presence of stairs, meshes in the spillway and chute section were tri / pave. The boundary conditions were velocity inlet for inlet flow, free pressure inlet for free surface flow, outlet pressure for outflow and wall for floor and stairs. The numerical model has been calibrated applying experimental data extracted from the physical model of the Zhaveh spillway. It is stepped spillway with a height of 85 m and located on the Zaveh river at the junction of the Gavrood and Gheshlagh rivers at 35 km south of Sanandaj city. The spillway with a crest length of 55 m and a side slope of 1.2V:1H (50.19 degrees) located on the body of the Zhaveh dam.

Findings:

 The results indicated in addition to agreement between laboratory and numerical data, having steps alongside the chute spillway can reduce significantly the length of the boundary layer which is developed from the spillway crest and encountered with the flow surface from where the flow air entrainment is initiated. So the inception point related to air entrainment is located further upstream. Analysing the flow pattern indicated that due to aeration after the inception point, the flow depth and velocity in stepped spillway increases and decreases respectively compared to a chute spillway. Flow aeration causes the cavitation index to become much higher than the critical value (0.2) in the entire stepped spillway thus the risk of cavitation occurrence and the relevant damages are reduced considerably. While it was observed that the possibility of cavitation occurrence was high on the chute spillway (without steps) starting from 56 m downstream of the spillway crest. As a result, cavitation erosion was more likely expected on the chute spillway surface. Also for design discharge, the flow energy was effectively dissipated alongside the stepped spillway in comparison to chute spillway with a discrepancy of 46%.

Conclusion:

 The application of the stepped spillway would be more appropriate and economical than the chute spillway due to the various advantages mentioned above and also there is no risk of cavitation on the Zaveh stepped spillway, while in the end of the chute spillway Corresponding to it, we may encounter cavitation erosion.