Air-water flow properties on stepped spillways for embankment dams: Aeration, energy dissipation and turbulence on uniform, non-uniform and pooled stepped chutes

Abstract

Stepped spillways are a common design for flood release facilities of embankment dams. The steps increase the energy dissipation and aeration performances. In the past couple of decades several  experimental studies provided design guidelines and yielded insights into the complex air-water  flow structure. Despite this research, an optimum design for embankment dam stepped spillways is  still unknown and the interplay between free-surface aeration, cavity recirculation and energy  dissipation is still not completely researched. The present thesis focused therefore on two objectives, i.e. the investigation of aeration and energy dissipation on alternative stepped  spillway designs and the improved understanding of the microscopic and macroscopic two-phase flow  properties. Twelve different stepped spillway configurations were investigated in three different experimental facilities with slopes of 8.9° and 26.6° comprising flat uniform steps, non-uniform steps, pooled steps, porous pooled steps and combination of flat and pooled steps. The experiments comprised the observation of the flow patterns for a wide range of discharges and measurements of  the full range of air-water flow properties with phase-detection intrusive probes for all step  edges downstream of the inception point in transition and skimming flows.The experimental findings on the flat stepped spillways agreed well with previous studies and  showed scale effects for both Froude and Reynolds similitudes. All air-water flow data were in  accordance with self-similar equations and an extrapolation to prototype scale seemed possible.  Detailed analyses of the microscopic air-water interfaces within the bulk of the flow for air  concentration of 50% highlighted the effect of the free-surface aeration upon the flow processes in the intermediate flow region. The air-water flow properties on the non-uniform stepped  configurations agreed well with the corresponding flat stepped spillways, but larger aeration and  small instabilities were observed downstream of a large step drop. The experiments with acoustic displacement meters on a non-uniform stepped spillway showed the possibility to  record the free-surface profiles and fluctuations in both aerated and non-aerated flow regions. On the pooled stepped spillways, some instabilities were observed for small discharges including pulsations in  the first step cavity. For the 8.9° slope pooled stepped spillway, the instabilities impacted upon the air- water flows comprising jump waves propagating downstream and irregular cavity recirculation and ejection processes. These instationary air-water flows were also reflected in the air-water flow properties and unrealistically large turbulence levels were recorded. A triple decomposition approach for the square wave voltage raw signal was developed to identify the contribution of the slow and fast fluctuating motion upon the turbulence velocity fluctuations in the instationary air-water flows. The triple decomposition was also applied to hydraulic jumps highlighting the possibility for further applications in (pseudo-)periodic air-water flows. With introduction of pores into the pool weir walls, the instabilities disappeared and the flow became more stable, but the aeration within the step niche was reduced. The air-water flow processes within the step cavities for the flat and pooled stepped spillways were investigated in detail at several positions along and within the step cavities. The experiments highlighted the cavity recirculation and the momentum exchange between shear layer and mainstream flows. Self-similarities of the shear layer velocities were observed independently of cavity shape and pooled weirs. The observations of microscopic air-water flow properties and of the integral turbulent scales showed the cavity recirculation processes.The comparative analyses of all stepped spillway configurations in the present thesis showed a strong energy dissipation and aeration performance. The largest rate of energy dissipation rate was observed for the pooled stepped spillway with flatter chute slope. However, the instable flow processes might not allow a safe operation and the preferred design is the flat uniform stepped spillway. For more complex stepped spillway designs, the present results provided design guidelines and highlighted the importance of physical modelling before implementation in a prototype environment. The comparison of the present experiments with previous data for embankment stepped spillways identified a chute slope of 21.8° as optimum for air entrainment. The aeration efficiency was calculated based upon air-water flow measurements and a self-similar relationship between aeration efficiency and energy dissipation rate was found. The findings highlighted the important role of the intermediate flow region for the energy dissipation performance.