Influence of air entrainment on rock scour development and block stability in plunge pools

Influence of air entrainment on rock scour development and block stability in plunge pools The impingement of high-velocity water jets on plunge pools, such as it occurs downstream of spillways of high-head dams, may cause scour on the rock foundations. The scour process is the result of complex and consecutive physical phenomena. Turbulence plays a major role, especially in the jet development during the trajectory in the air, in the diffusion process in the pool, in the pressure fluctuations at the water-rock interface, and in the pressure propagation inside rock fissures. These phenomena cannot be reproduced in Froude-based reduced-scale models without significant scale effects. Hence, observations on prototype-scale models and consideration of dynamic, fluctuating pressures are the only way to correctly assess the effect of jet impingement on the development of rock scour. Air entrainment greatly influences the whole process. Air is entrained in the jet during the travel through the air and is also entrained in the plunge pool at impact. The air bubbles in the plunge pool influence energy dissipation. The bubbles may also enter rock fissures, where they change properties of pressure wave oscillation and amplification. The correct description of air bubbles behavior in plunge pools and their influence on the pressure fluctuations on the water-rock interface and inside underlying fissures under prototype conditions has never been achieved. Air has properties such as buoyancy, compressibility and solubility in water, which are difficult to evaluate and require a comprehensive research. Since 1998, the Laboratory of Hydraulic Constructions (LCH) of the Ecole Polytechnique Fédérale de Lausanne (EPFL) develops a broad research to accurately describe the principles of rock scour due to high-velocity jets. An innovative experimental facility was built, which reproduces prototype jet velocities. Past developments were made to describe rock fissure break-up, block ejection, the influence of pool bottom geometry, amongst others topics. The present research enhances the knowledge of air entrainment properties, air bubbles behavior and their influence in plunge pool bottom pressures, and tie the former work all together. Experiments were undertaken under near-prototype scale jets to measure air concentrations and air bubble velocities throughout different depths and radial distances in the jet diffusive layer, as well as dynamic pressures and displacements around a block embedded on the plunge pool bottom. Submerged jets, actively aerated at the nozzle, were reproduced to provide results with a known total aeration in the pool, whilst plunging jets, actively aerated as well, were tested to represent a more realistic scenario. Furthermore, the block was either fixed on the bottom or free to move, representing different degrees of opening and interlocking of the rock fissure network found in practice. This study reveals that the air entrained by the jet influences the pressures on the bottom in two opposed ways. On one hand, the air-water mixture can be interpreted as a jet with a reduced apparent density, and thus with lower momentum. The consequence is