ABSTRACT

The impact of a turbulent shear layer generated by the diffusion of plunging jets may scour the riverbed downstream of water releasing structures, weirs or waterfalls. Large recirculation currents may as well create undesirable erosion of the riverbanks. The turbulent mixing and air entrainment in the pool by plunging jets represent a major interest for water quality, since they contribute to the processes of degradation of organic matter and dilution of pollutants. These phenomena are directly related with the type of macro-turbulent flow in the pool, which often depends on its geometry. Riverbed and bank erosion have been observed in nature and real-size facilities, as well as investigated in reduced-scale laboratory facilities. Jet diffusion and impact loads in plunge pools have mainly been studied in pools with flat bottom (Cola 1966; May and Willoughby 1991; Ervine et al. 1997; Bollaert and Schleiss 2003). However, pool geometry may considerably differ in practice, as a function of the joint set patterns and the degree of fracturing of the rock mass (Figure 1). Extensive experimental work with idealized scour geometries using quasi-prototype velocity jets and aeration conditions have been performed by Manso (2006). The understanding of the physical processes governing the interaction between

the pool flow patterns and jet diffusion may help engineers inferring the behaviour of a given plunge pool and providing adequate solutions for the mitigation of scour and bank erosion induced by recirculation currents. This paper presents results of experimental investigations with plunging jets in laterally confined plunge pools using near-prototype jet velocities and aeration conditions. Due to the difficulties of performing direct measurements of pool flow characteristics under such extreme conditions of air concentration and high Reynolds numbers, the approach consisted in measuring dynamic wall pressures. The analysis of the fluctuating characteristics of bottom pressures allows presenting evidence of the main flow features. This

analysis is correlated with direct observations of the flow motion.