Rafael Duarte

Experimental Study on Dam-Break Waves for Silted-Up Reservoirs

The overall dam-break phenomenon is currently well understood for reservoirs filled with water. However, many dams face serious problems of reservoir sedimentation. Thus, regarding risk management, the influence of a large amount of sediment in the reservoir on dam-break wave propagation is of great importance. Experimental investigations were performed to investigate the influence of the silting degree of reservoirs and grain size distribution on wave heights and velocities. Dam-break waves in silted-up reservoirs are the result of a two-phase flow formed by a layer of water over a layer of saturated sediments in which complex phenomena occur simultaneously. Sediments flow such as a viscous fluid from the dam breach until complete stoppage, and clear water flows over the sediments creating bed load transport. The final sediment deposition cone, the flow of the positive front, and maximum wave heights were studied. An empirical formula is proposed to describe the celerity of the positive wave front in the case of silted-up reservoirs.

Dynamic response of an embedded block impacted by aerated high-velocity jets

ABSTRACT Uplift of blocks from the rock mass at the bottom of plunge pools is the main process of the scouring phenomenon caused by high-velocity jet impact. Although the influence of water and rock on dynamic block ejection has been investigated, no systematic research has been done regarding the influence of air entrainment. This study presents theoretical developments and experimental investigations of the vertical displacements of a block embedded in the pool bottom impacted by aerated plunging jets. The theoretical model reproduces well the measured block vibrations, especially for high-velocity jets where the properties of the air–water mixture inside the fissures are steadier. Block displacements reduce with jet aeration for shallow pools, but, inversely, they increase with jet aeration for deep pools. This confirms that jet aeration reduces jet momentum due to a lower apparent density, but may also increase bottom pressures due to lower velocity decay along the pool depth.

An Enhanced Physically Based Scour Model for Considering Jet Air Entrainment

Abstract Based on systematic experiments on the influence of air entrainment on rock block stability in plunge pools impacted by high-velocity jets, this study presents adaptations of a physically based scour model. The modifications regarding jet aeration are implemented in the Comprehensive Scour Model (CSM), allowing it to reproduce the physical-mechanical processes involved in scour formation concerning the three phases; namely, water, rock, and air. The enhanced method considers the reduction of momentum of an aerated jet as well as the decrease of energy dissipation in the jet diffusive shear layer, both resulting from the entrainment of air bubbles. Block ejection from the rock mass depends on a combination of the aerated time-averaged pressure coefficient and the modified maximum dynamic impulsion coefficient, which was found to be a constant value of 0.2 for high-velocity jets in deep pools. The modified model is applied to the case of the observed scour hole at the Kariba Dam, with good agreement.