On large dams, spillways are used to evacuate excess inflow discharges from the upstream reservoir to the downstream tailwater. When passing over the spillway the high potential energy of the water is transformed into kinetic energy and must be dissipated, which generally happens in stilling basins or plunge pools. Failing to dissipate sufficient energy can lead to erosion or scour in these structures. If this process is left uncontrolled, the structural safety of the foundation might be endangered and therefore the stability of the whole dam. During construction of the Calderwood Dam in the USA, a scour hole of 15 meter occurred because the weir controlling the water level in the pool was still under construction. At the Kariba Dam, the rock in the plunge pool has eroded to a depth of 70 meters below the original riverbed. Various spillways with different dissipation structures exist, the focus of this thesis is on the free-falling nappe initiated on a sharp crested weir in combination with a man-made plunge pool. When not all energy is dissipated before reaching the pool bottom, the remaining nappe exerts dynamic pressure on it, potentially causing scour to develop. Being able to predict the characteristics of these pressures is crucial for designing structures that can withstand them. In practice this is often done by performing pressure measurements on scale models and extrapolating the results to prototype scale. This is done by using appropriate scale factors or by making the measurements dimensionless. However, due to
the inability of achieving full similitude between scale and prototype models, scale effects influence these measurements. The objective of this study was to quantify the influence of these scale effects on pressure measurements performed downstream of the free-falling nappe. To fulfil this, 2 scale models of the same prototype but of different scale were constructed in the Hydraulics Lab of the University of Liège. On both models pressure measurements were performed and their results compared. Due to scale effects the properties of the dimensionless pressures are different across the models. In absence of a water cushion, the mean in the centre is overestimated on the small model and the
fluctuations around this mean underestimated. Additionally, when comparing the empirical density histograms, on the large model two peaks are observed. One close to the fall height and one close to 0, while on the small scale model only the peak close to the fall height appears. Furthermore, the impact location of this centre is further downstream on the large model than on the small one. With increasing water cushion height in the plunge pool, two phases affecting impact pressures related to effective and non-effective cushions can be observed on the small model. For the non-effective cushion, the cushion does not reduce impact pressures compared to the absence of a cushion. In contrast, during the effective phase, a reduction in impact pressures occurs, and this reduction increases with increasing cushion height. This distinction is absent on the large model, where any cushion height
reduces impact pressures. This reduction continues as the cushion height increases.
To increase the understanding of these scale effects, future research should focus on several aspects. If results from other scale models are published, scaling relationships could be developed to better predict pressures across different model sizes. Additionally, conducting measurements on existing prototypes, where controlled test conditions can be replicated, could be an option. Further investigation is needed to understand the cause of the peak observed near zero in the density histograms of the large model. Since air concentration in the flow has been identified as a potential primary cause of these scale
effects, qualitative research is necessary.