Spillway Stilling Basins and Plunge Pools Lining Design

The stability of slab protection at the bottom of spillway stilling basins or plunge pools downstream of large dams came to the great practical interest when the protection under the hydraulic jump stilling basins in some hydraulic plants was seriously damaged by floods smaller than maximum design value. This thesis offers novel practical design criteria to define the concrete thickness of spillway stilling basins and plunge pools lining. In the case of spillway stilling basins, the study presents a new experimental procedure that can define the global instantaneous uplift force. Results from detailed experiments of the statistical structure of turbulence pressure fluctuations at the bottom of hydraulic jumps is reported. Here, the whole spatial correlation structure of the fluctuating pressure field is required in order to evaluate slab stability. This is computed via simultaneous acquisition of the point pressure fluctuations on a dense grid in the hydraulic jump region, requiring a severe experimental work. As an alternative, one can evaluate the pressure spatial correlation structure via auto-correlation using one point pressure acquisition and applying the Taylor hypothesis. To adopt the Taylor hypothesis, one must know the pressure propagation celerity in space that can be obtained by comparing the whole spatial pressure correlation with the pivot point pressure auto-correlation. The experiments were performed by simultaneous pressure acquisitions at the bottom of a hydraulic jump for Froude numbers of the incident flow ranging from 4.9 to 10.3. From experiments, a criterion to define the pressure celerity as a function of the incident flow velocity is presented. The results highlight a good agreement between the relevant pressure statistical parameters as measured and the ones computed using the Taylor hypothesis. The comparison between the slab thicknesses, as computed via Taylor hypothesis, with the ones retrievable in literature, as obtained by direct force measurement on instrumented slabs in laboratory conditions, highlights the accuracy of the proposed approach that presents undeniable practical advantages. While this simplified approach based on Taylor hypothesis is used to assess the pressure field acting on the slab, the pressure propagation at the lower surface of the slab is evaluated using a 3D model based on unsteady flow analysis of seepage through porous media. By this approach, it is possible to consider the effect of finite thickness foundation layers, typical in the case of earth dams, rock-fill dams and in other dam types. Slabs with unsealed joints are considered and compared to the case of sealed joints. The dynamic behavior of anchored slabs is also investigated. These results are relevant to a robust and safe design and maintenance of stilling basins downstream of large dams. In the case of plunge pools, the stability of concrete slabs or rock blocks under the impact of an impinging jet is theoretically analyzed, with reference to the mean characteristics of the flow field: pressure and velocity. In cases when the mean components are relevant respect to the fluctuating ones this analysis is exhausting. In other cases, a separate evaluation of the fluctuation effects in lining design is treated, by means of experimental evidences. The mean dynamic pressure at the bottom depends strongly on the impingement angle that assumes a relevant role in the design of floor protections. In plunge pools, that are confined upstream by the presence of the drop structure, the impingement angle is theoretically determined by mass balance and momentum conservation, resulting independent on the jet entrance angle at the plunge pool water surface. The theoretical results are compared with literature experimental evidences and numerical simulations. This highlights the capability of the proposed theoretical framework to correctly interpret the physical phenomena and to produce suitable engineering approaches.