E. Bollaert

Scour of rock due to the impact of plunging high velocity jets. Part II : Experimental results of dynamic pressures at pool bottoms and in one- and two-dimensional closed end rock joints

This paper presents the experimental results of dynamic pressure measurements at simulated plunge pool bottoms and underlying rock joints, due to plunging high velocity jet impact. Emphasis is given on the mean and the fluctuating part of the dynamic pressures, to the extreme pressure values, and to the spectral content of the fluctuations. Particular attention is also paid to the relationship between pool bottom pressures and the pressures they generate inside underlying rock joints. Based on data analysis in one- and two-dimensional rock joints, it was found that high velocity plunging jets are able to generate oscillatory and resonance pressure waves inside the joints. These non-linear transient phenomena propagate at wave celerities that depend on the air content of the air-water mixture inside the joint. This air content is directly related to the plunge pool air content and to instantaneous pressure fluctuations inside the joint. The resulting amplification of pool bottom pressures inside rock joints is believed to be a key for a better assessment of scour formation in rock.

Experimental and numerical modelling of sedimentation in a rectangular shallow basin

Abstract Numerical simulation of flows in shallow reservoirs has to be checked for its consistency in predicting real flow conditions and sedimentation patterns. Typical flow patterns may exhibit flow separation at the inlet, accompanied by several recirculation and stagnation areas all over the reservoir surface. The aim of the present research project is to study the influence of the geometry of a reservoir on sediment transport and deposition numerically and experimentally, focusing on a prototype reservoir depth between 5 and 15 m as well as suspended sediment transport. A series of numerical simulations is presented and compared with scaled laboratory experiments, with the objective of testing the sensitivity to different flow and sediment parameters and different turbulence closure schemes. Different scenarios are analyzed and a detailed comparison of preliminary laboratory tests and some selected simulations are presented. The laboratory experiments show that suspended sediment transport and deposition are determined by the initial flow pattern and by the upstream and downstream boundary conditions. In the experiments, deposition in the rectangular basin systematically developed along the left bank, although inflow and outflow were positioned symmetrically along the centre of the basin. Three major horizontal eddies developed influencing the sediment deposition pattern. Although asymmetric flow patterns are privileged, a symmetric pattern can appear from time to time. This particular behaviour could also be reproduced by a two-dimensional depth-averaged flow and sediment transport model (CCHE2D). The paper presents numerical simulations using different turbulence closure schemes (k-ɛ and eddy viscosity models). In spite of the symmetric setup, these generally produced an asymmetric flow pattern that can easily switch sides depending on the assumptions made for the initial and boundary conditions. When using the laboratory experiment as a reference, the most reliable numerical results have been obtained with a parabolic depth-averaged eddy viscosity model. This model appeared to be the only one that was able to reproduce the strongly asymmetric flow behaviour observed during the experiments.

Influence of Plunge Pool Geometry on High-Velocity Jet Impact Pressures and Pressure Propagation inside Fissured Rock Media

The geometrical development of unlined plunge pools downstream large dams depends on both local geology and type of plunging jet and its diffusion pattern. Experimental investigations in plunge pools with different lateral confinements are presented. They show that the pool geometry influences plunging jet diffusion, air entrainment in the pool, and, as a consequence, impact pressures at the water-rock interface and inside the fissured rock mass. Results include impact pressures definition for variable pool depths, jet velocities, and pool geometries, depending explicitly on the lateral confinement of the jet. The main pool flow features are described focusing on their contribution to energy dissipation. Should the lateral confinement limit the development of macroturbulent rollers around the plunging jet (instead of the pool depth), the mean pressures at impact are considerably reduced, as well as RMS pressures in transitional and deep pools. For shallow pools, RMS pressures increase as a result of enhanced jet development.

Comprehensive system analysis of a multipurpose run-of-river power plant with holistic qualitative assessment

Alpine rivers have been channelized by significant river training works in the past two centuries and are now disconnected from their natural environment. In addition, their flow regime is often affected by hydropower plant operation. Also the risk of flood damages is increasing continuously due to urbanization requiring additional flood protection measures. Nevertheless, such trained rivers still have high potential for renewable energy production. Furthermore, there is often a need for biotope restoration and creation of leisure infrastructures. New hydraulic schemes on such rivers have a chance to obtain public acceptance only if they are designed as multipurpose projects, which can alone ensure high synergies between different goals. Multipurpose projects are complex systems and have to be assessed with an appropriate global approach. Based on a network thinking approach, this article presents a global qualitative system analysis specially adapted for a typical multipurpose run-of-river power plant for the six project themes involved: (1) hydraulic scheme and river flow regime, (2) energy, (3) economy, (4) leisure activities, (5) groundwater and (6) ecology. The qualitative network thinking method developed by Gomez and Probst for business strategies is, for the first time, applied and enhanced for the assessment of such a multipurpose hydraulic scheme. Each theme, i.e. purpose of the project, is analysed separately, followed by a comprehensive study of the six themes combined together. Based on a network representation of the global system, three groups of factors are distinguished describing the sizes, the operations and the goals of the project. The size factors characterize the main geometrical aspects of the hydraulic structures, which can define the best layout of the project. The operation factors allow the optimization of the management of the reservoir. Finally, the objective factors characterize the synergies obtained by the multipurpose project. The developed methodology is illustrated with a case study of a multipurpose hydroelectric run-of-river power plant.

Evaluation of high-velocity plunging jet-issuing characteristics as a basis for plunge pool analysis

This paper presents experimental work on the characteristics of jets issuing from water-releasing structures. Systematic experimental tests have been performed at near-prototype velocities of up to 30 m/s using a cylindrical jet nozzle. Dynamic pressures were measured along the diameter at the nozzle outlet with an acquisition rate of up to 2 kHz, for different upstream supply conditions. These measurements allowed assessing the mean and the fluctuating characteristics of the jet. The influence of supply circuit secondary currents, aeration and geometrical contraction on jet turbulence intensity and velocity profiles is discussed. The initial turbulence intensity varies from 2 to 8%, whereas the kinetic energy correction factor ranges from 1.0 to 1.1. Statistical analysis of pressure measurements shows that jet core pressures follow a Gaussian distribution fairly well for non-exceedence probabilities between 0.1 and 0.999. The experimental results and an extensive literature survey are used to define issuance parameters relevant for engineering practice.

Fluid-Structure Interaction Analysis Applied to Oil containment Booms

Note: [452] Reference LCH-CONF-2008-035 Record created on 2008-02-22, modified on 2017-11-27

Discussion of "Effect of jet aeration on hydrodynamic forces on plunge pool floors"

Note: [653] Reference LCH-ARTICLE-2009-010doi:10.1139/L08-143View record in Web of Science Record created on 2009-06-26, modified on 2016-08-08

Hybrid Modelling Approach to Study Scour Potential at Chancy-Pougny Dam Stilling Basin

Chancy-Pougny is a run-of-river dam on the Swiss-French border constructed in the early 1920s. Since its inauguration, the operation of the four spillway gates was responsible for a progressive erosion of the stilling basin. Thus, a hybrid modelling was performed to study the scour potential and to determine adequate solutions to maintain future scour within acceptable levels. Visual observations on the physical model identified a strong recirculation in the non-symmetrical basin, that combined with the outflow from the spillway gate, lead to an enhanced specific discharge and to the formation of a turbulent vortex impacting the rocky foundation. Some measures to limit the recirculation, including a free-standing wall and various configurations of concrete prisms for scour protection, were tested on both the physical and numerical model. The pressure and water depth measurements resulting from the physical model were used within the numerical model to determine the corresponding scour potential for each of the tested mitigation measures. A solution containing a layer of randomly distributed concrete prisms laid on the basin’s current bottom was identified through this study, proving the importance of both numerical and physical approaches in hydraulic engineering

Discussion of Closure problem to jet scour

The author is to be complimented on his interesting paper. We continue to be in awe of the contribution Isaac Newton made to physics, and it is wonderful to see how his principles can be applied to solve modern problems. The author illustrates application of his equation to scour problems in both non-cohesive soils with diameters less than 0.0125 m and to a rock scour problem at Kariba Dam. The value of coefficient c2v, required in Eq. (14) to calculate the scour depth, is a function of material properties (Hoffmans 1998). Its value decreases with increasing diameter until reaching a diameter of 0.0125 m, after which c2v is constant at 2.9. According to the author, the coefficient should assume a constant value of 2.9 for particle diameters in excess of 0.0125 m, which includes intact rock formations. According to the discussers, the latter finding may be an oversimplification for rock scour estimation. Predicting scour of rock formations requires quantification of both the scour resistance offered by the rock and the relative magnitude of the erosive power of the plunging jet. Both tasks are complex. The erosive capacity of jets is affected by jet issuance velocity, drop height, jet break-up characteristics, pool depth and entrained air. Rock scour mechanisms are similarly complex. Bollaert (2002) identified dynamic impulsion, brittle fracture and fatigue failure as the principal mechanisms leading to rock scour. Abrasion is an additional failure mechanism that is usually ignored. The scour resistance of rock formations is conventionally assessed by considering the unconfined compressive strength of the rock, its block size, shear resistance between rock blocks and its dip and dip direction, in addition to quantifying the rock’s fracture toughness and fatigue failure properties. To the best of our knowledge, two methods currently provide a practical means to analyse scour of rock, taking account of all the variables indicated above. Currently, the erodibility index method (EIM) (Annandale 1995, 2006) and Bollaert’s (2002) comprehensive scour method (CSM) are applied to analyse scour of rock formations subject to plunging jets. Joint application of these methods provides a means of cross-checking results and better understanding mechanisms that can lead to scour of rock. The discussers are therefore of the opinion that the author’s suggestion to quantify scour resistance of rock formations universally by the single parameter c2v may not be fully justified. Rock formations have various properties and react differently to the erosive capacity of plunging jets. For example, granitic rock can be massive forming large rock blocks and is usually very hard, while sedimentary rock may be less resistant because of lower compressive strength and, often, smaller block sizes. The scour depth for these two rock types will differ if subjected to the identical jet. Figure D1 provides ratios between the author’s equation, observed scour and calculated scour. For example, the ratio Hoff/EIM represents the ratio between scour depth estimates using the author’s equation and the EIM. Similarly, Hoff/Obs and Hoff/CSM relates Hoffmans’ equation estimates to observed scour depths and scour depths estimated with the CSM, respectively. It is noted that the comparison between observed scour and Hoffmans’ estimates vary considerably, from as low as about 40% to as high as about 260%. The comparison between calculated values (i.e. in the absence of observed scour) is provided for informational purposes. Journal of Hydraulic Research Vol. 49, No. 2 (2011), pp. 276–282 doi:10.1080/00221686.2011.568205 # 2011 International Association for Hydro-Environment Engineering and ResearchKeywords: Jet scour Note: [767] Reference EPFL-ARTICLE-166919doi:10.1080/00221686.2011.568602 URL: http://www.journalhydraulocresearch.com/ Record created on 2011-06-16, modified on 2016-08-09

Numerical Modeling of Scour at Bridge Foundations on Rock

The present paper presents an application of the Comprehensive Scour Model (CSM) to quarrying and plucking of fractured rock at bridge piers. Numerical modeling of rock block plucking has been performed within the framework of the National Cooperative Highway Research Program Project NCHRP -24-29. A two-phase transient numerical model simulates the potential movements of the block as a function of flow turbulence and stream power in the scour hole around the bridge pier. The hydraulic action on the rock blocks is automatically adapted during formation and growth of the scour hole. Both the ultimate scour depth and the scour threshold flow velocity are determined as a function of the shape, dimensions and protrusion of the rock block, of the average upstream river bed slope and of the angle of the rock joints. The numerical model points out the influence of turbulent eddies and block protrusion on rock block uplift.