Dam Safety. Overtopping and Geostructural Risks

This reprintshows recent advances in dam safety related to overtopping and the prevention, detection, and risk assessment of geostructural risks. Related to overtopping, the issues treated are: the throughflow and failure process of rockfill dams; the protection of embankment dams against overtopping by means of a rockfill toe or wedge-shaped blocks; and the protection of concrete dams with highly convergent chutes. In the area of geostructural threats, the detection of anomalies in dam behavior from monitoring data using a combination of machine learning techniques, the numerical modeling of seismic behavior of concrete dams, and the determination of the impact area downstream of ski-jump spillways are also studied and discussed. In relation to risk assessment, three chapters deal with the development of fragility curves for dikes and dams in relation to various failure mechanisms

Analysis of the Hydrodynamics of rectangular plunging Jets and the subsequent scouring in broken-up Rock Beds

Thesis (MEng)--Stellenbosch University, 2016.ENGLISH ABSTRACT: The scour mechanisms of rock are highly variable, equally so the geotechnical and hydrodynamic conditions of each study area, which complicate the prediction of scouring. Cases such as Kariba Dam and Ricobayo Dam are examples where, due to inadequate scour prediction methods, scouring caused major damage to the downstream riverbed. Rock scouring due to plunging jets is thus an important area of study. Various studies have been done to predict the scour hole depth and its extent using physical laboratory models and their subsequent empirical formulas. A classification method, the Erodibility Index method (EIM) by Annandale (1995), was also developed to give an indication of the depth of scouring. Physically based methods, such as the Comprehensive Scour Method (CSM) including the Quasi-steady impulsion method (QSI) by Bollaert (2002 and 2014), tried to incorporate several of the rock scour mechanisms, to determine both the depth and extent of the scour hole. The current study focused on using a 1:40 physical laboratory model to ascertain the applicability of using PVC blocks to replicate rock blocks and the subsequent scouring thereof. The drop height, as well as the tailwater, varied between the different tests. The PVC blocks were able to replicate the scour hole to a relatively good extent and could sustain steep slopes replicating the repose angle of rock. The subsequent scour holes from the physical laboratory models were compared to the depths calculated using empirical formulas and the classification method by Annandale (1995) (EIM). The methods overestimated the depths, and the EIM was found to be very sensitive to both the hydrodynamic and geotechnical boundary conditions. The physically based method proposed by Bollaert (2002) (CSM) overestimated the scour hole extent, as compared to the physical laboratory model scour hole, but the scour hole profile (shape and depth), was in agreement. The use of Computation Fluid Dynamics (CFD) to simulate hydraulic problems has become more viable, due to advancements in computational power. The hydrodynamic characteristics of the jet in the air and in the plunge pool was modelled in 2D using FLUENT, as the scour mechanisms of rock cannot presently be modelled using commercial CFD codes. The hydrodynamic conditions in the air were modelled with good comparison to both that of the physical laboratory model, as well as the calculated conditions of Stellenbosch University https://scholar.sun.ac.za -iiithe jet in the air, while the hydrodynamic conditions (velocity and pressure) in the plunge pool were overestimated in comparison to current methods available, as used in the EIM as well as the CSM and QSI methods, due to possible flow confinement and deflection effects. In conclusion, the study firstly confirmed the applicability of using PVC blocks to model scouring due to plunging jets in rock beds, and secondly it established the use of current scour prediction methods in validating small scale scour hole profiles (shape and depth). The use of 2D CFD modelling, in predicting the hydrodynamic conditions of the plunging jet in the air and plunge pool, was also introduced with relative success.AFRIKAANSE OPSOMMING: Die voorspelling van erosie is gekompliseerd as gevolg van erosie meganismes van rots wat baie wisselend is, asook die unieke geotegniese en hidrodinamiese kondisies van elke area wat bestudeer word. Gevalle soos Kariba Dam en Ricobayo Dam is goeie voorbeelde waar, as gevolg van onvoldoende erosie voorspellings metodes, die rots erosie grootskaalse skade aan die stroom-af rivierbedding veroorsaak het. Rots erosie wat deur vallende water strale teweeggebring is, is dus ‘n baie belangrike area en vereis deeglike navorsing. Verskeie studies is al gedoen om die diepte en omvang van die resulterende erosiegat te bepaal deur gebruik te maak van fisiese modelle en die daaropvolgende emperiese formules. ‘n Klassifiserende metode, die Erodibility Index method (EIM) deur Annandale (1995), is ontwikkel om ‘n indikasie van die erosie diepte te gee. Fisies gebaseerde metodes, soos die Comprehensive Scour Method (CSM) insluitend die Quasi-steady impulsion method (QSI) deur Bollaert (2002 en 2014), het soveel moontlik gepoog om van die rots erosie meganismes te inkorporeer, om beide die diepte asook die omvang van erosie van die erosie gat te bepaal. Die huidige studie is toegespits op die gebruik van ‘n 1:40 fisiese model om die toepaslikheid van die gebruik van PVC blokke om rots blokke na te boots en die daaropvolgende erosie vas te stel. Die valhoogte, asook die plons dompel waterhoogte, is verander tussen toetse. Die PVC blokke was instaat om die erosie gat redelik goed na te boots en kon ‘n steil gradiënt volhou aan die sykante van die erosie gat. Die daaropvolgende erosie gate van die fisiese model is vergelyk met die dieptes soos bepaal deur die emperiese fomules, asook die dieptes van die klassifiserende metode deur Annandale (1995) (EIM). Die metodes het die dieptes oorskat en die EIM metode was baie sensitief in terme van die hidrodinamika en geotegniese grens kondisies. Die fisiese gebaseerde metodes, soos ontwikkel deur Bollaert (2002) (CSM), het die erosie gat se omvang oorskat in vergelyking met die fisiese model. Die erosie gat profiel het wel ooreengestem met die profiel van die fisiese model. Die gebruik van Computation Fluid Dynamics (CFD) om hydroliese probleme te simuleer is deesdae meer uitvoerbaar, as gevolg van die vooruitgang in rekenaartegnologie en kapasiteit. Die hidrodinamiese Stellenbosch University https://scholar.sun.ac.za -veienskappe van die water straal in die lug, asook in die dompel poel, is gemodelleer in 2D, deur gebruik te maak van FLUENT. Die erosie meganismes kan huidiglik nie deur kommersiele CFD sagteware gemodelleer word nie. Die gemodelleerde hidrodinamiese toestand van die water straal in die lug het goed ooreengestem met beide die fisiese model asook die berekende eienskappe. Die hidrodinamiese aspekte in die dompel poel (druk en snelheid), is oorskat in vergelyking met die huidig beskikbare metodes om die hidrodinamika van die water straal te bepaal soos gebruik in die CSM en EIM metodes. Die geskatte waarde van die hidrodinamiese toestande was groter – dit kan toegeskryf word aan die moontlike effek van defleksie en beperking van watervloei. Ter opsomming, die studie het eerstens vasgestel wat die toepaslikheid van PVC blokke is om rots erosie as gevolg van water strale te modelleer en tweedens het die studie vasgestel dat huidige erosie vooruitskattings metodes gebruik kan word vir klein skaalse modelle. Die gebruik van 2D CFD modelle, om die hidrodinamiese kondisies van water strale in die lug en dompel poel te beraam, is met redelike sukses toegepas

Estimation of the Maximum Scour Depth at Bridge Pier under Effects of Debris Accumulations using NF-GMDH Model and Evolutionary Algorithms

Rivers accumulate huge amounts of floating debris including the trunk, branches and leaves during the floods, leading to increase the depth of local scour around bridge piers. A large number of the laboratorial and field studies have been performed to understand the mechanism of scouring phenomenon under floating debris. Over two past decades, different types of the artificial intelligence methods have been used to estimate the maximum scour depth around bridges piers. In this study, the Neuro-Fuzzy model based on group method data handling (NF-GMDH) was used to estimate the scour under effect of debris accumulations. The NF-GMDH network was developed using evolutional algorithms: genetic algorithm (GA), particle swarm optimization (PSO), and gravitational search algorithm (GSA). Parameters effective on the maximum scour depth included average velocity of upstream flow of the bridge pier, critical velocity of river bed sediments, depth of flow in section without debris, thickness of submerged debris, debris diameter, average particle size, pier diameter, and channel width. After training and experiencing each NF-GMDH models, the performances of each one was evaluated through statistical parameters. The results showed that the models proposed had better performance compared with emperical relationships. NF-GMDH-PSO (R=0.8413 and RMSE=0.37) and NF-GMDH-GA (R=0.8407 and RMSE=0.3640) had relatively similar performance. Finally, sensitivity analysis indicated that the ratio of pile diameter (D) to mean diameter of bed sediments (d50) has the most influence on determination of maximum scour depth

Hydraulic Structures: Useful Water Harvesting Systems or Relics?

Throughout the ages, the construction of hydraulic structures has supported the development of human civilisation. The rate of construction of new water projects in Europe and North America has dropped during the last few decades, and many of the original water harvesting system mega projects are now near, or even past, their original intended design lives. The question therefore arises whether the existing systems are redundant relics from the past that have reached their sell-by date, or do they still have an important role to play in modern society? The high level of activity, even at a junior engineer and researcher level, in scientific meetings on hydraulic structures demonstrates that water harvesting systems are highly relevant to modern day needs. In these proceedings, the topics presented are driven by an urgent demand for practical solutions to real-world problems involving design, construction, maintenance and operation of systems for water management and water harvesting. It is clear that the demands on hydraulic structures are now imposed by a wide range of stakeholders, and the papers contained herein cover applications including flood protection, stormwater management, navigation, industrial water consumption, infrastructure, and energy dissipation. The IJREWHS'10 workshop addressed both conventional and innovative aspects of hydraulic structures, their design, operation, rehabilitation and interactions with the environment. The workshop provided an opportunity for young researchers and engineers, typically post-graduate students, but also young researchers and engineers in both public and private sectors, to present ideas, plans, and preliminary results of their own research in an inspiring, friendly, co-operative, and non-competitive environment. The proceedings contain 15 papers including a keynote lecture paper involving 37 authors from 6 countries and 2 continents, plus 6 session reports and 9 pages of photographs of hydraulic structures from around the world, in addition to the photographs of the workshop and of the technical visit. Each paper was peer-reviewed by a minimum of two experts. The proceedings regroup a keynote lecture paper on the hydraulic jump flow phenomenon, 8 technical papers dealing with the design of hydraulic structures and their physical modelling, 3 papers discussing the interactions between sediment processes and hydraulic structures, and 3 papers related to the numerical modelling of open channel flows. These are followed by 6 discussions reports. Innovative designs for overflow weirs are presented in two papers. An improved understanding of performance of existing hydraulic structures is discussed in several papers on flow characteristics and hydrodynamics along and downstream of spillways. The requirement for water in industrial applications is addressed by papers on optimization of applied designs for navigation, hydropower and cooling water intakes. As human development encroaches further on the natural water systems, the interaction between hydraulic structures and nature, and minimization of the potential impacts of structures on natural river processes, is presented in the papers on bridge pier scour. Finally, the use of computer programs to assist in studying, understanding and designing of water related infrastructures is addressed in three papers on numerical modeling techniques

The influence of pool geometry and induced flow patterns in rock scour by high-velocity plunging jets

The dissipation of energy of flood discharges from water releasing structures of dams is often done by plunging jets diffusing in water and impacting on the riverbed downstream. The construction of expensive concrete structures for energy dissipation can be avoided but the assessment of the scour evolution is mandatory for dam safety. The scour growth rate and shape depend on the riverbed geology. The geometry of scour may influence the turbulent flow pattern in the pool, the dynamic loadings acting on the rock interface, and the pressures propagating inside rock joints. Up to present, dynamic impact pressures at the pool bottom have been investigated mainly in pools with flat bottom and are therefore described as function of the pool depth and the characteristics of the jets only. To approach the conditions found in practice, non-flat plunge pools and turbulent two-phase jets are investigated in this research work. This fundamental investigation focuses on the interaction between the development of plunging jets in the water and the geometry of the plunge pool. The influence of laterally confining jet diffusion is investigated by means of experimental work in near prototype conditions, in terms of jet velocities and air entrainment in the pool. Different pool geometries typical of prototype conditions are tested and compared with a reference pool with flat bottom. Pressure measurements at the jet outlet, at the pool bottom and inside a closed-end fissure are presented. The main emphasis of the text is on the analysis and the description of physical processes. The integration of the findings in existing scour estimation models is discussed. The thickening of the water cushion downstream, artificially or by scour, is investigated for fully controlled jet issuance conditions. The dissipation of jet energy is estimated based on measurements of mean impact pressures and is compared with results from an analytical model. The developed model features jet diffusion in limited-depth pools and is tentatively applied to turbulent two-phase jets. Agreement is good in the early stages of scour and in deep flat pools. For pool depths about the jet core development length (i.e. transitional pools), analytical estimates are quite sensitive to the initial assumptions on the centreline velocity decay, dimensions of the impinging zone and pool aeration. The findings highlight the limitations of existing empirical laws in representing the diffusion of turbulent two-phase jets in pools with flat bottom. Turbulent impact pressures are also investigated for increasing pool depths. Based on an evaluation of high-order statistical moments and autocorrelation functions of pressure fluctuations at stagnation, jet development conditions at impact are distinguished in core and developed impact conditions. Core impact conditions are typical of shallow pools and generate negatively asymmetric distributions at stagnation. The end of core development is associated with highly intermittent flow conditions, with important pressure fluctuations (high kurtosis). For developed impact conditions, pressure fluctuations at stagnation are positively asymmetric. A Gaussian distribution fits satisfactorily the data, save for extreme high and low (cumulated) probabilities. Air-water measurements are carried out at selected points in pools with at bottom. They allow describing the behaviour of air bubbles before, at, and aside stagnation. Void fraction estimates close to the entry of rock fissures show that air bubbles reduce in size under the influence of the high-pressure gradient at stagnation. The characteristic dimensions of the air bubbles close to the bottom are small compared to typical entry dimensions of rock fissures. The investigations conducted in pools with flat bottom are used as a reference scenario in the investigation of plunge pools with more realistic geometries. The experimental results show that mean impact pressures at the pool bottom are lower in laterally confined pools than in equivalent pools with flat bottom. The length of core development can be reduced, depending on the degree of confinement and pool depth. Enhanced pool turbulence is described by power spectra density and probabilistic distribution functions of impact pressures. It is concluded that the flow currents created by deflection of the jet on the lateral boundaries of the pool may interfere with the development of the jet, generate additional dissipation in the water column and hinder the propagation in depth of air bubbles in the pool. For shallow and transitional laterally confined pools, pressure fluctuations may have more energy than in corresponding pools with flat bottom. Power spectra of pressure fluctuations have higher energy content in the intermediate frequency range (e.g. 10 a 100 Hz). Extreme positive pressures increase. In terms of scour, there is a trade-off relatively to flat pools: there is hardly core impact and persistent hydro-fracturing (because mean impact pressures are lower), but fracturing may occur if high low-persistence pressure peaks are generated inside rock fissures (by transients due to enhanced impact pressure fluctuations). For deep laterally confined pools, the energy of pressure fluctuations is lower than in pools with flat bottom. Extreme positive pressures are similar, but increase in relative terms to the total energy of pressure fluctuations. Negative extreme pressures are lower. The most relevant flow features in laterally confined pools are identified using direct observations of flow patterns and in-depth analysis of the characteristics of turbulent pressures at impact. Large-scale pool flow features like surface oscillations, shear eddies and air-water ejections are described. The evolution of geometry-induced flow patterns and dynamic loading with scour development, for variable width of confinement and pool depth, are presented for four typical scour scenarios. The role of the deflected upward currents and shear eddies in the dissipation process depends on the degree of confinement, jet velocity and pool depth. The closer they are to the plunging jet, the higher is the dissipation of energy before impact. Recirculation currents may enhance jet development by either pushing upward currents into the jet. The dynamic pressure measurements performed inside closed-end fissures allow concluding that the dynamic response of rock fissures varies with the turbulent character of impact pressures at the rock interface. It is shown that the dimensions of the entry of the fissure play an important role in filtering turbulent pressure fluctuations in the transition from the pool into the fissure: the larger the dimensions, the lower are the frequencies filtered out. It is observed that the energy of pressure fluctuations inside the fissure is always higher than at the entry, for all pool configurations tested. The energy of pressure fluctuations inside rock fissures is lower in narrow confined pools for transition and deep pools, but higher in shallow pools, compared with equivalent pools with flat bottom. This is also valid for positive extreme pressures. Negative extreme pressures are generally lower. Amplification of pressure peaks is observed inside a closed-end fissure for both shallow and deep pools; it depends of the degree of jet development, i.e. of relative pool depth, pool geometry and jet turbulent characteristics (and, indirectly, of the amount of entrained air reaching the bottom of the pool). Therefore, transient pressure peaks generated inside fissures are a potential agent of scour in laterally confined pools, from shallow to deep. Amplification occurs due to the development of transient flows inside the fissure, that may include column separation. The occurrence of resonance inside fissures is investigated numerically. Multiple resonant harmonics are replicated solving numerically the waterhammer equations inside the fissure with the hydraulic impedance method. A probabilistic-based event analysis is developed to correlate the probability, persistence, duration and energy content of extreme pressure pulses. It is shown that pulses with high extreme (cumulated) probabilities have low persistence and high energy content. The concept of relevant probability is outlined, allowing for the selection of pressure events that should effectively be considered for the propagation of rock fissures or for the dynamic uplift of rock blocks. The role of extreme pressure events in the scouring processes of crack propagation and block displacement is discussed. In conclusion, the experimental investigation of jet diffusion in pools with flat bottom and laterally confined pools shows the importance of pool flow patterns in the definition of impact pressures and transient pressures inside rock fissures. It provides detailed information on hydrodynamic processes involved in rock scour, as well as several contributions to engineering practice, in terms of jet issuance conditions, empirical relationships for impact pressures and recommendations for the design of pre-excavated pools

Advances in Hydraulics and Hydroinformatics Volume 2

This Special Issue reports on recent research trends in hydraulics, hydrodynamics, and hydroinformatics, and their novel applications in practical engineering. The Issue covers a wide range of topics, including open channel flows, sediment transport dynamics, two-phase flows, flow-induced vibration and water quality. The collected papers provide insight into new developments in physical, mathematical, and numerical modelling of important problems in hydraulics and hydroinformatics, and include demonstrations of the application of such models in water resources engineering