Dam Safety Assessment Research Articles

Experimental Study on Permeability Characteristics of Geotubes for Seepage Analysis on Safety Assessment of Dams

Experimental Study on Permeability Characteristics of Geotubes for Seepage Analysis on Safety Assessment of Dams

Analysis of periodicities in long‐term displacement time series in concrete dams

The hydrostatic-season-time (HST) model is a widely used method in the safety assessment of dams and for the estimation of reversible and irreversible deformations due to different load scenarios. After implementing an optimal HST-model to observational data, the residual time series can still expose some underlying periodicities. In our contribution, the underlying periodicities and their contribution to residual minimisation are further analysed by the Lomb–Scargle normalised periodogram frequency method. The extended HST-model, obtained by adding additional sinusoidal terms for the statistically most significant frequencies, presents a slight statistical improvement on the optimal HST-model. Statistically significant frequencies may not be justified or correlated to some physical process of the dam, but they make it possible to identify a pattern in the analysed period. The analyses are performed for the radial direction of long-term displacement time series measured at the topmost reading station of an inverted pendulum system in the central cantilever of the Alqueva concrete arch dam.

Transverse cracking in embankment dams resulting from cross-valley differential settlements

This article investigates transverse cracking in embankment dams resulting from cross-valley differential settlements during and post-construction, in which concentrated leak erosion may initiate. Based on case studies and numerical analyses, it is shown that valley profiles with a step in the upper part of the abutment and subject to large post-construction settlements within the embankment, or within compressible soil in the foundation, are most likely to be subject to cracking. Estimated localised strains at the crest > 1%, and in particular >2%, are likely to be indicative of conditions conducive to transverse cracking. Smaller strains but greater than about 0.25% may also result in cracking but they are likely to be narrower and shallower. The article also gives general guidance on the likely width and depth of cracking. The information presented is only suitable for preliminary dam safety assessments. For detailed dam safety assessments and risk assessments where decisions are to be made on whether risks are tolerable, or upgrades required, site-specific cross-valley numerical analyses should be carried out. These detailed analyses should model the cross-valley profile accurately, material parameters more rigorously than has been done in the parametric studies reported in this article.

DAMS SAFETY INDEX (DSI): A REASSESSMENT OF THE PARAMETERS OF ANALYSIS

ABSTRACT Dams provide a water supply, irrigation and power generation. However, the economic, social and environmental impacts are serious when they rupture. It is important that dams continue to function properly and are constantly evaluated for their safety and functionality. Contrary to general belief, small dams are the ones that present the most problems, because they are often built without adequate design and construction methods. One of the methods to evaluate the safety of a dam is through the Dam Safety Index (DSI), which may not entirely avoid subjectivity in dam safety assessment but does propose balanced individual subjectivities in determining the weights of the index criteria. The DSI can also assist managers with preventive maintenance schedules in a dam safety assessment. This work aims to reassess the importance of the parameters of the DSI based on a new weighting of these criteria. The modification of the DSI shows a new bias in the weighting of the criteria that composes the index, not only considering technical aspects, but also legal and environmental ones.

The Usage of Agent Based Modelling in Flood Evacuation during Dam Failure

With the advent of latest technology has enabled disaster management related efforts to be more efficient and effective. Agent based modelling has been utilized in crowd management context especially related to simulation of evacuation passage during the outbreak of any untoward incidents. In dam safety assessment, the number of loss of life during the flood indicates the severity of the catastrophe. Therefore, it is important for any dam owners to estimate the required warning time to ensure minimization of fatalities in the event of a disaster. The objective of the research is to estimate the loss of life at the downstream area of the Sultan Abu Bakar Dam when Probable Maximum Flood (PMF) scenario occurs. The adopted model is known as Life Safety Model (LSM), which is an agent-based model that uses the concept of dynamic interaction between the 2d-hydraulic and the receptor. A case study has been conducted focusing the population of Bertam Valley in Cameron Highlands, Pahang, West Malaysia. The outcome of this modelling has indicated that successful evacuation depends on the response rate of the victims towards the given warning prior to the disaster. It is shown that the response time might affect the fate of people during the evacuation. The application of Life Safety Model in this simulation proves its capability in estimating the loss of life as a planning measure to face the actual events.

Dam safety assessment using 2D electrical resistivity geophysical survey and geological mapping

A combination of Geological mapping and two-dimensional electrical resistivity (2D-ER) surveys was applied to study the Unilorin Dam and its environment. The purpose of the study was to investigate the dam for structural anomalies that may compromise the purpose and safety of the dam. Field equipment for the study comprised a SuperSting R8/IP Multi-Electrodes Resistivity Meter, 84 metallic electrodes and the accessories, compass clinometer, and portable GPS equipment. Geological data were processed and plotted to obtain a Geological Map and Rosette Diagram that were used for structural interpretations. The structures interpreted include an asymmetric fold and a strike-slip fault. The 2D resistivity data were processed and tomographically inverted to obtain the resistivity models of the subsurface around the dam. Interpretations of 2D resistivity models showed that the dam reservoir floor is underlain by competent basement rocks, however, the basement rock is weathered in some places. Patches of low resistivities structures interpreted as seepages, fractures and water-saturated cavity were delineated in different sections of the dam. The surface structural elements from geological study support the subsurface structures interpreted in the 2D resistivity models. Weathered structures, fractures, and seepage in the reservoir floor constitute areas of excessive water loss in the dam. The cavity delineated in the dam foundation is a potential threat to the dam’s safety. The dam section with cavity has been referred to structural engineers for detailed study.

Confidence-Based Assessment of Safety of Irrigation Reservoir in Vietnam

Recently, in Vietnam deterministic design method has been applying for assessment of dam safety which confidence for the works and the headworks are not quantified. Under this study audience are introduced to the number of existing dams in Vietnam and relevant studies on confidence in the field of irrigation. The authors have applied theories of irrigation works, the theory of statistical probability and the theory of system analysis in developing algorithms for assessment of confidence level II for irrigation reservoir headworks in conditions in Vietnam. Based on detailed algorithms and block diagrams, the authors have built program for design of safety probability for incident mechanisms for individual works and the entire system of Phu Ninh reservoir in Quang Nam province. Analysis of the influence of random variables on individual incident mechanisms, the impact of incident mechanisms on the confidence of the works and the level of effect caused by the works on the overall confidence of the system brought the authors about solutions that improve the confidence of the system in case of new design or system repair.

Piezometric level prediction based on data mining techniques

The safety assessment of dams is a complex task that is made possible thanks to a constant monitoring of pertinent parameters. Once collected, the data are processed by statistical analysis models in order to describe the behaviour of the structure. The aim of those models is to detect early signs of abnormal behaviour so as to take corrective actions when required. Because of the uniqueness of each structure, the behavioural models need to adapt to each of these structures, and thus flexibility is required. Simultaneously, generalization capacities are sought, so a trade-off has to be found. This flexibility is even more important when the analysed phenomenon is characterized by nonlinear features. This is notably the case of the piezometric levels (PL) monitored at the rock–concrete interface of arch dams, when this interface opens. In that case, the linear models that are classically used by engineers show poor performances. Consequently, interest naturally grows for the advanced learning algorithms known as machine learning techniques. In this work, the aim was to compare the predictive performances and generalization capacities of six different data mining algorithms that are likely to be used for monitoring purposes in the particular case of the piezometry at the interface of arch dams: artificial neural networks (ANN), support vector machines (SVM), decision tree, k-nearest neighbour, random forest and multiple regression. All six are used to analyse the same time series. The interpretation of those PL permits to understand the phenomenon of the aperture of the interface, which is highly nonlinear, and of great concern in dam safety. The achieved results show that SVM and ANN stand out as the most efficient algorithms, when it comes to analysing nonlinear monitored phenomenon. Through a global sensitivity analysis, the influence of the models’ attributes is measured, showing a high impact of Z (relative trough) in PL prediction.

A multicriteria fuzzy approximate reasoning approach for risk assessment of dam safety

The benefits of dams are unquestionable, with the water impoundment upstream and flood-control downstream usually being sufficient to increase investments and promote growth. However, their presence brings risks to the downstream population and the environment. This paper employs a fuzzy approximate reasoning for dam safety risk assessment, initially using a FAHP to weight the importance of the indicators suggested by a dam expert and then adopts linguistic variables in a two-stage risk assessment. In the first stage, we compute the risk of dam collapse, hereinafter called dam break, through indicators such as design premises and safety procedures. In the second stage, we evaluate the combined effect between the risk of a dam collapse and the socioeconomic and environmental impact on the surrounding area, hereinafter called potential risk. The danger scale of dam break and the hazard scale of potential risk were constructed. The risk assessment of the Simplicio Hydroelectric plant, located in Brazil’s Southern region, showed that the risk classification of the proposed method attached great importance to the downstream consequences. The accuracy of the evaluation results was proved through a study of the Oroville Dam, whose spillway faced a failure event. These findings indicate the importance of extracting the knowledge of experts and incorporating it in the model specification through fuzzification of variables and the rule-based construction.

Research on Safety Assessment Method and System of a Heightened Concrete Face Rockfill Dam in Zhejiang Province

In view of many special analysis and long working period of the dam safety appraisal of water conservancy system, taking a heightened concrete face rockfill dam in Zhejiang Province as an example, starting from the analysis of the main impervious structure system affecting engineering safety, comprehensively using safety monitoring, inspection and detection methods, a relatively convenient and efficient dam safety assessment method and system has been established. The evaluation results show that the concrete face rockfill dam can play functional benefits according to design requirements, and the safety condition is good. The evaluation conclusion is of great significance for the reservoir operation management unit to quickly grasp the dam safety traits. The evaluation method and system have reference significance for the continuous development of the dam safety appraisal work in new era.

Experimental investigation of interfacial erosion on culvert-soil interface in earth dams

The erosion of soil particles within a dam, by water that seeps through the dam, is one of the most common causes of failure in levees and earth dams. This phenomenon can be more complex and dangerous when interacting with internally affiliated structures, such as water-supply culverts, due to the potential development of seepage channels through the weak interfaces. However, the erosion in this particular case has not been thoroughly investigated. In this paper, therefore, a seepage apparatus is designed to simulate and investigate the progressive failure mechanism of interfacial erosion on the culvert-soil interface of earth dams. This further leads to a detailed investigation of the threshold and failure hydraulic gradients of interfacial erosion by accounting for the influence of two critical geotechnical factors, i.e., the degree of compaction and the clay content. Furthermore, numerical analyses are carried out to reproduce the seepage-induced interface stress-displacement behaviour, as a supplement to the experimental investigations. Finally, emphasis is placed on studying the protective effects of two typical mitigation measures (the filter layer and impermeable rings) against interfacial erosion. The main conclusions can be drawn as follows: (1) The interfacial erosion in the unprotected cases shows a three-stage progressive failure as the hydraulic gradient increases, i.e., the stable, transition and failure stages, while that of the protected cases shows a four-stage failure, i.e., the stable, transition, hardening and failure stages. (2) The threshold and failure hydraulic gradients of interfacial erosion are found to be in the ranges of 0.771–1.467 and 0.974–1.994, respectively, for the investigated cases. (3) The critical hydraulic gradients are seen to be in a nonlinear relation with the degree of compaction or the clay content. (4) The experimental results show the effective protective effects of employing the two mitigation measures against interfacial erosion by blocking the formed seepage channels. The research provides critical references for the safety assessment of embankment dams when considering interfacial erosion failures at culvert-soil interfaces.

Hydrological Risk Analysis of Dams: The Influence of Initial Reservoir Level Conditions

In this paper, we present a method to assess the influence of the initial reservoir level in hydrological dam safety and risk analysis. Traditionally, in professional practice, the procedures applied are basically deterministic. Several physical processes are defined deterministically, according to the criteria of the designer (usually in the conservative side), although there is a high degree of uncertainty regarding these processes. A relevant variable is the reservoir level considered at the beginning of flood events. Hydrological dam safety assessment methods traditionally assume that the reservoir is initially full when it receives the design flood, thus, staying in the conservative side when designing a new dam. However, the distribution of reservoir levels at the beginning of flood episodes takes more importance for evaluating the real risk for the dams in operation. We analyzed three different scenarios—initial reservoir level equal to maximum normal level, equal to a maximum conservation level, and following the probability distribution from the historical records. To do so, we presented a method applied to a gated-spillway dam located in the Tagus river basin. A set of 100,000 inflow hydrographs was generated through a Monte Carlo procedure, by reproducing the statistics of the main observed hydrograph characteristics—peak flow, volume, and duration. The set of 100,000 hydrographs was routed through the reservoir applying the Volumetric Evaluation Method as a flood control strategy. In order to compare the three scenarios, we applied an economic global risk index. The index combines the hydrological risk for the dam, linked to the maximum water level reached in the reservoir, during the flood routing, and the flood risk in the downstream river reach, linked to the discharge releases from the dam. The results showed the importance of accounting for the fluctuation of initial reservoir levels, for assessing the risk related to hydrological dam safety. Furthermore, a procedure to quantify the uncertainty associated with the effects of initial reservoir level on hydrological dam safety, has been proposed.

A Combinatorial Procedure to Determine the Full Range of Potential Operating Scenarios for a Dam System

Traditional dam safety assessment tends to place the focus on estimating the probabilities of failure for the system based on a few subjectively-chosen operating scenarios. The techniques used to assess these systems rely on linear chains of events and are incapable of considering component interactions, feedbacks and non-linear behaviour. This paper uses a systems approach to develop a new technique that puts possibilities first and is capable of generating an exhaustive list of potential component operating state scenarios for the system. The scenarios can then be simulated using a Monte-Carlo approach to determine a wide range of system behaviour (outcomes) for each scenario, taking into account component interactions and feedbacks. A component operating states database is presented where the system can be broken down into various levels of detail and the operating states and causal factors for each component can be defined. The Cartesian product of each operating state set is used to derive the full range of potential operating scenarios, with each scenario consisting of a single operating state for every component in the system. The list of scenarios can be used as an input to a simulation model which can then be run many times for each scenario, using Monte-Carlo inputs which vary the timing and severity of events as well as the inflows. The approach for automated scenario generation is demonstrated on a simple and complex representation of a hydropower system and the scenarios are generated and counted. Results show that increasing system complexity results in exponentially increasing numbers of potential operating scenarios, with the simple system having a total of 1.1 × 106 operating states in comparison to the complex system which has 1.83 × 1027. The approach presented in this paper (a) reduces the subjectivity associated with traditional dam safety assessments through automated scenario generation, and (b) improves the ability to understand component interaction and feedbacks by describing a Monte Carlo simulation approach which can be used for scenario simulation. By understanding how the system responds to the full range of potential operating conditions, dam owners and asset managers can make informed decisions relating to the improvement of operating strategies and implementation of system upgrades.

The Value of Using Unofficial Measurements of Rainfall: The Dublin Storm and Flood of June 1963

Rainfall measurements are vital for the design of hydraulic structures, climate change studies, irrigation and land drainage works. The most important source of design rainfall data comes from convective storms. Accurate assessment of the storm rainfall requires a fairly dense network of raingauges. In 1963, such a storm took place over Dublin in Ireland. However, the existing raingauge network was insufficient to identify both the depth and pattern of rainfall. An appeal was made by Met Eireann for additional unofficial rainfall data. The result was remarkable in that the estimated maximum rainfall depth was found to be more than double the official value and that the resulting depth area analysis suggested a rainfall volume over a large area much bigger than the original isohyet map indicated. This result has huge implications for the estimation of maximum rainfall and dam safety assessment, especially in countries where the raingauge network has a low density. This paper first provides a description of the synoptic conditions that led to the storm, second an analysis of the rainfall data and how the unofficial measurements produced a very different depth area relationship; third, the social consequences of the resulting flood are described. Fourth, the storm is then placed in the context of other storms in the British Isles Finally the implications for rainfall measurement, gauge density and an example of how revised estimates of probable maximum precipitation (PMP) have been used to improve the safety and design standard of a flood detention dam are discussed.

Safety Assessment Technology of Concrete Arch Dam

Most of China’s reservoir dams were built before the 1980s, long-term safety issues of which have become increasingly prominent. In recent years, with the “high dam, large reservoir, long-distance water diversion” water conservancy projects are under way, the safety assessment of some major water conservancy projects will be a major long-term problem. Taking Gomal Zam Dam as an example, this paper introduced and summarized the safety assessment technology of concrete arch dams, which will provide a reference for the safety assessment and technical development of arch dams in the future.

Review article: Climate change impacts on dam safety

Abstract. Dams as well as protective dikes and levees are critical infrastructures whose associated risk must be properly managed in a continuous and updated process. Usually, dam safety management has been carried out assuming stationary climatic and non-climatic conditions. However, the projected alterations due to climate change are likely to affect different factors driving dam risk. Although some reference institutions develop guidance for including climate change in their decision support strategies, related information is still vast and scattered and its application to specific analyses such as dam safety assessments remains a challenge. This article presents a comprehensive and multidisciplinary review of the impacts of climate change that could affect dam safety. The global effect can be assessed through the integration of the various projected effects acting on each aspect of the risk, from the input hydrology to the calculation of the consequences of the flood wave on population and assets at risk. This will provide useful information for dam owners and dam safety practitioners in their decision-making process.

Application of geophysical methods in a dam project: Life cycle perspective and Taiwan experience

There is a growing demand for using non-destructive geophysical techniques to internally image dam condition and facilitate the early detection of anomalous phenomena. Near surface geophysical techniques have advanced significantly in the last few decades, and can play a significant role in the siting, construction, and operational safety and sustainable management of dams. Application of engineering geophysics in site characterization during feasibility investigation phase is already part of the standard of practice. This paper introduces newer applications of engineering geophysics during construction phase, dam safety assessment, and sustainable management, including quality control of compacted soils, investigation of abnormal leakage in an earth dam, evaluation of an aged concrete dam, geophysical health monitoring for a newly-constructed dam, and monitoring of sediment transport for sediment management. The applications were presented with more emphases on the needs of dam engineering and adapting appropriate geophysical methods to make assessment more effective and consequential. The collage of these case studies is to broaden the view of how geophysical methods can be applied to a dam project throughout a dam's life cycle and strengthen the linkage between geophysical surveillance and engineering significance at all stages.

Assessment of dam safety using the approximate entropy method

One of the most important research focuses in today’s dam engineering is related to health monitoring of reservoir dams. A novel method named Approximate Entropy (ApEn) is introduced to evaluate the operation condition of gravity dam by monitoring data in this paper, which has been proved to be very successful while applied in many other disciplines, such as the fields of economics, medicals and mechanical, etc. The theoretical framework of ApEn together with a modified programming algorithm are proposed first, and then a quantitative relationship between the horizontal displacement of 14 different measure points located on a practical gravity dam is established. The results of the practical applications have shown that ApEn can effectively and efficiently diagnose the health state of the dam, with high robustness and strong ability to resist noise.

Quantitative risk assessments – a wellbeing perspective

This paper seeks to demonstrate that the processes used in dam safety assessments are not too dissimilar to those used in the life and death situations encountered in the medical profession. Through a personal account, the paper explores how the processes used within our profession for risk assessment can also be applied to the investigation, diagnoses and treatment of prostate cancer.

Reinforcement Analysis of Toe Blocks and Anchor Cables at the Xiluodu Super-High Arch Dam

High stress concentrations near an arch dam–foundation interface may cause cracking and failure, particularly in super-high arch dams. Appropriate reinforcement measures are usually needed at the dam toe to increase the shear strength and bearing capacity of the rock mass foundation. In this paper, the criteria and procedure for reinforcement design of dam toe are first given, as priority issues in the overall safety assessment of super-high arch dams. A complete reinforcement analysis method covering the calculation of reinforcement force, effect evaluation and monitoring validation is proposed. With the Xiluodu super-high arch dam as an example, the reinforcement analysis under the numerical simulation and physical model tests performed are described in detail. The results show that the reinforcement measures adopted, such as the toe block and pre-stressed anchor cables, contribute to decrease the local high stresses at the dam toe, as well as increase the cracking initiation factor and the overall safety factor of the dam–foundation system. The reinforcement force loss of anchor cables during the subsequent construction stage has a limited effect on the dam performance. In terms of improving the stability at dam toe, the reinforcement of anchor cables is suggested to be implemented as early as possible. The reinforced dam is finally verified by field monitoring during the operation period. No anomalies in deformation, stress and seepage response near the dam are identified, and the arch dam is shown to perform as desired.