Dam Safety Concepts

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Dam Safety Concepts

Proefschrift

ter verkrijging van de grad van doctor aan de Technische Universiteit Delft,

op gezag van de Rector Magnificus prof.ir. K.C.A.M. Luyben, voorzitter van het College voor Promoties,

in het openbaar te verdedigen op donderdag 11 december 2014 om 15:00 uur

door Jasna Duricic

Master of Science in Hydraulics and River Basin Managment geboren te Sarajevo, Bosnia and Herzegovina

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Dit proefschrift is goedgekeurd door de promotoren Prof.drs.ir. J.K. Vrijling

Prof.dr.ir. P.H.A.J.M. van Gelder

Samenstelling promotiecommissie:

ISBN: 978-94-6186-407-9

Printed by: Sieca Repro, Delft, The Netherlands

Cover image: “Dreams of Nature” Symbolism from Van Gogh to Kandinski (Source: http://www.jpekker.nl/wp-content/uploads/2012/03/Gallen-Kallela_Lake-Keitele.jpg)

Rector Magnificus, voorzitter

Prof. drs. ir. J.K.Vrrijling Technische Universiteit Delft, promotor Prof. dr. ir. P.H.A.J.M. van Gelder Technische Universiteit Delft, promotor Prof. dr. ir. M.J.F. Stive Technische Universiteit Delft

Prof. ir. T. Velinga Technische Universiteit Delft

Prof. ir. C.A. Willemse SBM Schiedem B.V., Netherland

Assoc. prof. dr. I. Popescu Technische Universiteit Delft/UNESCO-IHE Assist. prof. dr. T. Erdik Istanbul Technical University, Turkey

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S

UMMARY

The majority of dams constructed in the world are dams that can be categorized as embankment dams. Throughout history we can point to many failures of dams, and embankment dams in particular. Nowadays it is clear that the goal to construct sta-ble dams has not been achieved, even with advanced technologies and construction techniques available. There are always unexpected factors which can produce unfore-seen problems, and also most of the suitable sites are already utilized. This might in-crease the probability of failure in the future; therefore constructors will have to face more complicated geological conditions.

The main reasons for failures are inconsistency between design (design hypothesis) and reality (unpredictability encountered on the site or during construction), natural processes like flash floods, rock and landslides, earthquakes and deliberate human ac-tions. Research on failure case histories and lessons learned from them provide im-portant answers and leads to improvement in dam safety approach.

The concerns of dam safety are related to dam procedures that will avoid a dam break or diminish the probability of a dam break or any other abnormal event. Most of dam safety decisions are based on the predictions of the probability of dam failure and of resulting loss-of life. In this thesis the intention is to highlight the methodolo-gies for dam safety decisions.

Previously, risk and uncertainty methods have been applied for safety assessments of hydraulic structures but they were restricted to some extent. In this thesis an im-provement of the methodology proposed by Hsu et al. (2011) is suggested in three as-pects. The first is the development of multivariate flood frequency analyses in which the annual maximum peak discharges and the surface runoff are modelled as a bivari-ate distribution function. The second is the treatment of the initial reservoir level as a random variable, and the third is the overtopping assessment sample zone which is divided jointly by multivariate flood frequency, wind frequency and initial stage fre-quency, generating eight sub domains.

Damage of property due to dam break discharge is certain, but loss of life depends on the flooded area and population. Therefore, analysis of dam breaching and the result-ing floods are crucial for reduction of potential for loss of life and damage of property. Further in this thesis the breach itself and methodologies applied to quantify the peak discharge due to breaching are highlighted. Many computer models are capable of

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simulating dam-break hydrographs and routing these hydrographs downstream.

How-ever, dam break analysis models normally require certain geometric and temporal characteristics of the dam breach as inputs for the model. An alternative approach to estimating these parameters has been the use of case study data to develop empirical-regression relationships relating the peak discharge of the breach to the dam height and/or reservoir-storage volume. An efficient model based on Kriging methodology is proposed to forecast peak discharge at certain height and volume of water respective-ly, behind the dam at failure.

The time of occurrence and magnitude of floods is very difficult to predict while it is possible to predict fairly accurately the propagation of the flood wave along the river, once that a flood wave is generated at some upstream location in the river (in the case of this thesis due to dam break). The dam-break induced loads and their effects on buildings are of vital importance for assessing the vulnerability of buildings in flood-prone areas. A comprehensive methodology, for risk assessment of buildings subject to flooding, is nevertheless still missing. The intention of this thesis is to take a step forward by following previous research. A new and efficient variable that can take into account both the shape of the structure and flow conditions is proposed and new and practical formula for predicting the mean normalized force is suggested for different types of obstacles, which is missing in previous research.

As a part of a dam safety assessment, an empirical breaching model is coupled with a numerical model in order to achieve a more accurate prediction. However, empirical models provide only peak discharge calculation, neglecting breach development in time. In this thesis a numerical model is constructed and a hydrograph is calculated based on dam breach development and failure time whereas breach parameters are calculated based on empirical model.

Outcomes of this thesis can contribute to a growing tendency to assess the safety lev-els of existing dams based on risk and uncertainty analysis using mathematical and statistical models with multivariate flood frequency analysis. Previously designed dams should be checked with at least a bivariate analysis especially in the determina-tion of spillway discharge coefficient and dam crest level. Breaching development is a complex process; therefore many assumptions have to be made in order to describe the process as close as possible, in the same time the structural uncertainties related to failure increase.

In this thesis an efficient formula for better understanding and application of dam-break flow induced forces on structures of different cross sections is suggested with a new parameter that describes the shape of the obstacle which is named “shape of in-fluence”. The importance of distinguishing the breach initiation time and breach de-velopment time is also highlighted in this thesis. So far, breach initiation time has not been considered as a distinct parameter in the majority of dam case studies, and it is not input in most of the numerical models. We could see that distinguishing between these two phases is still a difficult task because the guidance available for determina-tion of the breach initiadetermina-tion time is very limited. In reality we can only predict breach development.

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S

AMENVATTING

Het merendeel van de dammen die in de wereld gebouwd zijn, zijn dammen die als dijk dammen gecategoriseerd kunnen worden. Door de geschiedenis heen kunnen we het falen van talloze dijken zien, vooral bij dijk dammen. Tegenwoordig is het duide-lijk dat het doel om stabiele dammen te bouwen, zelfs met geavanceerde technologie-en technologie-en bouwtechniektechnologie-en, nog steeds niet is gerealiseerd. Er zijn altijd onverwachte fac-toren die onvoorziene problemen veroorzaken, en ook zijn de meest geschikte locaties al gebruikt. Dit kan de kans op falen in de toekomst doen toenemen; constructeurs zullen daardoor meer gecompliceerde geologische omstandigheden onder ogen moeten zien.

De belangrijkste redenen voor falen zijn de inconsistenties tussen ontwerp (ontwerp-hypothese) en werkelijkheid (onvoorspelbaarheid opgetreden op het bouwterrein of tijdens de bouw), natuurlijke processen zoals overstromingen, rots- en aardverschui-vingen, aardbevingen en opzettelijke en weloverwogen menselijke activiteiten. Onder-zoek naar voorgeschiedenissen van falen, en de hieruit geleerde lessen, biedt belangrij-ke antwoorden en leidt tot verbetering van de aanpak van de veiligheid van een dam. Dit zou niet mogelijk zijn zonder een overzicht van gebeurtenissen in het verleden. De bezorgdheid over de veiligheid van een dam staat in verband met factoren die een dam doorbraak voorkomen of de kans op breuk van een dam of een andere abnormale gebeurtenis verminderen. De meeste veiligheidsbeslissingen van een dam zijn geba-seerd op de voorspellingen van de waarschijnlijkheid van het falen van een dam en het daaruit voortvloeiende verlies van mensenlevens. In dit proefschrift ligt de nadruk op de procedures van besluitvorming met betrekking tot de veiligheid van een dam. Voorheen zijn voor de beoordeling van de veiligheid van hydraulische constructies methoden van risico en onzekerheid toegepast, maar ze waren tot op zekere hoogte beperkt. In dit proefschrift wordt een verbetering van de methodologie, voorgesteld door Hsu et al. (2011), aanbevolen in drie opzichten. Het eerste aspect is de ontwikke-ling van multivariate overstromingsfrequentie analyses, waarin de jaarlijkse maximale piekafvoer en de oppervlakteafvoer worden gemodelleerd als een bivariate verdelings-functie. De tweede is de behandeling van het begin niveau van het reservoir als een stochastische variabele , en de derde is de beoordeling van de overtopping toetsings-domein, die gezamenlijk wordt gedeeld over de multivariate overstromingsfrequentie, windfrequentie en de frequentie van het begin niveau en hiermee acht sub-domeinen genereert.

Schade aan eigendom, te wijten aan dam doorbraak, is zeker, maar het verlies van mensenlevens hangt samen met het overstroomde gebied en de bevolking. Analyse

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van dam breuk en de resulterende overstromingen zijn essentieel voor een

verminde-ring van potentieel verlies van mensenlevens en schade aan eigendommen. In dit proefschrift ligt de nadruk ook op de breuk zelf en de toegepaste methodologie om het hoogste punt van overstroming te kwantificeren. Veel computermodellen zijn in staat om dam-breuk afvoerverlooplijnen en een stroomafwaarts routering van deze af-voerverlooplijnen te simuleren. Echter, dam breuk analysemodellen vereisen meestal bepaalde geometrische en tijdelijke kenmerken van de dam breuk als input voor het model. Een alternatieve benadering voor het inschatten van deze parameters, is het gebruik van gegevens van casestudies om empirische-regressie verbanden, gerelateerd aan de piek van de breuk met betrekking tot damhoogte en/of het reservoir-opslagvolume af te leiden. Een efficiënt model, gebaseerd op de Kriging methodologie, wordt voorgesteld om de piek afvoer op een bepaalde hoogte en de hoeveelheid water, respectievelijk, achter de dam tijdens het faalmoment, te voorspellen .

Het juiste moment en de omvang van overstromingen is zeer moeilijk te voorspellen, terwijl het mogelijk is om vrij nauwkeurig de voortplanting van een overstromingsgolf door de rivier, wanneer een overstromingsgolf wordt gegenereerd op een stroomop-waartse locatie in de rivier, te voorspellen (in het geval van dit proefschrift, als gevolg van dam doorbraak). De door dam-doorbraak veroorzaakte belastingen en de gevol-gen daarvan op gebouwen zijn van vitaal belang voor de beoordeling van de kwets-baarheid van gebouwen in overstromingsgevoelige gebieden. Een uitgebreide methodo-logie voor de evaluatie van risico van gebouwen die door overstroming getroffen kun-nen worden, is echter nog steeds afwezig. De bedoeling van dit proefschrift is een stap voorwaarts te zetten door het opvolgen van voorafgaand onderzoek. Een nieuwe en ef-ficiënte variabele wordt voorgesteld, die in staat is rekening te houden met zowel de vorm van een constructie als met de stroom karakteristieken, en een nieuwe en prak-tische formule wordt voorgesteld voor het voorspellen van de gemiddelde genormali-seerde kracht voor de verschillende soorten obstakels, dat in voorafgaand onderzoek ontbrak.

Als onderdeel van de beoordeling van de veiligheid van een dam, wordt een empirisch breuk model gekoppeld aan een numeriek model om een meer nauwkeurige voorspel-ling te bereiken. Echter, empirische modellen bieden alleen piek afvoer berekeningen en verwaarlozen de ontwikkeling van een breuk in een tijdsperspectief. In dit proef-schrift worden een numeriek model en een afvoerverlooplijn berekend op basis van de ontwikkeling van een dam doorbraak en het falen van de dam in tijd.

De resultaten van dit proefschrift vormen een bijdrage tot een toenemende tendens om de veiligheidsniveaus van al bestaande dammen te beoordelen op basis van risico en onzekerheidsanalyse met behulp van wiskundige en statistische modellen met een multivariate overstromingsfrequentie analyse. Bestaande dammen moeten worden ge-controleerd met tenminste een bivariate analyse, met name bij de vaststelling van de overlaat afvoercoëfficiënt en het niveau van de top van de dam. De ontwikkeling van een doorbraak is een complex proces; veel aannames moeten worden gemaakt om het proces zo precies mogelijk te beschrijven, terwijl de structurele onzekerheden gerela-teerd tot falen toenemen.

In dit proefschrift wordt een efficiënte formule voor een beter begrip en toepassing van door dam breuk stroming geïnduceerde krachten op constructies van

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verschillen-v

de doorsneden voorgesteld met behulp van een nieuwe parameter die de vorm van de

obstakels beschrijft, de zogenaamde “Shape of influence.” Het belang van het maken van een onderscheid tussen het begin van een breuk en de tijd van de ontwikkeling van de breuk wordt ook in dit proefschrift benadrukt. Tot nu toe is het moment van het begin van een breuk niet als een afzonderlijke parameter in beschouwing genomen in de meerderheid van studies, evenmin is het ingevoerd in de meeste numerieke mo-dellen. We konden zien dat het maken van een onderscheid tussen deze twee fasen nog steeds een moeilijke taak is omdat de aanwezige richtlijnen voor de bepaling van het moment van het begin van een breuk zeer minimaal zijn. In werkelijkheid kunnen we alleen de ontwikkelingstijd van een breuk voorspellen.

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C

ONTENT

Summary _____________________________________________ i Content _____________________________________________ vii Chapter 1 Introduction _____________________________________ 1 1.1 Background ____________________________________________ 1 1.2 Objectives of thesis _______________________________________ 3

1.3 Outline of the thesis and approach _______________________________ 3

Chapter 2 Embankment dams and failures __________________________ 5

2.1 Introduction ___________________________________________ 5

2.2 General dam classification ___________________________________ 5

2.3 Key developments in the history of embankment dams ___________________ 6

2.4 Characteristics of embankment dams _____________________________ 8

2.5 General aspects of failures and incidents ___________________________ 9

2.5.1 Statistics of failure of embankment dams _________________________ 11

2.6 Failures by overtopping ____________________________________ 12

2.6.1 Failure mechanisms of overtopping ____________________________ 12 2.6.2 Case histories of breaching by overtopping failures ___________________ 15

2.7 Lessons learned from historical dam failures ________________________ 19

2.7.1 Problems caused by overtopping and inadequacy of spillways and outlets _______ 20 2.7.2 Dam foundation condition _________________________________ 21 2.7.3 Monitoring _________________________________________ 21

2.8 Discussion and conclusions __________________________________ 21

Chapter 3 Theories of probabilistic calculations ______________________ 23

3.1 Introduction __________________________________________ 23

3.2 Safety of structures ______________________________________ 23

3.3 Uncertainties in reliability assessment ____________________________ 24

3.4 The concept of reliability ___________________________________ 26

3.5 The methods of failure probability estimation ________________________ 27

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3.5.2 Second order method ____________________________________ 30 3.5.3 Simulation methods _____________________________________ 30 3.6 Regression analysis ______________________________________ 31 3.6.1 Models ____________________________________________ 31 3.7 Variable selection _______________________________________ 33 3.7.1 Effects of multicollinearity _________________________________ 33 3.7.2 Multicollinearity diagnostics ________________________________ 35 3.7.3 Biased estimation (dealing with multicollinearity) ____________________ 35 3.7.4 Computational methods for variable selection (Stepwise methods) ___________ 38 3.7.5 Cluster analysis _______________________________________ 39

Chapter 4 Probabilistic assessment of embankment dam overtopping __________ 41

4.1 Introduction __________________________________________ 41

4.2 Previous research on probabilistic assessment of overtopping _______________ 42

4.3 Case study: the Akyayik dam _________________________________ 43

4.4 Methodology __________________________________________ 45

4.5 Reliability analysis and overtopping probability ______________________ 47

4.6 Flood frequency analysis and bivariate distribution ____________________ 48

4.6.1 Calculation of flood frequency _______________________________ 49 4.6.2 Initial reservoir level (hirl) _________________________________ 53 4.6.3 Wind speed _________________________________________ 53

4.7 Reservoir routing _______________________________________ 56

4.7.1 Wind setup (hwws) ______________________________________ 56

4.8 First evaluation of probabilistic assessment _________________________ 57

4.8.1 Comparison of overtopping risk calculations based on univariate and bivariate flood frequency _______________________________________________ 58 4.8.2 The effects of spillway discharge coefficient on overtopping probability ________ 58 4.8.3 The effects of spillway discharge coefficient on maximum reservoir water level ____ 59 4.8.4 The effects of dam crest level on overtopping probability ________________ 60 4.8.5 The effects of joint peak discharge and runoff volume on overtopping probability __ 61 4.8.6 The effects of initial reservoir level on overtopping levels ________________ 62 4.8.7 The effects of wind speed on overtopping _________________________ 63

Chapter 5 Predicting peak breach discharge due to embankment dam failure using

Kriging methodology ______________________________________ 65

5.1 Introduction __________________________________________ 65

5.2 Models for calculating peak breach discharge ________________________ 66

5.2.1 Parameters of peak breach discharge ___________________________ 67 5.2.2 Discussion of existing models and new approach _____________________ 68

5.3 Restrictive assumptions in regression analysis _______________________ 68

5.3.1 Testing of regression analysis reliability __________________________ 69 5.3.2 Data used __________________________________________ 72

5.4 kriging methodology ______________________________________ 73

5.4.1 Basic principles of geostatistical methods _________________________ 73 5.4.2 Assumptions for prediction of random processes with dependence ___________ 73 5.4.3 Semivariogram _______________________________________ 73 5.4.4 Geostatistical estimator Kriging ______________________________ 74

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5.4.5 Ordinary Kriging ______________________________________ 75

5.5 Hypothetical case study ____________________________________ 77

5.6 Application ___________________________________________ 82

5.6.1 Evaluation of the proposed model _____________________________ 84

Chapter 6 Mean normalized force computation for different types of obstacles exposed to dam break flow using statistical techniques _________________________ 93

6.1 Introduction __________________________________________ 93

6.2 Dam break wave description _________________________________ 93

6.3 Forces on structures due to dam break wave (theoretical background) __________ 95

6.4 Experiments __________________________________________ 98

6.5 Statistical analysis _______________________________________ 101

6.5.1 Basic statistics and correlation analysis __________________________ 101 6.5.2 Multiple Linear regression model _____________________________ 102 6.5.3 Stepwise Multiple Linear Regression Model _______________________ 103 6.5.4 Principal-Component Analysis _______________________________ 104 6.5.5 Cluster Analysis_______________________________________ 105 ____________________________________________________ 106

6.6 Evaluation of the statistical techniques ___________________________ 107

6.7 Additional parameter definition _______________________________ 107

6.8 Evaluation of the proposed model for mean normalized force computation _______ 111

Chapter 7 Numerical modelling of the akyayik dam break and flood propagation ___ 113

7.1 Introduction __________________________________________ 113

7.2 Breach modelling _______________________________________ 113

7.2.1 Breach shape ________________________________________ 114 7.2.2 Breach development in time ________________________________ 115

7.3 Introduction to dam break modelling ____________________________ 115

7.3.1 One dimensional MIKE 11 model _____________________________ 117

7.4 Akyayik dam failure calculation _______________________________ 117

7.4.1 Topographical data of the project area __________________________ 118 7.4.2 Input hydrograph ______________________________________ 120 7.4.3 Dam breach parameters prediction ____________________________ 121 7.4.4 Peak discharge prediction by Mike 11 and comparison with Froehlich _________ 121

Chapter 8 Conclusions and recommendations ______________________ 125 Appendix ___________________________________________ 128 References __________________________________________ 131 List of symbols ________________________________________ 138 List of abbreviations _____________________________________ 142 List of figures ________________________________________ 143 List of tables _________________________________________ 146

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Acknowledgments ______________________________________ 148 Curuculum Vitae ______________________________________ 150

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Chapter 1 I

NTRODUCTION

1.1 B

ACKGROUND

Dam failure disasters usually cause destruction of properties and environment, as a result of which large scale economic investments for reconstruction are needed. In re-cent decades, many countries have experienced floods that may cause dams to over-top. For example, failure of Vajont dam in Italy in the year 1963 caused 2600 deaths. Moreover, failure of Teton dam in USA in 1976 induced hundred deaths and an eco-nomic loss that reached about 1 billion dollars, and finally, in China, the failure of Gouhou dam in 1993 resulted in 300 deaths. According to the statistical analysis of 534 dam failures from 43 countries before 1974, earth-rock dam failures account for the largest proportion of all failures. Overtopping constitutes 49% of all earth dam failure causes. This is the reason why embankment dam failure due to overtopping is studied in this thesis.

Thousands of dams have been constructed around the world thoroughout the centu-ries and hundreds of them have failed leaving devastation behind. To mitigate such risks, dam owners and regulators analyse and inspect dams to identify potential fail-ure modes and prevent them. However, in design practice there is not a program or regulations for preventing failure with high certainty. The potential for loadings to exceed design limits cannot be eliminated, therefore apart from inspection and ana-lysing, another important aspect of risk mitigation is simulation of potential failures and planning based on them.

When designing a large dam there are two main goals: (1) the dam must be stable and (2) the structure must be constructed as economically as possible. The two ob-jectives are against each other: ensuring stability by over-design increases the costs, while cost cutting methods could lead to unsafe structures (De Wrachien and

Mam-If anyone be too lazy to keep his dam in proper condition, and does not so keep it; if then the dam breaks and all the fields be flooded, then shall he in whose the dam break occurred be sold for money, and the money shall replace the corn which he has caused to be ruined.

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bretti, 2009). In order to satisfy both criteria the structure should be constructed by economical optimal probability of failure.

Nowadays it is clear that the goal to construct stable dams is not always achieved, even with advanced technologies and construction techniques available. The reasons are in facts that there are always unexpected factors which can produce unforeseen problems, and also most of the suitable sites are already utilized. That simple reason might increase the probability of failure in the future because constructors will have to face more complicated geological structures. Obviously, advanced technologies must be amply applied and improved during investigation, design and construction of dam.

The main reasons for failures are inconsistency between design (design hypothesis) and reality (unpredictability encountered on the site or during construction), natural processes like flash floods, rock and landslides, earthquakes and deliberate human ac-tions.

There is an increasing interest in dam break hydraulics and hydrology, having in mind potential of occurrence of extreme meteorological events due to rapidly chang-ing climate. Observations made in recent years warn that global climate change will have devastating impact (e.g. Intergovernmental Panel on Climate Change (IPCC) assessment reports). According to IPCC the decreasing ice volume in Arctic, increas-ing of meltincreas-ing of Greenland ice sheet and global risincreas-ing of mean sea level are the most evident consequences of global warming (Figure 1.1). In many regions, changing presipitaion or melting snow and ice are altering hydrological systems, affecting water recources in terms of quantity and quality. Glaciers continue to shrink almost world-wide because of climate change, affecting runoff of water recources downstream. Cli-mate change is causing permafrost warming and thawing in high latitude regions and in high-elevation regions (IPCC 2014).

New trends in climate change impose new uncertainties regarding maximum flows that can occur during life time of dams. It may produce new safety uncertainties as well, because previously considered as safe dams; many of them are now exposed to risk of overtopping triggered by natural hazards.

Figure 1.1 The global climate of the 21st_{century will depend on natural changes and the} re-sponse of the climate system to human activities (IPCC, 2001).

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Legislation and safety criteria for dams vary significantly throughout the world, be-cause despite improved engineering knowledge and construction quality still full non-risk guarantee is not possible. The concerns of dam safety are related to internal dam procedures that will avoid a dam break or diminish the probability of a dam break or any other abnormal event. Reclamation Safety of Dams Act (1978) set detailed guide-lines for the Bureau of Reclamation, one of the federal government’s largest dam own-ing agencies in USA. This act authorized Bureau of Reclamation to preserve struc-tural stability of its dams and related facilities by performing modifications (Wahl et al. 2010). In the United States today, most dam safety decisions are based on the predictions of the probability of dam failure and of resulting loss-of life. Similar legis-lation has been accepted in countries around the world

There is presently both a need and opportunity to achieve significant improvements in technology used to analyze embankment dam breach processes. The potential benefits to be achieved may significantly aid assessment risk studies (with importance of thresholds of dam failure, probabilities of failure and consequences of failure).

1.2 O

BJECTIVES OF THESIS

The objectives of this thesis can be summarized as follows:

1. Comparison of the existing methodologies and development of bivariate proba-bilistic assessment of the probability of overtopping embankment dam failure. 2. Review of the existing methodologies concerning peak discharge prediction and

development of a new method based on geostatistics.

3. Comparison of the new model with the physically based models in literature. 4. Statistical investigation of effective parameters for predicting mean forces on

buildings due to dam failure and development of new approach.

5. Numerical model investigation of dam breach peak discharge for predicting flood hydrographs.

1.3 O

UTLINE OF THE THESIS AND APPROACH

This thesis consists of eight chapters. Chapter 2 gives literature review of the history of dams and dam failures, focusing on embankment dams and overtopping as it is the most common failure mode for embankment dams.

Chapter 3 contains theory of probabilistic approach and other statistical methods

used in the thesis.

Chapter 4, the probabilistic analysis is performed. In the literature, univariate

fre-quency analysis of peak discharges is usually employed. However, floods are inherent-ly caused by multivariate random hydrological variables such as inflow volumes and duration of hydrographs. Hence, the conventional approaches cannot mimic the whole

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flood event by only considering the peak floods. There should be joint consideration of flood peaks, volumes, and durations. In this study, bivariate frequency analysis is employed. Akyayik dam of Turkey is employed as a case study with real time data. Probability of dam break failure is calculated. An efficient Monte Carlo Simulation (MCS) with Latin Hypercube (LH) sampling technique is developed based on bivari-ate flood frequency analyses of flood discharges and corresponding flood volumes. Both the probabilistic effects of wind events and initial reservoir levels are taken into account as random variables. In addition, the effects of spillway discharge coefficient (Dc) and optimum dam height are also discussed.

Chapter 5, after determining the overtopping probability failure in Chapter 4, peak breach discharge due to embankment dam failure is predicted when the dam overtops. In the literature, many parametric breach models based on regression techniques have been developed so far. In the literature many parametric breach models based on re-gression techniques have been developed so far. In this study, an efficient model is proposed to forecast peak discharge based on height and volume of the water behind the dam at failure by using the Kriginig approach.

Chapter 6, dam break induced wave with current loading and their effects on

build-ings are assessed. This part is an improvement of the most recent research in the lit-erature. To this aim, (1) five statistical procedures including: simple correlation anal-ysis, multiple linear regression model, stepwise multiple linear regression model, prin-cipal component analysis and cluster analysis are used to study relationship between mean normalized force on structure and other related variables; (2) a new and effi-cient variable that can take into account both the shape of the structure and flow conditions is proposed; (3) a new and practical formula for predicting the mean nor-malized force is suggested for different types of obstacles, which is missing in the pre-vious research.

Chapter 7, numerical flood modeling is applied. The method of Froehlich (1995) is

applied for dam breach parameter estimation. Linear breach progression is applied. The downstream effects of Akyayik dam failure have been estimated with the Dam break module of MIKE 11. Dam break setup consists basically of a reservoir, the dam structure and a single channel. The topography of the river is described as accurately as possible through many cross sections, and particularly when they are changing rapidly. Model results are compared with the physical based model in the literature.

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Chapter 2 E

MBANKMENT DAMS AND FAILURES

2.1 I

NTRODUCTION

The greatest part of constructed dams belongs to embankment dams and there are many examples of their failures throughout history. Before giving insight in failures, some historical facts about embankment dam development are presented, followed by characteristics of the dam as a structure. As introduction to failures some general as-pects are given in Section 2.5, together with explanation of terms like: “incident”, “failure “and “accident” which are important for understanding “behaviour” of dams. Distribution of failures in different types of dams and modes is given in order to un-derstand contribution of embankment dam failures. Following this statistical evidence of embankment dam failures by different failure modes is presented. Further, more at-tention is given to failures by overtopping and some historical failures are discussed, as it would be subject of interest in further chapters of this thesis. Finally the past experiences are summarized in Section 2.7 and some measures are suggested in order to improve design, construction, operation and monitoring during lifetime of the dam. The aim of this chapter is to give relevant background information on embankment dams and their failure characteristics.

2.2 G

ENERAL DAM CLASSIFICATION

International Commission on Large Dams (ICOLD) divided dams into two main groups based on the construction characteristics such as type of material and design solutions for the dam structure. Each of these groups is further subdivided:

 Embankment dams:

• Built of earth and/or rock (erodible type) and resist the water pressure by their weight. If the material is not inherently water tight, they are covered with an impervious material or have a watertight core.

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• Built of concrete (nonerodible) and mixed type.  Concrete dams:

• Gravity dams; they have a roughly triangular cross-section and also re-sist water pressure by their weight. These are the most widespread type of concrete dams, accounting for two thirds of the total.

• Arch dams; they transmit most of the water load into the valley sides or large concrete thrust blocks.

• Buttress dams: they have the water load transmitted to triangular but-tresses parallel to the direction of river flow.

• Multiple arch dams; they consist of a number of small arches bearing on buttresses.

2.3 K

EY DEVELOPMENTS IN THE HISTORY OF EMBANKMENT DAMS

The oldest known dams of the world were located at the foot of the Djebel Druze Mountains bordering Jordan and Syria (Viollet, 2004). On the Jordanian side there was a settlement constructed by the end of the fourth millennium BC. The only wa-ter resource for Jawa settlement was the flood running down from the canyon called Wadi-Rajil, with annual discharge of 2x106 _m3_{/s (Helms, 1981). Natural depressions}

which collected this runoff were dammed to form artificial reservoirs, and 3 km long canal was constructed allowing the diversion of water from the floods of Wadi-Rajil into a system of 8 connected reservoirs, with a total storage capacity of about 5x104

m3_{/s (Schnitter, 1994;} _{Viollet, 2004). One of the reservoirs, which stored half of the}

total volume of water, was impounded by a 4.5 m high and 80 m long dam. This structure consists of two stone walls with an earth fill between. The stability of the structure was assured by a downstream embankment. The base of the water face was protected against water-backpressures by stone-paved surface (see Figure 2.1). This feature is called an apron in modern terms and the protection it provides was rein-vented only in modern times (Helms, 1981; Schnitter, 1994).

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On the Syrian side, at the site of Khirbet el-Umbashi, eight successive settlements were built from about from 3000 to 1500BC. This site had a water management sys-tem similar to that in Jawa which included canals, dams and reservoirs (Viollet, 2004).

The oldest high dam in the world, Sadd-el-Kaffara, was built in Egypt in the Old Kindom around 2600 BC (Garbrecht, 1997). It is located in the Wadi Garawi on the eastern bank of the Nile, 30 km south of Cairo. The purpose of the dam was to retain water from relatively moderate but frequent floods. Sadd-el Kafara was intended to reach the height of 14 m; was about 110 m long and 98 m wide at its bottom and it was the first dam of such size known (Figure 2.2).

Figure 2.2 Remains of Sadd-el-Kafara dam (Scnitter, 1994)

The dam had an earthfill core between two rockfill sections covered with limestone ashlars on its upstream and downstream faces. The total volume of fill material was about 87000 m3_{; and the construction period is estimated to be in the order of 10}

years (Schnitter, 1994). Due to inexperience dam was overdesigned and actually con-struction was never accomplished. There was no diversion tunnel to release water from the river around the dam site and the first flood destroyed it while still under construction. Engineers of the time were discouraged by consequences of this dam col-lapse so that there was no further dam construction in the following few centuries. Later on it was constructed and the dam served only to retain moderate floods, and usually it was breached by extreme floods which occurred once in 50 years. This dam was in use 1300 years. Once the dam failed and caused displacement of 50 000 people it was never repaired again.

In post medieval Europe the Saint Ferrol dam was constructed and with its 36 m of height it was the highest embankment dam in the world for a long time. The primary was project of the Languedoc canal in France which connected the Mediterranean Sea with the Atlantic Ocean. The purpose of the reservoir was to compensate the losses of water in the canal during the dry season. The construction of the dam was completed in 1675. It can be considered as an earthen dam with a masonry core because the wa-ter retaining wall was supported by a downstream lower embankment in case of ex-treme discharges.

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Figure 2.3 The Saint Ferreol Dam sketch (Schnitter, 1994)

Embankment dam engineering has evolved throughout centuries and major develop-ments are tracked since 1940s with the development of soil mechanics, geotechnical and hydraulic engineering. A large number of embankment structures have been built with purposes of flood defence, water supply, power generation, transportation and sediment retention. Because these structures can sustain only limited safety levels and are subject to decay, they may fail owing to various triggering mechanisms, particu-larly with a high probability of failure under extreme conditions (Costa, 1985; Foster et al., 2000; Allsop et al., 2007).

2.4 C

HARACTERISTICS OF EMBANKMENT DAMS

The embankment dams are the oldest and most comon world wide, especially the lower ones with a height between 10 and 15 m, making 83% of the world total, and presently they are the large majority of newly constructed dams in the world. More than 70% of large dams registered by ICOLD, with height of 15 m or more and/or capacity of the reservoir not less than 106 _m3_{, are embankment dams.}

Construction of embankment dams has economic advantages because there are no specific requirements for the foundation like for the concrete dams and construction materials can generally be provided from the dam site.

Homogenous earth dams have a dam section consisting almost entirely of one type of material (Figure 2.4 (a)). In homogeneous earthfill dams, the dam body should exhib-it resistance against both; load transmexhib-itted by the reservoir and seepage forces wexhib-ith provided drainage facilities (Deangeli et al., 2009). The dams that are made up of rel-atively impervious central zone called the core and outer zones that provide the structure with the required stability are called zoned earth dams (Terzaghi and Peck, 1967).

The term rockfill dam refers to a dam in which the major portion of the pressure ex-erted by the impounded water is transmitted onto the foundation through a rockfill (Terzaghi and Peck, 1967). The fill material is available from natural boulder deposits or obtained from stone quarry. Nowadays a variety of construction materials is avail-able, therefore we can find cross-section of rockfill dam as a rockfill dyke with up-stream slope covered with a membrane or a concrete slab and also as a zoned earth dam with an impermeable core constructed by modern technology. Rockfill dams can be classified into three categories (Deangeli et al., 2009) based on the configurations

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of dam sections: rockfill dams with central cores, rockfill dams with inclined cores and membrane–faced rockfill dams shown in Figure 2.4 (b,c and d).

In order to prevent loss of soil by erosion due to seepage through embankment a filter zone should be provided in any type of rockfill dam. Rockfill dams with earth cores, which are basically similar to zoned earth dams, represent most of the highest em-bankment dams, because a good quality rockfill provides free drainage and higher shear strength (Singh and Varshney, 1995).

Figure 2.4 (a) Homogenous earth dam; (b) rockfill dam with centrally located core; (c) rock-fill dam with an inclined core; (d) rockrock-fill dam with a facing (after Narita, 2000).

2.5 G

ENERAL ASPECTS OF FAILURES AND INCIDENTS

International Commission on Large Dams (ICOLD, 1995) defined the term “failure” as collapse or movement of a part of the dam or its’ foundation, so that the dam can-not retain water. In general a failure results in breaching and releasing of large quan-tities of water, imposing risks on the people and/or property downstream. Thus they differentiate the term “incident” from the “failure” referring to all the troubles that may occur to a dam but not ended with “failure”. The term “accident” represents all malfunctions of the structure that could evolve to “incident” or even “failure” but appropriate actions can prevent this negative development.

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ICOLD carried out extensive surveys on large dam accidents. Large dams are defined as dams which are more than 15 m in height (measured from the lowest point in the foundations to the crest of the dam). As large dams can be considered dams between 10 and 15 m in height which meets one of the criteria: (1) the crest length is not less than 500 m, (2) the capacity of the reservoir is not less than 106 _m3_{, (3) the maximum}

discharge is not less than 2000 m3_/s.

The National ICOLD Committees have reported 176 failures related to 17 406 dams registered in the world.

If we take a look at concrete dams we can see that overall failure rate of concrete dams is lower than that of embankment dams. It is noted that failure rate of concrete dams significantly decrease with service age and development of technology and relia-bility of design and construction. The main reasons of embankment dam break in general are overtopping and piping through the dam or foundation.

Researches on failure case histories and lessons learned from it provide important an-swers and improvement in dam safety approach. It would not be possible without having an overview of past events. In Figure 2.5 (a) failures of different types and heights of dams are presented. Marcello et al. (2009) related number of reported fail-ures with time of their construction. Out of the 17 406 registered dams, 5268 were constructed before 1950. One hundred and seventeen of them which make 2.2% have failed. Within period 1951-1986 there were 12 138 large dams completed out of which 59 failures were reported, which is approximately 0.5% (in Figure 2.5 (b)). The failure rate has significantly reduced in the last years. The failure rate of dams prior to the year 1900 was higher than 4%.

Figure 2.5 (a) Number of dam failures per typology and height (b) typology, height and time period (ICOLD, 1995).

ICOLD (1995) reported that approximately 70% of the reported failures happened within the first 10 years of life of the dam. Failures that occurred during construction or shortly after the first filling of the reservoir contribute to more than half of this percentage (refer to Figure 2.6).

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Figure 2.6 Number of dam failures during the first 10 years of life (ICOLD, 1995). Interesting to note is that China reported 3% of failures out of 80 000 dams complet-ed within period 1950-1980. These are mostly embankment dams <30 m with reser-voir capacity of 0.1 – 106 _m3_{and failure rate decreased significantly after 1980.}

2.5.1 Statistics of failure of embankment dams

In Table 2.1 after Foster et al. (2000), the overall statistics of failures for all failure modes is presented, separating failures during operation. From the historical evidence average frequency of failure of large embankment dams is estimated to be 1.2% dur-ing service life time of the dam, which means that 136 embankment dams failed out of 11 196 large embankment dams constructed up to 1986, excluding China and Ja-pan pre-1930. This number decrease slightly to 1.1% during life time of the dam for the failures occurring only while the dam was in operation. Based on historical rec-ords annual probability of failure of large dams in recent times is estimated to be 4.5x10-4_{per dam per year. If failures occurred during constructions are excluded than}

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Table 2.1 Overall failure statistics for large embankment dams up to 1986, excluding dams constructed in Japan pre-1930 and in China (Source: Foster et al., 2000)

2.6 F

AILURES BY OVERTOPPING

The most common type of failure of embankment dams is overtopping. It may occur for different reasons such as: excessive inflows into the reservoir due to heavy rainfalls or the failure of upstream reservoir, landslide into reservoir, extreme waves and surg-es, inadequate design structure and maintenance of the structure, debris blockage of outlet or spillway and flood channel, and settlement of the embankment crest. In ad-dition, as Milly et al. (2002) stated, overtopping failures are likely to occur with in-creasing frequency attributable to a significant increase in global warming that, ac-cording to climatologist, will result in strong variability and extremes in precipitation patterns. In this thesis the failure by overtopping will be further studied.

2.6.1 Failure mechanisms of overtopping

Overtopping is usually the consequence of an extreme flood and often the cause of partial or complete failure. Embankment dams are a kind of structures which is very

Mode of failure

No. of cases % failures (where known) Average frequency of failure _(x10-3₎

All failures Failures in

operation All fail-ures Failures in operation All failures Failures in operation

Overtopping and Appurtenant Overtopping 46 40 35.9 34.2 4.1 3.6 Spillway-gate 16 15 12.5 12.8 1.4 1.3 Subtotal 62 55 48.4 47.0 5.5 4.9 Piping Through embankment 39 38 30.5 32.5 3.5 3.4 Through foundation 19 18 14.8 15.4 1.7 1.6

From embankment

in-to foundation 2 2 1.6 1.7 0.18 0.18 Subtotal 59 57 46.1 48.7 5.3 5.1 Slides Downstream 6 4 4.7 3.4 0.54 0.36 Upstream 1 1 0.8 0.9 0.09 0.09 Subtotal 7 5 5.5 4.3 0.63 0.45 Earthquake-liquefaction 2 2 1.6 1.7 0.18 0.18 Unknown mode 8 7

Total no. of failures 136 124 12.2(1.2%) 11.1(1.1%) Total no. of failures

where failure mode unknown

128 117 No. of embankment

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sensitive to overtopping depending on the materials and configuration and even envi-ronmental conditions. A concrete embankment dam will break instantaneously when the structure or even a portion of structure loses stability under certain impacts. An earth embankment will fail gradually (breaching process) due to erosion of materials by water flow or waves. Therefore determination of the earth embankment breach characteristics (e.g. width, shape, peak outflow, failure time) is important and chal-lenging, requiring the prediction of complex interactions among soil, water and struc-ture (ASCE/EWRI Task Committee on Dam/Levee Breaching, 2011).

Powledge et al. (1989) distinguished three hydraulic flow regimes and erosion zones in overtopping flow (Figure 2.7).

Figure 2.7 Flow and erosion regimes in embankment overtopping according to Powledge et al. (1989).

In the subcritical flow region on the dam crest, erosion will occur only if the material is highly erodible because the energy slopes and velocities and tractive stresses are relatively low. In the region of transition to supercritical flow, on the downstream portion of the crest, energy slopes and tractive stresses are higher. Erosion can take place at the downstream edge of the crest. The third zone of erosion is at the down-stream face of the dam in supercritical regime flow. The flow accelerates until it reaches uniform flow conditions and the tractive shear stresses exceed a critical re-sistance that keeps material together in place. The erosion process can potentially ini-tiate at any point of the slope but usually it takes place at the toe of the dam.

De Almeida Manso (2002) describes processes induced by overtopping of an earthfill dam as two kinds of flow that may develop: an infiltrating flow throughout dam body and free surface flow along the dam’s face. Depending on embankment structure,

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du-ration of overflowing and magnitude of the overtopping, the relation between these two types of flow change resulting in different failure modes.

For noncohesive embankments the critical overtopping erosion mode is usually pro-gressive surface erosion while for cohesive embankments it is headcut erosion. Nonco-hesive heavily compacted embankments may also erode in the form of headcut and the point of transition between the headcut and surface erosion mode is not well de-fined (ASCE/EWRI, 2011). Erosion usually starts at the toe and advances upstream, deepening free surface and degenerating bank sides thus decreasing width of the crest. This is the process of back cutting or retrogressive erosion. It usually ends with the vanishing of embankment crest, which enables breach flow to increase considerably. Consequently, breach itself is lowering and widening until headwater is depleted and tailwater is increasing.

For composite embankments when overtopping occurs, erosion starts on the down-stream slope in the form of surface erosion or headcut migration and develops to-wards the core or floodwall and eventually leads to the failure.

The headcut migration is a three dimensional phenomenon that depends on the em-bankment geometry, the hydraulic load and soil properties, the hydraulic load and soil properties, and it is not even a statics problem. What’s more, the headcut migra-tion is a dynamic process involving variamigra-tion of the flow and geometry (Zhao et al., 2013). The schematized representation of headcut migration is given in Figure 2.8., with all forces acting on eroded block during headcut migration as water weight G0, flow stress F and tail water pressure P, together with the block weight G and em-bankment cohesion forces N1 and N2, where N1 and N2 are cohesive forces among soil particles.

Figure 2.8 Sketch of headcut migration (Zhao et al., 2013).

Depending on the duration and the magnitude of flow different situation may be en-countered for homogenous embankment as presented in Figure 2.9.

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Description Schematic representation of breaching

1

In case of short duration of overtopping there will be no saturation in the em-bankment or at least hardly any. In case of low flows embankment could in-filtrate the entire volume of overflow.

2

When the overflow increases more rap-idly than the infiltrating flow, water will run along the dam slope. Free sur-face flow will progressively erode the slope at the dam toe.

3

For a long duration, infiltration might occur during time enough to raise the saturation line, leading to internal ero-sion and shallow surface sliding. Water will spring out along the downstream face of the dam, independently of the magnitude of free-surface flow. The de-stabilizing uplift pressures generated in the embankment by this seepage flow will unbalance the equilibrium of some parts of the embankments that will subsequently slide.

Figure 2.9 Schematic presentation of the failure mode of a homogeneous earth fill dam sub-ject to overtopping with corresponding description (after de Almeida Manso, 2002).

2.6.2 Case histories of breaching by overtopping failures

Belci dam

The Belci dam was built at the Tazlau River in Romania in 1962 as an earthfill struc-ture with clay core, 18.5 m height, 422 m length and a storage capacity of 12.7 x 106

m3_{. The central longitudinal section of the dam consisted of a concrete spillway which}

included four flood gates of flap type. The two middle piers had in the lower part two tainter gates used as bottom outlets. At the maximum water level, the spillway ca-pacity was 850 m3_{/s (Marcello et al., 2009). Hydrological measurements for}

calcula-tions and estimation of design flood had been collected from a gauging station 10 km upstream from the site and even ten years before construction, even though since 1950 the peak measured values exceeded the assumed design values on several occa-sions.

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In 1970 a peak inflow of 980 m3_{/s caused overtopping of the dam and its left wing}

was partially eroded. New hydrological calculations were provided after that partial dam failure, which estimated a significantly higher peak inflow as 1515 m3_/s.

Never-theless, the spillway capacity was never re-designed, because the dam was classified from high to middle risk.

On 28 July 1991 heavy rainfall occurred in the upstream catchment where four moni-toring systems were installed, meanwhile there was no rain at the dam site. The tele-phone lines failed and it was not possible to send flood warnings to the dam site. The rain at the dam site started at 10 p.m. At 11:50 p.m. the dam officer could not open the bottom outlet more than 40 cm because of failure of electric power system. The emergency power was also unavailable. The outlet could not be open manually because it was blocked by wood. The overtopping started at 2:15 a.m. of 29 July. The water overcame the dam crest by 50 cm. At 6:00 a.m. the water level in the reservoir started to fall presumably because of the beginning of the dam erosion process. At 7:15 a.m. the storage was practically empty (Marcello et al. 2009). More than 20 peo-ple were killed and 119 houses were destroyed.

The outflow measured by a water gauge downstream was 1200 m3_{which was even}

lower than 1515 m3_{/s of newly estimated design flood. The investigations carried out}

some days later put into evidence that initial dam break occurred at the same area where the dam had been affected by erosion in 1970. The final size of the breach was 112 m length and 15 m deep. It was also shown that repair had produced the effect of natural overflow section. A cable that had been dug along the dam crest also had a negative and accelerating influence with regards to the process and dynamics of the breach formation (IWP and DC, 2009). Even if all outlets were functioning, the over-topping would have been inevitable except the effects would not be so destructive.

Tous dam

The Tous dam is located at river Jucar in Spain. It was designed and built as a flood defence structure with irrigation and regulation functions. Its construction started in 1958 following a project of 80 m tall concrete dam. During foundation works geotech-nical conditions revealed a problem and construction was stopped in 1964. It was con-tinued in 1974 according to modified design, in which the central part had to be changed to a loose material as clay core, and finally finished in 1978 (Alcrudo, 2003) The dam was earth-gravity with 70 m of height, 400 m crest length and crest level 98.5 m, corresponding to a maximum capacity of 115 hm3_{. The old Tous dam in}

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(a)

(b)

Figure 2.10 The old Tous dam (a) in operation; (b) after failure (Source: La Presa de Tous, J.L. Utrillas, ed. MOPTyMA).

The dam was provided with radial gates to regulate spillway in the central part with capacity of 7000 m3_{/s and the bottom outlet had a capacity of 250 m}3_{/s. The second}

stage of the project would have increased the crest level to 142 m providing maxi-mum capacity in excess of 500x106 _m3_.

On 20 October 1982 when the dam was still under construction but almost complet-ed, heavy rain occurred in the river Jucar basin close to Tous dam. Heavy rain quick-ly filled the reservoir up to 600 hm3 _{that exceeded its capacity. The estimated inflow}

was 5000 m3_{/s and the gates of the spillway had to be opened. The electric network}

was out of order, as it usually happens due to such weather conditions, and one of the gates was under repair and the other could not be started. It was even impossible to raise them manually.

The overtopping started at 17:00 p.m.; the water rose 1.10 m above the crest at 19:15 p.m. After 16 hours it was clear that gates were not able to operate and the dam was

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overtopped. Within 1 hour it was washed out by erosion of a greater part of shoulders and central core (Figure 2.10 (b)).

The dam break wave overflowed already filled river bed ad its floodplain. Peak dis-charge was estimated at 1500 m3_{/s. The wave flooded a populated area of about 300}

km2_{, where 8 people lost their lives and huge damage was estimated.}

A new Tous dam was reconstructed on the same site using remaining part of the clay core material, which had proved significantly high resistance to water flow.

Taum Sauk upper dam

The Taum Sauk Plant is located in Missouri State (USA). It is a reversible pumped storage system used to supplement the generation and transmission facilities of the hydroelectric plant. The facilities at Taum Sauk Plant consist of a ridge top upper reservoir, a shaft and tunnel conduit, the powerhouse and lower reservoir (Figure 2.11). The plant is in operation since 1963 and at the time it was one of the largest of that type.

Figure 2.11 Overview of Taum Sauk plant, lower and upper reservoir (Source: Hollenkamp et al., 2011).

The plant’s upper reservoir is impounded by a dike and the lower reservoir is im-pounded by a dam across the Black River. The dike was a concrete-faced dumped rockfil dam with a maximum height in the range of 25.6 m. The upper reservoir had no spillway and no drainage area and its capacity is 5.4x106 _m3_{. Water filled the}

res-ervoir by pumping and direct rainfall (Hollenkamp et al., 2011). In Figure 2.12 (a) full upper reservoir can be seen. The plant is equipped with two reversible pump-turbine units capable of generating 450 MW. The plant is a typical reversible system, which means to generate in the mid-morning by releasing water from the upper reser-voir through the turbines to the lower reserreser-voir; to pump from the lower reserreser-voir back to the upper reservoir in the afternoon and generate in the evening; to pump again from the lower to the upper reservoir in the early morning. During fall, spring and winter months the generation occurs only once per day.

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(a) (b)

Figure 2.12 Upper reservoir view (a) view of breached dike (b) (Source: Hollenkamp et al., 2011).

December 14, 2005, the northwest side of the upper reservoir breached over a width about 213.36 m. About 5.7x106_m3_{. of water were released within 25 minutes. A view}

of the breach is shown in Figure 2.12 (b). According to investigation of the Public Service Commission established after the failure, it was stated that the cause was de-ficiency in the control system of the upper reservoir water level during filling phase: a computer software problem caused the reservoir filling even though it was already at its maximum water level. The overtopping was inevitable and a rapid increase in pore pressure at the dike/foundation interface brought to the failure. The owner’s decision to continue operating Taum Sauk after the discovery of the unreliability of the pie-zometers readings, upon which the upper reservoir control system was based, was considered very imprudent (Marcello et al., 2009).

No casualties were reported; nevertheless the breach produced significant property and environmental damage. The lower reservoir accommodated a huge amount of the flood, avoiding damage to a larger extent.

The upper reservoir was replaced with a roller compacted concrete dam together with changes in the safety management structure for the whole system.

2.7 L

ESSONS LEARNED FROM HISTORICAL DAM FAILURES

In the last 50 years the number of dam failures has been reduced significantly thanks to improvement of design, construction and monitoring during operation. Most of these improvements are attributable to lessons learned from the past incidents and failures. Past experiences given rise to the improvement of safety criteria which were adopted by designers, constructors and dam operators. Based on this, national legis-lations for dam safety have been formulated and consequently international recom-mendations which represent a reference to whole dam engineering community. On oc-casion of 80th_{Anniversary of ICOLD in 2008, the bulletins were issued which}

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In the following paragraphs only few general remarks on causes of failures and measures applied in order to avoid unwanted situations will be discussed.

2.7.1 Problems caused by overtopping and inadequacy of spillways and outlets

Historical accidents and failures of dams of different types and size proved that over-topping had been caused mostly by inadequate spillway and outlet capacity and also ill-functioning of gates. Usually incomplete or inaccurate hydrological data together with insufficient analysis were usually found to be reason of improperly designed spillways and outlets. It was only after the 1950ies that the situation has improved because of experiences gained by analysing incidents and failures. The most usual measures that should be undertaken in order to avoid failures are numbered here:

1) The location of spillways has to be considered and analysed with special care: Faults or slide prone areas have to be avoided.

2) The functioning of outlets is essential in case of flood or if problems arise at the dam site: In such situations the capability of outlet works to

re-lease the water and to lower the water level in the reservoir can make the dif-ference between saving and losing a dam. Outlets should be designed to have sufficient capacity to allow timely release of water during a flood. Of course, conservative criteria for sizing flood control works can raise the project cost appreciably; but economizing on spillways can produce very severe problems and higher costs in case of incidents and failures (Marcello et al., 2009).

3) Many defects disclosed at dams are caused by malfunctioning of ap-purtenant structures such as spillways, outlets or conduits, particu-larly important is location of outlets and conduits: It is especially

pro-nounced in embankment dams and foundations, were such rigid structures placed in soft materials can be damaged by differential settlement and cause uncontrolled release of water. After many incidents attributed to poor compac-tion of backfill around the conduits at the base of several dam failures, nacompac-tion- nation-al laws and internationnation-al recommendations have been issued which established standard that such facilities should be placed on sound rock.

4) Particularly important is timely functioning of gates, valves and conduits during flood: It is necessary to exercise with equipment under

op-erating conditions. It is necessary to provide additional sources of power which will enable equipment to operate in the conditions of heavy rainstorms. Fre-quent testing and maintenance of such system is fundamental for their reliable functioning during emergency situations.

5) Embankment dams are much more vulnerable to overtopping than concrete dams: A suitable protection of the downstream slope could prevent

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2.7.2 Dam foundation condition

Past experiences show that an inadequate foundation caused large number of dam failures. A geological investigation has to precede the dam design, and special atten-tion should be paid to the rock characteristics such as faults and joints. The experi-ences from some old dams built on faults or close to them taught us that presence of faults should be avoided for the construction of a new dam.

Another aspect to be considered is the solubility of the rock at the dam foundation and the reservoir site. Attention should be paid to the areas where outlets and spill-ways will discharge.

With utmost attention the seepage control in the foundation should be considered. Safety of a dam depends on how efficient, designed and maintained, is drainage sys-tem which is supposed to cut off seepage that could induce pressures or erosion in the foundation.

2.7.3 Monitoring

Experience proved that the first filling is one of the most critical phases in a dam life during which many failures occur. For this reason a system for close monitoring the first filling should be set up. At the beginning it should be focused on detecting any deviation from the foreseen dam behaviour. Every year the knowledge of the owner of the structure grows and the probability of failure of the structure falls (Vrijling and Van Gelder, 1996).

Possible anomalies can be detected and interpreted according to a sound cause-effect model capable to include all possible variables concurrent to define the behaviour of the dam-water-foundation system (Marcello et al., 2009). Both statistical and deter-ministic models can be applied in the frame of automatic monitoring surveillance. Surveillance has to continue after the first filling of reservoir is completed and the dam is in operation, because it takes few years before foundations and structures are adjusted to the loading. ICOLD bulletins provide guidance as monitoring requires various instruments to be installed, which is usually done based on professional’s ex-perience, and measures have to be performed at dam site.

2.8 D

ISCUSSION AND CONCLUSIONS

In this chapter we were able to see see that embankment dams are the oldest and most widespread type of dam. They are generally divided into two main categories based on type of soil used, as construction material as earthfill and rockfill dams. We learned that overall failure rate of embankment dams is higher than of concrete dams, because embankment dams are a kind of structures which are very sensitive to mate-rials and configuration impacts and even environmental conditions.

The most common type of failure of embankment dams is overtopping. Case studies presented showed that embankment dam failures occurred in different parts of the

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world and different year; although we can say that in the last 50 years the number of dam failures has been reduced significantly thanks to improvement of design, con-struction and monitoring during operation. It is shown that reasons for failures were inadequate spillway and outlet capacity, ill-functioning of gates or mistakes in the monitoring procedures. Only a few case studies are presented here, with failure modes significant for further work in this thesis.

Important conclusions from this chapter are already discussed in Section 2.7, in les-sons that we learned from incidents and failures.