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COMMISSION INTERNATIONALE DES GRANDS BARRAGES

------- VINGT-CINQUIÈME CONGRÈS

DES GRANDS BARRAGES Stavanger, Juin 2015

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PROPOSING STANDARD OF SOCIETAL RISK CRITERIA FOR DAMS IN IRAN (*)

Seyedeh Fatemeh FAKHRMOOSAVI Sharif University of Technology, Tehran, Iran

Ali NOORZAD

Faculty of Water and Environmental Engineering, Shahid Beheshti University & President, Iranian Committee on Large Dams, Tehran, Iran

Mohsen GHAEMIAN

Faculty of Civil Engineering, Sharif University of Technology, Tehran, Iran

IRAN

1. INTRODUCTION Assessment of consequences caused by dam failure is an essential part of

a dam safety study. Each dam failure may cause a lot of damages to governments and people’s life and properties. So dam safety evaluation have great significance to any society. Dam safety is influenced by something more than simple engineering concepts. Actually, a dam failure is a complicated process that includes natural events such as earthquake, flood and human or construction defects.

International committee on large dams expresses as a philosophy for dam

safety, “the safety of a dam manifests itself in being free of any conditions and developments that could lead to its deterioration or destruction. The margins which separates the actual condition of a dam, or the conditions it is designed for, from those leading to its damage or destruction is a measure of its safety” [1].

(*) Proposition d’un standard de critères de risque de la société pour les barrages en Iran

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As was mentioned, dam safety problem is always attached to the dam and isn’t only dependent on initial design and construction conditions. Dam safety is including every activity of probable defects and their consequents and recognition, reduction, elimination and treatment of them in a reasonable way. Although many countries in the world have done a lot of researches in the context of dam safety, there isn’t any efficient study in this concept for Iranian dams. So it is essential to study on safety of dams in Iran. In this way, obtaining a standard for the safety limit of dams in Iran and explanation of a simple method that is applicable in this country despite the shortage of data and facility, has great significance.

Several events may threat the safety of a dam such as a flood, earthquake,

Tsunami, movement of floating ice, volcano, human error, explosion, far and terrorism, internal instability, landslide and etc. Among them flood and earthquake is specifically considered because of their generality and major damages. Other events is being checked depending on their situation and probability. To this end, level of damages due to hazards which mentioned above should be determined and compared with available standards.

2. LITERATURE REVIEW The ICOLD committee on dam safety issued Bulletin 130 in 2005, “Risk

Assessment in Dam Safety Management- a Reconnaissance of Benefits, Methods and Current Applications”. In this bulletin two fundamental principles were presented as aspects of properly designed dam safety management program, one of them is equity- the right of individuals and society to be protected- and the other is efficiency- the need that society has to distribute and use its available resources in such a way to gain maximum benefit. According to this bulletin, the level of safety can be considered as bounded by two extremes, the maximum level of safety that is physically achievable at any cost and a minimum level below which the dam cannot withstand normal operating conditions. Between these bounds, safety decision making involves striking a balance between the risks and the benefits and between social equity and economic efficiency [2]. The diagram shown in Fig. 1 is a conceptual definition of the level of safety.

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Fig. 1

Conceptual definition of the level of safety [3] Définition conceptuelle des niveaux de sécurité (Bowles et al 2007)

This committee in a guideline by the title of “ Dam Safety Management:

Operational Phase of the Dam Life Cycle” presents the frequency- number (F-N) curve which is shown in Fig. 2 as an example of societal risk requirements for existing dams. In this figure the risks applied on dams are divided into three section of intolerable, tolerable if ALARP is required and negligible.

Fig. 2

NSW DSC Societal Risk Requirements for Existing Dams [4] Exigences en matière de risques sociétaux pour des barrages existants – NSW

DSC (New South Wales – Dam Safety Committee)

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3. SOCIETAL RISK CRITERIA

Societal risk is often represented with F-N curves which is probability that

more than N lives are lost versus number of fatalities resulting from dam failure. A standard is needed to decide the acceptability of risks incurred by the dams. According to the nature of the hazards that dams pose, broadly acceptable risks in the F-N curves isn’t applicable to these kinds of facilities. So the standard line for dams is a boundary between the tolerable and unacceptable regions. The standard line can be described with the following formula for dams in Iran:

[ 1 ]

1 − 𝐹𝐹𝑐𝑐(𝑁𝑁) =

1𝑁𝑁2 10−1

N is the number of life that is lost and Fc is the probability that less than N

life is lost. This limit line in comparison with the line for other countries is shown in Fig. 3. It can be seen that the standard line for Iran is up to lines of some other countries which means the lower requirements for dams in Iran because of unwillingness to accept higher values of safety in Iran.

Fig. 3

The suggested standard for dams in Iran in comparison with the standards of other countries

Norme proposée pour les barrages en Iran comparée aux normes d’autres pays

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The societal risk criteria can also be shown with f-N curves which represents the annual probability of dam failure (f) versus number of fatalities (N). The annual probability of dam failure is much more accessible than F which is the probability of N or more fatalities. So f-N curves may be useful for primary studies in Iran.

In the formula of the standard line, N is supposed to be an integer number.

It isn’t in agreement with the current methods of life loss estimation. Methods that are based on fatality rates often yield to non-integer numbers. So the numbers should round up or down [5]. Since the number of fatality is an infinite number, the limit line shown in Fig. 3 can be replaced by a set of points with coordinates (N, fc(N)) and (N, Fc(N)) for N = 0,1,2, . . . , NPAR where NPAR is the total amount of people at risk. So by the assumption of discrete number of fatality and according to F-N formula, it can be written that:

[ 2 ]

N2(1 − Fc(N)) = 10−1

The same formula can be written for N+1 fatalities:

[ 3 ] (N + 1)2(1 − Fc(N + 1)) = 10−1

The left side of both formula is equal so:

[ 4 ] ⇒ N2(1 − Fc(N)) = (N + 1)2(1 − Fc(N + 1))

Then it can be resulted in:

[ 5 ] N2(Fc(N + 1)−Fc(N)) = (2N + 1) (1 − Fc(N + 1))

It can be written according to the definition of the probability of exactly N

fatalities that:

[ 6 ] [Fc(N + 1) − Fc(N)] = fc(N) By substitution of Eq.[ 6 ] and Eq.[ 3 ] in Eq.[ 5 ] it can be resulted that:

[ 7 ]

𝑓𝑓𝑐𝑐(𝑁𝑁) = (2𝑁𝑁 + 1)𝑁𝑁2(𝑁𝑁 + 1)2 10−1

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4. CASE STUDY A case study has been performed to show the application of the frequency-

number of fatality curves. In this study the data available from a dam located in north-east of Iran is depicted in f-N curves and checked out with the standard determined in previous section.

Golestan dam in 13 kilometer east of Gonbad city in north-eastern part of

Iran experienced and controlled many devastating floods in last years. The number of fatalities and damages caused by two floods occurred on august 2001 and 2002 were too high. On August 2001 a flood with return period of 1000 years occurred on Gorganrud River that killed 240 people in the downstream. Also the flood occurred on august 2002 with return period of 100 years resulted in 43 loss of life. Golestan dam tolerated another severe flood in 2005 with return period of 200 years and life loss of 40 people.

The data of these floods are depicted in comparison with the standard line

in Fig. 4. As it can be seen in Fig. 4, all the data from those floods are upper than the limit line, so the performance of the Golestan dam against such floods were unacceptable and the safety of the dam should be increased. This can be by actions like preparation of emergency action plan, public education, design of appropriate and adequate spillways, depletion of the storage and etc.

Fig. 4

Application of f-N criteria in Golestan dam’s safety evaluation Application du critère f-N pour évaluer la sécurité du barrage de Golestan

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5. CONCLUSION

In this study, the concepts of dam safety were reviewed and common

methods of representing societal risk as a main criteria of dam safety were mentioned. Then a standard for acceptable level of safety for dams in Iran were suggested. Since the depiction of F-N curves has some difficulties, the f-N curves are another way to show societal risk criteria that is more applicable. Golestan dam as a case study was checked out according to f-N standard. Although this dam prevented catastrophic mortality which often occurred before its construction, its safety wasn’t satisfactory in selected floods for this study. So the method presented in this paper can easily be used for dams in Iran despite the inaccessibility of data in this country.

REFERENCES

[1] Bulletin 59, Dam Safety Guidelines, ICOLD, 1987. [2] Bulletin 130, Risk Assessment in Dam Safety Management A Reconnaissance

of Benefits, Methods and Current Applications, ICOLD, 2005. [3] BOWLES, D.S., GIULIANI, F.L., HARTFORD, D.N.D., JANSSEN,

J.P.F.M., MCGRATH, S., POUPART, M., STEWART, D. AND ZIELINSKI, P.A. ICOLD Bulletin on Dam Safety Management, Keynote paper in Proceedings of the NZSOLD/ANCOLD Workshop on “Promoting and Ensuring the Culture of Dam Safety”, Queenstown, New Zealand, 2007.

[4] Bulletin 154, Dam Safety Management: Operational Phase of the Dam Life Cycle, ICOLD, 2011.

[5] Zielinski, P., Tolerability of societal risk criteria, The international journal on hydropower & dams, 2014, Vol. 21 , No. 4, pp. 63-67.

SUMMARY Safety management and risk assessment of dam failure have great

significance to governments. Estimating the safety level of existing dams is an important part of dam safety management program in order to improve their safety and to achieve an acceptable level and to provide emergency action plans for probable failures. Reaching this goal requires a good sense of the actual conditions of the dam, identification of hazards and selection of the appropriate scenario of safety level estimation in accordance with these conditions. In this

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paper, the frequency- number curves have been presented as a method of quantification of societal risk criteria and developed in order to be practicable for estimating the safety level of existing dams in Iran. Then a standard has been proposed and applied on a case study (Golestan Dam) to evaluate the applicability of the method in Iran.

RÉSUMÉ La gestion de la sécurité et l’évaluation des risques de rupture de barrages

ont une importance majeure pour les gouvernements concernés. L’estimation du niveau de sécurité de barrages existants représente une part importante du programme de gestion de sécurité des barrages afin d’améliorer leur sécurité et d’atteindre un niveau satisfaisant et d’élaborer des plans d’action en cas de rupture possible. Atteindre ce but demande une bonne connaissance de l’état actuel du barrage, l’identification des dangers et la sélection de scénarios correspondants appropriés pour l’élaboration des niveaux de sécurité. Dans cette présentation, les courbes fréquence-nombres sont présentées comme méthode de quantification pour les critères de risque sociétaux puis développées afin d’être utilisables pour estimer les niveaux de sécurité de barrages existant en Iran. Finalement une norme est proposée et appliquée à une étude de cas (barrage de Golestan) afin d’évaluer l’applicabilité de la méthode en Iran.