TOTAL RISK ANALYSIS OF DAM AND APPURTENANT STRUCTURES IN A BASIN AND A CASE STUDY

Transcription

1 TOTAL RISK ANALYSIS OF DAM AND APPURTENANT STRUCTURES IN A BASIN AND A CASE STUDY Prof. Dr. Hasan TOSUN Osmangazi University Civil Engineering Department Bati Meşelik Eskişehir Turkey htosun@barajguvenligi.org ABSTRACT Total risk of dam and appurtenant structures in a basin is systematically analyzed by considering the various factors of hazard. These factors can mainly be categorized into three groups: (1) the factors resulted from structure such as settlement, seepage, leakage, and internal erosion. (2) the factors resulted from dam site such as seismic activity, landslide, rockfall and (3) flooding. These hazards can be analyzed in detail and some design and construction measures should be taken into account to increase safety. The non-structural measures such as alarm system and planning for land usage can be considered to reduce the risk of flooding. There are also other factors such as sabotage and action of war that their threats are unpredictable. In other word, these factors are unfortunately not considered in the content of conventional risk-analyzing methods. A risk-analyzing method identifies the weak element or elements of a complex dam system. Safety can be increased by means of improvement of these elements. This paper outlines the main principles of total risk analysis of dams in a basin, discussed some issue of conventional methods and introduces the results of a study which was performed for the large dams of Euphrates basin in Turkey. Key words: basin, dam, hazard and total risk 1.INTRODUCTION The total risk for dam structures mainly depends on the seismic hazard rating of dam site and the risk rating of the completed structure. The seismic hazard of a dam site can be rated as based on the peak ground acceleration. This value derived from the defined earthquake produces the main seismic loads. For preliminary study, the existing map of seismic zones can be used to estimate the seismic hazard of a dam site. The risk rating of the completed structure based on the capacity of reservoir, the height of dam, the evacuation requirements and the potential downstream damages.

2 478 INTERNATIONAL CONGRESS ON RIVER BASIN MANAGEMENT In general, the seismic and risk ratings are evaluated separately [ICOLD, 1989]. Recently two factors were combined to define total risk factor of dam structures [Bureau, 2003]. The type of dam is important parameter acting on total risk rating. ICOLD [1989] stated that safety concerns for embankment dams subjected to earthquakes involve either the loss of stability due to a loss of strength of the embankment of foundation materials or excessive deformations such as slumping, settlement, cracking and planer or rotational slope failures. To obtain preliminary information about seismic parameters, the simplified procedures can be used. If the materials used in embankment are not susceptible to loss of strength and the hazard and risk ratings are low, the simplified analyses are entirely sufficient to define the seismic evaluation parameters. The safety concerns for concrete dams subjected to earthquakes involve evaluation of the overall stability of the structure, such as verifying its ability to resist induced lateral forces and moments and preventing excessive cracking of the concrete. For analyzing the loads, different procedures are performed. In the simplified analyses, peak ground motion parameters and response spectra are sufficient to define the seismic evaluation parameters. It is suggested the finite element method to be used for analyzing of most dams in high risk or hazard class. In Turkey most of dams under operation and construction are of embankment type because of physical factors and economic reasons. The construction of concrete dam is rare when compared with that of embankment dams. The total number of large dams constructed in Turkey is over 1200 on the basis of data of National Organizations [DSI, 2002 and KHGM, 2004]. It is a common thought that embankment dams, which are well compacted according to the specification, are suitable type when constructed in the regions having high seismic activity. In general, strong ground shaking can result instability of the dam and strength loss of foundations. Active faults, which are very close to the foundation of dams, have the potential to cause damaging displacement of the structure. There are some examples in Turkey which were damaged during the earthquakes occurred in past [Tosun, 2002]. Author thinks that some evidence of aging process has been observed on old concrete dams in Turkey. This paper deals with an evaluation of seismic hazard and total risk analyzing methods and a case study involving the analyses of large dams in Euphrates basin in Turkey. 2.METHODS OF ANALYSIS The total risk-analyzing methods for dam structures primarily have two separate components. For the seismic hazard analysis of a particular site, all possible sources having seismic activity should be identified and their potential should be evaluated in detail. The study about seismic activity includes the deterministic and probabilistic seismic hazard analyses. The deterministic seismic hazard analysis considers a seis-

3 BASIN WATER MANAGEMENT 479 mic scenario and includes four-step process. It is very simple procedure and gives rational solutions for large dams because of providing a straightforward framework for evaluation of worst ground motions. The probabilistic seismic hazard analysis is widely used and considers uncertainties in size, location and recurrence rate of earthquakes. Kramer [1996] states that the probabilistic seismic hazard analysis provides a framework in which the uncertainties can be identified, and combined in a rational manner to provide a more complete picture of the seismic hazard. For obtaining more accurate solution, a computer program can be used for probabilistic and deterministic assessment of seismic hazard. The seismic sources are identified and recurrence of relationship of earthquakes is estimated. An extensive survey as well as taking into account the available literature should be performed and the data about historical and 20 th century instrumentally recorded earthquakes for Turkey and vicinity should be considered as a basis for seismic hazard analyses Erdik et al, 1985 and Yücemen, 1982). The earthquakes occurred within last 100 years should be taken into account for estimating seismic parameters. Due to the unavailability of strong motion records, various attenuation relationships are considered [Ambrayses, 1995; Campbell, 1981; Campbell and Bozorgnia, 1994; and Joyner and Boore, 1981]. Seismic hazard parameters can be estimated by using the method developed by Gumbel and Gutenberg-Richter methods [Boore and Joyner, 1982]. For all analyses, Operational Based Earthquake (OBE) and Maximum Credible Earthquake (MCE) should be defined and the peak ground acceleration should be determined by Maximum Design Earthquake (MDE) given by ICOLD [1989]. For existing dams, Safety Evaluation Earthquake (SEE) is considered [FEMA, 2004]. There are various methods to quantify the total risk factor of any dam. One of them, which was recommended by ICOLD [1989], considers the seismic hazard of dam site and risk rating of structure separately. According to this method, seismic hazard of the dam site regardless of type of dam can be defined from low to extreme and classified into four groups. In other words, it introduces a quick way of rating for seismic hazard. The hazard class of a dam site obtained from this method provides a preliminary indication of seismic evaluation requirements. As based on the ICOLD classification, potential risk of dams consists of structural components and social-economics components. The first one is mainly based on capacity of reservoir and height of dam. The second one can be expressed by evacuation requirement and potential downstream damage. Total risk factor is defined as a summation of risk factors for capacity, height, evacuation requirement and potential damage. As based on total risk factor, four risk classes are defined as low, moderate, high or extreme. Risk classification of dam provides more detail information for selection of seismic evaluation parameters and methods to be used for analysis. It is notified that special considerations for safety are recommended for the large dams having a height above 90 m and a storage capacity grater than 1200 hm 3.

4 480 INTERNATIONAL CONGRESS ON RIVER BASIN MANAGEMENT Second method to quantify the safety of any dam defines total risk factors, which depends on the dam type, age, size, downstream risk potential and the dam vulnerability [Bureau and Ballentine, 2002]. Total Risk Factor (TRF) includes a downstream hazard factor, which has two different components. Bureau [2003] also suggests using a simple classification chart for estimating downstream hazard factor, when it is difficult to obtain sensitive values of risk factors for downstream evacuation requirements and downstream damage, respectively. For obtaining the seismic vulnerability of rating, a curve developed by Bureau and Ballentine [2002] is used. By means of this curve, a Predicted Damage Index (PDI) can be obtained. The PDI value depends on the Earthquake Severity Index (ESI). As based on ESI value, PDI can be estimated from a graph and then the Predicted Damage Factor (PDF) is calculated. The local magnitude of causative earthquake is considered as well as peak ground acceleration to estimate the seismic vulnerability of dams. The last step for assessment techniques is to rank the dams by TRF and to assign risk classes. Bureau [2003] states that the risk class can be used to establish the need for more detailed seismic evaluations and to estimate priorities for such evaluations. 3. CASE STUDY-DAMS OF EUPHRATES BASIN The Euphrates basin, which is the largest one of 26 basins throughout all country, has a water yield resources of 31.6 km 3 per year. The Euphrates River, formed from the Karasu and Murat tributary rivers, is the main river of the basin and has 2800 km in length and crosses Iraq to join the Tigris, where it flows into the Persian Gulf. Its average discharge in upstream and Syria border is 650 m 3 /s and 950 m 3 /s, respectively. In the basin, thirty-two large dams have been designed to exploit the energy and irrigational potential of the basin (figure 1). The physical properties of these dams are given in table 1. Twenty-five of them were entirely completed. Seven of them are under construction. The last two dams are under designing stage. A dam series on the main river, namely Karkamis, Birecik, Ataturk, Karakaya, Keban and Ozluce have been completed. The large ones such Alparslan I, Kigi, Uzuncayir and Yazici are under construction. It is estimated that they will entirely finished at the end of Alparslan II and Konaktepe are at designing stage now. It is expected to start their construction in The basin includes the main part of Southeast Anatolian Project, which is a multi-sector and integrated regional development effort approached in the content of sustainable development. The project encompasses such sectors as irrigation, hydraulic energy, agriculture, rural and urban infrastructure, forestry, education and health. The water-resources development component of the project envisages the construction of 22 dams and 19 hydraulic power plants and irrigation of 1.7 million hectares of land. The total cost of the project is estimated as 32 billion US dollar. The

5 BASIN WATER MANAGEMENT 481 total installed capacity of power plants is 7476 MW and the annual energy production reaches 27 billion kwh. Figure 1. Large dams of Euphrates basin considered for this study. In the basin most of dams under operation and construction are of embankment type because of physical factors and economic reasons. The Euphrates basin is situated at the region having very complex geology and also very active seismisity. For this study, the geology of basin was simplified and represented by seven separate units ranging in age from Precambrian to Quaternary [Tosun et al, The oldest units, which are commonly made up of various metamorphic schists, are seen at the central part of basin. They have no problem with water leakage and foundation stability in dam sites. But small landslides were observed along the main bank of river. The younger units are mainly composed of sedimentary rocks including marl, mudstone and limestone and shale alternation. Limestone, as base rock for dams, has resulted to significant leakage problem in some dam sites. The metamorphosed sedimentary rock such as marble also created the leakage problem, because of being jointed, fissured and faulted and including many cavities. Quaternary aged units are composed of volcanic and sedimentary rocks. Recent deposits called as alluvium can be seen along river basins and lakes In the basin, North Anatolian Fault (NAF) Zone and East Anatolian Fault (EAF) Zone are dominant features to explain its structural geology [Şaroğlu et al, 1992]. The NAF zone is one of the best-known strike-slip faults in the world, because of its significant seismic activity and well-developed surface features. It has approximately 1500 km-length with extending from eastern Turkey in the east to Greece in the west

6 482 INTERNATIONAL CONGRESS ON RIVER BASIN MANAGEMENT and forms the part of boundary between the Eurasian Plate to the north and Anatolian plate to the South [Bozkurt, 2001]. Its wide ranges from a single zone of a few hundred meters to multiple shear zones of 40 km. The EAF Zone, which has a 550 km length is approximately northeast trending, sinistral strike-slip fault zone. It comprises a series of faults arranged parallel to the general trend. It is a transform fault forming parts of boundaries between the Anatolian and Arabian Plates [McKenzie, 1972]. The NAF Zone joins with the EAF Zone at Karlıova in the basin and forms a typical triple junction. This zone produces very large earthquakes resulted on thousand souls and very severe damages. Table 1. Properties of dams considered for this study Dam Height from Function (*) (**) Completed Volume of dam Reservoir Type river (m) year (***) fill (x1000m 3 ) capacity (hm 3 ) Alpaslan I 88.0 E RF u/c Alpaslan II I+E+FC RF u/c Ataturk I+E RF Birecik 53.5 I+E CG+RF Boztepe 64.0 I EF u/c Camgazi 39.0 I EF Cat 64.5 I RF Dumluca 24.0 I EF Erzincan 73.0 I EF Gayt 30.7 I EF Gulbahar 59.0 I EF u/c Hacihidir 32.0 I RF Hancagiz 45.0 I EF Kalecik 33.9 I EF Karakaya E CA Karkamis 22.5 E+FC CG+EF Kayacik 45.0 I RF Keban E CG+RF Kigi E RF u/c Konaktepe E RF u/c Kuzgun I+E RF Medik 42.0 I RF Mursal 49.5 I+E EF Ozluce E RF Palandoken 44.0 I+WS EF Patnos 31.7 I EF Polat 51.0 I EF Surgu 55.0 I EF Tercan 56.0 I+E EF Uzuncayir 58.0 E EF u/c Yazici 83.5 I EF u/c Yoncali 81.0 I RF u/c * E: Energy I: Irrigation FC: Flood control WS: Water supply ** RF: Rockfill EF: Earthfill CG: Concrete gravity CA: Concrete arch *** u/c: under construction

7 BASIN WATER MANAGEMENT 483 For the seismic hazard analyses of the dam site in the basin, a detailed study was performed to identify all possible seismic sources, As a result of detailed evaluation, total area covering all basins was separated into eleven seismic zones. The number of earthquakes having a magnitude on the basis of surface wave (Ms) which is greater than 4.0 is 264 in the basin along last 100 years.. The numbers of earthquakes with Ms that are greater than 5.0 and 6.0 are 55 and 7, respectively. There are only two earthquakes having a magnitude greater than 7.0 in the basin. The first one is the Erzincan earthquake with Ms of 7.9 in The second one is the Karlıova earthquake with Ms of 7.0 in The seismic hazard analyses have been performed for 32 dams in the basin. The results of analysis are given in table 2. The results indicate that peak ground acceleration (PGA) changes within a wide range (0.01g and 0.564g). Seventeen dam sites have low hazard rating and identified as the hazard class of I. Twelve dam sites are classified into the class of II with moderate hazard rating. According to ICOLD (1989) classification, if PGA value is greater than 0.25g and the energy source is closer than 10 km from the dam site, it is classified as hazard class IV with hazard rating of extreme. Erzincan dam, which is located at the northern part of basin, has a hazard class of IV. It is very close to epicenter of the catastrophic 1992 earthquakes and can be subjected to a PGA of 0.564g with a Maximum Design Earthquake (MDE) of 7.9. For Alparslan I and Kigi dams, the value of PGA is greater than 0.25g, but they are not closer than 10 km from the energy source. Therefore, they are identified into the hazard class of III. Alparslan I dam s risk can be regarded as that of Erzincan dam, when considered the adverse geotechnical properties of foundation soil. It is well known that the foundation material of this dam is composed of soft rocks, which have low slake durability. The large dams which are under the influence of near source zone are located to very close the North Anatolian Fault that is the most active zone in Turkey.

8 484 INTERNATIONAL CONGRESS ON RIVER BASIN MANAGEMENT Table 2. Hazard class and rating for all dams within the basin Dam MDE (*) PGA (**) Critical Zone Hazard Class Hazard rating Alpaslan I IV III High Alpaslan II IV II Moderate Ataturk X II Moderate Birecik X I Low Boztepe IX I Low Camgazi XI I Low Cat X II Moderate Dumluca XI I Low Erzincan II IV Extreme Gayt IV I Low Gulbahar IV I Low Hacihidir X I Low Hancagiz X I Low Kalecik IX II Moderate Karakaya IX II Moderate Karkamis X I Low Kayacik X I Low Keban IX II Moderate Kigi IV III High Konaktepe IV II Moderate Kuzgun III II Moderate Medik X I Low Mursal I II Moderate Ozluce IX I Low Palandoken III II Moderate Patnos VII II Moderate Polat IX II Moderate Surgu X IV Extreme Tercan IV II Moderate Uzuncayir IV III High Yazici VIII II Moderate Yoncali IX I Low * MDE: Maximum design earthquake (based on local magnitude) **PGA: Peak ground acceleration in g There are important dam structures in the basin such as Karkamis, Birecik, Ataturk, Karakaya, Keban and Ozluce dams. The dams, which are located on the main river of basin, can cause very serious conditions for downstream life and property, when they damaged or failed. For Birecik and Karkamis dams, the PGA values were estimated very low. In other words, they have safe sites when considered their seismic activity. Atatürk dam, which is the largest dam of Turkey with a storage capacity of hm 3, is also located on the site having very low seismisity. Karakaya dam, which is the unique concrete dam of basin, is subjected to relatively high earthquake loads as a value of 0.132g with a MDE value of 7.0.

9 BASIN WATER MANAGEMENT 485 Surgu, Polat and Cat dams were located at the west part of basin. Their location is frequently jointed, fractured and faulted. The dam sites for last two are identified as the hazard class of II with a moderate value of PGA, even if the MDE value is low. However, Sürgü dam is classified into hazard class of IV. The PGA value is 0.170g for Polat dam, while it is 0.211g for Cat dam. The PGA value of Sürgü dam is estimated as 0.256g. However, the authors point out the fact that these dams are under the influence of local near-source zone and have high risk rating for earthquake conditions. The damage on the Dogansehir earthquake with Ms of 5.8 on Surgu dam absolutely confirms the author s thought (Tosun, 2002). Table 3. The results of potential risk analyses of dams within the Euphrates basin Site Influence (*) Dam MDE PGA TRF Risk Class Definition Alpaslan I III High Alpaslan II II Moderate Ataturk II Moderate Birecik II Moderate Boztepe II Moderate Camgazi II Moderate Cat III High Dumluca II Moderate Erzincan III High Gayt III High Gulbahar II Moderate Hacihidir II Moderate Hancagiz II Moderate Kalecik II Moderate Karakaya II Moderate Karkamis II Moderate Kayacik II Moderate Keban III High Kigi III High Konaktepe III High Kuzgun III High Medik II Moderate Mursal II Moderate Ozluce III High Palandoken III High Patnos III High Polat III High Surgu III High Tercan III High Uzuncayir III High Yazici II Moderate Yoncali II Moderate *MDE: Maximum design earthquake (based on local magnitude) PGA: Peak ground acceleration in g **CRF: Capacity risk factor HRF: Height risk factor ARF: Age risk factor *** DRI: Downstream damage risk index ERF: Evacuation requirements factor ****TRF: Total risk factor

10 486 INTERNATIONAL CONGRESS ON RIVER BASIN MANAGEMENT The total risk for dam structures mainly depends on the seismic hazard rating of dam site and the risk rating of the dam structure. Bureau (2003) method, which considers dam type, age, size, downstream damage potential and evacuation requirements, was utilized to realize the risk analyses of basin. It recommends four separate risk classes ranging from I (low risk) to IV (extreme risk) as based on the Total Risk Factor (TRF). If TRF is between 2 and 25, the risk class of dam is I (low). If TRF is ranging from 25 to 125 and from 125 to 250, the risk classes of dam are identified as II (moderate) and III (high), respectively. If TRF is greater than 250, the risk classes of dam are considered as IV (extreme). Following Bureau s method, all large dams are classified in risk classes II and III that mean moderate and high risk rating. The solution obtained from Bureau method is more rational than those estimated by ICOLD method. The results of potential risk analyses of the dam within the Euphrates basin are totally given in table 3. The values of TRF range from 55.0 to It means that there is no any dam having a risk class of IV and I in the basin. There are twenty dams at risk class of II and twelve dams at risk class of III. In other words, half of total dams are approximately identified as the risk class of III, while the rest are identified as class of II. The dams having high-risk class are concentrated on the certain part of basin. This study identified at least eleven large dams of the Euphrates basin, which must be reanalyzed by selecting appropriate seismic parameters. They are Alparslan I, Çat, Erzincan, Gayt, Keban, Kigi, Konaktepe, Kuzgun, Ozluce, Palandoken, Polat, Surgu, Tercan and Uzuncayir dams. Rehabilitation design and construction measures, if necessary, may follow after in cases where the dams are found deficient seismically. Especially Kigi, Alparslan I and Erzincan dams should seismically be reanalyzed with considering local attenuation relationships of dam sites. Keban, Kigi and Ozluce dams, which are identified as risk class of III with high risk and have a structural height greater than 100 m, were mainly designed for producing electricity. They also have large reservoirs and under construction or operation stages. Therefore, these dams have an important role on Turkish economy and high risk for downstream life. A detailed evaluation is made for these dams in the following paragraphs. 4. CONCLUSION Large reservoirs, which are constructed near urbanized area, have a potential risk for downstream life and property. Therefore, total risk analyses must also be performed for the existing dams. The seismic analysis is the main component of total

11 BASIN WATER MANAGEMENT 487 risk analysis. The author believes that seismic criteria and analysis parameters for dams should be selected more conservatively than for conventional structure. The seismic performance of embankment dams closely depends on the nature and state of compaction of the fill and foundation materials. There can be some stability problems due to earthquake shaking for embankment dams. Aging is an important aspect for old concrete dam. As a result of the study, it seems necessary that a National Safety Program for dam structures should be prepared in Turkey. The priority of proposal should be for dams, which are under the effect of near seismic zone. For old embankment dams of Euphrates basin, detail seismic analyses must be performed and all structures, which have high risk potential, should be redesigned to provide their earthquake resistance and in general to protect public safety and property. REFERENCES Ambraseys, N.N., (1995) The Prediction of Earthquake Peak Ground Acceleration in Europe Earthquake Engineering and Structural Dynamics, V.24, Boore, D.M. and Joyner, M.B., (1982) The Empirical Prediction of Ground Motion Bull. Seism. Soc. Am., V.72, N.6, Bozkurt, E., Neotectonics of Turkey-a Synthesis. Geodinamica Acta, 14, Bureau, G.J., (2003) Dams and Appurtenant Facilities in Earthquake Engineering Handbook edited by Chenh, W.F and Scawthorn, C. CRS press, Bora Raton Bureau, G and Ballentine, G.D., (2002) A Comprehensive Seismic Vulnerability and Loss Assessment of the State Of South Carolina Using HAZUS. Part VI. Dam Inventory and Vulnerability Assessment Methodology 7 th National Conference on Earthquake Engineering., July 21-25, Boston, Earthquake Engineering Research Institute, Oakland, CA. Campbell, K.W., (1981) Near-Source Attenuation of Peak Horizontal Acceleration Bulletin Seism. Soc. Am., V.71, N.6, Campbell, K.W. and Bozorgnia, Y., (1994) Near-source attenuation of peak horizontal acceleration from Worldwide Accelerograms Recorded from 1957 to 1993 Proc. On Fifth U.S. National Conference on Earthquake Eng. Chicago, Illinois, July 10-14, DSI (2002) National Catalogue on Dams and Hydroelectric Power Plants in Turkey. State Hydraulic Works, Ankara. Erdik, M., Doyuran, V., Gulkan, P., Akkas, N., (1985) Evaluation of Earthquake Hazard in Turkey with Statistical Approach. Middle East Technical University Earthquake Engineering Research Center, Ankara, 116 p (in Turkish).

12 488 INTERNATIONAL CONGRESS ON RIVER BASIN MANAGEMENT FEMA, 2005, Federal Guidelines for Dam Safety-Earthquake Analyses and Design of Dams. Federal Emergency Management Agency. ICOLD (1989) Selecting Parameters for Large Dams-Guidelines and Recommendations. ICOLD Committee on Seismic Aspects of Large Dams, Bulletin 72. Joyner, W.B. and Boore,D.M. (1981) Peak Korizontal Acceleration and veleocity from Strong-Motion records Including Records from 1979 Imperial valley, Bullettin of Seisnological Society of America, V.71, N.6, KHGM, 2004, List of dams constructed by KHGM General Directorate of Rural Services, Ankara (unpublished) Kramer, S.L., (1996) Geotechnical Earthquake Engineering, Prentice-Hall, Upper Saddle River, NJ 653 p. McKenzie, D.P., (1972) Active Tectonics of the Mediterranean Regions Geophys. J.R. Astr. Soc. 30, p. Saroglu, F., Emre, O., and Kuscu, I., (1992) Active Fault Map of Turkey General Directorate of Mineral Research and Exploration, Ankara. Tosun, H., (2002) Earthquake-Resistant Design for Embankment Dams Publication of General Directorate of State Hydraulic Works, Ankara, 208 p (in Turkish) Tosun, H. and Seyrek, E., Seismic hazard analyses and risk classification of large embankment dams in Turkey. Dam Safety 2005-Annual Conference of ASDSO, Orlando. Tosun H. and Seyrek, E., (2006). Seismic studies for embankment dams in Turkey Int. Water Power and Dam Construction, Feb Tosun, H., Zorluer, İ., Orhan, A., Seyrek, E., Savaş, H. and Türköz, M. (2007) Seismic hazard and Total Risk Analyses for Large Dams in Euphares basin, Turkey Engineering Geology 89, Yucemen, S., (1982) Seismic Risk Analysis Publication of Middle East Technical University, Ankara, 160 p (in Turkish).