By Hasan Tosun
To determine the safety of dams in southeast Turkey in the event of an earthquake, the Turkish Association of Dam Safety conducted a seismic risk analysis on 42 structures. Analysts found five of the dams would potentially be unsafe during an earthquake. Further analysis is planned to determine work needed to increase the structures’ stability.
Earthquake safety evaluation for dams depends on both the seismic hazard rating of the site and the risk rating of the structure. The seismic hazard of the site can be determined using the peak ground acceleration (PGA), which is a measure of the ground motion. The risk rating of the structure is based on structural (reservoir capacity and dam height) and socioeconomic (evacuation requirements and potential for downstream damage) components.
In general, the seismic and risk ratings are evaluated separately. Recently, combining the two factors allowed determination of the total risk factor for 42 large dams in the Euphrates-Tigris Basin in Turkey. The total risk factor is defined as a summation of risk factors for capacity, height, evacuation requirements, and potential damage. Then, based on the total risk factor, four risk classes are defined as low, moderate, high, or extreme. Determining the risk class of a dam aids in the selection of seismic evaluation parameters and the method or methods to be used to analyze its seismic safety.
As a result of this analysis, researchers developed a seismic hazard map showing the PGA values for the region, calculated based on the defined earthquakes for each dam site. This map includes all existing dams in the basin as well as those planned. Researchers concluded that five large dams in the basin have a high or extreme hazard classification.
Based on this assessment, work must be done to reevaluate the seismic safety of these dams. We recommend that the dam owner perform a dynamic analysis for the dam based on the data obtained from the seismic hazard analysis and improve the structure, if necessary.
A review of the Euphrates-Tigris Basin
Turkey is one of the world’s most seismically active regions. In the past decade, more than 50,000 residents have died during earthquakes, commonly as a result of building failures. Many dams have suffered significant structural damage, although no failures have been reported. The most well-known earthquake damage to a dam in Turkey occurred during the 1986 Dogansehir Earthquake, with a magnitude of 5.8 on the Richter scale. The epicenter of the earthquake was only about 5 kilometers from Surgu Dam. Damages to the embankment dam – consisting of 10-centimeter-wide longitudinal cracks at the upper part of the crest and as much as 20 centimeters of settlement at the crest – were a result of the strong motion of the fault.
The Euphrates-Tigris Basin, the largest river basin in Turkey, yields about 53 cubic kilometers of water per year. The basin is in a region with very complex geology and active seismicity. The geological setting and the data used to analyze earthquake safety of dams in the basin are based on previous geological investigations and engineering geological studies performed in this region.1,2
For the study described in this article, researchers represented the geology of the basin by seven separate units, ranging in age from Precambrian to Quaternary. The oldest units, which are made up of metamorphic schists, are in the central part of basin. There is no problem with water leakage or foundation stability at the dam sites in this part. However, small landslides can be observed along the main bank of the rivers. The younger units are mainly composed of sedimentary rocks including marl, mudstone, limestone, and shale alternation. These are soft and soluble rocks that have low durability.
The Euphrates-Tigris Basin is affected by the North Anatolian and East Anatolian fault zones. The North Anatolian Fault zone is one of the best-known strike-slip faults in the world. It is about 1,500 kilometers long and extends from eastern Turkey to Greece. It ranges in width from a single zone a few hundred meters wide to multiple shear zones 40 kilometers wide. This fault zone produces large earthquakes, which have caused the deaths of a thousand people, as well as severe damage to buildings.3 This fault zone joins the East Anatolian Fault zone at Karliova and forms a typical triple junction (the cross point of three linear fault systems).
The East Anatolian Fault zone, which is 550 kilometers long, is a northeast-trending, sinistral strike-slip zone. It is comprised of a shear zone that includes a series of faults parallel to each other. Within the basin, it extends from the triple junction at Karliova to the Kahramanmaras area in the southwest, where it meets and forms a triple junction with the Dead Sea Fault zone. The left-lateral slip along the East Anatolian Fault zone contributes to the westward extrusion of Anatolia. Several pull-apart basins (plains developed by tension forces) complicate the fault zone. There are also conjugate fractures, folding, and a considerable thrust component.4,5,6 This system has produced large, serious earthquakes. There are many dams located on or close to this shear zone.
Dams in the basin
The Euphrates-Tigris Basin includes the main part of the US$32 billion Southeast Anatolian Project, a development that incorporates irrigation, hydropower, agriculture, rural and urban infrastructure, forestry, education, and health. The water resources component includes building 22 large dams and 19 hydro plants with a total capacity of 7,476 mw to irrigate 1.7 million hectares of land. Work on this development began in 1976 and is expected to be complete in 2012.
There are two important rivers in the basin. The 2,800-kilometer-long Euphrates River is the main river. The Euphrates River crosses Iraq to join the Tigris River, where it flows into the Persian Gulf. Average discharge in the river upstream from where it joins the Tigris River is 650 cubic meters per second (cms), compared with 950 cms where the combined rivers cross the Syrian border.
On the Euphrates River and its tributaries, 32 large dams have been designed to impound water for hydro projects and to provide irrigation. Twenty-three are entirely completed. Seven are under construction.6 It is estimated that they will be entirely finished at the beginning of 2009. Construction of the last two dams on this river is scheduled to begin by the end of 2008.
On the Tigris River and its tributaries, ten large dams are planned. Five of them have been completed and two (Ilisu and Hakkari) are under construction. The other three – Cizre, Cukurca, and Silvan – are being designed. Construction of these three dams is expected to begin by the end of 2009.
Analyzing seismic risk
The total risk posed by an earthquake to a dam and its appurtenant structures depends on various hazards, including:
- Factors affecting the structure, such as settlement, seepage, leakage, and internal erosion;
- Factors affecting the site of the dam, such as seismic activity, landslide, and rockfall; and
The goal of this research was to evaluate seismic hazard and local site effects (i.e., liquefaction of soil, the position of ground water, etc.) for 42 large dams – with a structural height greater than 30 meters – in the Euphrates-Tigris Basin (see Table 1).
Seismic hazard is the main factor affecting the total risk for dams. The seismic hazard of a site can be based on the PGA, which is derived from the defined design earthquake for the site and produces the main seismic loads. For a preliminary study, an existing map of PGA can be used to estimate the seismic hazard. The PGA map also can be used for a dam site with a low risk class, which is defined based on reservoir capacity, dam height and age, evacuation requirements, and potential for downstream damages.7,8
For a thorough seismic hazard analysis of a site, all possible seismic sources are identified and their potential is evaluated in detail, as based on existing guidelines.9,10 The study of seismic activity for dam structures includes deterministic and probabilistic seismic hazard analyses.
The deterministic seismic hazard analysis involves a four-step process. This simple procedure works well for analyzing large dams because it provides a straightforward framework to evaluate the worst ground motions. Due to the unavailability of strong motion records in Turkey, various attenuation relationships are adopted to calculate the PGA acting on dam sites.11,12,13,14 There are two methods available to estimate the PGA acting on a site. Specifications for dam construction in Turkey suggest using the deterministic seismic hazard analysis, which was done for this study. For this study, two predictive relationships for horizontal PGA were considered.11,13
The probabilistic seismic hazard analysis considers uncertainties in size, location, and the recurrence rate of earthquakes. This type of analysis provides a framework under which uncertainties can be identified and combined to provide a more complete picture of the seismic hazard.15 The seismic hazard parameters to define the maximum credible earthquake for each zone were estimated using probabilistic methods. For this study, the parameters were estimated using the computer program DAMHA, developed at the Earthquake Research Center at Eskiàžehir Osmangazi University, following the Gutenberg-Richter and Gumbel methods.16,17
The Gutenberg-Richter method estimates the magnitudes of future earthquakes, using a statistical scheme in which all past earthquakes, of almost any size and number, are included in the computations. It is assumed that the seismic history data contains sufficient information about the magnitudes of the past earthquakes of the selected zone, and a straight-line relationship is defined between the number of earthquakes and their magnitudes.
The Gumbel method is similar to the Gutenberg-Richter method. However, using this method, the lack of historical data and the deficiencies of the relevant earthquake files prepared by the various state organizations are compensated for by selecting only the maximum earthquake magnitude for any particular year.
To identify the seismic sources and estimate the recurrence interval of earthquakes, researchers used a seismic zonation map of Turkey. Through an extensive survey by the National Disaster Organization and a literature search, researchers identified several sources to help analyze the seismic hazard of dams in Turkey.18,19,20 Data from historic and 20th century earthquakes for Turkey and the vicinity, collected by the National Disaster Organization, were considered as a basis for the seismic hazard analyses. The earthquakes that occurred within the past 100 years were used to estimate seismic parameters. Throughout the study, seismic zones and earthquakes within a radius of 100 kilometers around each dam site were considered. As a result of the detailed evaluation, the area covering the basin was separated into 11 seismic zones.
The Turkish Association of Dam Safety analyzed seismic risk at 42 dams in the Euphrates-Tigris River Basin, including Ozluce Dam show here, to determine their safety in the event of an earthquake.
The operating basis earthquake, which is the earthquake that produces the ground motions at the site that can reasonably be expected to occur within the service life of the dam, was defined using the probabilistic methods mentioned above. The maximum credible earthquake, which is the largest earthquake magnitude that could occur along a recognized fault or within a particular seismotectonic province or source area, was defined for each zone. Researchers chose the controlling maximum credible earthquake as the most critical value for the dam site. Finally, researchers determined the maximum design earthquake, which is characterized by a level of motion equal to that expected at the dam site from the occurrence of the maximum credible earthquake.21 In this case, the maximum design earthquake is equal to the maximum credible earthquake. Most of the large dams in Turkey were analyzed using these definitions.22
For all analyses performed throughout this study, the PGA was deterministically obtained by considering the maximum design earthquake value, which is an earthquake with a 10 percent probability of being exceeded in 50 years. The DAHMA program can execute all the procedures mentioned above, then map the results using a geographic information system (GIS) program to obtain the equivalent PGA values for the area being investigated.
Results of the analysis
In the Euphrates-Tigris Basin, 344 earthquakes have had a magnitude – on the basis of a surface wave – of greater than 4.0. The number of earthquakes with magnitudes greater than 5.0 and 6.0 are 66 and nine, respectively. Only two earthquakes with a magnitude equal to or greater than 7.0 have occurred in the basin – in 1939 and 1949.
Table 2 on page 42 shows the results of the seismic hazard analyses for the 20 dams in the basin classified as moderate, high, or extreme risk. The system used to assign a hazard class to the risk at each dam is based on one developed by the International Commission on Large Dams (ICOLD).7
The table does not include 22 dam sites having a low hazard rating, identified as a hazard class of I.
Fifteen dam sites have a moderate hazard rating, classified as II.
A high hazard rating, classified as III, occurs for sites with a PGA value greater than 0.25 gravity but source of seismic energy that is more than 10 kilometers from the site. Dams with this rating are Alpaslan I, Kigi, and Uzuncayir.
An extreme hazard rating, classified as IV, occurs if the PGA is greater than 0.25 gravity and the energy source is less than 10 kilometers from the dam site. Erzincan, in the northern part of the basin, has a hazard class of IV. It is close to the epicenter of a catastrophic 1992 earthquake and can be subjected to a PGA of 0.564 gravity, with a maximum design earthquake of 7.9. Surgu, which was damaged during the Dogansehir earthquake with a magnitude of 5.8 in 1986, also has a hazard class of IV, with a PGA of 0.256 gravity.
Interestingly, Alpaslan I’s risk rating is similar to Erzincan when the adverse geotechnical properties of foundation soil are considered. The foundation material of this dam is composed of claystone and siltstone.
There are many dams in the basin – such as Atatà¼rk, Birecik, Karakaya, Karkamis, and Keban – that can result in serious conditions for downstream life and property if they are damaged or fail. For Birecik and Karkamis dams, the PGA values are low, so they are judged to be seismically safe sites. Atatà¼rk is the largest dam in Turkey, with a storage capacity of 48,700 cubic hectometers (hm), but it also is on a site having low seismicity. On the other hand, Karakaya, which is a concrete dam, is subjected to relatively high earthquake loads (with a PGA of 0.132 gravity) when compared with other key structures in the basin.
All the dams on the Tigris River and its tributaries, with the exception of Hakkari, have a low hazard class. The Hakkari site was classified as moderate risk. Ilusu, which will have the second largest reservoir in Turkey when it is completed in 2010, poses low seismic activity and is classified as low risk.
The Polat and Cat dams, in the west part of basin, are in the zone of the East Anatolian Fault that is frequently jointed, fractured, and faulted. The dam sites are classified as moderate risk. The PGA is 0.170 gravity for Polat and 0.211 gravity for Cat. Surgu, which was damaged by the Dogansehir earthquake, is also included within this zone. Consequently, Polat and Cat dams could be similarly influenced by local near-source zones and thus have a high risk rating for earthquake conditions. The damage to Surgu from the Dogansehir earthquake confirms this premise.23
As a result of seismic hazard analyses, the author developed a map showing the equivalent PGA values for the large dams in the basin. A GIS program, namely MapInfo Professional from MapInfo, was used to build this map. The PGA values were plotted on the map, and these values were interrelated to estimate equivalent curves, which indicate points with the same PGA values. This map shows that the most critical area in the basin is very close to the North Anatolian Fault zone, because of the greater PGA values. However, there are also some isolated areas showing different seismological behavior. The most critical one is the area that includes the Surgu, Polat, and Cat dams, along with numerous small dams. The other critical area is on the eastern part of the basin, which includes the Alpaslan I and Alpaslan II dams.
Applying the results
Applying a risk analysis method allows identification of the weak element or elements in a complex dam system. Safety then can be increased by improving these elements. Based on this analysis, the most critical zone for the basin is the North Anatolian Fault. Several dams in this region – Erzincan, Kigi, Tercan, Uzuncayir and Alpaslan I – are earth structures and have critical PGA values. Erzincan and Alpaslan I are less than 10 kilometers from the fault zone, meaning they are under the effect of a near source zone. In addition, the maximum design earthquake value for the first three dams is greater than 7.2.
Most of the dams, which are near active seismic zones, can be damaged or fail during earthquakes.
All the dams that are within the shear zone in the west part of the basin are under the influence of a local near-source zone. The damage or failure risk is high for these dams, when subjected to a loading of a near field earthquake with moderate magnitude (5.9 to 6.1 gravity). Thus, their seismic positions need to be re-evaluated in detail.
The results of this research provide an understanding of the seismic effects on the dams in the Euphrates-Tigris Basin. The next step for the researchers will be to perform more detailed seismic hazard and dynamic analyses using the finite element method. Then researchers can suggest that dam owners improve the stability of large dams in the basin, if necessary.
Dr. Tosun may be reached at Earthquake Research Center, Civil Engineering Department, Osmangazi University, Bati Meselik Eskisehir, Turkey; (90) 222-2394215; E-mail: htosun@barajguvenligi. org.
Hasan Tosun, Prof. Dr., is a professor and geotechnical consultant in the Civil Engineering Department at Osmangazi University in Turkey and president of the Turkish Association of Dam Safety. He performed the analysis described in this article.
This article has been evaluated and edited in accordance with reviews conducted by two or more professionals who have relevant expertise. These peer reviewers judge manuscripts for technical accuracy, usefulness, and overall importance within the hydroelectric industry.
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