Risk Management of Large Dams under Operation Stage and Case Studies for Cascade System

Abstract

In recent specifications, there are regulations stating that there should be no accidents on construction sites. In fact, construction-related risks have cultural, social, and technological dimensions, and risks can be minimized by appropriate identification of these factors. Although there are no great social and cultural uncertainties in the applications carried out so far, the technological dimension, especially the use of digital technologies, has been at a minimum level. Currently, the construction industry investigates how social-technical systems interact during the construction of all large engineering structures, including dam structures, and their effects on risks at construction sites. However, after the completion of the construction of these structures, management of the risk posed by the relevant structures for the environment is a very separate issue. This study focuses on the total risk of dam structures that have been constructed and are in the operational stage. Due to damage or failure of a dam, downstream life is affected and huge hazards may occur. In fact, major disasters may occur depending on the amount of water to be discharged. For this purpose, the risk of dams is defined according to their type, height, amount of water they store, age of the dam, downstream evacuation requirement, and potential damage area. In newly presented methods, local ground effects are also included. In addition, the fact that the relevant structure is in a cascade system affects the degree of hazard that will occur and increases the total risk of the system. In this study, the parameters affecting the total risk of dam structures in the operational stage are summarized, various total risk methods are mentioned, the total risks for two separate rivers located in the largest basin of the country are discussed and the necessary measures to be taken on the basis of risk management are suggested. Finally, a new concept is introduced for risk management of dams in the cascade system.

1. Introduction

Every construction project, regardless of size, scope, complexity, nature, or location, has unique risks. Failure to manage relevant projects correctly also increases the risk and causes losses to the balance sheet. It should be emphasized that a risk, even if small, in the project may create uncertainties during the implementation phase and lead to negative consequences that may affect the success of the project. Additionally, new risks may arise at different stages of a project. However, it is clear that high risks waste resources, increase project costs, reduce efficiency, and extend project completion time. Therefore, it is of great importance to correctly identify and manage risks at every stage of the project. Risk definitions are of greater importance in the management of water resource structures such as dams and storage facilities, which are built for public service purposes and are vital for living life downstream.

Although dams provide benefits such as domestic and industrial water supply, electrical energy production, and agricultural irrigation, they may cause some environmental impacts. These affect communities through resettlement and other socio-economic impacts, result in environmental concerns, and create sedimentation problems in the long term. In addition to all the negative consequences, if they collapse, they can cause major disasters resulting in great loss of life and property. These problems and concerns need to be alleviated through proper planning and incorporating various improvement techniques according to the demands of society. Hariri-Ardebeli [1] suggests that these expectations can be most effectively met by implementing systematic risk management that includes factors such as sustainability and public participation during dam planning, construction, and operation stages.

According to 2016 data in the USA, there are over 90,000 small or large dams and approximately 18% of them are defined as high-hazard dams. It has also been observed that the average lifespan of dams is over 50 years. It has been stated that at least 64 billion dollars are needed to meet the current safety requirements of these dams across the country [1]. The number of large dams in Turkey is close to 1500 and their average age exceeds 30 years. These dams have costs of up to 10 billion dollars for changing physical conditions and updated safety requirements. Significant damage occurred in 40 dams by two major earthquakes (Mw7.8 and Mw7.6) on February 06, 2023 [2, 3] and the cost of the damage to these dams was determined as 4.0 billion dollars [4]. All these developments necessitate the effective use of water resources for social life (water and food safety as well as the danger of downstream life), and the need for all risks (especially during the operation phase) to be well identified and managed effectively.

Safety concern for a dam structure is mainly affected by ground motion and result in loss of stability and some physical defects in the structure [5]. Pre-planning for dams and earthquake mitigation generally uses simplified procedures. An assessment of the overall stability of the structure should be included. Simplified procedures are generally used for pre-planning earth-rockfill dams. The analysis of concrete dams exposed to earthquakes must include an assessment of the stability of the entire structure to withstand lateral forces and moments [6, 7]. Various methods from simplified analysis to sophisticated techniques are used to analyze seismic loads acting on concrete structures. It should be noted that seismic analysis is the first step in determining the overall risk of a dam structure.

This chapter considers a portfolio of dams on two tributaries of the main river of Euphrates Basin. The main River, with its tributaries of Murat, Perisuyu, and Karasu has a 2800 km length. It flows through Iraq into the Gulf in the south. The flows upstream and on the Syrian border are 650 cubic meters per second and 950 cubic meters per second, respectively. Within the basin, 40 large dams are constructed to use the basin’s energy and irrigation potential. This article assesses the overall risk of 11 large dams in the two Euphrates River cascade systems. In other words, this study summarizes the safety of dam structures during the operational phase, especially with regard to the risks to life downstream, highlights the methods used for the overall risks of the structures involved, and highlights the risks posed by dams.

The author discusses individual structures and cascading structures in the country’s largest watershed. Table 1 introduces the general characteristics of dams. This paper refers to some studies done previously [9, 10, 11]. The author and his co-workers have completed many international and national projects about dam safety and risk management of earth structures relating to water resources and introduced a lot of studies in congress, symposium, and journals. These studies mainly concentrate on dam safety concepts in majoring and risk management of dams and their appurtenant structures in minoring [12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28].

#	Dam	River	Height from river bed (m)	Function (^*)	Type (**)	Completed year	Reservoir capacity (hm^3)	Installed capacity (MW)
1	Keban	Euraphate	163	E + F + I	CG + RF	1975	31,000.0	1330
2	Agri-Yazici	Murat	84	I + WS	EF	2001	202.9	—
3	Alparslan-1	Murat	91	E	RF	2009	2993.0	160
4	Alparslan-2	Murat	108	E + I + WS	ACEF	2020	2431.0	280
5	A.Kalekoy	Murat	93	E	CG + ACRF	2020	516.5	500
6	Beyhan-1	Murat	84	E	RCC	2015	369.1	582
7	Beyhan-2	Murat	42	E	CG	—	78.9	264
8	Gayt	Murat	36	I	EF	1998	23.0	—
9	Gulbahar	Murat	66	I	EF	2003	14.1	—
10	Patnos	Murat	38	I	EF	1992	33.4	—
11	Y.Kaleköy	Murat	138	E	RCC	2018	783.8	636.6
12	Kalecik	Peri	34	I	EF	1987	12.5	—
13	Kigi	Peri	146	E	RF	2016	507.6	140
14	Konaktepe	Peri	119	E	RF	u/d***	450.0	201
15	Ozluce	Peri	124	E	RF	1998	1075.0	170
16	Pembelik	Peri	75	E	HF	2015	358.0	127
17	Seyrantepe	Peri	34	E	EF	2008	23.0	56
18	Tatar	Peri	73	E	EF	2013	300.0	128
19	Uzuncayir	Peri	58	E	EF	2009	308.0	82

Table 1.

Characteristics of large dams on two tributaries of Euphrates [8].

^**

RF: Rockfill EF: Earthfill CG: Concrete gravity HF: Hardfill ACRF: Rockfill with asphaltic core ACEF: Earthfill with asphaltic core.

^***

Under designing.

2. Methods and materials

Risk management is a key issue in dam construction [1]. Technically speaking, risk is described as the likelihood and severity of adverse impacts acting on life, environment, and property. According to an international specification [29], the risk is generally predicted by combining the probability of its occurrence and the impact of the scenario on the associated outcome and defined by a simple formula (Eq. (1)). In this equation, P means conditional probability depending on variables X and Y and also C represents outcomes [9].

$$Risk=\int P\left[LoadEvents\right]\times P[Responses|Loads]\times C\left[Loads,Responses\right]$$E1

In the construction industry, the term “risk” can primarily refer to economic concerns, but can also refer to life loss. Technical specifications mostly use a “risk curve” to measure safety. Seismic hazard, on the other hand, represents the uncertain relationship between seismic intensity level and the probability that at least excitation level will occur at the given location. The hazard curve represents a graph that correlates the seismic intensity of a particular location on the horizontal axis and the annual exceedance frequency in vertical axis [7].

There are various techniques to quantify a dam’s overall risk factors. The ICOLD method takes into account seismic hazards belonging to dam site and structure’s risk separately [30]. In this method, the seismic hazard can be classified into four groups, regardless of the dam type [31, 32]. This is an easy manner to evaluate earthquake hazard via way of means of figuring out the hazard class. ICOLD introduces that the whole risk of dams includes structural and socio-monetary factors [33]. The first is especially based upon at dam height and ability of the reservoir. The second one is the capability for downstream damage and evacuation requirements. The summation of these four risks represents the Risk Factor (RF). Four danger classes, which can be used for deciding on seismic parameters of floor motion, are described on the idea of the RF values. ICOLD states that big dams (peak is extra than 90 meters and a storage potential of extra than 1200 hm³) require unique protection considerations [30].

In general, a dam hazard evaluation must be decided primarily based totally on reservoir capacity, dam height, evacuation requirements, and capability downstream damage. Generally, seismic evaluation and risk evaluation are evaluated separately. However, Bureau [33] combines those elements to outline the overall hazard factor for dam structures (TRF), as given in Eq. (2). In the equation, TRF refers to total risk factor. The first three phrases cowl dam-based elements, and HRF covers dam-based elements. Downstream risk as a feature of population and properties at risk is described as a feature of specific elements (Eq. (3)) [34, 35]. In this equation, ERF is the evacuation demand component and DRI is the downstream damage risk index. The Downstream Evacuation Requirement (ERF) component relies upon the population at risk. The Downstream Damage Risk Index is primarily based on the value of private, commercial, and industrial properties in the capability flood path. Finally, the PDF shows the anticipated damage component (Predicted Damage Factor), which is a feature of the dam’s determined overall performance in addition to the location-based seismic hazard.

$$TRF=\left[\left(CRF+HRF+ARF\right)+DHF\right]\times PDF$$E2

$$DHF=ERF+DRI$$E3

Vulnerability evaluation represents the location-based seismic risk described by the Predicted Damage Factor (PDF), which is assigned to every dam consistent with Eq. (4). In this equation, PDI is the Predicted Damage Index, calculated the usage of the dam fragility curve advanced with the aid of using Bureau and Ballentine [36]. The PDI relies upon the dam type and the seismic risk of the dam site and is calculated as a feature of the Earthquake Severity Index (ESI), which represents the predicted ground motion on the dam site as given with the aid of using Eq. (5). In this equation PGA is peak ground acceleration in g; and M is the Richter or moment magnitude.

$$PDF=2.5\times PDI$$E4

$$ESI=PGA\times \left(M-4.5\right)$$E5

The author proposed a qualitative evaluation framework that shows the relative impact of dams within a cascade system. Five different risk rates (very low, low, moderate, high, and very high) are defined for every structure in the system. The principles are listed in Table 2 according to dam type, reservoir storage capacity, and dam location. All dams fall into two basic groups: “rigid” and “flexible.” “Flexible” refers to earth-fill dams, rock-fill dams, and combinations thereof; “rigid” refers to concrete dams (gravity, arch, and buttress), roller-compacted concrete dams, and hardfill dams. Another factor considered is the number of downstream dams that may fail. This is based on the relative storage capacity of the dam. The dam risk assessment considered how much damage downstream dams would suffer if the dam completely collapsed. This model is based on the number of dams in the downstream area and assumes that the rigid dam acts as an overall spillway structure and is not damaged. For composite types, it may make sense to consider weaker ones. The author first mentioned this problem in ICOLD symposiums held in Prague and Chania in 2017 and 2019, respectively [37, 38] and in a recent study published in an international book project [39]. This problem was also considered by some scientists from east-south Asia [40, 41].

#	Dam type*	Number of dams that are potentially under failure downstream (N)	Risk ratio for cascade system (RRCS)
1	Rigid	—	Very low
2	Flexible	—	Low
3	Flexible; Rigid	<2; 1–3	Moderate
4	Flexible; Rigid	2–3; 3–4	High
5	Flexible; Rigid	≥3; ≥4	Very high

Table 2.

Risk ratio for dams of a cascade system.

*

Flexible means earthfill dams, rockfill dams, and their combination while rigid describes concrete dams (gravity, ach, buttress), roller compacted dams and hardfill dams.

3. Case studies and discussion

In the study, risks in dams that are in operation are discussed and risk management of dams in individual and cascade systems is emphasized. For this purpose, some dams on the Upper Euphrates river located in the east of Turkey were examined by considering two separate cascade systems. There are 11 dams on these two separate systems, located on the Murat river and Perisuyu tributaries in the Upper Euphrates. The country’s second-largest reservoir (Keban dam), and two dams on the Munzur stream, which is considered a part of the Perisuyu system have also been included to the system considered in the study. Individual and cascade systems of dams were evaluated on the basis of total risks and recommendations were made for risk management. All analyses were carried out depending on the seismo-tectonic models in the regions [42, 43, 44]. The projects and their results, which were done previously, have been considered in this study to describe the problem in this study [20, 45, 46].

The first cascade system is located on the Murat river. This system records a total of six separate large dams having heights between 42 and 127 m. The structures located on the main river from downstream to upstream are Beyhan-2, Beyhan-1, Lower Kalekoy, Upper Kaleköy, Alparslan-2, and Alparaslan-2 dams (Figure 1a). The total installed power of these dams, which were built mainly for energy production, is 2337 MW and it is planned to produce 5740 GWh of electrical energy annually. In addition, together with some other dam structures on the tributaries, it is envisaged to irrigate an area reaching 120,000 ha. Two large dams located upstream were constructed in embankment type (rock fill and asphaltic core rockfill) and the other four dams were in rigid type (concrete gravity, roller compacted dam). Figure 1a shows locations and some basic features of the cascade structure of the dams. The related data about dam characteristics are given in Table 1.

Figure 1.

Cascade systems of tributaries of main river of Euphrates basin: (a) Murat river and (b) Peri-Munzur river [8].

The system evaluated in this study and forming the second cascade structure of the basin is located on Perisuyu. These structures, whose heights vary between 34 and 146 m; It consists of Kığı, Özlüce, Pembelik, Seyrantepe, and Tatar dams from upstream to downstream. Two separate dams (Konaktepe and Uzunçayır) on the Munzur stream, which constitutes an important side branch of this system, were also considered as a part of the system. All of these dam structures, except one (Pembelik dam), were planned as fill type (rock fill and earth fill). The total installed power of these dams is 624 MW and they have the potential to produce 1766 GWh of electrical energy annually (Table 1 and Figure 1b).

First of all, seismic hazard analyses were carried out for all dams considered in the study and the seismic parameters that should be taken into account in terms of total risk were determined. On this basis, the hazard rates for the 11 dams taken into consideration are in the “high-very high” range (Table 3). Based on the ICOLD method, the risk factor was 36, which is the highest value of the related classificaiton (excluding one dam), and the risk class was determined as between III and IV, and the risk rate was predicted as “high-very high.” According to the Bureau method, total risk factor values vary between 143.3 and 229.1, and the Risk Class and Risk Ratio values for all dams are III and “High,” respectively (Table 3).

#	Dam	Hazard analysis		Total risk [30]			Total risk [33]			Risk for cascade system*
		Class	Hazard ratio	Risk factor	Risk class	Risk ratio	Total risk factor	Risk class	Risk ratio	V (hm^3)	N	Type	RRCS
Murat tributary
1	Alparslan-1	IV	Very high	36	IV	Very high	148.2	III	High	2993	2	RF	High
2	Alparslau-2	III	High	36	IV	Very high	201.0	III	High	2431	1	ACRF	High
3	Y.Kaleköy	IV	Very high	36	IV	Very high	168.7	III	High	784	1	RCC	Moderate
4	A.Kalekoy	III	High	36	IV	Very high	143.3	III	High	517	—	ARCF	Low
5	Beyhan-1	IV	Very high	36	IV	Very high	173.5	III	High	369	—	RCC	Very low
6	Beyhan-2	IV	Very high	36	IV	Very high	173.5	III	High	79	—	CG	Very low
	Keban	II	Moderate	36	IV	Very high	126.4	III	High	31,000
Munzur tributary
1	Konaktepe	III	High	36	IV	Very high	168.0	III	High	450	1	RF	Moderate
2	Uzuncayir	III	High	36	IV	Very high	212.3	III	High	308	—	RF	Low
	Keban	II	Moderate	36	IV	Very high	126.4	III	High	31,000
Perisuyu tributary
1	Kigi	III	High	36	IV	Very high	176.1	III	High	508	3	RF	Very high
2	Ozluce	IV	Very high	36	IV	Very high	174.5	III	High	1075	2	RF	High
3	Pembelik	IV	Very high	36	IV	Very high	229.1	III	High	358	2	HF	Moderate
4	Seyrantepe	IV	Very high	24	III	High	162.9	III	High	23	1	EF	Moderate
5	Tatar	III	High	36	IV	Very igh	147.5	III	High	300	—	EF	Low
	Keban	II	Moderate	36	IV	Very high	126.4	III	High	31,000

Table 3.

Risk for individual and cascade systems for dams of the basin.

(^*)

V=Storage capacity at Maximum Water Level; N: Number of dams at which are potentially under failure in downstream; ype: RF = Rockfill; EF = Earthfill; CG = Concrete gravity; HF=Hardfill; ACRF = Rockfill with asphaltic core; RCC = Rollered Compacted Concrete RRCS = Risk Ratio for Cascade System.

The author believes that the risk of dams during the operation stage should be evaluated together with all elements within the cascade system. For this purpose, he introduced a new proposal. The results of this study showed that individual risk definitions of all dams within the two selected cascade systems were formed similarly. However, it is seen that the dams in the cascade system do not affect the system at the same impact value in terms of the stability of the system. Within the principles suggested by the author, among six large dams in the cascade system of Murat river, Alparslan-1 dam has and Alparslan-2 have a “high” risk rate while Yukarı Kaleköy dam poses “moderate” risk. Lower Kaleköy, Beyhan-1, and Beyhan-2 dams are represented with “low” and “very low” risk rates.

In the Perisuyu-Munzur cascade system, the risk rates of Kığı and Özlüce dams were found to be “very high” and “high,” respectively. Konaktepe, Pembelik, and Seyrantepe dams have a “medium” risk rate, and Uzunçayır and Tatar dams have a “low” risk rate.

Keban dam, which is located downstream of these two cascade systems and has a large storage capacity (31,000 hm³), has the location and capacity to compensate for the flooding resulting from all dams upstream. It is necessary to realize a specific program of operation for Keban dam and others located downstream to provide a stable condition for all system upstream.

4. Conclusions

Risk management during the operation phase of dam structures differs from the risk management of other engineering structures. Therefore, instead of independent risk assessments of structures, it is appropriate to evaluate the risk of the cascade structure as a whole. The safety of cascade dams must be considered with a new concept from that of single dams. Collapse of a cascaded dam can lead to flooding or collapse of downstream dams. The author assumes that the probability of dam failure within a cascade structure is greater than that of a single dam. Concrete dams cannot fail when they are overtopped by flood waves. Because they operate as a spillway during flooding. There are so many river power plants constructed as concrete structures, which had a functional operation, without damages or minor damages after the Wenchuan earthquake in 2008. Similar cases have been observed in Kahramanmaraş earthquakes with Ms. of 7.6 and 7.8 occurred on February 06, 2023. An embankment dam can fail to release reservoir water when it is overtopped by a flood wave. Therefore, embankment dams in cascade systems are more dangerous structures when compared with concrete dams for unexpected cases. Within the scope of this study, a risk identification has been made for the cascade structure and a qualitative assessment basis that reveals the most critical structure(s) in the system has been proposed. For the study, Özluce and Kigi dams are the critical structures when considering the Perisuyu-Munzur cascade system while Alparslan-1, Alparslan-2, and Y. Kaleköy dams are ones for Murat River cascade system of Upper Euphrates basin. The risk concept for the cascade system of dams, which was proposed in this study should be developed in the future. The author states that the proposal is just a beginning stage to start the discussion on risk management for dams of cascade systems in dam engineering.

Conflict of interest

The author declares that there is no conflict of interest.