Special report: How safe will Diamer Basha dam be?
In the morning hours of October 8, 2005, a magnitude 7.6 earthquake violently shook the mountainous regions of northern Pakistan. In less than a minute of strong ground shaking, over 87,000 people were killed, among them 19,000 children, many of whom were in school buildings which collapsed. Three and a half million people were left homeless and thousands of buildings plus much of the infrastructure suffered severe damage.
Despite the widespread damage and high economic losses, reconstruction was done quite efficiently and today very few signs of this earthquake can be seen in the most severely hit city of Muzaffarabad, except for the scars from huge landslides. Most people are unaware that the magnitude of this event was still less than that of a worst-case earthquake that could occur in this highly seismic region.
Experts who have been studying the Himalayan Tectonic Interface belt for decades are predicting future earthquakes of magnitudes up to M=8.7 in the region, resulting in estimates of over a million casualties! Due to rapid growth of the population and the economy, the seismic risk is expected to grow exponentially with time if no urgent actions are taken.
Two of the foremost experts on seismic aspects of large dams raise troubling questions about the current design of Pakistan’s proposed mega-dam project
As strong earthquakes cannot be predicted reliably, the only way to reduce the seismic risk and to protect the people is to build houses and other structures which can resist the effects of strong earthquakes. Seismic building codes and guidelines for different types of structures exist and are used worldwide.
Following the Kashmir Earthquake, Pakistan managed to update its building code — with at least one of the authors of this article — participating in the effort, to include many modern requirements for earthquake safe design and construction of structures. However, the main challenge is to effectively enforce these guidelines and codes, which can generally be done easily for new buildings and structures.
On the other hand, equal attention must be paid to existing structures, which is a more challenging process. The identification, evaluation and upgrading of seismically deficient existing structures will be a long-term process, requiring not only significant investment, but first requiring an increase in public awareness about the danger of occupying deficient structures which may not have been designed to safely resist earthquakes, or which were designed with methods that are outdated today. An additional difficulty is the quality of work done by contractors, which is often an even greater concern. Government authorities must first acknowledge the risk and then start taking adequate steps to address it. Ignoring or minimising earthquake risk is not a solution; in fact, it is a prescription for disaster.
Along with rapid national development in nations such as Pakistan, which are located in highly earthquake prone regions, new risks emerge from the construction of major public works and infrastructure projects, such as power plants and large storage schemes for hydropower generation, flood control, water supply and irrigation. It is important that these essential projects do not lead to an undue increase in seismic risk.
A particularly important category of projects exposed to high seismic hazard are the large dams, as these must be built where water is available and can be stored economically. In the case of other types of projects, such as thermal or even nuclear power plants, sites can be selected where the local conditions are much more favourable than at dam sites.
The main water storage projects planned in Pakistan are in the upper reaches of the Indus River and its tributaries. This is a region of very high seismicity located literally on top of the subduction zone of a continental tectonic interface, which has been the cause of the creation of the highest mountain ranges of the world, the Himalayas and Karakoram ranges.
One of the mega projects planned in that region is the Diamer Basha Dam, which is slated to be one of the tallest and highest concrete gravity dams in the world. Among the main technical challenges for this project, and other similar projects in this region, are the high seismicity, high flood discharge during the monsoon season, landslides and the difficult geological and topographic site conditions. Additionally, there are major logistical challenges with the transport of huge quantities of construction materials, along with serious security issues.
The primary planning and design issues for the Diamer Basha Dam project are structural safety and sustainability. This implies that the dam must be designed to safely operate for a service-life of well over 100 years. Today, as an example, the new Brenner railway tunnel between Austria and Italy will be designed for a service-life of 200 years. Similar criteria must be used for Diamer Basha. Not only does this mean the utilisation of top-quality construction materials, especially concrete, and high-quality construction works, but it also obligates the concerned authorities to ensure the highest level of care, competence and diligence during the feasibility, planning and design phases of such a project.
Experts who have been studying the Himalayan Tectonic Interface belt for decades are predicting future earthquakes of magnitudes up to M=8.7 in the region, resulting in estimates of over a million casualties.
A highly sophisticated and suitably conservative assessment of the seismic hazard, prepared and overseen by top-level international specialists, must be formulated and implemented for the detailed design of this new dam. Such a process is necessitated by the fact that a mega-dam of this type, in a region such as its location, has not yet been designed and built anywhere in the world.
Moreover, there is very little data from even somewhat similar types of dams located in very high seismic regions that might otherwise add a level of confidence to an ordinary or customary seismic hazard assessment and seismic design process for this world-precedent concrete gravity dam. It must be noted that strengthening the dam once it has been completed will be extremely costly, therefore, there is no alternative to creating and implementing a robust, unquestionably vetted, state-of-the-art earthquake safe design for this project.
It is well known to experts in this field that the seismic hazard is more complex than the flood hazard, as it includes ground shaking, fault movements and landslides. The latter cause impulse waves, or tsunamis, in the reservoir. Although a lot is said about climate change, to deal with the consequential changing flood hazard, from the engineering point of view, is less complex than dealing with the seismic hazard.
As mentioned above, there are multiple effects from strong ground shaking that must be included while calculating the seismic hazard for a high-quality dam, which must allow for the high uncertainties in seismic hazard. One key aspect of seismic safety for large dams is the need to control the water level in the reservoir after a strong earthquake and the provision of low-level outlets to lower the reservoir, in case there are doubts about the safety of a dam — especially from large aftershocks which occur following a strong earthquake.
In addition to the seismic-resistant design of the main dam body, one must also focus on seismic-resistant designing of the appurtenant structures and the electro-mechanical equipment that form the entire dam complex. The elements of the facility that are required to control and lower the water level in the reservoir after a strong earthquake must be operational after an earthquake. If this is not the case, the dam may be overtopped days or weeks after a destructive earthquake.
After reviewing the available material related to past studies and the present design of the Basha Dam project, and having discussed these issues at length with renowned seismic experts who have actually worked on the Basha Dam project, plus other dam projects in Pakistan, here is a summary of our thoughts on the matter as follows:
As a concrete gravity high dam located in one of the most intense tectonic regions of the world, the Diamer Basha Dam project represents a tremendous opportunity for Pakistan to achieve something that is heretofore unprecedented in the world. However, along with the potential benefits and opportunity come some seriously high risks which must be properly recognised and adequately managed.
The unique nature of the project and the consequences of possible earthquake-induced damage — or even total failure — of this mega-dam demand that extraordinary care is taken in assessing the seismic hazard, developing the seismic design basis and design criteria and executing the detailed seismic design of the dam and its appurtenant parts.
For such a process to be properly executed, there is no alternative but to engage top-level international seismic specialists, not just dam engineers, to perform the highly specialised seismic analysis and design work. It is common practice to also engage an independent third-party peer review standing committee, which includes experts in the seismic design and safety assessment of large dams as well as flood and hydrology experts. This committee remains fully integrated and active throughout the design and construction phases of the project.
Since the 2004 feasibility study of the Basha Dam Project, and even thereafter, two important earthquakes have occurred, i.e. the 2005 Kashmir Earthquake and the 2008 Wenchuan Earthquake in China, which provided new aspects of the seismic hazard for large storage dams in mountainous regions. These are the large number of landslides and rockfalls, which have been neglected or underestimated in most dam projects internationally.
Furthermore, greater consideration is now being given, in the field of seismic design, to the vertical component of seismic accelerations. Thus, the current seismic hazard assessment might be incomplete.
From the information gathered, it seems that the maximum seismic acceleration proposed for the dam design is less than half of the actual value experienced during the 2005 Kashmir Earthquake! Therefore, further in-depth investigations are still needed, as even with the best of intentions, the country may be headed towards the creation of a situation of unduly high seismic risk, potentially jeopardising billions of dollars and perhaps thousands of lives downstream.
The bulk of the work completed thus far on this project was done during the tenures of the preceding two governments. Many Pakistanis seem to believe that the recent elections have brought to power a new government which is reportedly much more committed to meritocracy, excellence, judiciousness, transparency and accountability in public affairs. If so, the current government must take immediate measures, along the lines of those recommended above, to ensure that such hugely expensive and critical public works projects of national importance are indeed designed, overseen and built according to the highest standards available in the world today. The Pakistani people deserve nothing less.
Saif M. Hussain is a Pakistani-American structural/earthquake engineer practicing in California. His project experience includes Hub and Kalabagh Dams as well as post-Kashmir Earthquake research, reconstruction projects, training and capacity building programs. He is also a founding member of the United States Resiliency Council.
Martin Wieland is Chairman of the Committee on Seismic Aspects of Dam Design of the International Commission on Large Dams. He is a civil engineer based in Switzerland, who has been involved in several large dam projects located in highly seismic regions, mainly in Asia.
Header photo: The 285-metres-high Grande Dixence gravity dam in Switzerland, which was completed in 1961, is currently the world's highest gravity dam. The Diamer Bhasha and Dasu dams will have a similar height.
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Published in Dawn, EOS, April 7th, 2019
Risky business
By: Khurram Husain
Eos asked Wapda, the lead authority overseeing the construction of Diamer Basha dam, about the seismic design of the project, and whether they have factored in the heightened risk of earthquakes in the region when drawing up the design parameters. Specifically two questions were posed to the authority, both asking for the range of ground shaking that the dam has been designed to withstand.
Earthquakes involve two kinds of ground shaking that have to be taken into consideration when designing structures in hazardous zones. There is vertical acceleration, the speed with which the ground is expected to rise and fall in a possible earthquake, and the horizontal acceleration, which is the speed with which the ground is expected shake from side to side in an earthquake. Eos asked Wapda what range of vertical and horizontal acceleration they have used in the design parameters of Diamer Basha dam.
In vertical acceleration, Wapda replied saying the range they have factored is 0.146g as the operating basis for an earthquake, and 0.247g as the maximum design earthquake that the structure will be designed to withstand. In horizontal shaking, the operating earthquake assumption used in the structure’s design is 0.22g and the maximum design earthquake parameter is 0.37g.
The water and power development authority’s seismic design parameters indicate they may be woefully inadequate
Wapda explained that they arrived at the values by measuring the level of shaking that was observed at the dam site during the October 2005 earthquake in Kashmir. They say that two strong motion accelerographs have been installed at the dam site since 2002, and after August 2007, they have 10 additional micro seismic stations within a 50km-diameter of the dam site to also measure the small, almost imperceptible earthquakes that occur in that region with some regularity. Their response says that the accelerographs at the dam site measured horizontal ground shaking of up to 0.11g on rock and 0.015g on alluvium during that event. Their response adds that “[s]eismic criteria after 2005 earthquake have been duly considered in the design of the dam and its structures.”
But the data raises very important questions. The ranges identified by Wapda are more or less consistent with the seismic tolerance ranges given in the Building Code of Pakistan 2007. In that code, the peak horizontal ground acceleration that should be accounted for in Zone 3 of Pakistan (where the dam site is located) is 0.24g to 0.32g. The Wapda range for the dam structure is very similar to this.
There are two big problems that this data has. First is that the Building Code of Pakistan 2007 specifically says that its tolerance ranges are not applicable to dams as well as a host of other structures that are larger than small houses, including power stations and transmission lines, military installations, tunnels and pipelines and more. In fact, the code specifically says that its provisions are for “buildings and building-like structures” only.
The second big problem is that the ground shaking parameters are taken from the October 2005 earthquake, which occurred hundreds of miles away from the dam site. The question naturally arises: what if the next earthquake is closer to the dam site? The ground-shaking in an earthquake decreases by the square of the distance as one moves away from the epicentre. If done properly, the seismic design parameters should assume that the epicentre of the next mega earthquake will be directly under the dam itself, then draw up a tolerance range and multiply it by five, at least, to arrive at a safe provision for the seismic hazard that the dam faces.
At the moment, the seismic design parameters that Wapda has derived for the dam and its associated structures (power house and tunnels, transmission lines, etc) is grossly inadequate to the seismic risk that the structure is exposed to.
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The writer is a member of staff
Published in Dawn, EOS, April 7th, 2019
Until March 11, 2011 no people died from the failure or damage of a large water storage dam due to an earthquake. However, during the magnitude 9.0 Tohoku earthquake in Japan in 2011, an 18.5-metres-high embankment dam failed and the flood wave created by the release of the reservoir caused the loss of eight lives.
Earthquakes have always been a significant aspect of the design and safety of dams. A large storage dam consists of a concrete or fill dam with a height exceeding 15 metres, a grout curtain or cut-off to minimise leakage of water through the dam foundation, a spillway for the safe release of floods, a bottom outlet for lowering the reservoir in emergencies, and a water intake structure to take the water from the reservoir for commercial use. Depending on the use of the reservoir there are other components such as a power intake, penstock, powerhouse, device for control of environmental flow, fish ladder, etc.
During the Richter magnitude 8 Wenchuan earthquake of 12 May 2008, 1803 concrete and embankment dams and reservoirs and 403 hydropower plants were damaged. Likewise, during the 27 February 2010 Maule earthquake in Chile of Richter magnitude 8.8, several dams were damaged.
Excerpts from a policy paper prepared by the International Commission on Large Dams
However, no large dams failed due to any of these two very large earthquakes.
What earthquake action does a dam have to withstand?
In order to prevent the uncontrolled rapid release of water from the reservoir of a storage dam during a strong earthquake, the dam must be able to withstand the strong ground shaking from even an extreme earthquake, which is referred to as the Safety Evaluation Earthquake (SEE) or the Maximum Credible Earthquake (MCE). Large storage dams are generally considered safe if they can survive an event with a return period of 10,000 years, i.e. having a one percent chance of being exceeded in 100 years. It is very difficult to predict what can happen during such a rare event as very few earthquakes of this size have actually affected dams. Therefore, it is important to refer to the few such observations that are available.
The main lessons learnt from the large Wenchuan and Chile earthquakes will have an impact on the seismic safety assessment of existing dams and the design of new dams in the future. There is a basic difference between the load-bearing behaviour of buildings and bridges on the one side, and dams.
Under normal conditions, buildings and bridges have to carry mainly vertical loads due to the dead load of the structures and some secondary live loads. In the case of dams, the main load is the water load which, in the case of concrete dams with a vertical upstream face, acts in the horizontal direction. In the case of embankment dams, the water load acts normal to the impervious core or the upstream facing. Earthquake damage of buildings and bridges is mainly due to the horizontal earthquake component.
Concrete and embankment dams are much better suited to carry horizontal loads than buildings and bridges. Large dams are required to be able to withstand an earthquake with a return period of about 10,000 years, whereas buildings and bridges are usually designed for an earthquake with a return period of 475 years. This is the typical building code requirement, which means the event has a 10 percent chance of being exceeded in 50 years.
Depending on the risk category of buildings and bridges, importance factors are specified in earthquake codes, which translate into longer return periods, but they do not reach those used for large dams.
Moreover, most of the existing buildings and bridges have not been designed against earthquakes using modern concepts, whereas dams have been designed to resist against earthquakes since the 1930s. Although the design criteria and analyses concepts used in the design of dams built before the 1990s are considered as obsolete today, the reassessment of the earthquake safety of conservatively designed dams shows that, in general, these dams comply with today’s design and performance criteria and are safe. In many parts of the world, the earthquake safety of existing dams is reassessed based on recommendations and guidelines documented in bulletins of the International Commission on Large Dams (ICOLD).
Seismic hazard is a multi-hazard
Earthquakes represent multiple hazards with the following features in the case of a storage dam: ground shaking causes vibrations and structural distortions in dams, appurtenant structures and equipment, and their foundations; fault movements in the dam foundation or discontinuities in dam foundation near major faults can be activated, causing structural distortions; fault displacement in the reservoir bottom may cause water waves in the reservoir or loss of freeboard; rockfalls and landslides may cause damage to gates, spillway piers (cracks), retaining walls (overturning), surface powerhouses (cracking and puncturing and distortions), electromechanical equipment, penstocks, masts of transmission lines, etc; mass movements into the reservoir may cause impulse waves in the reservoir; mass movements blocking rivers and forming landslide dams and lakes whose failure may lead to overtopping of run-of-river power plants or the inundation of powerhouses with equipment, and damage downstream; ground movements and settlements due to liquefaction, densification of soil and rockfill, causing distortions in dams; and abutment movements causing sliding of and distortions in the dam.
Effects such as water waves and reservoir oscillations (seiches) are of lesser importance for the earthquake safety of a dam. Usually the main hazard — which is addressed in codes and regulations — is the earthquake ground shaking. It causes stresses, deformations, cracking, sliding, overturning, etc.
However, some of the other hazards, which are normally not covered by codes and regulations, are also important. Therefore, in the earthquake design of dams all seismic hazard aspects must be considered and depend on the local conditions of a storage dam project.
What could happen in the case of strong earthquakes?
The experience with the seismic behaviour of large dams is still limited. Four dams, two concrete, an earth core rockfill and a concrete face rockfill dam, with heights exceeding 100 metres were damaged during the Wenchuan earthquake. Elsewhere, there are another five concrete dams over 100 metres in height, which were damaged due to strong ground shaking.
In concrete dams, the damage was mainly in the form of cracks but also joints can open up leading to the release of water from the reservoir. In modern embankment dams, the damage is mainly by deformations and cracks along the crest that can eventually lead to internal erosion and piping through the dam.
However, we have to be aware that each dam is a prototype located at a site with special site conditions and hazards. Therefore, based on the observation of the earthquake behaviour of other dams it is still very difficult to make a prediction of the damage that could occur in a particular dam.
At this time, we are still in a learning phase as very few large modern dams have been exposed to strong earthquakes. In the case of the Wenchuan earthquake, a large number of rockfalls took place, which caused significant damage to dams and appurtenant structures. Surface powerhouses were particularly vulnerable to rockfalls in the steep valleys in the epicentral region of the Wenchuan earthquake.
The greatest hazard of a dam is the water in the reservoir. Therefore, in the seismic design of dams, we have to ensure the safety of the dam under full reservoir condition. Although an arch dam may be more vulnerable to the effect of ground shaking when the reservoir is empty, this case is not critical for the safety of the people living downstream of the dam but it is, of course, an important economical issue for the dam owner if the dam should fail.
Can earthquakes be triggered by storage dams?
There are a number of cases where earthquakes were triggered by the reservoir. The main prerequisites for reservoir-triggered seismicity (RTS) are: (i) the presence of an active fault in the reservoir region, or (ii) existence of faults with high tectonic stresses close to failure. The filling of the reservoir in a tectonically active region may merely cause the triggering of an earthquake which, in any case, would occur at a later date. The occurrence of RTS has been mainly observed in reservoirs with a depth exceeding 100 m. Six events with Richter magnitudes of more than 5.7 have been observed up to now. The largest magnitude was 6.3. In two cases, concrete dams were damaged, i.e. Hsinfengkiang buttress dam in China and Koyna gravity dam in India. Since records did not exist on the local seismicity prior to dam construction, there are still doubts whether these large earthquakes have actually been triggered by the reservoirs.
If a large dam has been designed according to the current state-of-practice, which requires that the dam can safely withstand the ground motions caused by an extreme earthquake, it can also withstand the effects of the largest reservoir-triggered earthquake. However, RTS may still be a problem for the buildings and structures in the vicinity of the dam, because they would generally have a much lower earthquake resistance than the dam. In the great majority of RTS, the magnitudes are small and of no structural concern.
The issue of RTS is always discussed in connection with large dams. RTS was also considered in connection with the Wenchuan earthquake as the Zipingpu reservoir is located close to the ruptured segment of the Longmenshan fault. However, there is no conclusive evidence that the earthquake was triggered by the filling and the operation of the reservoir.
What are the main concerns with respect to the earthquake safety of large storage dams?
The main concerns are related to the existing dams, which either have not been designed against earthquakes — this applies mainly to small and old dams — and dams built using design criteria and methods of analyses which are considered as outdated today. Therefore, it is not clear if these dams satisfy today’s seismic safety criteria. There is a need that the seismic safety of existing dams be checked and modern methods of seismic hazard assessment be used. This will result in a dam which will perform well during a strong earthquake.
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Published in Dawn, EOS, April 7th, 2019