Mumbai: India has 4,407 large dams of which more than 1,000 would be 50 years or older by 2025, a new study has shown.

Older dams pose greater safety risks, cost higher in terms of maintenance and have declining functionality due to sedimentation, stated a study by the Canada-based United Nations University Institute for Water, Environment and Health, released in January. Climate change is also likely to accelerate the ageing of dams, it said.

India must conduct a cost-benefit analysis of its ageing dams, and conduct timely safety reviews in order to ensure their operational and ecological safety, as well as the safety of those who inhabit the areas downstream, experts told IndiaSpend.

Key findings

All over the world, many large dams built in the 20th century may start to show signs of ageing, and many may already be operating at or beyond their design life, the study said. For India, 2025 is set to be a big year as more than 1,000 dams would turn roughly 50 years or older.

Fifty years is not a defining age for all dams because the design life also depends on factors such as construction and maintenance, the study said, adding that a well constructed and well maintained dam can go up to a 100 years. However, 50 is roughly the age when a dam may begin to show signs of ageing. In some cases, dam components such as gates and motors may need to be replaced after 30 to 50 years.

Therefore, an assessment of the structure should be carried out, and, if needed, the dam should be decommissioned, states the study.

Do dams have a life expectancy?

About 55% of the world's dams are in just four Asian countries, including India. "Dams play a significant role in water supply, energy production, flood control, and irrigation," Duminda Perera and Vladimir Smakhtin of the United Nations University and authors of the study told IndiaSpend. "However, we identified that most of the water-storage dams are getting older, leading us to bring this study to the surface. There are no other studies to discuss this emerging threat on a global scale."

Structural problems can occur at any stage in a dam's lifespan, but without regular inspections and repairs, ageing increases the risks, said Perera and Smakhtin. "In our report, we selected 50 years as the arbitrary age for a dam to show ageing signs," they said, adding that several kinds of age-related problems can crop up, such as decay or deterioration of the structural materials used in construction, ageing of other components such as gates and spillways, and sedimentation (when silt settles at the bottom over time, reducing the dam's storage capacity). Most studies have focused on ecological degradation--the negative impact of man-made structures on biodiversity and the environment--due to dam construction, and not on the safety of ageing dams, they said.

With the careful implementation and regulation of practices recommended by the International Commission on Large Dams (ICOLD), dam structures could last hundreds to thousands of years, said Pramod Narayan, project director at Dam Rehabilitation and Improvement Project (DRIP). "With proper engineering, good construction and regular maintenance, the dams that we design and build today can continue to serve many generations to come," he said, and contested the study's conclusions saying, "Modern high hazard dams, such as the ones constructed after 1980, have been designed for extreme events such as 'probable maximum' flood and 'maximum credible' earthquakes that have a probability of one in thousands of years. Moreover, the average age of Indian dams is 42 years, which is much younger in comparison to some of the other leading countries."

India's waters

India has 4,407 large dams, the third highest number in the world after China (23,841) and the USA (9,263). Over 1,115 large dams will be about 50 years old by 2025. In less than 30 years, by 2050, over 4,250 large dams would pass 50 years of age, with 64 large dams being 150 years old, the study said, citing a 2019 paper published in the Economic & Political Weekly.

In 2018, ICOLD had modified its definition of "large dams", but India continues to use the previous definition. Hence, the dams considered in the study--which was based on ICOLD's World Register of Dams--are a subset of the number listed by the Indian government. India has 5,334 large dams, according to 2019 data from the Jal Shakti ministry's Central Water Commission.

India's dams are more vulnerable to deterioration because a large proportion of them are earthen--built by compacting successive layers of earth, and not concrete--and are hence more prone to ageing, said Himanshu Thakkar, coordinator of South Asian Network on Dams, Rivers and People (SANDRP). "Secondly, the country gets concentrated rainfall every year for a designated time period as opposed to distributed rainfall, which contributes to the dams' vulnerability. Thirdly, siltation, which is the accumulation of silt and debris behind the reservoir, leads to a reduction in the storage capacity of the dams. The actual siltation rate of the dams is higher than what is estimated in the proposals. Since siltation rates determine the costs and benefits of a dam, operators tend to underestimate the costs and overestimate the benefits when pushing for sanctions. The Central Water Commission's recent study on the Srisailam project on the Krishna river also found that the dam's storage capacity was reduced as a result of siltation. There are several more studies that clearly show that the actual siltation rates are several times higher than what was estimated," added Thakkar.

Furthermore, in India, the downstream areas are often exposed to flood disasters even without a dam breach, in which water creates an opening in a dam due to rapid erosion of a section of the embankment. Non-breach flooding from dams happens when they are not operated properly. "Monsoon is the time when dams fill up. What usually happens is that dams are filled up right at the beginning of the rainy season instead of at the end, which means that in case of heavy rainfall in the upstream as well as downstream areas, the dam would have to release water downstream causing heavy flooding in the downstream areas," Thakkar said, adding, "The safety performance of India's dams is much worse than what the UNU report states."

Flooding caused 44% of dam failures in India, while the remaining were caused by other factors, including inadequate spillway capacity, piping and poor workmanship, as per the Central Water Commission.

Another important dimension is how climate change affects ageing dams. The changing rainfall patterns and fluctuations mean the inflow into the dams cannot be predicted and may be a lot different than what was assumed during the process of designing, warned K.J. Joy, senior fellow at the Society for Promoting Participative Ecosystem Management and convenor, Water Conflict Forum. "When it comes to dam safety, earlier design parameters may not hold anymore," he added.

The example of the 126-year-old Mullaperiyar Dam on the Periyar River in Kerala downstream to Tamil Nadu is cited in the UNU report. A gravity dam with a full reservoir level of 152 ft, it shows significant structural flaws and might be at a risk of failure even as an interstate conflict between Kerala and Tamil Nadu rages over it with the former asking for the water levels to be steady whereas the latter demanding water levels to be increased. A gravity dam is one which is designed to withstand water by its own weight and resistance. The weight and width of the base prevent the dam from overturning when subjected to the force of impounded water.

The government of Kerala carried out hydrological review studies between 2006 and 2011 that concluded that the Mullaperiyar Dam is unsafe for passing the estimated probable maximum flood limit. However, the Central Water Commission scrutinised Kerala's report and declared it as not well-founded. The vulnerability of earthen dams has to be reassessed, said Thakkar.

The Mullaperiyar Dam also happens to stand atop a seismically active area categorised in Zone III (moderate risk) by the seismic zone mapping of India, said R. Ajayakumar Varma, former chief scientist at the National Centre for Earth Science Studies. "This area is close to a large number of major faults in this region, some of which are active and others have the potential to become active," Varma told IndiaSpend. "This is a high hazard dam as per the criteria fixed by the Central Water Commission and its catastrophic potential is very high considering the instability of the region. In this context, it is extremely necessary to get the structural stability of the dam verified… especially so because the Mullaperiyar is a century-old weak dam."

Apart from this, some of the Himalayan dam systems, including the Tehri Dam, are in an active seismic area given that the Himalayan mountain system is constantly changing and growing giving rise to several tectonic movements, said Joy. The Koyna Dam in Maharashtra and the downstream Warna Dam on the Warna river, a tributary of Krishna, also lie in a highly sensitive area. A major earthquake in 1967 had led to widespread damage, post which Koyna became a point of study for reservoir-induced seismicity, he added.

Decommissioning dams

Decommissioning is the process of dam removal or demolition and letting the river flow its natural course. It is often undertaken when a dam suffers from structural flaws, its maintenance costs are mounting, concerns public safety or poses ecological risks.

In India, the concept has not caught on as decommissioning of dams is considered a rather sacrilegious act, said Joy. "It has largely been a discussion limited to environmentalists and civil society groups such as activists," Joy told IndiaSpend. "It is not right to make a blanket statement saying all dams have outlived their purpose. A case-by-case assessment should be made by reviewing dams that have reached 50-60 years of age and an informed decision should be taken."

However, decommissioning a dam also has multiple socio-economic impacts. "The cost of dam removal is influenced by many site-specific factors, including treatment of accumulated reservoir sediments, stream restoration, and loss of operational benefits such as flood control, water supply, power, etc.," said Narayan of DRIP. "While there are exceptions, the experience of our industry largely has been that regular refurbishment and good upkeep will be more beneficial to society and less expensive than dam removal."

Besides the economics of it, a risk management decision such as decommissioning is also based on public safety and environment factors. The larger emerging risk than ageing is the increase in unregulated downstream development without consideration of the corresponding risk escalation, warned Narayan. "Society has been quick to privatise development gains and socialise development risks. That is, profits from flood-plain development are kept by the developers, while damages from developing in known flood risk areas are left at the mercy of insurances and government reimbursement of losses," he added.

Ensuring dam and public safety

Projects that pose serious safety risks should be listed and then decommissioned in a phased manner through a consensus of all the stakeholders, suggested Joy.

The most important aspect in ensuring dam safety is the existence of accountability and transparency while taking into consideration the views of the real stakeholders--the people living downstream from the dams, who are the most at-risk group in case of a breach, said Thakkar. "We need management committees for every dam where fully independent voices and experts beyond the government agencies as well as those representing the vulnerable downstream communities will have a say in the dam safety policy paradigm."

In terms of the operational safety, the rule curve, that decides how a dam is supposed to be operated and is created when a dam is proposed, needs to be upgraded at regular intervals on the basis of environmental changes such as siltation and rainfall pattern since these would change the frequency and intensity of incoming flood into the dam as well as the spillway capacity, agreed both Joy and Thakkar. "The rule curve also needs to be in the public domain so that the people can keep a check on its correct functioning and can raise questions in its absence," added Thakkar.

Moreover, in India every river has multiple dams along its course, so a cumulative assessment of every upstream and downstream dam needs to be in place to ensure dam safety in terms of operations, he said.

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