Dams induce severe floods

They were built for flood protection by attenuating rivers’ flow

By Bharat Lal Seth
Published: Wednesday 14 September 2011

Each year, floods, to greater or lesser degree, affect about 7.5 million hectares in India, a little over two per cent of the country's surface area. What's more, over five times this area is considered prone to floods, caused when rivers breach their banks.

But this year, and of late, it's not just heavy rains, but the operation of dams that have induced floods, when water released from a dam reservoir is beyond the carrying capacity of channels downstream. A dilemma faced by dam operators is that filling the reservoir early in the monsoon may result in excess discharge and floods later on, while delay in filling the reservoir risks not having enough water for drinking and irrigation the next lean season.

Operators bend curve rule

September has emerged as a risky period for dam-induced floods. This was seen in the case of the Hirakud dam on the river Mahanadi this year. Large quantum release from its reservoir displaced over 2 million people in 20 of the state’s 30 districts. The India Meteorological Department’s (IMD) analysis of the data of the past 100 years says there is no deviation, suggesting a significant change in rainfall patterns. “But yes, excessive variability and localised events towards end-August and September are forcing release of huge quantum of water from dam reservoirs,” says Ajit Tyagi, director general of IMD. But extreme and variable weather patterns are not the only menace.

Many dams are built for the purpose of flood protection, to attenuate flow and its impacts downstream. The inflow and outflow of water from a reservoir are determined by an operating schedule or rule curve, which maintains a flood cushion in expectation of inflows expected to fill the reservoir. “Rule curve revision may be required periodically, but changing weather patterns is rarely the reason for it. More often, rule curve revision is required due to changed priorities and reduced carrying capacity of the downstream channel,” says Chetan Pandit, chief engineer with the Central Water Commission.

But the operation of the reservoir during an extreme event, like a severe flood, is not determined solely by the rule curve. It is determined by the reservoir level at the start of the flood. Water level may be temporarily allowed to exceed the upper limit prescribed by the rule curve for that date, to absorb a part of the flood, and thus provide flood moderation, says Pandit. The problem arises when the rule curve is not abided prior to an extreme event and a larger release is deemed necessary.

According to activists, the operators sitting in the hot spot are often, as in the case of the Ukai dam releases that flooded Surat in Gujarat in 2006, slow in reacting to information on inflow of water and rainfall in the catchment. The typical official response in the case of such events is: there was a sudden influx of water due to incessant rains in the catchment; there is no way to avert these disasters. According to a study by the South Asia Network of Dams Rivers and People, a non-profit, neither the flood cushion of the Ukai dam, nor the operation rules were reviewed in light of the decrease in the carrying capacity of the downstream channel.

Based on the past observations of inflow as the monsoon progresses, rule curve advises a minimum and a maximum level for each date, with a view to maximize the probability of having a full reservoir at the end of the monsoon. The balance that is not absorbed by the flood cushion is released through dam gates or a spillway to ensure safety of the civil structure.

Faulty operation

In theory, this works; all multi purpose dam projects are expected to attenuate floods, by keeping the reservoir levels optimal. But the postulation is dependent on proper operations of the reservoir schedule. Often, this is not followed and the reservoir is filled beyond levels prescribed in the rule curve. “This is happening with Hirakud and even in the Krishna basin where Andhra Pradesh as the downstream state is apprehensive it wouldn't get the requisite floodwaters to fill its reservoirs later on,” says Hanumantha Rao, former engineer-in-chief with the Andhra Pradesh irrigation department. “This fear complex compromises flood cushioning, putting lives and public property at risk,” says Rao. The reservoir operation should carefully build up the storage levels, allowing for controlled release of floodwater.

“There is an inherent contradiction in multi-purpose dam projects that look to maximise storage for irrigation and energy generation for the following year,” says Shripad Dharmadhikary, founder of Manthan Adhyayan Kendra, a non-profit that researches water issues. The other problem with dams, says Dharmadhikary, is that they attenuate lower magnitude floodwater, thereby creating a sense of security. “So when the peak floods come, the damage is far greater than otherwise”.

"Dams and embankments are an outdated and too often counter-productive way to manage floods,” says Lori Pottinger of International Rivers, a US non-profit working to defend the rights of communities that depend on rivers. “Flood problems caused by poor management of dams are likely to worsen as climate change brings more intense storms,” she says. Dams built to moderate floods, says Pandit, release incoming waters over a longer period of time. Often, there is no inundation, while a very high flood may impinge a reservoir that is almost full. “But even then the inundation is invariably less than what would have been if there was no reservoir,” says Pandit.

Setting the guidelines

A committee was constituted in the 1980s by the Ministry of Irrigation, to review the existing practices of maintenance of dams in various states and to evolve standard guidelines for the same. This committee is also setting the principles for reservoir operation.

The report, published in 1982, says that it is essential to release water as soon as flood-like situations develop. The report admits that “the existence of a dam upstream of the floodplain may either mitigate the extent of the calamity or accentuate it.” It is essential, therefore, to develop operating rules with built-in factor of safety and adequate and efficient warning system to formulate accurate forecasts on the time lag when rain in the catchment results in increased inflow to the reservoir.

The report made its way to the Parliament as the Bill on Dam Safety, 2010. The Ministry of Water Resources drafted it and defined a dam failure as: “such failures in the structures or operation of a dam which may lead to uncontrolled release of impounded water resulting in downstream flooding, affecting the life and property of the people.” In the event of such a failure, the owner of the dam is responsible, says the bill. In August earlier this year, a standing committee, while presenting its recommendations to Lok Sabha, recommended a statutory provision for punishment, saying it will be a deterrent and add to the sense of responsibility among the dam operators. The bill is yet to be passed.

“In case of a large reservoir mid-river, the catchment below the reservoir may be capable of generating a flood comparable to the design flood at the reservoir. This is true of the Hirakud and of many other reservoirs. Sometimes, it may so happen that there is a local flood from the rainfall in the catchment below the reservoir, and at the same time it becomes necessary to release large quantity of water from the reservoir,” says Pandit. If this happens, there can be severe inundation.

But Pandit insists dams control floods. “No flooding has no news value, and therefore moderation of many successive floods by a reservoir goes unnoticed,” he says.



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