Why Hirakud dam failed to check flood

The river has strayed from its flood control route

By Richard Mahapatra
Published: Wednesday 14 September 2011


There is no sign of floods abating in Odisha. In the space of a fortnight, the number of people affected by the deluge in the Mahanadi river, the state's largest riverine system, increased from one million to 2.2 million on September 13. Two-thirds of the state's 30 districts are affected.

The flood has impacted all those districts along the Mahanadi which are downstream of Hirakud multipurpose dam in Sambalpur district. This happened despite the fact that the dam, commissioned in 1958, was meant to control floods in the delta areas. Right now, the water level is receding in west Odisha, while the districts along the coasts like Cuttack, Jagatsinghpur and Puri continue to be marooned because of flood waters. A massive relief operation is underway amid allegations about its inadequacy.

The Hirakud dam, just a few inches short of its maximum storage level—the reservoir level was 629.08 feet (191.7 metre; one foot equals 0.30 metre) against the full reservoir level of 630 ft—opened 59 gates on September 9. It unleashed a flood considered the worst ever in the state. The flood of 2008 was the last such severe flood after the one in 1982. The gates remained open for two consecutive days. Going by what officials in charge of the dam have to say, unprecedented rainfall in Chhattisgarh, which accounts for 85 per cent of the river's catchment area, led to a huge inflow of water into the dam's reservoir that used to be hardly ever full. On September 7, the dam's storage level was at 625.60 feet when 10 gates were open. This meant that within 48 hours the Mahanadi received five times more water.

During this time, the entire state received severe rainfall because of low pressure building up over the region. This compounded the severity of the flood. Within four hours of the opening up of the gates, Sambalpur, which is immediately downstream of the Hirakud, got flooded. In places like Sonepur, a district headquarter town some 80 km from the dam, the river's level rose by a foot an hour during the night of September 9. Going by the flood level markers in the town, the current flood broke the record of the past 53 years. By September 11, the delta areas got inundated. The full moon of September 12 causing high tides in the Bay of Bengal severely restricted the outflow of flood water into the sea.

Flood lingers

Before details about the exact degree of devastation emerges, there is a deluge of another kind. Experts, and the public in general (their voices are more subdued), attribute the current flood entirely to the mismanagement of the Hirakud dam, the state's biggest instrument of flood control. The gist of their criticism is this: for 23 years the dam has not changed its flood control strategy while the rainfall pattern has undergone major changes in local areas.

The experts believe the dam is responsible for the severity of the flood, a perception that has been around since 1982. Since then, almost all severe floods in the state have been triggered by the abrupt opening of the dam's gates—2002, 2008 and 2011. There is a widespread demand that the dam authorities junk the strategy called the rule of curve (more on this later) and adopt a completely new one based on the present rainfall pattern both upstream and downstream of the dam. This involves an agreement and flood coordination with Chhattisgarh state; currently there is no such mechanism. In fact, during the period between September 5 and 9, most of the dams on the Mahanadi in Chhattisgarh were 90 per cent full following heavy rainfall. In the absence of a coordination mechanism, the state released water, thus leading to sudden increase in water flow to the Hirakud.

The inflow of water from Chhattisgarh is not the sole reason. The dam has no mechanism to monitor flow of water into it from many tributaries of the Mahanadi. According to Saroj Kumar Mishra, Chief Engineer of the dam, during the time of this heavy flooding, only 177,000 cusecs (cubic feet per second) or 49,560 cumecs (cubic metre per second) of water was received from Chhattisgarh; at the time the dam was receiving an inflow of 1.1 million cusecs (308,000 cumecs). The Ib and Bheden rivers that join the reservoir contributed 100,000 cusecs (28,000 cumecs) each. Rest of the water has come from other small rivers and rivulets that flow into the Mahanadi in the upper catchment. But there is no system in place with the water resources department to monitor flow of water from these rivers into the reservoir.

A curve in the rule

What is also being debated is the 'rule curve' that every reservoir follows to control flood. It is a mechanism to regulate outflow and storage in such a fashion that there is no need for sudden release of water in case of heavy inflow. This also ensures maximum storage post-monsoon for power generation and irrigation. The rule is based on rainfall pattern both upstream and downstream of the dam. This rule is changed keeping in mind the water demands and water inflow in the reservoir. In 1988, Hirakud got a new rule of curve that still holds.

In case of Hirakud, the reservoir level should be 590 feet in July and maintain dead storage level till September 1. This is to regulate flood as most of the rainfall arrives during this phase. By this rule, the reservoir should attain full reservoir level by end of September. Let's see how the rule has been violated this year. Going by the current phase, the dam had a storage level of 595 feet in the first week of July. It increased to 596.85 ft on July 20. On August 1 it reached 607.27 ft in contrast to recommended level of 590 ft. The next week the level rose to 620 ft, again, against recommended level of 606 ft. The reservoir was, in fact, nearing full reservoir level in the first week of September, instead of the end of October.

For a week preceding September 9, there have been regular warnings from the India Meteorological Department about heavy rainfall in both Chhattisgarh and Odisha. Only in the last week of August, the dam opened 10 gates while retaining near full reservoir level. In such a case, there was no other way than to open all possible gates of the reservoir for its own safety. Rest is part of the state's disaster filled history.

In 2008, a similar situation led to severe flooding. It was the worst flood after the one in 1982. All through August, the authorities filled the dam reservoir. On September 18, it was almost full, as on September 9, 2011. Rains in the catchment created a situation wherein the dam's gates had to be opened in a hurry. Floods inundated 19 districts. There were debates and protests because the flood of 1982 was also caused in a similar manner. The lesson was never learnt. Out of the 19 major floods experienced in the state in the recent past, 14 were downstream of the Hirakud and nine were caused by sudden release of water from the dam. What's more, in the past one decade, the frequency of floods have increased: there have been five major floods, and all of them have been attributed to the dam storing water in violation of the rule curve. 




Subscribe to Daily Newsletter :

Comments are moderated and will be published only after the site moderator’s approval. Please use a genuine email ID and provide your name. Selected comments may also be used in the ‘Letters’ section of the Down To Earth print edition.