Sardar Sarovar Dam
Source: By Aditi Raja: The Indian Express
The Sardar Sarovar Narmada Dam is a terminal dam built on the Narmada river at Kevadia in Gujarat’s Narmada district. Called the ‘lifeline of Gujarat’, it usually has no water for irrigation during summers. However, this year, in the ongoing summer, the dam released about 1.3 Million Acre Feet (MAF) water for irrigation between 1 April and 31 May in its command area of 21.29 lakh hectares.
And for the first time in the history of the dam, as many as 35 dams and reservoirs, close to 1,200 check dams and 1000 village tanks have been filled with Narmada water this year, according to the Sardar Sarovar Narmada Nigam Ltd (SSNNL).
As of 3 June, the dam had 122.72 metres with live storage of 1,711 million cubic metres. With an inflow of about 15,000 cusecs, the total outflow from the dam is at around 43000 cusecs —of which 12,965 cusecs is being released after generation of power from the Canal Head Power House and 30,361 cusecs from the Riverbed Powerhouse.
River Narmada is a classic case of Integrated River Basin Planning, Development, and Management, with water storage available in all major, medium, and minor dams on the main river and its tributaries, shared amongst four party states – Gujarat, Rajasthan, Madhya Pradesh and Maharashtra — in the ratio stipulated by the 1979 award of the Narmada Water Dispute Tribunal.
Out of the 28 MAF capacity of Narmada basin, Gujarat has been awarded a share of 9 MAF, while Madhya Pradesh has 18.25 MAF, Rajasthan 0.50 MAF, and Maharashtra 0.25 MAF. The power benefits from the project are to be shared thus: Madhya Pradesh at 57 per cent, Maharashtra at 27 per cent, and Gujarat at 16 per cent.
In 2017, the dam was raised to a height of 138.68 meters (spillway level until 2017 was 121.92 meters) and 30 gates were installed. The dam achieved its Full Reservoir Level (FRL) for the first time in 2019. It also attained FRL in the monsoon of 2020 but SSNNL officials say that the live (utilisable) water storage capacity of Sardar Sarovar Dam does not even make up for 50 per cent of the annual water needs of the party states and, therefore, the water management at Sardar Sarovar becomes critically dependent on the regulated releases from the upstream reservoirs in Madhya Pradesh, where hydropower generation ensures water inflow from time to time.
The water management initiatives that helped harness water
During the monsoon from July to October, the reservoir operation is well synchronised with the rain forecast in the catchment area. The strategic operation of River Bed Power House (RPBH) ensures that minimum water flows downstream into the sea and maximum water is used during the dam overflow period, which is not calculated in the annual water share. These measures help in maximizing the annual allocation of water share. Similarly, in non-monsoon months, the measures for efficient use of the allocated share typically include minimising the conventional and operational losses, avoiding water wastage, restricting water-intensive perennial crops, adopting of Underground Pipelines (UGPL); proper maintenance of canals and structures and operation of canals on a rotational basis. In SSP, about 60 percent of canals constructed so far are UGPL.
How has Full Reservoir Level (FRL) helped?
Although the Sardar Sarovar Dam, after attaining its full height, was inaugurated in September 2017, it could not be filled up to the FRL of 138.68 meters in 2017 and 2018 due to monsoon deficit. However, good rainfall in the catchment in 2019 and 2020 ensured that it achieved FRL for two consecutive years. “The live storage capacity of the Sardar Sarovar Dam increased by 3.7 times after the permission to close the gates was received in 2017. Its real benefit is realised now with the dam filled upto FRL for two consecutive years, an SSNNL official said.
The annual share allocated to Gujarat during the last two water years was 8.86 MAF (million acre-feet) in 2019 and 10.08 MAF in 2020, respectively. “However, in 2019-20, reservoir operation and water management were constrained a lot because it was the first time that the dam was to be filled to full capacity and stringent safety considerations were to be followed in order to check the strength of the structure for the first time, the official added.
Has the Covid-19 lockdown helped in preserving water in the basin?
The industrial consumption of the water from the Narmada dam is very less as compared to other uses. “Out of the 9 MAF awarded to Gujarat, the water quantity earmarked for industrial use is only 0.2 MAF, which is roughly only 2 per cent. The present utilisation of water by the industries is 0.07 per cent MAF even during full operational years in normal times. Therefore, the lockdown or partial closure of industries has not impacted the storage levels much, the SSNNL official says.
How would the summer level of over 121 meters of the dam reservoir help in the functioning of the powerhouses during the next water year, beginning on 1 July?
A water year is counted from 1 July to 30 June. “A comfortable water level at the beginning of monsoon can definitely lead to higher hydropower generation during non-summer months as we have experienced during the last two years. In the water year 2019-20, 4784 MU (million units) of hydropower was generated and in the current year 2833 MU are generated so far with June to go. It is noteworthy here that in a single month of September 2019, 988 MU were generated, and at present also, four turbines of RBPH (river bed power house) are in operation. Madhya Pradesh and Maharashtra are getting 57 per cent and 27 per cent share respectively,” the SSNNL official said, adding, “However, considering the limited storage capacity available at Sardar Sarovar Powerhouse and the requirements of the party states, it is not possible to forecast the scenario of the hydro power generation for the next water year as much will depend on the coming monsoon.”
Does the Garudeshwar Weir, located 12 km downstream from Narmada dam, help in maintaining water level in dam reservoir?
The RBPH discharges almost 42,000 cusec at its peak operation phase, which would be wasted by getting drained downstream into the river and eventually going into the sea. The 200 meter wide and 32.75 meter high Garudeshwar Weir can contain 850 lakh square meter water that is released downstream after hydro power generation at the underground RBPH, stationed 165 meters from the dam, on the right bank of the river. The RBPH has six Francis-type reversible turbines, each of 200 MW installed capacity, to recycle this water stored in the Weir during non-peak hours of the grid because the power consumption per minute of reversing the water back from the Weir is more than the per unit generation capacity. While the Garudeshwar Weir does not directly help in maintaining the water level in the main dam, the storage of water after the generation of hydropower is of help in the non-monsoon season. This water stored in the Weir also helps maintain a water level in the river around the Statue of Unity, where there is a ferry service called the Ekta Cruise.
How is the dam spillway operated to maximize storage in the dam reservoir and mitigate the risk of flood as seen in 2020?
The SSNNL explains that the operation of Dam Spillway Gates is a specialized and complex issue, involving domain expertise and experience in hydrology, flood routing, and hydraulics. “It is about striking a balance between the safety of the dam as well as the population and environment located downstream and the likelihood of having scarce water storage. The dam has to have an adequate flood absorption capacity by maintaining cushion levels and must also harness the available flood water in order to ensure that there is no water scarcity. Ideally, as a general guideline, a major dam should not be filled more than 60 per cent as of 31 July, more than 75 per cent on 31 August, and more than 85 per cent on September 15. Therefore, excess flood water received after attaining these levels is allowed to flow downstream by opening the gates. Each spillway gate level is decided after duly considering the storage and flood absorption capacity in the upstream dams, the rain forecast, flood conveyance capacity of the river downstream, and balancing hydropower generation with power grid requirements, SSNNL says.
