Dr. B.P Das Expert Consultant FAO, UN Former Engineer In Chief & Chief Advisor Department of Water Resources, Odisha Former Vice Chairman, EAC for River Valley & HE Projects, MoEF Chairman, Expert Body for studying Uttarakhand Disaster, MoEF Eco-system Benefit Rendered by Large Hydro Projects of India & Consequential River Rejuvenation

ROLE OF WATER STORAGE IN RIVER REJUVENATION AND SUSTAINING E-FLOWS Ecosystem benefit rendered by large Hydro Projects of India and Consequential River Rejuvenation Brain Storming Session-1 Dr. B. P. Das Former Chief Advisor & Engineer-in-Chief, WR, Odisha Former Vice Chairman, EAC for River Valley & HE Projects, MoEF Chairman, Expert Body for studying Uttarakhand Disaster, MoEF Chairman, Monitoring Committee, Indira Sagar & Omkareswar Command Area in Narmada Basin, MoEF Expert Consultant to FAO April 5, 2016

General Note Dams constructed across rivers for multiple uses result in abstraction of a large proportion of freshwater to meet consumptive use. Consequently the riverine reach below the dam gets deprived of its need to sustain the aquatic ecosystem particularly in lean season. While this is generally true for heavily harnessed rivers in arid basins, such as Amu- Darya, Syr-Darya draining to Aral Sea, Colorado, Indus and Nile the situation is absolutely different for tropical rivers in India in general and peninsular rivers in particular. The benefit to the river for sustaining aquatic biodiversity and general riparian use is presented for a few typical multipurpose projects of India.

General Note For rivers harnessed in peninsular India, 20% of MAF flows in non-monsoon in the riverine reach below dams against 5- 10% MAF that was flowing prior to construction of dams. This is substantiated by reference to pre and post dam hydrological scenario for two large dams in Odisha, Hirakud Dam on Mahanadi and Rengali Dam on Brahmani, Nagarjuna Sagar and Srisailam Dam on Krishna. Positive contribution of major dams in India to riverine health and sustenance of aquatic ecosystem in addition to enhancing the capability of the downstream reach to meet industrial / drinking water need and flushing out pollutant is established. For major dams almost 32-50% of the monsoon flow gets impounded in reservoirs & stored water at end of monsoon gets released after power generation synchronizing with Rabi / Summer irrigation need of the downstream. The estuarine region of the harnessed river is particularly benefited.

General Note Benefit of Farakka Barrage The Farakka Barrage across river Ganga in Murshidabad district of West Bengal was constructed in 1970 to replenish the freshwater flow in river Hoogly for improving navigability to Calcutta port. The feeder canal from Farakka brought in 850 m3/s perennially into Hoogly estuary, feeding the Sundarban mangroves, which forms the largest chunk of 4,000 km2 in India. As Ganga (Padma) flowed to Bagladesh causing dearth of freshwater the mangroves were slowly perishing, with salinity increasing to ppt, detrimental for the mangroves. With increased freshwater flow from Farakka barrage, not only the mangroves flourished due to decrease of salinity by ppt but also the fishery production went up from 7,000 Ton (1970) to almost 70,000 Ton currently attributable to the healthy mangroves in the Sundarban.

Mahanadi Basin Hirakud Dam

Non-monsoon Flow Below Hirakud Dam Hirakud Dam intercepts 83000km2 of 1,48,000km 2 of overall basin Mean Annual Flow 50 MAcft Monsoon Flow 47 MAcft Non-monsoon (without Hirakud Dam) 3 Macft Non-monsoon flow ( ) deduced from Hirakud data Macft Post Dam non-monsoon flow actual( ) 5 Macft Comparison of actual/derived flow at delta head in pre & post dam situation

Brahmani River Basin Rengali Dam

Non-monsoon Flow Below Rengali Dam Rengali Dam intercepts 25,250km 2 of 39,268km 2 of overall basin Mean Annual Flow 20,000Mm 3 Monsoon Flow 18,000Mm 3 Non-monsoon (without Rengali Dam) 2,000Mm 3 Non-monsoon flow( ) would have been 1123Mm 3 Post Dam non-monsoon flow actual( ) 3434Mm 3 Comparison of actual/derived flow at delta head in pre & post dam situation

Krishna River Basin Nagarjunasagar dam

Non-monsoon Flow Enhancement in Krishna Basin Basin Area – km 2 Annual yield of Krishna River – 78100Mm 3 (CWC) Bachawat Award(1976) accepted 75% dependable yield as 63900Mm 3 Non-monsoon flow of Krishna River – 6409Mm 3 Two Mega Projects in Andhra Pradesh Nagarjunasagar intercepts km 2 Mean annual flow 42360Mm 3 (CBIP) Live storage of 6940Mm 3. Srisailam intercepts km 2 Mean annual flow 52160Mm 3 Live storage of 7080Mm 3 Major Projects in Karnataka Hipparagi Barrage, Alamatti Dam, Narayanpur Dam, Tungabhadra Dam Major Projects in Maharastra Koyna Dam

Post monsoon Study of Krishna Basin Nagarjuna Sagar (Operational 1974) As on 31 st Oct. live storage available is 6940 Mm 3 Srisailam(Operational 1984) As on 31 st Oct. live storage available is 7080 Mm 3, Which flows down to Nagarjuna Sagar excluding an abstraction of maximum 200 Mm 3 in the non-monsoon Consumption Rabi requirement for ultimate command areas of 1 Mha of Nagarjuna Sagar is about 2000 Mm 3. Overall flow available at Nsagar is around 13000Mm 3. Average flow below Nagarjunasagar Dam upto Krishna barrage Non-consumptive water available11000Mm 3 Add D/S contribution of 44000km 2 and return flow3500Mm 3 On the average almost Mm3 flows from NSagar dam to Krishna barrage at Vijayawada (in the post 1985 period) against 7000Mm3 prior to construction of the dam helping the aquatic environment and significantly stabilizing the delta command.

Enhancement of Non-monsoon Flow in Krishna Basin

Conclusion Hydrological basin studies of three major east flowing rivers, Mahanadi, Brahmani and Krishna have been presented. It is clearly established that in predam situation the tail reach of the rivers used to convey 5 to 15% of the annual flow with insignificant flow during summer months March through June. After impounding by the dams the non-monsoon release and the downstream basin contribution have resulted in substantial increase (20-25% of the mean annual flow) helping the aquatic environment of the entire riverine reach in the downstream. The major dams are thus contributing positively to river rejuvenation and aquatic environment.

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