Subansiri river near Daporijo in Arunachal Pradesh.

A N Mohammed, Miren Verma

The Subansiri river:

Subansiri is a major right bank tributary of river Brahmaputra originating in the Central Himalaya Range at an altitude of 5340 m. The total drainage area of the 375 km long Subansiri river up to the confluence with the Brahmaputra is about 37000 sq. km out of which 40% area lies in Tibet. The catchment, crescent-shaped with its concavity on the western side, has 4500 sq. km covered with snow and 24116 sq. km rainfed. The contribution of the river Subansiri is estimated to be about 10 per cent of the total discharge of the river Brahmaputra observed at Pandu near Guwahati.

The hydrology:

The average annual rainfall in the Subansiri basin is 2356 mm and 4600 mm at the dam site. A major portion of rainfall in Arunachal Pradesh occurs in the monsoon season and heavy precipitation is generally limited to the South-Western parts. Some rainfall occurs in the post-monsoon months and in winter season also. River gauges have been established since 1956 on the Subansiri river system. NHPC Ltd also established G&D sites near the dam site and at Chouldhowaghat (about 12 km d/s of the dam site) which are operational since 2001.

The average annual flow at the dam site is 44024 MCM (Million Cubic Metre) @ 1396 cumec (cubic meter per second). The floods in the Subansiri River are observed due to heavy rains in the catchment from May to October. A maximum flood of 18790 cumecs at Chouldhowaghat (July 1972) and 13800 cumec at Gerukamukh (2011) was observed in the Subansiri River.  The 1 in 100, 25 and 10 years return flood in the Subansiri river are 19600, 14500 and 12400 cumec (cubic meter per second) respectively. The spillway design flood has been estimated at 37,500 cumec. The average annual sediment load at the dam site is estimated as 20 MCM based on long-term observed data.

2000MW Subansiri Lower Hydroelectric Project.

The 2000MW Subansiri Lower Hydroelectric Project (SLHEP) is under construction at Gerukamukh, on Arunachal Pradesh and Assam border. The SLHEP envisages the construction of a 116 m high dam with gross storage of 1365 MCM. The river bed level elevation at the dam site is 94 m (above mean sea level). The Probable Maximum Flood (PMF) hydrograph has been routed through the reservoir by impinging at Full Reservoir Level (FRL) at 205 m and accordingly Maximum Water Level (MWL) of 208.25 m has arrived. The Minimum Draw Down Level (MDDL) is 181 m.

Land acquisition and compensatory afforestation:

Like many other developmental activities, the proposed project, while providing planned power generation could also lead to a variety of adverse environmental impacts. However, by proper planning at the inception and design stages and by adopting appropriate mitigatory measures in the planning, design, construction and operation phases, the adverse impacts have been minimized to a large extent, whereas the beneficial impacts are maximized.

The area under submergence at FRL is 3436 ha (hectares). No homestead land is coming under submergence. As such, there is no displacement due to the project. However, part of the cultivable land of two villages namely Gengi and Siberite in the Lower Siang District of Arunachal Pradesh had come under submergence. 77 families from the above two villages were declared as Project Affected Families (PAFs) by District Authorities. The Rights & Privileges of the PAFs have been protected as per the provisions under the Rehabilitation & Resettlement (R&R) Policy 2008 of Arunachal Pradesh.

A total of 3999.30 ha of forest land, 3183.00 ha in Papum Pare, Lower Subansiri, Upper Subansiri and West Siang districts of Arunachal Pradesh and 816.30 ha in Dhemaji district of Assam diverted for construction of Lower Subansiri Hydroelectric Project by the NHPC Limited. The area in the state of Assam is forest land in the Dhemaji district free of any habitation or agricultural activity. The diversion of forest areas had been compensated by afforestation over an area of 8000 ha of degraded forests through the Compensatory Afforestation Fund Management and Planning Authority (CAMPA) of respective State Forest Departments.

Environmental flow:

The downstream impacts raised by the Assam Expert Group in 2010 and NGT petitioners are managed after deliberation by different committees formed for SLHEP. A cumulative impact study of the Subansiri basin conducted by the Central Water Commission in Dec 2014 recommends the release of 240 cumec (cubic meter per second) flow for the protection and conservation of aquatic biodiversity including dolphins in downstream. NHPC Ltd. decided to run one turbine continuously to ensure minimum release of 240 cumec.  The distance between the dam toe and the start of the tail race outlet of the first unit is around 250 m only. This stretch shall always be filled with back water when one turbine runs continuously for releasing 240 cumec discharge during non-monsoon season. Depending on the running of a number of turbines 5 to 10 m depth shall always be available. Therefore, the short stretch of the river from the dam-toe up to the power house release point shall not be dried up at any time.

Impact on downstream and Majuli Island due to flow variations during peaking operation:

Subansiri Lower HEP is envisaged as a peaking power station producing 2000 MW of peaking power for a minimum of 4 hours daily in a year of 90 % dependable flow during non-monsoon periods.  Peaking power operations result in diurnal variations in the downstream flow. The peaking discharge gets moderated over time along the river due to valley storage and stream routing progressively. The maximum peaking discharge is only 2579 cumec in lean season at the dam site, which will further reduce as it approaches downstream.  Thus, the lean season flow will have much lesser velocity to have any impact on the erosion of the river banks in comparison to monsoon season discharge/velocity.

Based on the findings of different studies/reports, there is no significant impact anticipated in the downstream or Majuli Island due to the fluctuation of water. Protection measures have been adopted up to 30 km as per TEC (Thatte and Reddy Committee) suggestions. NHPC Ltd is taking up necessary measures in association with the water resources department of Assam to protect the river banks in a 30 km stretch downstream of the dam. Thatte-Reddy Committee in its report mentioned, “It is seen from the routing of peaking discharges through the river that by the time Subansiri approaches Majuli, the flow gets steady and there will be hardly any noticeable fluctuations.” “Subansiri Lower HEP will only attenuate the floods. Majuli bank erosion cannot be attributed to floods on account of Subansiri Lower HEP. The fluctuations on account of peaking mode of operation do not carry up to Majuli. They die down in river stretch of about 30 km from Subansiri Lower HEP due to valley storage”.

Flood moderation:

The design flood for Subansiri Lower HEP is Probable Maximum Flood (PMF) as per codal requirement for dam safety. As per IS 4410, “Probable Maximum Flood (PMF) is defined as that flood estimated to result if the most critical combination of severe meteorological and hydrologic conditions considered reasonably possible in the region were to occur.” The design flood has been estimated by both the Probabilistic and Deterministic approach and a higher design flood value 37500 cumec of the two methods has been adopted. All Committees agreed that the design flood estimation is in order and the spillway was adequately sized. The present planning of Subansiri Lower HEP includes the flood control aspect in conjunction with the Subansiri Upper (SUP) and Subansiri Middle (SMP) projects and significant flood moderation can be achieved by all these three projects together. The flood control aspect has been planned for the project during Detail Project Report (DPR) stage itself.

For the purpose of flood moderation, the reservoir level of Subansiri Lower HEP shall be kept at an elevation 190 m i.e 15 m below the full reservoir level of 205 m in most of the monsoon period with storage of 442 MCM. This will cause regulated flow in the downstream for the medium floods. The carrying capacity of Subansiri river is about 7000 cumec. If more than 7000 cumec water comes, excess water will be stored in the reservoir to relieve downstream areas from flood. If a flood of 12000 cumec comes, 5000 cumec will be stored at the reservoir releasing 7000 cumec in the downstream controlling flood for about 24 hours. If the flood of less than 12000 comes, flood control will be for more time and if more than 12000 comes flood can be controlled for less time. The reduction in medium floods (10 to 100 years return period floods) shall be around 30-40%.    

Subansiri river near Chauldhowa Ghat in Assam.

Sediment management:

The annual sediment load at the Subansiri Lower dam site is 20 MCM. During flood season, the sediment inflow in Subansiri is about 85% of the annual sediment load. In the Subansiri Lower reservoir, by keeping the spillway crest at low level (El 145 m) and maintaining a lower reservoir level during most of the monsoon, the sediment transport capacity will be enhanced and will not allow major portion of the incoming sediment to settle down.  All the opinions expressed by the experts indicate that by the time the dam become operational a stable sediment regime similar to the original natural condition would be reinstated. Thatte and Reddy Committee in its report agreed that “There is no good enough reason for the downstream to be concerned. It should rather welcome the dam from “sediment” considerations.” “Sluice spillway as provided in Subansiri Lower HEP is in accordance with the best International practice for sediment control. Since the dam will arrest only bed load, besides a very small part of suspended load, no problem is envisaged on the downstream of dam on account of sediment transport.”

Flood forecasting and reservoir operation:

Flood forecasting in a river basin is a specialized field. Central Water Commission (CWC) and Indian Metrological Department (IMD) in conjunction carry out flood forecasting in many river basins. On persuasion of NHPC Ltd with CWC and IMD, flood forecasting for the Subansiri basin was included in the 12th 5-year plan. Flood forecasting for Subansiri Lower HEP is operational since 2017. An advisory for an inflow forecast of 5 days with a time series of river discharge is being issued by CWC during monsoon season. For Early Warning System (EWS), Automatic Water Level (AWL) Recorder with auto transmission facility/telemetry (GSM based) has been installed at Tamen, about 73 km upstream of dam site along the river on Kamla river and at Daporijo, about 70 km upstream of dam site along the river on Subansiri river.

These stations give approx. 6 hours lead time for flood forecasting. The river inflow at upstream G&D sites Daporijo and Tamen and at Subansiri Lower HEP dam site is being monitored 24×7 at the EWS master control room, Faridabad. The Water Level at upstream of the dam, flood forecast by CWC at Subansiri Lower HEP dam and rainfall forecast by IMD, JAXA and MOSDAC is also being monitored continuously by NHPC Ltd. Any untoward event or unusual increase in water level upstream of the dam will be reported with a sufficient lead time for timely action ensuring the safety and security of the downstream population. The Disaster Management Plan and SOP on the “Adopting Measures for Protection of People Living in Downstream Area for Emergency such as Sudden Release of Water from Dam” is being implemented by NHPC Ltd for each project.

Reservoir operation manuals have been developed for all the NHPC Ltd projects/Power Stations on the basis of predominant and expected discharges taking into cognizance the actual discharging capacity of the spillways bays that is obtained generally through physical model studies. This manual contains the elaborate spillway gate operation protocol for any given inflows in the reservoir. The step-wise operation of gates is given in an easy-to-read table, which is easy enough to be understood by the gate operator.

This process of gate operation for managing the floods in the downstream is in practice by NHPC Ltd in all its projects ranging from a 25 m high barrage to a 140 m high dam and has stood the test of time. The reservoir level is monitored closely during flood season. Water Level recorders are installed at the location of all the intakes and at the extreme spillway bays to monitor the reservoir levels accurately.

Automation in plant operation:

NHPC Ltd has introduced automation in controlling all its power stations. Automation is the use of control systems, intelligent electronic devices, instruments and new communication technologies to enhance better monitoring and controlling of the system to reduce the human interface with system. The automatic system reads the information on the equipment status operation, and then activates commands to various equipment of power plant and responsible for Auto sequencing. SCADA (Supervisory Control And Data Acquisition) control system has been installed to communicate with hardware and software, and then to integrate power plant automation systems with almost unlimited connectivity.

The system will collect the latest information and communication technologies (ICT), and multiple communication channels for the interoperation of various control system in the powerhouse and substation automation system. SCADA is used for telecontrol in electrical power system for controlling electrical transmission grids, powerhouse control system like turbine controller, protection system, excitation system and other geographically widespread control systems from the power plant control room as well as from NHPC Ltd.’s Master Control Room in Faridabad. The operator of the automatic system will still have to make the necessary changes to the commands or controls, based on the needs or production demands, flood control etc from time to time.

Downstream river bank protection/ erosion control works:

To mitigate the downstream impact of the 2000 MW Subansiri Lower HEP, the Assam Expert Group had suggested river bank protection measures up to 15 km downstream. Joint Steering Committee had also recommended that necessary protection works in the portion of 15 km in downstream from the dam site be carried out at locations and details worked out by NHPC Ltd, Brahmaputra Board and Water Resources Department, Government of Assam. NHPC Ltd has decided to carry out protection works up to 30 km in downstream. In order to implement the best possible anti-erosion measures in downstream, a detailed study of bank erosion and protection measures had been undertaken by Department of Water Resources Development & Management (WRDM), IIT Roorkee.

Downstream river bank protection works under progress.

In view of the recommendation of the Technical Expert Committee (TEC) and Joint Steering Committee (JSC) for downstream impact due to the Subansiri Lower HE Project in July 2012, NHPC Board resolved for allocation of a downstream development package of Rs 470 Crores, out of which Rs 145 crores for downstream Subansiri river bank protection/ erosion control measures up to 30 km downstream, Rs 320 crores for development works in downstream areas and Rs 5 crores for social awareness & mass awakening program. An additional fund of Rs188.24 crores has been provided for balance downstream river protection works up to 30 km downstream of dam axis in January 2022 and proposed timeline for the project to be completed by December 2024. Protection measures has been implemented by NHPC Ltd in consultation with the Water Resource Department, Government of Assam and design-quality assurance is being monitored by IIT, Guwahati.  Till date, river protection work has been completed up to 25 km.

Downstream development works:

For the development works taken up by NHPC Ltd for nearby local stakeholders, a baseline survey conducted by the Institute of Rural Management, Anand (IRMA), Gujarat, engaged by NHPC Ltd as a consultant for designing and strategizing implementation of sustainable livelihood interventions in Dhemaji and Lakhimpur districts of Assam. IRMA conducted a baseline survey which inter alia included Focused Group Discussion (FGD) & Fuzzy Cognitive Mapping (FCM) in 125 revenue villages, 3125 nos. of household survey and village-level survey in 354 villages. Based upon the multi-criterion analysis, IRMA identified livelihood intervention in piggery, sericulture, handloom, dairy, poultry, handicraft, agriculture and horticulture.

NHPC Ltd has been implementing the schemes commencing from June 2018 initially on sericulture, piggery and handloom with cooperative modalities through Network Partners, identified by IRMA, whose timeline has been extended up to June 2024 which include social mobilization, sensitization, capacity building, training, workshops, exposure visit, technical know-how & up-gradation etc. All beneficiaries are women farmers and more than 5000 local female members are being engaged in this lively hood intervention.

Farmer Producer Company viz. SAAR Pig Producer Company Ltd., SAAR Handloom Producer Company Ltd. & SAAR Eri Silk Producer Company Limited has already been registered under Company Act. SAAR Pig Producers Company Ltd is a 2500 women members company funded by NHPC Ltd. whereas Sericulture Producer company has 2000 women members under its downstream developmental initiatives. Once Common Facility Center (CFC) completed, this facility will have India’s biggest pig breeding farm with 400 high-quality pigs apart from the scientific slaughter house, nursery units, furrowing sheds, feed mill and storage units as well as the CFC shall be a training center for future interventions by the company. Till now, 2943 and 1470 families in Dhemaji and Lakhimpur districts respectively are benefitted under the schemes.

Sl. No.DescriptionTargetAchievedDhemajiLakhimpur

Fellowship to the meritorious students:

Under NHPC Fellowship scheme Assam, a total of 408 students of Assam domicile have been selected for scholarships to pursue higher studies in various courses like M.Tech/ ME/ M.Sc from IIT/ NIT/ IISc, degrees in Engineering, MBBS, MBA etc  for the Academic Session from 2015, 2016 and 2017.  

Master Plan for Gogamukh, Dhemaji, Lakhimpur, Silapathar and Dhakuakhana:

School of Planning & Architecture (SPA), Delhi has been engaged as a consultant for preparation of master plan for Gogamukh town development. After consultation with District Authorities, concerned line departments and stakeholders, SPA submitted the Final Draft Detailed Project Report for the master plan for Gogamukh town in June 2019. Preparation of four Master Plans for Dhemaji, Lakhimpur, Dhakuakhana and Silapathar and two District Development Plan for Dhemaji and Lakhimpur districts through SPA, Delhi are under preparation with a total financial involvement of Rs 3.57 Crore. SPA started the implementation in consultation with the District Authorities along with concerned district-level officers of all line-up departments, local bodies, other stakeholders and NHPC.

CSR initiatives in project area:

Under NHPC Ltd’s CSR-SD initiatives, various programs/schemes for more than Rs135 crores have been implemented for the welfare of the local populace of Assam and Arunachal Pradesh. The schemes included sustainable development works in education, safe drinking water supply, healthcare, vocational training, rural development, sports, arts and culture sectors in the nearby project areas.

A N Mohammed is a consultant at NHPC Ltd and Miren Verma is a general manager at NHPC Ltd.

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