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% of the 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 name Subansiri is derived from the Sanskrit word svar?a, meaning gold, and is applied to the river only after the confluence of the Chayul Chu and Tsari Chu rivers in the Upper Subansiri district in Arunachal Pradesh, India. Chayul Chu flows for about 50 km in Tibet before entering India. After traversing through the Miri Hills of the outer Himalayas (Siwalik foothills), the Subansiri debouches into the plains of Assam near Dolungmukh in Dhemaji district. Before entering the plains, Subansiri cuts a steep gorge of unique beauty through the Siwalik rocks of the Arunachal Himalayas. The river bed falls from a height of 4206 m to 90 m near Dolungmukh in the foothills and joins the Brahmaputra River at Jamugurighat in the Lakhimpur district at an elevation of 70 m. The river Subansiri in Arunachal Pradesh flows through Upper Subansiri, Lower Subansiri, and Kamle districts and in Assam through the North Lakhimpur and Dhemaji districts.
Throughout its journey from the Central Himalayas to the Arunachal foothills, the Subansiri river receives the discharges of numerous mountainous big and small streams. The number of tributaries is higher in the Siwalik foothills than in other zones. The main tributaries of the Subansiri include Kamala, Boginadi, Ranganadi and Dikrong.
The hydrometeorology
Subansiri basin extends from tropical to temperate zones and therefore the area exhibits a great diversity in rainfall characteristics. In the Northern and Central Himalayan tracts, precipitation is scarce on account of high altitudes. The south-east part of the Subansiri basin comprising the sub-Himalayan and the plain tract in Arunachal Pradesh and Assam, lies in the tropics. Precipitation occurs in this region in copious quantities due to North-East as well as South-West monsoon. Particularly the South-West monsoon causes very heavy precipitation in the whole of this region from May to October with July-August in general being the high flood months.
The annual rainfall typical in the Subansiri basin normally varies from 630 mm at Raga Station in Kamle district, Arunachal Pradesh to 4600 mm at Gerukamukh, Assam along the foothills in South-Eastern parts of Arunachal Pradesh. A major portion of precipitation is in the shape of rainfall in Arunachal Pradesh, which occurs in the monsoon season and heavy precipitation is generally limited to South Western parts. Some rainfall occurs in the post-monsoon months and in the winter season. The heavy rainfall in the catchment is usually associated with the synoptic situations of North-South oscillations of the axis of monsoon trough during July and August, activation of monsoon trough by westerly waves when the trough is close to the foothills and movement of depressions from the Bay of Bengal towards Eastern side of Assam.
The climate in the entire region of the Subansiri basin is humid and fairly uniform. The maximum temperature is 27.3°C and minimum (-) 8°C. The relative humidity at these stations ranges from a maximum of 94% to a minimum of 62%. The basin experiences extremely low temperatures during winter months.
The hydrological budget of Himalayan rivers is dominated by monsoon rainfall and snowmelt. The hydrological data for Subansiri river are estimated as mean annual rainfall of 1300 mm, Mean Annual Discharge (rain + snow ? evaporation) 1100 cumec (cubic meter/sec), snowmelt contribution of 13.6%, evaporation 11.5%, May-October rainfall and snowmelt contribution to annual discharge 79.4% and 7.7% respectively (Bodo Bookhagen et al, 2010). The average annual discharge of Subansiri river is 44024 MCM (million cubic meter). The contribution of the river Subansiri is estimated to be about 10 percent of the total discharge of the river Brahmaputra at Pandu near Guwahati.
Historical floods
From the discharge data, 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 cumecs at Gerukamukh (2011) were observed in the Subansiri river.
The research paper ‘Seven centuries of reconstructed Brahmaputra River discharge demonstrate underestimated high discharge and flood hazard frequency’ by Mukund P. Rao et al published in ‘Nature Communications; in November 2020 recorded 12 historical flood events in Brahmaputra valley in the years 1787, 1842, 1858, 1871, 1885, 1892, 1900, 1902, 1906, 1910, 1918, 1922 and 6 recent instrumental period flood years in 1966, 1987, 1988, 1998, 2007, 2010. Floods in 1987, 1988, 2007, and 2010 along with the latest inundation from flooding in 2020 have caused large fatalities, permanent loss of livelihoods, and the displacement of thousands of people, in addition to raising regional food security concerns due to famine from damaged crops.
Landslides and channel migration:
The Subansiri river traversing through the Miri Hills of the outer Himalayas (Siwalik foothills), the Subansiri debouches into the plains of Assam near, Dolungmukh in Dhemaji district. Before entering the plains, Subansiri cuts a steep gorge through the Siwalik rocks of the Arunachal Himalayas. Amongst a variety of natural resources, soil losses are the first and foremost problem in the Himalayas, which causes siltation problems in the plains. The loss of productive substrate is both due to direct human development actions and indirect ecosystem changes. The direct human development action pertains to cultivation and development activities like road construction. The indirect human-induced ecosystem changes like changes in land use and excessive use of natural resources are causing erosion of the productive soil from the already depleted system.
Subansiri river in the plain section represents one of the most dynamic and unstable alluvial rivers in the Brahmaputra valley. The reach of the river downstream from Arunachal hills, especially towards its mouth, has a sluggish course subjected to intense braiding of the channel.
The great Assam Earthquake, of 1950 triggered a large landslide forming an artificial dam on the Subansiri river near the Sippu Nalah confluence at 9 km upstream of the Subansiri Lower Project site which dried up the river in downstream and breached after 3 days and the sudden release of the stored water causing large scale damages in downstream areas of Assam killing hundreds of people by the surging waters.
Studies showed that there had been two landslides on both banks and assessed the mass of slides 19 and 10 MCM on the right and left banks respectively. The associated flood episodes are primarily responsible for bringing a major change in the hydrologic regime of the Subansiri river. The river had left the original channel near the Gogamukh Chariali and started flowing almost 5 km away towards the east near Chouldhowa.
Floods and bank erosions are the main driving forces for channel migration. Large discharge and heavy sediment load during floods cause the river to become extremely unstable because of which it consistently migrates laterally. Satellite imagery in 1984, 1994, 2004 and 2014 showed that the Subansiri river has shifted about 8 km in the last 40-45 years @ 200 m per year towards its right side downstream of Gogamukh.
Future scenario after completion of Subansiri Lower Hydroelectric Project:
The 2000 (250×8) MW Subansiri Lower Hydroelectric Project (SLHEP), India’s second-largest hydroelectric project (HEP) in terms of installed capacity, is at the advanced stages of construction. It is being built by NHPC Ltd, a leading CPSU at the foothill of Gerukamukh on the border of Arunachal Pradesh and Assam, India. The 116m high concrete gravity dam will create a reservoir of 1365 MCM capacity.
Floods in the Subansiri river are observed due to heavy rains in the catchment from April/May to September/October. Maximum and minimum discharge at Gerukamukh are recorded at 13820 and 188 cumec. The 100, 25, and 10 years return floods in the Subansiri river are 19600, 14500 and 12400 cumec respectively. The spillway design flood has been estimated at 37500 cumec which is also the Maximum Probable Flood (PMF).
The designed discharge of each turbine is 322.4 cumec and eight turbines need 2579 cumec to produce 2000 MW electricity. The water level of Subansiri is minimal in February and it increases from April/May and discharge becomes more than 2579 cumec. At this stage, all the turbines can be run from the river flow continuously for at least 6/7 months from April/May to September/October. Discharge above the 2579 cumec shall be released through the spillway gates. At this period, inflow to and outflow from the reservoir is the same maintaining natural flow in downstream. From September/October onwards, flow decreases below 2579 cumec and running time for turbines also will be decreased proportionately.
One turbine shall be running continuously at part load to release a minimum of 240 cumec of e-flow for the sustenance of river ecology which is higher than the observed minimum flow of 188 cumec. Water in excess of e-flow will be stored for up to 20 hrs depending on the daily river flow rate in the reservoir and all the turbines shall run for a minimum of 4 hrs during peak energy demand. During peaking, diurnal flow variation would be 240 to 2579 cumec, and fluctuations in water level are estimated to be from 3 m at the dam site to 20 cm at 30 km downstream respectively.
Since the 1950 Great Earthquake, almost every year several waves of flood water devastate Assam bringing untold misery to the people and eroding its economy. Floods of various magnitudes have been seen in the Subansiri river in Assam. River flow above 7000 cumecs creates flood waves in downstream of the Gerukamukh. There has been provision for flood control in the downstream in Subansiri Lower Project (SLP). The elevation above the mean sea level of the dam top is 210 m and Full Reservoir Level (FRL) is 205 m. The reservoir level shall be kept at an elevation of 190 m, 15 m below FRL for a flood cushion of 442 MCM during the monsoon period. It will control a 10-year return flood of 12400 cumecs in downstream areas by absorbing 5000 cumecs over 24 hours.
Sediment management:
The annual sediment load at the Subansiri Lower dam site is 20 MCM. During flood season, the sediment inflow in the Subansiri river is about 85% of the annual sediment load. In Subansiri Lower reservoir, by keeping the spillway crest at a low level (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 maintaining a stable sediment regime similar to the original natural condition.
The river bank protection works in a 60 km stretch in downstream of the Subansiri Lower Dam being implemented by the NHPC Ltd, shall stop the acute bank erosion and the river migration problems of the Subansiri river. The downstream river bank people living in the completed portion of 30 km are free from the river bank erosions that compelled them to frequently shift their houses away from the bank.
Dam being located in the foothill, the types of species of fish are different in upstream and downstream and no fish migration is observed. The Fisheries Management Plan being implemented envisages the development of hatcheries for the rearing of the fish seed of Golden Mahseer, Snow Trout and Indian Major and Minor Carps and the ranching of fingerlings in Subansiri river upstream of SLP-Dam as a conservation measure for the sustenance of riverine ecology. To mark the commencement of the activity, ranching of about 6000 fingerlings of Indian Major Carps was undertaken in the Subansiri River downstream of the dam in June 2024. Modification of 59 km stretch of the river from swift flowing river to artificial lake due to reservoir formation, expected to cater many new varieties of fishes. The reservoir may become a haven for numerous migratory birds.