Natural Disasters and Hydro-Excitement in the Upper Tista Catchment: A Serious Concern!

By: Staff Reporter

New Delhi, August 31 (G’nY News Service): Trans-boundary Tista Basin forms a part of the larger Brahmaputra Basin in the Eastern Himalaya. The river drains a total geographical area of about 12159 km². Around 2004 km2 of the basin (about 17 percent) area lies in Bangladesh with the rest being in India. In recent times, traditional symbiotic and intimate human-environment relationship in the Tista Basin has been increasingly put to danger by diverse undercurrents of development. This has resulted in imbalances in the environment and various ecological systems there in. Besides other forms of development including improper expansion of agriculture and irrigation, unscientific construction of roads and buildings and unplanned urbanization, Central and Provincial Governments of India are vehemently underway with series of hydropower projects particularly within Sikkim-Darjeeling catchment of the basin.


The mega hydropower projects proposed in the area are part of the Government of India’s program of dam construction to create another 200 billion cubic metres of storage through the 50000 MW hydroelectric initiatives launched in 2003. Considering huge untapped hydro potential of the Tista and its numerous perennial tributaries, Central and Provincial governments see huge opportunity to mobilize flow of capital investment through public, private or joint sector.
Therefore, apart from development of various small, mini and micro-hydel projects, several mega projects have been awarded to NHPC, NTPC and private developers in the last one and half decades. From these projects, the State governments of Sikkim and West Bengal will get 12 percent of free power. Reportedly, Sikkim hopes to yield approximately INR 2000 crores per annum by tapping into ‘the enormous hydroelectric potential’ of the basin within the State. Besides contributing to the growth and development of the country, Sikkim visualizes of a prosperous Sikkim with the revenue earned.

However, the number of mega projects in Sikkim allotted to public and private sectors has been lowered radically from around 27 in 2007 to 16 in 2015. Important reasons cited by the government include critical social, ecological, geological and financial considerations. Reportedly, the Sikkim-Nepal earthquake of September 2011 played significant role in bringing down sizeable numbers of hydro-projects in the Sikkim Himalaya. The data recorded in the official documents indicate that number of ongoing mega projects came down from around 25 in 2010 to about 18 by 2012. Accordingly, the identified hydro capacity of the Sikkim Himalaya has been lowered from over 5200 MW to around 4200 MW as per the recent reassessment study of the Central Electricity Authority (CEA, 2014).  In the Darjeeling region, out of three mega projects one (TDLP III) is already commissioned. Others are in various stages of construction. Further, the Chief Minister of West Bengal has recently announced four more projects in Darjeeling region of the Tista basin. Consequently, the Sikkim-Darjeeling catchment is now expected to produce over 6000 MW of electricity within the next few decades.

The biggest concerns at the moment are the varied impacts of such gigantic development ventures on the regional environmental security in this fragile region and its neighbourhood. The recent tectonic events in Sikkim (2011) and Nepal (2015) and the consequent disasters have further challenged the very idea and future of mega hydropower dams in the region. These seem to have either been overlooked or their impacts underplayed by respective EIA reports.

There are concerns that building hydro-dams may lead to river-induced seismicity in this geologically young and seismically active region. The Darjeeling-Sikkim catchment of the basin is located in the high-risk seismic zone IV of the Indian seismic zoning map and therefore had been active seismic region in historical times. Recent major earthquakes in Nepal (April 2015) and Sikkim (September 2011) measuring 7.8 and 6.8 in magnitudes have clearly exposed the region’s wherewithal with regard to earthquake disaster. The Nepal earthquake ofApril 25, 2015 and series of aftershocks thereafter have reportedly damaged about 14 hydropower plants across Nepal resulting in a loss of 150 megawatt (MW) of electricity. In this regard, Sunkoshi Hydropower plant has apparently suffered serious damage with its 3-km canal suffering from multiple leakages. Environmentalists, activists and researchers in the region have long been warning against too many constructions of mega dam projects in the upper Tista catchment.

The fragile geology coupled with mega hydro-dams could induce earthquakes and the resultant landslides and flash floods could result into a disaster.  The central government, provincial governments and hydropower companies may, however, dismiss the earthquake related concerns as fear mongering. Yet, a contingency plan for disaster management in the event of earthquake is a far cry for almost all the hydro-projects in the area! Scholars across the world have reported the performance of various types of dams under earthquake shaking. Their studies show that concrete dams may be subject to severe cracking, movement and opening of joints that may render the dam unserviceable or may require major repairs.

It may further be noted that Sikkim-Darjeeling segment of the Tista basin is featured by a number of active and dormant landslides. A cursory glance at landslide statistics gives us a fearful idea of the enormity of damage done and the ever-present threat to life and property in the region. In the last one-century more than 10000 slides have been registered in Darjeeling region alone. Thousands of lives have been lost and the overall economic development of basin negatively impacted.  Mention should be made that heavy and spontaneous rain on June 30 and July 01, 2015 triggered a string of landslides across Darjeeling Himalaya killing over 40 people. According to Praful Rao, President, Save the Hills, ‘Kalimpong was pummeled by torrents of rain starting from approximately 20:00hrs. I watched the clouding as it formed over us and remained almost stationery much like the clouding over Uttarakhand in 2013. Kalimpong received almost half (226 mm) the entire July month’s average rainfall (548.7mm) in the 06 hours’. Darjeeling Together, an initiative of the people of Darjeeling to help the people affected by the recent landslides, has placed the following preliminary figures on the immediate impacts of landslides: villages affected: 165, people affected: 94797, houses damaged: 1907, people in relief camps: 2360. Initial report of the district administration has calculated property loss to the tune of INR 12 crores in the Darjeeling region.

According to a recent study of Wadia Institute of Himalayan Geology (Dehradun), the Sikkim Earthquake (2011) triggered several hundred landslides in Sikkim, Darjeeling, Nepal, Tibet and Bhutan Himalaya. In the Indian Territory, the earthquake-triggered landslides were reported as far as 100 km away from the earthquake rupture zone. Within Sikkim, the study reported over 350 new landslides in the post earthquake period.

Notably, after water impoundment and pondage by the dam, water level in the area rises considerably. As a consequence, the strength parameters of the slope mass weakens and it may become susceptible to destabilization thus triggering new landslides and further destabilisation of already active slides. A live example of such a situation is seen along the National Highway between Tista Bazar and 27 Mile near Rambi in Darjeeling region where TDLP-III regularly impounds water for several weeks, although NHPC claims to be a run-of-river project (RoR), threating livelihood security of the riparian settlements.

River Erosion
Of all the Himalayan Rivers, Tista reportedly has the highest sediment yield.  The effects of erosion and sedimentation provide favorable conditions to river shifting. River Kosi has shifted by about 150 km to the west during the last two centuries. According to Hunter’s statistical account of Bengal, Tista originally was a river of Ganga basin. In 1787, due to incessant rain followed by heavy flood and devastating earthquake Tista shifted its course to Brahmaputra basin. If such sudden river capture occurs today, thousands of villages will be swept away in a gigantic flash flood inflicting incalculable human and environmental disasters.

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