The devastating landslips were caused by the undercutting of fragile hillsides for highways rather than by dams, which actually helped mitigate the floods

The natural calamity of June 16 through 19 that devastated the whole of Uttarakhand and large areas of Himachal Pradesh and western Uttar Pradesh — an area of almost 20,000 — was one of extremely rare severity among all the hydro-meteorological disasters to have struck India.

Intense point rainfall exceeding 200 mm occurred within a 24-hour spell and continued with lesser intensity for a few more days over several locations in the Alakananda and Bhagirathi basins. Two stations, Devprayag and Srinagar, recorded 283 mm and 320 mm in this spell (source: IMD) with rainfall progressively increasing toward Kedarnath in the Mandakini sub-basin. Such short-duration intense rainfall in June has a statistical recurrence interval exceeding 500 years.

In the past, more intense rainfall — exceeding 300mm in a day — has occurred over extensive areas in the peninsular basins of the Mahanadi (1982, 1994, 2009), the Godavari (2003), the Krishna (2009) and the Narmada (1968, 1970 and 1994). Overbank flooding of 10 to 30,000 of flat terrain occurred in these basins, causing extensive agricultural damage and loss of hundreds of lives. In contrast, lower intensity rainfall leading to loss of several thousands of lives and colossal property damage in Uttarakhand can be attributed to poor geology and extremely steep topography of the terrain leading to massive landslips.

The heavy rain on the glacier above Kedarnath led to increased glacial melting with a massive landslip, reported as 1,200 feet long by Dave Petley, Professor of Geography at Durham University, U.K. The downslide burst the Chorabari moraine lake, which then caused catastrophic debris flow down Mandakini and Alakananda rivers.

The human intervention in the region over the last 10 years also played a major role in intensifying the disaster. To cater to the unprecedented growth religious tourism, a large network of new highways and road-widening schemes are cutting into the toes of delicate, fragile and marginally stable slopes that hug the highways on river edges. Highways — around 500 km long — on the banks of the Alakananda and the Bhagirathi are constantly being widened at narrow stretches, leaving extremely steep conglomerates of rock and soil exposed to the vagaries of the weather. The recent spurt of construction of hotels on river edges, particularly on low over-banks prone to flooding, has further deteriorated the over-bank stability. The heavy tourist traffic adds to the destabilising process.

Sequence of failure

A logical sequence of failure can be inferred as follows: massive and progressive landslips on the banks of the rivers following the intense battering of storm rainfall; the crevices and fractures on marginally stable undercut slopes, which had no specific protective drainage measures, were flooded by thick sheet of rain water; trees which provided cohesion were uprooted by widening crevices; rain water penetrating into root openings further segregated the rock mass leading to massive slope failure.

Subsequent to the cloud burst on June 16-17, a very high flood stage caused saturation of low overbanks and heterogeneous slopes of poor strength, and with receding of water level, pore water pressure built up inducing slips. Such failures continue progressively to upper levels.

Environmentalists have pointed to hydro-projects in the region as having a role in intensifying, even initiating the slips, and have alleged that the Ministry of Environment and Forests (MoEF) while giving clearance to such projects has been glossing over some critical environmental issues arising from them. Such an observation needs prudent and unbiased analysis.

The Himalayan region is attractive for hydro-power generation because all the rivers in Jammu and Kashmir, Himachal Pradesh, Uttarakhand, Sikkim and Arunachal Pradesh descend from around 3,500m to 500m in a short, 200-km stretch. This water wealth is nature’s gift and a bounty for these relatively underdeveloped States, and for the country as a whole. This is not to say that abstraction of fresh water by blocking of rivers for power generation is being indiscriminately allowed disregarding either the geotechnical/seismic safety of the terrain or the riparian need of the river to support the needs of humans as well as terrestrial and aquatic ecosystem down the river.

This is precisely what the MoEF ensures by conducting, through an accredited consultant, an impact study and evolving mitigation measures, along with a cumulative study of adjoining projects followed by a public hearing. Through a critical and stringent examination of all environmental documents, the Expert Advisory Committee of the MoEF recommends environmental clearance (EC) in conjunction with the clearance of technical formalities of the projects by the Central Water Commission, Central Electricity Authority and Geological Survey of India. Issuing an EC, which is required for forest clearance, is neither a standalone activity nor does the prerogative rest solely with the EAC. A project normally takes five to eight years to go from the concept stage to getting the EC from the EAC.

Minimal impact

The selection of hydro projects in the Himalayan region was carried out in early 2000 by the Central Electricity Authority in consultation with the Central Water Commission. Against 50 feasible projects with the potential of 20,000 MW, the present status of hydro development in the Alakananda and Bhagirathi basins is that only four major projects — Tehri, Maneri-Bhali-I & II and Vishnuprayag of 3164MW capacity — have been commissioned. Another five projects are in different stages of implementation. Less than 40 km of riverine stretch has been impacted by these projects out of 800 km of main river and tributaries. No project component or its vicinity has suffered from landslip-induced failure, except flooding of debris into partially completed components.

On the contrary, the Tehri reservoir on the Bhagirathi held back the incoming devastating flood which attained a peak of 7000 cubic metre per second (cumec). And since the release was restricted to a mere 400 cumec — causing the reservoir to rise by 25 metre a day — the flood damage below the Tehri dam was minimal. Without the Tehri dam the combined flood of the Alakananda and the Bhagirathi would have exceeded 25,000 cumec at Rishikesh, possibly wiping out the prosperous urban river stretch at Rishikesh, Haridwar and Saharanpur.

It would be naive to say that the EAC of the MoEF, with members of proven domain expertise, is giving ECs without due diligence to hydro or river valley projects of which irrigation projects constitute a large share. The path ahead should be to learn lessons by an informed debate, analysing the technical issues of the hazard dispassionately. The immediate need is to enforce flood-plain zoning below Rudraprayag on the Alakananda and Maneri Bhali on the Bhagirathi and disallow permanent structures in flood-prone zones. The flattening and stabilisation of all highway slopes dictated by geological consideration is of highest priority.

Courtesy: The HIndu