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Landslides in downstream areas far from the dam may not be catastrophic: expert

By Ajit Patowary

GUWAHATI, June 3 - Landslides in the downstream of the under-construction 2000-megawatt (MW) Lower Subansiri Hydroelectric Power Project (LSHEP), far from its immediate vicinity, is not that risky, contrary to the landslides taking place in the upstream of its reservoir or dam. Any landslide in the filled-up reservoir area may cause heavy damage to the dam and structures associated with it, thus possibly resulting in a tsunami-like situation. But, that probably won�t happen if the landslide takes place in the downstream areas far-off the reservoir/dam, said Prof Sarada Kanta Sarma, a London-based globally famous earthquake engineer.

Prof Sarma was contacted in the wake of the May 26 landslide incident in the LSHEP project site.

Prof Sarma, a professor emeritus in engineering seismology at the Imperial College, London, said examining the safety of the slopes upstream of the dam is mandatory for those connected with the project. In the immediate vicinity of the dam, even in its downstream areas, these checks are obligatory.

Before building a dam, it is absolutely essential to check the safety or stability of the slopes in the entire upstream region, and, if necessary, taking measures to improve the slopes� safety, either before or with the progress of dam construction. But, this should not be left till the completion of dam construction, he said.

Safety or stability of the slopes depend on two factors � the strength of soil, rocks, or rock joints vis-a-vis the applied load. Applied load is the gravity and the seismic load. Strength of the soil depends on the geological materials and the water present inside. Seismic load also affects soil strength, reducing it due to the cyclic nature of the load.

Water affects slope strength considerably. This is why landslides occur in the rainy season. If earthquake occurs during rainy season, it amplifies the load, and, the slope strength diminishes under the impact of both the water and the earthquake. Thus, slope failure increases considerably when earthquakes occur in the rainy season. Here, one may consider the case of the 1950 Assam earthquake, when too many landslides occurred in the Himalayan region.

A dam is built with a factor of safety in calculations. The earthquake factor is a probabilistic number in the design. Therefore, there is every likelihood of the design factor exceeding some time during the structure�s life. Usually, design details take this into account. But if the seismic load exceeds considerably, compared to the design factors, failure is very likely to happen. Therefore, it is essential to get the design factor right.

Failures of slopes in upstream regions of dams are common. This usually happens soon after the filling of reservoirs even where there were no such slope failures. In Assam, where slope failures are so commonplace with or without earthquakes, it is imperative that slopes are checked for stability concurrently with the progress of dam construction, he said.

Referring to Italy�s Vajont Dam, where in 1963 the dam structure did not fail, he said the wave of water overtopping the dam caused heavy damage to the downstream areas killing about 2000 people. The dam now stands as a monument of human folly, and not used, he said.

LSHEP Executive Director (ED) Arvind Bhat said the May 26 landslide occurred on the left bank of the confluence point of the Deonala channel with the Subansiri, around 500 metres away from the dam. Deonala carries the water from the LSHEP diversion channel.

The landslide resulted in landmass slipping in an area around 50 metres in length and two metres in width. It resulted from non-completion of slope repairs due to COVID-19 lockdown. Though it has not threatened the dam, it has totally damaged the road leading to the dam�s left bank. This was, however, apprehended. Therefore, a tunnel road�s drawings were prepared in March. It�s construction is on, with six months� deadline for completion, the ED said.

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