Foundation, Concrete and Earthquake Engineering

Seismic Vulnerability of Tipaimukh Dam

The Vajont dam, Italy: October 1963
An earth took place and reservoir of this dam began to fill One tremor set off landslides that plunged into the reservoir, creating a huge wave that overtopped the dam by 110 metres. About two minutes later, the town of Longarone was leveled and almost all of its 2,000 inhabitants killed. 

This example is mentioned to visualize the risk of constructing Tipaimukh Dam in the Brahmaputra valley and its adjoining hill ranges. Due to the colliding Eurasian (Chinese) and Indian tectonic plates, this region is seismically very unstable and the region has seen some major. Above example indicates that even the Tipaimukh Dam remain intact after earthquake the downstream may wash out.

The Vajont Dam Failure : Longarone Village
Currently, the only mandatory risk assessment requirement is to conduct a ‘dam-break analysis’ which predicts the effects of flooding downstream, in case the dam actually breaks. Let us take the example of earthquakes. Currently the focus is only on whether the dam will withstand the earthquake. Occasionally, the issue of whether the water reservoir itself can induce seismic activity is discussed. While these are both very important aspects, they are not the only earthquake associated risks as far as dams are concerned.

The Vajont Dam: A view from  village Longarone
Researchers in the Northeast have been highlighting overall impacts of earthquakes on river systems, which can increase risks to and from existing large dams. Dam engineers are quick to point out that a particular dam may survive a major earthquake, but even assuming that the actual structure is able to withstand a powerful tremor, quake-induced changes in the river system may have a serious impact on the viability of the project itself, as several basic parameters vis-à-vis the regime of rivers, and the morphology and behaviour of channels, may change. The last two major earthquakes in the region (1897 and 1950) caused landslides on the hill slopes and led to the blockage of river courses, flash floods due to sudden bursting of these temporary dams, raising of riverbeds due to heavy siltation, fissuring and sand venting, subsidence or elevation of existing river and lake bottoms and margins, and the creation of new water bodies and waterfalls due to faulting.

Location of Tipaimukh Dam
Location of Tipaimukh Dam
 Analysis of the available scientific data clearly indicates that the neotectonism of the Brahmaputra valley and its surrounding highlands in the eastern Himalayas has pronounced effects on the flooding, sediment transport and depositional characteristics of the river and its tributaries, which in turn has a bearing on the long-term viability of dams. The earthquake of 1950, for example, raised the bed level of the Brahmaputra at Dibrugarh by at least three metres (10 feet) leading to increased flood and erosion hazard potential in the river. Brahmaputra expert, Dr. Dulal Goswami, says: “A single earthquake event could cause sedimentation equivalent to several decades of normal sedimentation during the high flow period.” This could certainly render many of the proposed dams economically unviable as dam life is intricately connected with rates of sedimentation.

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