In the Himalayan mountainous region of India, flash floods are nothing new. The residents of the Himalayas have witnessed several such incidents before,during and after the monsoon. On Tuesday, 5 August,at around 1.30 p.m., the inhabitants of Dharali, Uttarakhand, witnessed yet another devastating flash flood. Again, we lost many fellow citizens and suffered damage to numerous national assets. At any gauge station, if the water level rises more than one metre above the pre-determined danger mark, it is considered a flood. When the floodwater level exceeds 10 metres above the danger level, it is classified as a devastating flood.

The water level in the Dharali flash flood undoubtedly exceeded this limit. The scale of destruction caused by a flood depends on the water volume, the height of the flood level, and the nature of land use by human settlements along the riverbanks. In mountainous regions, flash floods occur due to various reasons. Sometimes, in a narrow valley, very heavy rainfall over a short period of time can trigger such events. For instance, the village of Harsil near Dharali experienced such an incident in July 2012. At other times, large volumes of water suddenly rush down from higher Himalayan glaciers or glacial lakes into narrow valleys, creating devastating floods.

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These events are called Glacial Lake Outburst Floods (GLOFs). Examples include the Kedarnath disaster of 16 June 2013 and the Lhonak Lake Disaster of 4 October 2023 in Sikkim. Another example occurred on 7 February 2021, when a glacier collapse in the Tapovan-Reni area triggered flash floods in the Dhauliganga and Alaknanda rivers. Such floods can also result from the sudden breach of man-made reservoirs or dams. Scientists have found that in some narrow rivers of the mountain regions, massive landslides sometimes block river channels, forming what are called natural temporary dams.

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Later, due to heavy rainfall, the large volume of water stored behind boulders and rock masses builds up pressure, causing the dam to break, and........

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