A glacial lake outburst flood (GLOF) is a type of outburst flood caused by the failure of a dam containing a glacial lake. An event similar to a GLOF, where a body of water contained by a glacier melts or overflows the glacier, is called a jökulhlaup. The dam can consist of glacier ice or a terminal moraine. Failure can happen due to erosion, a buildup of water pressure, an avalanche of rock or heavy snow, an earthquake or cryoseism, volcanic eruptions under the ice, or massive displacement of water in a glacial lake when a large portion of an adjacent glacier collapses into it. Increasing glacial melting because of climate change, alongside other environmental effects of climate change (i.e. permafrost melting) mean that regions with glaciers are likely to see increased flooding risks from GLOFs. This is especially true in the Himalayas where geologies are more active. A 2023 study found 15 million people at risk from this hazard, mostly in China, India, Nepal, Pakistan, and Peru.
Definition
A glacial lake outburst flood is a type of outburst flood occurring when water dammed by a glacier or a moraine is released. A water body that is dammed by the front of a glacier is called a marginal lake, and a water body that is capped by the glacier is called a sub-glacial lake. When a marginal lake bursts, it may also be called a marginal lake drainage. When a sub-glacial lake bursts, it may be called a jökulhlaup. A jökulhlaup is thus a sub-glacial outburst flood. Jökulhlaup is an Icelandic term that has been adopted into the English language, originally referring only to glacial outburst floods from Vatnajökull, which are triggered by volcanic eruptions, but now is accepted to describe any abrupt and large release of sub-glacial water. Glacial lake volumes vary, but may hold millions to hundreds of millions of cubic metres of water. Catastrophic failure of the containing ice or glacial sediment can release this water over periods of minutes to days. Peak flows as high as 15,000 cubic metres per second have been recorded in such events, suggesting that the v-shaped canyon of a normally small mountain stream could suddenly develop an extremely turbulent and fast-moving torrent some 50 metres (160 ft) deep. Glacial Lake Outburst Floods are often compounded by a massive river bed erosion in the steep moraine valleys, as a result, the flood peaks increase as they flow downstream until the river reaches, where the sediment deposits. On a downstream floodplain, it suggests a somewhat slower inundation spreading as much as 10 kilometres (6.2 mi) wide. Both scenarios are significant threats to life, property and infrastructure.
Monitoring
The United Nations has a series of monitoring efforts to help prevent death and destruction in regions that are likely to experience these events. The importance of this situation has magnified over the past century due to increased populations, and the increasing number of glacial lakes that have developed due to glacier retreat. While all countries with glaciers are susceptible to this problem, central Asia, the Andes regions of South America and those countries in Europe that have glaciers in the Alps, have been identified as the regions at greatest risk. There are a number of imminent deadly GLOF situations that have been identified worldwide. The Tsho Rolpa glacier lake is located in the Rolwaling Valley, about 110 kilometres (68 mi) northeast of Kathmandu, Nepal, at an altitude of 4,580 metres (15,030 ft). The lake is dammed by a 150 metres (490 ft) high unconsolidated terminal moraine dam. The lake is growing larger every year due to the melting and retreat of the Trakarding Glacier, and has become the largest and most dangerous glacier lake in Nepal, with approximately 90–100 million m3 (120–130 million yd3) of water stored.
Examples
Asia
India
In June 2013, Kedarnath in Uttarakhand witnessed flash floods along with a GLOF caused by Chorabari Tal, killing thousands of pilgrims, tourists and residents who came to visit the place.
Pakistan Pakistan has more than 7000 glaciers, which is more than anywhere else in the world, except for the polar regions. As of 2018, more than 3,000 glacial lakes had formed in Gilgit-Baltistan, with 30 identified by the UNDP as posing an imminent threat of glacial lake outburst flooding. In 2017, a "Scaling up of Glacial Lake Outburst Flood Risk Reduction in Northern Pakistan Project" was continued. In 1929, a GLOF from the Chong Khumdan Glacier in the Karakoram caused flooding on the Indus River 1,200 km downstream (a maximum flood rise of 8.1 m at Attock).
Bhutan
GLOFs occur with regularity in the valleys and low lying river plains of Bhutan. In the recent past, flash floods have occurred in the Thimphu, Paro and Punankha-Wangdue valleys. Of the 2674 glacial lakes in Bhutan, 24 have been identified by a recent study as candidates for GLOFs in the near future. In October 1994, a GLOF 90 kilometres (56 mi) upstream from Punakha Dzong caused massive flooding on the Pho Chhu River, damaging the dzong and causing casualties. In 2001, scientists identified Lake Thorthormi as one that threatened imminent and catastrophic collapse. The situation was eventually relieved by carving a water channel from the lip of the lake to relieve water pressure.
Nepal
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