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The ominously swelling torrent of grey water churns down the gully as onlookers wait to see how bad it will get.
First large rocks and then boulders the size of cars can be seen tumbling amid the growing cascade.
Suddenly an unstoppable wall of water and rock surges round the valley bend, obliterating a concrete bridge and roaring on down the valley.
The video captured recently in northern Pakistan shows the terrifying destructive force of a growing natural threat to millions of people.
Glaciers found in the country’s awe-inspiring mountain ranges are melting as global temperatures rise. The meltwater collects as glacial lakes that are often only held back by thinning ice walls, or fragile earth banks.
When these barriers break, perhaps when pressure gets too much or an earthquake strikes, in an instant millions of tons of water can be unleashed into the populated valleys below.
As the mass of water known as a glacial lake outburst flood, or GLOF, careers down steep-sided valleys it accelerates and picks up debris.
It takes little imagination to consider what would happen when the accelerating wall hits a town or village.
“It’s an outburst of debris and glacial meltwater which then takes down everything on its path,” Abduvakkos Abdurahmanov, a United Nations technical specialist in Pakistan, told the Telegraph.
In 2010 the Booni Gol Glacier, near Chitral, generated such an outburst flood that killed 1,980 people, injured an additional 2,946 more, and destroyed some 1.6 million homes. Thousands of acres of scarce agricultural lands were damaged.
Research by the United Nations Develop Programme (UNDP) estimates millions of people are now at risk from such flooding.
Pakistan’s site at the confluence of the Himalaya, Hindu Kush and Karakoram mountain ranges means it has at least 5,000 glaciers, more than any country outside the polar regions.
Glaciers feed the Indus River system, the country’s precious water lifeline. But data from the past 50 years shows that all but about 120 of the glaciers show signs of melting. In the past 80 years, meteorologists say the average temperature in mountainous Gilgit-Baltistan has increased by 1.4C (2.5F) compared to a rise of 0.6C (1F) in the lower-lying provinces.
The melting ice has created some 3,000 glacial lakes and a survey has found 33 considered at risk of bursting in Gilgit-Baltistan and Khyber Pakhtunkhwa, threatening seven million people.
With so much of these areas inaccessible and unsurveyed, the figure for glacial lakes, including those that form a risk to people, could be higher. As Pakistan’s population grows sharply and agricultural land becomes more scarce, people are also moving further up valleys into the danger zone.
The government of Pakistan and UNDP have now launched a joint £29m ($37m) project to try to protect these people.
The plan involves building defences, but also forming early warning networks and creating an inventory of dangerous lakes.
Defences at key points on a flood’s potential path can deflect or slow its force. They can also stabilise vulnerable slopes to prevent floods triggering landslides and debris that add to the flood’s impact.
The engineers toolkit will includes walls made of cages of rocks, like those sometimes seen in coastline sea defences, terracing, channels to divert flows, dams, and tree-planting on deforested slopes.
Mr Abdurahmanov said the defences may not survive the vast natural force of the floods, but they only needed to work once.
He said: “It can be destroyed at the spot, but it’s done it’s job, basically it’s reduced the impact.”
Advance warning can also save lives. When a glacial lake burst in Gilgit-Baltistan in July, residents of Badswat village, in Ishkoman valley at the foot of the Hindu Kush, saw the flood wipe out homes, roads and bridges, as well as crops and forest. A timely evacuation meant nobody had died though.
Training teams of local residents to look for warning signs and giving them communications gear could lend valuable time. Search and rescue skills will also be taught for the aftermath of floods.
Widespread cutting down of forests has added to the flooding problem in Pakistan’s mountains. Deforestation for timber and firewood has left behind precarious scree slopes that can be swept away in floods, while it has also robbed Pakistan of trees considered essential to absorb carbon emissions.
Imran Khan has pledged a nationwide “10 Billion Tree Tsunami” planting campaign over the next five years.
A 2017 global climate change risk index ranked Pakistan as the seventh most under threat country in the world. Rising temperatures are expected to diminish harvests, erode coastlines, and cause more extreme weather like cyclones, floods and droughts.