A VAST torrent of black, sulphurous water and ice from a remote sub glacial lake receded yesterday after peaking with force that damaged two long bridges on Iceland's vital coastal ring road, officials said.
Scientists had been predicting the cascade from Lake Grimsvotn, which was swollen in early October by a volcanic eruption under the Vatnajokul ice cap, Europe's largest glacier.
But they said the flow of melt water was faster and much more violent than they expected when it finally broke on Tuesday.
"The flood reached its peak late last night and is now receding," Dr Magnus Gudmundsson, a geophysicist at the University of Iceland in Reykjavik, said. "We estimate the water was flowing at 45,000 cubic metres (1.6 million cubic feet) per second at the peak of the flood at about 2230 GMT."
As light returned in the shortening northern hemisphere day, scientists planned to fly over the flooded area, a treeless uninhabited plain in south eastern Iceland, to assess the damage, in particular a partially wrecked 900 metre bridge over the river Skeidara.
An Icelandic radio reporter said two other bridges, including a 400 metre span over the Gigjukvis river, had been wiped out.
Only one of four bridges along the 35 km stretch of the coastal road, which provides a crucial link for the truck transport of fish catches from remote ports, was left untouched by the torrent.
The government had expected the cost of the flood in a worst case scenario to be at least 1.5 billion Icelandic crowns (£14 million) assuming all bridges were wiped out.
But the actual cost to public finances will be limited.
Most of the costs will be borne by an insurance fund set up by the government after a volcanic eruption in 1973.
Pressure under the Vatnajokul Glacier had been building up since the volcano erupted, bursting through the ice cap and sending a column of ash and steam up to 10 kms high.
The Prime Minister, Mr David Oddson, who saw the torrent at first hand on Tuesday afternoon, said that "in just four hours this has knocked us back 20-30 years in terms of our road building endeavours".
The eruption melted part of the ice cap, filling the sub glacial Grimsvotn Lake to bursting point. Most recent estimates put the volume of melted water collected in Grimsvotn at over 3,000 billion litres.
"Everything happened much faster than expected. It was the biggest flood this century in terms of how fast it flowed and how fast it reached its peak," Dr Gudmundsson said.
"The reason why it was so quick is probably because the water level was so high. The weight of the ice is normally sufficient to contain the lake but now the water lifted the ice cap " he laid.
A seismograph close to the glacier recorded a tremor late on Monday that indicated the water was moving, Dr Gudmundsson said.
The volcano went quiet in mid October but the North Atlantic island, with a long history of natural disasters, had remained on alert for massive flooding.
Authorities have been reinforcing dykes and building channels through the road to direct the flood.
Hydrologist, Dr Arni Snorrason, was measuring river levels at the foot of the glacier when the floods began around 8 a.m. (local time) on Tuesday.
"We had to run for our lives when it broke through. It's picking up speed much faster than we expected," he told National Icelandic Radio.
The Vatnajokul glacier stretches over 8,300 sq km and has a depth in parts of 1,000 metres.
It is about 250 km east of Reykjavik and about the same distance south of the Arctic Circle.