On a clear Friday in August, a helicopter flies across the Sima Valley in Eidfjord in Hardanger, Norway. On board is builder Harald Kleiveland and two of his carpenters, as well as a load of materials. They have been hired by the local trekking association to repair the woodshed at the cabin by Lake Demmevatn.
"From the helicopter we could see Lake Nedre Demmevatn below us," says Kleivedal, who seized the photo opportunity. On Saturday, he also took pictures of seven hikers who were walking along the lake.
"But the next time we looked down on the lake, it was gone. A very strange experience!"
The only sign that not all was as it should be this late summer weekend was a loud noise from the Rembesdalskåka glacier tongue.
Climate change means new challenges.
Historical sources tell tales of major flood disasters in the Sima Valley, caused by jökulhlaup – a term used for flash floods that occur when trapped water finds a way under the glacier.
"This is what happened in August," says Vidar Riber, head of Hardanger power plant group.
"The hot summer caused heavy melting on the glacier, at the same time as the water temperature rose," says Riber. "The combination of thin ice and warmer water enabled the water to melt a channel under the Rembesdalskåka glacier tongue. As a result, all the water disappeared over the course of a few hours, and ended up in Statkraft's reservoir Lake Rembesdalsvatn."
Statkraft's readings, which show how the water from Lake Nedre Demmevatn flooded into Lake Rembesdalsvatnet over a few hours, were initially ignored because they were unrealistic.
"The figures showed that the water level rose significantly over a few hours in the morning on Monday, 25 August," says Riber. "All in all, a couple of billion litres disappeared from Lake Nedre Demmevatn."
Everything was nice and quiet, until this summer.
In the past, the people of the Sima Valley were used to floods every late summer, as water would lift Rembesdalskåka and flow down to Lake Rembesdalsvatn, which flooded.
A powerful flood in 1893 caused significant damage, and led to a 380-metre long bypass tunnel being blasted in 1899 to prevent future disasters.
"In 1937, the tunnel was clogged, and there was a new, major disaster," says Riber. "In four or five hours, 12 billion litres of water flooded the Sima Valley, taking houses and fields with them. It destroyed the entire valley."
In 1938, a new bypass tunnel was ready. This lowered the water level in Lake Nedre Demmevatn.
"Since then, Lake Nedre Demmevatn has been but a puddle," says Riber. "A lot less water, and the reservoir in Lake Rembesdalsvatn, has removed the risk of any major floods."
However, the disappearing lake has had consequences for Statkraft.
"It means the loss of 250 meters of elevation difference, which means a loss of money," says Riber. "If this is permanent, we need to do something to lead the water into the tunnel."
We lose 250 meters of elevation difference, which means a loss of money. If this is permanent, we need to do something to lead the water into the tunnel.
1: Lake Nedre Demmevatnet
2: Lake Midtre Demmevatnet
3: Lake Øvre Demmevatnet
4: Bypass tunnel completed in 1899
5: Bypass tunnel completed in 1938
6: Tunnel to Lake Holmavatnet, completed in 1978
7: The Demmevasshytta cabin
8: Lake Rembesdalsvatnet
When Lake Nedre Demmevatn (1) disappeared, the water did not flow like it should have through the tunnel (6) to Lake Holmevatnet, but under the Rembedalskåka glacier tongue and into Lake Rembedalsvatnet (8).
Senior engineer Hallgeir Elvehøy of the Norwegian Water Resources and Energy Directorate (NVE) believes climate change is the cause of the disappearance of Lake Nedre Demmevatn in August.
"The thickness of the glacier tongue Rembesdalskåka varies with the climate," explains Elvehøy. "During the Little Ice Age around 1750, the glacial front ran halfway out into what is Lake Rembesdalsvatnet today, but since the 1800s it has gradually shrunk."
The influx of local water and glacial water ensures that Lake Nedre Demmevatn will always be refilled.
"When Rembesdalskåka was larger, water followed a route over the ice or along the glacier's edge when Lake Nedre Demmevatn flooded," he adds. "However, when the water ran underneath the ice, and all the water disappeared in a short period of time, jökulhlaup occurred, causing major flood disasters."
In the 1900s, the glacier shrunk even more, causing the 1937 disaster. "As the new tunnel was ready, the water level in Lake Nedre Demmevatn was lowered 50 metres," says Elvehøy.
"Everything was nice and quiet, until this summer."
Elvehøy believes the tunnel created by the water through the ice will close during the winter. He also thinks that we will see a jökulhlaup again. However, when and where is difficult to predict.
"We need to be prepared for this to happen again," he says. "There is a clear connection with climate change, and an issue we will see more of in the future, both for power plants and unregulated waterways. It all depends on the terrain under the glacier, and we do not know how this is until the glacier pulls back. The only thing we can do is to monitor the glacier's development."
Elvehøy says that if people had been present where the jökulhlaup occurred at Lake Nedre Demmevatn, the outcome might have been much more dramatic.
"It all happens so fast. You might hear a loud bang, and then the water comes rushing," he says. Elvehøy believes the loud noise Kleivedal heard could have been the glacial front collapsing. The glacial front was 40 to 50 metres high, and was supported by the water in the lake. When the water disappeared, the front became unstable and eventually collapsed.
Text: Sissel Fantoft
Photo: Statkraft, Tommy Andresen
Also published in Statkraft's magazine People & Power no. 4/2014
Norway's sixth largest glacier, located in Ulvik and Eidfjord municipalities in Hordaland County, in the very north of the Hardangervidda mountain plateau.
At its thickest, the glacier is 380 metres thick.
The glacier has shrunk significantly during the 1900s. The front of the glacier has pulled back several hundred metres. In 1925, the top of the glacier was at 1876 metres above the sea. In 1995, it had shrunk to 1863 metres.
An Icelandic term that has been adopted into the English language. It describes any large and abrupt release of water from a sub glacial or pro glacial lake/reservoir.
Source: Store norske leksikon
10. Nov. 2014