Glacial lake outburst flood

From Wikipedia, the free encyclopedia

Hubbard Glacier, Alaska squeezes towards Gibert Point on May 20, 2002. The glacier is close to sealing off Russell Fjord at top from Disenchantment Bay at bottom.

In this Hubbard Glacier image from July 16, 2002, the glacier has closed off Russell Fjord from Disenchantment Bay. The waters behind the glacier rose 61 ft (18.6 m) in 10 weeks, creating a short lived Russell Lake.

The Hubbard Glacier is overwhelmed on August 14, 2002 in the second largest GLOF in historical times.

A glacial lake outburst flood (GLOF), also known as a jökulhlaup in Icelandic, can occur when a lake impounded by a glacier or a terminal moraine dam fails. This can happen due to erosion, a buildup of water pressure, an avalanche of rock or heavy snow, an earthquake or cryoseism, or if a large enough portion of a glacier breaks off and massively displaces the waters in a glacial lake at its base.

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.^[1]

There are a number of imminent deadly GLOFs situations that have been identified worldwide. The Tsho Rolpa glacier lake is located in the Rolwaling Valley, about 110 kilometers (68 miles) northeast of Kathmandu, Nepal, at an altitude of 4580 m (15,026 ft). The lake is dammed by a 150 m (492 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 to 100 million m³ (117 to 130 million yd³) of water stored.^[2]

A Jökulhlaup is a glacier outburst flood, principally from an ice-dammed lake. Jokulhlaup is an Icelandic term that has been adapted into the English language, and originally only referred to a glacier outburst flood triggered by a volcanic eruption, but now is accepted to describe any abrupt release of water dammed by a glacier. The most famous are the immense Jokulhlaup released from the Vatnajokul Ice Cap in Iceland. The peak water release from a lake that develops around the Grimsvotn Volcanic Crater in the center of the Vatnajokul ice cap generates flows that exceed the volume of the Mississippi River. The outbursts have occurred in 1954, 1960, 1965, 1972, 1976, 1982, 1983, 1986, 1991 and 1996. In 1996 the outburst was very large yielding 6,000 cubic meters per second at peak flow. Some jokulhlaups release annually. Lake George near the Knik River had large annual outbreaks from 1918 to 1966. Since 1966 the Knik Glacier has retreated and an ice-dam is no longer created. Lake George might resume annual floods if the glacier thickens again and blocks the valley (Post and Mayo, 1972).

A GLOF occurred September 6 to September 12, 2003, from Grasshopper Glacier in the Wind River Mountains, Wyoming. Flow at the gage site, 14 miles downstream, rose from 7.1 m³/s (250 feet³/s) base flow to a peak of 37 m³/s (1300 feet³/s) on September 9. The Downs' Fork Bridge was over-topped, woody debris was left on the bridge, and its abutments were damaged.^[3]

Jökulhlaups have also been known to occur when a volcano erupts under a glacier. The ice over the volcano melts because of the heat, causing water to form a lake under the remaining ice-cap. Then the ice-cap collapses or the water breaks through the barrier in front of it, and there follows a more or less disastrous flooding of the land below the mountain. These events are similar to a type of lahar.

It is not by chance that the term Jökulhlaup comes from the Icelandic, as the south of Iceland has very often been victim to such catastrophes. This was the case when in 1996, the volcano under the Grímsvötn lakes belonging to the Vatnajökull glacier erupted, and the river Skeiðará flooded the land in front of Skaftafell National Park. The Jökulhlaup reached 45,000 m³s^-1 and destroyed parts of the Hringvegur (road no. 1). After the flooding some icebergs 10 m high could be seen on the banks of the river where the glacier run had left them behind (see also Mýrdalsjökull).

Immense prehistoric GLOFs, known as the Missoula Floods, occurred in North American's Columbia River watershed towards the end of the last ice age. They were the result of periodic breaches of ice dams in present day Montana, resulting in the draining of a body of water now known as Glacial Lake Missoula.