GLACIAL OUTBURST FLOOD / AVALANCHE

GLACIAL OUTBURST FLOOD / AVALANCHE

Subject: Geography

Context : A glacial lake burst, a cloud burst or an avalanche, the impact of climate change or “development” — scientists are not sure what triggered the sudden surge of water near Chamoli in Uttarakhand Sunday morning that briefly raised fears of a repeat of the 2013 disaster in the state.

Concept:

Glacial Lake Outburst Flood

• A glacial lake outburst flood (GLOF) is a release of meltwater from a moraine– or ice-dam glacial lake due to dam failure.
• GLOFs often result in catastrophic flooding downstream, with major geomorphic and socioeconomic impacts.

GLOFs have three main features:

• They involve sudden (and sometimes cyclic) releases of water.
• They tend to be rapid events, lasting hours to days.
• They result in large downstream river discharges (which often increase by an order of magnitude).

Direct Causes

• The following direct causes of glacial lake outburst floods were documented:
• Rapid slope movement into the lake
• Heavy rainfall/snowmelt
• Cascading processes (flood from a lake situated upstream)
• Earthquake
• Melting of ice incorporated in dam/forming the dam (including volcanic activity-triggered jökulhlaups)
• Blocking of subsurface outflow tunnels (applies only to lakes without surface outflow or lakes with a combination of surface and subsurface outflow)
• Long-term dam degradation

Glacial Lakes

• There are two main settings in which glacial lakes form:
• Behind moraine dams
• Behind ice dams.

Moraine Dammed Lakes

• When a glacier retreats, it can leave behind a large impression in the ground that fills with water, turning it into a lake.This is typically known as a moraine.
• Moraine-dammed lakes form during periods of glacier retreat from a moraine.
• As a glacier margin retreats, water collects in the topographic low between the ice-front and the abandoned frontal and/or lateral moraine.
• These lakes can be impounded by an unstable pile of debris and buried ice.
• Most existing moraine-dammed lakes were formed when mountain glaciers began to retreat from large moraine ridges constructed during the Little Ice Age.

Ice Dammed Lakes

• In contrast to moraine-dammed lakes, ice-dammed lakes form when drainage is blocked by a glacier that advances or becomes thicker.
• Consequently, ice-dammed lake growth is closely related to glacier mass balance and climate.
• Ice-dammed lakes form wherever a glacier blocks the drainage of meltwater.

AVALANCHE

• Avalanche is a rapid downslope movement of a large detached mass of snow, ice, and associated debris such as rocks and vegetation.
• Small avalanches, or Sluffs, occur in large numbers, while large avalanches that may encompass slopes a kilometer or more in length with millions of tons of snow, occur infrequently but cause most of the damage.
• Humans have been exposed to the threat of sliding snow for as long as they have inhabited mountainous regions.

Types of Avalanches:

• Loose snow avalanches-formed in snow with little internal cohesion among individual snow crystals.
• Slab avalanches-Originate in snow with sufficient internal cohesion to enable a snow layer, or layers, to react mechanically as a single entity.
• Wet snow avalanches-These move more slowly than dry ones and are seldom accompanied by dust clouds.
• Direct action avalanches –These are the immediate result of a single snowstorm. They usually involve only fresh snow.
• Climax avalanches-Result of a series of snowstorms or a culmination of weather influences. Their fall is not necessarily associated with a current storm or weather situation.