hlaup

Hlaup is a term used in glaciology and Icelandic hydrology to describe a rapid release of water from a glacier-dammed lake. In English-language literature the phenomenon is commonly called a glacial outburst flood or jökulhlaup. The Icelandic root hlaup means "run" or "surge," reflecting the sudden onset of water flow.

Causes and mechanisms: A hlaup begins when meltwater accumulates in a lake held back by an ice dam. The dam can fail due to overtopping, hydrofracturing, or rapid melting beneath or within the ice, sometimes aided by volcanic or geothermal heating that weakens the glacier. When the dam bursts, a pulse of water surges downstream, often following subglacial channels or tunnels that route the flow.

Characteristics: Hlaups are marked by a sudden rise in water level, very high discharge, and rapid onset that can last from hours to days. They transport large amounts of sediment from the glacier, giving the water a milky or turbid appearance. The flood wave can travel long distances and cause rapid erosion, debris flows, and temporary changes to river channels and floodplains.

Impacts and monitoring: The effects can include damage to infrastructure such as roads, bridges, and hydropower installations, as well as landscape changes from erosion and sediment deposition. Monitoring often involves hydrological networks, remote sensing, and hazard mapping to provide early warnings and mitigate damage.

Examples and regional context: Iceland provides the best-known examples of hlaup, particularly those linked to volcanic activity beneath Vatnajökull. Similar processes occur in other glacier-covered regions where lakes behind ice bodies drain catastrophically, producing glacial outburst floods.