Slushflow hazard and mitigation in vesturbyggd, northwest Iceland

Abstract

Slushflows - flowing mixtures of water and snow - threaten several residential areas within Vesturbyggo, a municipality of 1250 people located in the Vestfirdir peninsula in Northwest Iceland. An overview of the problem is given and particular attention is drawn to the catastrophic events of January 22nd 1983 in Patreksfjörour, when two slushflows damaged or destroyed 20 houses, killing four of the twelve people caught in the them. As the runout zones of the slushflows are densely populated the local authority requested an appraisal study for slushflow defences. For the main hazard zone below Geirseyrargil the work included an evaluation of potential starting zones, hazard areas, mitigative measures and cost estimates compared with the value of the involved properties. Slushflow activity in Vesturbyggo and other parts of Iceland is associated with cyclonic activity during winter as well as spring break-up periods. Most commonly, slushflows are released in drainage channels and streams, but they may also start from inclined depressions. Snow-embanked, water-saturated snowfields are other potential starting zones. Whether a snowpack will reach a critical stability during rain and snowmelt depends on the complex interaction between geomorphic factors, snowpack properties and the relative rate of formation and discharge of free water in the snowpack in potential starting zones. By analysis of meteorological records from the autumn of 1982 and January of 1983 the snowpack development prior to the events and stability of the snowpack at the start of the critical weather period in January 1983 are estimated.