Slope processes and related risk appearance within the Icelandic Westfjords during the twentieth century

International audienceIn North-western Iceland, records of slope processes were increasing during the twentieth century. Few dramatic events during the last decades highlighted the danger due to slope dynamics, leaving local populations in a risk situation that was merely unknown before 1970. The recent snow-avalanche, debris-flow and rock-fall activity underlined that the most frequent processes are not these with the largest human impact. In fact, the most catastrophic events were the extreme ones, following directly from a low frequency and a high magnitude. The purpose of this paper is to draw a parallel history of natural hazard and residence spatial extension, for an accurate understanding of the present-day risk situation, as the population growth markedly increased during the same time. Different quantitative and qualitative methods are applied. Geomorphological investigations locate the main threaten areas, in the path of slope processes release evidences, i.e. suitable slope morphology and/or inherited/actual forms. By a collection of dating data, as historic records and lichenometrical analysis, the frequency of given magnitude events is known. Climatic analysis clarifies the triggering meteorological conditions of slope processes and offers an overview of climate fluctuation during the investigated period; wind speed and direction is critical to hazardous snow-avalanche departure and snowmelt is crucial for debris-flow release. The findings clearly indicate that a combination of spatial expansion of inhabited areas and a lack of slope processes knowledge at the expansion time led to a recent and progressive risk appearance due to snow avalanches (including slush flows), debris flows and rock fall in most towns and villages of North-western Iceland