No abstract

Debris flows in the Gleivarhjalli area in northwestern Iceland occurred after a sudden and intensive snowmelt period during 10–12 June, 1999. The area, in the northwestern part of the town of Ísafjörvur, was chosen for a detailed study. Meteorological data and bedrock conditions, triggering mechanisms and geomorphological and human impacts were examined. This paper describes and emphasises the role of rapid snowmelt as a mechanism for the release of debris flows in a subpolar basaltic fjord setting. Post‐event mapping of erosional and depositional landforms showed strong geomorphic impacts of debris flows and their role in mass transfer in a mountainous environment. The estimated denudation rate for the singleevent is 0.29 mm/km2. The use of a new lichen growth curve provides relative dating of previous unreported events. Finally, the paper estimates the mean return period for debris‐flow events in the Gleivarhjalli area as 4–5 years, thus constituting a serious threat to the community.

International audienceIn de-glaciated areas, para-glaciation (i.e. the conditioning of landscapes by prior glaciation) has often been considered a major predisposing factor in landslide occurrence; its consequences have been particularly well identified at a fine scale (especially on bedrock jointing). Hitherto, the relative impacts of para-glaciation on hillslope dynamics at a regional scale had nevertheless not been quantified statistically. We examine Skagafjörður area (northern Iceland) where landslides are widespread (at least 108 were mapped in an area of c. 3000km2).We compare the role of para-glaciation (debuttressing, influence of post-glacial rebound) with that of classic factors (topography, lithology, etc.) in landslide occurrence and location, using a spatial analysis based on a chi-square test. On the one hand, the results highlight that landslides are over-represented in areas where post-glacial rebound was at its maximum, with a stronger concentration of landslides in the northern part of the fjord. On the other hand, the distribution of landslides did not show any clear relationship with the pattern of glacial debuttressing. Tschuprow coefficient highlights that the influence of post-glacial rebound on landslide location is higher than the combined influence of slope gradient, curvature or geological structure. This result is supported by our initial evidence for the timing of landslides in the area: most landslides occurred during the first half of the Holocene, and a period of hillslope instability was initiated when the post-glacial uplift was at its maximum. Finally, the mechanisms that link post-glacial rebound and landsliding as well as the geomorphic impacts of landslides, are discussed