Lahar, floods and debris flows resulting from the 2010 eruption of Eyjafjallajökull: Observations, mapping, and modelling

Historic, post-eruptive debris flows of remobilised volcanic ash are rare in Iceland, being restricted to explosive eruptions. Volcanic ash slurry from the southern slopes of the icecapped Eyjafjallajökull volcano on 19 May 2010 is the first lahar observed in Iceland since the 1947 Hekla eruption. This study focuses on the volume of sediment transported, the size and hydrological behavior of watersheds, and the resulting erosion. The analysis is based on: (1) direct measurements of the 19 May lahar; (2) direct measurements of ash fallout; (3) aerial and ground-based imagery; (4) topographic data from an airborne LIDAR survey; (5) airborne synthetic-aperture radar; and (6) precipitation data. The volume of the lahar in the Svadbælisa channel was estimated at 200,000 m³. This flow originated from crown and flank failures, similar to slab avalanches, with water-saturated, fine-grained ash as the slip surface. Several ash-laden floods occurred in Svadbælisa and neighboring channels during the summer of 2010. None, however, were as saturated as the 19 May lahar. An increased number of small debris flows were also recorded some blocking roads to farms. Precipitation during the summer of 2010 was not higher than average and therefore does not explain this increased erosion. Large quantities of volcanic ash mantle the lower slopes of the icecap. Ash in the ablation zone is expected to be transferred down-slope in the next few years inducing the erosion to the root of the mountain endangering homes and infrastructure. Fieldwork during the summer of 2010 has resulted in a map showing the volume of ash above and below the ablation zone of the main catchments and recorded erosion events. This data was used to assess the hazard and the need for immediate actions.