The Geomorphological Legacy of the Rockslide and Outburst Flood in Steinsholtsdalur in 1967

Abstract

High-magnitude floods are extreme geologic agents, transforming landscapes through erosion and deposition within a matter of minutes. On January 15th, 1967, a rockslide fell onto the Steinsholtsjökull outlet glacier and into its proglacial lake on the northern flank of the Eyjafjallajökull ice cap in South Iceland. The rockslide caused a catastrophic glacial lake outburst flood down Steinsholtsdalur valley, the Steinsholtshlaup, producing a unique geomorphological imprint along the flood path. The primary goal of this study was to analyze the post-flood geomorphology with drone surveys, Digital Elevation Models (DEMs) of pre- (1960) and post-flood (1967) surfaces, volumetric calculations, and field mapping. Palaeohydraulic calculations, rockslide volumes, and hummock morphology were used to assess landform formation and the redistribution of debris, and the flow hydrology of the flood. Our results show that the initial rockslide displaced ~20 M m3 of bedrock, which was deposited and reworked throughout Steinsholtsdalur, forming three distinct sets of large sediment hummocks, boulder erratics, boulder bars, and kettle holes. The morphology of these features was analyzed concerning their deposition post-Steinsholtshlaup to advance our understanding of calculated flood parameters (e.g., mean flow depth, average flow velocity, and discharge). This study extends our understanding of the variety and complexity of outburst floods from rockslides into proglacial lakes, which may occur more frequently in the future due to impacts of climate change in high-mountain regions.