Tenth-century subglacial eruption flood modelling at Sólheimajökull, Southern Iceland

Abstract

Jökulhlaups (glacial outburst floods) are commonly linked to volcanism underneath Mýrdalsjökull. These mega-floods can cause rapid landscape change, infrastructure damage and human disturbance. The 920 AD Katla eruption caused a dual hazard event, which then altered the flood pathways of Katla jökulhlaups forever. Accounts of this specific event were retold in the Icelandic Book of Settlement (Landnámabók). A duel between two powerful wizards, using magic to alter the path of the food, is physically depicted in the landscape today.

Geographic information system (GIS) methods were implemented to visualise an idealised pre-eruption landscape. HEC-RAS flood modelling tools were used to explore the quantitative aspects of this significant flooding event. By utilising zonal statistics for volumetric understanding, the erosional and depositional impact was quantified and then validated against 2D unsteady flow models. This project finds that the 920 AD jökulhlaup generated ~89.4 × 106 m² of erosional volume, compared to only ~61,000 m² of depositional volume. These volumetric calculations agree with field observations throughout the study site. The erosional impact coincides with previous jökulhlaups such as Katla 1918 and Eyjafjallajökull 2010; however, the overall magnitude is primarily influenced by differences in catchment area size within the ice cap and the storage capacity of the ice-dammed lakes. The 920 AD predicted simulation generated a maximum discharge of ~11,000 m³/s, which agrees with paleo-discharge trends recorded within literature, which range between 10³ m and 10⁵ m³/s. This project investigates the different flood characteristics and the effect that terrain and storage areas have on flood pathways and velocity changes.