Field evidence and hydraulic modeling of a large Holocene jökulhlaup at Jökulsá á Fjöllum channel, Iceland

Abstract

Field investigation and hydraulic modeling of the Jökulsá á Fjöllum outflow channel in the northern highlands of Iceland suggest a larger than previously modeled jökulhlaup catastrophic release of glacial floodwaters probably occurring just after early Holocene deglaciation. Although earlier investigations described a similar large paleoflood event, our hydraulic model parameter estimates and floodplain inundation maps correlate with new field evidence presented here. Due to its temporal and voluminous outflow we consider potential jökulhlaup sources and mechanisms and also its relevance as an Earth analog to Mars fluvial geomorphology and processes. In this study, we reconstruct this large jökulhlaup event using HEC-GeoRAS to extract three-dimensional channel geometry and the HEC-RAS hydraulic model. Depositional and erosional landforms across the 435–485km2 flood inundation area provide field evidence of high water lines (trimlines) required for hydraulic model constraints. Hydraulic modeling results related to this field evidence and the unambiguous inundation of Ferjufjall along the Mt. Herðubreið reach gives a conservative peak discharge rate of 2.2×107m3s−1 and a mean flow velocity of 14.9ms−1. By comparison, this is larger than the 1.8×107m3s−1 peak discharge of the Kuray paleoflood in the Altai Mountains of Siberia, which is the largest previously documented paleoflood on Earth. This study suggests that this paleoflood through the Jökulsá á Fjöllum channel is the largest known on Earth.