| Summary: | Jökulhlaups are the consequence of a sudden and significant release of meltwater from the edge of a glacier. Such floods are sourced commonly from ice-dammed lakes, but occasional volcanic eruptions beneath ice can produce intense jökulhlaups due to prodigious rates of meltwater release. Globally, volcanogenic jökulhlaups have caused fatalities and damage to infrastructure within effected catchments. Here, we present the results of one-dimensional hydraulic modelling of the inundation area of a massive, hypothetical jökulhlaup on the Jökulsá á Fjöllum River in northeast Iceland; the floodwater source for this simulation is an eruption within the ice-filled caldera of Bárðarbunga: an active volcano beneath the Vatnajökull ice cap. Remotely sensed data were used to derive a digital elevation model and to assign surface-roughness parameters. We used a HEC-RAS/HEC-GeoRAS system to host the hydraulic model; to calculate the steady water-surface elevation; to visualise the flooded area; and to assess flood hazards. Maximum discharge was set notionally at 180,000 m 3 s −1 and the duration and volume of the jökulhlaup were placed at 39 h and 14 km 3 , respectively. During the simulated rise to maximum discharge, the mean velocity of the jökulhlaup was 2.8 m s −1 over a distance of 120 km. At the height of the jökulhlaup an area of 460 km 2 was inundated. Modelling results showed that, along short reaches, stream-power values exceeded 11,000 W m −2 such energy conditions would have allowed boulders up to 10-m in diameter to be mobilised by the jökulhlaup. Unsteady flow was simulated along a 22-km reach of the flood tract and it revealed strong spatial and temporal variations in flood power. Besides providing insight into the erosional and depositional effects of a volcanogenic jökulhlaup, the modelling results enable estimates of the relative timing and location of likely flooding hazards. Copyright Springer Science+Business Media B.V. 2007 Glacial outburst flood, Iceland, Hydraulic modelling, GIS
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