Summary

Jökulhlaups, or glacier outburst floods, are complex flood phenomena, with hydraulics that vary considerably spatially and temporally. However, jökulhlaups occur too suddenly, are too powerful and often too infrequent and remote for direct measurements of hydraulics to be made. Thus various palaeohydraulic methods have been applied to reconstruct jökulhlaup hydraulics from geomorphological and sedimentological evidence. However, these techniques fail to sufficiently characterise transient jökulhlaup hydraulic phenomena, in both space and time. A detailed understanding of these transient hydraulics is important for understanding rapid landscape change, high-magnitude flood mechanisms of erosion, transport and deposition, and hence jökulhlaup hazard management.

Therefore this paper reconstructs transient jökulhlaup flow phenomena using boulder clusters, the slope-area method and a depth-averaged two-dimensional (2D) hydrodynamic model. Kverkfjöll volcano on the northern edge of Vatnajökull, Iceland, provides the study site. Jökulhlaups inundated anastomosing bedrock valleys and exhibited transient hydraulic phenomena including sheet or unconfined flow, channel flow, flow around islands, hydraulic jumps, multi-directional flow including backwater areas and hydraulic ponding. Reconstructions of these jökulhlaups indicate peak discharges of 50–100 000 m3 s−1, which attenuated by ∼65% within 20 km. Frontal flow velocities were ∼1.6 m s−1 but as stage increased velocities reached 5–15 m s−1. Shear stress and stream power reached 1 × 104 N m−2 and 1 × 105 W m−2 respectively. Flows were largely supercritical due to steep channel gradients and shallow flows and highly turbulent due to high hydraulic roughness. Kverkfjöll jökulhlaups thus achieved geomorphological work comparable to that accomplished by ‘megafloods’.