Hydrodynamics and geomorphic work of jökulhlaups (glacial outburst floods) from Kverkfjöll volcano, Iceland

Abstract

AbstractJökulhlaups (glacial outburst floods) occur frequently within most glaciated regions of the world and cause rapid landscape change, infrastructure damage, and human disturbance. The largest jökulhlaups known to have occurred during the Holocene within Iceland drained from the northern margin of Vatnajökull and along the Jökulsá á Fjöllum. Some of these jökulhlaups originated from Kverkfjöll volcano and were routed through anastomosing, high gradient and hydraulically rough channels. Landforms and sediments preserved within these channels permit palaeoflow reconstructions.Kverkfjöll jökulhlaups were reconstructed using palaeocompetence (point measurements), slope–area (one‐dimensional), and depth‐averaged two‐dimensional (2D) hydrodynamic modelling techniques. The increasing complexity of 2D modelling required a range of assumptions, but produced information on both spatial and temporal variations in jökulhlaup characteristics. The jökulhlaups were volcanically triggered, had a linear‐rise hydrograph and a peak discharge of 50 000–100 000 m3 s−1, which attenuated by 50–75% within 25 km. Frontal flow velocities were ∼2 m s−1; but, as stage increased, velocities reached 5–15m s−1. Peak instantaneous shear stress and stream power reached 1 × 104 N m−2 and 1 × 105 W m−2 respectively.Hydraulic parameters can be related to landform groups. A hierarchy of landforms is proposed, ranging from the highest energy zones (erosional gorges, scoured bedrock, cataracts, and spillways) to the lowest energy zones (of valley fills, bars, and slackwater deposits). Fluvial erosion of bedrock occurred in Kverkfjallarani above ∼3 m flow depth, ∼7m s−1 flow velocity, ∼1 × 102 N m−2 shear stress, and 3 × 102 W m−2 stream power. Fluvial deposition occurred in Kverkfjallarani below ∼8 m flow depth, 11 m s−1 flow velocity, 5 × 102 N m−2 shear stress, and 3 × 103 W m−2 stream power. Hence, erosional and depositional ‘envelopes’ have considerable overlap, probably due to transitional flow phenomena and the influence of upstream effects, such as hydraulic ponding and topographic constrictions, for example.Holocene Kverkfjöll jökulhlaups achieved geomorphic work comparable to that of other late Pleistocene ‘megafloods’. This work was a result of steep channel gradients, topographic channel constrictions, and high hydraulic roughness, rather than to extreme peak discharges. The Kverkfjöll jökulhlaups have implications for landscape evolution in north‐central Iceland, for water‐sediment inputs into the North Atlantic, and for recognizing jökulhlaups in the rock record. 2D hydrodynamic modelling is likely to be important for hazard mitigation in similar landscapes and upon other glaciated volcanoes, because it only requires an input hydrograph and a digital elevation model to run a model, rather than suites of geomorphological evidence and field‐surveyed valley cross‐sections, for example. Copyright © 2006 John Wiley & Sons, Ltd.