GLACIER OUTBURST FLOODS (JÖKULHLAUPS) FROM KVERKFJÖLL, ICELAND: FLOOD ROUTEWAYS, FLOW CHARACTERISTICS AND SEDIMENTARY IMPACTS

Jökulhlaups with peak discharges of 10⁵ - 10⁶m³s^-1 have occurred during the Holocene along the Jökulsá á Fjöllum river in north-eastern Iceland. Controversy surrounds their source, cause, frequency and characteristics. Kverkfjöll is a glaciated volcano on the northern margin of the Vatnajökull and is a discrete source of meltwater into the Jökulsá á Fjöllum. This study presents evidence of jökulhlaup flows from Kverkfjöll and methods used in their reconstruction. Flow reconstructions will be used to quantify the spatial and temporal variation in jökulhlaup flow characteristics (depth, velocity, shear stress, energy and stream power). Jökulhlaup routeways are distinguished from non-flood surfaces by identifying landforms and sediments diagnostic of jökulhlaups and alternative processes. The geologically controlled topography of the area has significantly influenced the routing of floods from Kverkfjöll. In some localities flows were of a magnitude sufficient to spill over low divides into neighbouring valleys, producing an anastomosing complex of flow routeways. Within channel evidence of jökulhlaups includes erosional features such as dry waterfalls and cataracts, linear grooves, sculpted bedrock bedforms, obstacle scour marks and streamlined hills. Depositional evidence includes bars, terraces and slackwater deposits. Several pits excavated into slackwater deposits reveal multiple floods differentiated by tephra horizons. Differential flood surface weathering and the presence of flood-washed surfaces intercalated with lava flows again suggest multiple jökulhlaups from Kverkfjöll. Imbricated boulder clusters and matrix-supported boulder-rich sedimentary units reflect fluid sedimentation and non-Newtonian flows respectively. Such turbulent and complex flows are partly due to flood generation mechanisms, flood routeway characteristics and variations in sediment availability between and within jökulhlaups. Our study of jökulhlaup flows within this distinctive volcanic landscape may be of use in identification and reconstruction of floods on the flanks of glaciated volcanoes elsewhere. Knowledge of jökulhlaup flow behaviour is crucial for jökulhlaup hazard prediction, management and mitigation.