RECONSTRUCTION OF THE LARGEST HOLOCENE JÖKULHLAUP WITHIN JÖKULSÁ Á FJÖLLUM RIVER, NE ICELAND

This paper presents a reconstruction procedure for glacial outburst floods (jökulhlaups). Remote sensing and GIS methods were combined with geomorphological and sedimentological evidence to enable reconstructions of jökulhlaup flow magnitude and hydraulics. Here we present a comprehensive 1D hydraulic model (HEC-RAS/HEC-GeoRAS) of a jökulhlaup emanating from the northern margin of Vatnajökull, NE Iceland. So far, few studies have provided estimates of peak discharge based on slope-area calculations and assumptions of total flood volume. Peak discharges range from 0.25 x 106 to 1 x 106 m3s-1, with total flood volumes exceeding ~10 km3. To date, there have been no systematic attempts to reconstruct the hydraulics of Jökulsá á Fjöllum jökulhlaups using hydraulic modelling techniques.

Present-day topography of the 80 x 40 km study area is characterised by an ERS-InSAR DEM with 25x25x1m resolution. Palaeotopography was determined by modifying the DEM to take account of the effects of topographic change associated with the accumulation of post flood lava flows, aeolian deposition and tectonic displacement. Filtering of distorted elevation information on the river channel was undertaken with the aid of a water mask. Subsequently, channel topography was derived from present-day channel width and discharge. Land cover roughness (Manning’s n) was derived from remotely sensed data and subsequently converted to pre-flood roughness using known land cover (e.g. lava flow) age. Flood hydrograph was calibrated iteratively from wash limit evidence. Thus, flood parameters was modelled and matched with field evidence.

Reconstructions of Jökulsá á Fjöllum jökulhlaups using HEC-RAS provides a much more detailed picture of palaeoflow characteristics than peak discharge calculations derived solely from simple slope-area techniques. Our jökulhlaup reconstruction method allows inundation areas to be mapped, calculation of local hydraulic properties and assessment of the role of a number of potential hydrograph shapes. Modelling results will be used to constrain the source and release mechanisms of Jökulsá á Fjöllum jökulhlaups as well as providing a starting point for assessing the capacity of these floods for the formation of highly distinctive flood landscapes within an active volcanic zone.