Lidar measurements of jökulhlaup source areas on Mýrdalsjökull in 2011 and on Hofsjökull in 2013: implications for jökulhlaup dynamics

Digital elevation models (DEMs) of several jökulhlaup source areas were measured during the mapping of Icelandic glaciers with aerial lidar in 2008--2013. This included i) the jökulhlaup in Múlakvísl from Mýrdalsjökull on 9 July 2011, which destroyed the bridge on the ring road, and ii) the jökulhlaup in Vestari-Jökulsá from Hofsjökull in late August 2013. Comparison of these DEMs with lidar measurements from before the jökulhlaups reveals valuable information about the volume of the floods and the nature of subglacial water flow from jökulhlaup source areas created by subglacial geothermal activity. DEM differencing shows lowering of the glacier surface of Mýrdalsjökull along several kilometres of the subglacial flood path from the main cauldrons that released the initial flash flood, which is most likely caused by melting of the glacier sole by warm flood water. It is not possible to derive an explicit estimate of the temperature of the flood water near the main cauldrons, but the DEM differencing indicates temperature on the order of 10 °C to tens of degrees Celsius. In recent years, three jökulhlaups have been observed from Hofsjökull and cauldrons have formed in the ice cap surface at three different locations. These measurements and observations of several other jökulhlaups in Iceland in recent years show that the initial temperature of jökulhlaup flood water may be quite variable from near the freezing point of water to tens of degrees Celsius. This variable temperature is likely to have important consequences for jökulhlaups dynamics and may be part of the explanation for the highly variable development of jökulhlaups. The jökulhlaup in Múlakvísl in 2011 and from Hofsjökull in 2013, as well as several other jökulhlaups from Mýrdalsjökull and some other glaciers and ice caps in Iceland, highlight the importance of improving the understanding of jökulhlaups and related natural hazards.