Outburst Flood in Skeiðararsandur, Iceland: Sedimentation And Landscape Recovery

The November 1996 jökulhlaup that emerged from the Vatnajökull ice cap onto Skeiðararsandur was the highest-magnitude flood ever recorded on the largest active glacial outwash plain. Introduction Outburst floods, also known as jökulhlaup, are examples of high magnitude, low frequency sudden releases of massive amounts of water and are in involved in the formation of outwash plains. The main sources of the melt water can come from subglacial environments, ice marginal, englacial or supraglacial stores, and some floods are produced by volcanic activity and rain if there is not enough water storage.

The meltwater is produced by subglacial volcanic activity and jökulhlaup occurs after significant amount of water is produced and the breakdown of ice and breached moraine dams. Outwash plains are formed through the deposition of glacial sediments by meltwater outwash at the limit of the glacial boundary. It is more common that this deposition of sediments is done by jökulhlaup than by braided river facies caused by ablation related flow. The glacial outwash plain Skeiðararsandur in Iceland is the largest active plain in the world. Skeiðararsandur extends from the boundary of the Skeiðararjökull outlet glacier of the Vatnajökull ice cap to the Icelandic coast. The stratigraphy of the glacial plain is categorized by the massive coarsening of sequences of clast sustained cobbles and pebbles. Their deposition was likely the result of multiple jökulhlaup events that occurred historically every 1-7 years and are related to volcanic activity.

The Skeiðararsandur flood of 1996 was caused by a fissure eruption at Vatnajökull in northern Iceland. Eruptions began on September 30 and lasted for 13 days which resulted in the melting of 3km3 of ice, the erupted magma formed a hyaloclastite ridge 6-7km long and 200-300m vertically beneath 500-750 m of ice. The meltwater, from subglacial eruptions and geothermal heat, accumulated in the subglacial Grimsvotn Caldera lake causing the failure of the ice dam due to the rise of hydrostatic pressure. The eventual failure of the ice dam on November 4th in the eastern margin of the caldera discharged a record jökulhlaup that flowed for approximately 50 km sub glacially Skeiðararjökull before rupturing onto the Skeiðararsandur. The peak discharge of the jökulhlaup was estimated to be about 2590 m3s-1 in the proximal zone and approximately 490 m3s-1 40 km downstream. The Skeiðararsandur has an area of 1350 km2 and the flow combined to flood 75% of the outwash plain. Three large braided stream systems, Gigjukvisl, Skeiðara, and Nupsvotn, carried the meltwater and sediments to the Skeiðararsandur. The initial flows were measured at 6ms-1 carrying large quantities of debris lobes which was continuously supplied even to the dying stage of the flow.

Discussion

The Skeiðararsandur has been divided into four areas which are sorted based on the coarseness of sediments which are gradually decreasing as the distance grew from Skeiðararjökull. Closer to the glacier are tills, ranging from coarse gravel to fine gravel, while the lower segments of the sandur are dominantly composed of sand-sized grains. The hydrograph of the jökulhlaup showed the rapid rise and gradual fall of the discharge of meltwater and sediments. The sedimentary and geomorphological impacts lead to the creation of poorly sorted structureless matrix supported deposits, massive sand units, clasts supported units, ice-proximal cobbles, rip up clasts, kettle-holes, and steep sided kettle-holes. These sedimentary sequences are distinctive of rapid rising stage deposition. During the deposition large scale gravel bars and bedload sheets are moved, the progress of these bedforms were expedited by the high discharge acceleration rates and high bedload transportation rates throughout the rapid rising stage.

The jökulhlaup had pronounced effects on the three main braided stream systems channeling through the glacier. The Nupsvotn and Skeiðara rivers both splay off from the glacier margins and lead directly to the outwash plains. While the Gigjukvisl river is supplied by meltwater from central and west central portions of the Skeiðararjökull. The water then collects in the proglacial zone before being transported to the sandur. The Gigjukvisl was converted to a series of deepened channels from what was a previously low relief braided channel network.

The Haoldukvisl and Seluhusskvisl overflow channels become relevant during jökulhlaup as the water accumulates in the proglacial zone. In the upper portion of the Skeiðara there is evidence of lateral re-working of the braided channels and bars that show small changes in the topographic relief. The Gigjukvisl system displayed substantial elevation loss upstream and is distinguished by the removal of bars and channel infilling downstream. The two river systems upstreams are different from each other since Gigjukvisl experienced the largest net elevation losses while Skeiðara net losses were relatively small. Downstream both systems showed similar net losses. The three river systems gradually become wider and less distinct closer to the sandur. Within 3-8 km of the ice front the river braid channels are shifting creating a complicated network that merge during increased flow rates. The amount of sediment taken to the proximal zone of the sandur are controlled by the ice marginal depression and the ice parallel drainage network. The bed material fine quickly when the distance is increased from the ice front and if the Gigjukvisl River is lower than the Skeiðara River, and it’s based off the slope of the main ice-marginal channel within proglacial zone. The averaged net elevation increase across the central sandur was 22cm, but by 2001 half of that increase in net elevation had disappeared. At the current rate of removal, the increase in net elevation will have been wiped out around a decade after deposition. Sea level and the elevation of the glacier bed potentially have a larger effect on the topography than the total amount of sedimentary material transported by jökulhlaup.

Conclusion

The jökulhlaup caused widespread sediment deposition and net aggradation in the distal areas of the Skeiðararsandur. Regardless of the relentless erosion the jökulhlaup was significant in the transportation of large quantities of sediments to the glacier outwash plains. It transformed the three braided river systems due to the mass flow of melt water and sediments. But the lasting effects are short lived as the erosion rate of the material deposited is going to be gone within the following decades.