Ice fracturing during jökulhlaups: implications for englacial floodwater routing and outlet development

Theoretical studies of glacial outburst floods (jökulhlaups) assume that: (i) intraglacial floodwater is transported efficiently in isolated conduits; (ii) intraglacial conduit enlargement operates proportionally to increasing discharge; (iii) floodwater exits glaciers through pre-existing ice-marginal outlets; and (iv) the morphology and positioning of outlets remains fixed during flooding. Direct field observations, together with historical jökulhlaup accounts, confirm that these theoretical assumptions are not always correct. This paper presents new evidence for spatial and temporal changes in intraglacial floodwater routing during jökulhlaups; secondly, it identifies and explains the mechanisms controlling the position and morphology of supraglacial jökulhlaup outlets; and finally, it presents a conceptual model of the controls on supraglacial outbursts. Field observations are presented from two Icelandic glaciers, Skeiðarárjökull and Sólheimajökull. Video footage and aerial photographs, taken before, during and after the Skeiðarárjökull jökulhlaup and immediately after the Sólheimajökull jökulhlaup, reveal changes in floodwater routing and the positioning and morphology of outlets. Field observations confirm that glaciers cannot transmit floodwater as efficiently as previously assumed. Rapid increases in jökulhlaup discharge generate basal hydraulic pressures in excess of ice overburden. Under these circumstances, floodwater can be forced through the surface of glaciers, leading to the development of a range of supraglacial outlets. The rate of increase in hydraulic pressure strongly influences the type of supraglacial outlet that can develop. Steady increases in basal hydraulic pressure can retro-feed pre-existing englacial drainage, whereas transient increases in pressure can generate hydraulic fracturing. The position and morphology of supraglacial outlets provide important controls on the spatial and temporal impact of flooding. The development of supraglacial jökulhlaup outlets provides a new mechanism for rapid englacial debris entrainment

Icelandic jökulhlaup impacts

Iceland contains the worlds largest and best-documented active glacial outwash plains or sandar that have been studied since the 19th century. Vigorous subglacial volcanic activity and the presence of numerous ice-dammed lakes, make Iceland the prime location for the study of glacier outburst floods or jökulhlaups and their geomorphological and sedimentological impact. Increasing attention is being focused on large jökulhlaup channels related to both modem and ancient processes. Jökulhlaup impact within Icelandic bedrock channels has so far received little attention despite the fact that such channels are abundant as sandar in Iceland. Despite clear descriptions within Icelandic literature of jökulhlaup impact on glacier margins, there have been attempts to link jökulhlaup feeder system dynamics with processes and products in the proglacial outwash system. This chapter presents the latest research on the impacts of jökulhlaups generated by three main mechanisms: volcanic, ice-dammed lake drainage, and glacier surge. This chapter identifies and discusses the main advances in the understanding of jökulhlaup impact in Iceland

Rapid sediment entrainment and englacial deposition during jökulhlaups

Correspondenc

An unusual jökulhlaup resulting from subglacial volcanism, Sólheimajökull, Iceland

Jökulhlaups (glacial outburst floods) are frequent in glaciated terrain. Jökulhlaups exhibiting a sudden rise to peak discharge have not been subject to detailed investigation. A volcanically-generated flood burst from Sólheimajökull, Iceland in July 1999. This paper accounts for the causes, characteristics and impacts of this flood. Pre- and post-flood fieldwork was carried out at Sólheimajökull allowing the reconstruction of mean flow velocities and peak discharge. Flood onset was rapid, rising to a peak discharge of 4.4 x 103 m3s−1 (± 1.2 x 103 m3s−1) within one hour. High basal water pressures resulted in floodwater bursting through the glacier surface. Ice rip-up clasts containing glacial diamict provided evidence of floodwater contact with the glacier bed. Within the glacier, jökulhlaup sedimentation occurred within hydrofractures and conduits generating complex hydrofracture fills and esker ridges. In the proglacial zone, regions of flow expansion associated with rapid reductions in sediment transport capacity controlled the locations of major jökulhlaup sedimentation. A large fan composed of material of up to boulder size was deposited at the glacier snout. Two ice-marginal basins filled and drained during the jökulhlaup. One of the basins, Jökulsárgil, emptied rapidly during the flood, via ice-dam flotation, accentuating peak jökulhlaup discharge. High rates of downstream peak discharge attenuation during the July 1999 jökulhlaup provides an analogy with flash flood hydrographs in semi-arid regions and the catastrophic failure of man-made dams. The July 1999 jökulhlaup was initially triggered by the subglacial volcanic eruption, but the characteristics of the flood were accentuated by within-event meltwater storage and release. This unusual jökulhlaup provides an important addition to our understanding of the spectrum of distinctive jökulhlaup characteristics