A coupled sheet‐conduit mechanism for jökulhlaup propagation

Abstract

[1] The largest glacier outburst flood (jökulhlaup) ever recorded in Iceland occurred in 1996 and came from subglacial lake Grímsvötn in Vatnajökull ice cap. Among other noteworthy features, this flood was characterized by an unprecedentedly high lake level prior to flood initiation, extremely rapid linear rise in lake discharge, delay between the onset of lake drainage and floodwater arrival at the glacier terminus, formation of short-lived supraglacial fountains, and initially unchannelized outbursts of floodwater at the terminus. Observations suggest that the 1996 flood propagation mechanism was fundamentally different than that of previously observed floods from Grímsvötn. We advance a new model whereby floodwater initially propagates in a turbulent subglacial sheet, which feeds a nascent system of conduits. This model is able to explain key observations made of the 1996 jökulhlaup and may shed light on other outburst floods that do not conform to the standard model.

Fresh arguments against the Shaw megaflood hypothesis. A reply to comments by David Sharpe on “Paleohydraulics of the last outburst flood from glacial Lake Agassiz and the 8200 BP cold event”

“…He seems to imply that contemporary sheetlike floods, such as that associated with the 1996 outburst from subglacial lake Grı´msvo¨tn in Vatnajo¨kull ice cap, are modern analogues to the sheet-like floods invoked in the megaflood hypothesis. Although we ignored the possibility of a sheet-like flood from Lake Agassiz we do not, in general, rule out this possibility (e.g., the recent paper by Flowers et al (2004), of which Bjo¨rnsson and Clarke are co-authors, on the hydraulics of the 1996 Grı´msvo¨tn flood). Our motivation for not entertaining the possibility of a similar sheet-like flood from glacial Lake Agassiz is that we consider it highly unlikely that the flood proceeded in this manner.…”

Hazard assessment investigations in connection with the formation of a lake on the tongue of Unterer Grindelwaldgletscher, Bernese Alps, Switzerland

“…This work was extended mainly by Spring and Hutter (1982), Clarke (2003) and Flowers et al (2004). Even though the model we use is based on the Spring-Hutter equations, for illustration, we give a brief introduction to the simpler Nye (1976) model for which Ng and Björnsson (2003) derived an approximate solution.…”

Subglacial flood path development during a rapidly rising jökulhlaup from the western Skaftá cauldron, Vatnajökull, Iceland

“…The lake is assumed to be hydraulically connected to the atmosphere through water passageways such as crevasses or moulins. The volume of water in the lake and surrounding waterways, V w , is the sum of (i) the lake volume, taken to be the integral of the elevation of the bottom of the ice shelf, z b , over the area of the lake, and (ii) the water volume in the passageways between the lake and the atmosphere, which we assume can be expressed as water level in the passageways, z w , multiplied with their effective area, S, by analogy with the traditional way to express ground water storage in terms of a storage coefficient (Fetter, 2001). Therefore, variations in V w , may be expressed as…”