Jökull


Jökull - 01.06.2000, Page 19

Jökull - 01.06.2000, Page 19
An ice-dammed lake in Jökulsárgil: predictive modelling and geomorphological evidence Fiona S. Tweed Division of Geography, Staffordshire University, College Road, Stoke-on-Trent, Staffordshire, ST4 2DE, U.K. Abstract — At Sólheimajökull, southern Iceland, a river has previously been dammed by the glacier, forming an ice-dammed lake in Jökulsárgil from which major jökulhlaups last occurred in the 1930s. Currently, the river maintains a tunnel through the glacier for most ofthe yecir. Geomorphological evidence indicates that an ice-dammed lake is occasionally formed in Jökulsárgil at present, probably when the river discharge is low. This indicates that tunnel closure occurs for a part of the year, resulting in the formation of an ice-dammed lake that could present the risk offlooding on drainage. This paper discusses tunnel closure processes and ice-dammed lake formation using models oftunnel dynamics, in conjunction withfield data, and identifies the physical conditions required for tunnel closure at Sólheimajökull. The models predict that tunnel closure is most likely in the winter when river discharges are low enough to be incapable of ojfsetting conduit closure by ice overburden pressure. Ice conduit cross-sectional geometries commonly specified in existing models are unrepresentative ofthe tunnel at Sólheimajökull which has a broad and low cross-sectional form with a much lower strength than the circular or semi-circular tunnels often used in existing models. INTRODU CTION Subaerial and subglacial ice-dammed lakes are prevalent in Iceland (e.g. Grænalón, Grímsvötn) and the sandar bear witness to the power and impact of the jökulhlaups that result from the often catastrophic drainage of such lakes (Einarsson et al. 1980; Maizels, 1989, 1991; Russell et al. 1997). The floods (jökul- hlaups) that are frequently the product of ice-dammed lake drainage can cause damage to property, land and transport links and have implications for the installati- on of hydro-electric power stations and pipelines (e.g. Young, 1980; Rist, 1983; Sturm and Benson, 1985; Björnsson, 1992). The recent outburst from Gríms- vötn in 1996 highlights the destructive nature of this type of flooding and illustrates the importance of developing predictive criteria by which the formati- on of ice-dammed lakes and their flooding probability can be assessed. This paper analyses the potential for ice-marginal ice-dammed lake formation at Sólheima- jökull. Geomorphological evidence for the temporary existence of an ice-dammed lake is presented and simple models are used to analyse the broad thres- holds required for such a lake to occur. SÓLHEIMAJÖKULL: BACKGROUND AND HYDROLOGICAL CHARACTERISTICS Sólheimajökull is a 9 km long, non-surging outlet glacier of Mýrdalsjökull southem Iceland (Figure 1). Sólheimajökull has not always echoed the behaviour of other glaciers in the area, sometimes advancing whilst other outlets from Mýrdalsjökull have retrea- ted. Dugmore and Sugden (1991) ascribe this behavi- our to ice divide migration; as the ice cap thins, the catchment area of Sólheimajökull increases, thus resulting in glacier expansion. Ice divide migration is cited as a possible reason for the advance of Sólheima- jökull in the 1980s. The total glacier advance since JÖKULL No. 48 17

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