Jökull - 01.06.2000, Blaðsíða 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