Jökull - 01.01.2001, Blaðsíða 21

Changes in the Gígjukvísl river channel
sion at the Jökulsá River outlet of about 8.5 m per
year. This erosion threatens the road (Route One) and
two powerlines as discussed by Jóhannesson (1994,
1995). Mýrdalssandur (Figure 1) has been created by
jökulhlaups from the Katla volcano, with peak dis-
charge of the order of 100,000-300,000 m

s


(Jóns-
son, 1982; Maizels, 1989a,b, 1991, 1993a,b, 1995;
Tómasson, 1996). The 34 km long road across Mýr-
dalssandur (Route One) is not designed to withstand
such an event.
Skeiðarársandur (Figure 1) in southeast Iceland is
the “typesite” for jökulhlaups (Thórarinsson, 1939,
1974; Churski, 1973; Björnsson, 1988, 1992; Guð-
mundsson et al., 1995), and different features of their
impact have been recorded by Klimek (1972, 1973),
Galon (1973), Nummedal et al. (1987), Boothroyd
and Nummedal (1978), Maizels (1991,1993a,b) and
Russell and Marren (1999). These studies describe the
sedimentology and geomorphology of Skeiðarársand-
ur without isolating the geomorphic change caused by
a single jökulhlaup. The November 1996 jökulhlaup
is thus unique as it is the first large jökulhlaup on
Skeiðarársandur since 1938 and the geomorphic con-
ditions before and after the jökulhlaup are well docu-
mented.
The November 1996 jökulhlaup on Skeiðarár-
sandur caused damage to roads and bridges which has
been estimated at about 1,000 million ISK (15 million
USD). The main geomorphic impact of the jökul-
hlaup occurred within the Gígjukvísl channel (Fig-
ure 1). This river captures water drained from out-
lets at the central part of the glacier, whereas the two
rivers to the east and west of Gígjukvísl, the Súla
and Skeiðará respectively, emerge from single lateral
conduits. Both these lateral channels have previously
conveyed numerous jökulhlaups and therefore the ge-
omorphic change during the 1996 event was minimal.
The sediments produced by this jökulhlaup have been
described by Russell and Knudsen (1999a,b). This
paper presents maps of the Gígjukvísl channel be-
fore and after the November 1996 jökulhlaup. Herein,
these maps are used to give an overview of the geo-
morphic changes within the Gígjukvísl river channel
during the jökulhlaup.
THE NOVEMBER 1996 JÖKULHLAUP
A volcanic eruption beneath the Vatnajökull ice cap
began on September 30th, 1996 (Guðmundsson et
al., 1997). Meltwater travelled subglacially into the
Grímsvötn subglacial caldera lake until it reached a
critical level for drainage (Björnsson, 1997). The
jökulhlaup began on the most easterly outlet river,
the Skeiðará, at 0730h on November 5th. The jökul-
hlaup began in the Gígjukvísl river at around 1015h
on November 5th and reached the Súla bridge at
1613h on November 5th (Snorrason et al., 1997) (Fig-
ure 1). The jökulhlaup in the Súla river had the
shortest duration and was observed from aerial pho-
tographs to have waned considerably by 1200h on
November 6th. The jökulhlaup had its longest du-
ration in the Gígjukvísl river channel persisting well
into November 7th (Snorrason et al., 1997). The flood
reached a peak discharge of 40,000–50,000 m

s


within 15 hours, forming the shortest rising limb on
discharge charts for any jökulhlaup recorded from the
Grímsvötn caldera (Björnsson, 1997). In this flood,
3.2 km

of water drained from the Grímsvötn caldera
within a period of 40 hours (Björnsson, 1997). At
the flood peak, a backwater lake containing 0.06–
0.1 km

of water was temporarily stored upstream of
the Gígjukvísl moraine constriction (Russell et al.,
1999, Russell and Knudsen, 1999a) (Figure 1).
The 1996 jökulhlaup was similar in magnitude to
the 1934 and 1938 jökulhlaups which drained 4.5 km

and 4.7 km

, respectively, with peak discharges of
25,000–30,000 m

s


(Björnsson 1992; Guðmunds-
son et al., 1995).
PRODUCTION OF THE MAPS
A map of Skeiðarársandur, based on the August, 1997
aerial photographs, was produced by Ísgraf for the
Public Roads Administration following a tender. The
map covers the total width of Skeiðarársandur from
just south of Route One to just north of the Skeiðarár-
jökull glacier terminus (Figure 1). The work included
aerial photography and the production of gridpoints
with 10 m spacing. In the tender documents the height
of flight is required to be 3000 m. Average error, as
specified in the tender documents, was required to
be less than

0.5 m for plan coordinates (x, y) and
JÖKULL No. 50 21