Jökull - 01.12.1999, Page 47

Channel cross-sectional area changes and peak discharge
calculations in the Gígjukvísl river during the November 1996
jökulhlaup, Skeiðarársandur, Iceland
Andrew J. Russell1, Óskar Knudsen2, Judith K. Maizels3 and Philip M. Marren1
1) Department of Earth Sciences, Keele University, Keele, Staffordshire, ST5 5BG, UK. a.j.russell@keele.ac.uk
2) Klettur Consulting Engineers, Bfldshöfða 12, 112 Reykjavík, Iceland. E-mail: oskar@vedur.is
3) Department of Geology and Geophysics, Grant Institute, University of Edinburgh,
West Mains Road, Edinburgh, EH9 3JW, UK. E-mail: j.maizels@btinternet.com
Abstract — This paper presents the results offieldwork undertaken before and immediately after the Novem-
ber 1996jökulhlaup on Skeiðarársandur, Iceland, to determine both the geomorphic impact ofthis spectacul-
arflood on the Gígjukvísl river channel and to provide an independent estimate ofpeak jökulhlaup discharge
in this channel. This study provides a new perspective on the wide range ofpeak discharge estimates publish-
edfor the November 1996 jökulhlaup. The primary geomorphological impact ofthe November 1996 jökul-
hlaup on the Gígjukvísl river catchment was erosion of the narrow channel through the moraines to create a
much wider channel with a less sinuous course. Channel enlargement, downstream ofthe moraines, was ac-
companied by net aggradation in the bed of the former channel thalweg. Waning stageflow and hence eros-
ion was concentrated on the eastern bank of the Gígjukvísl river. Peak jökulhlaup discharge reconstructed
for the Gígjukvísl river using the slope-area technique amounts to 19,500 m^s~l±5,000 m3s~-\ The Gígju-
kvísl river is therefore thought to have carried between forty and fifty percent of the peak jökulhlaup dis-
charge. Calculated Manning's n values of 0.045-0.152, normally associated with very high resistance in
rivers, suggest that ice blocks significantly retarded theflow in this relatively low gradient channel. Remov-
al of substantial volumes of glacier ice within the nearby push-moraines acted as an important additional
supply of ice to the Gígjukvísl channel. The impact ofthe November 1996 jökulhlaup was severe, due both
to this being tlie largest everflow through this river course and the deposition oflarge numbers ofice blocks
within the river channel.
AIMS AND BACKGROUND
This paper aims to: (1) determine the geomorphic
impact of the spectacular November 1996 jökulhlaup
on the Gígjukvísl river channel, Skeiðarársandur, Ice-
land; and (2) to provide an independent estimate of
peak jökulhlaup discharge in this channel. As such
this study provides a new perspective on the wide
range of peak discharge estimates published for the
November 1996 jökulhlaup.
A volcanic eruption beneath the Vatnajökull ice
cap began on September 30í/l, 1996 (Einarsson et al.,
1997; Guðmundsson et al, 1997; Björnsson, 1997).
Over the next month 3.8 km3 of meltwater travelled
subglacially into the Grímsvötn subglacial caldera
lake until it reached a critical level for drainage
(Björnsson, 1997). The jökulhlaup began in the most
easterly outlet river, Skeiðará, at 0730h on Novem-
ber 5th and had a total volume of 3.6 km3 (Björnsson,
1997). Published instantaneous peak discharge fig-
ures for all outlets range from c. 45,000 m3s_1 based
on the rate of volume reduction in Grímsvötn (Einars-
son et al, 1997), to 53,000 m3s_1 based on observa-
tions of flow in the proglacial area (Snorrason et al.,
JÖKULL, No. 47 45