Jökull - 01.12.1999, Page 59

kvísl channel carried between forty and fifty percent
of the peak jökulhlaup discharge. Calculated Man-
ning's n values of 0.045-0.152 are normally associ-
ated with very high resistance in rivers such as that
created by heavy vegetation growth within the main
channel (Henderson, 1966). Observations of large
numbers of ice blocks grounded, rolling and sliding
within the rising stage flows on November 5th illus-
trate the potential of ice blocks to significantly retard
jökulhlaup flows in this relatively low gradient chan-
nel. That numerous ice blocks were available dur-
ing higher stage flows is graphically illustrated by the
spectacular (5.7*106 m3) amount of ice removed by
tunnel roof collapse from the main Gígjukvísl sub-
glacial outlet (Russell et al., in press). Removal of
substantial volumes of glacier ice within the nearby
push-moraines (Figure 6b) is also likely to have sup-
plied large amounts of ice to the Gígjukvísl chan-
nel. The impact of the November 1996 jökulhlaup
was severe, due both to this being the largest ever
flow through this relatively recent river course and the
deposition of large numbers of ice blocks within the
river channel.
ACKNOWLEDGEMENTS
Fieldwork during summer 1996 was carried out as
part of Keele University Iceland Expedition 1996.
Research visits immediately before and after the
jökulhlaup were funded by an UK NERC grant
(GR3/10960) awarded to A. J. R. in October 1996.
Fieldwork during 1997 was funded by grants awarded
to Ó. K. by the Icelandic Public Roads Administra-
tion and the Research Council of Iceland. Dolf de
Jong and Helen Fay are thanked for valuable assis-
tance and discussion in the field. Dr. Magnús T.
Guðmundsson provided us with valuable discussion.
Dr. Helgi Jóhannesson and Dr. Hreinn Haraldsson are
thanked for checking our calculations and for valuable
comments on this manuscript. Within Keele Univer-
sity Department of Earth Sciences, Richard Burgess
is thanked for photographic reproduction and, Peter
Greatbach, David Wilde and Terry Doyle are thanked
for their cartography.
ÁGRIP
BREYTINGAR Á FARVEGI GÍGJUKVÍSLAR í
JÖKULHLAUPINU í NÓVEMBER 1996
Miklar breytingar urðu á farvegi Gígjukvíslar í jök-
ulhlaupinu í nóvember 1996. Hér er þessum breyt-
ingum lýst á svæðinu frá jökulgörðunum sem jökull-
inn stóð við um síðustu aldamót og niður fyrir þjóð-
veg. Með rannsóknunum fæst einnig ný sýn á há-
marksrennsli í Gígjukvíslarfarvegi en aðrir hafa einn-
ig sett fram tilgátur um þetta rennsli. Megin land-
mótun hlaupsins var á áður þröngum bugðóttum far-
vegi í gegnum jökulgarðana sem varð mun breiðari
og beinni ásamt stækkun farvegarins neðan jökulgarð-
anna sem var samfara hækkun botns í dýpsta hluta far-
vegarins. Þegar hlaupið rénaði varð rof á austurbakka
Gígjukvíslar. Hámarksrennsli Gígjukvíslar metið út
frá halla og flatarmáli er 19.500 m3 s_ 1 ±5.000 m3 s_ 1.
Um Gígjukvísl hefur því farið milli fjörtíu og fimm-
tíu prósent af hámarksrennsli jökulhlaupsins. Reikn-
að Mannings n gildi 0,045-0,152 tengist venjulega
mjög mikilli mótstöðu í ám: það bendir til að ísjak-
ar hafi truflað rennsli verulega í þessum hallalitla far-
vegi. Flutningur íss úr jökulgörðunum jók verulega
ís í farveginum. Landmótun jökulhlaupsins í farveg-
inum var stórkostleg bæði vegna þess að jökulhlaupið
var það stærsta sem um hann hefur farið og vegnaþess
hve mikill ís settist þar.
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