Jökull - 01.12.1980, Page 23

Comparison of Sediment load Transport in the Skeiðará
Jökulhlaups in 1972 and 1976
HAUKUR TÓMASSON, SVANUR PÁLSSON AND PÁLL INGÓLFSSON,
National Energy Authority,
108 Reykjavík
ABSTRACT
The charactenstics of the Skeiðará jökulhlaups are
briefly descnbed. A summary of the data on the
quantity of suspended sediment load is presented in
Table 1. The three methods which have been used in
the sampling of sediment are found to give highly
different amounts of 'coarse sediment load. The jökul-
hlaup of 1976 was the first one in Skeiöará in which
more than one sampling method was applied.
In the jökulhlaup of 1976 a good dis-
charge/sediment rating curve and a fairly uniform
spatial distribution of the sediment were obtained. On
the other hand in 1972 the rating curve was poorly
defined and the spatial distribution non-uniform.
In the jökulhlaup of 1972 a layer of sand and
coarse silt was deposited in the flood channel, but not
in 1976. The deposit is too fine-grained to have been
laid down by running water. The present authors
explain it as having been deposited by way of anchor
ice which is also considered responsible for the
vanation in the sedimenl concentration from one place
to another. Similarly, such deposition may explain
some very fine-grained layers in ancient alluvial
sedimenls.
INTRODUCTION
The Grímsvötn jökulhlaups are vast floods
caused by draining of the Grímsvötn sub-
glacial lake which is situated in the western
part of the Vatnajökull ice-cap. The floods
appear in the glacier rivers Skeiðará, Gígju-
kvísl and Súla on Skeiðarársandur and some-
times they spread over a great part of the
sandur plain, see Fig. 1. Because of these jök-
ulhlaups the above rivers were first bridged as
late as 1973 — 74. Skeiðará is by far the most
voluminous of the glacier streams on Skeiðar-
ársandur. Floods in that particular stream are
hlaups. Key: SKO: Skeiðará at glacier outlet,
SKG: Skeiðará at dikes. — Mynd 1. Yfirlits-
mynd. Hlaupfarvegir Skeiðarárhlaupa. SKÚ: Skeið-
ará við Útfall. SKG: Skeiðará við garða.
JÖKULL 30. ÁR 21