Jökull - 01.06.2000, Page 26

Fiona S. Tweed
Figure 3. Likely tunnel cross-sectional form at Sólheimajökull. - Líklegtþversnið afútfalli jökuláa undan Sól-
heimajökli.
collapse would lead to tunnel closure at the lowest
discharges but not necessarily at the higher end of the
discharge range.
Figure 4. Conduit geometry assumed by Hooke et al,
1990. - Þversnið afvatnsrás við jökulbotn samkvœmt
Hooke et al, 1990.
OTHER TUNNEL CLOSURE PROCESSES
Temporary blockage of the tunnel by falling ice
blocks is possible at Sólheimajökull; ice can be obser-
ved to calve from the area around the southem ex-
it portal where the river sometimes undercuts the
glacier. Given the compressive flow across the tunnel
axis, it is unlikely that ice blocks detach from the roof
within the tunnel proper, and at the northern portal
entrance in Jökulsárgil the main components of ice
flow are across the tunnel axis and southwards which
seldom results in ice falls around the tunnel entrance.
Thus, should falling ice blocks occur anywhere, it
is likely that they do so around the exit portal. This
process was observed to dominate during the summer
months when river discharges tend to be sufficiently
high to float the ice blocks away from the area tunnel
portal, although water was occasionally observed to
back up behind the blocks eventually giving rise to a
small flood surge.
Lawler (1989) describes the occurrence of “heart-
beat” effects in the Jökulsá, comprising rapid dischar-
ge decreases proceeded by a sharp change to higher
stages prior to the restoration of previous flow levels.
This type of event may be linked to the temporary
24 JÖKULL No. 48