RD&D-Programme 2004 - SKB

The hydromechanical modellings that were carried out /21-19/ gave the following results
regarding conditions at depth in the rock mass beneath the ice sheet:
• When a warm-based continental ice sheet with considerable meltwater production expanded
over the studied area, the water pressure increased rapidly during the first 1,000 years, in
particular within conductive fracture zones.
• A high horizontal hydraulic gradient was obtained during the same period, due in part to the
compression of the pore system caused by the ice load.
• When the ice sheet completely covered the area, a large downward gradient arose.
• When the ice then retreated, the gradient reversed and became upward.
• Where the hydraulic conductivity of the rock mass was very low, residual high water
pressures could remain at depth for a long time.
• Since the ice load’s contribution to the stress was compensated to a great extent by high
water pressures, the change in effective stress was relatively moderate.
Shoreline displacement is above all a result of glaciations, both directly due to the isostatic
downwarping of the crust by the weight of the ice, and indirectly due to the fact that eustasy is
affected by the fact that sea water is bound up in or released from continental ice sheets. The
empirical model of shoreline displacement referred to above /21-3/ provides a well-underpinned
picture of the sequence of events following the retreat of the most recent continental ice sheet.
It provides support for interpreting present-day conditions and can also be used to predict what
will happen in the immediate future.
Programme
The programme is divided into three subprogrammes:
• Scandinavian ice sheets.
• Stress states in the earth’s crust.
• Shoreline displacement.
Scandinavian ice sheets
The project “Basal conditions and hydrology of continental ice sheets” was started in 2002 and
is planned to be concluded in 2006/2007. An ice sheet’s basal conditions and meltwater production,
and the water pressures and flows it generates, are of central importance for the thermal,
hydrological, mechanical and chemical conditions in the repository. The purpose of the project
is to provide a both spatially and temporally high-resolution picture of the basal conditions and
the hydrological system beneath continental ice sheets. The project includes both processes that
control the subglacial hydrology of ice sheets and the glaciological processes that control the
spatial and temporal variation of this hydrology. The subglacial hydrology system is supplied by
water from the following two sources:
• Melting of basal ice, due to for example deformation heat and geothermal heat.
• Meltwater from the ice surface, generated by the prevailing climate.
Basal melting occurs in warm-based areas which can be very large and exist over long spans
of time. The variations in this melting, caused by the general dynamics of the ice, are relatively
slow. Extensive basal meltwater production can occur beneath ice streams, common during the
Weichselian Glaciation. Surface melting generates much more water than the basal meltwater
system, but in a smaller, frontal area. The variations in water production from surface melting
are rapid. These variations may be great on both a daily and a seasonal basis, and generate great
variability in both pressure and flow. The two systems may overlap in the frontal portions of the
ice sheets. One of the project’s goals is to establish a model that includes both basal and surface
meltwater production.
296 RD&D-Programme 2004