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waves in recent years. Figure 2 shows measurements of Hodge on Nisqually Glacier which
indicates the rapid passage of a velocity wave downstream.
Mention should also be made of wave ogives, although we will not deal with them
in this lecture. They are bands (also known as Forbes bands) which propagate below
ice-falls, and are due to the annual ablation cycle.
1.3 Surges
Perhaps the most spectacular form of wave motion is the glacier surge. Surges are large
scale relaxation oscillations of t.he whole length of a glacier. They are roughly periodic,
with periods on the order of 20-100 years. During a long quiescent phase, t.he glacier is
over-extended and t.hin. Ice accumulation causes the glacier to thicken upstream, while
the over-extended snout thins and retreats. Eventually, a critical thickness is reached,
and the glacier slumps rapidly downslope again. These surges will typically last only a
year or two, during which time the velocit.y may increase a hundred-fold. The glacier
snout will then advance by several kilomet.res.
A typical (and much st.udied) example is t.he Variegated Glacier in Alaska. Its surge
periodicity is about. twenty years, while its surges last about two years. The glacier, of
length twenty kilometres and dept.h four hundred metres, advances some six kilometres
during its surge, at measured speeds of up to 65 metres per day. Such large velocities
can only occur by basal sliding, and detailed observat.ions during the 1982-3 surge showed
that the surge was mediat.ed by an alt.erat.ion in the basal drainage system, which had the
effect of raising water pressure dramat.ically. A dynamic model suggests, in fact, that the
oscillations are caused by the competit.ive int.eraction between the basal sliding law and
the hydraulics of the subglacial drainage syst.em. When the ice is relatively thin (hence
the driving shear stress is low) the drainage occurs through a network of channels incised
into the ice at the glacier bed - called Rot.hlisberger channels. These allow effective
drainage at quit.e low water pressures (hence high effective pressures) and thus also low
velocities. At higher driving stresses, however, an instability forces the channel system
to close down, and the basal water is forced into cavities which exist between the ice and
bedrock prot.uberances (such cavities are well-known t.o exist). The water flow is reduced,
and the sudden increase in water pressure causes a sudden increase in ice velocity - the
surge. The transit.ion front. bet.ween t.he linked cavit.y drainage system and the channel
system is nucleat.ed near the maximum depth, and propagates rapidly both upstream and
downstream, at (measured) speeds on the order of hundreds of met.res per hour. At the
end of the surge, t.he channel drainage system is re-established. Figures 3 and 4 show a
vert.ical view of Variegated Glacier in pre- and post-surge states.
Our underst.anding of t.he Variegated surges relies on the concept of drainage switch
waves in recent years. Figure 2 shows measurements of Hodge on Nisqually Glacier which
indicates the rapid passage of a velocity wave downstream.
Mention should also be made of wave ogives, although we will not deal with them
in this lecture. They are bands (also known as Forbes bands) which propagate below
ice-falls, and are due to the annual ablation cycle.
1.3 Surges
Perhaps the most spectacular form of wave motion is the glacier surge. Surges are large
scale relaxation oscillations of t.he whole length of a glacier. They are roughly periodic,
with periods on the order of 20-100 years. During a long quiescent phase, t.he glacier is
over-extended and t.hin. Ice accumulation causes the glacier to thicken upstream, while
the over-extended snout thins and retreats. Eventually, a critical thickness is reached,
and the glacier slumps rapidly downslope again. These surges will typically last only a
year or two, during which time the velocit.y may increase a hundred-fold. The glacier
snout will then advance by several kilomet.res.
A typical (and much st.udied) example is t.he Variegated Glacier in Alaska. Its surge
periodicity is about. twenty years, while its surges last about two years. The glacier, of
length twenty kilometres and dept.h four hundred metres, advances some six kilometres
during its surge, at measured speeds of up to 65 metres per day. Such large velocities
can only occur by basal sliding, and detailed observat.ions during the 1982-3 surge showed
that the surge was mediat.ed by an alt.erat.ion in the basal drainage system, which had the
effect of raising water pressure dramat.ically. A dynamic model suggests, in fact, that the
oscillations are caused by the competit.ive int.eraction between the basal sliding law and
the hydraulics of the subglacial drainage syst.em. When the ice is relatively thin (hence
the driving shear stress is low) the drainage occurs through a network of channels incised
into the ice at the glacier bed - called Rot.hlisberger channels. These allow effective
drainage at quit.e low water pressures (hence high effective pressures) and thus also low
velocities. At higher driving stresses, however, an instability forces the channel system
to close down, and the basal water is forced into cavities which exist between the ice and
bedrock prot.uberances (such cavities are well-known t.o exist). The water flow is reduced,
and the sudden increase in water pressure causes a sudden increase in ice velocity - the
surge. The transit.ion front. bet.ween t.he linked cavit.y drainage system and the channel
system is nucleat.ed near the maximum depth, and propagates rapidly both upstream and
downstream, at (measured) speeds on the order of hundreds of met.res per hour. At the
end of the surge, t.he channel drainage system is re-established. Figures 3 and 4 show a
vert.ical view of Variegated Glacier in pre- and post-surge states.
Our underst.anding of t.he Variegated surges relies on the concept of drainage switch