Summary
Summary Subglacial hydrology
- Module
- Glaciology
- Institution
- Cambridge University (CAM)
Exam notes on the subglacial hydrology of mountain glaciers, and how the switch from a distributed to channelised system occurs.
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Intro• interested
in
glacier
hydrology
given
the
power
of
glaciers
to
influence
runoff
and
chemical
weathering
and
so
act
as
a
potential
geohazard
(Sharp,
2005).
• influence
drainage
configurations
on
water
storage
and
flow
(Benn
and
Evans,
2010).
• understanding
-‐
indirect
methods
(Sharp,
2005).
o radio-‐echo
sounding,
monitoring
boreholes
and
runoff
properties
and
artificial
tracing
(Sharp,
2005).
• Winter-‐spring,
meltwater
discharge
of
temperate
glaciers
increases
2
orders
of
magnitude,
hydrological
system
must
adapt
accordingly
(Fountain
and
Walder,
1998).
• subglacial
drainage
system
shifts
accordingly
from
a
‘distributed’
system
in
which
water
transport
to
the
tongue
is
slow
and
inefficient
to
a
‘channelized’
system,
which
transports
water
faster
and
more
efficiently.
Distributed
system
• 3
components
-‐
relative
hydrological
each
component
vary
space
+
time
(B
and
E,
2010)
o pervasive
water
film
§ exist
between
ice
and
the
underlying
sediment
§ transport
water
produced
at
the
bed
by
geothermal
heating
and
sliding
friction,
or
waters
involved
in
regelation
around
small
obstacles
on
the
glacier
bed
(Weertman,
1964).
§ Stable
up
to
very
small
thicknesses
of
around
4mm
o permeable
till
enabling
groundwater
flow,
§ Permeable
and
porous
subglacial
sediment
hypothesised
function
confined
aquifer
drain
water
from
glacier
bed
(Boulton,
1974)
§ probably
minor
role
(Sharp,
2005).
• Measured
hydraulic
conductivities
of
subglacial
sediments
are
low
(Fountain
and
Walder,
1998).
• Most
calculations
-‐
Darcian
flow
through
aquifers
of
thicknesses
around
0.1m
with
a
hydraulic
gradient
of
∼0.1
are
not
even
able
to
transport
discharge
of
the
winter
period
(Alley,
1989).
• Therefore
the
aquifer
will
become
saturated
and
drainage
systems
will
operate
on
its
upper
surface
(Sharp,
2005).
o a
linked
cavity
network
§ major
role
in
distributed
system
(Fountain
and
Walder,
1998)
§ Form
-‐
sliding
ice
separates
from
the
glacier
bed,
formation
favoured
by
rapid
sliding
and
high
bed
roughness
(Nye,
1970).
§ Lliboutry
(1976)
2
types
• autonomous
cavities
–
hydraulically
isolated
stagnant
meltwater
• interconnected
cavities
-‐
active
part
§ nonarboresecent
cavity
networks
-‐
linked
by
small
bedrock
(Nye)
channels
which
are
incised
up
to
0.2m
into
the
glacier
bed
(Nye,
1973).
§ Studies
recently
deglaciated
bedrock
surfaces
-‐
large
numbers
of
Nye
channels,
beneath
at
least
some
small
alpine
glaciers
(Sharp
et
al,
1989;
Fountain
and
Walder,
1998).
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