Geoscience Reference
In-Depth Information
5.4.3 Geomorphic Variables
Estimates of thickness, volume of rock contained in the rock glacier, rates of cliff
retreat, formation age, and average thickness of ice were calculated (Table
5.3
).
These variables can improve our understanding of the current characteristics and
provide some useful information about Holocene erosional processes and recent
activity. Based on the surface slope and height of the front slope, the rock glacier
has an average thickness of about 54 m. If the debris content (excluding ice and
void spaces) is assumed to be 30-50% of the total volume, the volume of rock
ranges from 5,540,400 to 9,234,000 m
3
(Table
5.3
)(Barsch
1996
). When con-
sidering the source area of this rock, 17-28 m of rock must have been removed
from the cliff walls surrounding the rock glacier. This corresponds to a cliff retreat
rate of about 1.7-2.8 mm/yr, assuming the rock glacier began forming 10,000
years ago.
Glen's flow law has been modified to account for the weight of overlying rock;
this provides a method to estimate the thickness of ice contained within rock glaciers
(Konrad et al.
1999
). The equation assumes that basal sliding is not a major com-
ponent of the flow of the rock glacier. Using this formula, mean ice thickness could
range from 1-10 m for California rock glacier, assuming that 2-5 m of surface debris
exists over the entire surface of the rock glacier (Table
5.3
).
Table 5.3
Geomorphic variables estimated from analysis of GIS data and rates of flow
Variable
Calculation
Formula
Mean thickness
54.1 m
H
v
(cos
α
), where
H
v
=
height of the front
slope,
α =
slope of upper surface
Volume of debris
5,540,400-
9,234,000 m
3
(Arg) (Drg) (T), where
A
rg
=
rock glacier
area,
D
rg
=
debris content (0.3-0.5),
T
=
mean thickness
Thickness of bedrock worm
17.0-28.3 m
(Vrg)/(As), where
V
rg
=
volume of the
rock glacier,
A
s
=
area of source
Cliff retreat rate
1.70-2.82 mm/yr
(
T
b
)/(10,000), where
T
b
=
thickness of
bedrock
ρ
i
d
(
n
+
1)
1
v
(
n
+
1)
ρ
d
n
+
1
ρ
d
ρ
i
d
,
Mean ice thickness with 2 m
of surface debris
9.8 m
+
−
)
n
2
A
(
ρ
i
g
sin
θ
Mean Ice Thickness
with 3 m of Surface Debris
6.9 m
where
v
=
surface velocity,
g
=
acceleration of gravity,
ρ
=
density
i
of ice (900 kg/m
3
),
ρ
=
surface debris
d
(2700 kg/m
3
)
Mean ice thickness with 4 m
of surface debris
3.9 m
From Konrad et al. (
1999
)
Mean ice thickness with 5 m
of surface debris
1.0 m
-
