Geology Reference
In-Depth Information
24
Scarp Evolution
New Zealand
north facing
Topographic Profiles:
New Zealand &
Idaho
New Zealand
south facing
Idaho
north facing
A
Idaho
south facing
Fig. 11.7
Scarp degradation in contrasting
climates.
A. Topographic profiles of 15- to 20-kyr-old
fluvial scarps in New Zealand and in Idaho
showing rounding of corners and relatively
straight riser slopes. Equator-facing scarps at
both sites degrade faster than pole-facing
scarps. B. Modeled slopes for three New
Zealand profiles, shown in the left inset. Right
inset shows hillslope transport rule, in which
the linear dependence of hillslope transport on
slope at low slopes is modified by more
efficient transport as slopes approach a
threshold slope. Modified after Roering
et al.
(2001). The effective diffusivity of the
landscape in New Zealand is shown to be
roughly half that in Idaho, such that scarps
degrade about twice as fast in Idaho. Modified
after Clarke and Burbank (2010b).
40
Transport Rule
46 m
Q=kS(1/(S-S
c
)
2
)
0.6
46 m
20
12 m
2 m
Q=
k
S
0
0.5
Slope, S
-60
0
6
0
12 m
Distance (m)
0.4
0.3
0.2
2 m
0.1
B
-100
-80
-60
-40
-20
0
20
40
60
Distance (m)
not well described by diffusion was operating in
the early stages of scarp decay. Rapid failure of
the scarp by slumping and talus formation leads
to angle-of-repose slopes typically within a mat-
ter of decades to centuries - times much shorter
than the presumed ages of the scarps (Pierce and
Colman, 1986). This angle-of-repose slope is,
therefore, likely the more appropriate initial con-
dition for diffusion-based modeling of the scarp.
Estimates of scarp age are made knowing that a
slight lag is associated with the development of
the angle-of-repose slope. Alternatively, one may
postulate a process rule that mimics the genera-
tion of the angle-of-repose slope and start the
calculation from the initial oversteepened step.
Examples of this treatment include that in
Anderson and Humphrey (1989), Howard (1997),
and Howard
et al
. (1994), where the downslope
sediment flux departs from a linear relation with
slope angle, increasing rapidly as the slope steep-
ens toward the angle of repose.
Non-uniform diffusivity
Late glacial fluvial terrace scarps in central Idaho
(Pierce and Colman, 1986) and New Zealand
(Clarke and Burbank, 2010b) display a
dependence of the diffusion coefficient on the
orientation of the slope (slope aspect; Fig. 11.7).
These studies document a several-fold difference
between north- and south-facing slopes, which
they attribute largely to the dependence of vege-
tation density on solar radiation and moisture
availability. Both studies also note a dependence
