Geology Reference
In-Depth Information
migration could account for up to 30% of the
total erosion. Hillslope failures are more likely
following knickpoint passage, and, despite
steady base-level fall, sediment fluxes vary
cyclically by 20-30%. The dynamism of these
artificial landscapes is striking!
A
Knickpoint Migration
3.0
1.0
0.8
0.5
0.3
Numerical experiments
The apparent conflict between Gardner's (1983)
observation that knickpoints degenerate into
uniform slopes during stream-table experi-
ments using homogeneous material and field
observation that at least some knickpoints in
homogeneous bedrock appear to be rather
long-lived (Wobus
et al
., 2006a) suggests that
other approaches, including numerical models,
may be useful in investigating knickpoint
migration and persistence. Some recent models
(Crosby
et al
., 2007) exploit a stream-power
formulation of the form of Eqn 8.5 in which
m
=
0.5 and
n
=
1 to calculate the change in height
at each point along a channel profile (Fig. 8.11).
Initially, the model is run forward to allow
development of a detachment-limited profile that
is adjusted to base-level fall at a steady rate of
1 mm/yr. Once an equilibrium profile is attained,
the rate of base-level fall is accelerated 10-fold to
10 mm/yr for 5 kyr (equivalent to 50 m of base-
level change) and then is restored to 1 mm/yr
(inset in Fig. 8.11A). Following restoration to the
background rate of base-level fall, this model
predicts that the knickpoint that formed in
response to rapid base-level fall will migrate
upstream as a kinematic wave with a speed that
is proportional to the area (and, therefore, dis-
charge) upstream of the knickpoint. Notably, the
0.1
0.0
0.2
0.4
0.6
0.8
1.0
Normalized discharge
B
“Complex” Response
terrace
incision
initial
c
hanne
l
time
2
time
1
braided
channel
terrace
time 3
aggradation
time
4
C
baselevel lowering events
250
"
“Complex” Response
200
baselevel
3
baselevel
4
150
100
50
baselevel 2
baselevel
1
0
0.0
0.2
0.4
0.6
0.8
1.0
Time
Fig. 8.10
Stream-table experiments on the effects of
base-level lowering.
A. Rate of knickpoint migration in a stream-table
experiment as a function of discharge (or drainage area)
contributing to a site. These data suggest that knick-
points migrate 20 times faster for a catchment about
twice as large (twice the discharge). B. Schematic
cross-sections of a channel responding to a knickpoint
migrating past it. Alternating intervals of incision and
deposition are sometimes termed “complex response.”
Time 1: prior to knickpoint passing, small alluvial
terraces flank the channel. Time 2: immediately after
knickpoint passed, channel has incised through the
alluvium and the underlying bedrock; lateral migration
causes it to widen its valley. Time 3: increased sediment
load from upstream causes the bed to aggrade. Time 4:
decreased sediment load causes incision of the bed.
C. Sediment yield resulting from multiple events of
base-level lowering on a stream table. Note the high
variance in yield, the secondary peak that occurs after
the initial rapid drop in yield, and the gradual damping
of the oscillations in sediment fluxes. Modified after
Schumm
et al.
(1987).
