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timing and location of channel bifurcation is con-
trolled stochastically as a function of the cross-
valley slope of the floodplain adjacent to the
channel belt relative to the down-valley slope and
of annual flood discharge. To examine how the
model responds to extrinsic controls, the model
was run under conditions of changing base level
and increasing sediment supply. Rises and falls in
base level and increases in sediment supply
occurred over 10,000 years. Rising base level
caused a wave of aggradation to move up-valley,
until aggradation occurred over the entire valley.
Frequency of bifurcations and avulsions increased
with rate of base-level rise and aggradation rate.
Kleinhans
et al
. (2008) modelled numerically
duration of the avulsion time, which is assumed
to be determined by the width to depth ratio of the
channel, the upstream bend radius determining
the gradient advantage for one bifurcate over the
other and the length of the bifurcates. According
to Kleinhans
et al
. (2008) the model explains how
combination of variables may result in the
observed large variation of avulsion duration in
historical and geological data.
In summary, experimental and numerical stud-
ies show that aggradation rate is an important
driver for the frequency at which autogenic pro-
cesses occur. Furthermore, these process studies
show that absolute values of frequencies depend
on the backfilling rate of the channel, which
depends strongly on the accumulation space that
needs to be filled, but also depends on local fac-
tors that determine the hydrology. It is important
to note at this point that all experimental model-
ling discussed here showed that avulsion fre-
quency is certainly not constant with aggradation
rate or varies around a mean value, as assumed in
the modelling of Leeder (1978).
1
Sheets
et al.
(2002)
Stage 1
Stage 2
Stage 3
0.1
Experiments herein
X1
X4
X3
0.01
0.001
0.0001
0.00001
10
100 000 10 000
Flow occupation (sec)
100 000 10 00 000
Fig. 2.
Change in effective aggradation rate as a function of
duration of flow occupancy for all sections in the three
experiments. From Ashworth
et al
. 2007 with data from
stages 1 to 3 from Sheets
et al
. (2002).
the alluvial fan surface. While in the first case all
supplied sediment aggraded on the fan surface,
in the second case part of the supplied sediment
bypassed the fan surface and aggraded in the
subaqueous delta.
Ashworth
et al
. (2007) plotted flow occupancy
against effective aggradation rates including the
data from Sheets
et al
. (2002). The plot shows a
near-inverse relationship and demonstrates that
with the highest aggradation rates the channels
still have time to distribute sediment sufficiently
over the braid plain. With the lowest aggrada-
tion rates, the channel can be seen to migrate
predominantly laterally with, rare, sudden shifts.
Ultimately, a perfect inverse relationship between
effective aggradation rates and flow occupancy
must exist if the braidplain aggrades evenly over
its entire surface. Ashworth
et al
. (2007) compiled
their data and that of Sheets
et al
. (2002) in a dia-
gram (Fig. 2) to demonstrate that aggradation rate
is an important driver of flow occupancy and thus
frequency of autogenic processes. In spite of the
fact that the avulsion process is of a different kind
than in the alluvial fan studies of Sheets
et al
.
(2002), the correlation is excellent.
For low-gradient rivers, Karssenberg & Bridge
(2008) modelled bifurcation and avulsion fre-
quency three dimensionally by simulating sedi-
ment transport by the diffusion equation. The
ALLOGENIC CONTROLS ON GENERIC
AUTOGENIC BEHAVIOUR
The experiments on three-dimensional alluvial
stratigraphy by Hickson
et al
. (2005) were designed
to investigate the influence of allogenic controls
on architecture in alluvial successions. They
concluded that the alluvial architecture of their
models is controlled, very strongly, by externally
forced facies migrations, hence by changes in sedi-
ment supply, base level or subsidence. Sea-level,
climate and tectonics change the accumulation
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