Geoscience Reference
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T/T
eq
[-]
0
0.4
0.8
1. 2
1.6
2.0
1. 0
Slow
allogenic
change
0.8
0.6
Low-gradient rivers
Moderate-gradient rivers
Steep-gradient fans
Experimental fans
0.4
Fast
allogenic
change
0.2
0.0
Fig. 6.
Dimensionless plot of aggradation rate by normalised sediment yield q
in
/q
out
against time (T) relative to the time that
the fluvial system requires to reach grade (T
eq
). Sediment transport is calculated with a non-linear diffusion equation using
different exponents. The low-gradient rivers are simulated with the linear diffusion equation (exponent m = 1, based on
Paola
et al
. 1992). The steepest curve is calibrated against experimental results, for which the equilibrium slope is much
steeper than for the deeper natural streams. The dotted curves intermediate of experimental fans and low-gradient rivers
are estimations based on numerical interpolation between the steep experimental slopes and those of low gradient rivers
(see Postma
et al
., 2008). If the time period of allogenic forcing is much faster than equilibrium time of the river system then
the frequency of avulsion will change with it. If it is slow then there will be no significant change (see text for further
explanation).
aggradation rates are enhanced and if the period
of change is much slower than T
eq
, there will be
little change in aggradation rate (see also Paola
et al
. 1992; Van Heijst & Postma, 2001). Hence, slow
changes, as imposed for instance by regional tec-
tonics, will hardly affect the aggradation rate so
that the system remains in, or close to, the keep-
up stage. Yet, rapid progradation of a delta lobe
and subsidence near a fault scarp can have a sig-
nificant effect on the accumulation space of the
fluvial system and may bring it back into the start-
up stage (Fig. 6). The experiments by Hickson
et al
. (2005) illustrate this point beautifully: fast
subsidence is counterbalanced by high aggrada-
tion rates and slow subsidence rates by low aggra-
dation rates. This causes fluvial systems not to
migrate towards places with highest subsidence
rates unless aggradation rates cannot keep up with
the subsidence.
The analysis above leads us to a new working
hypothesis that predicts the change in autogenic
frequency: the rate of change (i.e. fast or slow
change) in allogenic forcing relative to the equilib-
rium time related to the morpho-dynamically
active part of the river system is the dominant
driver of the rate of change in aggradation and
herewith the change in frequency of autogenic
behaviour. Slow changes in aggradation rate do
not change autogenic behaviour significantly,
whereas fast change does. However, it is not to say
that other parameters, like peat growth in adjacent
floodplains, flood frequency, storm surge frequen-
cies and others cannot be important in causes for
a change in avulsion rate. Yet, it would be inter-
esting to test the launched hypothesis and meas-
ure aggradation rates in delta plains and plot them
against reconstructions of avulsion frequency.
IMPLICATIONS
At the scale of the channel belt, Leeder (1978)
attempted to establish fundamental connections
amongst subsidence, avulsions and channel belt
sandstone bodies stacking density. He suggested
that channel-belt stacking density and hence
connectivity is
inversely
correlated to temporal
(vertical) changes in sedimentation rate and that
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