Geoscience Reference
In-Depth Information
(A)
(B)
XES 02
XES 05
3.2
3.5
3.1
sediment
storage
3.3
sediment
storage
3
sediment
release
3.1
sediment
release
0.5
1
channelised flow
channelised flow
0.8
0.4
0.3
0.6
0.4
0.2
sheet flow
sheet flow
0.2
0.1
sheet flow
0
0
10
11
12
13
14
15
16
17
18
80
82
84
86
88
90
92
94
96
98
100
run time [hr]
run time [hr]
Fig. 2.
Time series data of shoreline migration (top panels) and wetted fraction of the deltaic surface (bottom panels) in
(A) XES 02 and (B) XES 05.
using a bimodal sediment mixture at the University
of Pennsylvania. The sediment mixture is com-
posed of 80% acrylic sand (D = 300 µm) and 20%
granite chips (D = 2 mm). This bimodal mixture
was designed to maximise the contrast between
grain size and density, thus enhancing the thresh-
old effect for sediment transport. Fluctuation in
the wetted fraction associated with strong chan-
nelisation and backfilling in the fluvial surface is
consistent with the other experiments presented
above. The time necessary for the surface to reor-
ganise through refilling incised channels and
searching for new paths was shown to scale with
the following form (Reitz
et al
., 2010):
XES 02 and 05 experiments. The XES 02 experi-
ment had five times greater sediment supply,
which would cause a shorter time duration between
the autogenic cycles, than that shown in XES 05.
XES 02 showed an increase in the wetted fraction
over 2 to 3 hour backfilling/storage events whilst
the XES 05 shows 8 to 10 hour events (Fig. 2).
The sediment to water discharge ratios for the
XES 02 and 05 experiments were kept constant
at ~ 0.01, which generated similar delta top sur-
face slopes of 0.036 and 0.048, respectively.
However, this small increase in delta topset slope
induces a fair decrease in the size of the fluvial
sediment buffer (i.e. potential volume capacity to
hold sediment during a storage event) and forces
less sediment on the topset potion of the delta to
be accommodated and released to offshore (Kim &
Jerolmack, 2008). Direct and indirect measure-
ments for the slope fluctuations in XES 02 and 05,
due to the fluvial autogenic processes, indicate
averaged 0.004 and 0.0027 slope changes across
the basin, respectively (11% and 6% changes
compared to the averaged topset slopes), to
account for the observed shoreline fluctuations.
The amount of sediment that is either stored or
released in the delta top surface thus defines
the magnitude of the shoreline fluctuation over
the autogenic process (Kim & Jerolmack, 2008).
XES 02 shows a larger amount of sediment
HBs
Q
T
~
(1)
ap
s
where
T
ap
denotes the characteristic avulsion (auto-
genic process) time scale,
H
is the vertical length of
alluvial cutting by the release events, which can be
roughly scaled with flow depth,
B
is the horizontal
length of basin, scaled with total channel width,
s
denotes the downstream shoreline position as a
function of time equivalent with the basin size in
the downbasin direction and
Q
s
is sediment sup-
ply. This time scale captured the sediment storage
and release cycles in their experiment as well as
correctly predicting autogenic frequencies in the
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