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delta switching examples known from the
Mississippi delta (Coleman & Gagliano, 1964;
Scruton, 1960) but human engineering can also
cause this reduction of fluvial input (McManus,
2002; Syvitski & Saito, 2007). Both subsidence and
reduced upstream sediment supply (and associated
deterioration of wetlands, e.g. due to distorted
balance of nutrient supply and salt water intru-
sion) induce topographic changes that, in turn,
will change local characteristics of the incoming
wave field, which itself may be constant in deep
water. Both processes are the main causes for
increased wave power in the Mississippi delta
(Day
et al
., 2007) and presumably also in other del-
tas. The increased water depth and lack of natural
buffer allow the waves to propagate and cause fur-
ther retrogradation. No compaction of the deposits
and no other sediment reworking processes are
included in the simulation, hence the focus is on
deltaic wave reworking only.
Several scenarios are set up to analyse the effect
of wave reworking under fluvial discharge
conditions with different magnitudes (Table 2).
All scenarios have equal parameter settings and
initial conditions but the imposed boundary con-
ditions vary. The Base case scenario is defined by
sediment reworking under perpendicular incident
quiet-weather short-crested waves (Hs = 1 m) only.
No fluvial discharge is taken into account. The
Base case simulation is repeated with a range of
different fluvial discharge boundary conditions to
mimic a gradual or partially successful upstream
avulsion (cases
Q
low
and
Q
high
). For the scenario
with low fluvial input (case
Q
low
), a simulation
with a discharge of 500 m
3
/s is conducted and for
the scenario with high fluvial input (case
Q
high
), a
discharge of 2000 m
3
/s is applied. The latter
discharge is identical to the discharge during the
formation of the initial delta. Sediment transport
in river channels is mainly governed by flow
velocity and therefore increases with river dis-
charge. For these scenarios, the grid of the Base
Table 1.
Model parameters.
Parameter
Value
Timespan simulated
44 months
Timestep
30 s
Morphologic
acceleration factor
60
Grid resolution
50 m × 50 m
Flow grid size
167 × 200
Wave grid size
170 × 382
Morphological characteristics
Sediment transport
formula
TRANSPOR2004 for both
sediment fractions
Specific density (both
sediment fractions)
2650 kg m
−3
Median grain size
(sediment fraction 1)
125 µm
Dry bed density
(sediment fraction 1)
1600 kg m
−3
Median grain size
(sediment fraction 2)
50 µm
Dry bed density
(sediment fraction 2)
500 kg m
−3
Number of underlayers
75
Maximum thickness
underlayer
0.1 m
Thickness of transport
layer
0.2 m
Suspended transport
factor
1.0
Bed-load transport factor
1.0
Wave-related suspended
transport factor
0.3
Wave-related bed-load
transport factor
0.3
Spin-up interval before
morphological changes
60 min
Table 2.
Model scenarios.
Overview case settings
Boundary conditions
Wave climate
(offshore)
River discharge
(upstream)
River sediment load
(upstream)
Scenario
Description
Base
case
Wave reworking with
no river discharge
Perpendicular,
H
s
= 1 m, T
p
= 5 s
-
-
Case
Q
low
Wave reworking with
low fluvial input
Perpendicular,
H
s
= 1 m, T
p
= 5 s
500 m
3
s
−1
Equilibrium sediment
concentration for silt -
and sand fraction
Case
Q
org
Wave reworking with
high fluvial input
Perpendicular,
H
s
= 1 m, T
p
= 5 s
2000 m
3
s
−1
Equilibrium sediment
concentration for silt -
and sand fraction
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