Geoscience Reference
In-Depth Information
case scenario is extended in the landward direc-
tion to include part of the river system (Fig.  2).
The discharge is imposed directly at an already
present (main) channel.
(Fig. 4B). The remnants of the decoupled distribu-
taries can be identified from the depth-averaged
flow velocity field, by weakly-channelised flow
zones on the delta plain and complex small-
scale gyres at their termini. The depth-averaged
suspended-load transport fields for the sand and
silt fractions (Fig. 4C) show an overall dominance
of the silt fraction; a large amount of silt is trans-
ported in an offshore direction by two counter-
rotating gyres on either side of the longitudinal
delta axis (cf. longitudinal drift divides commonly
present at major headlands). Maximum suspended
sand transport is concentrated at the tip of the lon-
gitudinal delta axis, while maximum suspended
silt transport is distributed along the delta front,
at  either side of the longitudinal delta axis. After
45 months, overall flow field and suspended trans-
port fields at the delta front have become more axi-
ally symmetric, though the signature of the initial
delta front can still be observed from the wave
height distribution (Fig. 4D). Furthermore, the major
distributary is now fully disconnected from deeper
water; however, its former configuration can be
traced from the local wave height gradient, which is
lower compared to the overall steeper decline. In
addition, note that the roughness of the wave height
distribution decreases with simulated time.
Wave reworking and transport leads to typical
sediment sorting patterns. Silt is transported as
suspended load over a broad area in both the long-
shore direction and offshore direction and settles
in deeper waters, mainly off-axis at the delta front
RESULTS
Base case scenario
The initial fluvio-deltaic morphology of the delta,
which contains multiple distributaries dissecting
the delta plain (Fig. 2), rapidly adjusts to the imposed
wave conditions by infilling of distributaries with
silt caused by onshore transport that is driven by
wave asymmetry (Fig.  3). Large volumes of silt are
entrained by waves at the delta front. Overall, wave
reworking and associated wave-induced currents
cause erosion and smoothing of the delta front
(Fig.  3). As expected, the highest transport rates
are observed along the delta front due to longshore
currents as a result of longshore gradients in wave
height (Fig. 4). Consequently, the shoreline perturba-
tion gradually diminishes as the delta front diffuses
alongshore with respect to the incoming waves.
Fig.  4 displays the significant wave height,
depth-averaged flow velocity field and the depth-
averaged suspended load for both silt and sand
fractions, for two specific time steps (t = 2 and
t = 45 months). After 2 months, all distributaries,
except for the main one, are disconnected from the
retrograding shoreline (Fig.  4A), which is charac-
terised by relatively strong, front-aligned currents
(A)
(B)
t 1
t 5
t 15
t 45
total deposition/erosion
N
Silt
deposition
4
Delta
front
erosion
0m
-4
Channel infill
0
-2
-6
Silt
deposition
2 km
-10
Fig. 3. Morphologic development of the Base case scenario at four distinct time intervals. Timesteps (t) are given in months.
To the right: cumulative sedimentation and erosion at t = 45 months.
 
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