Geoscience Reference
In-Depth Information
N
Base case
Initial
2 km
Q low
Q high
Fig. 10. Maps indicating the connected reservoir compartments. Each colour represents an individual reservoir compartment
(See Table 3 for details) for all four scenarios (Initial, Base case, Q low and Q high ) . See text for further explanation.
scenarios. The initial scenario results in a compart-
mentalised reservoir consisting of three main inter-
fluve areas directed in an on-offshore direction
(Fig.  10). The silt-filled channels form potential
flow barriers between the high-quality reservoir
areas. The connectivity between reservoir volumes
is increased by the formation of a wave-induced
sand-rich shoreface that improves the connectivity
of the isolated sands.
The decrease in the rate of morphological
change over time in the three cases suggests that
they move towards an equilibrium situation
(Fig.  11A, C and E). The hypsometric curves,
defined as the cumulative distribution of bed level
in the modelled area, tend towards stabilisation
when the rate of change decreases per time step.
Erosion of the delta front in the Base case occurs
to a depth of 7 metres (Fig. 11B). This erosion is
solely due to wave reworking. Beyond 7 m water
depth aggrading conditions prevail as the eroded
sediments are re-deposited in deeper water. In the
fluvial input cases ( Q low and Q high ) the erosion
extends to 10 m in depth (Fig.  11C and E). The
increased retrogradation in shallow water com-
pared to the base case scenario is due to the higher
efficiency of the transport mechanism for silt
induced by the offshore-directed fluvial discharge.
The net offshore transport of silt results in an
increased sediment loss at the shallow shoreface.
Channel scouring, migration and extension related
to fluvial processes ( Q low and Q high ) are responsible
for the hypsometric changes in areas where the
water depth exceeds 7 m.
The initial rapid adaptation of the shoreface pro-
file to the imposed wave climate suggests that
deltas may be very vulnerable, especially if their
deposits are composed of fine grained sediments.
However, our simulations show that, in time, the
sorting mechanism induced by wave reworking
results in a gradual increase of shoreface sands that
act as a protective layer to wave-induced erosion,
DISCUSSION
This study highlights the effects of wave rework-
ing on a fluvial-dominated delta based on numer-
ical simulations. Relevant physical processes
underlying flow transport and sediment trans-
port are included. When comparing the simula-
tion results to real-world data, several similarities
are observed. For example, in data from the
Mississippi delta (e.g. Atchafalaya Bay, Louisiana;
Neill & Allison, 2005; Roberts et al ., 2005) similar
silt and sand transport conditions are observed.
Furthermore, the partial preservation of silts by
reworked sand deposits is observed along the
'Isles Dernieres' barrier islands in the Mississippi
delta, where a protective sand cap prevented ero-
sion of the underlying fines (Dingler & Reiss,
1990). A comparable sand cap also plays a key
role in a conceptual morpho-sedimentary model
for Louisiana coastal restoration and barrier
retreat developed by Campbell (2005).
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