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initially remain open but over time only the main
distributary channel stays active (Fig. 6C and D).
All other distributary channels are filled-in with
silt. The relatively high discharge combined with
a steady decrease in the number of distributary
channels results in increasing flow velocities
(1.3 m/s to 1.5 m/s) in the main distributary chan-
nel. This causes severe scouring. Consequently,
the pathway of the main distributary channel
orientation switches to the hydraulically most
efficient route, i.e. through the basin centre.
The main distributary channel also transports
sand, as bed load, and silt, as suspended load, off-
shore. Additional sand transported by the river is
deposited immediately downstream of the river
mouth, thereby forming a subaqueous levee and
incipient mouth bar (Fig. 6C; centre-right panel).
As the sand-ridge aggrades it gradually blocks the
outgoing current causing enhanced deposition,
thereby deflecting the main distributary channel.
The southern delta front is not influenced by fluvial
input and mainly silt is transported in a southern
direction. The degree of wave-influence thus not
only changes temporally, due to changes in river-
ine supply, but also spatially, in agreement with
field observations (Bhattacharya & Giosan, 2003).
Sediment sorting occurs more distinctively than
in the Base case and the
Q
low
scenarios. The removal
of large volumes of silt from the delta front by the
scouring distributary channel leads to a more
sandy character of the retrograding delta. When
channel switching occurs, abandoned sand ridges
are gradually reworked in landward direction and
lateral direction and are expected to reconnect to
the shoreline (Fig. 6C and D). These ridges help to
prevent further erosion by reducing wave impact
on landward deposits. Furthermore, the formation
of sandy shoreface-like deposits prevents the silty
substrate from further erosion as observed in the
Base case scenario (Fig. 3).
except for areas with local features such as chan-
nel infills or areas where coastline realignment
takes place by long shore transport. Therefore, the
large-scale deltaic development of the scenario
presented above is driven by long shore transpor-
tation of sand.
LINKED PROCESS AND RESERVOIR
MODELLING
The simulated initial and wave-reworked del-
taic stratigraphy can be imported into reservoir
modelling software for post processing to obtain res-
ervoir characteristics. Here we used the Petrel 2008
seismic and simulation software by Schlumberger.
The simulated stratigraphy, which has a resolution
of 50 m by 50 m by 0.1 m, is up-scaled to 50 m by
50 m by 0.25 m. We have isolated the deltaic sedi-
ments by assuming that the final stratigraphy,
including the channels, is overlain by silt thus creat-
ing a heterogeneous synthetic deltaic reservoir of
10,000 m by 5000 m by 15 m.
Synthetic wells
Fig. 7 shows the net-to-gross ratio (N/G) at the
palaeo-surface of the reworked delta deposit for
the Base case scenario as well as the position of
five synthetic wells (D3D01 to D3D05). A series of
fence diagrams for all scenarios shows the strati-
graphic heterogeneity of the deposits (Fig. 8). The
substrate is a homogeneous mixture of sand and
silt. Fig. 9 shows synthetic stratigraphy encoun-
tered in the five wells for the four scenarios. The
synthetic wells all show both fining and coarsening
upward cycles which are typical for fluvio-deltaic
stratigraphy. Well D3D01 is characterised by sub-
strate-overlying fine-grained prodelta sediments
that gradually change into delta front sediments.
This coarsening-upward succession is overlain by
delta plain sediments which typically fine upward.
Well D3D01 is unaffected by wave reworking in the
Base case scenario and hardly affected by wave
reworking in the
Q
low
scenario. However, it is fully
reworked during the
Q
high
scenario where the pro-
gradational delta sequence is replaced by a typical
coarsening upward shoreface succession.
Well D3D02 shows a similar stratigraphic pattern
as that in well D3D01 but in this case it is more
proximal and therefore has a higher N/G ratio. As
mentioned before, significant reworking occurred
only in the
Q
high
scenario; however, in this well the
Sediment transport directions
Our model results indicate that cross-shore sedi-
ment transport is an order of magnitude lower
than long shore sediment transport. Long shore
transport gradients are large because of the curved
deltaic shoreline. Cross-shore transport is most
relevant in areas where the cross-shore profile
(shoreface) is out of equilibrium, either becoming
too gentle or too steep in relation to the induced
waves. The constant wave forcing ensures that the
cross-shore profiles are generally in equilibrium,
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