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with a transgressive shoreline setting. Van Maren
(2005) concluded that onshore transport of sedi-
ment, induced by wave asymmetry, is of prime
importance to the initiation of subaqueous barriers.
Subsequent (subaerial) stages of barrier(−spit)
transformation were hypothesised to be character-
ised by alongshore currents and overwashes, while
river (flood) flow was suggested as a destabilising
mechanism to barrier settlement. The role of
typhoons in barrier destruction was considered
less significant (van Maren, 2005). Although van
Maren (2005) investigated the morphodynamics of
a real-world delta under riverine and wave forcing,
enabling the formulation of a concept for cyclic
constitution of barriers on delta plains, the stratal
response of the sediment surface was not addressed.
boundaries of the computational domain, drive
the system. The shallow water equations are
discretised on a staggered grid (cell size 50 m ×
50 m) with the water level points defined at the
cell centres and the velocity components at the
midpoints of the cell faces. An alternating direc-
tion implicit time integration (ADI) method is
used to solve the shallow water equations
(Stelling & Leendertse, 1991). A fixed time step of
30 s is applied, which meets the requirements of
the Courant number for turbulent flow.
A wave climate is imposed as an offshore
boundary condition at the downstream boundary
of the computational domain. Simulation of wind-
generated short-crested waves is performed by the
third-generation SWAN model (Booij
et al
., 1999).
SWAN (Simulating WAves Nearshore) computes
the propagation of short waves in coastal regions
based on the discrete spectral balance of action
density and driven by boundary conditions and
winds. The flow and wave modules of Delft3D
communicate via the 'online' coupling method, in
which the flow and wave modules communicate
with each other at each time step, establishing a
two-way wave-current interaction. Wave condi-
tions are updated each (hydrodynamic) hour. The
lateral boundaries of the basin are defined as
Neumann boundaries, thus imposing an along-
shore water level gradient as an open boundary
condition to allow water levels and currents to
develop freely (Roelvink & Walstra, 2004). The
ocean side of the basin (alongshore direction) is an
open water level boundary, which defines the
water level (Fig. 2).
For the simulations including fluvial input (low
and high fluvial input cases are
Q
low
and
Q
high
respectively) the river discharge and sediment
load are defined as an upstream boundary condi-
tion (Fig. 2). The river discharge is taken constant
(500 m
3
s
−1
and 2000 m
3
s
−1
of respectively the
Q
low
and
Q
high
cases) during a simulation. The sediment
load is defined as an equilibrium concentration
depending on the hydrodynamic conditions in
the cells immediately downstream of the inflow
boundary.
The sediment dynamics are calculated for two
sediment fractions; a fine sediment fraction with
weakly cohesive characteristics, which represents
silt, and a coarser sediment fraction that represents
non-cohesive fine sand. The sediment fractions
are defined by specific and dry bed density and
the median grain diameter (d
50
). The fine silt-
like sediment fraction has a specific density of
MODEL DESCRIPTION AND SET-UP
In this study, a high-resolution depth-averaged
(2DH) hydrodynamic model is combined with a
three-dimensional stratigraphic module (Fig. 1),
which are both part of the Delft3D numerical
modelling suite. Delft3D is a physics-based numeri-
cal model for applications in coastal, river and
estuarine areas (Lesser
et al
., 2004). For the hydro-
dynamics of this study, non-steady flow and trans-
port phenomena (flow-module) and wind-generated
short-crested waves (wave-module) are simulated.
The stratigraphy is simulated on the same computa-
tional grid as the hydrodynamics as the product of
deposition and erosion over time. Flow, sediment
transport and bottom updating are all executed at
each time step (Roelvink, 2006).
Hydrodynamic processes are simulated by
solving the unsteady depth-averaged shallow-
water equations. Fluvial discharge and wave cli-
mate, imposed at the upstream and downstream
Bathymetry
Boundary
conditions
Waves
Flow
Sediment transport
Morphological
acceleration
Bed change
Fig. 1.
Schematic overview of Delft3D -online (adjusted
after Roelvink, 2006).
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