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##

Computational grid and time window

The **computational grid** must be defined by the user. The orientation (direction) can be chosen
arbitrarily.

If the computational grid extends outside the input grid, the reader is referred to Section 2.5.3
to find the assumptions of SWASH on depth, current, water level, wind, bottom friction, porosity, grain size,
structure height and vegetation density in the non-overlapping area.

The spatial resolution of the computational grid should be sufficient to resolve relevant details of the wave
field. Usually a good choice is to take the resolution of the computational grid approximately equal to that
of the bottom or current grid.

SWASH may not use the entire user-provided computational grid if the user defines exception values on
the bottom grid (see command `INPGRID BOTTOM`) or on the curvilinear computational grid (see command
`CGRID`).
A computational grid point is either

__wet__, i.e. the grid point is included in the computation since it represents water or
__dry__, i.e. the grid point is excluded from the simulation since it represents land which may vary
as moving shoreline or
__exceptional__, i.e. the grid point is permanently excluded from the computations since it is so
defined by the user. This provides a means to make a line of dams or screens through the computational domain,
separating the flow on both sides. This line of thin dams may represent a small obstacle with subgrid dimensions
that possibly influence the local flow (e.g. breakwater, jetty, or small harbour).

It must be noted that for parallel runs using MPI the user must indicate an exception value when
reading the bottom levels (by means of command `INPGRID BOTTOM EXCEPTION`), if appropriate, in order to obtain
good load balancing.

The **computational time window** must be defined by the user. The
computational window in time must start at a time that is early enough that the initial state of SWASH has
propagated through the computational area before reliable output of SWASH is expected. Before this time
the output may not be reliable since usually the initial state is not known.

The computational time step must be given by the user. Since, SWASH is
based on explicit schemes, it is limited by a Courant stability criterion (which couples
time and space steps). Moreover, the accuracy of
the results of SWASH are obviously affected by the time step. Generally, the time step in SWASH should be
small enough to resolve the time variations of computed wave field itself. Usually, it is enough to consider
the time variations of the wave boundary conditions.

** Next:** Input grid(s) and time
** Up:** Choice of grids and
** Previous:** Introduction
** Index**
The SWASH team 2017-04-06