The bathymetry, current, water level, bottom friction (if spatially variable), wind (if spatially variable),
atmospheric pressure, porosity regions, grain sizes, heights of porous structures (if spatially variable)
and vegetation density (if spatially variable) need to be provided to SWASH on so-called input grids. It is
best to make an input grid so large that it completely covers the computational grid.
When the atmospheric pressure is included, it must be combined with space varying wind. They may be read from a meteorological file. Space varying wind and pressure is of particular importance for the simulation of storm surges.
In the region outside the input grid SWASH assumes that the bottom level, the water level, bottom friction, atmospheric pressure, stone diameter and vegetation density are identical to those at the nearest boundary of the input grid (lateral shift of that boundary). In the regions not covered by this lateral shift (i.e. in the outside quadrants of the corners of the input grid), a constant field equal to the value at the nearest corner point of the input grid is taken. For the current and wind velocity, SWASH takes 0 m/s for points outside the input grid, while for porosity and structure height, SWASH takes 1 and 99999 (i.e. emerged), respectively, for points outside the input grid.
One should choose the spatial resolution for the input grids such that relevant spatial details in the bathymetry, current, bottom friction, wind and pressure are properly resolved. Special care is required in cases with sharp and shallow ridges (sand bars, shoals, breakwaters) in the sea bottom and extremely steep bottom slopes. Very inaccurate bathymetry can result in very inaccurate wave transformation or flooding and drying.In such cases the ridges are vitally important to obtain good SWASH results. This requires not only that these ridges should be well represented on the input grid but also after interpolation on the computational grid. This can be achieved by choosing the grid lines of the input grid along the ridges (even if this may require some slight "shifting" of the ridges) and choosing the computational grid to be identical to the input grid (otherwise the ridge may be "lost" in the interpolation from the bottom input grid to the computational grid). An alternative is to smooth the bottom gradients. But this should be done in a way that the quality and feature of the bathymetric data is more or less the same.
In SWASH, wind, pressure and bottom friction may be time varying. In that case they need to be provided to SWASH in so-called input time windows (they need not be identical with the computational, output or other input windows). It is best to make an input window larger than the computational time window. SWASH assumes zero values at times before the earliest begin time of the input parameters (which may be the begin time of any input parameter such as wind). SWASH assumes constant values (the last values) at times after the end time of each input parameter. The input windows should start early enough so that the initial state of SWASH has propagated through the computational area before reliable output of SWASH is expected.
Finally, one should use a time step that is small enough that time variations in the wind, pressure and bottom friction are well resolved.