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Input grids

The most important input grid is the bathymetric grid. This grid represents the bottom level at each grid point with land points defined as negative while wet points are defined as positive. The resolution of bathymetric grid is not necessarily the same as that of the computational grid. It is advised to avoid extremely steep bottom slopes or sharp obstacles as much as possible. Some kind of smoothing or re-interpolation is therefore recommended.

Jetties, piers and quays, or other impermeable walls, may be schematized either

From a stability point of view, the second option is the best one. However, the first option may be a better choice when, for instance, wave diffraction around the berm of the quay need to be simulated accurately. To avoid unrealistically high surface elevation around the quays or possible instabilities due to steep slopes, this first option may be combine with a larger threshold of the water depth; see Section 5.4.6 and command SET DEPMIN. The default value of this threshold is 0.05 mm. Depending on the horizontal grid sizes ($ \Delta$x, $ \Delta$y), and thereby the actual slope, this threshold may be increase to 0.1 mm or even 1 mm in order to get a stable solution. The higher the grid resolution, the higher this threshold should be set. But be careful as this higher threshold may negatively influence mass conservation.

Instead, however, a combination is also possible, i.e. to place the porosity layer on top of the quay walls, and having a volumetric porosity of 20% and a grain size of 0.1 m.

A rubble mound breakwater must be schematized by means of porosity layers. These layers must be placed inside the computational domain. Rubble mound breakwaters have a typical porosity value of (n=) 0.4, while the stone size of the armour layer is typically 0.5 m. The berm of the breakwater can be specified by means of the structure heights (relative to the bottom). In case of two or more breakwaters in the domain, both porosity and structure height are thus spatially varied, and so they need to be inputted by means of input grids. Also stone diameters need to be specified as well.

Alternatively, the porosity layers may be placed on top of the impermeable core of the breakwater which, in turn, is schematized by adapting the bottom level. In addition, the berm of the breakwater is schematized by including its slope in the adapted bathymetry.

The width of the breakwater should be at least four times the grid size of the computational grid. So, the grid resolution should be high enough. When choosing a too coarse grid size, it may lead to an overestimation of the transmission and an underestimation of the reflection.

This way of schematization permits to simulate partial reflection and transmission of the waves through breakwaters. Wave reflection at a breakwater is typically determined by wave energy dissipation on the slope and wave penetration into the breakwater. Both processes are equally important, and thus both slope angle and porosity are important governing parameters for the wave reflection.

Using the command INPGRID BOTTOM EXCEPTION, one can introduce permanently dry points in the computational grid. 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. It must be noted that for parallel runs using MPI the user must indicate an exception value for bottom levels, if appropriate, in order to obtain good load balancing.

The water depth should be uniform along the wavemaker boundary where incident waves are imposed.

If tidal currents are significant over the computational domain, the spatial distribution of the currents u(x, y) should be specified as an input grid.

next up previous index
Next: Initial and boundary conditions Up: Setting up your own Previous: Computational grid   Index
The SWASH team 2017-04-06