MIKE 21 BW, User Manual, Page: 146 / 196

Reference Manual
146 MIKE 21 BW - © DHI
Step 4
If other parts of your model area include porous structures the generated
porosity map may look like the map shown in Figure 5.33. As the wave and
depth conditions typically vary inside a harbour most often you have to use
different values of porosity along the porous structures. You simply divide
your entire model area into a number of sub-areas characterised by having
reasonable uniform wave and depth conditions (typically 10-20 sub-areas).
For each of these sub-areas a porosity value is determined using the MIKE
21 Toolbox programme Calculation of Reflection Coefficient. For each of the
defined sub-areas the original porosity (0.5 in this example) is replaced by the
new and more representative value using the Grid Editor.
Figure 5.33 Example of a porosity map covering other areas where partial reflection
will take place
Step 5
Finally, you ensure that the overall porosity map looks “OK”. For harbour
entrance areas as illustrated in Figure 5.33 (by the circle) it is recommended
to reduce or remove the porosity layer as illustrated in Figure 5.34 (using the
Grid Editor). Otherwise the wave penetration may be underestimated. This is
particularly the case for small entrance widths.