Calibration Parameters
147
Figure 5.34 Example of porosity map correction at harbour entrance areas.
Hint
In order to obtain a quick rough estimate of the wave disturbance inside a
harbour it is a good idea to run a simulation using a porosity map with a con-
stant porosity along all porous structures. Such a map can usually be estab-
lished within less than one man-hour. The results of this simulation (wave
height distribution) can be used subsequently for an improved estimate of the
porosity.
Wave reflection
Wave reflection from e.g. structures, or a beach, can be modelled by applying
porosity values (less than 1) at the grid points representing the structures. If
no porosity values are specified, the reflection will be 100% from land points.
As the porosity results in dissipation of energy, the reflection may be con-
trolled by the porosity values and the number of porosity layers.
If the porosity values are not backed up by land values, the porosity layers
will result in both reflection and transmission through the layers representing
e.g. a permeable breakwater.
If no reflection is wanted, sponge layers should be applied - absorbing all
wave energy entering the layers.
What is a typical value of the reflection coefficient?
Typically, the reflection coefficient for a rubble mound structure is 30-50%.
More information about reflection, absorption and transmission characteris-
tics can be found in Thompson et al (1996). Figure 5.35 shows typical reflec-
tion coefficient ranges for short period waves.
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