MIKE 21 BW - Recipe for Generation of Porosity Maps

Reference Manual
144 MIKE 21 BW - © DHI
Porosity values are used to model either partial reflection and/or transmission
through structures. If porosity values are backed up by land, partial reflection
will take place. Conversely, (partial) transmission will also take place if the
porosity values are not backed up by land points.
The porosity values at open water grid points should be set to unity (i.e.
porosity = 1.0).
The porosity should only be set to less than unity along structures where you
want to include the dissipation effect of porous flow.
Recipe for generation of porosity maps
An effective procedure for establishment of porosity maps in connection with
application of MIKE 21 BW 2DH in wave disturbance analysis is outlined
below.
Step 1
Open your bathymetry file (dfs2-file) and change the land value (e.g. 5) along
all structures having partial reflective properties to another value/code (e.g.
3). This is illustrated in the figure below (for a sub-area of the bathymetry) in a
case with a pile-type quay structure, where partial reflection will occur.
Figure 5.30 The bathymetry land value next to a water point is changed to a
value/code representing stretches with partial reflection
Step 2
The porosity value required to obtain the desired reflection coefficient (in this
case approximately 0.4) is determined using the MIKE 21 Toolbox pro-
gramme Calculation of Reflection Coefficient as shown in Figure 5.31. In this
example we assume the wave and depth conditions along the quay can be
considered as almost uniform (H
m0
~ 0.2 m, T
p
~ 8 s and h ~ 8 m). An
absorber width of 15 m (3 grid points wide porosity layer) results in a porosity
of n ~ 0.5.