In addition to specifying spectral parameters, the user has to select additional
parameters to describe the directional distribution of wave energy. The
different directional distributions are discussed in Appendix C.
Type of Directional Spreading Function
Standard Deviation, σθ (deg) The standard deviation of the Wrapped-
Normal directional spreading function. The allowable range is from
5 deg to 50 deg with 10 deg representing narrow-band swell-type condi-
tions and 30 deg representing broader, local wind-generated seas.
Spreading Index, s Describes the degree of directional spreading for the
Cosine-power directional distribution. The allowable range of the
spreading index is from 0 to 65 with s = 2 representing broad, local
wind-generated seas and s = 15 representing narrow-band swell-type
Principal Wave Direction, θp (deg) This is the direction corresponding
to peak of the directional spreading function. The wave direction is
defined in a clockwise manner from the northern boundary of the grid as
shown in Figure 6. The principal direction has to be within 30 deg of
the normal to the wave generation boundary.
Maximum Propagation Direction, θmax (deg) This is the maximum
wave propagation direction relative to the normal to the wave generation
boundary as shown in Figure 7. The directional distribution is truncated
at θmax and renormalized. As pointed out by Sand and Mynett (1987),
θmax also defines the limited area in the computational domain over
which homogenous conditions exist, i.e., all wave directions are
included. Diffraction effects will further reduce the size of the spatially
homogenous region. The recommended value is two to three times the
standard deviation of the directional distribution.
It should also be noted that when using more than one wave generation boundary,
different spectral/directional parameters might be specified for the individual
boundaries. However, the duration of the synthesized or input time record must
Step 7. Current Field
If tidal currents are significant over the computational area, the spatial
distribution of the currents U(x,y) should be stored as an ASCII file in the grid
file format (.grd) described in Appendix D. The user should then enter the name
of the file containing the velocity data.
Chapter 4 Setting Up and Running BOUSS-2D