Q: How can I change wave spreading in MOSES?
REV 6.01

A: The short answer is you should not do this. In general, the default provides a conservative answer and there are problems with spreading in the time domain. This is further discussed in the frequency domain frequently asked questions section.

If you must change it, use the -SPREAD option on the &ENV command. Here one specifies an exponent, EXP which is used in the wave spreading function, F:

           F(THET) = COS(H - THET)**EXP for -pi/2 < THET < pi/2

           F(THET) = 0                   Otherwise

Here H is the mean wave heading. Now, for a heading of 65 degrees away from the mean heading the spreading would be as follows:

     COS(65)**2   =   0.1786   for EXP = 2

     COS(65)**4   =   0.0319    for EXP = 4

     COS(65)**200 =   1.55e-75 for EXP = 200

The default in MOSES is to assume unidirectional waves, using the EXP value of 200. I believe this is commonly referred to as "long crested waves". Any values for EXP lower than 200 will result in less conservative results, so this practice should be considered carefully. As I understand it, "short crested waves" refers to waves where wave spreading is considered, meaning some value less than 200 for EXP.