Most waves in the oceans and coastal area are two dimensional of nature. Consequently modelling of waves have largely been carried out by means of two-dimensional models and hence, contained in the MIKE 21 package. Today there are five wave modules included in MIKE 21, each with their particular area of application. The models can be divided basically into two groups:

• models based on wave action concept
• models based on the momentum concept

The choice of wave model depends on the purpose of the application whether e.g. if wave conditions or wave transformation phenomena are the point of interest.

Furthermore, the choice depend on a number of other parameters such as whether the waves are regular or irregular, generated offshore of by local wind, or if phenomena like diffraction, bottom friction, nonlinear effects or wave-current interactions are important.