The accurate modelling of the wave field distribution around Wave Energy Converters (WECs) has been a relevant subject of research in the recent years. A considerableamount of works has studiedarrays of WECs employing methodologies such as Boundary Element Methods (BEMs) based on potential flow or wave propagation models. This study presents an investigation on the modelling of the perturbed wave field by a flap type WEC in a time-dependent mild-slope equation model. The capability of modelling the diffracted wave intrinsically in the mild-slope equation model and the implementation of a coupling technique with a BEM are shown. In the last section a representation of the flap by applying the sponge layer technique is carried out. The understanding of all these techniques with their advantages and disadvantages is relevant for a further development of the wave field modelling. The improvement of the state of the art would allow to quantify more accurately the wake effect of a WEC farm.