Ocean Energy Europe has estimated that 100 GW of ocean energy capacity (wave and tidal) could be deployed in Europe by 2050. Along with the European targets it is expected that large farms of Wave Energy Converters (WECs) will be installed in the sea and, as part of the consenting process for their installation, it will be necessary to quantify their impact on the local environment. The objective of this study is to improve the assessment of WEC farms impact on the surrounding wave field (wake effect) through the use of a numerical coupling methodology. The methodology consists of a Boundary Element Method (BEM) solver to obtain the wave perturbation generated by the WEC farm for the near-field accounting for the wave-body interactions within the farm whilst a Wave Propagation Model (WPM) based on the mild-slope equations determines the wave transformation in the far-field. The near-field solution obtained from the BEM solver is described as an internal boundary condition in the WPM and then it is propagated throughout the WPM numerical domain. The internal boundary is described by imposing the solution of the surface elevation and velocity potential at the free-surface at each instant of time along a line surrounding the WEC farm. As a case study the methodology was applied to flap type WECs that are deployed in shallow water conditions. The validation of the technique was done first for a single flap and then for a farm of 5 flaps. Once validated, a realistic scenario was assessed by quantifying the impact of irregular sea states composed of long crested waves on a large WEC farm composed of 18 flaps and located on a real bathymetry. The irregular waves were obtained by superposing the regular wave field solutions for all wave frequencies represented in the considered sea state based on the linear water wave theory. Within the limits of this theory these simulations demonstrate the versatility of the methodology to accurately represent the impact of a WEC farm on the surrounding wave climate. The influence of the peak period and the spacing between flaps on the WEC farm wake effect was assessed as well.