It is expected that large farms of Wave Energy Converters (WECs) will be installed and as part of the consenting process it will be necessary to quantify their impact on the local environment. The objective of this study is to improve the state-of-the-art of the methodologies to assess the impact a WEC farm has on the incoming wave field through the use of a coupling methodology. A Boundary Element Method (BEM) solver is used to obtain the near-field wave solution accounting for the wavebody interactions within the array of WECs and a Mild-Slope Equation (MSE) model is used to assess 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 MSE model and then propagated throughout the domain. The internal boundary condition is described by imposing the solution of the surface elevation in the area where the farm is located. The methodology is applied to flap type WECs that are deployed in shallow water conditions. The validation of the technique is done first for a single flap and then for an array of 5 flaps. Finally, a mild-slope bathymetry and the influence of the changing depth on the wave transformation is assessed in order to prove the versatility of the method to be applied to real scenarios.