In this thesis, a general method to predict the notional permeability for various structure lay-outs is developed. A numerical model is used to find a characteristic value for different structure lay-outs: the three structures for which Van der Meer determined the P-value, and a fourth structure of which the P-value has to be predicted. The P-value of the fourth structure is predicted by interpolation of the characteristic value of this structure between the characteristic values found for the structures of Van der Meer. The numerical model used is the RANS-VOF model OpenFoam. The developed prediction method uses the total wave induced water pressure over the armour layer as characteristic value. The prediction method is validated for a structure lay-out for which (Kik, 2011) found a P-value of 0.37 by means of physical model testing. A P-value of 0.38 was found by (Kluwen, 2012) for the same structure lay-out by means of more physical model testing. The prediction method predicts a P-value in line with the P-value found by (Kik, 2011) and (Kluwen, 2012). The predicted P-value is 0.35 with this prediction method. A sensitivity assessment has been performed by varying the wave conditions, the porosity of the porous layers of the structure and the Forchheimer parameters applied in the numerical simulations. Results show that the prediction is not very sensitive to changes in the wave conditions or changes in the Forchheimer parameters. And although not confirmed by tests, it seems that the prediction method is not very sensitive for changes in the applied porosities of the porous layers of the structure with unknown P-value. Therefore, the method seems to be a robust method and can be practically used in engineering projects. With this new prediction method for the notional permeability of a breakwater, it is possible to make a more accurate estimate on the notional permeability for more complex rubble mound structures. A more accurate estimate of the notional permeability will improve the accuracy of the initial design of the armour layer in early stages of a project. A next step to further improve the predictions of the notional permeability and to gain a better understanding of the damage of the armour layer of a rubble mound breakwater, is to research the correlation between the wave induced water pressure over the armour layer, the discharge through the armour layer and the flow velocity on top of the armour stones and to research how the combination of these three parameters has influence on the armour layer stability.