In JUMELET [2010] a method with a physical basis (so called “Volume Exchange model”) to determine the ‘notional’ permeability coefficient P was developed. The ‘notional’ permeability coefficient was previously introduced in the stability formula of the armour layer; see VAN DER MEER [1988]. In this latter study this coefficient was empirically based for three different structures. Due to the limited validity it is difficult to apply a coefficient for different breakwater configurations. The Volume Exchange model determines the influence of the core permeability by computing the difference between the surface wave run-up on an impermeable core and a permeable core. The volume of water that flows into the core causes a reduction of the wave run-up. Reduction of the wave run-up is not only caused by infiltration, but also by the slope surface roughness and energy dissipation inside the pores of the armour layer. To investigate the influence of the above three mentioned factors physical model tests have been conducted. The tests were carried out in the wave flume in the water laboratory at Delft University of Technology. On four different configurations (smooth impermeable slopes, rough impermeable slope, armour layer on an impermeable core and permeable core) tests were conducted. In the analysis of the results the influence of the surface roughness, energy dissipation in the pores of the armour layer and the reduction of the surface wave run-up due to the inflow into the core could be determined. Besides, the surface wave run-up also the wave run-up on the core is measured. The results showed that the slope surface roughness has no influence on the wave run-up, when the waves are of the surging breaker type. Also, the surface wave run-up is not reduced by a permeable core. Wave run-up measurements showed the same wave run-up height for armour layers on an impermeable and a permeable core. Wave run-up on the core showed a considerable difference between run-up on an impermeable core and a permeable core. Therefore, in the Volume Exchange model the wave run-up on the core should be considered. The adjusted Volume Exchange model is used to determine a formula for the permeability coefficient. This has led to the conclusion that the permeability coefficient is dependent on the Iribarren number and the structural configurations and /or properties.