This study describes a theoretical approach of a physical description of the notional permeability factor in the stability formulae of Van der Meer [1988]. Caused by the empirical character of these stability formulae a physical description is not available for the notional permeability factor. In practice this leads to ambiguities in determining the value of this factor. To give this factor a physical description a volume-exchange-model was introduced to express the effect of core permeability on the external wave run-up process.