Permeable coastal structures typically contain granular filters in one or more layers. These filters are normally geometrically tight to prevent material washout. A cheaper alternative is a geometrically open filter where only minimal base material loss or settlement occurs. In Van Gent and Wolters (2015) physical model tests were performed to estimate the amount of erosion and accretion of sand underneath one or two layers of rock. Here, a numerical model has been applied to compute hydraulic gradients at the internal sand-rock interface for conditions in which the structure is exposed to wave action. The results show that the model can accurately compute hydraulic gradients at the internal rock-sand interface and that the position of erosion and accretion of sand can be predicted. The performed parameter analysis shows which parameters affect the hydraulic gradients and which parameters do not. Moreover, processes that contribute to erosion and accretion have been analysed.