Recent research has shown that also Tetrapods can be applied in single layer application under certain conditions (D'ANGREMOND, ET AL [1999], VANDENBOSCH, ET AL [2002]). However, the placing density of the Tetrapods has to be larger than in a normal single layer application. The recently developed Xbloc? is applied in a single layer. Both in case of the single layer Tetrapods, as well as the single layer Xbloc (see also KLABBERS ET AL [2003]) the porosity of the top layer is more than in case of a normal riprap structure. The porosity of the Xbloc is about 50%, and the porosity of the normal riprap structure is about 40%. For Xbloc, the roughness, porosity as well as the geometry of the pores differs from a standard riprap slope. In the model tests of the Xbloc, the visual impression of the slope seemed smoother than in case of a Tetrapods layer. It is expected that the overtopping quantities over single layer Tetrapods or over Xbloc armour may deviate from well known overtopping formulae. Van Der Meer makes a distinction between breaking and non-breaking wave conditions. The model tests showed that the overtopping volume increases with the increase of wave height, decrease of freeboard. The overtopping also showed an increase with the decrease of the wave steepness. Klabbers ET AL [2003] found that wave overtopping in case of Xbloc increases with both wave height and wave length. The wave steepness could therefore not be considered as governing parameter for wave overtopping. It was however found that, the asymmetry of the incoming waves; quantified in shallow water by the Ursell parameter had a significant effect on the wave overtopping. Therefore, a new type of overtopping formula has been developed which includes the Ursell parameter. The performed overtopping tests in this work confirmed the use of this equation for the Xbloc unit. After comparing the test results with the Van Der Meer formula and the Xbloc formula, it seemed that the Xbloc formula showed a better fit with the test results, as can be seen in the following figures. Thus for single layer Tetrapod units and Xbloc units, the Xbloc overtopping formula is more suitable to use than the Van Der Meer formula.