in the under layer from erosion due to waves. Besides empirical formulae derived from physical model testing, the interaction between breakwater elements (submerged or emerged) and waves as well as the filtration of the fluid into the porous breakwater has been investigated amongst others by MEDUS, based on RANS equations coupled with a RNG turbulence model. Xblocs are typically applied on an armour slope steepness between 3V:4H and 2V:3H. Unlike natural rock, the hydraulic stability does not increase at shallower slope inclinations, because, in that situation, the interlocking effect is reduced. Standard Xbloc sizes vary between 0.75m3 (significant wave height up to Hs = 3.35m) and 20m3 (Hs = 10.0m). It is noted that the given relation between design wave height and volume size is valid for the concept stage only. Further parameters as foreshore slope, crest configuration, construction equipment, etc. can have an important effect on the recommended unit size. For detailed design, in particular for non standard situations, physical model tests are essential and normally carried out to confirm overall stability and functional performance of a breakwater (wave overtopping and/ or wave penetration). The effect of interlocking is apparent when comparing a rock revetment with a modern single layer unit for average boundary conditions, while taking into account the lower specific density of concrete compared to most natural rock commonly used in breakwater construction. Assuming that natural rock would be placed at identical slope steepness, the individual rock weight would require to be three times as high, compared to Xbloc units. Rock is generally to be placed as double layer, thus the volume of armour material which needs to be quarried, stored, handled, transported and installed can be enormous for a larger breakwater exposed to significant wave action. Due to the interlocking effect the weight, and thus the volume, of single layer armour units is considerably less compared to an armour consisting entirely of rock. In addition, units are normally fabricated near or at project site, so that transport issues are less critical. Production of armour units The Xbloc consists of non-reinforced concrete, similar to other single layer armour units. Ordinary concrete C25/30 is normally appropriate for the production of Xbloc armour units. However, often concrete of higher strength is applied for other reasons, e.g. early strength for faster de-moulding, ice loads, etc. By omitting reinforcement, time and costs are cut and the armour units are less vulnerable