At breakwater and seawall projects at Port St Francis and Scarborough breakage of single layer interlocking armour units was observed. It is generally assumed that breakage of single layer armour units has a significant negative effect on the hydraulic stability of a rubble mound breakwater. The significant decrease of interlocking capacity and mass of the broken units would lead to displacement of these units and surrounding units. The broken parts of the damaged units would act like projectiles. The waves would "throw" these broken parts back and forth to the armour layer. More armour units would break due to the impact of these broken parts leading to rapid damage progression of the armour layer and finally to failure of the total construction. This damage behaviour has however never been confirmed. The main objective of this research is to determine the effect of single layer armour unit breakage on the hydraulic armour layer stability and potential damage progression. A 2-dimensional model of a rubble mound breakwater with typical cross section was tested with individual and clustered positioned broken Xbloc armour units around the still water line. The residual stability of the armour layer was determined. The armour unit displacement and damage progression was assessed. It is concluded that breakage of single layer armour units has a significant negative effect on start of damage of the armour layer. Breakage of single layer armour units has no significant effect on failure of the armour layer. This damage behaviour leads to a long and gradual damage progression. This type of damage progression looks more like the damage progression of an armour layer consisting of rip-rap rock. The majority of the broken parts show little to no movement. It is therefore unlikely that rapid damage progression occurs due to broken parts damaging other units.