Influence of armour porosity on the hydraulic stability of cube armour layers

•Armour porosity affects the hydraulic stability of armour layers.•Armour stability formulae rarely include armour porosity or packing density.•Packing density of randomly-placed cube armours is a relevant design factor.•A 1.2-power relationship was found for stability number and packing density.•Packing density must be explicitly reported for small-scale tests and prototypes.

Armour placement and packing density directly affect construction costs and hydraulic performance of mound breakwaters. In this paper, the literature concerning the influence of armour porosity on the hydraulic stability of single- and double-layer armours is discussed. Qualitative and quantitative estimations for the influence of armour porosity and packing density on the hydraulic stability are given for the most common concrete armour units. The analysis focuses on specific 2D hydraulic stability tests of double-layer randomly-placed cube armours with different armour porosities and permeable core. The stability number showed a 1.2-power relationship with the packing density for double-layer randomly-placed cube armours considering armour unit extraction and Heterogeneous Packing. The literature review and experimental results with small-scale breakwater models protected with a variety of armour units clearly indicate that a significant increase in armour porosity above the recommended values substantially decreases armour hydraulic stability. To avoid uncontrolled model effects, packing density should be routinely measured in small-scale tests, and armour placement techniques should be monitored at prototype scale. The actual packing density obtained in small-scale models and prototypes has to be explicitly reported, because packing density significantly affects hydraulic stability during service time.