Surface roughness measurement for the assessment of damage dynamics of existing and upgraded cube-armored breakwaters

Behavior of existing or upgraded rubble-mound breakwaters with non-conventional geometry, layering and surface porosity may be quite different from what predicted by state-of-art design approaches, and traditional techniques for damage estimation may be not sufficient. In the present work, a novel SfM-based technique for the analysis of damage mechanisms of cube-armored breakwaters is proposed, which is based on the evaluation of descriptors of the macro and micro-roughness of the armor layer. The first one represents the waviness of the armor slope, whereas the latter one is a proxy of the armor layer surface porosity. A systematic experimental investigation was performed, considering different upgrading options for a harbor rubble-mound breakwater with cube-shaped armor units, which involve, besides the rise of the wave wall, the addition of extra armor blocks similar or smaller than the existing ones. Damage measurements acquired through traditional laboratory techniques were combined with the armor surface roughness descriptors. The results of the novel analysis of the cube armor layer macro-roughness revealed that the processes which cause the displacement of the outer blocks are influenced by the level of waviness of the armor slope. Moreover, the quantitative analysis of the micro-roughness enabled the automated identification of four damage mechanisms experienced by the outer armor layer: (i) static equilibrium; (ii) incipient instability; (iii) bulk displacement; (iv) individual displacements. The use of the proposed roughness parameters makes simple and immediate the quantitative comparison between the performances of different upgrading solutions.