Homogeneous Low-Crested Structures (HLCS) with pre-cast concrete units are a new coastal structure to protect beaches from wave attack. HLCS are a natural based solution which can be used to restore degraded coral reefs areas where not only the coastal protection is important but also the regeneration of the ecosystem. The placement grid of HLCS plays a relevant role on their hydraulic performance affecting the wave transmission and hydraulic stability. Within the Project HOLOBREAK, 5-layer Cubipod HLCS trunks with H/V≈2:1 were tested in the wave flume of the Laboratory of Ports and Coasts (30 x 1.2 x 1.2 m) at the Universitat Politècnica de València. The 5-layer Cubipod HLCS was constructed following a triangular equilateral placement grid with porosity p[%]=50 on a 4% bottom slope. The model showed a total height around 20 cm with Cubipods with nominal diameter D_n[cm]=3.79 In total, 100 tests were carried out with four wave steepnesses (s_0p=0.01, 0.02, 0.03 and 0.04) and four crest freeboards (R_c[cm]=-3, 0, 5, 10 cm). The tests measured the layer thickness, armor stability, wave reflection and wave transmission of the structure. The wave height (H_m0) and wave period (T_p) at the toe of the structure were obtained using the SwanOne model. In this study, Neural Network models are used to create a new wave transmission estimator valid for 5-layer Cubipod HLCS with R²=0.91 within the ranges-2<R_c/H_m0<3.6 and 0.008<s_0p<0.035.

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The retreat of coral reefs during decades, and their progressive degradation, is a clear indicator of the poor health of coastal ecosystems, which in turn affects the stability of nearby beaches protected by these reefs. In addition to their importance in the resilience of coastal ecosystems, coral reefs also produce on average 1,000 m³/km² of biogenic sediment per year, which is key for the long-term stability of neighbouring sandy beaches. Sea-sand-sun tourism is critical for the social and economic development of many coastal communities worldwide and often this is directly related to the quality of the natural resources. The focus of the HOLOBREAK Project is the study of Homogeneous Low-Crested Structures (HLCS), a new type of coastal structure composed of large rocks or pre-cast concrete units. These are similar to low-crested structures (LCS) but without a core, mimicking the protection provided by coral reefs to adjacent beaches. HLCS also provide an increased porous substrate that allows colonization of the structure by local species. The placement grid of the HLCS is significant in economic, logistic and functional perspectives; affecting the feasibility of construction, the costs involved and its effectiveness as beach protection. In this study, various placement grids were analysed. The results of the physical 3D tests were calibrated and validated against numerical placement tests, based on a Bullet Physical Engine (BPE). Good agreement was found, with a global mean relative error of 2.75%, indicating that the BPE model is a valuable tool for the feasibility analysis of different placement grids.

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Armor erosion due to wave attack has been studied intensively since it is considered the main failure mode of mound breakwaters. Cube-armored mound breakwaters in depth-limited breaking wave conditions are common in practice but have received limited attention in the literature. In this study, 2D physical tests were performed on non-overtopped double-layer randomly-placed cube-armored mound breakwater models with armor slope cotα = 1.5 and bottom slope m = 2% in breaking wave conditions. Using the experimental results, a new hydraulic stability formula was derived with a coefficient of determination R² = 0.85 based on a power relationship between the armor damage and the stability number and the dimensionless water depth. A lower hydraulic stability was found for the front slope of non-overtopped cube-armored structures in breaking wave conditions when compared to formulas given in the literature.

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Sea level rise due to climate change, as well as social pressure to decrease the visual impact of coastal structures, have led to reduced crest freeboards, and this increases the overtopping hazard. In previous studies, pedestrian safety during overtopping events was assessed considering the overtopping layer thickness (OLT) and the overtopping flow velocity (OFV). This study analyzed the statistics of OLT and OFV on mound breakwaters without crown walls during severe wave storms. Small-scale 2D physical tests were conducted on mound breakwaters with dimensionless crest freeboards between 0.29 and 1.77, testing three armor layers (single-layer Cubipod®, and double-layer cubes and rocks) in depth-limited breaking wave conditions and with two bottom slopes. Neural Networks were used to develop new estimators for the OLT and OFV exceeded by 2% of the incoming waves with a high coefficient of determination (0.866 ≤ R² ≤ 0.876). The best number of significant figures in the empirical coefficients of the new estimators was determined according to their variability. The 1-parameter Exponential and Rayleigh distribution functions were proposed to estimate the extreme values of OLT and OFV with 0.803 ≤ R² ≤ 0.812, respectively.

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Mound breakwaters are usually designed to limit the mean wave overtopping rate (q) or the maximum individual wave overtopping volume (V_max). However, rarely do studies focus on wave overtopping volumes on breakwaters in depth-limited breaking wave conditions. This study analyzes 2D physical tests on mound breakwaters with relevant overtopping rates (0.33 ≤ R_c/H_m0 ≤ 2.83) and three armor layers (Cubipod®-1L, rock-2L and cube-2L) in depth-limited breaking wave conditions (0.20 ≤ H_m0/h_s ≤ 0.90) and with two bottom slopes (m = 1/25 and m = 1/50). The 2-parameter Weibull distribution was used to estimate V_max* = V_max/(gH_m0T₀₁ ²) with coefficient of determination R² = 83.3%. In this study, the bottom slope (m = 1/50 and m = 1/25) did not significantly influence V_max or the number of overtopping events, N_ow. During the design phase of a mound breakwater, q is required to use the methods given in the literature to estimate V_max. Thus, q must be estimated for design purposes when measured q is not available. In this study, CLASH Neural Network (CLASH NN) was used to estimate q with R² = 63.6%. If the 2-parameter Weibull distribution proposed in this study is used to estimate V_max with q estimated using CLASH NN, the prediction error of V_max* is R² = 61.7%. With the method presented in this study, the ratio between estimated and measured V_max* falls within the range 1/2 to 2 (90% error band) when q is estimated with CLASH NN. The new estimators derived in this study provide good predictions of N_ow and V_max with a method simpler than those in the literature on overtopped mound breakwaters in depth-limited breaking wave conditions on gentle sea bottoms (1/50 ≤ m ≤ 1/25).

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In recent years, mound breakwater design has had to consider social concerns about the visual impact of coastal structures as well as the rise in sea levels due to climate change. Additionally, mound breakwater design must consider pedestrian safety when facing overtopping loads and the subsequent hazards. The overtopping hazard on dikes has been studied considering the overtopping velocity and the overtopping layer thickness on the crest. This paper proposes a new method to estimate the overtopping layer thickness on the crest of mound breakwaters based on 123 physical tests of conventional low-crested mound breakwaters with three different armors. The new method is based on formulas given in the literature to estimate wave run-up for mound breakwaters and overtopping layer thickness on the crest of dikes which was exceeded by 2% of the incoming waves, but using the roughness factors 0.33 (Cubipod ®-1L), 0.48 (rocks-2L) and 0.35 (cubes-2L) and the empirical coefficients 0.51 and 0.89. The new method explained 79% of the variance in the observed overtopping layer thickness exceeded by 2% of the incoming waves (rMSE=0.21).@en