## 2 Materials and Methods
### Laboratory Model  
The 2D experimental tests were performed in a 25.00 m long, 1.50 m wide and 1.20 m deep wave flume (Figure 3), where a down-scaled model of OBREC, with a scale factor of 1:30 (Froude scale), as compared to the prototype dimensions, was installed. The wave flume was horizontal for the first 6.50 m, starting from the piston wave paddle. After a step of 0.35 m, a sloping ramp, with a length of 9.00 m, conveyed the wave flow toward the OBREC model. The latter was placed on a horizontal invert. At the end of the flume, a dissipative gravel beach with a slope included between 1:4 and 1:5 was positioned in order to absorb the wave energy transmitted at the rear side of the model and to minimize the reflection in the channel. The flume was separated in two subsections by an axial guiding wooden wall, with a total length of 5.00 m. The herein considered model had a flat ramp and it was placed in one of these subsections, whereas a similar model with a curved ramp was located in the adjacent flume subsection. Lateral and frontal views of the two OBREC models are displayed in Figure 4.  
The model was made up of a rubble-mound foundation and a steel superstructure with a thickness of 5 mm, by which the flat ramp, the reservoir and the vertical wall were built. The model was installed in the flume in different steps: First, the steel component was fixed to the channel, with the base placed at 0.30 m above the wave flume bottom. The rubble-mound foundation, with the different layers made out of rock material, was then installed under the fixed structure. Figure 4: Lateral (**a**) and frontal (**b**) view of the two configurations tested in the wave flume. Figure 3: Layout of the 2D wave flume at the Aalborg University (wave flow direction from right to left). The rubble-mound materials were chosen to ensure the stone stability under the wave action and to reproduce the main hydraulic behavior of the structure. The porous media below the OBREC base was composed of three layers with nominal diameter \(\mathrm{D_{n,50}}\). In detail, the rubble-mound foundation consists of a core with \(\mathrm{D_{n,50}}=5\) mm, filter layers with \(\mathrm{D_{n,50}}=20\) mm and the seaward and leeward external armor layers with \(\mathrm{D_{n,50}}=50\) mm. Figure 5 shows a cross-section of the OBREC model with the frontal ramp in the flat configuration [26]. The latter had a planar face slope of \(\alpha=34^{\circ}\) with reference to the horizontal. In the sketch of Figure 5, the following notation is assumed: \(\mathrm{B_{r}}\) is the emerged reservoir width; \(\mathrm{B_{s}}\) is emerged sloping plate width; \(\mathrm{B_{b}}\) is the lower side reservoir width; \(\mathrm{B_{base}}\) is the base width; \(\mathrm{\Delta B_{rs}}\) is the horizontal distance between the ramp crest and the crown wall; \(\mathrm{d_{w}}\) is the ramp height; \(\mathrm{d_{d}}\) is the submerged shaft; \(\mathrm{h}\) is the water depth at the toe of the structure; \(\mathrm{h_{res}}\) is the vertical distance between the base reservoir and the channel bottom; \(\mathrm{R_{r}}\) is the crest freeboard of the ramp; \(\mathrm{R_{c}}\) is crest freeboard of the crown wall; \(\mathrm{\Delta R_{c}}\) is the vertical distance between the top of the crown wall and the ramp crest; and \(\mathrm{h_{w}}\) is the crown wall height with reference to the reservoir bottom. It is worth noting that the nose (highlighted by a circle) on the crown wall is also represented in Figure 5.  
Table 1 summarizes the range of the main model geometrical parameters. An accumulation box with a water level gauge was installed at the rear side of the structure, with the twofold aim, to measure the wave overtopping volumes and to control the evacuation pumps. As depicted in Figure 6, the overtopping discharge flowed along a ramp, with a width of 0.202 m, on the top of the crown wall. Then, the flow entered the accumulation box, which was connected to a tailwater reservoir by a PVC pipe placed at the same level of the OBREC reservoir bottom.