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Porous breakwater structures are widely used as protection against waves for ports and harbors as well as for general coastal protection. Often the breakwater structure is designed as a porous structure that allows water to flow through the structure while the wave energy is removed. These structures prevent coastal erosion and ensure safe and functioning harbors. As such, it is of high importance that these structures remain stable under extreme wave action. On the other hand, regarding the high complexity of wave interaction with porous structures, it is required to provide a rich understanding and conduct fundamental studies on the mechanism of rubble mound breakwaters subjected to incident waves. The existing literature review on wave interaction with rubble mound breakwaters reveals two mechanisms inside the porous media of such structures and outside the structure on the armor layer. Both could make destructive forces affecting the structure’s stability. In the present study, the stability of the multi-layer berm breakwater has been studied by considering the effect of run-up/run-down flow on the armor layer of a breakwater and, consequently, the flow inside the porous media as it gets filled and empty during wave run-up and run-down. Both numerical and experimental methods are used to investigate the flow inside and outside the multi-layer berm breakwater. Validation of numerical results is conducted through the use of experimental results, including the water level fluctuation outside the structure and also the water pressure variations inside the breakwater. The Flow-3D software has been used to simulate and solve the governing equations on the flow. FLOW-3D numerically integrates RANS (Reynolds Average Navier-Stokes) equations using the Volume of- Fluid (VOF) method to track the free surface. It has been thoroughly tested for coastal hydrodynamics problems. Various turbulence models are available, and the results presented here are based on the RNG turbulence model. The time series of pressure variation in the porous media indicates that as the flow infiltrates, the created pressure damping and also its variation at a constant elevation have been significantly decreased due to energy dissipation during the infiltration. Moreover, at the maximum run-down and exfiltration process, the created flow from inside to outside would be approximately perpendicular to the structure’s slope at about the maximum run-down elevation. Results reveal that the pressure gradient due to changes in the flow field of porous media would lead to a pressure force toward the outside during the run-down and a pressure force toward the inside during the run-up. It has been found that the positive pressure gradient force toward outside during the run-down and also the perpendicular force of flow during the exfiltration occurred simultaneously and acted as an active force to destabilize the stones. More to the destabilizing mechanism of stones, the effect of flows occurred during the run-up, and run-down on the armor layer have been investigated using the recorded images during the experimental tests.

Keywords: Multi-layer berm breakwater, Numerical simulation, Experimental modeling, Run-down, Porous media.

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