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Wave impact has been identified as the main source of failure for coastal breakwater. While wave transformation over low to mild sloping beaches have been widely studied, shoaling, breaking, reflection and dissipation over steep and very rough slopes are much less documented, especially in situ. The Artha breakwater, south-west of France, is a composite breakwater sitting on top of a complex, steep bathymetry. The results of in-situ measurement of wave transformation in the block armor of this structure are presented here.Recorded Presentation from the vICCE (YouTube Link): https://youtu.be/T8IINpfcPUM

Impulsive loads are difficult to predict due to the extremely non linear response to incident wave conditions. This behavior is related to air dynamics and local wave shape. The present study focuses on the latter.We propose a new method to predict pressure peaks generated during breaking wave impacts. In this method, the plunging jet is assimilated to two equivalent triangular jets (wedge impact) with variable inclination in time. The basic case of wedge impact is first studied in the paper. Semi empirical laws relating pressure peak and incident angle are derived based on numerical results obtained with a Navier-Stokes model . The more general case of a breaking wave is then investigated. By making an analogy with the wedge impact case and inverting the relation obtained before, we computed the location where the equivalent angles have to be taken on the free surface. We show that these points correspond to the minimum curvature section on the free surface. In another simulation of a breaking wave, we finally show how the relation can be applied to give a first approximation of the pressure peak only based on the free surface local shape.

In this paper, we study the wave impact process with a multi-fluid Navier-Stokes model (THETIS). Preliminary simulations have been conducted, first on a plunging wave generated by unstable Stokes initial condition, and second, involving a dam breaking bore impact. In both cases, a convergence study shows pressure peak results instability when using different meshes. This is due to the incapacity of the model to ensure, after a certain time of computation, the exact same surface profile at impact when simulating a specific case with different meshes. This instable numerical behavior is somehow similar to peak pressure instabilities observed in experiments. This similarity shows the critical role played by local free surface shape at impact on impulsive loads. When initializing the model with a specific interface right at impact, convergence is observed and the pressure peaks are correctly assessed by the code for moderate intensity impact. However, further improvements are still needed especially regarding the interface tracking technique to simulate the most violent impacts involving the weaker dead rise angles. The paper also encourages us to use numerical simulations preferably to study impact flow at local scale.