Large concrete block displaced by a rogue wave : analysis of an event incidentally captured by a photographer

Large boulders lying at abnormally high levels are common proxies to determine the occurrence of extreme past geophysical events such as storm or tsunami. Empirical formulas are derived to reconstruct the event conditions associated to the boulder displacement (e.g., Nott, 2003). Up to now, to the best of our knowledge, no one directly identified the individual wave responsible for the movement of a large boulder. In this paper, we report such an observation and analyze the associated hydrodynamic conditions.

On the 2/28/2017, a large concrete block of about 50T, protecting the Artha breakwater in Saint Jean de Luz (south west of France) was displaced from the block armor unit to the crest of the structure during a large storm event. The vertical displacement of the block is assessed to a minimum of 5.3m. This type of event is quite rare (last known date is 1951), nevertheless, this time, the scene was by chance captured by a photographer taking pictures of the storm at the same moment. This allowed assessing the approximate time at which the event occurred and analyze the corresponding data available locally at this time namely : waves (integrated parameters and free surface signal 1 km off-shore), water level and the associated photographs.

First, the water level is found to be close to its maximum (high tide with high tidal coefficients). During the storm, the significant wave height evolved between 5.61 to 7.47m, and the maximum wave height from 8.53 to 11.11m. Surprisingly, at the time of the block displacement, these parameters were not maximum (i.e., 6.8m and 8.53m, respectively). Nevertheless, the investigation of the free surface signal shows the existence of a large wave of more than 14m high and about 25s long approximately 1min30 before the block displacement, not detected by the zero-crossing wave buoy algorithm. This individual wave satisfies the definition criterion of a rogue wave (i.e., H_wave/H_s>2.2).

With 1D phase resolving wave simulations, we first validated the wave propagation time from buoy to breakwater (i.e., 1min30) and assessed the reflection component at the buoy. The latter was considered as sufficiently moderate at the time of observation to consider the 14m recorded wave as an incident one. Next, we used a combination of two models nested : SWASH (1D) and the Navier-Stokes OLA-FLOW (2DV), to simulate the event considering the free surface signal as input. The hydrodynamical conditions obtained with the computations were finally compared to the existing empirical formulas and discussed.

The simulations show that the long rogue wave keeps its coherence up to the breakwater and generates a strong and abnormally long duration flow above and within the blocks (i.e. the porous medium). This result strengthens the idea that the force duration (along with force magnitude), is an important factor to consider to predict large boulder or concrete block displacement. This study also shows a possible original effect of rogue waves in the nearshore area.