*4.2. Wave Storm Parameter Selection*

As discussed above, the selection of the optimal threshold is an open issue. Consequently, there are several procedures for selecting an appropriate threshold value. Among these, one of the most widely used in practice is the percentile method, which, like the others, incorporates a certain degree of subjectivity but has the advantage of simplicity. Furthermore, some authors attribute a fairly good degree of robustness to this procedure, depending on the specific use (e.g., [30,31]).

In line with previous comments, the approach used to select the threshold, *Ht*, has been conditioned by three main factors. The first is the specific interest of the study in storms capable of causing substantial damage on sandy beaches. The second is the knowledge of the prevailing wind and wave conditions in the archipelago, and the third is the observed general slight overestimation of significant wave height by the model, mainly during stormy conditions. Accordingly, after trying different quantiles (Q95.0, Q99.0, Q99.5 and Q99.9) to select a threshold considering these aspects, it has been observed that using percentiles lower than or equal to Q99.5 led to the identification of a substantially high number of storms for an area with a rather moderate wave climate and subjected to

strong sheltering effects against the most frequent storms, generally coming from the NW directional sector, as will be discussed in Section 4.5. Finally, the quantile Q99.9 has been identified as the most appropriate threshold for the objectives of the study.

Considering the importance of the occurrence of wave storms coinciding with high tide conditions, the minimum allowable temporal distance between storms, *ISDmin*, was set at 48 h, which includes four low and high tide levels and is close to the average duration of atmospheric disturbances in the North Atlantic [11,17]. In the same vein, the minimum duration of the storm has been stated as the duration of a complete tidal cycle, *Dmin* = 12 h.
