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The detachment and mobilization of boulders from rocky shore platforms by waves involves complex geomorphic and hydrodynamic processes. Understanding these processes requires precise information on the rates and patterns of movement of these megaclasts scaled against the wave conditions that generate boulder mobility. Repeat photogrammetry and structure-from-motion (SfM) models commonly used in geomorphic analyses are an interesting option for monitoring boulder dynamics. In this study, we used unmanned aerial vehicle (UAV)-based digital photogrammetry and SfM differential models to identify recent boulder movements over a rocky shore platform in Laghdira, Morocco. Combining these results with data on storm occurrence in the study area allowed us to identify storm waves as the unique driver of the dislodged and mobilized boulders. The identified storm event had a significant wave height of 5.2 m. The UAV models were built from imagery captured in September and December 2019 using a DJI MAVIC PRO PLATINUM, and we used QGIS to produce 2D and 3D model outputs. The exploitation of the 2D model differentials allowed us to appreciate the response of the boulders to the storm waves and to determine platform volumetric changes and, therefore, boulder mobility. The 3D models were valuable in determining the mode of transport of the boulders. Mobility patterns included sliding, overturning with no further mobility, and rotation and saltation, as well as boulder breakup. Storm waves did not have a preferential impact on any particular boulder shape, size category, or position at the outer edge of the platform. These results highlight the utility of combining UAV surveys with identified storm events, which are much more frequent than tsunamis, in determining observed boulder initiation and mobility.

The Atlantic coasts of Morocco are exposed to tsunami risk. Although this risk level is low because of the rarity of tsunamis in the region, a future event would be catastrophic for the Moroccan society and economy because of the numerous issues at stake. In this paper, bathymetric/topographic data and parameters of three known tsunamigenic faults (Marquês do Pombal Fault, Gorringe Bank Fault and Horseshoe Fault) were used to simulate a potential tsunami event in the city of El-Jadida (Morocco) using MIRONE software. The simulation results of the worst-case scenario were then exploited to perform a quantitative risk assessment, using demographic and economic input data. Results show that choosing the Horseshoe Fault as source of the simulated event produces the largest tsunamis; with maximum wave heights ranging from 10 to 27 m. These results are found to be consistent with historical records and computer model simulations from previous studies. Quantitative risk assessment results indicate that the city of El-Jadida is exposed to a high risk of loss of lifes and loss of property with values of the order of 34 lives/year and 14 M$/year, respectively. For this reason, we underline the dire need for the implementation of tsunami risk prevention and mitigation strategies that should prioritize the protection of high-risk areas of the city and its population from a possible future catastrophic event.