A temporal assessment of boulder mobility on a limestone rocky coast in Qawra, Malta (Central Mediterranean)

The dislodgement and transportation of boulders on rocky coasts by storm waves has been the subject of an increasing number of studies in the last decade. Research has been approached from different perspectives, such as numerical models that hindcast wave heights that produce movement, structural control on boulder production, and innovative monitoring and change detection techniques. However, the complex dynamics between boulders and wave forces that determine boulder mobility remain insufficiently understood.

This ongoing study aims to document instances of boulder displacement and quantify their frequency over a two-year observation period. Monitoring is being conducted at three coastal sites across the Maltese Islands (Central Mediterranean): one in Qawra (northern coast) and two in Marsascala (southern coast). Data acquisition is conducted periodically using unmanned aerial vehicles (UAVs), with subsequent 3D model reconstruction performed using Agisoft Metashape. Each newly generated model is compared to its predecessor to detect changes in boulder positions.

Observed boulder movement is evaluated based on key parameters including size, morphology, initial location, displacement distance, and direction of transport. These transport events are then correlated with wave conditions recorded during the interval between successive UAV surveys.

Preliminary findings from the first six-month monitoring phase at Qawra are being presented here. These results suggest that boulder mobility is influenced by a combination of factors, including boulder morphometry and location, coastal topography, wave energy, and wave direction relative to shoreline orientation.