Coastal Hazard Assessment through 3D Mesh Analysis of Coastal Boulder Deposits from Extreme Wave Events in Greece

Coastal boulders have been identified as significant markers for the assessment of extreme wave conditions and forces in past events, thus providing crucial insights into the dynamics of coastal hazards. This study examined a coastal boulder field located near Pounta in the Gulf of Laconia (Southern Peloponnese, Greece). The boulders were studied in terms of their lithology, geometrical shape, distribution by UAV imagery, and axis orientation along the coast. 3D LiDAR measurements in combination with rock density analysis were carried out to calculate their masses. The investigation was achieved through open-access methodologies, encompassing readily accessible LiDAR recording technology, with the objective of substantiating the replicability of such studies. An essential aspect of the study involved the detailed specification of previously published data for the region, providing correct locations and characteristics of the boulders, besides employing high-resolution field data and refined analytical techniques for the estimation of wave heights and velocities.

We mapped and analyzed >250 boulders along a ~600 m-long coastal stretch with the furthest inland boulders being located at ~200 m at 6-7 m asl. Boulders were imbricated and show overturning marks (e.g., rock pools) indicating transport during an extreme wave event. Some boulders are deposited on the bedrock, while further inland located boulders are partly embedded in sand-grade sediments, indicating joint transport and deposition of boulders and sediments. Preliminary results from UAV mapping show grouping of boulders based on differences in size and potentially by different events (e.g., AD 1303 Crete earthquake). The largest imbricated boulders (~3.3 t) have been moved a few meters inland, while smaller boulders (up to several hundred kilograms) were transported further inland.

With reference to the existing literature, we found that the volume analyses carried out overestimated the masses of the coastal boulders, thus calculated wave energies to transport or even move boulders were erroneous. We updated existing mathematical approaches to calculate the properties of the waves transporting the analyzed boulders by adding the properties of the transport medium itself that sets the boulder in motion. An approach to take this into account is implemented here for the calculation, accordingly affecting the studied wave energies of historic high-energy events (i.e., storm surges or tsunami).

The study area is characterized by complex coastal geomorphology by staircase-like raised marine terraces, which is susceptible to effects brought by sea-level rise and is particularly sensitive to hazards triggered by extreme wave events. The absence of protective infrastructure in this area necessitates the crucial role of studies that investigate the potential hazard of extreme wave events in coastal risk assessment.