Beyond the boundaries of feasible engineering geological solutions: stability considerations of the spectacular Red Beach cliffs on Santorini Island, Greece

The outstanding Red Beach in Santorini, a famous volcanic island in the Aegean Sea in the territory of Greece, exhibits extended rockfall instabilities along its cliffs, placing its highly frequented touristic zones at high risk. This study aimed to generate an engineering geological interpretation of these instabilities and to evaluate the degree of the rockfall potential in relation to the natural evolution of the beach. A detailed field survey of the engineering geological conditions and thorough scanning of the cliffs using terrestrial scanning (LiDAR) combined with accurate geodetic survey façade plans, enabled the detection of cinematically unstable sections. A semiautomated qualitative method for evaluating the site-specific landslide potential was also performed here using an unmanned aerial vehicle, which executed several flights around the research area 3.5 years after LiDAR scanning. The resulting products, namely a digital surface model, an orthophotograph and point clouds, were compared with the LiDAR data to evaluate the behavior of the volcanic formations at the cliff and the rockfall potential. The Red Beach cliff, mainly composed of volcanic scoria cones, was found to be very challenging case for determining a feasible engineering geological solution. Its numerous requirements with respect to rockfall control and stabilization in conjunction with the necessity of maintaining the landscape’s natural beauty and preserving the adjacent archeological site further complicated the problem. However, stabilization to prevent rock falls will provoke a disruption of the balance between the sea-induced erosion and the supply of material that originates from rock falls of the natural slopes toward the beach that could put the existence of the Red Beach in significant danger. A feasible engineering geological solution for stabilizing the cliffs was investigated by evaluating all the possible protective and prevent measures. However, whether any of these measures are acceptable is doubtful.