Factors Triggering Sea Cliff Instability Along the Slovenian Coasts

Abstract

FURLANI, S., DEVOTO, S., BIOLCHI, S and CUCCHI, F., 2011. Factors Triggering Sea Cliff Instability Along the Slovenian Coasts. In: Micallef, A. (ed.), MCRR3-2010 Conference Proceedings, Journal of Coastal Research, Special Issue, No. 61, pp. 387–393. Grosseto, Tuscany, Italy, ISSN 0749-0208.The cliff retreat is the sum of the sustained action of marine and continental factors. Their interrelations depend on the geological and environmental settings of the area. The studied cliffs are cut in the Eocene Flysch, a turbiditic succession composed by centimetric—metric sandstones with millimetric-centimetric interbedded silty marlstones, almost horizontal in the study area. The low resistance of the rock mass allows the rapid retreat of the cliffs and the development of wide shore platforms.This work aims at evaluating sea cliff retreat and the factors that trigger the collapse of material and its removal from the cliff foot. Different methods have been used: a detailed characterization of the geomechanical properties and the quality of rock masses, the susceptibility to rock falls and a photographic surveying.The sea cliff retreat in five sites along the Slovenian coast, in the Northeastern Adriatic Sea, has been studied through the comparison of more than 7000 pictures collected since 1998, at precise time steps and under any weather conditions. These data have been subsequently compared with archaeological data, historical maps and the characterization of the geomechanical features of rock masses in order to determine the factors triggering the cliff retreat. Photographic surveying displays a rapid but complex behaviour of the cliff retreat: during long stable-weather periods, cliff modifications are very low and groundwater solution or landslides have been observed. Major changes in the cliff face have been observed after great storm events, because of a complex interaction between marine and non-marine factors.The integrated method proposed here, including the photographic surveying and the geomorphological-geomechanical characterisation of rock masses is a valuable method for understanding and evaluating the mechanisms of cliff retreat.