Geomorphological risk evaluation in the heterogeneous formations (flysch) along the Liguria rocky coast

High rocky sea cliffs are landforms affected by abrupt and quick geomorphological changing. Their stability depends on many factors: the characteristics and strength of the rock mass, the presence and conditions of discontinuities, the geometric relationships between joints and slope, weathering, the sea wave action and human intervention. Their analysis is essential to evaluate the susceptibility to coastal instability. In this study we analyze the factors that contribute to the stability of rocky coasts and we propose a method to develop coastal hazard and risk maps. The existing geomechanical classifications are not complete to represent the dynamics of a sea cliff, as they do not take into account the action of the sea waves. Therefore a new classification, defined as Sea Cliff Mass Rating (SCMR), based on the index of Slope Mass Rating (SMR) by Romana, has been studied, introducing a new parameter that takes into account the sea wave action. Beside the quality of the rock and the condition of discontinuities, the new classification consider sea wave direction, wave length (L), wave height (H) and sea bottom depth, in order to evaluate breaking wave or broken wave and wave energy. Five classes of susceptibility of coastal instability have been identified for classifying the sea cliffs according to their quality and state of stability. This new rating has been tested along the coastal stretches between Genova and Chiavari (Eastern Liguria) and between Sanremo e Capo Mele (Western Liguria), both characterized by a marly limestones rock masses (Sanremo and Antola Flysches). These results were used to prepare a coastal geomorphological risk map, just considering the elements at risk present in a buffer zone of 30 m, starting from the edge of the sea cliff scarp. Crossing in a matrix the classes of susceptibility to coastal instability with those of the elements at risk, we have identified five classes of coastal risk (very high, high, medium, low and very low), which have showed to be consistent with what was observed on the field.