CISA, Cliff Instability Susceptibility Assessment: valutazione qualitativa della propensione al dissesto delle falesie. Il caso del centro storico di Polignano a Mare (Bari, Italy)

The downtown of Polignano a Mare lies on a small promontory characterized by a carbonate cliff ranging between 13 and 22 m height constituted by Calcare di Bari at the base and by Calcarenite di Gravina at the top. The cliff is vertical or locally overhanging and continues 2-5 m below sea level. The coastal stretch studied is characterised by five wide caves and a series of smaller cavities, different in shape and height along the cliff face, even below sea level. At place, the cliff is prone to instability phenomena due to a complex geostructural setting and sea wave action. In this study, we present a new multidirectional method for assessing cliff instability susceptibility at a given scale (CISA, Cliff Instability Susceptibility Assessment). The method is based on the “expert evaluation” approaches and is completed by morphological analysis, morphoevolutive models, geomechanical surveys and bythe estimate of the critical heights for vertical cliffs using the lower bound theorem of limit analysis. The coastal stretch was divided in 8 sectors based on morphological affinity and then 23 incidence parameters were considered and determined. These parameters were divided in four categories: geomechanical (12), morphological (5), sea-wave (5) and anthropogenic (1). For each parameter 5 classes of rating were proposed; the cliff classification, in terms of instability susceptibility, was obtain from the total rating which represents the summation of the single rating of the individual parameter.