I sinkhole di Piano dell'Acqua (San Basile - Calabria settentrionale). Indagine sulle fasi di attivazione su base storica, geomorfologica, ed idrologica

In the territory of San Basile (Northern Calabria), five sinkholes have recently been identified at "Piano dell'Acqua", and analyzed in historical, geological, geomorphological, and hydrological terms. The study area is characterized by Pliocene conglomerate and sand, dislocated by tectonic lines. The sinkholes are limited in size, with maximum diameter of 10 m, and maximum depth of 2.5 m; two of them present elongated shapes, whilst the remaining three are circular. Initially, sinkhole phenomena were investigated in the whole territory by means of multitemporal aerial photos, and the outcomes from this analysis were checked in the field. At the same time, historical analyses were performed to collect and critically evaluate the existing information and testimonies about the age of occurrence of the surveyed cases. As a result, it could be assumed that two out of the five sinkholes developed during the winter 2000-2001, with likely rapid formation; two of the remaining cases probably originated during the middle-late 70's, as also suggested by the age of vegetation hosted within them; c) the last case opened sometime between February 2001 and November 2007. According to the collected testimonies, a further phase of sinkhole development might have occurred in the first half of the past century, but no field evidence of such ancient phase could be found. Since no earthquake has been identified as possible trigger for any of the sinkholes at Piano dell'Acqua, the origin of the studied phenomena may be related to sub-cutaneous erosion, within an area that is known to be rich in groundwater. Local changes in the water table, either related to climate or man-induced activities, may have triggered the development of the sinkholes - that may therefore be ascribed to the suffosion or dropout types. Hydrologic investigations have recently been carried out to analyse in detail the climatic conditions of the three periods mentioned above, in which the considered sinkholes probably originated. For such periods, the recorded rainfalls and drought phases have been analysed in terms of "Standardized Precipitation Index" (SPI), by computing the deficit of precipitation for different temporal scales. Furthermore, by applying the "method of the runs", the return periods of the drought phases have also been evaluated. In this paper, after a short description of the study area and of the considered sinkholes, the results of the hydrological analyses are commented, and the climatic characteristics of the assumed periods of origin of the sinkholes are discussed in detail.