Morphodynamics and morphological changes of the last 50 years in a badland sample area of Southern Tuscany (Italy)

摘要

The purpose of this geomorphologic study was to analyse the recent morphologic evolution of calanchi badlands over a 50 years' period (1955-2006) and to evaluate the mean erosion rate in areas situated in Southern Tuscany. These areas are characterised by Pliocene marine claystone which have undergone and continue to undergo erosion processes and a rapid morphologic evolution. Our morphodynamic investigation was based on systematic field surveys conducted over a 12 years' period (1994-2006) and photogeological analysis of a 30 km(2) wide area. The systematic field surveys demonstrated that in the calanchi badland areas, erosion processes due to sheet-wash and channelled water act together gravitational processes, mainly represented by landslides and solifluction. Our research indicated that the effects of gravitational processes have increased over the last 10 years while sheet-wash action has been less intense over that same period. This is demonstrated by the progressive increase of flows at the top of the calanchi badland slopes which determined the general rounding of sharp crests and the partial filling of several cuttings. In addition, our morphodynamic analysis strongly suggests that anthropogenic action should not be overlooked or underestimated as a major force in shaping the landscape in the study area over the last 50 years. Quantitative evaluation of the morphodynamic evolution based over a 50 years' period, was carried out in the "Torrente Cacarello" sample area, chosen as representative of the whole study area. The comparison between 1955, 1976 and 2006 detailed geomorphologic maps (scale 1 :5,000), obtained from the aerial photographs and from the recent (2006) field survey, allowed us to estimate the morphological differences which occurred in this period. Detailed analysis of the three geomorphological maps of the sample area obtained for each period, were compared through a GIS software and enabled us to quantify the following different morphological processes that influenced the variation found in these areas: 1) the calanchi badland area increased over the last 50 years; 2) mass movements prevail over sheet wash erosion; 3) calanchi badlands in their most recent morphological state are shaped by numerous scattered small landslides. The geomorphological study in the sample area was completed by direct morphodynamic analysis: measurements of erosion were taken in order to quantify the mean erosion rate. The sample area was monitored during the period Jul. 00-jan. 06. Morphographic and morphometric analyses showed that the calanchi badlands are characterised by constant and quick shrinking of their edges caused by landslides. Selected measurement stations located on slopes indicated an erosion rate ranging from 6 to 6.5 cm/y. The mean erosion rate measured in the sample area of this study was comparable to the mean erosion rate indicated in our earlier studies of neighbouring areas of Southern Tuscany; the mean erosion rate in both of these studies was significantly higher than those indicated by other authors who have studied Italian badlands.