Entità dei processi di denudazione e variazioni morfologiche recenti nell’area di Radicofani (Toscana meridionale, Italia).

ABSTRACT: CICCACCI S., DEL MONTE M., LUPIA PALMIERI E.& SALVATORE M.C., Erosion rate and recent morphological change in the Radicofani area (Southern Tuscany, Italy) The present work is part of a wider research project conducted over the past two decades by the Dipartimento di Scienze della Terra of the University of Rome “La Sapienza”, with aim of studino the recent geomorphological evolution and morphodynamic aspects of some areas in central and southern Apennines. The goal of the present research is (1) to analyse in detail what are the main erosion processes acting on hillslopes, and (2) to estimate their relative rates. This paper presents the results of geomorphological investigation related to the Radicofani area characterized by intense morphogenetic processes and morphological change recorded in the past 50 years. The examined area, situated in the Tuscan Preapennine, near Siena, extends for about 100 km² around the town of Radicofani close to the watershed of Orcia and Paglia rivers. The Pliocene marine claystones outcropping in this area have been uplifted during the Quaternary to several hundreds of meters up sea-level, and they are now suffering particularly intense erosion, lending to considerable badland formation. The tectonic history for this area is representative of the general history of orogenesis in the northern Apennines. Cretaceous to early Oligocene deposition occurred in a compressive foreland as the Tuscan and Ligurian Thrust sheets verged eastward in a large, subaqueous accretionary wedge. Emergence of the wedge in the Miocene was accompanied by extensional faults and formation of local graben. These graben filled with marine and terrestrial deposits. More recently, in the Quaternary, there has be regional arching of the Apennines, perhaps locally influenced by volcanism, to raise Pliocene marine deposits far above sea level. These deposits upl now outcrop at 750 meters above sea-level and show, in some cases, a clear stratification with strike about N-S and dip direction about E-ENE, a few degrees steep; in the western part of the area, the stratification is nearly horizontal or it has a western dip direction. Jointing follows a few preferential directions that seem to condition the main valley downcutting and the intense denudational processes. The analysis of landforms, surface deposits and morphogenetic processes of the study area was carried out through a detailed field survey, and compiled on a Geomorphological Map at a scale of 1:10.000 ( enclosed). The survey was systematically carried out for sixteen years (1988-2004). It allowed to gather detailed information about active morphological dynamics and evidence that the area is characterised by intense short-term morphological changes. In order to cast these data in a long-term context, an aerial-photograph interpretation analysis was made using the aerial photos of the GAI flight of 1954 and the EIRA flight of 1976; thus geomorphological characteristic of the study area was reconstructed for those years. The comparison of these situations with the one illustrating the present landscape, quantifies the morphological evolution of the area in the last 50 years . The geomorphological study has been completed by morphodinamic analysis: direct and indirect measurements have been carried out in order to have quantitative information on the erosion rate in the examined area. For direct measurements of the erosion simple metallic instruments have been used, placed in two chosen sample-areas with a badland sharp morphology. These areas represent a natural laboratory, where short-term evolution and seasonal change of landforms generated by fluvial and hillslope processes could be accurately studied, and where erosion rate could be directly measured. The metallic instruments (stakes and disks) have been placed in significant zones , but poorly accessible in order to avoid tampering. They have been used as datum points for accurate and continuous measuring, to collect a large series of data. Direct measurements have been integrated with indirect measurements carried out on the basis of the aerophotographic analysis. For a sample-area, digital elevation models (DEM), obtained using a digital video plotter (DVP, Leica) and the aerial phothographs of EIRA flight (1976) and Tuscany Region flight (1994), have been constructed. The comparison between 1976 and 1994 DEM allows to obtain a “difference map”, showing the variation of topographic surface in nineteen years and then to calculate the mean erosion rate in the chosen area. Results of geomorphological analysis evidence that important denudation processes are active in most of the studied area. Erosive processes due to channelized flow (which are responsible for the several deep valley downcutting), to sheetwash (responsible for the frequent badland and “biancana” morphologies), and to gravity (cause of continuous and frequent mass movements, both superficial and deep, active also on gentle slopes) seem to be the particularly intense. The 1988-2004 field survey has proved that processes and landforms in the sample area are involved in continuous and rapid short-term changes. The comparison of the 1954 and 1976 landscapes with the 2004 one, evidence many macroscopic morphological differences. The “biancana” areas have been reduced, by the bulldozer working over the last 50 years (especially during the 60s and in the 70s). These areas now occupy only 30% of their previous 1954 surface. “Calanchi” badlands have been less affected by human activity, because of their greater steepness, while have been mainly modified by natural causes. “Calanchi” badlands intensely shaped by runoff waters in 1954, with a characteristic network of deep incisions, steep slopes and sharp ridge-lines, seem to have today a less sharp morphology , due to an increasing impact of gravitative movements on the morphosculptural evolution. Data obtained from direct field measurements of denudational processes evidence that the mean values of erosion rate due to sheetwash range from 1,5-2 cm/year on “biancana” badland areas to 2-3 cm/year on “calanchi” badland slopes. For these slopes analyses show that the value of erosion rate increase up to 6 – 7 cm/year as a consequence of very intense mass movements . Indirect measurements confirm direct ones, shoving mean erosion rate values of about 5,5 cm/year in the above mentioned nineteen years term. KEY WORDS: Erosion, Badlands, Morphodynamics, Tuscany, Italy