Erosion and recent morphological change in a sample area of the upper Orcia River Basin (Southern Tuscany, Italy

Abstract

Qualitative and quantitative geomorphological investigation has been carried out in a restricted area of the upper Orcia River basin (about 10 km²) called «Landola sample area», near Siena, NE of the Monte Amiata volcano (Tuscany). The Pliocene marine claystones outcropping in this area have been uplifted during the Quaternary up to several hundreds of meters above sea-level and are now rapidly eroding, resulting in one of the highest values of suspended sediment loads in Italy. Since 1988 detailed geomorphological field surveys hare been performed in the Orcia River basin; and beginning in 1993 monitoring of specific denudational processes on slopes characterised by rapid erosion has been completed. These slopes 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 the erosion rate could be directly measured, showing in the most extreme cases a lowering of topographic surface of a few cm/year. The values of some morphometric parameters have also been determined, in order to estimate erosion rate indirectly. Spatial variations of this modelled rates of erosion have been compared with field measurement to confirm the extreme values. To extend the results of this research over a longer period of time, aerial pictures taken over several decades have been analysed. The comparison between the 1954 map, drawn by air-photo interpretation, and the 2002 map show many macroscopic morphological differences. Biancane badlands have been reduced by bulldozers working (especially in the 60s and in the 70s). These areas now occupy only 30% of their previous 1954 surface. Calanchi badlands have been greatly modified by natural causes. In the last 50 years, hillslope processes have become dominant compared to sheet wash and channalized flow in the calanchi badlands evolution. Direct measuring of the denudation rate, mainly related to superficial running waters, have determined 2-3 cm/year mean values for calanchi badlands and 1,5-2 cm/year for biancana areas. Indirect data confirm these results, showing that mean erosion rates for small drainage basins are close to 1 cm/year, and often exceed this value.