Dissesto idrogeologico e interventi forestali nell'alta valle del fiume Orcia, comune di Radicofani, provincia di Siena
Author(s)
Castaldi, Fabio
Date Issued
March 11, 2011
Type
Doctoral Thesis
Abstract
The areas consisting of the early Pliocene clayey deposits are often affected by badlands erosion in the central Apennine hills. The Pliocene claystones cover 21000 ha only in the River Orcia basin, southern Tuscany (province of Siena): this landscape is called Crete Senesi.
The area of Radicofani, in the upper valley of the River Orcia, is mainly formed by early Pliocene clayey sediments (River Paglia Unit). The slopes are widely affected by geomorphological hazard consisting of very frequent badlands erosion and landslides. Many reforestations were carried out in the last fifty years for a total area of 320 ha in this scenario. Frugal conifers (Cupressus arizonica, Cupressus sempervirens, Pinus nigra and Mediterranean pines) in monospecific or mixed stands were chosen for these works.
The goal of our study is to evaluate through a multidisciplinary approach the impact of reforestations on the geomorphological instability of nine small drainage basins belonging to the rivers Paglia and Formone watersheds. The geological, geomorphological, hydrological and vegetational characteristics were analyzed by a diachronic confrontation between the pre – reforestation and post – reforestation situations. Each elaboration has been carried out for the year 1954 (pre - reforestation) and year 2007 (post – reforestation).
A DEM from historical cartography was constructed and compared with the actual DEM. The difference between the two DEMs leads to estimate the erosion and deposition annual rates for each drainage basin.
The aerial – photo interpretation and the processing in GIS system allowed to quantify the surface change in the areas affected by badlands, land use, adopting the CORINE Land Cover methodology, and the variation of the erosion rates through the reconstruction of the drainage networks. Furthermore, the landslide areas are related to some factors that are determinant in susceptibility to instability: slope, aspect, land use and lithology. This relationship has allowed the allocation of weights for the predisposing factors, in order to determine which among them has more influence on the instability and to calculate the Landslide Index.
Beside the diachronic investigation, eighteen circular sample plots of ten meters radius in forty years stands have been carried out. The number of plants, mean diameter, basal area and average height and dominant for the five different forest types analyzed have been determined. The forest survey, along with the evaluation of the fragmentation of reforestation cores, allowed to evaluate the adaptability of different forest species used in this typically clayey substrate.
The differences between the pre and post reforestation situations are remarkable. There is an increase of arable land and forest areas, while the degraded areas with little or no vegetation decrease. In parallel there is a significant reduction in the erosion rates and badlands areas. Although the geomorphological hazard seems to have been mitigated, extensive areas affected by instability are still remaining, in particular the Landslide Index shows a strong susceptibility in areas where the River Paglia Unit outcrops, with slopes between 10° and 20° and used for agricultural purposes.
The reforestation cores show a consistent fragmentation due mainly to the choice of plantation sites. The forest stands show a satisfactory cover despite dendrometric values indicate poor fertility classes and naturalization is present only in few stands. However there is no significant correlation between erosion rates and forest surface, due to the excessive fragmentation of cores, but also to the low reforested surface in the stream basins as compared to the total extension. The land reclamation seems to have had more weight in reducing erosion rates in these clayey substrates.
The results of this study provide an important tool to understand the morphological dynamics of badlands areas in relation to forest vegetation and for planning and land management.
Additional information
Dottorato di ricerca in Scienze e tecnologie per la gestione forestale e ambientale
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