Contributo dell'analisi geomorfica quantitativa allo studio dei processi di denudazione nel bacino idrografico del Torrente Paglia (Toscana meridionale - Lazio settentrionale)

Contribution of Quantitative Geomorphology in the study of denudation processes in the drainage basin of Torrente Paglia (Tuscany, Latium)

Abstract

The possible relations existing between morphometric parameters of drainage basins and the type and intensity of denudational processes were investigated. To this end, the Torrente Paglia basin was examined; it constitutes the headwaters of the Fiume Paglia, which joins the Fiume Tevere to the southeast of Orvieto. The studied basin is located in the graben of Radicofani, which trends NW-SE and is delimited by the Monte Cerone horst to the east and by the Monte Civarella-Monte Elmo horst to the west. The basin shape and the pattern of its drainage network strictly depend on the regional structural conditions. Denudational forms and processes that characterize the Torrente Paglia drainage basin were identified based on a careful geomorphological survey; gravity and surface running waters were found to be the most important morphogenetic agents. The type and intensity of these processes are functions not only of local climatic conditions but also of the characteristics of the outcropping lithotypes. The latter consist of four main units: mainly carbonate formations of the "Tuscan series" (Upper Triassic-Eocene); flysch formations with more or less marked argillaceous components (Cretaceous-Eocene-Oligocene); argillaceous and sandy-argillaceous formations of the "Tuscan neogenic cycle" (Lower Pliocene); volcanic formations linked to the Quaternary volcanism of Monte Amiata, the Radicofani apparatus, and the Vulsini Complex. To single out the relationships existing between denudational processes and the main morphometric characteristics of drainage basins and networks, some particularly interesting parameters were examined. Such parameters, calculated for the whole basin and for its main partial basins, are: drainage density, relief energy, and some parameters expressing the degree of hierarchization of drainage networks (in particular, index and density of hierarchical anomaly). Hypsometric analysis was also performed; the aim was to evaluate the equilibrium stage of the studied basins, on which the magnitude and the rate of denudational processes depend. The most interesting observations derived from the study of the degree of hierarchization of drainage networks and from the hypsometric analysis. On the basis of data obtained so far, it is possible to affirm that when linear erosion processes prevail, the drainage networks are better organized, while their disorganization gradually increases as slope denudational processes become more important; the less organized networks were observed wherever "calanchi" occur. The hypsometric analysis showed that hypsometric curve trends and the values of the relevant integrals strictly depend on the type of prevailing denudational processes: the observations made allowed the proposal of an interpretative model of the curves themselves; this model integrates the traditional model proposed by STRAHLER, which referred to the "geomorphic cycle."