Direct and indirect evaluation of denudation rates in Central Italy

This paper presents the main results of a series of geomorphological studies, aimed to the quantitative direct and indirect evaluation of denudation processes in Central Italy. These studies, which started in some sample areas since long ago, were further deepened and extended to a wider number of drainage basins, representative of a variety of morphostructural conditions. The complex cause/effect relationships among factors influencing both the morphogenesis and the plano-altimetric configurations of drainage basins resulted in a large variability in the values of the denudation index (Tu), which ranged from 200 to 2500 t/km 2/year. Very high values (Tu > 10,000 t/km 2/year) were obtained for small catchments belonging to the wider basins of Ombrone River and Tevere River, where fast morphological modifications occur due to the action of surface running waters and gravity. In the drainage basins affected by the most powerful denudation processes direct measurements of denudation were performed: several monitoring stations were equipped with iron stakes and disks to record the local modifications of the topographic surface with time. Whereas some of these stations have been working continuously since 1993, some others have been running only periodically. Direct measurements pertinent to periods longer than a decade indicated that mean annual values of denudation range between 1 and 2.5 cm/year for slopes affected by sharp-edged and/or rounded edged badlands. Higher denudation rates (3–4 cm/year) were obtained for shorter observation periods; still higher rates were recorded for short periods at the gully bottom or in coincidence with mass movements before bench marks were turned up or damaged. Denudation processes are likely to occur discontinuously. They show a step-like trend with time and are triggered by rainfall events distributed over a number of consecutive days, though not greatly intense. Particularly, their amount should overcome the threshold of 60 mm per 5 consecutive days. Below this threshold the surface runoff does not seem to reach the power necessary to induce significant modifications of the topographic surface in the study area.