Estimating Long to Short-Term Erosion Rates of Fluvial vs Mass Movement Processes: An Example from the Axial Zone of the Southern Italian Apennines

Estimates of erosion rates related to fluvial and landslides processes have been performed for several temporal ranges on the basis of different viable methods. The study area includes three catchment sub-basins of the north-eastern flank of the Agri Valley, a tectonically active morpho-structural low located in the axial zone of the southern Apennines and affected by desertification processes in its middle part. Long and mid-term rates have been estimated evaluating the missing volume of eroded rocks, on the grounds of GIS-aided calculations linked to geomorphological markers corresponding to ancient base-levels of the erosion. Short-term rates have been calculated converting the parameter related to the turbid transport of streams (Tu), derived from the quantitative geomorphic analysis, adopting an average value for the specific weight of the outcropping rocks, Such an approximation has to be considered acceptable because of the presence of conditions of lithological homogeneity in two of the chosen basins. In the light of this, the third basin, characterized by a certain geological heterogeneity, has been assumed as test-site with regard to the other two sub-basins. The average value of the long-term erosion rate from the entire study area is 0.25 mm/y, mainly due to fluvial processes. The comparison of the erosion rates related to the fluvial network activity from early Pleistocene to Present with those related to the mass movements occurred in a more recent (and shorter) time-span indicates that mass movements contribute just for the 1% of the whole erosion estimated in the two homogeneous basins, but they refer to a short time-span (10 ky to Present). A theoretical extrapolation to the past (0.5 to 1 My) allows to consider the contribution of landslides to the total erosion inside the catchment areas as almost equal to the fluvial aliquot. The conversion of the Tu values shows that the short-term rates are generally greater than the rates calculated on the basis of the “volumetric” method, being the average value about 0.46 mm/y. This may mean that the present-day trend of the linear erosion rate could increase respect to the long- and mid-term trend as a response to climate triggering (i.e. more rainfalls and/or sediment supply), or simply represent an instantaneous trivial fluctuation in the long-lasting behavior of the fluvial nets under variable climate conditions. The comparison of all these data with the uplift rates calculated on a regional scale (0.6-1.1 mm/y) confirms that the southern Apennines may represent a non-steady state system.