Erosive processes in the Turbolo River catchment (northern Calabria)

This work is focused on preliminary data obtained from a qualitative-quantitative study on denudational processes (soil erosion and mass movements) in the Turbolo River catchment (northern Calabria), representing a leftbank tributary of the Crati River. This catchment has been selected as a test area, because its geological, geomorphological and climatic features make it similar to wide sectors of Calabria, where overall soil erosion susceptibility is high. It extends over an area of about 30 km2 ranging from 75 to 1015 m a.s.l. The basin is characterized by steep slopes shaped on Palaeozoic metamorphic rocks in its western portion, whereas terraces and gentle landforms are shaped on the Neogene-Quaternary sedimentary terrains (sandand gravel-dominated with clays in suborder) exposed in the eastern and wider reaches. The climate is typically Mediterranean, mean annual rainfall approaching 1130 mm and mean annual temperature close to 15 °C. The soil temperature regime is thermic to mesic, associated with a xeric to udic soil moisture regime (the milder and less contrasting conditions occurring in the uplands). This work started with air-photo interpretation, followed by a geomorphological survey in the field: the main morphodynamic processes affecting the landscape were described and mapped. Sheet and rill wash dominate, especially in the eastern lowland, whereas badlands rarely occur in the western clayey terrains. The whole sedimentary infilling is also affected by occasional gully erosion. Mass-wasting processes affect upland and lowland areas, landslides being triggered both on the weathered crystalline rocks and the sedimentary terrains, the latter also showing soil creep and soil slip phenomena. The data obtained were integrated, elaborated and spatialized in a GIS in different thematic layers. On the basis of a DEM, various morphometric parameters were calculated. In addition, key relationships between the main morphodynamic processes and relief properties and land-use were investigated. Moreover, a quantitative geomorphic analysis of the drainage network, obtained from all the surface drainage trunks detected on topographic maps (1:25,000 scale), was carried out to estimate the overall annual sediment yield and consequently the denudation rates affecting the catchment. This approach is based on the assumption that denudation rates can be approximated to the suspended sediment yield of the drainage system. Long-term research and literature in this field supports that drainage density and related parameters indicating hierarchical organization degree and anomalies of river systems can be successfully applied to evaluate denudation rates, expressed by the denudation index (Tu). Our results estimate for the Turbolo River catchment an amount of soil loss reaching about 1700 t/km2/year.