Integrative assessment of Badland erosion dynamics in the Oltrepo area

The present work is the result of three years of investigations on soil erosion forms and features in the Oltrepo Pavese, Northern Apennines, Italy. The aim of the work is to review from a modern and scientific point of view the badlands which crop out in the study area as well as to improve methodologies to study the sediment dynamics in badland areas. Badlands are the result of a complex interaction between sub-surface and surface runoff soil erosion processes and are a hotspot for biodiversity and geodiversity. In addition, badlands have always been a fundamental environment for soil erosion investigations. This work is based on the following four principal steps: i) the geological and structural characterisation of the study area, ii) the description of badland forms and features, iii) a probabilistic approach to determine soil erosion susceptible areas in the Oltrepo Pavese and iv) the assessment of suspended sediment dynamics at catchment scale. This study highlights a complex geological and structural sector of the Northern Apennines characterised by soft sedimentary bedrock materials that are prone to be eroded by running water. Initially, a litho-structural map was assembled, and the geological formations of the study area were grouped according to their lithology. The map represents a homogeneous base of information to classify from lithological point of view the badlands of the study area and will become a raster-base for spatial multilayer analysis. Subsequently, a geological, geomorphological and morphometrical classification of the badlands which crop out in the study area was performed though field survey and detailed terrain analysis based on Digital Terrain Models (DTM). The Oltrepo Pavese badlands were classified in type A and B according to their morphology and vegetation conditions. The badlands show high heterogeneity and can be closely related with melange bedrock, claystone and interstratified rocks. Furthermore, the badlands show the typical characteristics of Apennine badlands even if certain morphological differences were noted. This study also highlights the importance of the rainfall characteristics and land use changes playing an important role in the development and stabilisation of the badland forms and features. The land use change induced by planting operations (afforestation) and the reduction of agricultural activities in the area, as well as the reduction of the precipitation amount leads to a shrinking of badlands. Though a detailed terrain analysis and the application of the Maximum Entropy model (MaxEnt) three susceptibility maps were generated for the badland and rill-interrill erosion forms. The predictor analysis has highlighted that the more important predictors (i.e. lithology, land use, elevation) can significantly explain the diversity between calanchi type A and B. However, less significant predictors e.g. Vertical Distance to Channel Network, Valley Depth and Catchment Area are fundamental to understand the development of the two morphotypes. Finally, the study reveals for the first time, the dynamics between precipitation, discharge and suspended sediment transport in a small watershed basin sited in the Northern Apennines. A laser diffraction instrument was installed at the outlet of a small watershed basin deeply interested by aquatic erosion and the sediment diameter and concentration was evaluated with respect to rainfall. The initial moisture condition, hydrophobicity, vegetation cover, and physical conditions of the basin play a fundamental role in the assessment of sediment dynamics. Finally, the study reveals the importance of a correct land management to reduce badland erosion in the Apennine region.