Quantitative study of piping processes in badland areas of the Ebro Basin, NE Spain

In the Ebro Basin, piping is an ubiquitous erosion process in badland areas, occurring in two main situations: on slopes of exposed Tertiary clays and in recently incised silty-clay Holocene valley-fill sediments. The quantitative importance of the main factors controlling piping processes in those conditions was obtained by statistical analysis based on multiple and stepwise regressions. Piping erosion is strongly related to a high sodium absorption ratio (SAR) which favours clay dispersion. However, the statistical analysis indicates that the chemical characteristics and the physical-chemical behaviour are a necessary condition but are not sufficient on their own to develop piping processes. On the slopes, piping is controlled by the physical and chemical properties of the material such as the dispersion index, the granulometric fraction smaller than 4 μm, pH and crack density. In the valley-fill sediments, piping is mainly controlled by the sulphate concentration in the water extract from saturated paste, which is also associated with high SAR values, and the dispersion index. In both geomorphic situations, piping is always related to high crack density and high topographic gradient.