The causes of piping in a set of abandoned agricultural terraces in southeast Spain

This study aims to identify some of the factors contributing to the piping process in abandoned terraces in seven sites located in the Murcia Region (southeast Spain). The local landscape has the appearance of badland geomorphology, possessing deep valleys which until the 1970s were terraced and cultivated. The terraces have now been abandoned to the piping process. The research focuses on the relationship between the size and position of pipes and the physical and geochemistry properties of the local highly dispersive marl lithology; and the extent to which the reworking of the materials for terrace cultivation has exacerbated piping erosion.
Morphologically, the terracing in the stream beds has resulted in a series of gentle benches separated by steeper risers with variable ‘drop’, (riser height). A series of flat infiltration surfaces result, and because the resculpting produces a reduced bulk density infill with enhanced throughflow, infiltrating water is forced along steep hydraulic gradients to a low outfall at the base of the riser.
Paralleling studies of land levelling and terrace abandonment in similar materials elsewhere, piping in the study area is shown to be closely related to the physical and chemical characteristics of the materials its sodicity and dispersive character. Considerable differences in structure, texture, mineralogical composition and material's surface crust and subsoil geochemical properties emerged from ‘site signature’ plots, and these results suggest that agricultural processes affect the dispersive character of the crust materials, presumably because agricultural activity increases the buffering effect of organic acids on the dispersed clay. This is supported by the observation that on the one site that was levelled but not used for agriculture, a stable organically-distinct crust like the abandoned agricultural sites has not formed. The changes to the material's aggregate stability and bulk density properties when terracing occurs is examined, but insufficient evidence is available from the data collected to unpick the causes of aggregate stability changes in the study plots. The way in which differences between crust and subsoil enhance infiltration is discussed, and the role of the terrace ‘drop’ in influencing hydraulic gradients and pipe development is illustrated. A possible index to describe the relationship between evacuated material volumes and the controlling parameters discussed above is tested.
It is concluded that terracing which is intended as a conservation practice has, in dispersive materials, actually enhanced piping, leading to the abandonment of farming.