Exploring the frequency-size relationships of pipe collapses in different morphoclimatic regions

Abstract

Piping is an important land degradation process that occurs in almost all climatic zones. This study analyses for the first time the frequency-size distribution of different types of pipe collapses (PCs): sinkholes and compound depressions related to the coalescence of sinkholes (compound sinkholes and elongated blind gullies). Exploring the frequency-size relationships of PCs provides critical morphometric information about this hazardous erosion and ground-stability process, as well as its impact on hillslope denudation, hillslope-channel coupling and landscape development. The analysis has been performed with field-based inventories of PCs from five catchments located in two different morphoclimatic environments: one in the semi-arid Ebro Depression, NE Spain (Valpalmas catchment), and four in the humid Bieszczady Mts., SE Poland. In total, 724 PCs were mapped, among which 335 PCs in Valpalmas and 389 PCs in the Bieszczady Mts. The morphometric analyses were based on the major axis (the maximum length or width) and area of the PCs. Despite differences between study areas from NE Spain and SE Poland (related to climate conditions, rainfall regime, soil water storage capacity, spatial pattern of pipe networks, vegetation cover and land use), empirical cumulative frequency curves of PCs show similar patterns in all study sites indicating similar overall effects of piping erosion. The size distribution of the inventoried PCs can be modelled satisfactorily with logarithmic functions with high goodness of fit values (R2 ≥ 0.93) for each type of PC and the combined set of PCs (sinkholes and compound sinkholes/blind gullies) mapped in each area and in the five catchments together. The PCs mapped in Valpalmas are displaced towards larger dimensions attributable to local factors. The applicability and limitations of frequency-size relationships of PCs for geomorphological studies and hazard assessment are discussed.