Analysis of lateral sediment connectivity and its connection to debris flow intensity patterns at different return periods in the Fella River system in northeastern Italy

Abstract

A wide variety of issues are now being addressed using the concept of connectivity, which has initiated the development of various methods to assess a river's relationship to its catchment. This study tests two well-established methods, the Effective Catchment Area (ECA) and the Index of Connectivity (IC) in the study area of the Fella River in northeastern Italy, to gain an idea of their potentials, limitations and ability to represent connectivity patterns observable in the field. The results show that both methods provide largely agreeing outputs, which widely match field observations. Disagreement is mainly found where human-induced features, especially roads, encroach the rivers system. Focusing on a natural hazard background, the study furthermore approaches the issue of events of different frequencies and magnitudes and their representation in terms of connectivity. This is done by correlating debris flows at varying return periods with the IC, which seemed more fitting for this comparison due to the differentiation between different intensities of connectivity. Over the entire catchment, patterns of debris flow intensities (DFI) only agree weakly with the patterns of the IC, however, debris flows reaching the main channel show strong correlations with IC values. This can be traced back to the fact that connectivity focuses on a catchment's relationship with the river and does not include processes that happen in those parts of the catchment not directly linked to the main channel network. The IC is therefore able to represent patterns of processes reaching the main valley very well but cannot be used to explain or even predict the occurrence of processes that have no direct spatial connection to the river.