Geomorphic indexing of landslide dams evolution

Landslide dams are rather common events in hilly and mountainous areas and they occur when a landslide reaches a valley floor closing the riverbed. If they form a lake basin, unstable landslide dams can have catastrophic consequences when they occur in upstream of populated regions. Landslide dam behavior is not completely understood yet, however several studies suggested implementing geomorphological index in order to assess their formation and evolution. These indexes result from the composition of two or more morphological attributes that characterize the landslide (e.g. landslide volume or length) and the involved river valley (e.g. valley width).The objective of this work is the definition of a procedure, based on the joint use of different indexes, to assess landslide dams evolution over large areas (e.g. entire river catchment or even a region or a nation) and in short times, in order to be used for emergency response or for planning activities.About 300 landslide dam events collected in Italy were analyzed and some state-of-the-art geomorphological indexes were applied to characterize the damming phenomena at the national scale. To overcome some limitations of the aforementioned indexes, we introduce two new indexes: the Morphological Obstruction Index and the Hydromorphological Dam Stability Index. The former combines the river width and the landslide volume, and it can be used to identify the conditions associated to dam formation discriminating between circumstances where a landslide dam is formed and circumstances where it is not. The latter uses a simplified stream power formulation (combining the upstream catchment area and the local slope gradient) to account for the river energy. This index allows evaluating the stability of a dam in near real time as soon as it occurs and can be used to discriminate between stable and unstable dams.If compared with the reviewed state of the art indexes, the newly proposed ones show an improvement in the forecasting effectiveness and have the advantage of being based on morphometric input parameters that can be easily and quickly assessed on a distributed way even over large areas. We propose a tool that is based on the joint use of the newly proposed indexes and that can be used to provide fast and effective assessment on landslide dam formation and stability during emergency or planning activities.