Assessing Landslide Dams Evolution: A Methodology Review

In hilly and mountainous regions, landslide dams can be recurring events involving river networks. A landslide dam can form when sliding material reaches the valley floor and closes a riverbed causing the formation of a water basin. Unstable landslide dams may collapse with catastrophic consequences in populated regions because of the resulting destructive flooding wave released. To prevent these consequences, the assessment of landslide dam evolution is a fundamental but not easy task, because of the complex interaction between watercourse and slope dynamics. Several researchers proposed geomorphological indexes to evaluate dam formation and stability for risk assessment purpose. These indexes are usually composed by two or more morphological parameters, characterizing the landslide (e.g. sliding material volume or velocity) and the river (e.g. catchment area or valley width). In this work, a procedure to evaluate landslide dam evolution is applied and reviewed. About 300 obstruction cases occurred in Italy were analyzed with two recently proposed indexes, the Morphological Obstruction Index (MOI) and the Hydromorphological Dam Stability Index (HDSI). The former, which combines the landslide volume and the river width, is used to identify the conditions that lead to the formation of a landslide dam or not. The latter, which combines the landslide volume and a simplified formulation of the stream power (composed by the upstream catchment area and the local slope), allows a near real time evaluation of the stability of a dam after its formation. The two indexes show a good forecasting effectiveness (61% for MOI and 34% for HDSI) and employ easily and quickly available input parameters that can be assessed on a distributed way even over large areas. The indexes can be combined in a convenient procedure to assess, through two subsequent steps, the final stage in which a landslide dam will evolve.