The formation and impact of landslide dams – State of the art

The blocking of river courses by mass movements is very common in mountainous areas with deep and narrow valleys. Landslide dams may pose serious threats to people and their livelihoods downstream in the case of abrupt dam failure. Since the publication of benchmark reviews of Costa and Schuster (1988) and Korup (2002), there is a growing number of studies focusing on the formation, stability, and short-term impacts of landslide dams. This review combines the insights of all these studies, builds on current concepts of landslide dams, and suggests ways to unify terminologies and classifications. We furthermore present a new worldwide database compiled from literature data. It contains 410 landslide dams >1 million m3 in volume that were formed since 1900 since these have the most complete data entries. These data show that dam longevity is, among other factors, correlated with the type of landslide forming the dam. Those formed by rock/debris avalanches and rockslides have longest lifespans. However, the influence of landslide type or material on dam longevity decreases with time after dam formation. To ensure consistency in the next database generation, we suggest guidelines for data collection to provide a solid basis for evaluating dam stability and governing factors. A preliminary classification matrix for landslide dam stability that combines topographic setting and the internal structure of the dam body is another outcome of our review. Furthermore, an evaluation of the various geomorphic stability indices proposed in the literature regarding their suitability and limitations in assessing dam formation and stability shows that they predict the probability of dam formation reasonably well, but that their application to longevity estimates requires further assessment. The geomorphic impacts of landslide dams in the short-, medium- and long-term are summarized and illustrated with key examples. Finally, for a better understanding of the factors controlling dam stability, we recommend to (1) include dam composition and sedimentary structures in future case studies, (2) maintain and update the worldwide database for sound statistical analyses, (3) refine landslide dam stability indices and test them for different landslide types, and (4) study hazard cascades related to multiple dams in one watershed. For long-term landscape evolution studies, we suggest to (5) quantify terrestrial sediment flux related to landslide dams, (6) detect ancient landslide dams in river profiles, and (7) further exploit the sediment archives in former impoundment areas.