Contribution of dendrochronology to the determination of magnitude–frequency relationships for landslides

The determination of magnitude–frequency (M–F) relationships is necessary for landslide hazard assessment. Ideally, this is achieved by using inventories of past landslide events recorded by technical units or aerial photo interpretation. However, direct sources of landslide data are often unavailable. Dendrogeomorphological techniques allow us to determine the frequency of events, but few attempts have been made to estimate landslide magnitude by analysing the spatial distribution of disturbed trees. This paper addresses M–F determination for rockfalls, debris flows and complex slides obtained by dendrogeomorphology. The cases presented require the appropriate understanding of both the geomorphological context and the disturbance caused by landslide motion on trees. Both small and large rockfalls can be identified and dated by reconstructing the trajectories in the forest stand. The larger the rockfall size, the longer the path, the higher the number of affected trees and the greater the area of deposition of rock fragments. The area of damaged trees is proposed as an indicator of the rockfall magnitude. The frequency and extent of a series of debris flows were reconstructed by mapping their deposits using relative dating criteria and by dating damaged trees. Local and global mudslide reactivations were deduced by sampling various tree generations and by analysing the location of the disturbed trees in the landslide units. Magnitude–cumulative frequency curves were determined for the three cases.