Evidence for repeated re-activation of old landslides under forest

摘要

Local reactivations of landslides in forests are rarely reported in landslide catalogues. The occurrence of hillslope sections with fresh morphological landslide features in forested old, deep-seated landslides, however, suggests that landslide reactivations are not restricted to residential areas. In this study, a dendrogeomorphological analysis of beech stands was used to investigate the periods of reactivation of a deep-seated rotational slide in the Koppenberg forest (Flemish Ardennes, Belgium). The relation to rainfall and the correspondence to landslide reactivations reported in a nearby built-up area were also analysed. A dendrometrical study preceding the dendrochronological analysis proved that, compared with the nearby reference site, trees on the Koppenberg forest landslide site were significantly more inclined and showed more knees, indicating that the landslide site has not stabilized yet. As the sampled trees are younger than the landslide, dendrochronology did not allow determination of the year in which the landslide was initiated, but analysis of two different tree ring width parameters (i.e. ring eccentricity and growth change) calculated for trees sampled on the Koppenberg landslide and the reference site proved to be of great help in determining the temporal sequence of landslide reactivation. During the past 80 years, several periods indicative of local reactivations (i.e. 1943-1945, 1949-1952, 1967-1970, 1972-1977, 1979-1981, 1988-1997) were found within the investigated landslide, but delineation of the spatial extent of the reactivations during these indicative periods was not straightforward. These periods generally correspond to years with above-average rainfall. Finally, the fact that at least 34% of the years indicative of reactivation of the Koppenberg forest landslide correspond to a year in which a landslide reactivation was reported in the Flemish Ardennes suggests that in built-up areas, apart from anthropogenic interventions, natural triggering factors remain very important.