Original paper

Decadal-scale analysis of ground movements in old landslides in western Belgium

Dewitte, Olivier; Van Den Eeckhaut, Miet; Poesen, Jean; Demoulin, Alain

Abstract

More than 150 large deep-seated landslides have been mapped in the Flemish Ardennes. Slope instabilities that have occurred in this hilly area of western Belgium during the last decades correspond to ground movements within these pre-existing landslides. In order to identify the mechanisms and controlling factors of these ground movements, a good knowledge of their spatial and temporal distribution is critical. 13 landslides affecting two hills were investigated based on several DTMs extracted by aerial stereophotogrammetry and spanning the 1952-1996 period. Vertical ground displacements were measured at each pixel by DTM subtraction with a confidence value of ± 0.70 m. Horizontal displacements were also estimated within the landslides and along the head scarps through topographical profiles. Most observed movements displayed patterns typical of rotational landslides. Vertical and horizontal displacements vary in magnitude both spatially and temporally, with respective ranges −7.4 m-+3.8 m and 0-14 m. Many displacements are materialized in the field. We distinguished two kinds of slope processes, corresponding to either reactivation at a deeper level or shallower motion. The former re-uses pre-exiting surfaces of rupture located at depths of ∼15-20 m and is associated with the largest subsidence and uplift. They are also smaller reactivations confined at the landslide head. The other displacements consist in (1) earth flows occurring in the zone of accumulation sometimes as a consequence of large upslope reactivations, and (2) small failures occurring randomly. While most movements were triggered by intense rainfall, their spatial and temporal distribution is strongly related with the nature of the vegetation cover and the human activity.