Analysis of slope processes in the Vallcebre landslide (Eastern Pyrenees, Spain) by means of Cross Correlation Function applied to high frequency monitoring data

Abstract. The Vallcebre landslide is a large slow moving translational slide in the Eastern Pyrenees (Spain). In this work, the Cross-Correlation Function (CCF) method was used in order to quantitatively investigate the time-lagged correlation between high frequency monitoring data on rainfall, piezometric and displacement with the objective to evidence hydro-mechanical processes occurring along the slope. The CCF is a signal processing tool for measuring similarities of time-series waveforms as function of an applied time-lag. Specifically, it was applied in Vallcebre landslide to a 3 years long time series of monitoring data, from 1999 to 2001, with a sampling frequency of 20 minutes. Data were measured in three boreholes instrumented with automated wire-extensometers and piezometers and a rain gauge. The boreholes are lined up down the slope and following the displacement direction, which allowed investigating transfer of landslide mass and groundwater along the slide. Several combinations of time series were analysed: rainfall vs. displacement; rainfall vs. piezometric depth; piezometric depth vs. displacement. Moreover, correlation analysis of displacement and piezometric depth between boreholes was also performed. The CCF analysis highlighted and constrained in time a dual triggering mechanism in which factors controlling movement change along the slide: movement in the lower landslide zone is predominantly influenced by toe erosion whereas in the intermediate and upper landslide zone movement is mostly controlled by groundwater recharge and flow.