SPATIAL DATA ANALYSIS FOR THE ASSESSMENT OF SLOPE INSTABILITY; APPLICATION TO EIA FOR A CASE STUDY IN THE VERGARA-EIBAR AREA, GUIPUZCOA, SPAIN

摘要

The assessment of both hazards and impacts of human activities on geomorphological processes must be based on a detailed analysis of terrain dynamics and behaviour, prior to the proposed new actions. Knowledge of former behaviour provides extremely useful clues for predicting probable impacts. In the case of slope instability processes, the analysis must include mapping of conditioning factors and past movements and identification of triggering factors. This work presents the results of an analysis carried out in the Eibar-Vergara area. The analysis was based on the construction of digital maps for a variety of data layers. Thematic maps included the main conditioning factors of slope movements: surficial geology, geomorphology, DTM (from which several derivative maps were obtained), land cover, type and distribution of past landslides. Spatial data analysis was carried out by means of Favourability Functions, on the basis of statistical relationships between former movements and conditioning factors, to determine landslide susceptibility. The analyses were performed and tested through two types of comparisons: a) spatial comparison; a part of the study area was analysed and compared with another, used as test area; b) temporal comparison; movements prior to a certain date were analysed and later movements used as test group. Landslide susceptibility maps thus obtained were then used as a basis for EIA of a proposed motorway. The least impacting alternative was selected through the intersection of potentially unstable areas and motorway routes. Using simple predictive models, based on a series of scenarios, potential impacts were estimated.