Hazard zoning for landslides connected to torrential floods in the Jerte Valley (Spain) by using GIS techniques

The Jerte Valley is a northeast- southwest tending graben located in the mountainous region of west central Spain ( Spanish Central System). Mass movements have been a predominant shaping process on the Valley slopes during the Quaternary. Present day activity is characterized as either ' first- time failure' ( shallow debris slides and debris flows) or ' reactivations' of pre- existing landslides deposits. A delineation of landslide hazard zoning within the Valley has been carried out by using the detailed documentation of a particular event ( a debris slide and a sequel torrential flood, which occurred on the Jubaguerra stream gorge), and GIS techniques. The procedure has had four stages, which are: ( 1) the elaboration of a susceptibility map ( spatial prediction) of landslides; ( 2) the elaboration of a map of ' restricted susceptibility' in the particular case of slopes that are connected to streams and torrents ( gorges); ( 3) the elaboration of a digital model which relates the altitude to the occurrence probability of those particular precipitation conditions which characterized the Jubaguerra event and ( 4) the combination of the probability model with the ' restricted susceptibility map', to establish ' critical zones' or areas which are more prone to the occurrence of phenomena that have same typology as this one.