Regional prediction of landslides in the Tramuntana Range (Majorca) using probability analysis of intense rainfall

摘要

The Tramuntana Range on the Island of Majorca has suffered numerous, damaging episodes caused by landslides. The historical inventory of these movements shows a clear correlation between landslide occurrence and intense rainfall. Most of the slides have occurred after short intense storms. The slope movements are mainly shallow failures, debris flows, debris slides and soil slips. Regional hazard assessment is carried out by combining the probability of the landslide rainfall threshold with the landslide susceptibility in the area. The correlation of the known dates of historical landslides since 1956 with rainfall data provided by the rain gauges nearest to the location of the slides, reveals that most of the movements take place when maximum 24-hour rainfall values are around 130 mm or above. This is the triggering threshold considered in the present work. A statistical analysis of intense rainfall has been carried out in the area, using the Gumbel probability distribution function, which allows us to obtain the maximum 24-hour rainfall values that are to be expected for return periods of 5, 10, 25 and 100 years and to locate the areas where the rainfall threshold is exceeded. In order to predict medium-term hazard, we have considered the probability of the occurrence of episodes of intense rainfall, exceeding 130 mm in 24 hours, for a period of 25 years. Prior to this work, a susceptibility study was made on the area, based on the inventory of the movements and the analysis of the different conditioning factors of the instability. A susceptibility map at 1 :25,000 scale was designed which provides information on the spatial probability of occurrence of the landslides predicted in the area. As our objective was to determine the degree of hazard in the study area, we performed a spatial-temporal superposition on the general map of susceptibility and the probability map of intense rainfall exceeding 130 mm, for a return period of 25 years. This methodology allows us to obtain a hazard map which constitutes a feasible tool for regional hazard assessment.