Abstract
In the research field on landslide hazard assessment for natural risk prediction and mitigation, it is necessary to know the characteristics of the triggering factors, such as rainfall and earthquakes, as well as possible. This work aims to generate and compare the basic information on rainfall events triggering landslides in two areas with different climate and geological settings: the Loja Basin in southern Ecuador and the southern part of the province of Granada in Spain. In addition, this paper gives preliminary insights on the correlation between these rainfall events and major climate cycles affecting each of these study areas. To achieve these objectives, the information on previous studies on these areas was compiled and supplemented to obtain and compare Critical Rainfall Threshold (CRT). Additionally, a seven-month series of accumulated rainfall and mean climate indices were calculated from daily rainfall and monthly climate, respectively. This enabled the correlation between both rainfall and climate cycles. For both study areas, the CRT functions were fitted including the confidence and prediction bounds, and their statistical significance was also assessed. However, to overcome the major difficulties to characterize each landslide event, the rainfall events associated with every landslide are deduced from the spikes showing uncommon return periods cumulative rainfall. Thus, the method used, which has been developed by the authors in previous research, avoids the need to preselect specific rainfall durations for each type of landslide. The information extracted from the findings of this work show that for the wetter area of Ecuador, CRT presents a lower scale factor indicating that lower values of accumulated rainfall are needed to trigger a landslide in this area. This is most likely attributed to the high soil saturation. The separate analysis of the landslide types in the case of southern Granada show very low statistical significance for translational slides, as a low number of data could be identified. However, better fit was obtained for rock falls, complex slides, and the global fit considering all landslide types with R2 values close to one. In the case of the Loja Basin, the ENSO (El Niño Southern Oscillation) cycle shows a moderate positive correlation with accumulated rainfall in the wettest period, while for the case of the south of the province of Granada, a positive correlation was found between the NAO (North Atlantic Oscillation) and the WeMO (Western Mediterranean Oscillation) climate time series and the accumulated rainfall. This correlation is highlighted when the aggregation (NAO + WeMO) of both climate indices is considered, reaching a Pearson coefficient of –0.55, and exceeding the average of the negative values of this combined index with significant rates in the hydrological years showing a higher number of documented landslides.
Highlights
Landslides are considered to be one of the most serious hydrological hazards, producing isolated or catastrophic events that result in costly damage and high rates of causalities
In the case of the Loja Basin, the ENSO (El Niño Southern Oscillation) cycle shows a moderate positive correlation with accumulated rainfall in the wettest period, while for the case of the south of the province of Granada, a positive correlation was found between the NAO
The method used in this paper is of the second type, where empirical rainfall thresholds are defined by exploring the rainfall events associated with the occurrence of landslides
Summary
Landslides are considered to be one of the most serious hydrological hazards, producing isolated or catastrophic events that result in costly damage and high rates of causalities Quantitative reviews on their effects at different territorial scales can be found in the literature (e.g., [1,2,3,4,5,6]). Daily rainfall data were provided short-term, and the date of occurrence of the landslides can be extracted from literary sources such as the press, libraries, published scientific papers, and books The precision of these models depends heavily on the quantity of data available to characterize the hydrological scenarios producing landslides. After collecting the necessary data, the empirically-based models are expressed by the correlation of precipitation measurements (accumulated rainfall, duration, and intensity) related to individual or multiple rainfall events leading (or not) to landsliding. The CRTs have been obtained by combining the parameters of rainfall event duration and a measurement of cumulative rainfall
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