Intense rainfall events are a major trigger for flooding and landslides
throughout regions with a Mediterranean climate. They create problems for settlements
and infrastructures built across their paths. Changes in landuse have also been important
in increasing or decreasing the intensity of the flood problem and the mobility of land
surface materials, especially changes in the intensity of grazing on mountain pastures or
historical deforestation and recent reforestation. This paper focuses on the rainfall
thresholds that distinguish common events from rare, hazardous events. During the last
few years, rainstorms of different intensities have occurred in the Central Spanish
Pyrenees, including one of exceptional character. Large, historical debris flows have
been studied, as well as the actual sediment transport in small experimental catchments.
This study shows that during the most frequent events suspended sediment transport is
the common geomorphic process. River bedload is mobilized in river channels several
times per year, while small rock avalanches and channelized debris flows have a return
period of at least 5 years. Hillslope debris flows are triggered by rainfall events with a
25-30 year return period. Reactivation of large, deep mass movements is linked to
rainfalls of around 100 year return period (between 130 and 160 mm in 24 hours).
Catastrophic geomorphic processes occur when precipitation exceeds a 100 year return
period, as was the case of the Biescas campsite disaster. Geomorphic processes
triggered by intense rainfall events have caused major damages and human disasters but
the hazards have been reduced by the introduction of several control measures,
including reforestation, the construction of check-dams, canalization of river segments
and improved flood forecasting
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