Rainfall and soil moisture conditions for the triggering of torrential flows at the Rebaixader catchment (Central Pyrenees)

Torrential flows, like debris flows and debris floods, can mobilize large volumes at high velocities in mountainous regions. Therefore, they represent an important erosional process and a significant hazard towards infrastructures and people (sometimes catastrophic).

Monitoring-based analysis is a crucial task to improve the understanding of the mechanisms triggering torrential flows and its propagation, which are necessary to implement early warning systems. The monitoring of triggering conditions generally focusses on rainfall measurements and the characterization of the critical rainfall conditions. However, rainfall data do not provide a complete picture of the physical processes involved. Very few studies include soil moisture and/or pore water pressure measurements to define the hydrologic response at the natural slopes of the catchment. In that respect, this study analyses both rainfall and soil moisture data at a Mediterranean-influenced torrential basin located in Central Pyrenees (the Rebaixader site).

The Rebaixader site has a high torrential activity, with 11 debris flows and 24 debris floods detected since 2009. The temporal distribution of rainfall episodes and torrential flows shows a clear shift between the most frequent rainfall episodes (beginning of June) and torrential flows (mid-July). This suggests that soil moisture conditions, depending on antecedent rainfall and/or snowmelt, affect the triggering of torrential flows. Regarding critical rainfall conditions, a previously published rainfall threshold was updated including total rainfall duration and mean intensity of 2009-2019 rainfalls. On the other hand, measured volumetric water content (VWC) was analysed for triggering and non-triggering rainfall events. Preliminary results show lower VWC increment on wetter soils at the beginning of rainstorms that triggered torrential flows. This indicates that soil saturates with lower rainfall amount if the soil is initially wetter; which subsequently generates higher runoff rate and therefore a higher erosion and transport energy that may trigger torrential flows. In addition, a slight trend was observed when comparing rainfall intensity and soil moisture; generally larger rainfall intensity is necessary to trigger torrential flows when soil is drier.  

The analysis of VWC data was more complicated in contrast to the one of rainfall data, since the time series are shorter (2013-2019) and the physical interpretation is not straightforward. Therefore, additional data are necessary to confirm and define soil moisture thresholds triggering torrential flows.