The stability and erosion of natural and man-made slopes is influenced by soil-vegetation-atmosphere interactions and the thermo-hydro-mechanical slope conditions. Understanding such interactions at the source of slope mass-wasting is important to develop land-use planning strategy and to promote environmentally adapted mitigation strategies, such as the use of vegetation to stabilize slopes and control erosion. Monitoring is essential for calibrating and validating models and for better comprehending the physical mechanisms of soil-vegetation-atmosphere interactions. We approached this complex problem by means of an experimental work in a full-scale monitored embankment, which is divided into four instrumented partitions. These partitions are North or South-faced and present a bare and vegetation cover at each orientation. Our main findings show that vegetation enhances rainfall infiltration and decreases runoff, which reduces slope stability and surficial erosion, while plant transpiration induces higher suctions and hence slope stability. Concerning thermal aspects, vegetation reduces the incidence of net solar radiation and consequently heat flux. Thus, daily temperature fluctuations and evaporation decreases. However, the effect of vegetation in the development of dryer soil conditions is more significant than the orientation effect, presenting higher drying rates and states at the North-vegetated slope compared to the South-bare slope.

Slope mass-wasting like shallow slides are mostly triggered by climate effects, such as rainfall, and soil–vegetation–atmosphere (SVA) interactions play a key role. SVA interactions are studied by a full-scale embankment with different orientations (North and South) and vegetation covers (bare and vegetated) in the framework of the prediction of climate change effects on slope stability in the Pyrenees. A clayey sand from the Llobregat river delta was used for the construction of the embankment and laboratory tests showed the importance of suction on the strength and hydraulic conductivity. Sixty sensors, which are mostly installed at the upper soil layer of the embankment, registered 122 variables at four vertical profiles and the meteorological station with a 5 min scan rate. Regarding temperature, daily temperature fluctuation at the shallow soil layer disappeared at a depth of about 0.5 m. There was great influence of orientation with much higher values at the South-facing slope (up to 55 °C at −1 cm depth) due to solar radiation. Regarding rainfall infiltration, only long duration rainfalls produced an important increase of soil moisture and pore water pressure, while short duration rainfalls did not trigger significant variations. However, these changes mostly affected the surface soil layer and decreased with depth.

Torrential flows (debris flows and debris floods) are mainly triggered by precipitation and soil hydrological processes. Most early warning systems in torrential catchments are rainfall-based. However, this approach can result in frequent false positives, due to its pure black-box nature, in which soil water conditions are neglected. We aim to contribute to the understanding of the conditions required for triggering torrential flows by considering also in situ measurements of soil water content. Herein, monitoring data of 12 years of rainfall and torrential flow occurrence (2009–2020) and 8 years of soil hydrologic conditions (2013–2020) in the Rebaixader catchment (Central Pyrenees, Spain) are analyzed. The dataset includes more than 1000 rainfall events and 37 torrential flows. First, rainfall thresholds using maximum rainfall intensity (I_max) and mean intensity (I_mean) are defined. For the 2013–2020 dataset, which includes 15 torrential events, the I_mean threshold predicted 2 false negatives and 73 false positives (positive predictive value, PPV, of 15.1%) and the best I_max threshold predicted also 2 false negatives but only 11 false positives (PPV of 54.2%). However, our observations confirmed quantitatively that the lower is the soil moisture the higher is the rainfall intensity to trigger torrential flows. Then, we combined I_max and volumetric water content at 15 and 30 cm depth to define a hydro-meteorological threshold. This latter threshold reduced false negatives to 1 and false positives to 8 and increased the PPV to 63.6%. These results confirm that soil hydrological conditions are key factors for torrential flow triggering and may improve early-warning predictions.

The instrumental monitoring of torrential catchments is a fundamental research task that provides necessary information to improve our understanding of the mechanisms of debris flows. While most monitoring sites include meteorological sensors and analyze the critical rainfall conditions, very few contain soil moisture measurements. In our monitoring site, the Rebaixader catchment, 11 debris flows and 24 debris floods were detected during the last 9 years. Herein, the initiation mechanisms of these torrential flows were analyzed, focusing on the critical rainfall conditions and the soil water dynamics. Comparing the temporal distribution of both rainfall episodes and torrential flows, the Kernel density plots showed maximum values for rainfalls at the beginning of June, while the peak for torrential flows is on July 20. Thus, the antecedent rainfall, and especially the soil moisture conditions, may influence the triggering of torrential flows. In a second step, a new updated rainfall threshold was proposed that included total rainfall duration and mean intensity. The analysis of soil moisture data was more complicated, and no clear trends were observed in the data set. Therefore, additional data have to be recorded in order to quantitatively analyze the role of soil moisture on the triggering of torrential flows and for the definition of thresholds. Some preliminary results show that the soil moisture at the beginning of a rainfall event affects the maximum increase of soil moisture, while a slight trend was visible comparing the initial soil moisture with the necessary rainfall amount to trigger a torrential flow.

Slope-mass wasting like shallow slides and surficial erosion are mostly triggered by climatic actions, where rainfall plays the most important role. The number of extreme weather events such as droughts and intense rainfalls has increased in the past decades due to climate change. Consequently, slope-mass wasting has become in recent years one of the most important environmental problems with many socio-economic repercussions. Slope mass-wasting may be the most dangerous geo-hazard in many mountainous regions and represents as well an important threat in artificial or man-made slopes like infrastructure and transportation embankments.The erosion and stability of slopes depend on the soil-vegetation-atmosphere (SVA) interactions and the thermo-hydro-mechanical soil conditions. Therefore, understanding  the SVA interactions and the processes leading to slope-mass wasting is crucial to promote sustainable, low cost, and environmental-friendly mitigation strategies on potentially unstable slopes, such as the use of vegetation. Moreover, it is necessary for developing a correct land-use planning strategy. In this study, SVA interactions are assessed by a full-scale monitored embankment divided into four partitions with North and South-facing slopes and with bare and vegetated slope covers at each orientation. Monitoring is a fundamental task for understanding the physical mechanisms related to SVA interactions and for calibrating and validating models. The monitoring started in 2017 and includes 60 sensors recording 122 variables every 5 minutes. These devices provide accurate information on the thermo-hydraulic response of bare and vegetated slopes at both North and South orientations. In addition to hydraulic variables like suction and soil moisture, which are measured at several depths, thermal and atmospheric variables are monitored: soil heat flux, soil temperature at different depths, air temperature, rainfall, wind speed/direction, solar radiation, etc. The results show that vegetation has a strong effect on both thermal and hydraulic slope response. On one hand, vegetation increases rainfall infiltration and induces a faster saturation of the soil, which may reduce slope stability (this effect should be counterbalanced with other phenomena not considered in this work, like raindrop impact protection and root soil reinforcement, among others). Such an increase in turn suggests that vegetation decreases runoff and hence reduces slope surficial erosion. On the other hand, vegetation increases in-depth suction by plant transpiration, which may increase soil strength and stability on slopes. Regarding thermal aspects, vegetation strongly reduces the incidence of solar net radiation. As a result, soil heat flux, daily temperature fluctuations and evaporation decrease. In addition, this research shows that North-vegetated slopes develop dryer soil conditions when compared to South-bare slopes. This shows that the vegetation transpiration induces higher soil drying rates than the solar radiation effects on a bare surface with full solar exposition (i.e. southward orientation). Therefore, these results recommend the implementation and maintenance of vegetated slopes as a sustainable solution for preventing soil erosion especially in sparse vegetated or bare areas and in present and forthcoming semi-arid regions.

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.