Triggering Debris Flows Research Articles

Probabilistic rainfall thresholds for triggering debris flows in a human-modified landscape

In the Carrara Marble Basin (CMB; Apuan Alps, Italy) quarrying has accumulated widespread and thick quarry waste, lying on steep slopes and invading valley bottoms. The Apuan Alps are one of the rainiest areas in Italy and rainstorms often cause landslides and debris flows. The stability conditions of quarry waste are difficult to assess, owing to its textural, geotechnical and hydrogeological variability. Therefore, empirical rainfall thresholds may be effective in forecasting the possible occurrence of debris flows in the CMB. Three types of thresholds were defined for three rain gauges of the CMB and for the whole area: rainfall intensity–rainfall duration (ID), cumulated event rainfall–rainfall duration (ED), and cumulated event rainfall normalized by the mean annual precipitation–rainfall intensity (EMAPI).The rainfall events recorded from 1950 to 2005 was analyzed and compared with the occurrence of debris flows involving the quarry waste. They were classified in events that triggered one or more debris flows and events that did not trigger debris flows. This dataset was fitted using the logistic regression method that allows us to define a set of thresholds, corresponding to different probabilities of failure (from 10% to 90%) and therefore to different warning levels. The performance of the logistic regression in defining probabilistic thresholds was evaluated by means of contingency tables, skill scores and receiver operating characteristic (ROC) analysis. These analyses indicate that the predictive capability of the three types of threshold is acceptable for each rain gauge and for the whole CMB. The best compromise between the number of correct debris flow predictions and the number of wrong predictions is obtained for the 40% probability thresholds. The results obtained can be tested in an experimental debris flows forecasting system based on rainfall thresholds, and could have implications for the debris flow hazard and risk assessment in the CMB.

Effect of antecedent-hydrological conditions on rainfall triggering of debris flows in ash-fall pyroclastic mantled slopes of Campania (southern Italy)

Mountainous areas surrounding the Campanian Plain and the Somma-Vesuvius volcano (southern Italy) are among the most risky areas of Italy due to the repeated occurrence of rainfall-induced debris flows along ash-fall pyroclastic soil-mantled slopes. In this geomorphological framework, rainfall patterns, hydrological processes taking place within multi-layered ash-fall pyroclastic deposits and soil antecedent moisture status are the principal factors to be taken into account to assess triggering rainfall conditions and the related hazard. This paper presents the outcomes of an experimental study based on integrated analyses consisting of the reconstruction of physical models of landslides, in situ hydrological monitoring, and hydrological and slope stability modeling, carried out on four representative source areas of debris flows that occurred in May 1998 in the Sarno Mountain Range. The hydrological monitoring was carried out during 2011 using nests of tensiometers and Watermark pressure head sensors and also through a rainfall and air temperature recording station. Time series of measured pressure head were used to calibrate a hydrological numerical model of the pyroclastic soil mantle for 2011, which was re-run for a 12-year period beginning in 2000, given the availability of rainfall and air temperature monitoring data. Such an approach allowed us to reconstruct the regime of pressure head at a daily time scale for a long period, which is representative of about 11 hydrologic years with different meteorological conditions. Based on this simulated time series, average winter and summer hydrological conditions were chosen to carry out hydrological and stability modeling of sample slopes and to identify Intensity-Duration rainfall thresholds by a deterministic approach. Among principal results, the opposing winter and summer antecedent pressure head (soil moisture) conditions were found to exert a significant control on intensity and duration of rainfall triggering events. Going from winter to summer conditions requires a strong increase of intensity and/or duration to induce landslides. The results identify an approach to account for different hazard conditions related to seasonality of hydrological processes inside the ash-fall pyroclastic soil mantle. Moreover, they highlight another important factor of uncertainty that potentially affects rainfall thresholds triggering shallow landslides reconstructed by empirical approaches.

Critical rainfall thresholds for debris flows in Sanyanyu, Zhouqu County, Gansu Province, China

Critical rainfall thresholds can be the key to ensuring effective debris-flow forecasting. They are significant for the study of the trigger mechanisms of debris flows, for the forecast of the characteristics of future events, and for the development of engineering guidance for mitigation. Using a hydrological approach, we first calculated the flood peak discharge at different frequencies and then the corresponding rainfall thresholds for the initiation of different scales of debris flows in Zhouqu County, China. This was followed by the establishment of a functional relation between the intensity and duration of rainfall events that trigger debris flows of different warning levels in two initial soil conditions (dry and moist). This, in turn, yielded four warning levels and two preliminary warning levels. For the two early soil conditions (dry and moist) in the Sanyanyu Gully of Zhouqu County, the Level I (red) warning values for rainfall triggering debris flow are 56 and 51 mm h −1 , respectively; the Level II (orange) warning values are 41 and 38 mm h −1 , respectively; the Level III (yellow) warning values are 32 and 30 mm h −1 , respectively; and the Level IV (blue) warning values are 24 and 22 mm h −1 , respectively. The Level V preliminary warning values are 17 and 16 mm h −1 , respectively; and the Level VI preliminary warning values are 10 and 9.5 mm h −1 , respectively. The rainfall intensity and duration were found to exhibit a power function relation, I = αD β , where the values of α and β vary with the warning levels. Rainfall events capable of triggering debris flows in the new thresholds and intensity–duration relations presented here can be used for forecasting purposes and in operational geohazard warning systems. These research results also provide a scientific basis for regional hazard mitigation and reduction in Zhouqu County.

Spatial features of debris flows and their rainfall thresholds in the Wenchuan earthquake-affected area

The 2008 Wenchuan earthquake greatly altered the thresholds for rainfall-triggered debris flows within the affected area. Debris flows were widely and densely distributed, and they exhibited a range of differing local rainfall thresholds. This study looked at 518 debris flow events that occurred post-earthquake in order to analyze their spatial characteristics. The duration D (in h) and the average rainfall intensity I (in mm/h) that triggered the debris flows were determined for 252 of these events in order to analyze the spatial features of rainfall thresholds. Results show that 49 % of debris flows occurred in the highest-intensity seismic zone, 58 % occurred within 10 km of active faults, and 49 % occurred in areas with humid climate. Rainfall thresholds in these three regions were persistently lower than others. Moreover, debris flows were most frequent in watersheds smaller than 5 km2, and rainfall thresholds tended to decrease with a decrease in watershed size. Given the abundant loose materials available throughout the study area, 11 extreme debris flow-prone sub-regions were selected to illustrate the spatial features of rainfall thresholds in relation to local climate conditions. The lowest and highest I–D thresholds in the sub-regions examined were I = 5.94D−0.70 (2 < D < 53) and I = 21.4D−0.58 (3 < D < 50), respectively. The lowest and mean rainfall intensities needed to trigger debris flows were power-related with the local maximum 1- and 24-h rainfall. By normalizing the rainfall intensity (I) by mean annual precipitation (MAP), the IMAP–D thresholds were determined. Normalized results showed that the lowest and highest IMAP–D thresholds were IMAP = 0.0034D−0.55 (2 < D < 53) and IMAP = 0.0090D−0.40 (3 < D < 51), respectively. Such results are useful for debris flow forecasting based on empirical rainfall thresholds and have implications for hazard and risk assessment in this region.

Analysis of rainfall preceding debris flows on the Smědavská hora Mt., Jizerské hory Mts., Czech Republic

In August 2010, extreme rainfall affected the north of the Czech Republic and caused regional floods and landslides. Three torrential debris flows originated in the Jizerske hory Mts., close to Bilý Potok on the north slope of the Smědavska hora Mt. The rainfall situation which triggered the debris flow was analyzed and compared with the rainfall situation in 1958 when a debris flow occurred in the same area. The rainfall data were obtained from rain gauges of the Czech Hydrometeorological Institute. Four rain gauges were chosen close to the Smědavska hora Mt. with data of daily amounts from 1983 to 2013 and 10-min intensity or hourly amounts from the specific period. The data from 1958 were available from three different rain gauges (only daily amounts). The data series were not complete so linear regression was applied to interpolate them. A number of analyses were carried out including daily rainfall, 2-day/3-day moving values, antecedent precipitation index (API) of 5/10/30 days, 10-min intensity, and hourly amounts, and the trigger factor of the debris flow in the study area was also investigated. It was determined that for the triggering of debris flows, both high API values as well as high-intensity short-duration rainfall is needed. It was documented that in cases of solely high API indices or high-intensity short-duration rainfalls, no debris flows were initiated.

Rainfall intensity–duration conditions for mass movements in Taiwan

Mass movements caused by rainfall events in Taiwan are analyzed during a 7-year period from 2006 to 2012. Data from the Taiwan Soil and Water Conservation Bureau reports were compiled for 263 mass movement events, including 156 landslides, 91 debris flows, and 16 events with both landslides and debris flows. Rainfall totals for each site location were obtained from interpolated rain gauge data. The rainfall intensity–duration (I–D) relationship was examined to establish a rainfall threshold for mass movements using random sampling: I = 18.10(±2.67)D −0.17(±0.04), where I is mean rainfall intensity (mm/h) and D is the time (h) between the beginning of a rainfall event and the resulting mass movement. Significant differences were found between rainfall intensities and thresholds for landslides and debris flows. For short-duration rainfall events, higher mean rainfall intensities were required to trigger debris flows. In contrast, for long-duration rainfall events, similar mean rainfall intensities triggered both landslides and debris flows. Mean rainfall intensity was rescaled by mean annual precipitation (MAP) to define a new threshold: I MAP = 0.0060(±0.0009)D −0.17(±0.04), where I MAP is rescaled rainfall intensity and MAP is the minimum for mountainous areas in Taiwan (3000 mm). Although the I–D threshold for Taiwan is high, the I MAP –D threshold for Taiwan tends to be low relative to other areas around the world. Our results indicate that Taiwan is highly prone to rainfall-induced mass movements. This study also shows that most mass movements occur in high rainfall-intensity periods, but some events occur before or after the rainfall peak. Both antecedent and peak rainfall play important roles in triggering landslides, whereas debris flow occurrence is more related to peak rainfall than antecedent rainfall.

Rainfall thresholds for the occurrence of debris flows in the Jiangjia Gully, Yunnan Province, China

The Jiangjia Gully (JJG), Yunnan Province, China, is one of several debris flow gullies that characterize the topography of mountainous areas in China. Rainfall-triggered debris flows are frequent in the JJG. In order to model and predict these debris flow occurrences, we analyzed precipitation parameters (including intensity–duration thresholds and rainfall event–duration thresholds) that triggered past debris flow events in the JJG. Since the JJG is unique, any model used for predicting debris flows in this gully must be developed using data from only this gully. Furthermore, the effects of normalized precipitation and antecedent precipitation on debris flows in the JJG were considered. Debris flow data and precipitation data were compiled from 47 debris flow events in 1993, 1994, 1998–2001, 2004 and 2006 in JJG. All of these debris flows were triggered following a precipitation event with a duration of 6h or less, with most of the precipitation events exhibiting a duration of 3 to 6h. Only three rainfall events with a duration of less than 1h and average intensity between 1.0 and 42.0mm/h produced debris flows. The 90%, 70% and 50% probability curves for debris flow events were then constructed. Intensity–duration (I–D) and rainfall event–duration (RE–D) thresholds were then used for debris flow modeling based on preliminary tests and comparisons. Antecedent precipitation was found not to be a significant factor in triggering debris flows in the JJG; however, intraday precipitation played a significant role. Normalized precipitation threshold curves from adjacent areas were not well-correlated with the patterns observed in the JJG. Determination of unique thresholds for each gully is necessary for developing an effective prediction system.

Estimation of debris flow triggering rainfall: Influence of rain gauge density and interpolation methods

The forecast of debris flow occurrence relies mainly on empirical rainfall intensity–duration thresholds, which are based on rain gauge observations. This work focuses on the effect of rainfall estimation uncertainty on the estimation of debris flow triggering rainfall events and on the identification of rainfall thresholds for debris flow occurrence. Specifically, the influence of rain gauge network density and the interpolation method on the estimation of debris flow triggering rainfall is investigated. These questions are examined using high-resolution, carefully corrected radar data to represent space–time patterns of true precipitation at the debris flow initiation points and in the surrounding area. Radar rainfall fields are sampled by simulated rain gauge networks, stochastically generated with varying rain gauge densities. Based on these networks, rainfall is estimated by using three rainfall interpolation methods: nearest neighbor (NN), inverse distance weighting (IDW) and ordinary kriging (OK). Results show that NN provides the estimates with bias smaller than IDW and OK but larger estimation variance. Overall, decrease in gauge density leads to increased underestimation and increased estimation variance of debris flow triggering rainfall. Rainfall estimation error leads to large underestimation of the intensity–duration thresholds. However, comparison of results shows that no particular benefit in intensity–duration threshold estimation is obtained by using approaches that are more complex than the NN method.

Dominant cloud microphysical processes of a torrential rainfall event in Sichuan, China

High-resolution numerical simulation data of a rainstorm triggering debris flow in Sichuan Province of China simulated by the Weather Research and Forecasting (WRF) Model were used to study the dominant cloud microphysical processes of the torrential rainfall. The results showed that: (1) In the strong precipitation period, particle sizes of all hydrometeors increased, and mean-mass diameters of graupel increased the most significantly, as compared with those in the weak precipitation period; (2) The terminal velocity of raindrops was the strongest among all hydrometeors, followed by graupel’s, which was much smaller than that of raindrops. Differences between various hydrometeors’ terminal velocities in the strong precipitation period were larger than those in the weak precipitation period, which favored relative motion, collection interaction and transformation between the particles. Absolute terminal velocity values of raindrops and graupel were significantly greater than those of air upward velocity, and the stronger the precipitation was, the greater the differences between them were; (3) The orders of magnitudes of the various hydrometeors’ sources and sinks in the strong precipitation period were larger than those in the weak precipitation period, causing a difference in the intensity of precipitation. Water vapor, cloud water, raindrops, graupel and their exchange processes played a major role in the production of the torrential rainfall, and there were two main processes via which raindrops were generated: abundant water vapor condensed into cloud water and, on the one hand, accretion of cloud water by rain water formed rain water, while on the other hand, accretion of cloud water by graupel formed graupel, and then the melting of graupel formed rain water.

Sensitivity of the initiation of debris flow to initial soil moisture

The initiation of debris flows is commonly attributed either to fluidization as a result of rainfall-induced landslides or to gully erosion induced by concentrated runoffs. A series of flume tests have been performed to show how the initial soil moisture influences the initiation of debris flows. At the start of each experiment, surface runoff was generated over loose granular deposits, triggering debris flows. These experimental debris flows enacted different scenarios according to the small variations among the initial soil moistures. In the loose granular deposits with initial soil moistures ranging from 1 to 5 %, most runoff water could infiltrate and trigger a landslide, which accelerated within 1 s to speed over 1 ms−1 and then transformed into a debris flow. In the same soil deposits with initial moistures >5 or <1 %, the debris flow was initiated by slow gully erosion with episodic events of damming and breaching due to small-scale landslides occurring on the side-slopes of the erosion valley. The slope failures were not triggered by positive pore pressure but by a decrease in suction due to the wetting of the soil. This suction decrease in initially unsaturated slopes explains why the transformation of these slope failures into debris flows are due not only to an increase of pore pressure leading to soil liquefaction, which is one of the expected triggering mechanisms, but also to a loss of the cohesive strength of the soil.

Experimental study on the triggering mechanisms and kinematic properties of large debris flows in Wenjia Gully

Debris flows are typically caused by natural terrain landslides triggered by intense rainfalls. If an incoming mountain torrent flows along sloping channels at high velocity, huge amounts of sediment (from landslide dams and eroded channel beds) will be entrained into the flows to form debris flows. It is likely that large debris flows are due to the failure of many landslide dams of different scales (due to bank slides or collapses), bed erosion, and solid transport. The catastrophic debris flows that occurred in Wenjia Gully (Wenchuan Earthquake Area), China on August 13, 2010 (two years after the mega earthquake), were caused by intense rainfall and the serious erosion of sloping channels. In the wake of the incident, experimental tests were conducted to better understand the process of sediment erosion and entrainment on the channel bed and the formation of debris flows. The results show that the bed erosion, bank collapses and channel widening caused by erosion accounted for the triggering and scale amplification of downstream debris flows in the Wenjia Gully event. This study illustrates how the hazardous process of natural debris flows can begin several kilometers upstream, and how such a complex cascade of geomorphic events (failure of landslide dams and erosion of the sloping bed) can lead to catastrophic discharges. Neglecting recognition of these hazardous geomorphic and hydrodynamic processes may result in high cost.

Topographical factors in the formation of gully-type debris flows in Longxi River catchment, Sichuan, China

There are three groups of factors: topography, geology and hydrology which have influence on the formation of gully-type debris flows. In this paper, a single representative factor (T-factor) for the topography is proposed, which can be used to define threshold values for debris flow formation. This study was carried out in the Longxi River catchment, located in the 2008 Wenchuan earthquake hit area. During a heavy rainfall event on August 13, 2010, 34 gully-type debris flows were triggered. In some catchments, no debris flows were triggered even though these catchments were in the vicinity of gullies with triggered debris flows. The formation mechanism of the triggered debris flows is the runoff-induced mechanism. The influence of the topography on the formation of debris flows in gullies with almost identical hydrological and geological conditions was isolated and analyzed. A new T-factor is proposed in this study as a topographical indicator which is a combination of the form factor, the average gradient of the stream, the catchment size, and the percentage of the catchment area with a slope gradient sensitive to trigger debris flows. Higher T-factor values are generally related to higher probabilities of debris flow formation. The T-factor can be used to predict the formation of debris flows in the sub-catchments of the Longchi River catchment. The T-factor was successfully validated in debris flow gullies with the same initiation mechanism in the Cida River catchment, Sichuan, China.

Radar rainfall estimation for the identification of debris-flow occurrence thresholds

Summary This work aims to evaluate the potential benefits and limitations of radar rainfall estimates for the identification of debris flow occurrence rainfall thresholds. Observations from a C-band weather radar and raingauge data are analyzed for seven convective rainfall events that triggered 117 debris flows in the Upper Adige river basin (Eastern Italian Alps). Four radar rainfall scenarios characterized by incrementally increasing accuracy are used for the estimation of rainfall intensity–duration thresholds. Error sources considered in the radar correction chain include beam blockage, attenuation and vertical profile of reflectivity. The impact of rainfall accuracy on the estimation of the intensity–duration thresholds is analyzed by comparing the thresholds derived from the various radar rainfall scenarios, using the rainfall estimates obtained from the application of the complete correction algorithm as a reference scenario. Results show that the application of the complete correction algorithm improves significantly the accuracy of radar rainfall estimates: Fractional Standard Error is decreased by 20%, Correlation Coefficient is increased by 24% relative to uncorrected data. Correction for atmospheric attenuation is the most important step in the correction chain. The use of uncorrected radar estimates leads to substantially underestimated thresholds with respect to the reference scenario; adjusting radar data for bias is not sufficient to overcome this problem. Radar rainfall estimates which are corrected but not adjusted with raingauge data are able to provide intensity–duration thresholds which are almost indistinguishable from the reference scenario. The derivation of the radar-based threshold is shown to be very sensitive to spatial location errors of rainfall and debris flows. Raingauge-based thresholds are severely underestimated with respect to the reference scenario. This clearly demonstrates the severity of the raingauge-based estimation problem for the derivation of debris flow triggering rainfall and highlights the benefits of using weather radar observations, at least for the case of short duration convective storms.

A numerical model for the simulation of debris flow triggering, propagation and arrest

In this paper, it is described the development and the assessment of a 1D numerical procedure for the simulation of debris flow phenomena. The procedure focuses on: (1) the rainfall triggering, and the effects induced on slope stability by both rainfall infiltration and groundwater dynamics; (2) the possible inception of debris flows during the propagation phenomenon itself, due to the actions exerted on the slope by the already triggered flowing masses; (3) the propagation phenomenon over complex topographies; (4) the non-Newtonian internal dissipative processes that develop within the sediment–water mixture; (5) the effects induced by the evolution of the boundaries where the propagation phenomenon occurs; (6) the run-out and arrest phenomena. In order to show the performance and capabilities of the model, the results of its application to an analytic test and to laboratory experimental tests are first analyzed, and finally, the application to a plausible debris flow scenario, taken from a real case study, is discussed.

Linking Land Cover Changes in the Sub‐Alpine and Montane Belts to Changes in a Torrential River

AbstractChannel changes are the consequence of changes in sediment yield from the slopes and in the connectivity between slopes and channels because of distinct land use and climate impacts. In this study, we investigated the characteristics and evolution of a short reach in the headwater of the Ijuez River, central–southern Pyrenees. Assessment of a series of sedimentary and geomorphic structures confirmed major changes to the valley bottom, mainly related to changes in the intensity of human activity. The oldest sedimentary structure is a terrace level located 3 to 4 m above the current alluvial plain. General deforestation, overgrazing and recurring fires in the montane belt (1100–1600 m a.s.l.) have led to increased soil erosion and connectivity, and to the triggering of debris flows that have been deposited on the fluvial terrace. Woody fragments from within the debris flows were dated using accelerator mass spectrometry 14C radiocarbon techniques (AMS), yielding ages between 100 and 115 cal years bp, which coincides with the period of maximum deforestation and human density in the Pyrenees. Depopulation and farmland abandonment since the beginning of the 20th century has resulted in generalized natural and artificial reforestation, a shrinkage of the eroded areas and a decline in connectivity between slopes and the channel. The most important consequence has been channel incision and narrowing, and the development of a sediment armour layer. Active sediment transport is continuing, although there has been a decrease in sediment yield from the slopes. Copyright © 2014 John Wiley &amp; Sons, Ltd.

Impact of uncertainty in rainfall estimation on the identification of rainfall thresholds for debris flow occurrence

Estimation of rainfall intensity–duration thresholds, used for the identification of debris flow/landslide triggering rainfall events, has been traditionally based on raingauge observations. The main drawback of using information from gauges is that the measurement stations are usually located far away from the debris flow initiation areas. In complex terrain where debris flows take place, the spatial variability of rainfall can be very high and this translates in large uncertainty of raingauge-based estimates of debris flow triggering rainfall. This work focuses on the assessment of the impact of rainfall estimation uncertainty on identification and use of rainfall thresholds for debris flow occurrence. The Upper Adige River basin, Northern Italy, is the area of study. A detailed database of more than 400 identified debris flow initiation points during the period 2000–2010 and a raingauge network of 100 stations comprise the database used for this work. The methodology examines the intensity–duration thresholds derived from a set of raingauges that are assumed to be located at debris flow initiation points (DFRs) and an equivalent set of raingauges assumed to have the role of the closest (to debris flow) available measurement (MRs). A set of reference rainfall thresholds is used to identify the rainfall events at DFRs that “triggered” debris flows (i.e. exceed the threshold). For these same events, the corresponding rainfall thresholds are derived from MR observations. Comparison between the rainfall thresholds derived from DFRs and MRs revealed that uncertainty in rainfall estimation has a major impact on estimated intensity–duration thresholds. Specifically, the results showed that thresholds estimated from MR observations are consistently underestimated. Evaluation of the estimated thresholds for warning procedures showed that while detection is high, the main issue is the high false alarm ratio, which limits the overall accuracy of the procedure. Overall performance on debris flow prediction was shown to be good for low rainfall thresholds and poor for high rainfall thresholds examined. Lastly, it was found that filtering out rainfall events with duration <12h may improve bias in estimated thresholds and performance for high rainfall thresholds.

Empirical atmospheric thresholds for debris flows and flash floods in the southern French Alps

Abstract. Debris flows and flash floods are often preceded by intense, convective rainfall. The establishment of reliable rainfall thresholds is an important component for quantitative hazard and risk assessment, and for the development of an early warning system. Traditional empirical thresholds based on peak intensity, duration and antecedent rainfall can be difficult to verify due to the localized character of the rainfall and the absence of weather radar or sufficiently dense rain gauge networks in mountainous regions. However, convective rainfall can be strongly linked to regional atmospheric patterns and profiles. There is potential to employ this in empirical threshold analysis. This work develops a methodology to determine robust thresholds for flash floods and debris flows utilizing regional atmospheric conditions derived from ECMWF ERA-Interim reanalysis data, comparing the results with rain-gauge-derived thresholds. The method includes selecting the appropriate atmospheric indicators, categorizing the potential thresholds, determining and testing the thresholds. The method is tested in the Ubaye Valley in the southern French Alps (548 km2), which is known to have localized convection triggered debris flows and flash floods. This paper shows that instability of the atmosphere and specific humidity at 700 hPa are the most important atmospheric indicators for debris flows and flash floods in the study area. Furthermore, this paper demonstrates that atmospheric reanalysis data are an important asset, and could replace rainfall measurements in empirical exceedance thresholds for debris flows and flash floods.

Debris flow activity related to recent climate conditions in the French Alps: A regional investigation

The primary objective of this study was to document the relationships between current climatic conditions and debris flow activity in the French Alps based on a large historical database of debris flow events covering 35years up to the present. The French Alps are composed of two contrasting geographic areas so two debris flow regions with different activity patterns were defined. For the period 1970–2005, the database contains 565 debris flow events in 87 catchments in the northern part of the French Alps, and in 150 catchments in the southern part. Possible links between debris flow and climate were investigated using two different approaches. The first approach was determining the rainfall thresholds responsible for triggering debris flow events by analysing the links between the intensity and the duration of rainfall events. The second approach used a probabilistic logit model to explore the links between the triggering of debris flow events and temperature and precipitation during the active debris flow period to identify inter-annual variability. Reanalysis data were used to document climate conditions in the two study areas.According to the results, in 80% of all debris flow events, precipitation was recorded during the three days preceding the event. However, in most cases, the quantity of precipitation associated with triggering of the debris flow was very low. Total precipitation exceeded 10mm in only 30% of all cases. We attribute this to the convective nature of summer precipitation, which is quite difficult to model. Probabilistic analysis of the debris flow inventory in the two regions revealed that different parameters were responsible for changes in annual debris-flow activity. In the northern part of the French Alps, the number of rainy days and the maximum daily temperature affected debris flow, while in the southern part the only significant factor was mean daily temperature during the period of debris flow activity (May–October). Model scores had an accuracy of 75% and 70% in the northern and southern Alps, respectively. Our observations revealed that the increase in the above parameters has influenced changes in debris flow activity in both regions, where the number of debris flow events has doubled over the last 35years.

Analyses of storm event triggering debris flows in Nagiso-Town, Nagano Prefecture, on July 9, 2014

Analyses of storm event triggering debris flows in Nagiso-Town, Nagano Prefecture, on July 9, 2014

Possible impacts of climate change on debris-flow activity in the Swiss Alps

This study uses a long dataset of past debris flows from eight high-elevation catchments in the Swiss Alps for which triggering conditions since AD 1864 have been reconstructed. The torrents under investigation have unlimited sediment supply and the triggering of debris flows is thus mainly controlled by climatic factors. Based on point-based downscaled climate scenarios for meteorological stations located next to the catchments and for the periods 2001–2050 and 2051–2100, we study the evolution of temperature and rainfall above specific thresholds (10, 20, 30, 40 and 50 mm) and durations (1, 2 or 3 days). We conclude that the drier conditions in future summers and the wetting of springs, falls and early winters are likely to have significant impacts on the behavior of debris flows. Based on the current understanding of debris-flow systems and their reaction to rainfall inputs, one might expect only slight changes in the overall frequency of events by the mid-21st century, but possibly an increase in the overall magnitude of debris flows due to larger volumes of sediment delivered to the channels and an increase in extreme precipitation events. In the second half of the 21st century, the number of days with conditions favorable for the release of debris flows will likely decrease, especially in summer. The anticipated increase of rainfall during the shoulder seasons (March, April, November, December) is not expected to compensate for the decrease in future heavy summer rainfall over 2 or 3 days.