Réf. Floris & al. 2010 - A

Référence bibliographique complète

FLORIS, M., D’ALPAOS, A., SQUARZONI, C., GENEVOIS, R., MARANI, M. 2010. Recent changes in rainfall characteristics and their influence on thresholds for debris flow triggering in the Dolomitic area of Cortina d’Ampezzo, north-eastern Italian Alps. Natural Hazards and Earth System Sciences, Vol. 10, 571–580.

Abstract: In this paper, the authors examine variations in climate characteristics near the area of Cortina d’Ampezzo (Dolomites, Eastern Italian Alps), with particular reference to the possible implications for debris-flow occurrence. The study area is prone to debris-flow release in response to summer high-intensity short-duration rainfalls and, therefore, it is of the utmost importance to investigate the potential increase in debris-flow triggering rainfall events. The critical rainfall threshold is agreed to be a crucial triggering factor for debris-flows. Data from a monitoring system, placed in a catchment near Cortina (Acquabona), show that debris-flows were triggered by rainfalls with peak rainfall intensities ranging from 4.9 to 17.4 mm/10 min.
The analyses of meteorological data, collected from 1921 to 1994 at several stations in the study area, show a negative trend of annual rainfall, a considerable variation in the monthly rainfall distribution, and an increase in the temperature range, possibly related to global climate changes. Moreover, high-intensity and short-duration rainfall events, derived from data collected from 1990 and 2008, show an increase in exceptional rainfall events. The results obtained in a peak-over-threshold framework, applied to the rainfall data measured at the Faloria rain gauge station from 1990 to 2008, clearly show that the interarrival time of over-threshold events computed for different threshold values decreased in the last decade. This suggests that local climatic changes might produce an increase in the frequency of rainfall events, potentially triggering debris flows in the study area.

Mots-clés
 

Organismes / Contact

• Department of Geosciences, University of Padova, Padova, Italy - mario.floris@unipd.it
• Department IMAGE, University of Padova, Padova, Italy


(1) - Paramètre(s) atmosphérique(s) modifié(s)
(2) - Elément(s) du milieu impacté(s)
(3) - Type(s) d'aléa impacté(s)
(3) - Sous-type(s) d'aléa
Rainfall   Debris flow  

Pays / Zone
Massif / Secteur
Site(s) d'étude
Exposition
Altitude
Période(s) d'observation
Italy Dolomites, Eastern Italian Alps Upper Boite River Valley, Acquabona catchment (near Cortina)     1921-1994
1990-2008

(1) - Modifications des paramètres atmosphériques
Reconstitutions

 

Observations

In the study area, data at the Cortina station between 1938 and 1994 show an increase in the annual mean maximum temperature of 3 °C, whereas the annual mean minimum temperature decreased slightly (by about 1 °C). Around the same period, changes in the amount of total annual rainfall and in the distribution of monthly rainfall can also be observed. At Podestagno, Cortina and San Vito stations, a negative trend in the total annual rainfall is displayed. (...) The distribution of monthly rainfall shows a remarkable variation in the period 1920–2008 at each of the four stations. Since about 1950, an increase in the mean monthly rainfall occurred during the autumn (October and November), while rainfall conditions in the summer (from June to August) remained constant. For Podestagno, Cortina and San Vito stations, a decrease in monthly rainfall from February to May can be observed. The results of this analysis suggest that the study area may have experienced a climate change during the second half of the past century.

Analysis of threshold exceedances:
It clearly emerges that the considered stations display quite different trends over the considered periods, because the number of over-threshold events and the number of exceedances are characterised by important variations among the five considered stations. In the case of Faloria and Podestagno, there is a marked positive trend in the number of both rainfall events and exceedances of the minimum triggering threshold. This indicates that, in the last 19 years, intense rainfall events have become more frequent. At Cortina station, the number of events increases, whereas the number of exceedances seems to remain quite constant throughout the considered period, possibly showing that the intensity of the over-threshold events is not undergoing important changes. Passo Falzarego station shows a fairly constant trend. Villanova station, unlike the other examined rain gauges, shows a slight negative trend in both the number of events and total exceedances.

All of the threshold exceedances observed in the considered period (1990–2008) occurred between May and October. Most events occur during the summer period, in particular, June, July, and August, whereas critical events very rarely occur in May and October. The distribution of exceedances is similar for Faloria, Cortina and Falzarego stations, with the maximum value in July, while for Podestagno station the maximum is in August. This shows, once again, the local variability of extreme rainfall event occurrence in the study area.

The analysis carried out in a Peak-Over-Threshold (POT) framework clearly shows that the interarrival time (1/λ) increases for increasing values of the considered threshold. Analogous analyses were carried out by considering two distinct periods, namely 1990–1998 and 1999–2008, to investigate possible variations in the frequency of over-threshold events, for instance related to climatic changes. It clearly emerges that, for every considered threshold value, the number of average annual over-threshold events increased in the second decade. In fact, the number of over-threshold events that characterises the period 1999–2008 is always greater than the number of over-threshold events in the period 1990–1998, showing that climatic changes could produce an increase in the number events potentially triggering debris flows. For example, for the critical threshold of 4.9 mm/10 min, the average annual number of over-threshold events increases from 2.8 in the period 1990–1998 to 3.9 in the period 1999–2008. The authors also performed the Wilcoxon rank sum test (Wilcoxon, 1945), which is equivalent to the Mann-Whitney U test (Mann and Whitney, 1947), a non-parametric test for assessing whether two independent samples of observations come from the same distribution. The null hypothesis is that the average annual number of over-threshold events obtained for the different thresholds in the period 1990–1998, λ1, and the average annual number of over-threshold events in the period 1999–2008, λ2, come from distributions with equal medians. The test shows that the null hypothesis can be assumed to be true with a probability less than 0.1, emphasising that the climatic changes observed at Faloria station produce overthreshold events characterised by different probability distributions.

Modélisations

 

Hypothèses

 


Informations complémentaires (données utilisées, méthode, scénarios, etc.)

This paper aims at examining variations in climate characteristics in the study area, with reference to the possible implications for debris-flow occurrence on a local scale. In the study area, rainfall intensity and debris-flow discharge are monitored in the experimental basin of Acquabona (Cortina d’Ampezzo, Italy), as described in detail by Galgaro et al. (2005). (...) The available historical data collected at all of the rain gauges within the area were considered. The objective of this study is to investigate the relationship between the variation in the frequency of rainfall events that may trigger debris flows and the changes in regional climatic and meteorological conditions. Such an analysis aims at determining whether the conditions favourable for triggering of debris flows are currently increasing or decreasing in the study area.

Climate:
To investigate, in detail, the main climatic features of the Cortina d’Ampezzo area, the authors considered the historical daily rainfall and annual temperature data, collected at four meteorological stations of the regional hydrological service, namely Podestagno, Cortina, Passo Falzarego and San Vito, located in the study area. The available rainfall data cover 1920 to 2008, while the temperature data have been recorded since 1938.

Debris-flow discharge and triggering rainfall threshold:
Rainfall data recorded at the Acquabona (Cortina d’Ampezzo) debris-flow monitoring system (Tecca et al., 2003) were analysed to assess the minimum rainfall triggering threshold for debris flow occurrence, as the Acquabona basin, due to its geomorphological, lithological and hydrological characteristics, can be considered as representative of the whole study area. The rain gauge is installed in the debris-flow initiation area, and sampled rainfall at 0.20 s (5 Hz). Rainfall data recorded from 1997 to 2001 were considered. During this period, seven debris-flow events occurred. The following variables were selected for the analysis of rainstorms and discharges in the Acquabona Basin: total rainfall, maximum rainfall intensity (over 10 min), and rainfall duration.

At the Acquabona catchment (Cortina d’Ampezzo), debris flows were generally triggered by rainfalls of high intensity and durations of less than one hour. The total amount and duration are considered from the beginning of the rainfall until the onset of the debris flow. In Acquabona, antecedent rainfall does not represent a significant factor for debris-flow occurrence because of the high permeability of the coarse debris. The authors, therefore, focused their analysis on single bursts, defined as continuous periods having rainfall intensities above 0.5 mm/5 min and containing at least one peak exceeding a second (and higher) intensity threshold. (...) Although thle limited number of cases does not allow to define a statisticallyvalidated triggering threshold for debris-flow occurrence, the data so far collected emphasize that debris flows do not occur with rainfall intensity below 4.9 mm over 10min, which represent the minimum triggering threshold.

Analysis of threshold exceedances:
From 1990, the regional meteorological network has been updated; the current rainfall dataset is composed of data collected with a temporal resolution of five minutes. Five automatic rain gauges, namely Faloria, Podestagno, Cortina, Passo Falzarego and Villanova, were considered in analysing the occurrence of rainfall events displaying characteristics similar to those of the events that triggered debris flows. According to the observations reported in the previous section, the rainfall events with maximum intensities higher than 4.9 mm/10 min were considered. Moreover, due to the fact that during a single rainfall event, the fixed threshold (i.e., 4.9 mm/10min) can be exceeded more than once, single 10-min rainfall values exceeding the minimum triggering threshold were also considered. The number of threshold exceedances was calculated between 1990 and 2008, by considering a sliding window of eight years, which allowed to consider overlapping periods containing a statistically significant number of exceedances. For each station, the results obtained from the analysis exhibits the total number of rainfall events and of values exceeding the minimum triggering threshold of 4.9mm/10 min, over each 8-year period.

The authors note that an approach based on the extreme value distributions, such as the General Extreme Value (GEV) distribution, to investigate the probability of occurrence of the considered rainfall threshold (i.e., 4.9 mm/10 min), cannot be applied, since such a threshold is on average exceeded more than three times per year in the considered period. An analysis carried out in a Peak-Over-Threshold (POT) framework (see, for example, the review paper by Lang, 1999) basically allows one to consider more than one rainfall peak per year in order to increase the available information with respect to procedures that use annual maximum rainfall data. Such an approach is particularly effective for this case because the interarrival time of the considered threshold is shorter than one year. Furthermore, the POT framework allowed to investigate the frequency of occurrence of over-threshold events for different threshold values, thus, extending the previously presented statistical analyses. In particular, the authors analysed the total number of exceedances over the considered period for different values of the critical threshold, and the average annual number of over-threshold events, λ, which is basically the inverse of the interarrival time.


(2) - Effets du changement climatique sur le milieu naturel
Reconstitutions

 

Observations

 

Modélisations

 

Hypothèses

 


Sensibilité du milieu à des paramètres climatiques
Informations complémentaires (données utilisées, méthode, scénarios, etc.)

 

 


(3) - Effets du changement climatique sur l'aléa
Reconstitutions

 

Observations

Debris flows are highly hazardous hydrological processes common in the Alpine environment, and the progressive increase in the socio-economic activity in these areas has led to an increase in the associated potential risk. In the Upper Boite Valley (Dolomites, Eastern Italian Alps), the significant increase in debris flow frequency in recent years (Armento, 2007) and the strong effect of rainfall on the generation of debris flows has prompted a re-definition of rainfall thresholds.

Modélisations

 

Hypothèses

 


Paramètre de l'aléa
Sensibilité des paramètres de l'aléa à des paramètres climatiques
Informations complémentaires (données utilisées, méthode, scénarios, etc.)

Rainfall-triggering thresholds are very important in landslide and debris-flow hazard evaluation because they form the basis of real-time instability early warning systems. A number of researchers [see references given in the study] have analysed precipitation data to determine rainfall thresholds for debris flow triggering. To establish such rainfall thresholds, both rainfall intensity and duration have been considered as possible indicators. Rainfall intensity and cumulative precipitation were considered as possible indicators to define critical rainfall thresholds for warning systems in Japan and Taiwan. Cluster analysis were applied to establish the rainfall threshold for all hazardous streams in Taiwan, which yielded a better disaster warning and prevention system. Not surprisingly, the published thresholds are distinctly different, as they are based on different datasets from areas with varying geological and climatic settings and, therefore, triggering thresholds should be developed for each region separately. Debris flows can be initiated from different processes, like slides or surface-water runoff. In the study area, channelised debris flows characterised by bed material mobilisation mainly occur, for which three essential factors are required: i) available sediment material, ii) steep stream slopes, and iii) surface-water runoff, mainly deriving from rainfall. The water inflows required for debris-flow occurrence are very sensitive to variations in the rainfall regime, which may be associated with climatic changes.

In the study area, debris flows occur as hillside flows or in channels, draining very steep rocky small catchments with no vegetation and almost no soil cover; channels are generally deeply incised in the thick talus slopes. The watersheds respond dramatically to high-intensity, short-duration, localised summer rainfalls, with intensities higher than 15mm in 30 min, rapidly generating high runoff discharge, generally above 1m3/s (Genevois et al., 2000). Debris flows generally start as a channel-bed failure caused by surface water flow [see references in the study]. This condition is quite common in the European Alps (Tognacca and Bezzola, 1997; Berti et al., 1999; Tognacca et al., 2000; Tecca and Genevois, 2009). Debris-flow occurrence in the study area is unaffected by the long-term antecedent precipitation due to the high permeability of soils.

This paper aims at examining variations in climate characteristics in the study area, with reference to the possible implications for debris-flow occurrence on a local scale. In the study area, rainfall intensity and debris-flow discharge are monitored in the experimental basin of Acquabona (Cortina d’Ampezzo, Italy), as described in detail by Galgaro et al. (2005). In the Cortina area, Genevois et al. (2003) mapped 325 debris-flow prone watersheds. Because of the homogeneity in the geological and geomorphological settings, the mechanisms of initiation and propagation of the debris flows can be considered to be quite similar. Nevertheless, the rainfall triggering events do not homogeneously affect the entire area because of the topographic and morphometric settings, such as slope aspect and the position in relation to the storm cell paths. For this reason, the available historical data collected at all of the rain gauges within the area were considered. The objective of this study is to investigate the relationship between the variation in the frequency of rainfall events that may trigger debris flows and the changes in regional climatic and meteorological conditions. Such an analysis aims at determining whether the conditions favourable for triggering of debris flows are currently increasing or decreasing in the study area.

Climate:
To investigate, in detail, the main climatic features of the Cortina d’Ampezzo area, the authors considered the historical daily rainfall and annual temperature data, collected at four meteorological stations of the regional hydrological service, namely Podestagno, Cortina, Passo Falzarego and San Vito, located in the study area. The available rainfall data cover 1920 to 2008, while the temperature data have been recorded since 1938.

Debris-flow discharge and triggering rainfall threshold:
Rainfall data recorded at the Acquabona (Cortina d’Ampezzo) debris-flow monitoring system (Tecca et al., 2003) were analysed to assess the minimum rainfall triggering threshold for debris flow occurrence, as the Acquabona basin, due to its geomorphological, lithological and hydrological characteristics, can be considered as representative of the whole study area. The rain gauge is installed in the debris-flow initiation area, and sampled rainfall at 0.20 s (5 Hz). Rainfall data recorded from 1997 to 2001 were considered. During this period, seven debris-flow events occurred. The following variables were selected for the analysis of rainstorms and discharges in the Acquabona Basin: total rainfall, maximum rainfall intensity (over 10 min), and rainfall duration.

At the Acquabona catchment (Cortina d’Ampezzo), debris flows were generally triggered by rainfalls of high intensity and durations of less than one hour. The total amount and duration are considered from the beginning of the rainfall until the onset of the debris flow. In Acquabona, antecedent rainfall does not represent a significant factor for debris-flow occurrence because of the high permeability of the coarse debris. The authors, therefore, focused their analysis on single bursts, defined as continuous periods having rainfall intensities above 0.5 mm/5 min and containing at least one peak exceeding a second (and higher) intensity threshold. (...) Although thle limited number of cases does not allow to define a statisticallyvalidated triggering threshold for debris-flow occurrence, the data so far collected emphasize that debris flows do not occur with rainfall intensity below 4.9 mm over 10min, which represent the minimum triggering threshold.

Analysis of threshold exceedances:
From 1990, the regional meteorological network has been updated; the current rainfall dataset is composed of data collected with a temporal resolution of five minutes. Five automatic rain gauges, namely Faloria, Podestagno, Cortina, Passo Falzarego and Villanova, were considered in analysing the occurrence of rainfall events displaying characteristics similar to those of the events that triggered debris flows. According to the observations reported in the previous section, the rainfall events with maximum intensities higher than 4.9 mm/10 min were considered. Moreover, due to the fact that during a single rainfall event, the fixed threshold (i.e., 4.9 mm/10min) can be exceeded more than once, single 10-min rainfall values exceeding the minimum triggering threshold were also considered. The number of threshold exceedances was calculated between 1990 and 2008, by considering a sliding window of eight years, which allowed to consider overlapping periods containing a statistically significant number of exceedances. For each station, the results obtained from the analysis exhibits the total number of rainfall events and of values exceeding the minimum triggering threshold of 4.9mm/10 min, over each 8-year period.


(4) - Remarques générales

In the last decade, several authors have suggested that climate changes are leading to a rapid increase in the frequency and intensity of extreme rainfall events (e.g., Gong and Wang, 2000; Easterling et al., 2000; Monirul Qader Mirza, 2002; Fauchereau et al., 2003; Fowler and Kilsby, 2003; Fowler et al., 2005; Sillmann and Roeckner, 2008). As a consequence, the frequency of rainfall-related phenomena, such as floods, landslides and debris flows, is changing all over the world (Eybergen and Imeson, 1989; Haeberli et al., 1990; Rickenmann and Zimmermann, 1993; Evans and Clague, 1994; Blijenberg, 1998; Van Steijn, 1996, 1999; Gärtner et al., 2003; Jomelli et al., 2007, 2009; Pelfini and Santilli, 2008), with potentially huge economic and social implications. However, the influence of global climate change on the increase in intense rainfall events in the Mediterranean area is still not completely understood (Bodini and Cossu, 2010). In fact, results from previous studies show contrasting trends of heavy rainfall events in the past century between western and eastern Europe (Alpert et al., 2002) and insular and peninsular Italy (Brunetti et al., 2001).


(5) - Syntèses et préconisations

Discussion and concluding remarks:
The first results of this preliminary study on critical rainfall thresholds for debris-flow occurrence and climatic changes in the Dolomitic area of Cortina d’Ampezzo suggest a number of conclusions that can be summarized as follows.

The increase in the number of potentially triggering rainfall events, as well as their intensity, suggests climate changes in the study area. In the past century, the considered area has experienced changes in temperature, in the amount of total annual rainfall, and in the distribution of monthly rainfall. Maximum annual temperatures increased by up to 3 °C, and the distribution of rainfall experienced important changes during the autumn (October and November).

The differences displayed by the analysis of data measured at different stations show a large variability in rainfall distribution, particularly for intense, short-duration rainfalls all over the study area. This variability does not seem to be related exclusively to the elevation but also to local morphological conditions. As a consequence, in order to investigate the relationship between rainfall characteristics and debris-flow occurrence, it is important to consider rainfall data recorded at meteorological stations close to the sites where debris flows occurred. A larger monitoring network is needed to develop forecasting models and to further investigate the presence of a climatic change.

As the mean frequency of rainfall events that can potentially trigger debris flows is 3.5 events per summer, it is necessary to use forecasting models that can account for this frequency and the possibility of major climate changes because models using only the annual maximum values, such as Gumbel or GEV, could underestimate the probability of occurrence, at least for small triggering thresholds.

Because of the marked variability displayed by the trends of intense rainfall events at local and global scales, more short-term rainfall data and further studies are needed to analyse the influence of global climate changes on the increase in intense rainfall events. In mountainous regions, rainfall data must be gathered from well-distributed rain gauge networks because of the influence of morphometric factors. At the global scale, and with the aid of remote sensing techniques, it will useful to analyse the correlations between regions with different climatic features, to understand the influence of global climate changes on precipitation patterns and mitigate the related social consequences.

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