Réf. Pelfini & Santilli 2008 - A

Référence bibliographique complète
PELFINI M., SANTILLI M. Frequency of debris flows and their relation with precipitation: A case study in the Central Alps, Italy. Geomorphology, 2008, in press, 10 p.

Abstract: This research deals with debris flows in an alpine environment, studied using dendrogeomorphological dating techniques, outlining their relation with precipitation, and analyzing possible changes in their frequency and intensity over time. The study area is the upper Valle del Gallo (Northern Italy), a typical high mountain environment dominated by mass wasting processes, where many debris-flow fans occupy the valley bottom. Dendrogeomorphological research was conducted on twelve of these fans and two channels located on slopes. Tree growth anomalies (abrasion scars, compression wood and abrupt growth changes) were used as dating methods. 239 debris-flow events between 1875 and 2003 were dated using 757 trees (Pinus montana Mill.). Analysis between dated events and precipitation suggests that debris flows in the study area could be triggered by 20-30 mm of rain concentrated in a few hours. The debris-flow frequency tends to increase gradually, but the highest value seems to have occurred in the period 1974-1983. This trend agrees with the historical occurrence of flooding events in Northern Italy as inferred by literature, and with similar studies conducted in the Swiss Alps.

Debris flows, dendrogeomorphology, Valle del Gallo, Italian Alps.

Organismes / Contact
Dipartimento di Scienze della Terra “Ardito Desio”, Sezione di Geologia e Paleontologia, Università di Milano, Via Mangiagalli 34, 20133 Milano, Italy.
manuela.pelfini@unimi.it. Tel.: +39 2 50315517.

(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
Temperature, Precipitation   Debris flows  

Pays / Zone
Massif / Secteur
Site(s) d'étude
Période(s) d'observation
Italy Alps Valle del Gallo (Lombardy)   1860-2050 m a.s.l. for fans 1875-2003

(1) - Modifications des paramètres atmosphériques
The climate of upper Valle del Gallo is defined as Alpine continental, with a mean annual temperature of 2.7°C and a mean annual precipitation of 828.5 mm. Precipitation is mainly distributed in the months between May and November, with numbers of rainy days concentrated between May and August. The temporal trend of the measured data for recent years shows an increase in the mean annual temperature, a decrease in the permanence and height of the snow cover and an increase in the amount and intensity of precipitation.

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

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

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

(3) - Effets du changement climatique sur l'aléa
Altogether 757 trees were used for dating 239 debris flows occurring between 1875 and 2003. The mean time gap between two consecutive events can be considered as meaningful only for the group A fans; it ranges between 3.1 and 5.0 years (4.1 years on average). The temporal distribution and the frequency of the debris flows can be analysed with good precision from 1887. Many events were recognised on different fans; in particular, the most meaningful dates, recorded on at least five fans, are 1887, 1888, 1899, 1917, 1941, 1951, 1955, 1962, 1970, 1975, 1977, 1980, 1986, 1991, 1997, 2000 and 2001. Some of the dated debris flows on certain fans (group A) have damaged a high number of trees; they can therefore be considered to be highly intensive events: debris flows which mobilized great amounts of debris and consequently affected great portions of the fans. Among these events, those that occurred in 1917, 1951, 1977 and 1991, which involved four fans, were particularly outlined.

For the years in which major debris flows occurred, the highest daily precipitation data ranges between 21.3 and 73.2 mm, with a mean value of 37.8 mm. However, years with particularly abundant maximum daily precipitation (even over 70 mm) do not always correspond to years of debris flows. On the contrary, some years with maximum rainfall values of only around 20 mm correspond to years of debris flows. The average daily maximum precipitation in the years with debris flows is 46 mm. This value has a return time of 2.2 years and is slightly longer than the mean interval of debris flows (1.6 years). It can be assumed that the debris flows are triggered by 20-30 mm of rainfall, probably concentrated in a short time, such as a few hours.

The frequency of all the dated debris flows since the end of 19th century shows a gradual temporal increase, reaching the maximum in 1974-1983 followed by a reduction. A similar trend can be noted for the number of years with debris flows per decade, independently of how many fans were affected, but after the peak of 1974-1983, another maximum can be found in 1994-2003. The general increasing trend could partially be related to the reduction of the amount and reliability of the data for older periods. However, the lessening of event frequency in the last few decades appears meaningful.

Similar studies show that debris-flow frequency has been greater in the past than in recent years. For example, in some French valleys, the 20-year frequency of debris flows from 1900 to 2000 increased until the period 1940-1959 but then decreased (Jomelli et al., 2003), although they suggest that the number of debris flows in the period 1900-1950was underestimated and that the following reduction in frequency is not statistically meaningful. Other research carried out on a fan in Switzerland indicates that the maximum frequency of debris flows took place between 1830 and 1922 (Gärtner et al., 2003), while a reduction started from 1995 (Stoffel and Beniston, 2006). Moreover, some climatic models forecast a possible reduction in debris-flow frequency in the future, since precipitation will decrease during summer but increase in spring or autumn (Stoffel and Beniston, 2006).

The results obtained from the dendrochronological dating of the debris flows in upper Valle del Gallo were compared with the historical recurrence of flooding events in Northern Italy during 1800-1997, in which the number of events fluctuated with mean intervals of 3-5 decades, with distinct peak periods around the 1850s and 1960s (Tropeano et al., 1999). The two datasets show a trend of parallel increase in the number of events in 1870-1960, which lasted until 1980 in Valle del Gallo, followed by decrease during the last few decades.

Paramètre de l'aléa
Sensibilité des paramètres de l'aléa à des paramètres climatiques
Informations complémentaires (données utilisées, méthode, scénarios, etc.)
The study area is the upper part of the Valle del Gallo, a valley located at the northeast extremity of Valtellina (Lombardy, Northern Italy, Central Alps). It is characterised by a wide and flat valley bottom constituted by the coalescence of many debris-flow fans of complex structure (Santilli et al., 2002).

Valle del Gallo vegetation is dominated by a forest of mountain pine (Pinus montana Mill.). Dendrogeomorphological research for dating debris flows was conducted on the nine main fans occupying the valley bottom, on three small fans located in a tributary valley, and on two channels located on the eastern slope. The samples are mainly cores taken with increment borers, from one to three for each tree. Special attention was paid to the occurrence of compression wood, corrasion scars and abrupt growth changes (considered to be valid only if coinciding with other types of marks). Altogether, 757 trees were used for debris-flow dating, while many other samples were disregarded.

A skeleton plot showing the time distribution of pointer years (Schweingruber et al., 1990) was constructed for each sampled tree used for the debris-flow dating. Finally, master plots outlining the number of damaged trees over time were created for each analysed fan. The dating of debris-flow events was based both on the number of damaged trees and on the quality of the collected samples. On six fans, detailed sequences of debris flows covering a long time period (1875-2003) and inferred by a great number of trees were obtained (group A); in contrast, the other eight fans only have a relatively small number of dated events, which are often recent and recognisable in fewer trees (group B). The growth curves of many samples were cross-dated with reference tree-ring chronologies in order to verify the correctness of the dating. All the growth curves were created by measuring the ring widths with the TSAP program (Rinn, 1996) or with image analysis using WINDENDRO software. The statistical cross-dating was executed with the COFECHA program (Holmes et al., 1986).

In this analysis, the dendrologic year from September of the previous year to August of the current year was considered and the months from November to April were excluded. Once the debris-flow dating of all the fans was obtained, the frequency of the events for intervals of 10 years was analysed. The total number of dated debris flows and the number of years in which they occurred, independently of their number, were both considered. Later, the debris-flow dating results were correlated to the precipitation data recorded at the Cancano weather station (1948 m a.s.l., some kilometres away from the debris-flow triggering areas). Daily precipitation data from 1936 to 2003 and maximum intensities of rain in 1, 3, 6, 12 and 24 h for the period 1936-1991, although with some gaps, are available.

(4) - Remarques générales

(5) - Syntèses et préconisations

Références citées :

Gärtner, H., Stoffel, M., Lièvre, I., Conus, M., Grichting, M., Monbaron, M., 2003. Debrisflow frequency derived from tree-ring analyses and geomorphic mapping, Valais, Switzerland. In: Rickenmann, D., Chen, C. (Eds.), Debris-Flow Hazard Mitigation: Mechanics, Prediction and Assessment. Proceeding of the 3rd International DFHM Conference, Davos, Switzerland, September 10–12, 2003, vol. 2. Millpress, Rotterdam, Netherlands, pp. 207–217.

Holmes, R.L., Adams, R.K., Fritts, H.C., 1986. Tree-ring chronologies of western North America: California, Eastern Oregon and Northern Great Basin with procedures used in the chronology development work including users manual for computer program COFECHA and ARSTAN. Chronology Series VI. Laboratory of Tree-Ring Research, University of Arizona, Tucson, USA. 182 pp.

Jomelli, V., Brunstein,D., Chochillon, C., Pech, P.,2003. Hillslope debris-flow frequency since the beginning of the 20th century in the Massif des Ecrins (French Alps). In: Rickenmann,D., Chen, C. (Eds.), Debris-Flow Hazard Mitigation: Mechanics, Prediction and Assessment. Proceeding of the 3rd International DFHM Conference, Davos, Switzerland, September 10–12, 2003, vol. 2. Millpress, Rotterdam, Netherlands, pp. 127–137.

Rinn, F., 1996. TSAP: Time Series Analysis and Presentation. Version 3.0. Reference Manual, 262 pp.

Santilli, M., Pelfini, M., Caccianiga, M., Comolli, R., Ravazzi, C., 2002. Late Holocene environmental evolution of the upper Valle del Gallo (Central Alps): an interdisciplinary study. II Quaternario 15, 187–208.

Schweingruber, F.H., Eckstein, D., Serre-Bachet, F., Bräker, O.U., 1990. Identification, presentation and interpretation of event years and pointer years in dendrochronology. Dendrochronologia 8, 9–38.

Stoffel, M., Beniston, M., 2006. On the incidence of debris flows from the early Little Ice Age to a future greenhouse climate: a case study from the Swiss Alps. Geophys. Res. Lett. 33, L16404. doi:10.1029/2006GL026805 . - [Fiche biblio]

Tropeano, D., Govi, M., Mortara, G., Turitto, O., Sorzana, P., Negrini, G., Arattano, M., 1999. Eventi alluvionali e frane nell'Italia Settentrionale. Periodo 1975–1981. Pubblicazione n. 1927 del GNDCI, Consiglio Nazionale delle Ricerche, Istituto di Ricerca per la Protezione Idrogeologica nel Bacino Padano, Gruppo Nazionale per la Difesa dalle Catastrofi Idrogeologiche. 280 pp.