Réf. Irmler & al. 2006 - A

Référence bibliographique complète
IRMLER R., DAUT G., MAUSBACHER R. A debris flow calendar derived from sediments of lake Lago di Braies (N. Italy). Geomorphology, 2006, Vol. 77, p. 69-78.

Abstract: In this study lake sediments were used for developing a high resolution calendar of past debris flow events. Debris flows frequently occur in the high-alpine catchment of the lake Lago di Braies (N. Italy). While the coarse grained material accumulates on the debris flow cones, the fine grained material is flushed into the lake. This material forms characteristic layers in the lake sediments, which are different from the typical annual lamination. By analysing thin sections of sediment, debris flow events of the last 2250 years were reconstructed with an annual resolution. Since debris flow frequency varies strongly, periods with high and low frequencies can be distinguished. However, neither a debris flow magnitude-frequency relationship nor a strong reaction of debris flow activity to past climate changes was identifiable in this study.

Mots-clés
Lake sediments, geochronology, debris flow frequency, debris flow magnitude, Lago di Braies.

Organismes / Contact
Friedrich Schiller-University Jena, Physical Geography, Institute of Geography, Löbdergraben 32, D-07743 Jena, Germany. ralf.irmler@uni-jena.de, Tel.: +49 3641 948827.

(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
    Debris flows  

Pays / Zone
Massif / Secteur
Site(s) d'étude
Exposition
Altitude
Période(s) d'observation
Italy Dolomite Alps Lago di Braies   ~1500-2300 m a.s.l. 250 BC - 2000 AD

(1) - Modifications des paramètres atmosphériques
Reconstitutions
 
Observations
 
Modélisations
 
Hypothèses
 

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

(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
Frequency
The debris flow calendar gives an impression of the debris flow events in the catchment of lake Lago di Braies over nearly 2250 years. During this time the recurrence interval of events varies between 1 and 127 years. At an average of every 16 years a debris flow was deposited.
- From 250 until 0 BC eight debris flows occurred, recurring with an irregular frequency of 8 and 81 years.
- During the next 250 years 21 debris flows occurred. Apart from a small gap (70–100 AD) the recurrence interval is regular with a variation between 8 and 18 years. - In the period between 250 and 500 AD seven events occurred. There exists a long period with no events between 290 and 403 AD.
- Between 500 and 750 AD 16 events took place; four of these occurring in the short period between 664 and 670 AD.
- During 750–1000 AD 11 debris flows occurred, whereas in 885 and 890 AD three flows were observed in the sediments.
- From 1000 to 1250 AD 18 debris flows have been observed. The return period is regular (with the exception of the last 10 years of this period) and varies between 9 and 21 years.
- From 1250 to 1500 AD a further 14 events were detectable. During this period is a 127 year gap where no event is recorded.
- From 1500 to 1750 AD altogether 22 events were recognized. Most of these events are concentrated at the beginning and at the end of this period. In the middle of this section, a small gap of approximately 60 years can be seen.
- In the last 250 years 23 debris flow events occurred in the catchment area of lake Lago di Braies, which took place from the beginning of the 19th century.

A better approximation of the debris flow event is shown the 100 year regressive moving average of the calendar. It clearly indicates periods with high and low activities. At average 6 events happened per 100 years. Longer periods with an above-average amount of events are the period between 60 and 300 AD, with an average of 8.0 events per 100 years, and 670–810 AD, with an average of 7.7 events per 100 years. The last millennium shows 3 periods with an above average frequency of debris flows: in 1240–1350 AD an average of 8.8 events per 100 years took place; from 1500 to 1620 AD an average of 11.3 debris flows occurred; and from 1740 to 1920 AD 12.3 events per 100 years are detected. These last two periods show the highest values of debris flows frequency, and in fact only the period 900–1240 AD reflects the mean frequency of debris flows.

Intensity
The magnitude of debris flow deposits, as reflected in layer thickness, varies between 305 and 1.9 mm with a mean value of 16.3 mm. The debris flow frequency was compared with the magnitude derived from thickness measurements from which the 100 year regressive moving mean was calculated. For the entire 2250 year period the average magnitude is 13.9 mm. Periods with an above-average magnitude are between 130–230, 1020–1125, 1540–1640, 1740–1850 and 1880–2001 AD. All events with extremely high magnitudes (N64 mm) took place during the last 500 years. In contrast to other studies no low frequency–high magnitude behaviour is identifiable. Only in the last century does mean debris flow activity reach an above average magnitude. After the long return periods in the 4th and 14 th century, no extremely high debris flow magnitudes occurred. By way of contrast, in periods with an above-average debris flow activity, an above-average magnitude is sometimes detectable. The missing low frequency–high magnitude behaviour, or vice versa, is also evident.

Relation with climate
A comparison with the primary climate anomalies of the last 2300 years was made: a Roman Climate Optimum (350 BC–400 AD); Peoples' Migration Period (450–750 AD); Medieval Warm Period (950–1250 AD); and Little Ice Age (1450–1750 AD). The appearance of debris flow events is not favoured by any climate trend. In the comparatively warm Roman Climate Optimum as well as in the cold Little Ice Age, the debris flow frequency is irregular. The debris flow events/100 years are subject to strong variance in both periods. By way of contrast, the frequency of debris flows in the cold Peoples' Migration Period and in the Medieval Warm Period is more regular and the events/100 years does not vary so much. A comparison of the debris flow calendar with the climate record of the last 80 years from a neighbouring meteorological station gives another perspective. Most years with debris flow events are colder and wetter than years without debris flows (Irmler, 2003). This observation was also made by Veit (2002) and Zimmermann et al. (1997).
Observations
 
Modélisations
 
Hypothèses
 

Paramètre de l'aléa
Sensibilité des paramètres de l'aléa à des paramètres climatiques
Informations complémentaires (données utilisées, méthode, scénarios, etc.)
Debris flow frequency
At long-term, the appearance of debris flow events is not favoured by any climate trend. At short-term, most years with debris flow events are colder and wetter than years without debris flows.
The lake Lago di Braies (1492 m a.s.l.) with a maximum area of nearly 36 ha and a catchment area of 30 km2 is situated in the Dolomite Alps, northeast Italy. Except in the north the lake is surrounded by high mountains, where the highest one is mount Croda del Becco (2810 m a.s.l.). Several debris flow cones are located on the steep face of this mountain. They are fed by steep chutes and former cirques at an altitude between 1800 and 2300 m a.s.l. The cones reach the southern and eastern part of the lake.

The new approach introduced in this paper is based on the observation that after a debris flow event fine grained material is washed into the lake and there forms characteristic layers that are different from normal, annually laminated sediments. By counting these laminae in a sediment core, it is possible to reconstruct a detailed debris flow calendar.

To gain information on water depths as well as on sediment structures, their distribution and thickness in the lake, intense sub-bottom profiling with a parametric echosounder (Innomar 96-light) was done. These data were the basis of site selection for subsequent coring. Altogether 35 gravity cores and 2 piston cores were taken. The high amount of gravity cores was necessary to estimate the distribution and thickness of the debris flow layers in the lake. Additionally the cores were used for selecting representative piston core locations. The first piston core was selected in order to obtain a long, undisturbed high resolution sediment sequence to reconstruct an almost complete debris flow calendar far back into the past. The other piston core was selected to recover a short compact sediment sequence from which to derive the age of lake formation.

The estimation of debris flow magnitude was done by measuring the thickness of the event layers. So these measurements are based on the hypothesis that the thickness of subaqueous debris flow sediments is directly related to the volume of the subaerial debris flow sediments. Visual documentation (color, grain size, structures) as well as geochemical and physical analyses at a 10 cm sampling interval were also conducted.

(4) - Remarques générales
 

(5) - Syntèses et préconisations
 

Références citées :

Irmler, R., 2003. Seesedimente als natürliches Archiv zur Erstellung eines Murkalenders am Beispiel des Pragser Wildsees (Norditalien). Ph.D. Thesis, University of Jena, Germany.

Veit, H., 2002. Die Alpen - Geoökologie und Landschaftsentwicklung. Verlag Eugen Ulmer, Stuttgart, p. 352.

Zimmermann, M., Mani, P., Gamma, P., 1997. Murganggefahr und Klimaänderung - ein GIS-basierter Ansatz, vdf Hochschulverlag AG ETH Zürich, p. 161.