Réf. Schneider & al. 2009

Référence bibliographique complète

SCHNEIDER, H., HÖFER, D., IRMLER, R., DAUT, G., MÄUSBACHER, R. 2010. Correlation between climate, man and debris flow events — A palynological approach. Geomorphology, 120, 48–55. Doi:10.1016/j.geomorph.2009.09.014

Abstract: Debris flows occur frequently in the high-alpine catchment area of lake Lago di Braies (1492m a.s.l.). The sediments of the lake are used for detecting and dating past debris flow events during the last 4000 years and their driving factors. Comparing palynological and sedimentological data the results suggest, that mainly heavy precipitation in winter and summer as well as snow melt events in spring enhance the release of debris flow events. Therefore especially longer periods with humid and cool climate conditions show a clearly higher debris flow activity. The pollen diagram documents human impact on the catchment area since the late Neolithic, but the anthropogenic activities influenced the event activity only indirectly. Grazing and deforestation reduced the retention of debris by vegetation and results in thicker debris flow layers in the lake sediments.

Mots-clés
Palynology, Debris flows, Alps, Climate change

Organismes / Contact

Friedrich-Schiller-Universität Jena, Physische Geographie, Institut für Geographie, Löbdergraben32, D-07743 Jena, Germany


(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   Torrential hazard Debris flow

Pays / Zone
Massif / Secteur
Site(s) d'étude
Exposition
Altitude
Période(s) d'observation
Italy North of the Italian Dolomite
Alps
High-alpine catchment area of lake Lago di Braies   1492m a.s.l. (lake Lago di Braies) ; catchment max. 2810m a.s.l. (mount Croda del Becco) Last 4000 years

(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

Influence of climate on debris flow activity:
Based on [palynological and sedimentological results, and published data indicating climate changes during the Holocene (see the study)] an increase in debris flow layers thicker than 10 mm is clearly visible in sections which were climatically characterised as more humid and cool-humid phases. It is to suggest that the clear decrease of anthropogenic indicators in this core sections points to worse conditions for human activity in the catchment, that means wetter and/or cooler conditions. The detected debris flow layers smaller than 10 mm (Irmler et al., accept.) show no correlation to the interpreted climatical effects.

It is to assume, that the debris flow activity is stimulated in longer periods with cooler and more humid conditions, because of higher precipitation and an increase in melt water flow. These conditions results in an accumulation of debris flow events. This result is in agreement with investigations of van Asch (1997) about the relationship between landslide activity and climate change.

But [the present] investigations showed, that debris flow events also occur during generally warm periods. During these periods they are triggered by a strong rainfall event, because a rapid increase in pore pressure is the precondition for a debris flow event (van Asch, 1997). Investigations by Pfister (1999) about the connection between monthly temperatures, precipitations and natural hazards during the last 500 years shows, that there is no correlation in general between single events and the weather conditions during the year. The weather tables of Pfister (1999) often show the same conditions in temperature and precipitation in years with and without debris flow events. In years with a debris flow event prevalently the yearly precipitation is not apparently, and only one strong rainfall event is responsible for the release of a debris flow run. That means humidity cannot be the only parameter for an event. Probably also the availability of debris flow material is one limiting factor, so longer cold periods with frost action could boost the process. In fact the pollen diagram shows an intensification of the debris flow activity in cold and humid periods.

[see also Influence of man on sediment change and debris flow activity, p. 7]

Conclusion:
Differences in debris flow activity seem to be predominantly caused by climatic conditions. However, the results of [the present] investigation suggest that not cooler or warmer conditions in general, but wetter conditions combined with drastic seasonal variations in temperature are the driving force for a higher debris flow activity.

It is supposed that the following combinations are responsible for a higher debris flow activity. Cold winters and very warm temperatures in March until May cause the melting of enormous amounts of snow. This process in combination with an increasing precipitation is the triggering factor for debris flow events. Drastic seasonal variations in temperature possibly force the thermal weathering processes and thus enhance the availability of material for debris flows.

Anthropogenic changes of the environment are only a weak factor for an increase of debris flow activities. The information gained from this study indicates that human influence changed the retention of debris in the catchment area as the result of a modified vegetation composition. So a disturbance of forests and a creation of agricultural areas are probably responsible for an increase in debris flow layer thickness.

Observations
 
Modélisations
 
Hypothèses
 

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

The sediments of the lake Lago di Braies (1492m a.s.l.) are used for detecting and dating past debris flow events during the last 4000 years and their driving factors, by comparing palynological and sedimentological data.


(4) - Remarques générales
 

(5) - Syntèses et préconisations
 

Références citées :

Pfister, Chr, 1999. Wetternachhersage. Paul Haupt, Bern, Stuttgart, Wien.

van Asch, Th.W.J., 1997. The temporal activity of landslides and its climatological signals. In: Matthews, J.A., Brunsden, D., Frenzel, B., Gläser, B., Weiß, M.M. (Eds.), Rapid mass movement as a source of climatic evidence for the Holocene: Paläoklimaforschung/Palaeoclimate Research 19, ESF Special Issue, vol. 12, pp. 7–16.