Réf. Zimmermann & al 1997 - A

Référence bibliographique complète
ZIMMERMANN M., MANI P., ROMANG H. Magnitude-frequency aspects of alpine debris flows. Eclogae geol. Helv., 1997, 90, 415-420.

Abstract: Based on historic documents, the event history for 17 mountain torrents in the Swiss Alps was evaluated. Four classes could be determined for the recurrence interval of the debris flow events. The magnitude is not necessarily dependent on the recurrence interval. The characteristics of the catchment basin (disposition) are mainly controlling the magnitude. In order to evaluate the effects of climatic change on the debris flow activity, knowledge about the magnitude and the frequency are necessary.

Mots-clés
Natural hazards, debris flows, magnitude, frequency, climate change.

Organismes / Contact
Geo, Geowissenschaftliches Buro, Neufeldstrasse 3, CH-3012 Bern
Forsting, Buro Teufen, Promenade 153, CH-7260 Davos Dorf
[From symposium "Natural Hazards" in the framework of the symposium Global Change of the Swiss-Academy of Natural Sciences, Zurich, 10-11 sept. 1996.]

(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
    Torrential events Debris flows

Pays / Zone
Massif / Secteur
Site(s) d'étude
Exposition
Altitude
Période(s) d'observation
Swiss Alps  

17 torrents

    ~ 400 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  
Observations

Depending on the ground disposition the recurrence interval can be regular (supply-limited cases). For catchment basins with a unlimited supply of debris the occurrence of debris flows is controlled by the triggering event and the pre-event conditions. The magnitude of the events is not necessarily related to the recurrence interval. Therefore, the concept which is widely applied in hydrology cannot be adopted for debris flows. The magnitude is mainly controlled by the debris sources or by the condition of the catchment basin.

Similar to Caine (1980) a threshold for the rainfall-related debris flows (intensity-duration relation) was developed for the Swiss Alps (Zimmermann et al. 1997). A total of 66 events (debris flows occurred during strong rainfall), 24 'non-events' (a strong rainfall occurred in a debris flow-prone area, however no activity in the channel was observed) and 23 threshold events (the torrent or some of the tributaries showed some minor debris flow activity, but in the main river only "normal" sediment transport occurred) could be classified.

Modélisations
 
Hypothèses

If changes in the climate are considered (temperature and/or rainfall regime) there are direct and indirect consequences for the occurrence of debris flows. Climatic variations may influence the variable disposition with regard to the frequency as well as the magnitude of the debris flow events. Within a cyclic system, for instance, the period between low and high disposition may be shortened due to more frequent storm events of a particular magnitude.

In two cases out of the 17 the past warming trend of the atmosphere may have had a direct influence on the debris flow activity : the most dramatic changes can be expected if the sediment availability alters. Only due to the melting of the Minstiger Glacier in the past 150 years the sediment source of the 1987 debris flow became exposed to erosion (Zimmermann & Haeberli 1992). The continuous degradation of under-ground ice (permafrost) has to be inferred under warmer climate (Haeberli et al. 1997). The stability of frozen debris sources may alter drastically. In the Ritigraben, for instance, the sediment sources are located in a permafrost environment. The time series of the debris flows in this torrent indicates that the probability for the occurrence has increased.


Paramètre de l'aléa
Sensibilité du paramètres de l'aléa à des paramètres climatiques
Informations complémentaires (données utilisées, méthode, scénarios, etc.)
Debris flows frequency and intensity Rainfall threshold

Information found in various archives permit to evaluate and estimate past debris flow events. In selected cases a period of about 300 to 400 years can be covered. To date 17 debris flow-prone rivers in the Swiss Alps could be evaluated. A total of 189 events were found. For most events the date of occurrence, an order of magnitude and the type of process could be classified.

1) Foioi, Guppenruns, Leimbach, Zavragia, Nant du Pissot, Steinlauibach. This group of torrents are producing debris flows in a more or less regular sequence, but with variable intervals from river to river. For the Leimbach, the period of inactivity was found to be 15 to 30 years, with mid-scale debris flows every 30 years. The variability of the magnitudes is relatively small. ln general, high-magnitude events (more than 100,000 m3) are not occurring in this first group of torrents. The material is eroded along the flow path.

2) Varuna, Nolla, Maschaenser Ruefe, Bubchser Ribi, Steinibach, Giswiler Laui. Torrents which are located in relatively weak rocks (rock of variable strength like shist) belong to the second group. Relatively short periods of a high activity (years to a few decades) are followed by periods of a low activity (several decades). The bulk of material in those rivers is being eroded along the flow path. It has to be expected that a major debris flow causes a significant disturbance of the system with unstable bed and banks. After such a disturbance even moderate rainfalls may trigger high-magnitude debris flows. The magnitude of events can reach several 100,000 m3.

3) Ritigraben, Dorfbach, Lammbach. These torrents are found in areas with abundant debris (moraines, talus, etc.). The occurrence of debris flows is irregular. The main sediment sources are found in the headwaters of the rivers. The variability of the magnitude can be large. Many mountain torrents in the inneralpine areas belong to this type.


4) Minstigerbach, Plaunca. The last group represents a singular event which have no historical parallels.


(4) - Remarques générales

 

(5) - Syntèses et préconisations

The information obtained through the analysis of historical documents can be very clear and accurate in one case but very vague in another. ln order to obtain a more precise picture of the frequency and magnitude of debris flows in a particular channel the work has to be continued. For an evaluation of the effects of an expected climatic change magnitude-frequency aspects are vital. The knowledge is still limited. More educational work is required to prove that the historical knowledge is not sufficient for an evaluation of the future hazards.

Références citées :

CAINE, N. 1980: The rainfall intensity-duration contrai of shallow landslides and debris flows. Geogr. Ann. 62A, 23-27.

HAEBERLI, W., WEGMANN, M. & VONDER MÜHLL, D. 1997: Slope stabilily problems re1ated 10 glacier shrinkage and permafrost degradation in the Alps; Eclogae geol. Helv. 90, 407-414. - [Fiche biblio]

ZIMMERMANN, M. & HAEBERLI, W. 1992: Climatic change and debris flow activity in high-mountain areas. A case study in the Swiss Alps. Catena Suppl. 22, 59-72. .

ZIMMERMANN, M., MANI, P., KIENHOLZ, H. & ROMANO, H.1996: Debris flows.ln: ECEnvironment Research Program, Contract EV5V-CT94-0453.


ZIMMERMANN, M., MANI, P., GAMMA, P., GSTEIGER, P., HEINIGER, O. & HUNZIKER, G. 1997: Murganggefahr und K1imaanderung - einGIS-basierter Ansatz. Vdf, Zürich.