Zimmermann & al 1997 - Magnitude-frequency aspects of alpine debris flows

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.

Organismes / Contact

(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

Massif / Secteur

Site(s) d'étude

Période(s) d'observation

(1) - Modifications des paramètres atmosphériques

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

Paramètre de l'aléa

Sensibilité du paramètres de l'aléa à des paramètres climatiques

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

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.

(5) - Syntèses et préconisations

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

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

(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.