Réf. Schneuwly & Stoffel 2008 - A

Référence bibliographique complète
SCHNEUWLY D. M., STOFFEL M. Tree-ring based reconstruction of the seasonal timing, major events and origin of rockfall on a case-study slope in the Swiss Alps. Nat. Hazards Earth Syst. Sci., 2008, Vol. 8, p. 203-211.

Abstract: Tree-ring analysis has been used to reconstruct 22 years of rockfall behavior on an active rockfall slope near Saas Balen (Swiss Alps). The authors analyzed 32 severely injured trees (L. decidua , P. abies and P. cembra) and investigated cross-sections of 154 wounds. The intra-annual position of callus tissue and of tangential rows of traumatic resin ducts was determined in order to reconstruct the seasonality of past rockfall events. Results indicate strong intra- and inter-annual variations of rockfall activity, with a peak (76%) observed in the dormant season (early October – end of May). Within the growth season, rockfall regularly occurs between the end of May and mid July (21.4%), whereas events later in the season appear to be quite rare (2.6%). Findings suggest that rockfall activity at the study site is driven by annual thawing processes and the circulation of melt water in preexisting fissures. Data also indicate that 43% of all rockfall events occurred in 1995, when two major precipitation events are recorded in nearby meteorological stations. Finally, data on impact angles are in very good agreement with the geomorphic situation in the field.

Mots-clés
Rockfall, frequency, seasonal timing, tree-rings reconstruction, Swiss Alps.

Organismes / Contact
Laboratory of Dendrogeomorphology, Department of Geosciences, University of Fribourg, Chemin du Musée 4, 1700 Fribourg, Switzerland. dominique.schneuwly@unifr.ch

(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
    Mass movements Rock fall

Pays / Zone
Massif / Secteur
Site(s) d'étude
Exposition
Altitude
Période(s) d'observation
Switzerland Valais Alps Schilt forest, near Saas Balen (46°09'06'' N, 7°55'27'' E) East-northeast ~1500-1900 m a.s.l. 1975-2006

(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
A total number of 154 injuries were dated for the period 1975–2006, resulting in an overall mean activity of 4.8 hits yr-1. Only five injuries occurred prior to 1985, when fewer than 75% of the sampled trees were present. This is why the seasonal timing or changes in rockfall activity will only be further analyzed for the period 1985–2006. As a result, the mean rockfall activity augments to 6.8 hits yr-1. There is a strong variation of rockfall activity between single years and no clear trend can be deduced from the rockfall series obtained. Rockfall activity was by far most important in 1995, when a total number of 66 wounds (43%) were recorded on the cross-sections, meaning that three out of four trees would have been injured. The second largest event was reconstructed for 2004, when a total of 23 injuries was observed on the cross-sections. In contrast, years with no injuries at all (1985, 1987 and 1989) and years with only one injury (1993 and 2000) were also observed.

A concentration of injuries has occurred during the dormant season of trees (76%). Rockfall is much more frequent during the period of earlywood formation (21.4%), i.e. from end of May to mid July – than during the period of latewood formation (only 2.6 %) i.e. between mid July and early October. When going into further intra-seasonal detail for the earlywood events, it can be seen that the most significant activity (13%) occurred in EE, followed by 6.5% of the events dated to ME and only 1.9% to LE. The numbers further decrease when analyzing the latewood events, where both EL and LL register only 1.3% of all rockfall events.

The massive concentration of rockfall activity in 1995 allows a more detailed seasonal analysis of rockfall activity within the year, indicating that there was three times more activity occurring during EE and ME as compared to the mean distribution. Apart from occasional freeze-thaw cycles or the melting of ice, it is possible that heavy precipitation events could have led to this concentrated and massive release of rocks and boulders. Meteorological data from a station in Zermatt (located 20 km to the south-west of the study site; SMI 2007) indicate indeed exceptionally heavy precipitation events for the dormant season of 1994–1995, with the heaviest daily precipitation sum for at least the last 25 years recorded on 5 November 1994. As the two rockfall sites in the two neighboring valleys (Stoffel et al., 2005, the present study site) presumably reacted to the same triggering events, one can suppose that the concentration of several precipitation events affecting large parts of the Valais Alps would be at the origin of the much higher frequency of rockfall in the first half of 1995.
Observations
 
Modélisations
 
Hypothèses
 

Paramètre de l'aléa
Sensibilité des paramètres de l'aléa à des paramètres climatiques
Informations complémentaires (données utilisées, méthode, scénarios, etc.)
Frequency, Seasonality
Freeze-thaw cycles and precipitation
The study was conducted in a forest near Saas Balen (46°09'06'' N, 7°55'27'' E) in the Valais Alps (Switzerland). The investigated forest stand is called “Schilt” and is located between 1470 and 1610 m a.s.l. on the east-northeast facing slope descending from the Lammenhorn (3189 m a.s.l.). The forested study site covers about 8000 m2 and consists predominantly of L. decidua trees, accompanied by approximately 10% of P. abies and single P. cembra trees. There is no anthropogenic influence visible in the forest stand. Rockfall frequently occurs on the slope, originating from disintegrated and glacially oversteepened cliffs at 1600–1900 m a.s.l. In the departure zone, bedrock consists of micaceous schists belonging to Penninic crystalline layers, dipping SSE with an angle of 20° (Bearth, 1973, 1980).

Virtually all trees at the forest stand show clearly visible and severe growth disturbances (GD) caused by past rockfall events. On the study site, scars were the most prominent sign of previous rockfall activity. Exclusively well-defined and clearly visible injuries were sampled at their maximum extension. In order to obtain a maximum of information, the sampling was realized using a handsaw, so as to obtain complete cross -sections from each injury. The cross-sections were analyzed using standard dendrochronological methods (Stokes and Smiley, 1968; Bräker, 2002). The dating of past rockfall activity was based on the position of callus tissue and tangential rows of traumatic resin ducts (TRD).

Following Stoffel et al. (2005), the intra-annual position of injuries and the adjacent callus tissue and TRD was determined and impacts dated to the dormant season (D), early earlywood (EE), middle earlywood (ME), late earlywood (LE), early latewood (EL) and late latewood (LL). The vegetation period locally starts at the end of May. The transition from LE to EL occurs in mid July. The period between October and May is called the “dormant season”. While trees will react almost immediately to injuring events during the vegetation period (Stoffel, 2008), reactions to a rockfall impact caused during the dormant season will only be apparent at the very beginning of the “new” tree-ring.

32 trees were felled (23 L. decidua, 8 P. abies, 1 P. cembra) on the study site and 123 cross-sections prepared, yielding a total of 154 injuries. Age of the selected trees averaged 25.8 yrs, with the oldest one having 44 and the youngest 12 annual rings. In total, 207 GD were identified on the cross-sections as a reaction to the 154 injuring impacts. TRD represented the most common reaction to impacts and were observed in 147 cases (95.5%).

(4) - Remarques générales
 

(5) - Syntèses et préconisations
 

Références citées :

Bearth, P.: Geologischer Atlas der Schweiz 1:25000, Niklaus, (Atlasblatt 71), Schweizerische Geologische Kommission, 1980.

Bearth, P.: Geologischer Atlas der Schweiz 1:25000, Simplon, (Atlasblatt 61), Schweizerische Geologische Kommission, 1973.

Bräker, O. U.: Measuring and data processing in tree-ring research – a methodological introduction, Dendrochronologia, 20, 203– 216, 2002.

SMI (Swiss Meteorological Institute): Annals of the Swiss Meteorological Institute, daily precipitation sums 1982–2007, Zurich, http://www.sma.ch, 2007.

Stoffel, M., Lièvre, I., Monbaron, M., and Perret, S.: Seasonal timing of rockfall activity on a forested slope at Täschgufer (Valais, Swiss Alps) – a dendrochronological approach, Z. Geomorphologie, 49, 1, 89–106, 2005.

Stoffel, M.: Dating past geomorphic processes with tangential rows of traumatic resin ducts, Dendrochronologia, in press, 2008.

Stokes, M. A. and Smiley, T. L.: An Introduction to Tree-Ring Dating, Chicago, University of Chicago Press, 73 pp., 1968.