Réf. Bussmann & Anselmetti 2010 - A

Référence bibliographique complète

BUSSMANN, F., ANSELMETTI, F.S. 2010. Rossberg landslide history and flood chronology as recorded in Lake Lauerz sediments (Central Switzerland). Swiss Journal of Geosciences, Vol. 103, 43–59. [Etude en ligne]

Abstract: The southern slopes of Rossberg mountain, Central Switzerland, on which one of the largest historic landslides of the Alpine region was released in 1806 ad (Goldauer Bergsturz), are prone to large-scale mass wasting processes. This has led to numerous sliding events, which are well-recognizable in the modern topography but lack accurate dating. In order to provide new insights into the timing and the processes associated with past landslides as well as into the frequency of exceptional flood events, long sediment cores were retrieved from the subsurface of Lake Lauerz that lies in the pathway of these landslides and that records strong runoff events with typical flood layers. Analyses of the recovered cores display a sedimentologic succession with variable fingerprints of past landslides and flood events, depending on the coring location within the lake. The landslide signature can be calibrated using the 1806 ad event: An organic-rich peaty unit, which is found in two cores located close to the rockmass impact, points towards a sudden, gravity spreading-induced lateral displacement of the swampy plain where parts of the rock mass were accumulating. This rapid lateral mobilization of soft sediments, and not the rock masses, acted as ultimate trigger for the reported ~15 m-high impulse waves on the lake. In the more distal areas, the 1806 ad event led to the deposition of a thick, organic-rich redeposited layer. The 10 m-long core from the distal basin covers a radiocarbon-dated ~2,000 years sedimentation history and contains a highly similar event layer that was deposited in 810 ± 60 ad. This layer is most likely the product of a major historic landslide, known as Röthener Bergsturz, which, based on scarce historical reports, was commonly dated to 1222 ad. In the 2,000 years record, [the authors] identify three periods with enhanced occurrence of flood turbidites dated to 580–850 ad, 990–1420 ad, and 1630–1940 ad. Among the 54 detected flood layers, 6 probably mark exceptionally heavy rainfall events that are dated to ~610, ~1160, ~1290, ~1660, ~1850, and ~1876 ad, the latter being associated to one of the most intense rainfall events ever recorded instrumentally in the region.

Mots-clés

Lauerz - Goldau - Lake sediments - Rock avalanche - Landslide succession - Impulse wave - Gravity spreading - Flood events

 

Organismes / Contact

• Department of Earth Sciences, Geological Institute, ETH Zürich, Zürich, Switzerland (e-mail: bussmannfelix@gmail.com)
Eawag, Swiss Federal Institute of Aquatic Science and Technology, Department of Surface Waters, Dübendorf, Switzerland (e-mail: flavio.anselmetti@eawag.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

 Précipitations

 

 Crues et inondations ; Mouvements de terrain

 Glissements de terrain

 

Pays / Zone

Massif / Secteur

Site(s) d'étude

Exposition

Altitude

Période(s) d'observation

 Suisse

Canton de Schwyz

Lake Lauerz ; Rossberg landslide ; Rivière Steiner Aa

 

 

Reconstruction de l’activité des crues sur 2000 ans

 

(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

[Conclusions] : The analysis of four long piston cores containing the lacustrine succession deposited in Lake Lauerz during the past 2,000 years revealed the following main conclusions:

1. The catastrophic impulse wave observed in the course of the 1806 AD rock avalanche on Lake Lauerz was triggered by a sudden, gravity spreading-induced lateral displacement of a considerable amount of swamp deposits into the lake basin. This contradicts previous suggestions that the direct impact of the slide mass into the water body caused the devastating wave. The mobilization of the swampy material was the product of a mechanical failure of these deposits due to the rapid loading caused by the accumulating rock mass. As a consequence, [the authors] propose a conversion from an originally subaerial into a partially submerged landslide–water body interaction to be the ultimate trigger of the reported impulse waves. Such a process has furthermore implications on impulse-wave hazard assessments, since even shore-distal rockfalls may cause devastating water movements.

2. Basinal landslide event layers can be distinguished from flood layers on the basis of a higher TOC content caused by organic macroremains and by lower bulk density values. A prominent mass movement-induced event layer is dated with 810 ± 70 AD and is proposed to represent the fingerprint of a major rock avalanche, the so-called Röthener Bergsturz. Based on our results, [the authors] suggest this mass-movement event to be substantially older than previously assumed.

3. The natural relocation of the Steiner Aa’s river mouth during a exceptional rainfall event in 1934 AD is clearly identifiable in two cores from the modern delta area and resulted in a significant increase of sedimentation rate at these particular locations. This increase contributed substantially to the rapid growth of the actual prograding delta fan. [The authors] assume that without any remedy measures, the remaining part of the shallow northwestern bay of Lake Lauerz will be separated from the rest of the lake as an isolated pond in less than 200 years.

4. [The authors] identify three periods with clusters of frequently occurring flood events, which are dated to 580–850 AD, 990–1420 AD, and 1630–1940 AD, each revealing a mean recurrence interval of major flood layers of ~20 years. These periods are separated from each other by comparably calm periods, during which almost no flood layers were deposited.

5. Assuming flood layer thickness to be a proxy for the magnitude of a heavy rainfall, 17 events within the past 2,000 years are supposed to reveal runoff events of higher intensity as the heavy rainfall in 1934 AD pointing towards the occurrence of highly catastrophic flood events during the analyzed period. Within these 17 flood layers, 6 are outstanding in terms of layer thickness and are dated to approximately 610, 1160, 1290, 1660, 1850, and 1876 AD, respectively. The most recent one is representing a well-known flood event involving the highest amount of precipitation ever recorded in numerous regions of the Swiss Plateau (Pfister 1999).

6. Traces of prehistoric mass wasting events (e.g. the postulated ‘Oberarther landslide’) were beyond the reach of the coring techniques applied in this study. However, this pilot work points towards the ability of using lake sediments for reconstructing past landslide activity. Additional drilling would probably be the key to further establish the prehistoric chronology of the Rossberg’s mass wasting succession.

Observations

 

Modélisations

 

Hypothèses

 

 

Paramètre de l'aléa

Sensibilité des paramètres de l'aléa à des paramètres climatiques

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

 

 

The main goals of the present study are (1) recognition of the sedimentological fingerprint created by the 1806 AD sliding event, (2) scanning the sedimentary archive for similar patterns that were created by comparable preceding events and (3) dating of detected events within the sedimentary record in order to reconstruct the mass movement history of Lake Lauerz. Further goals include (4) the reconstruction of flood event history based on recovered and dated flood layers and (5) reconstruction of the infilling rates to predict the future evolution in morphobathymetry of Lake Lauerz [...].

A single-channel reflection seismic survey was performed in order to (1) image the geometries of the lacustrine sediments, (2) detect possible event layers due to distinct seismic facies, (3) determine appropriate coring locations for the retrieval of both, short and long cores, and (4) generate a bathymetric map. […] In 2004, two short gravity cores and four long piston cores were retrieved from four different locations. […] Maximum length of the short cores was 1 m. […] The maximum coring depth of 9.77 m was achieved in the deepest part of the lake in the eastern basin. Physical core logging including p wave velocity, gamma-ray attenuation bulk density and magnetic susceptibility was performed on the non-splitted core sections in 5 mm-intervals […].Further core preparation included opening, photographing and visual description of the cores. […] No regular sampling interval but rather targeted sampling within visually outstanding sedimentary layers was performed. […] Furthermore, a coulometer was used for the determination of the total organic carbon (TOC) and total inorganic carbon (TIC; a measure for carbonate content). A total of five samples consisting of terrestrial plant macrofossils were extracted from the two basinal cores and submitted to 14C dating.

 

(4) - Remarques générales

 

 

(5) - Syntèses et préconisations

 

Références citées :

 Pfister, C. (1999). Wetternachhersage: 500 Jahre Klimavariationen und Naturkatastrophen (p. 304). Bern: Paul Haupt.