Risk analyses and risk management slope instabilities in alpine environments Stefan Eder (ILF Consulting Engineers Co. Ltd.) Gerhard Poscher (p+w geo-zt Ltd.) Christoph Prager (alp-s)
Contents Introduction General remarks on risk management of rapid slope movements such as huge catastrophic rockfalls Case study 1 Eiblschrofen rockfall Case study 2 Creeping slope Zintlwald Conclusions
Mass movements in Tyrol and surrounding temporal and spatial distribution Zintlwald Innsbruck Eiblschrofen
Prominent examples of huge catastrophic mass movements in Tyrol Comparison of project areas with reference systems as a part of the risk management process
Standards, geotechnical environment and authorisation procedures for land-use in Tyrol (Austria) regarding geohazards No defined standards for the risk estimation of geohazards Implementation of the geohazards in the so called Alpen-Konvention (Convention of the Protection of the Alps, Protocol on Soil Protection) and introduction of a checklist by the Geological Department of the Tyrolean government (Heissel et. al., 2004), which must be used for designs Geohazards like rockfalls are indicated in the land-use regulations only marked in brown, but without legal consequences, in contrast to areas marked in red which is an indication for floods and avalanche threats Design-events are defined by expert groups individually
Land-use and Standards (examples) TIRIS Unstable rock-masses (scarp area of the large Tschirgant rockslide)
Case study 1 Eiblschrofen rockfall (summer 1999) Eiblschrofen - scarp, accumulation areas and localities of retention dams
Mitigation measures - Protection dams Heigth: 25 / 15 m Dam Volumes: 130.000 / 50.000 m³ Retaining capacities: 220.000 / 80.000 m³ Gradient of 60 under the crest to reduce ski-jumping-effects
Mitigation measures - Monitoring systems Laser Scanner
Geologic profile base for event scenarios Schists Dolomite, Database: ILF Consulting Engineers on behalf of WLV Tyrol Autumn 1999: Protection dams erected, End of dam evacutations Monitoring system still activ Some remaining questions regarding land use and possible additional events
Conceptual model for the failure (event) scenarios Pult-plains Database: Gruppe Felsmechanik (GFM), 2003 Expert group Rock mechanics, on behalf of the WLV Tyrol.
Influencing factors Debris Volume of the event scenario evaluated with a 3D-Modell geo-byte Salzburg on behalf of WLV Tyrol Disintegration factor Exampleof thevolumeof themovingrock-mass with a disintegration factor of 1,5 Scenario Section Volume (m³) 1 East 274.500 Middle 210.000 West 351.000 Sum 835.500 2 East 412.500 Middle 433.500 West 684.000 Sum 3 Sum East Middle West 1.530.000 357.000 465.000 1.416.000 2.238.000 Accumulation geometry (accumulation scenarios) 4 East Middle 544.500 600.000 Retention capacity (m³) minimal accumulation middle accumulation maximum accumulation Sum West 1.750.500 2.895.000 Main dam 1.370.000 2.470.000 4.420.000 5 East Middle 720.000 1.002.000 Western dam 520.000 870.000 1.540.000 Sum West 2.752.500 4.474.500 Sum 1.890.000 3.340.000 5.960.000
Damage scenarios No damage the whole rockfall-debris is retented by the dams Stray boulders interaction of blocks in the rockfall mass may cause steep flight paths - some boulders can probably overcome the barrier dams Overflow of the dams this main hazard scenario has been evaluated by a comparison of the rockfall volumes and the retention space (- retaining capacity) of the dams Residual retention space [m 3 ] at: Failure Scenario 1 2 3 4 5 Min. Acc. Scenario 1,054,500 360,000-348,000-1,005,000-2,584,500 Medium Acc. Scenario Max. Acc. Scenario 2,504,500 5,124,500 1,810,000 4,430,000 1,102,000 3,722,000 445,000 3,065,000-1,134,500 1,485,500 http://www-geol.unine.ch/cours/geol/massflow.html
Event tree conceptual model
Comparison with well-investigated reference systems Comparisons to reference systems (geological and legal aspects) regarding the land-use show the need for the definition of design events. It is not common legal practice to use worst-case scenarios as design events, This has been verified by reference-projects regarding the authorisation aspects. It is based in the land-use-laws that expert groups (in Austria: WLV, Institution for Torrent and Avalanche control ) can decide whether land-use should be allowed or not, even in restricted areas. In similar projects the design events have been defined by expert groups for the dimensioning of measures and further land-use.
Case study 2 Creeping slope Zintlwald (summer 2005) Dr. G. Poscher, p+w geo zt, Pictures taken from Poscher, G., Mattle, B. & Henzinger, J. (2006): Hangbewegung Zintlwald: Geologisch-geotechnischer Bericht.- Unveröffent. Bericht i.a. des Amtes der Tiroler Landesregierung.
Investigation of damaged infrastructure Deformation rates (dm/d up to 1.3 m/d) Dr. G. Poscher, p+w geo zt, Pictures taken from Poscher, G., Mattle, B. & Henzinger, J. (2006): Hangbewegung Zintlwald: Geologisch-geotechnischer Bericht.- Unveröffent. Bericht i.a. des Amtes der Tiroler Landesregierung.
Measurements on-line Monitoring System Highly active area Slowly creeping land slide, deep creeping process Smaller events like local rock-falls Highly active area Smaller events at the bank of river Rosanna Dr. G. Poscher, p+w geo zt, Pictures taken from Poscher, G., Mattle, B. & Henzinger, J. (2006): Hangbewegung Zintlwald: Geologisch-geotechnischer Bericht.- Unveröffent. Bericht i.a. des Amtes der Tiroler Landesregierung.
Conclusions The presented risk analyses are based on different scenarios for the failure mechanisms, accumulation of debris volume and damage Semi-quantitative assessments have been used for the risk analyses Design-events should be defined by expert groups like foreseen in the landregulation-laws Worst-case scenarios have not been considered a design-event and are therefore neither a basis for measures, nor for decisions regarding future land-use Also reference systems regarding geology and authorisation processes should be taken into account Also the naturally occurring basic geohazards should be considered for a specific environment for a sustainable geohazard management Multidisciplinary automatic monitoring systems with alarm values are a powerful tool for this sustainable risk management
Thank you very much for your attention! Tschirgant ( G. Poscher, p+w geo zt gmbh) stefan.eder@ibk.ilf.com