Hazard Mapping and GIS: Simulating Avalanche, Debris Flow and Rockfall
Marc Christen, Perry Bartelt and Urs Gruber
November 26, 2008 - Geomundo, Brazil
The extreme avalanche winters of 1951 and 1999 in Switzerland clearly demonstrated the need for hazard mapping in mountainous areas. Hazard mapping helps in devising mitigation measures such as the construction of natural and artificial protection against avalanches, debris flow and rock-fall. The authors developed a new system called RAMMS (Rapid Mass Movements) comprising avalanche, debris-flow and rock-fall, protect and visualisation modules linked to a GIS.
The system enables prediction of avalanche run-out distance, impact pressure and flow velocity using a digital terrain model (DTM), and is coupled with a GIS to simplify specification of terrain and initial conditions. Automatic GIS schemes may determine the location and magnitude of the conditions under which the avalanche was released, as well as friction parameters. The GIS-based user interface allows specification of a multi-layered pack of snow that may be entrained according to an updated Grigorian–Ostroumov procedure. Three entrainment mechanisms are used: frontal ploughing, step-entrainment and basal erosion. Entrainment rates are based on such parameters as avalanche speed and strength of snow cover. The model was calibrated using extreme avalanche events from the catastrophic winter of 1999, events in the SLF database, and an artificially released powder-snow avalanche at the Vallée de la Sionne test-site.
Debris flow exhibits many modes of behaviour and may resemble either landslide or flood, depending on material, such as water and clay, and on the nature of the path of flow. Modelling is usually approached from the angle of both soil mechanics and hydraulics. A 2D-model is being developed to simulate run-out distance, velocity and flow-depth. The two-phase approach, based on shallow-water equations modified for granular flows, potentially allows for modelling the many modes of behaviour. However, details of frictional relationships and phase coupling for these flow types are not yet well understood. In most two-phase models developed so far, the Mohr–Coulomb description of stress–strain behaviour during the solid phase has been shown to be useful. Our model is based on a momentum-exchange concept, although the details of phase interactions remain a topic of intensive research. The friction parameters are therefore the internal and basal Coulomb friction angles, the pseudo-Chézy friction coefficient, a momentum exchange coefficient, bulk densities of the solid (equivalent dry bulk density) and fluid phases and initial volumetric concentrations. The resulting system of partial differential equations is strictly hyperbolic and is resolved using a finite-volume technique. Calibration is necessary to constrain model parameters. Our model will be calibrated using data from real debris-flow events recorded at our observation stations. This data includes front velocity and flow-height measurements and video observations, and coupled measurement of flow depth, fluid-pore pressure and normal and sheer forces. Before initialising a calculation, appropriate parameters can be selected specifying the nature of the debris flow, such as granular or muddy, and input volume via a hydrograph. Graphical output will allow overlaying simulated velocity, run-out and deposition profiles directly on topographic maps.
The Swiss Federal Institute (SLF) is developing RAMMS in co-operation with Creative Software Systems (CREASO). Since many mitigation measures protect against other hazards for which they were not originally intended, the common interface allows for a comprehensive evaluation of such measures. As the system is linked to a GIS environment, RAMMS provides a powerful, user-friendly tool for hazard mitigation studies in mountainous regions affected by gravity-driven, rapid mass movement.
Biography of the Author(s)
Marc Christen gained a degree in Civil Engineering from ETH, Zurich, in 1997. He has been a member of the Snow and Avalanche Research group (SLF) of the Swiss Federal Institute since then, currently in the post of IT specialist.
Perry Bartelt gained a masters degree in Civil Engineering from the University of Maryland in 1983 and a PhD from the Swiss Institute of Technology, Zurich, in 1989. He worked for Hilti AG for three years, until 1993. He is currently head of the research unit at SLF, where he has been employed since 1994.
Urs Gruber gained a degree in Geography and History from the University of Zurich in 1992 and a PhD at WSL Birmensdorf in 1998. He was a member of SLF until 2006.
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