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.21 On the predictability of snow avalanches Schweizer Jürg WSL Institute for Snow and Avalanche Research SLF, Flüelastrasse 11, CH-7260 Davos Dorf, email: schweizer@slf.ch Snow avalanche are rare events and large avalanches that cause damage or harm to people can be called extreme events. In the general context of prediction and predictably of extreme events, we will examine whether snow avalanches are predictable based on a few examples of forecasts at different scales. Clearly, avalanche predictability depends on scale. When the regional avalanche danger is High or Very High, an avalanche is likely somewhere in the region. However, even at such high danger levels, the release probability in a single avalanche path is well below 50% (typically on the order of 1-10%). This means that a single avalanche is a rare event, which is not predictable even when higher danger levels prevail. At the lower danger levels – relevant for backcountry recreation – the release probability is significantly lower. The low release probability does not mean that the risk is low. Even with low occurrence probability the risk might be too high to be acceptable so that comprehensive preventive measures (e.g. temporary road closures) are required. Though the quality of snow avalanche forecasts is typically rather poor, i.e. the skill score in a statistical sense is low, low probability forecasts can be useful if large values are at stake. .22 A portable radar interferometer for the measurement of surface deformation Strozzi Tazio, Werner Charles, Wiesmann Andreas & Wegmüller Urs Gamma Remote Sensing, Worbstrasse 225, CH-3073 Gümligen ({strozzi,cw,wiesmann,wegmuller}@gamma-rs.ch; www.gamma-rs.ch) Satellite radar interferometry has been used extensively for ground-motion monitoring with good success. In the case of landslides, glaciers and rock-glaciers, for example, space-borne radar interferometry has a good potential to get an overview on surface deformation. The role of space-borne radar interferometry as an element in a warning system is however constrained by the specific space-borne radar imaging geometry, the typical multiple-week repeat-interval, and uncertainties in the data availability. Most of these limitations can be overcome with an in-situ radar imaging system [1,2]. Gamma Remote Sensing has developed a portable radar interferometer that utilizes real-aperture antennas, each 2 meters in length, to obtain high azimuth resolution [3,4]. Images are acquired line by line while rotating the transmitting and receiving antennas about a vertical axis (Figure 1). The installation effort is relatively small and the instrument is portable and can be battery operated. Individual measurements can be taken in less than 15 minutes and the acquisition time is limited primarily by the speed of the rotational scanner. Each image line is acquired in approximately 2ms hence there is little or no movement of the scene to introduce temporal decorrelation. Phase differences between successive images acquired from the same location are used to determine line-of-sight displacements δ from the differential phase φ via the relationship δ = -λ⋅φ/4π where λ is the wavelength. The instrument operates at 17.2 GHz (Ku-Band, λ=17.4 mm) with a displacement measurement sensitivity better than 1 mm. The range resolution of the radar δr = c/2*B is determined by the 200 MHz bandwidth B and is equal to approximately 75 cm. Because this is a real-aperture imaging system, the azimuth resolution is determined by the antenna beamwidth and slant range R, δaz = R sin θ . In the case of our terrestrial radar, the azimuth beamwidth is 0.4 degree yielding an azimuth resolution of about 7m at a slant range of 1km. The instrument uses two receiving antennas with a short baseline to form an interferometer. Phase differences between simultaneous acquisitions by these antennas are used to calculate the precise look angle relative to the baseline, permitting derivation of the surface topography. Expected statistical noise in the height measurements is on the order of 1 meter. In this contribution the design, measurement principles and characteristics of the portable radar interferometer are presented. Results obtained in a series of experiments started in September 2007 over glaciers (e.g. Rhonegletscher and Gornergletscher) and landslides (e.g. Tessina, Pian San Giacomo) are also discussed. 26 Symposium 9: Natural Hazards and Risks
266 Symposium 9: Natural Hazards and Risks Figure 1. The portable radar interferometer deployed over the Gornergletscher showing the rotational scanner, antenna support structure, antennas, and microwave assembly. REFERENCES [1] Antonello G., N. Casagli, P. Farina, D. Leva, G. Nico, A. J. Sieber and D. Tarchi, Ground-based SAR interferometry for monitoring mass movements, Landslides, 1(1): 21-28, March 2004. [2] Pieraccini M., L. Noferini, D. Mecatti, C. Atzeni, G. Teza, A. Galgaro and Nicola Zaltron, Integration of Radar Interferometry and Laser Scanning for Remote Monitoring of an Urban Site Built on a Sliding Slope, IEEE Trans. Geosci. Remote Sensing, 44(9): 2335-2342, 2006 [3] Wiesmann A., C. Werner, T. Strozzi and U. Wegmüller, Measuring deformation and topography with a portable radar interferometer, 13th FIG International Symposium on Deformation Measurements and Analysis and 4th IAG Symposium on Geodesy for Geotechnical and Structural Engineering, Lisbon, Portugal, May 12-15 2008. [4] Werner C., A. Wiesmann, T. Strozzi and U. Wegmüller,Gamma’s portable radar interferometer, 13th FIG International Symposium on Deformation Measurements and Analysis and 4th IAG Symposium on Geodesy for Geotechnical and Structural Engineering, Lisbon, Portugal, May 12-15 2008. .23 Rockfall modelling applied to rockfall protection design Thüring Manfred Istituto Scienze della Terra, Scuola Universitaria Professionale della Svizzera Italiana, CP 72, CH-6952 Canobbio (manfred.thuering@supsi. ch) Rockfall modeling by computer simulations can be used to plan and design rockfall protection measures. RockSim3D is a recently developed software to simulate the three-dimensional process of rocks falling down a hill slope. The falling rocks are modeled as individual, dimensionless, spherical particles. The interaction with the digital representation of the terrain occurs at the center of the particles. The computer program is a stand-alone Microsoft Windows application, reads and writes ESRI raster and shapefiles as input and output and needs a GIS (geographic information system) for pre- and post-processing of input and output. The software is developed in Microsoft Visual Studio and uses the open-source library GDAL for reading and writing GIS vector and raster files. The main input of the software is a raster file which defines the digital terrain and the surface properties, and a shapefile
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