[1] The snow surface height was precisely measured, with a laser scanner, before and after the passage of two dry-mixed avalanches in Vallée de la Sionne during the winter of [2005][2006]. The measurements were used to calculate the depth of the deposited snow along each entire avalanche path with a height resolution of 100 mm and a horizontal resolution of 500 mm. These data are much more accurate than any previous measurements from large avalanches and show that the deposit depth is strongly negatively correlated with the slope angle. That is, on steep slopes the deposit is shallow, and on gentle slopes the deposit is deep. The time evolution of the snow depth, showing the initial erosion and final deposition as the avalanche passed, was also observed at one position using a frequency-modulated continuous wave radar. Measurements at a nearby position gave flow speed profiles and showed that the avalanche tail consists of a steady state subcritical flow that lasts for about 100 s. Eventually, the tail slowly decelerates as the depth slightly decreases, and then it comes to rest. We show that the dependency between the slope angle and the deposition depth can be explained by both a cohesive friction model and the Pouliquen h stop model.

ABSTRACT:Low cost mapping using UAV technology is becoming a trendy topic. Many systems exist where a simple camera can be deployed to take images, generally georeferenced with a GPS chip and MEMS attitude sensors. The step from using those images as information picture to photogrammetric products with geo-reference, such as digital terrain model (DTM) or orthophotos is not so big. New development in the field of image correlation allow matching rapidly and accurately images together, build a relative orientation of an image block, extract a DTM and produce orthoimage through a web server. The following paper focuses on the photogrammetric performance of an ultra light UAV equipped with a compact 12Mpix camera combined with online data processes provided by Pix4D. First, the step of image orientation is studied with the camera calibration step, thus the DTM extraction will be compared with conventional results from conventional photogrammetric software, new generation technique of pixel correlation and with reference data issued from high density laser scanning. The quality of orthoimage is presented in terms of quality and geometric accuracy. 2.1

Studies of large-scale sedimentary architecture are mainly based on the interpretation of two-dimensional photomosaics. This method cannot account for the natural rugosity of outcrop exposures, introducing errors in the measurement of geobody sizes and orientations. In the past, three-dimensional outcrop studies have relied on time-intensive fieldwork, with irregular sampling and low geometric accuracy. More recently, terrestrial laser scanning, or LiDAR (Light Detection and Ranging), has been widely applied to small-scale outcrops, but range and accessibility preclude its usage on larger-scale outcrops. Oblique helicopter-based laser scanning, however, allows the collection of tens of kilometres of outcrop sections in a relatively short time frame. In this paper, a procedure for collecting and processing such virtual outcrop data is outlined, and the application of the technique for extracting dimensions of fluvial geobodies from two large and otherwise inaccessible outcrops from Utah is presented. The results are compared to interpretations from more conventional photomosaicking of the same outcrops. Results show that the use of helicopter-based laser scanning enables geoscientists to rapidly acquire georeferenced data that can then be used for sedimentological interpretation and analysis on reservoir scales. It is concluded that helicopter-based laser scanning promotes sedimentological research and is well suited to capturing quantitative geometrical data from large outcrops.

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During winter 1999 three large avalanche events were triggered by explosives at SLF’s avalanche test site, Vallée de la Sionne, canton Valais, Switzerland. One important goal of these large-scale field experiments was to measure the release and deposition volumes of avalanches by photogrammetric methods. In this paper, the photogrammetric measurements of all three avalanches are summarized. For one avalanche event it was possible to realize the whole measuring procedure as planned, and to obtain volume measurements before and after the avalanche triggering In the other two avalanche events, the photographs before the triggering of the avalanche failed. Nevertheless the photographs taken after the avalanche provide valuable information on the fracture depth at the fracture line. The mean fracture depth of the largest avalanche was about 2.10 m, varying between 1 and 3.5 m over a width of > 1000 m. The total volume of the deposition of all three avalanche events was about 1300 000 m3. The deposits are distributed over a length of > 1000 m with depths up to 30 m. The difference between the released and deposited volumes proved that avalanches entrain a large amount of snow along the avalanche track. Furthermore, the snow distribution in the deposition zone provides important information about the behaviour of a dense flowing avalanche in the runout zone.