International Symposium on Mitigative Measures against Snow ...

<strong>International</strong> <strong>Symposium</strong> on Mitigative Measures against Snow Avalanches
Egilsstaðir, Iceland, March 11–14, 2008
Figure 2 Overview of the snow net test site “Hafelekar”.
2.1.1 Measurements on the snow nets
Figure 3 shows the measuring equipment installed on the snow nets. Measuring instruments
were only installed on one half of each net type. The axial and transverse forces in the base
point of the posts are measured with a load measuring pin of the type Magtrol LB236. For the
measurement of the tension forces in the anchors and all wire ropes, a special sensor
containing HBM KMR200 or KMR300 has been designed at the BFW. The inclination of the
supporting net and the posts is recorded by a biaxial inclinosensor HLPlanar NS-25/E2. The
data is recorded every thirty minutes with Campbell data loggers that are fixed on the posts.
Every four hours the measured values are directly transferred to an internet platform and
automatically edited in a graphical way.
2.1.2 Measurements on the snow cover
During field visits the snow distribution, snow depth and snow density were measured.
Besides the manual observation the temporal variation of snow depth was documented with a
terrestrial laser scanner (Riegl company, LPM-i2k). Laser ranging is based on measuring the
time-of-flight of a short laser signal from the instruments’ transmitter to the target and back to
the receiver. A detailed description of the laser scanner is given by Wiatr (2007). In order to
position the laser scanner five reflectors (tiepoints) have been installed within the test site.
Due to the high reflectivity this points appear as bright spots in the laser scan image, so the
exact coordinates of the laser scanner can be determined. The laser output is an irregular
scatter-plot, therefore data is transformed to a regular grid with the Nearest Neighbour
interpolation. To verify the results of the laser measurements, which means the difference
between summer and winter DEM (digital elevation model), snow depth was measured with
probing at several positions within the test site.
42 Snow loads on defensive snow net systems