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Automatised range-gated Doppler radar measurements of aspontaneously released avalanche at the Vallee de la Sionne

test site

L. Rammer1 , H. Schreiber2 , M. Hiller3 , F. Dufour3 , B. Sovilla3 , P. Gauer1 and M.A. Kern1∗

1Federal Research and Training Centre for Forests, Natural Hazards and Landscapes (BFW), Innsbruck, Austria2Institute for Communication Technology and Wave Propagation, Technical University Graz, Graz, Austria

3WSL Institute for Snow and Avalanche Research SLF, Davos Dorf, Switzerland

ABSTRACT. Range-gated Doppler radar measurements provide decisive informa-tion about the motion of an avalanche along the avalanche track. However, up to now,such measurements have been depending on artificial avalanche release: the devicehad to be started manually at the time of avalanche release.We present an automatised Doppler-radar setup that is installed it the bunker of theVallee de la Sione test site. The radar measurement is automatically triggered by thegeophone triggering device of the test site and autonomously performs range-gatedDoppler radar measurements of the avalanche velocity along the track.After shortly lining out on the principle of range-gated in Doppler-radar measurementsand on the analysis procedure, we present Doppler-radar data of an avalanche thatspontaneously released at the Vallee de la Sionne test site on 17 March 2009. Inparticular, we track the time evolution of the avalanche velocities in the range gatesalong the avalanche track to obtain a coherent picure of the avalanche motion from theevolved state to its standstill.

Keywords: avalanche dynamics, radar measurements, spontaneous avalanche

1 INTRODUCTION

Doppler radar measurements are an important toolto gain informationn on the velocity structure of largeavalanches. Following the pioneering Doppler radarmeasurements by H.U. Gubler at the Lukmanier pass(Gubler, 1986; Salm and Gubler, 1984), Doppler radarmeasurements were mainly performed at the largeavalanche field sites in Vallee de la Sionne in Switzer-land (Rammer et al., 2007) and Ryggfonn in Norway(Gauer et al., 2007). The appropriate interpretation ofDoppler radar velocity measurements of avalanchesis still amatter of debate: the radar wavelengt deter-mines the spatial resolution, i.e. the minimum sizeof particles that cause reflections contributing to the

∗Corresponding author’s address:Martin KernBFW Hofburg-Rennweg 16020 Innsbruck, AustriaTel: ++43 512 573933 5103; Fax: ++43 512 573933 5250;E-mail: martin.kern@uibk.ac.at

measured velocity spectrum. Accordingly, radar spec-tra have to be related to the dense or the suspendedparts of the measured avalanches. Advanced tech-niques allow to observe the evolution of velocity spec-tra along the track that can be divided into so-calledrange-gates (RG) (Rammer et al., 2007; Gauer et al.,2007). Up to now, operating staff was necessary tostart the radar measurement when the avalanche wasapproaching the measurement range. Consequently,avalanche velocity measurements with Doppler radarshave been confined to artificially released avalanches.We present a setup recently installed in the bunker atthe Vallee de la Sionne test site that is triggered bygeophones to autonomously acquire and store datafrom avalanches that spontaneously release at the testsite. After briefly introducing the Vallee de la Sionnetest site and the principle of range-gated Doppler radarmeasurements, we present velocity data from a spon-taneous avalanche that could be tracked along theavalanche path until its standstill.

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2 SETUP

2.1 The principle of range-gated Doppler radar mea-surements

The velocity of a reflecting body moving relative to aradar antenna can be determined using the so-calledDoppler frequency which is proportional to the veloc-ity. The Doppler frequency is defined as the the fre-quency shift between the transmitted radar signal andthe signal that is received after reflection at the movingobject. Snow avalanches do not move as rigid bodiesbut exhibit a velocity distribution. Accordingly, radarmeasurements on avalanches yield an intensity spec-trum of Doppler frequencies from which an intensityspectrum of the velocities of the moving snow withinthe target area of the radar beam can be derived.

Defining a chain of subsequent intervals alongthe avalanche track and periodically (and quasi-simultaneously) recording the Doppler frequencies ofa pulsed radar source from each of the sections re-sults in velocity spectra for all these sections along thetrack. This procedure is called ”range-gating” (see 1)and, by periodic repetition, allows to track the spatio-temporal evolution of the avalanche motion along itspath: assuming that an avalanche does not stop ina range-gate, it will apear in the next downhill range-gate (RG). This can be used to compute accelerationsalong the track, which is helpful to check and validatemodel predictions (Gauer et al., 2007). The pulsedDoppler radar (PDR) device of the Austrian FederalResearch and Training Centre for Forests, NaturalsHazards and Alpine Timberline (BFW) is operated inthe range-gating mode. Such a device is permanentlyinstalled at the VdlS field site.

Interpretation of radar spectra is a delicate matterthat is extensively discussed in (Rammer et al., 2007;Gauer et al., 2007). At this place, we confine our-selves to stating, that it makes a difference whetherthe intensity maximum of a spectrum, approximatelycorresponding to the velocity of the main mass, or themaximum velocity with an intensity above the noiselevel is considered, which rather can be regarded asa measure of the velocity of the fastest particles inthe respective range-gate (Gauer et al., 2007). If arange-gate starts to encounter a signal, this indicatesthat the avalanche front is entering the range-gateand, accordingly, the velocity considerations refer tothe avalanche front in this case.

2.2 The Vallee de la Sionne test site

At the SLF real-scale test site Vallee de la Sionne(VdlS) in Switzerland, natural and artificially released

avalanches can be investigated that flow down a∼ 2700 m long track with a vertical drop of 1300m. A 20 m high mast is situated at the start ofthe run-out zone of the track. This mast is instru-mented with measurement devices for flow velocity(Kern et al., 2009), impact pressure, density, flowheight and air pressure (within the powder part ofavalanches). FMCW (frequency-modulated continu-ous wave) radars mounted flush to the track surface atthree locations along the avalanche track record flowheights and the vertical flow structure of avalanches.Additional obstacles close to the mast are instru-mented with pressure gauges to study avalanche im-pact pressures (Schaer and Issler, 2001; Sovilla et al.,2008a,b). A manned measurement bunker at thecounter-slope houses the data-acquisition system andDoppler radars to track the avalanche (front) velocity.The site is described in detail by Issler (1999, 2003)and Sovilla et al. (2006, 2008a,b).

Depending on the snow and weather situation,two or three artificially released avalanches can berecorded each winter, though there have been win-ters in which no avalanches could be released.If avalanches spontaneously release, the measure-ments are triggered by geophone signals. However,radar measurements could not be performed automat-ically, as the radars are placed behind large steel shut-ters in the bunker which have to be manually openedbefore an avalanche release and need to be closedbefore the impact of the avalanche on the bunker, toprotect the radar antennas.

2.3 Automatised radar measurements

Pulsed and continuous radar measurements havebeen compared to optical velocity measurements fromthe mast and to videogrammetry velocity data of theavalanche front (Rammer et al., 2007). It could beshown that these different velocity measurement tech-niques yield consistent results. However, so far thepulsed and continuous radar measurements had to bemanually triggered and operated and, accordingly, de-pended on artificially released avalanches. To also al-low for automatised radar measurements, the antennaof the BFW pulsed Doppler radar (PDR) was placed atan outer side-wall of the measurement bunker and thetransmitter/receiver electronics and the data acquisi-tion setup was left in the bunker, where it is connectedto the geophone trigger device: the ground vibrationscaused by a spontaneous avalanche are registered bya geophone, which triggers the measurements at themast and the BFW pulsed Doppler radar (PDR).

The PDR of the BFW is operated at 5.8 GHz whichcorresponds to a target particle size of ≥ 5 cm. That

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Figure 1: Schematic of range-gated pulsed Dopplerradar measuremet at the VdlS test site.

is, loosely speaking, the radar beam is reflected onlyby larger snow particles, that make up the dense andsaltation layer of an avalanche. The radar pulses areemitted by a patch antenna with an horizontal and ver-tical aperture of 11◦ and 9◦, respectively. Starting at500 m distance from the antenna (i.e. from ≈ 150 mdownhill the mast), the avalanche track is divided into15 subsequent range-gates (RG) with a longitudinallength of 50 m, where the measurement mast approx-imately corresponds to RG 4 (fig. 1). Velocity spectraare retrieved from all range-gates each 0.163 s, allow-ing to trace the time evolution of the velocity distribu-tions in all RGs after they have been passed by theavalanche.

3 AVALANCHE OF 17 MARCH 2009

3.1 Weather and snow situation at time of release

In the week before 17 March 2009, the weather in therelease area of the VdlS test site was mild, day tem-peratures being above 0◦C, rising up to +5◦C on 16March and even to +7◦C on 17 March. The windswere moderate from north-eastern direction and thesnow-cover was almost isothermal, only the surfacefreezing overnight. On 17 March 2009 at 03:45 pm,a wet snow avalanche spontaneously released from arocky couloir at the southern part of the release area,below pt. Pra Roua. Note that the release was not aslab release but rather a loose snow release, probablyby wet snow falling from steep, rocky parts of the slope(fig. 2).

Figure 2: VdlS test site after spontaneous avalanchefrom 17 March 2009.

3.2 Automatic recording of the event

The spontaneous avlanche of 17 March representsa very special situation: though triggering the mea-surements at the mast by the geophone trigger, theavalanche did not reach the mast, so that no ve-locity and pressure data from this measurement de-vice are available. As there is no manually recordedvideo footage as well, the automatic PDR measure-ments represent the only record of the dynamics ofthis avalanche.

3.3 Data processing

The data recorded by the PDR device consist of timeseries of Doppler frequency intensity spectra

I(fD, t)i, i = 1 . . . 15 (1)

for each of the 15 RGs (labelled by i), where the timeseries are discrete with a time step of ∆t = 0.163 s.The Doppler frequency fD is the frequency shift be-tween the transmitted and the reflected radar signaland is proportional to the velocity u relative to the an-tenna:

u =cfD

2fscos α, (2)

where fs is the transmitted radar freqency, c is thespeed of light and α is the angle between the veloc-ity vector of the moving mass and the radar beam. Asthe avalanche is moving towards the antenna and theradar beam does not penetrate too far through snow,we can approximate c by the vacuum speed of lightand assume cos α ≈ 1. Using the proportionality be-tween velocity and Doppler frequency, the Doppler in-

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tensity spectra (1) can be converted into velocity inten-sity spectra

I(u, t)i, i = 1 . . . 15. (3)

Figure 3: Time evolution of velocity intensity spectraat RGs 14 (a), 10 (b) and 5 (c). The intensity is grey-scaled, black dots represent the weighted average ofthe velocity.

We plot the time evolution of the velocity spectra forthree RG (i= 5, 10, 15) in Figure 3, noting that thevelocity intensity spectra provide a more comprehen-sive picture of the avalanche motion than the averagedmean velocities extracted from the spectra do. For ex-ample, the time evolution of the spectra may exhibitbranches respresenting avalanche parts moving withdifferent mean velocities (see fig. 3(b)). Generally, thetime evolution shows a decrease of velocity, starting

with the high velocities of the avalanche front enteringthe RGs. The closer the RGs are to the antenna, theshorter the time series become. RG 5 represents thearea extending 50 m uphill from the mast. Here we ob-serve the shortest time series, representing the headof the avalanche, coming to a standstill right in front ofthe mast (see fig. 2).

4 CONCLUSION

We have presented a new experimental setupthat automatically performs automatically range-gateDoppler-radar measurements at the VdlS test site.The setup operates independent of artificial avalancherelease and manual triggering and therefore does notrequire presence of operating staff. Furthermore,it is able to capture avalanches that do not hit themeasurement mast (i.e. stop before). The methodof range-gating allows to trace avalanches along thetrack until standstill. We have presented data ofa spontaneously released wet snow avalanche thatdid not reach the mast so that the radar measure-ments represent the only available information on theavalanche motion. Ideally, measurement data fromthe mast and from the radar should be used comple-mentary and to calibrate the radar measurements us-ing the point-measurement data. Advanced analysistechniques allow for comprehensive information on theavalanche motion along the track.

ACKNOWLEDGMENTS

This work has been funded by BFW and, partially byan R’equip grant of the SNF. Many thanks are due tothe technical staff of BFW and SLF who made thispiece possible by their efficient and professional work.

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