Measurement of Avalanche Speeds and Forces: Instrumentation and Preliminary Results of the Ryggfonn Project (original) (raw)
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40 Years of NGI’s Full-Scale Avalanche Test-Site Ryggfonn
International Snow Science Workshop Grenoble Chamonix Mont Blanc October 07 11 2013, 2013
The Norwegian Geotechnical Institute (NGI) has been running full-scale avalanche experiments at the Ryggfonn test-site in western Norway for close to 40 years. In 1981, an avalanche catching dam was build in the runout zone to test the efficiency of this kind of mitigation measure. This feature makes Ryggfonn in a sense unique. During the years various kinds of instrumentation and structures have been placed along the avalanche track to gain an in-depth understanding of avalanche dynamics and the interaction of avalanches with constructions. This kind of avalanche measurements is the backbone for the development and calibration of numerical avalanche models. Full-scale avalanche tests are also required as reference for small-scale granular as well as for snow chute experiments. In this paper, we will give a brief summary of some of the hitherto obtained measurements from Ryggfonn.
Cold Regions Science and Technology, 2008
Impact pressures of eight snow avalanches measured at the Swiss avalanche test site Vallée de la Sionne are reported. Avalanche typologies varied between dense and powder. Measurements were performed on obstacles of different shape and dimension. High frequency pressure transducers, sampling at 7.5 kHz with circular diameters of 0.05, 0.10 and 0.25 m were mounted on a 20 m high tubular pylon and on a 5 m high steel wedge. To interpret the influence of sensor dimension on impact pressure measurements, the total pressure exerted on the steel wedge was recorded using two biaxial sensors and compared to the pressure recorded by the single pressure cells. On a small concrete wall, a 1 m 2 pressure plate mounted with 4 load-gauge bolts measured normal forces. At six locations along the tubular pylon (between 1-6 m above ground) optoelectronic sensors recorded the avalanche flow velocity. Flow depths were measured by mechanical sensors. Analysis of high resolution impact forces in combination with velocity measurements allowed us to reconstruct the flow structure. We combined impact pressure with avalanche structure to obtain load distribution and size effects for different avalanche typologies. Measured pressures are compared to existing guidelines procedures. It is shown that actual calculation formulas are not able to properly reproduce the measured pressure values and the load distribution.
Avalanche impact pressure on an instrumented structure
Cold Regions Science and Technology, 2008
Full-scale experiments have been conducted on the Lautaret avalanche test site (France, Hautes-Alpes) to quantify avalanche impact pressure. A structure with a flat surface is used as a large sensor and an inverse analysis procedure is developed to reconstruct the pressure applied on this obstacle. This method is validated by numerical simulations and in-situ impact hammer tests are performed to acquire the frequency transfer function of the sensor. Experimental measurements are carried out with the device during an avalanche and are processed to quantify the impact pressure. Results show that the loading rate can reach up to 100 kPa/s with a nominal pressure of 35 kPa for an avalanche with a front velocity of around 17 m/s and a density of around 100 kg/m 3. The nominal impact pressure of this avalanche, with a Froude number of around 5, is roughly consistent with the kinetic pressure estimation. Interactions between the flow and the structure form a dihedral snow deposit upstream of the obstacle (stagnation zone), modifying the shape of the obstacle and reducing the hydrodynamic drag pressure, which is nevertheless mainly depending on the velocity of the flow. Results are discussed on the basis of the drag coefficient, C r , to the Froude number, Fr, relationship which can be expressed as C r = 10.8 Fr − 1.3 from the data.
The upgraded full-scale avalanche test-site Ryggfonn, Norway
Measurements of full-scale avalanches are expensive and time con-suming, but are indispensable to gain in-depth understanding of the flow behavior of avalanches. They are needed to crosscheck the scaling used in small-scale experi-ments and also form the basis for developing and calibrating numerical models. The recent partial upgrade of NGI's Ryggfonn test-site is focused on the processes occurring during interaction between avalanches and a catching dam in the runout zone. These processes are crucial for the efficiency of this type of avalanche mitigation measure, which has been the focus of several small-scale experiments in recent years. But qualitatively and quantitatively good observations from real avalanches for a cross-comparison are rare. Therefore, two new masts were constructed at Ryggfonn. One is located about 10 m upstream of the foot of a catching dam and has a height of 15 m. The other stands on the crown of the dam and is 6 m high. In this way, we also hope to c...
On full-scale avalanche measurements at the Ryggfonn test site, Norway
Cold Regions Science and Technology, 2007
Avalanche measurements carried out at the Ryggfonn test site, Norway, during several winter seasons are analyzed with emphasis on recognizing different flow regimes and estimating flow densities. Measurements include impact pressure readings from load cells mounted at two locations within the track and stress readings from load plates flush with the upstream slope of a catching dam. Pressure measurements were combined with velocity estimates based on cross correlations between the load cell readings and, in several cases, on Doppler radar measurements. In most cases a saltation (fluidized) layer in front of a more dense part could be identified. Doppler radar measurements confirm a fast moving head, in some instants preceded by a slower snout, and decreasing speed from the head to the tail. Calculated accelerations (decelerations) indicate that the effective friction parameter varies strongly and depends on the flow regime.
On avalanche measurements at the Norwegian full-scale test-site Ryggfonn
Cold Regions Science and Technology, 2008
Avalanchemeasurementsandobservationsthatwerecarriedoutatthe Ryggfonntestsite,Norway,on16April2005areanalyzed.Thedataincludepulsed Dopplerradarmeasurements,impactpressurereadingsfromloadcellsmountedattwo locationswithinthetrackandstressreadingsfromloadplatesflushwiththeupstream slopeofacatchingdam.Theradarmeasurementswereusedtoderivevelocities andestimatesontheretardingacceleration.Theretardingaccelerationsshowawide discrepancywithcommonlyusedmodelassumptions.Pressuremeasurementswere combinedwithvelocitymeasurements.Themeasurementsinferthatcommonlyused dragfactorsarenotsufficienttodescribeforceexertbyslowmovingwetsnow.Mea-surementswithloadplatesimplyplasticfailureratherthenCoulomb-typefriction.Field observationoftheavalanchetracksuggestthaterosion/abrasiondueto(saltating) particlesisonepossibleentrainmentmechanism.
Le Centre pour la Communication Scientifique Directe - HAL - ENPC, 2018
The present paper describes preliminary re-analyses of field-based data on past welldocumented snow avalanches that have impacted an instrumented tripod structure in one of the paths at Lautaret avalanche test-site, France. The re-analyses done include data on velocity and pressure measurements, as well as new data on flow-depth measurements. The latter data was obtained with the help of re-analyses of pressure signals. The various techniques used and assumptions made are presented and discussed, which allows us to infer how the thickness of the dense flow, and both the velocities and pressures over depth, all change simultaneously over time. The present work pays attention to the gravity-dominated flow regime occurring after the passage of the avalanche front. That regime is characterized by a mean pressure on the tripod structure that is essentially controlled by the flow thickness, unlike the inertia-dominated regime which is deemed to be driven by the square of the flow velocity during the passage of the avalanche front. Moreover, a change in dynamics is well identified during the gravity-dominated flow regime, while moving from the avalanche core to the avalanche tail.
2009
The impact pressure of snow avalanches have been measured at the Vallée de La Sionne experimental test site using two different types of sensors. The first sensors consist of traditional piezoelectric load cells, with area of 80 cm 2 (diameter 10 cm), installed on the hillside of an instrumented pylon. A second "mechanical" type of sensors consist of a 125 cm 2 (5 x 25 cm 2) steel cantilever beams installed to the side of the pylon at different heights and extending into the avalanche flow. The beams are equipped with high precision strain gages to record the deformation histories during the loading by the avalanche. Pressure is extracted from measured deformations by deconvolution and the cantilever's frequency response function (FRF). The FRF is calculated from an Euler-Bernoulli beam model and validated by impact hammer in-situ tests. Pressures measured in the same avalanche by both sensors are compared and discussed in terms of sensor form, location and some other relevant parameters. As the two sensors are located at the same elevation and pair-wise close to each others having their "force sensing" surfaces differently oriented with a deviation angle of 23 degrees, it turned out that the two measurements can be combined to retrieve a rough estimate of average resultant force vector acting on a avalanche-snow control volume in the vicinity of the sensors. We estimate the modulus and the orientation of the force and discuss changes in these variables for different flow regimes.
The full-scale avalanche test site, Lautaret, France
The Lautaret full-scale avalanche test site in the southern French Alps has been used by IRSTEA (Cemagref) Research Institute since 1973. Over the recent years two avalanche paths are used to release small to medium avalanches 3 or 4 times each winter. Avalanche flows are generally dense, whether wet or dry, sometimes with a powder part. Main path n°2 (track length 800 m) is dedicated to avalanche dynamics. Within the flow of the avalanche, flow height and vertical profiles of pressure and velocity are measured along a 3.5 m tripod. The snow volume released in the starting zone is quantified by a differential analysis of laser scanning measurements set before and after triggering. A high rate positioning of the avalanche along the track is determined from terrestrial oblique photogrammetry. Above the dense layer, the saltation layer and the powder part are characterized by particles and air fluxes measurements. In path n°1 smaller in size, medium-size avalanches (track length 500 m) make this track of particular interest for experiments on structures. A macroscopic sensor-structure is set nearly 150 m downhill from the starting zone, that is, in the area where avalanches generally reach their maximum velocity. It consists is a one square-meter plate supported by a 3.5 m high steel cantilever fixed in the ground, facing the avalanche. Impact pressures are reconstructed from the cantilever deformations, while avalanche velocity is measured from optical sensors. Seismic signals generated by avalanches of those 2 paths are recorded by a 3-axial broadband seismometer. Around those experimental devices dedicated to the understanding of avalanche physics, a national and international partnership has been developed from years to years, including INSA de Lyon, CNRS and Université Joseph Fourier (France), Aalto University (Finland), Nagoya University (Japan), Boku University (Austria), IGEMA (Bolivia), OGS (Italy)
The full-scale avalanche test-site at Lautaret Pass (French Alps)
Cold Regions Science and Technology, 2015
The full-scale avalanche test site at Lautaret Pass in the southern French Alps has been used by IRSTEA-Cemagref Research Institute since 1972. Over recent years, two avalanche paths have been used routinely to release avalanches and study avalanche dynamics and interactions between avalanches and obstacles. Avalanche flows are generally dense and dry, sometimes with a powder cloud on top. Main avalanche path no. 2 is dedicated to studies on avalanche dynamics. Within the flow of the avalanche, flow height and vertical profiles of pressure and velocity are measured along a 3.5 m tripod. The snow volume released in the release zone is quantified by differential analysis of laser scanning measurements performed before and after triggering. High-speed positioning of the avalanche front along the track is carried out by terrestrial oblique photogrammetry. Above the dense layer, the upper layer of the avalanche is characterized by particle and air flux measurements. Avalanche path no. 1 is smaller in size and particularly well-suited to experiments on structures exposed to small to medium-size avalanches (b1000 m 3). A macroscopic sensor structure consisting of a one square-meter plate supported by a 3.5 m high steel cantilever beam is fixed in the ground, facing the avalanche. Impact pressures are reconstructed from the beam deformations and avalanche velocity is measured by optical sensors. For these experimental devices dedicated to improving our understanding of avalanche physics, a national and international partnership has been developed over the years, including