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... A major design change was introduced with the mark IV system when the laboratory waveform generator to produce the FMCW triangular wave was exchanged with a more compact and reliable direct digital synthesiser (DDS) [8]. Another capability of GEODAR is the phased-array receiver to localize the avalanche in cross-range, however, the large width of the avalanche made the array processing difficult [9]. In the mark V system, the receiver array was replaced with several directional antennas, each pointing at a slightly different cross-range angle. ...

... The maximum unambiguous range for an FMCW radar depends mainly on the largest resolvable frequency of the analog-to-digital converter (ADC), e.g., half the sample rate f adc gives a maximum range of r = 937 m for the given system that can be adopted to longer ranges easily. Furthermore, the transmitted signal strength must be sufficient as it decays with r −4 for point targets and r −3 for distributed targets like avalanches [9]. Additionally for long range radars, the pulse duration τ needs to be longer than the two-way propagation time 2 r c . ...

... However, external narrow bandpass filters may connect between the receiving antenna and the receiver unit. Such external filter is, for example, used in the GEODAR receiver chain to prevent saturation and signal leakage from a nearby pulse Doppler radar that operates at a similar frequency range [9]. ...

Radar measurements of gravitational mass-movements like snow avalanches have become increasingly important for scientific flow observations, real-time detection and monitoring. Independence of visibility is a main advantage for rapid and reliable detection of those events, and achievable high-resolution imaging proves invaluable for scientific measurements of the complete flow evolution. Existing radar systems are made for either detection with low-resolution or they are large devices and permanently installed at test-sites. We present mGEODAR, a mobile FMCW (frequency modulated continuous wave) radar system for high-resolution measurements and low-resolution gravitational mass-movement detection and monitoring purposes due to a versatile frequency generation scheme. We optimize the performance of different frequency settings with loop cable measurements and show the freespace range sensitivity with data of a car as moving point source. About 15 dB signal-to-noise ratio is achieved for the cable test and about 5 dB or 10 dB for the car in detection and research mode, respectively. By combining continuous recording in the low resolution detection mode with real-time triggering of the high resolution research mode, we expect that mGEODAR enables autonomous measurement campaigns for infrastructure safety and mass-movement research purposes in rapid response to changing weather and snow conditions.

... GEODAR is a FMWC phased-array radar system that is installed in the bunker at VdlS (Ash et al., 2014). The radar has a center frequency of 5.3 GHz and a bandwidth of 200 MHz, that is, the frequency spans 5.2-5.4 ...

... The range resolution is 0.75 m, and a complete view of the slope is collected at 111 Hz. A detailed description of the measurement principle and the radar design considerations can be found in Ash et al. (2014). Here we present data collected with the GEODAR Mark III system (Köhler et al., 2016). ...

Powder snow avalanches are typically composed of several regions characterized by different flow regimes. These include a turbulent suspension cloud of fine particles, a dense basal flow and an intermittency frontal region, which is characterized by large fluctuations in impact pressure, air pressure, velocity and density, but whose origin remains unknown. In order to describe the physical processes governing the intermittency region, we present data from four large powder snow avalanches measured at the Vallée de la Sionne test site in Switzerland, which show that the intermittency is caused by mesoscale coherent structures. These structures have a length of 3{14 meters and a height of 10m or more. The structures can have velocities as much as 60% larger than the avalanche front speed and are characterized by an
air/particle mixture whose average density can be as high as 20 kg/m3. This average density increases the drag on large granules by a factor of up to 20 compared to pure air, so that each structure can maintain denser snow clusters and single snow granules in suspension for several seconds. The intermittency region has importance for the dynamics of an avalanche, as it provides an efficient mechanism for moving snow from the dense layer to the powder cloud, but also for risk assessment, as it can cause large forces at large heights above the basal dense layer.

... In addition to the PD radar, another promising radar-technology-based sensor-the frequency modulated continuous wave radar, must be mentioned here. This type has also been extensively tested in the Vallée de la Sionne (Ash et al., 2014) and offers extremely high spatial resolution of up to 0.75 m (Köhler et al., 2016). The sensor has been shown to be particularly suited for applications in avalanche dynamics research . ...

As a consequence of their natural occurrence and the frequent formation of multiple surges with high sediment loads, debris flows are considered one of the most hazardous gravity‐driven mass movements in mountain regions. Field measurements of surge dynamics are an essential link in the chain of understanding fundamental process dynamics and engineering protection against debris flows. However, continuous information on the velocities of multiple consecutive surges within a single debris‐flow event with high temporal resolution is rare. In this study we present a new pulse‐Doppler radar (PD radar) for high‐resolution real‐time debris‐flow monitoring. We analyse PD radar data sets over a torrent length of 250 meters for two debris flows that occurred at the Gadria creek (IT), on July 26, 2019 and August 10, 2020. The radar data was validated with independently derived data from particle image velocimetry (DPIV) and manually tracked velocities. We observe that between surges the flow frequently comes to a complete halt and is re‐mobilized by subsequent surges, resembling erosion‐deposition waves in granular flows. In addition, our data confirm that surges can superimpose and merge. We anticipate that the outcomes of this work serve as a blueprint for future high‐resolution observations of debris‐flow surge dynamics with PD radar and that our findings provide new insights into the physical principles of natural debris flows.

... The measurement techniques used to capture the essentially smooth propagation velocities of fluid-type natural hazards (see for example ref. 25 for debris flows or refs. 26,27 for avalanches) cannot be employed because they do not have the spatial and temporal resolution to capture the sudden, short-duration impact phenomena governing rockfall motion. Several experimental studies of induced rockfall trajectories have been conducted 9,[28][29][30][31][31][32][33][34][35][36] . ...

The mitigation of rapid mass movements involves a subtle interplay between field surveys, numerical modelling, and experience. Hazard engineers rely on a combination of best practices and, if available, historical facts as a vital prerequisite in establishing reproducible and accurate hazard zoning. Full-scale field tests have been performed to reinforce the physical understanding of debris flows and snow avalanches. Rockfall dynamics are - especially the quantification of energy dissipation during the complex rock-ground interaction - largely unknown. The awareness of rock shape dependence is growing, but presently, there exists little experimental basis on how rockfall hazard scales with rock mass, size, and shape. Here, we present a unique data set of induced single-block rockfall events comprising data from equant and wheel-shaped blocks with masses up to 2670 kg, quantifying the influence of rock shape and mass on lateral spreading and longitudinal runout and hence challenging common practices in rockfall hazard assessment. The awareness of rock shape dependence in rockfall hazard assessment is growing, but experimental and field studies are scarce. This study presents a large data set of induced single block rockfall events quantifying the influence of rock shape and mass on its complex kinematic behaviour.

... On 18 November 2017, a magnitude 6.9 earthquake occurred in Milin County, Nyingchi City. The Milin earthquake triggered the bottom of the Gyala Peri glacier in the northern part of the Yarlung Zangbo River to become mobile and active, and movement sped up [57,58]. Coupled with recent climate change, the melting of glaciers or ice lakes resulted in a huge amount of sediment being deposited in the barrier lake. ...

As a “starting zone” and “amplifier” of global climate change, the Qinghai–Tibet Plateau is very responsive to climate change. The global temperature rise has led directly to an acceleration of glacial melting in the plateau and various glacier avalanche disasters have frequently occurred. The landslide caused by glacier avalanches will damage the surrounding environment, causing secondary disasters and a disaster chain effect. Take the disaster chain of the Yarlung Zangbo River at Milin County in Tibet on 17 and 29 October 2018 as an example; a formation mechanical model was proposed. The evolution mechanism for the chain of events is as follows: glacial melt → loose moraine deposit → migration along the steep erosion groove resulting in glacier clastic deposition then debris flow → formation of the dam plug to block the river → the dammed lake. This sequence of events is of great significance for understanding the developmental trends for future avalanches, landslides, and river blocking dam disasters, and for disaster prevention planning and mitigation in the Qinghai–Tibet Plateau.

... GEODAR is a high-resolution frequency modulated continuous wave radar and was first installed in winter season (Ash et al., 2014. The system has been continually improved and currently has a range resolution of 0.75 m at 110 Hz over the entire slope . ...

Large avalanches usually encounter different snow conditions along their track. When they release as slab avalanches comprising cold snow, they can subsequently develop into powder snow avalanches entraining snow as they move down the mountain. Typically, this entrained snow will be cold (T‾<-1∘C) at high elevations near the surface, but warm (T‾>-1∘C) at lower elevations or deeper in the snowpack. The intake of warm snow is believed to be of major importance to increase the temperature of the snow composition in the avalanche and eventually cause a flow regime transition. Measurements of flow regime transitions are performed at the Vallée de la Sionne avalanche test site in Switzerland using two different radar systems. The data are then combined with snow temperatures calculated with the snow cover model SNOWPACK. We define transitions as complete when the deposit at runout is characterized only by warm snow or as partial if there is a warm flow regime, but the farthest deposit is characterized by cold snow. We introduce a transition index Ft, based on the runout of cold and warm flow regimes, as a measure to quantify the transition type. Finally, we parameterize the snow cover temperature along the avalanche track by the altitude Hs, which represents the point where the average temperature of the uppermost 0.5m changes from cold to warm. We find that Ft is related to the snow cover properties, i.e. approximately proportional to Hs. Thus, the flow regime in the runout area and the type of transition can be predicted by knowing the snow cover temperature distribution. We find that, if Hs is more than 500m above the valley floor for the path geometry of Vallée de la Sionne, entrainment of warm surface snow leads to a complete flow regime transition and the runout area is reached by only warm flow regimes. Such knowledge is of great importance since the impact pressure and the effectiveness of protection measures are greatly dependent on the flow regime.

... GEODAR is a high-resolution frequency modulated continuous wave radar and was first installed in winter season 2009/10 (Ash et al., 2014). The system has been continually improved and currently has a range resolution of 0.75 m at 110 Hz over the entire slope (Köhler et al., 2018). ...

Large avalanches usually encounter different snow conditions along their track. When they release as slab avalanches comprising cold snow, they can subsequently develop into powder snow avalanches entraining snow as they move down the mountain. Typically, this entrained snow will be cold (T T > −1 °C) at lower elevations or deeper in the snow pack. The intake of thermal energy in the form of warm snow is believed to cause a flow regime transition. Measurements of flow regime transitions are performed at the Vallée de la Sionne avalanche test site in Switzerland using two different radar systems. The data are then combined with snow temperatures calculated with the snow cover model SNOWPACK. We define transitions as complete, when the deposit at runout is characterized only by warm snow, or as partial, if there is a warm flow regime but the furthest deposit is characterized by cold snow. We introduce a transition factor Ft, based on the runout of cold and warm flow regimes, as a measure to quantify the transition type. Finally, we parameterize the snow cover temperature along the avalanche track by the altitude Hs, which represents the point where the average temperature of the uppermost 0.5 m changes from cold to warm. We find that Ft is related to the snow cover properties, i.e. approximately proportional to Hs. Thus, the flow regime in the runout area and the type of transition can be predicted by knowing the snow cover temperature distribution. We find, that, if Hs is more than 500 m above the valley floor for the path geometry of Vallée de la Sionne, entrainment of warm surface snow leads to a complete flow regime transition and the runout area is reached by only warm flow regimes. Such knowledge is of great importance since the impact pressure and the effectiveness of protection measures are greatly dependent on the flow regime.

... In a next step, Randeu et al. (1990) also measured velocities from within the flowing avalanche using a pulsed Doppler RADAR and Gauer et al. (2007) used data of pulsed Doppler RADARs to obtain information on retardation within avalanche flows. Looking into the interior of snow avalanches over the entire slope at high temporal (111 Hz) and spatial (0.75 m) resolution was recently made possible ( Köhler et al., 2016) thanks to the use of pulsed Doppler radar for geophysical flow dynamics (GEODAR), pushing forward the initial technical developments and tests previously done on GEODAR (Vriend et al., 2013;Ash et al., 2014;Keylock et al., 2014). Continuing those in situ investigations and coupling non-intrusive GEODAR measurements with a series of more established experimental techniques, Köhler et al. (2018) take a step forward by producing and analysing a wide series of 77 avalanches, both artificially triggered and naturally occurring, at the avalanche test-site of Vallée de la Sionne (Switzerland) over the period 2010-2015. ...

Köhler et al. (2018) deploy a high spatial and temporal resolution GEODAR radar system to reveal the inside of snow avalanches over the entire slope. They detect a rich variety of longitudinal and slope normal flow structures across a data set of 77 avalanches recorded over 6 years. Distinctive features in the radar signatures permit the definition of seven flow regimes and three distinct stopping signatures, illustrating behaviours much richer than the conventional dichotomy between dense flow avalanches and powder snow avalanches. This presents modellers with the challenge of exploring the physics of these regimes, the transitions between them and their relationship with the surrounding conditions.

... Eight receiving antennas are arranged in a sparse-sampled linear array of 8 m base width and collect the avalanche signal. If the data from the eight receivers are postprocessed, the lateral position of the reflectors can be derived using beam-forming techniques [Ash et al., 2014]. Here, however, we averaged the signal of all receivers to improve the signal-to-noise ratio. ...

Five avalanches were artificially released at the Vallée de la Sionne test site in the west of Switzerland on 3 February 2015 and recorded by the GEOphysical flow dynamics using pulsed Doppler radAR Mark 3 radar system. The radar beam penetrates the dilute powder cloud and measures reflections from the underlying denser avalanche features allowing the tracking of the flow at 111 Hz with 0.75 m downslope resolution. The data show that the avalanches contain many internal surges. The large or “major” surges originate from the secondary release of slabs. These slabs can each contain more mass than the initial release, and thus can greatly affect the flow dynamics, by unevenly distributing the mass. The small or “minor” surges appear to be a roll wave-like instability, and these can greatly influence the front dynamics as they can repeatedly overtake the leading edge. We analyzed the friction acting on the fronts of minor surges using a Voellmy-like, simple one-dimensional model with frictional resistance and velocity-squared drag. This model fits the data of the overall velocity, but it cannot capture the dynamics and especially the slowing of the minor surges, which requires dramatically varying effective friction. Our findings suggest that current avalanche models based on Voellmy-like friction laws do not accurately describe the physics of the intermittent frontal region of large mixed avalanches. We suggest that these data can only be explained by changes in the snow surface, such as the entrainment of the upper snow layers and the smoothing by earlier flow fronts.

... • Frequency Modulated Continuous Wave Phased Array radar (henceforth referred to as the GEODAR radar) (Vriend et al., 2013;Ash et al., 2014). It can track the avalanche over the whole slope with a downslope spatial resolution of 0.75 m, and gives information on avalanche position, velocity and size. ...

... When a site is instrumented, measurements can be obtained continuously throughout the flow, but usually at discrete geographic locations [7,20,21]. Radar and other scanning methods are being tested to give more continuous measurements throughout the flow but this method is currently restricted to velocity, height and temperature measurements [22,23]. ...

Geophysical gravity-driven flows -- including avalanches, debris flows, pyroclastic flows and submarine turbidity currents -- are multiphase natural hazards that flow under the influence of gravity. Despite their differences, they share much of the same physics, having the potential to pick up material from beneath, a process called basal entrainment during which the flow may increase in volume and velocity manyfold. Due to their complexity and unpredictability there are still many unanswered questions about their mechanics, so that many of the theoretical models in use are based on insufficient data sets and may not apply to a general case. Here, basal entrainment by geophysical gravity-driven flows is studied by isolating the process in idealised laboratory experiments. The avalanche is simplified and controlled in such a way that any changes can be confidently attributed to the entrainment process alone. Further, the methods available in the laboratory allow the continuous, passive study of entrainment, so that for the first time full data sets of internal measurements are obtained, from experiments ranging from simple to complex. The data obtained is easily exploited for confirmation of the mathematical models developed in this thesis. Experiments which simulated entraining avalanches as Newtonian dam-breaks along a horizontal flume and as viscoplastic dam-breaks along an inclined flume both showed an increase in front position, dependent on the amount of material available, amongst other changes. The experimental results allow the development of a theoretical thin-film model in both cases which is solved numerically and compares favourably with the data obtained. The Newtonian model reproduced the flow characteristics excellently and the viscoplastic model successfully simulated the effects of entrainable material. The possibility of performing similar experiments using a granular suspension is also investigated, with promising results. This work shows that the effect of entrainment on gravity-driven flows can be quantified and modelled mathematically as a non-local transport process. This has implications for hazard modelling: if the quantity of available loose material is known, and its characteristics are similar to those of the flowing avalanche, the avalanche and the entrainable bed can be modelled as a continuous flow over a rigid base. Thus it is suggested that the models developed be tested in more realistic cases, e.g. in the case of an avalanche entraining material with different characteristics, or in a more complex geometry, in order to better mimic what happens in nature.

... Recently a C-band, phased array FMCW radar, Geodar, for imaging flowing avalanches was developed by UCL [3] to improve the quality of measurements of avalanche dynamics. The phased array consisted of eight off-the-shelf antenna elements whose positions along a linear axis were randomised to cover a wide aperture with reduced grating lobe level [4]. ...

Radar has proved to be a valuable tool for gathering velocity measurements of flowing avalanches. Geodar, a recently developed FMCW, phased array radar, successfully improved upon existing avalanche radar measurements and has to date provided the community with high-quality avalanche measurements over several winter seasons. Indeed, Geodar has recorded measurements of flowing avalanches in two-spatial dimensions for the first time. Following the success of Geodar, the authors are now redesigning the system to improve its performance in terms of sensitivity and array response. This paper summarises the issues encountered with the original design and describes how these issues have been addressed with the new design. This includes a drastic change to the receiver design, shifting the deramp processing onto the antenna board, and printing the antenna array and its associated RF receiver circuitry on a single PCB. The design is shown to dramatically improve the array sidelobe performance, and enhance the sensitivity of the receiver by reducing the transmission line losses in the RF chain.

... This sensor is connected to the same data acquisition system of the seismic sensor of cavern D with a common timebase. Furthermore, a Frequency Modulated Continuous Wave Phased Array radar (henceforth referred to as the GEODAR radar) is located at the shelter (Ash et al., 2014;Vriend et al., 2013). It can track the avalanche over the whole slope with a downslope spatial resolution of 0.75 m, and gives information on avalanche position, velocity and size. ...

Understanding the dynamics of snow avalanches is crucial for predicting their destructive potential and mobility. To gain insight into avalanche dynamics at a particle level, the AvaNode in-flow sensor system was developed. These synthetic particles, equipped with advanced and affordable sensors such as an inertial measurement unit (IMU) and global navigation satellite system (GNSS), travel with the avalanche flow. This study focuses on assessing the feasibility of the in-flow measurement systems. The experiments were conducted during the winter seasons of 2021–2023, both in static snow cover and dynamic avalanche conditions of medium-sized events. Radar measurements were used in conjunction with the particle trajectories and velocities to understand the behaviour of the entire avalanche flow. The dynamic avalanche experiments allowed to identify three distinct particle flow states: (I) initial rapid acceleration, (II) a steady state flow with the highest velocities (9–17 ms−1), and (III) a longer deceleration state accompanied by the largest measured rotation rates. The particles tend to travel towards the tail of the avalanche and reach lower velocities compared to the frontal approach velocities deduced from radar measurements (ranging between 23–28 ms−1). The presented data give a first insight in avalanche particle measurements.

For effective avalanche risk mitigation, numerical models with a correct description of snow rheology are needed. Conventionally, velocity in snow flow experiments is inferred by cross-correlating the voltage signals of paired sensors. The intention of this paper is to reconsider this problem to enhance processing of these data, leading to more effective estimates of fluctuating velocity quantities. The algorithm consists of a wavelet decomposition, a denoising step and a weighting method for the reconstituted signal. The resulting velocity time series are both consistent and informative, providing confidence that one can analyse not only the mean velocity profiles, but also the velocity distribution. Our approach is illustrated using a typical chute experiment undertaken at Col du Lac Blanc in the French Alps. Not only has the mean velocity profile a more complex shape than the bilinear one postulated from the results of the standard cross-correlation processing, but the probability distribution functions of the velocity at different heights is much more continuous and dispersed, revealing interesting new patterns of greater dynamical relevance.

This paper explores the use of an over-the-air deramping (OTAD) system as a solution for perimeter surveillance. Over-the-air deramping is a technique for wirelessly synchronising distributed passive FMCW radar nodes to a dual-frequency master FMCW transmitter node. Such a system allows simultaneous monostatic and multistatic measurements for improved clutter resilience, and multiple looks at a target for reduced susceptibility to signal fading due to target scintillation. To prove the latter, a simultaneous monostatic and OTAD bistatic node were set up with a 5m baseline and a walking person was measured. The results show that signal fading occurs in both the monostatic and the bistatic node, but rarely at the same time. Hence, combining the measurements from the two nodes gives a consistent response from the target. This demonstrates OTAD as a compelling system for a robust perimeter surveillance system.

The complex dielectric constant of snow has been measured at microwave frequencies. New and old snow at different stages of metamorphosis have been studied. The results indicate that the complex dielectric constant is practically independent of the strncture of snow. For dry snow, the dielectric constant is determined by the density. For wet snow, the imaginary part and the increase of the real part due to liqnid water have the same volumetric wetness dependence. The frequency dependence of the complex dielectric constant of wet snow is the same as that of water. A nomograph for determining the density and wetness of wet snow from its dielectric constant is given. A snow sensor for fielmd easurement of the dielectric constant has been developed. It can be used for determining the density and the wetneosfs snow bya singlem easurement.

ABSTRACT Constitutive,equations,for,the,properties,of snow,in avalanches,are,proposed. ,These,are,based,on the equations,for,a modified,Criminale,- Ericksen,- Filbey - fluid,- which,shows,a close,agreement,to published experimental,data,on granular,flow.,The normal,stresses are divided in three separate parts, effective pressure, pore,pressure,and,a dispersive,pressure.,The shear stresses are caused by cohesion, Coulomb friction and dynamic,stresses.,The resulting,terminal,velocity,is pro portional,to h3/2 (h = flow,height).,The validity,of the model has an upper limit for the slope inclination, defined,by the,ratio,of dynamic,shear,stresses,and dispersive pressures, which is also in agreement with experimental,results.,The discussion,of the,assumptions shows,that,the,model,probably,is reliable,in the,descrip tion of the flow, but the value of the material parame ters,need,to be,found,experimentally. MODELE CONTINU CALCULANT LES VITESSES D'ECOULEMENT D'AVALANCHES RESUME Des lois,de comportement,de la,neige,d'ava lanche,sont,proposées.,Ces lois,sont,basées,sur,les équations pour un fluide Criminale-Ericksen-Filbey, qui ont,montré,une,bonne,concordance,avec,des,données

Twelve well-documented dry-snow avalanches from the instrumented Ryggfonn path in western Norway were selected for back-calculations with several dynamical avalanche models. In each case, the run-out distance, the front velocity in the lower track, the extent of the deposits and the depth profile along a line are known. A 16m high and 100m wide retention dam in the run-out zone is often overflowed by avalanches but retains a considerable fraction of their mass. The tested models comprise a quasi-analytic block model, two 1D hydraulic models, a particle model with entrainment, and SAMOS, an advanced 2D/3D two-layer model. For each model, a wide range of friction parameters was needed to reproduce the twelve events, and none of the models matches the deposit distributions of all avalanches with fair accuracy. Explicit representation of the intermediate-density layer in dry-snow avalanches and accurate numerical schemes are expected to improve the modelling of dam interactions.

Use of formal risk analysis to assess avalanche danger is currently limited by a lack of knowledge of how avalanche impact pressures damage structures and cause fatalities. That is, the vulnerability component of risk is poorly specified. In this paper we outline a method for deriving vulnerability values as a function of position downslope for a range of avalanche sizes. The method is based on the weighted average of vulnerability and uses an avalanche-dynamics model embedded within a statistical framework. The models seem to behave in a consistent manner. By allowing avalanche size and stopping position to vary and calculating vulnerability as a function of distance from the stopping position, vulnerability values are less approximate than the assumption of a constant vulnerability value for each individual size. When the assumptions underlying the impact pressure - vulnerability relation are perturbed, the results seem to be robust. The method outlined here should provide a way for avalanche experts to reformulate danger zones based on return period and impact pressure so that they are set within a risk framework.Key words: risk, vulnerability, snow, avalanches, impact pressure.

Dry snow avalanches consist of two distinct layers. A dense-flow layer is superposed by a powder-snow layer, a cloud of relatively small ice particles suspended in air. The density of this suspension is one order of magnitude smaller than that of the dense flow. A simulation model for dry avalanches has been developed, based on separate sub-models for the two layers. The sub-models are coupled by an additional transition model, describing the exchange of mass and momentum between the layers. The fundamentals of the two-dimensional granular flow model for the dense flow and of the three-dimensional turbulent mixture model for the powder flow are presented. Results of the complete coupled model, SAMOS (Snow Avalanche MOdelling and Simulation), applied to observed catastrophic avalanche events, are discussed, and the prediction of powder-snow pressures acting on a tunnel bridge is briefly described. SAMOS is used routinely for hazard zoning at the Austrian Federal Service for Torrent and Avalanche Control.

Snow erosion and entrainment processes in avalanches are classified according to their mechanisms, the flow regimes in which they occur, and their spatial position within the avalanche. Simple, but process-specific, models are proposed for erosion by impacts, abrasion, plowing and blasting. On the basis of order-of-magnitude estimates, the first three mechanisms are clearly expected to be important. The fourth mechanism stipulates that the compaction of the snow cover ahead of the avalanche leads to the flow of escaping air just in front of the avalanche that may disrupt the snow cover and support formation of a saltation layer. The effects of this hypothetical mechanism resemble those of the plowing mechanism. All mechanisms depend strongly on the snow properties, but, with plausible parameter values, erosion rates at or above the experimentally found rates are obtained. The entrainment rate of an avalanche is most often limited by the shear stress needed to accelerate the eroded snow to avalanche speed.

1] The mountain snow cover is an important source of water but also leads to natural hazards, such as avalanches and floods. We use data collected during winters 1999/2000 to 2007/2008 by 239 automatic and manual measurement stations in Switzerland to highlight spatial characteristics of extreme snowfall. With the help of extreme value theory based on a ''peaks-over-threshold'' approach and a Poisson point process representation, we analyze spatial patterns and correlation characteristics. Our analyses show that a significant number of stations do not follow the Gumbel distribution. In particular, low altitude stations in the Swiss Plateau are heavy tailed because of rare extraordinary snowfall events. Spatial characteristics of extreme snowfall are compared to those of the mean snowfall. Altitudinal dependence and spatial distribution of mean and extreme snowfall are similar. Both mean snowfall and extreme snowfall show an increase of magnitude between 400 and 2200 m a.s.l. and a constant or slightly decreasing magnitude at higher altitudes. Below 1200 m a.s.l., the increase with altitude is stronger because of the rain-snow transition. Another finding is that the spatial correlation pattern of extreme snowfall is similar to that of mean snowfall, both of which are determined by the main climatological regions of Switzerland. An analysis based on those stations with a long record shows that extreme snowfall was 10% lower in the nine winters investigated than in the long-term period, but the main spatial characteristics of the two periods show no change.

Digital elevation models of glaciated terrain pro-duced by the NASA/Jet Propulsion Laboratory (JPL) air-borne interferometric synthetic-aperture radar (InSAR) in-strument in Greenland and Alaska at the C-(5.6 cm wave-length) and L-band (24-cm) frequencies were compared with surface elevation measured from airborne laser altimetry to estimate the phase center of the interferometric depth, or penetration depth, δp. On cold polar firn at Greenland sum-mit, δp = 9±2m at C-and 14±4m at L-band. On the ex-posed ice surface of Jakobshavn Isbrae, west Greenland, δp = 1±2 m at C-and 3±3 m at L-band except on smooth, marginal ice where δp = 15±5 m. On colder marginal ice of northeast Greenland, δp reaches 60 to 120 m at L-band. On the temperate ice of Brady Glacier, Alaska, δp is 4±2 m at C-and 12±6 m at L-band, with little dependence on snow/ice conditions. The implications of the results on the scientific use of InSAR data over snow/ice terrain is discussed.

Numerical avalanche dynamics models have become an essential part of snow engineering. Coupled with field observations and historical records, they are especially helpful in understanding avalanche flow in complex terrain. However, their application poses several new challenges to avalanche engineers. A detailed understanding of the avalanche phenomena is required to construct hazard scenarios which involve the careful specification of initial conditions (release zone location and dimensions) and definition of appropriate friction parameters. The interpretation of simulation results requires an understanding of the numerical solution schemes and easy to use visualization tools. We discuss these problems by presenting the computer model RAMMS, which was specially designed by the SLF as a practical tool for avalanche engineers. RAMMS solves the depth-averaged equations governing avalanche flow with accurate second-order numerical solution schemes. The model allows the specification of multiple release zones in three-dimensional terrain. Snow cover entrainment is considered. Furthermore, two different flow rheologies can be applied: the standard Voellmy–Salm (VS) approach or a random kinetic energy (RKE) model, which accounts for the random motion and inelastic interaction between snow granules. We present the governing differential equations, highlight some of the input and output features of RAMMS and then apply the models with entrainment to simulate two well-documented avalanche events recorded at the Vallée de la Sionne test site.

The velocity profile and basal shear force were measured for snow flowing down a chute 34 m long and 2.5 m wide. The flows were approximately steady by the end of the chute where measurements were taken and the angle was 32°. Measurements of the basal shear stress confirm approximate dynamic balance. The velocity profile was measured using optoelectronic sensors and showed a large slip velocity at the base, a shear layer of around 50 mm and an overlying plug-like flow of about 350 mm. The velocity profile is compatible with both a Herschel–Bulkley rheological model, which combines a constant critical stress with a power law dependence on the mean shear rate, and a Cross model where the effective viscosity varies between two limits. Estimates of the Reynolds number suggest that the flow is not turbulent. The measurements are used to estimate the distribution of energy dissipation and to show that its concentration near the base may locally melt the snow, and thus serve as an explanation for icy melt surfaces observed at the base of flowing avalanche tracks.

This paper deals with the assessment of physical vulnerability of civil engineering structures to snow avalanche loadings. In this case, the vulnerability of the element at risk is defined by its damage level expressed on a scale from 0 (no damage) to 1 (total destruction). The vulnerability of a building depends on its structure and flow features (geometry, mechanical properties, type of avalanche, topography, etc.). This makes it difficult to obtain vulnerability relations. Most existing vulnerability relations have been built from field observations. This approach suffers from the scarcity of well documented events. Moreover, the back analysis is based on both rough descriptions of the avalanche and the structure. To overcome this problem, numerical simulations of reinforced concrete structures loaded by snow avalanches are carried out. Numerical simulations allow to study, in controlled conditions, the structure behavior under snow avalanche loading. The structure is modeled in 3-D by the finite element method (FEM). The elasto-plasticity framework is used to represent the mechanical behavior of both materials (concrete and steel bars) and the transient feature of the avalanche loading is taken into account in the simulation. Considering a reference structure, several simulation campaigns are conducted in order to assess its snow avalanches vulnerability. Thus, a damage index is defined and is based on global and local parameters of the structure. The influence of the geometrical features of the structure, the compressive strength of the concrete, the density of steel inside the composite material and the maximum impact pressure on the damage index are studied and analyzed. These simulations allow establishing the vulnerability as a function of the impact pressure and the structure features. The derived vulnerability functions could be used for risk analysis in a snow avalanche context.

Knowing the path profile and the avalanche velocity variations with downstream distance makes it possible to deduce the bulk frictional force experienced by an avalanche during its course. This derivation was applied to 15 documented events reported in the literature. Three types of rheological behavior were identified: (1) the inertial regime, where the frictional force drops to zero; (2) the Coulombic frictional force, where the force is fairly independent of the avalanche velocity; and (3) the velocity-dependent regime, where the force exhibits a complicated (nonlinear and hysteretic) dependence on velocity. During its course an avalanche can experience one or several regimes. Interestingly, the Coulomb model can provide predictions of the velocity and run-out distance in good agreement with field data for most events, even though for some path sections the bulk frictional force departs from the Coulomb model. This result is of primary importance in zoning applications since it makes it possible to deduce avalanche velocities from a knowledge of the run-out distance. Its physical meaning is, however, not clearly demonstrated in this paper due to the lack of suitable data.

Microwave dielectric measurements of dry and wet snow were made at nine frequencies betweeo 3 and 18 GHz, and at 37 GHz, using two free-space transmission systems. The measurements were conducted during the winters of 1982 and 1983. The following parametric ranges were covered: 1) liquid water content, 0 to 12.3 percent by volume; 2) snow density, 0.09 to 0.42 g cm<sup>-3</sup>; 3) temperature, 0 to -5 deg C and -15deg C (scattering-loss measurements); and 4) crystal size, 0.5 to 1.5 mm. The experimental data indicate that the dielectric behavior of wet snow closely follows the dispersion behavior of water. For dry snow, volume scattering is the dominant loss mechanism at 37 GHz. The applicability of several empirical and theoretical mixing models was evaluated using the experimental data. Both the Debye-like semi-empirical model and the theoretical Polder-Van Santen mixing model were found to describe adequately the dielectric behavior of wet snow. However, the Polder-Van Santen model provided a good fit to the measured values of the real and imaginary parts of wet snow only when the shapes of the water inclusions in snow were assumed to be both nonsymmetrical and dependent upon snow water content. The shape variation predicted by the model is consistent with the variation suggested by the physical mechanisms governing the distribution of liquid water in wet snow.

Mini-avalanche systems were constructed both in a low-temperature laboratory and in a snowfield, and the behaviour of the flowing snow was observed in each case. Velocity profiles for the individual snow particles were determined and these implied that a viscous force, which has been neglected in most previous numerical simulations of snow-avalanche motion, needs to be taken into account for many avalanches. Kinematic viscosity coefficients for the fluidized snow were also measured using a modified Stormer-type viscometer. Substituting the dry-friction value and the kinematic viscosity coefficient for fluidized snow into the equation for avalanche motion, numerical simulation of natural events was achieved for the Shiai-dani region. Taking viscous resistance factors into account led to the conclusion that the magnitude of turbulent resistance of snow in avalanche systems is probably much smaller than that represented by the values previously in use.

The Savage-Hutter model is generalized by including a velocity-dependent drag in addition to the usual Coulomb dry friction at the base of the avalanche. Both linear and quadratic velocity dependencies are considered, with either constant or asymptotically constant drag coefficients for large thickness h. The singular nature of the constant coefficient model for small h is demonstrated and it is shown that the asymptotic model allows the tail of the avalanche to move at a finite velocity. The inclusion of velocity drag changes the stress state in the avalanche and new earth-pressure relations are derived and investigated.

A simple quasi one-dimensional model of flowing avalanches is presented. It is a further development of that used in the Swiss Guidelines for practitioners. It is shown that shearing in avalanche movement is concentrated near the ground and that, due to the geometrical roughness of the ground, a flow resistance proportional to the square of velocity must be taken into account in addition to dry friction. For the change of flow on changing slope angles it is demonstrated that under certain conditions for internal friction a “normal” flow on a flat lower part can no longer be attained; the avalanche behaves like a rigid body. The runout distance is in fair agreement with the Guidelines if a larger internal friction is used. The main differences lie in much smaller deposition depths and smaller velocities during runout.

A small avalanche path near the Bridger Bowl ski area in southwestern Montana has been instrumented to measure density, velocity and dynamic friction in a flowing avalanche. These measurements, made by an array of sensors mounted in the avalanche path, have been carried out for several dry-snow avalanches. Measurements of density were made using a capacitance probe that measures the dielectric constant of any material that passes in front of it. Through a calibration procedure, the dielectric constant of a given type of snow can be related to the density of that snow. Optical sensors were used to measure light reflected from the avalanche as it passed by the sensors. Signals from adjacent optical sensors were cross-correlated to determine velocity. Density and velocity measurements were made at several heights in the avalanche, with particular attention directed near the running surface. Results indicate that avalanche deformation is concentrated near the running surface where the snow density is found to be largest. Upward from the surface, the velocity gradient falls off greatly while the density also declines.
Finally, the dynamic-friction coefficient at the base of the avalanche was found by measuring shear and normal forces on a roughened 23 cm × 28 cm aluminum plate mounted parallel and flush with the avalanche running surface. The ratio of the shear force to normal force on the plate provides a measure of the dynamic-friction coefficient at the base of the avalanche.

Voellmy’s (1955) method for computing the run-out distance of a snow avalanche includes an unsatisfactory feature: the a priori selection of a midslope reference where the avalanche is assumed to begin decelerating from a computed steady velocity. There is no objective criterion for selecting this reference, and yet the choice critically determines the computed stopping position of the avalanche. As an alternative, a differential equation is derived in this paper on the premise that the only logical reference is the starting position of the avalanche. The equation is solved numerically for paths of complex geometry. Solutions are based on two parameters: a coefficient of friction μ; and a ratio of avalanche mass–to–drag, M⁄D. These are analogous to the two parameters in Voellmy’s model, μ and ξH. Velocity and run-out distance data are needed to estimate μ and M⁄D to useful precision. The mathematical properties of two–parameter models are explored, and it is shown that some difficulties arise since similar results are predicted by dissimilar pairs of μ and M⁄D.

In this paper we present a simulation approach to mapping avalanche risk with application to settlements in Iceland. Two simulation models are developed to calculate the probability of avalanches travelling a certain distance, and of the flow being a specific width. These two simulation models, in combination with knowledge of the average frequency of avalanche occurrence, the variability in avalanche direction and the degree of loss caused by an avalanche, permit risk values to be determined for the areas of concern.

Dense avalanches made of dry snow were studied as granular flows, through the development and use of a numerical model based on the shallow water theory. Friction was represented by a phenomenological law resulting from the recent progress in the field of snow avalanche constitutive laws. Using this friction formulation and assumptions similar to those employed in the shallow water theory, a simple model describing erosion and deposition was formulated and tested. The system of equations obtained was solved using a numerical scheme of finite volumes. The model was then tested on experimental data obtained in the laboratory. Relative good agreement was observed between the simulated and experimental data. An avalanche path where 153 avalanches were observed over the last century was chosen. The dry friction values have been determined providing the coincidence of the calculated and observed distances.We analysed the statistical distribution of the obtained dry friction coefficient and compared the obtained range to the range obtained by Cassassa et al. [Cassassa, G., Narita, H. Maeno, N., Shear cell
experiments of snow and ice friction, J. Appl. Phys., 69 (1991), pp. 3745–3755]. Afterwards, we studied the effect of a dam of different heights placed at two locations in the path and analysed its effectiveness in terms of volume reduction and run-out distance, demonstrating that the friction coefficient has a prevailing role on both the dynamics of the avalanche and the effectiveness of the dam

Geophysical mass flows, such as snow avalanches, are a major hazard in
mountainous areas and have a significant impact on the infrastructure,
economy and tourism of such regions. Obtaining a thorough understanding
of the dynamics of snow avalanches is crucial for risk assessment and
the design of defensive structures. However, because the underlying
physics is poorly understood there are significant uncertainties
concerning current models, which are poorly validated due to a lack of
high resolution data. Direct observations of the denser core of a large
avalanche are particularly difficult, since it is frequently obscured by
the dilute powder cloud. We have developed and installed a phased array
FMCW radar system that penetrates the powder cloud and directly images
the dense core with a resolution of around 1 m at 50 Hz over the entire
slope. We present data from recent avalanches at Vallee de la Sionne
that show a wealth of internal structure and allow the tracking of
individual fronts, roll waves and surges down the slope for the first
time. We also show good agreement between the radar results and existing
measurement systems that record data at particular points on the
avalanche track.

The Savage-Hutter model is generalized by including a velocity-dependent drag in addition to the usual Coulomb dry friction at the base of the avalanche. Both linear and quadratic velocity dependencies are considered, with either constant or asymptotically constant drag coefficients for large thickness h. The singular nature of the constant coefficient model for small h is demonstrated and it is shown that the asymptotic model allows the tail of the avalanche to move at a finite velocity. The inclusion of velocity drag changes the stress state in the avalanche and new earth-pressure relations are derived and investigated.

Mini-avalanche systems were constructed both in a low-temperature laboratory and in a snowfield, and the behaviour of the flowing snow was observed in each case. Velocity profiles for the individual snow particles were determined and these implied that a viscous force, which has been neglected in most previus numerical simulations of snow-avalanche motion, needs to be taken into account for many avalanches. Kinematic viscosity coefficients for the fluidized snow were also measured using a modified Stormer-type viscometer. Substituting the dry-friction value and the kinematic viscosity coefficient for fluidized snow into the equation for avalanche motion, numerical simulation of natural events was achieved for the Shiai-dani region. -from Authors

We test the behaviour of two statistical avalanche models and three hydraulic-continuum models of varying dimensionality against five reference events. For the hydraulic models, reference friction coefficients are produced that replicate the runout distance of the historical event. The model sensitivity to friction coefficients, release depth, release area and runout distance is also analysed.The hydraulic-continuum models yield similar reference coefficients on the simplest topography, but diverge for more complex paths, highlighting the importance of boundary conditions on model performance. The Coulomb friction (μ) shows a closer relation to runout distance than the turbulent friction (ξ). For models of this type, the debris deposition pattern is useful for selecting model coefficients, rather than relying purely on runout distances.The results of the sensitivity analysis are site-specific, again highlighting the importance of terrain as a model boundary condition. The release area seems to have less of an influence on model results than the fracture height and ξ. In general, the models are most sensitive to μ. At the end of the paper, we propose a scheme for avalanche hazard zoning that integrates the statistical and dynamic models, such that zoning can be undertaken with some confidence in model output.

Particle speed distributions in artificially- released dense flow avalanches from start to stop have been measured using oversnow vehicle based X-band micro wave doppler radars. Frequency-modulated continuous-wave X-band radars buried in the avalanche tracks provide localized flow height and slope-perpendicular particle speed profile recordings. So far, measurements on 20 ava lanches have been performed. Avalanche sizes varied from a few hundred to several tens of thousand cubic meters of snow, total drop height from release to runout from 150 m to 1000 m. Most avalanches were channeled at least in their middle part. The results show the following trends: The frontal speed of an avalanche increases with increasing size of the avalanche body. Transfer of avalanche snow from the avalanche body to the avalanche tail portion is highly effected by track roughness. If the body portion disappears, the avalanche front speed decreases rapidly and avalanches stop even at high slope angles. The relative velocities of particles in the flow increase with flow speed. The slope reactangular particle speed profile is roughly exponential. Computer modeling has been started, based on Haff's concepts for granular flows. An attempt is made to clearly separate material parameters from topographic and track roughness parameters.

A small avalanche path near the Bridger Bowl ski area in southwestern Montana has been instrumented to measure density, velocity and dynamic friction in a flowing avalanche. These measurements, made by an array of sensors mounted in the avalanche path, have been carried out for several dry-snow avalanches. Measurements of density were made using a capacitance probe that measures the dielectric constant of any material that passes in front of it. Through a calibration procedure, the dielectric constant of a given type of snow can be related to the density of that snow. Optical sensors were used to measure light reflected from the avalanche as it passed by the sensors. Signals from adjacent optical sensors were cross-correlated to determine velocity. Density and velocity measurements were made at several heights in the avalanche, with particular attention directed near the running surface. Results indicate that avalanche deformation is concentrated near the running surface where the snow density is found to be largest. Upward from the surface, the velocity gradient falls off greatly while the density also declines.
Finally, the dynamic-friction coefficient at the base of the avalanche was found by measuring shear and normal forces on a roughened 23 cm × 28 cm aluminum plate mounted parallel and flush with the avalanche running surface. The ratio of the shear force to normal force on the plate provides a measure of the dynamic-friction coefficient at the base of the avalanche.

A simple quasi one-dimensional model of flowing avalanches is presented. It is a further development of that used in the Swiss Guidelines for practitioners . It is shown that shearing in avalanche movement is concentrated near the ground and that, due to the geometrical roughness of the ground, a flow resistance proportional to the square of velocity must be taken into account in addition to dry friction. For the change of flow on changing slope angles it is demonstrated that under certain conditions for internal friction a “normal” flow on a flat lower part can no longer be attained; the avalanche behaves like a rigid body. The runout distance is in fair agreement with the Guidelines if a larger internal friction is used. The main differences lie in much smaller deposition depths and smaller velocities during runout.

This paper describes a frequency modulated, continuous wave (FMCW) microwave radar system used for different types of investigations in snow and avalanche research. Different semi-empirical equations describing transmission and backscatter of electromagnetic energy in snow are compared and applied to analyse the frequency domain spectra of the backscattered radiation. The FMCW scatterometers are either buried in the ground looking upward into the snow cover or are towed on skis looking downward into the snow. The backscatter of electromagnetic radiation from avalanche snow moving perpendicular to the radar beam is analysed to estimate the height of dense flow in the avalanche. The geometrical layering, density, water equivalence, settlement, total snow height, percolation of water through the snow cover and moisture content of the snow are determined from the backscatter of the stratigraphy of a static snow pack.

Voellmy’s (1955) method for computing the run-out distance of a snow avalanche includes an unsatisfactory feature: the a priori selection of a midslope reference where the avalanche is assumed to begin decelerating from a computed steady velocity. There is no objective criterion for selecting this reference, and yet the choice critically determines the computed stopping position of the avalanche. As an alternative, a differential equation is derived in this paper on the premise that the only logical reference is the starting position of the avalanche. The equation is solved numerically for paths of complex geometry. Solutions are based on two parameters: a coefficient of friction μ; and a ratio of avalanche mass–to–drag, M⁄D. These are analogous to the two parameters in Voellmy’s model, μ and ξH. Velocity and run-out distance data are needed to estimate μ and M⁄D to useful precision. The mathematical properties of two–parameter models are explored, and it is shown that some difficulties arise since similar results are predicted by dissimilar pairs of μ and M⁄D.

This paper presents the application of a pulsed Doppler radar for the measurement of dynamic properties of snow and ice avalanches. The instrument was developed and built by the Institute of Communications and Wave Propagation, Technical University Graz, and is operated by the Austrian Institute for Avalanche and Torrent Research (AIATR), Innsbruck. The data acquired by this radar during field campaigns are used to verify and optimize avalanche models and simulation tools. As well as describing the radar's technical principle, how the dynamic parameters are measured and presented to the user and how these data can be used to accomplish the determination of avalanche velocity and runout distance, the paper deals with the participation of the radar in a successful full-scale snow-avalanche experiment in Ryggfonn, Norway, and evaluates the collected data. This measurement campaign was a joint experiment by the Norwegian Geotechnical Institute, Oslo, and AIATR.

In winter 1998/99, high-frequency pressure measurements with 10 cm sensors mounted 1-19 m above ground were carried out in the upper run-out zone of the avalanche test site at Vallée de la Sionne, Switzerland. Two large dry-snow avalanches clearly revealed a three-layered structure, with surprisingly low pressures in the suspension (or powder-snow) layer. The height of the saltation layer varied between 1 and > 3 m. From the duration, impulse and frequency of single-particle impacts (observed in the saltation layer and intermittently in the dense flow), particle-size and velocity distribution functions as well as strongly varying saltation-layer densities were found. With improved methods for peak detection and correction for grazing impacts, pressure measurements will become a premier tool for testing granular flow models.

In this paper we present a simulation approach to mapping avalanche risk with application to settlements in Iceland. Two simulation models are developed to calculate the probability of avalanches travelling a ccrtain distance, and of the flow being a specific width. Thcsc two simulation models, in combination with knowledge of the average frequency of avalanche occurrence, the variability in avalanche direction and the degree of loss caused by an avalanche, permit risk values to be determined for the areas of concern.

While performing statistical-dynamical simulations for avalanche predetermination, a propagation model must reach a compromise between precise description of the avalanche flow and computation times. Crucial problems are the choice of appropriate distributions describing the variability of the different inputs/outputs and model identifiability. In this study, a depth-averaged propagation model is used within a hierarchical Bayesian framework. First, the joint posterior distribution is estimated using a sequential Metropolis-Hastings algorithm. Details for tuning the estimation algorithm are provided, as well as tests to check convergence. Of particular interest is the calibration of the two coefficients of a Voellmy friction law, with model identifiability ensured by prior information. Second, the point estimates are used to predict the joint distribution of different variables of interest for hazard mapping. Recent developments are employed to compute pressure distributions taking into account the rheology of snow. The different steps of the method are illustrated with a real case study, for which all possible decennial scenarios are simulated. It appears that the marginal distribution of impact pressures is strongly skewed, with possible high values for avalanches characterized by low Froude numbers. Model assumptions and results are discussed.

In order to study dry-snow avalanches, an intermediate scale device was built atthe col du lac blanc, a pass near the Alpe d''Huez ski resort, in the French Alps.It consists of a flow channel that can be artificially fed with snow through a hopper.The channel has been instrumented so as to measure height, normal and shear stressesat the bottom, and velocity profile within the flow. Through the variations in channelinclination and feeding rate, access to a wide range of slopes and heights is available.These measurements have been taken for a few dry snow flows at the end of the2001–2002 winter. The first results are presented here.

Pulsed Doppler radar measurements from several avalanche releases are analyzed to gain an in-depth understanding of avalanche dynamics. A pulsed Doppler radar emits short pulses and samples the echo in distinct time intervals, corresponding to distance intervals (range gates). Frequency analysis of the echo signals, exploiting the Doppler-effect, yields the velocity distribution within the width of a range gate. Thus, it is possible to gain information on the front speed along the track and information on the velocity versus time at a specific location along the track.In addition to the avalanche velocity, information on the magnitude of accelerations/decelerations along the track can be derived. To this end, the velocities of a pair of adjoining range gates are compared. The acceleration/deceleration values can give hints to parameters in the friction terms of commonly used numerical avalanche models. The derived retarding accelerations imply a behavior different to the one proposed in those models. They also indicate a dependency of the retarding acceleration on the stage of the flow.

Physical modelling is an important tool for obtaining insight into the internal flow structure of gravity currents such as snow avalanches, or, more practically, for estimating the dimensions of avalanche defense structures. We present a large-scale experimental setup, which allows the generation of avalanche-like gravity currents of snow under reproducible experimental conditions. The Weissfluhjoch chute is 34 m long, 2.5 m wide and can be run with up to 25 m3 of natural snow. We present two types of optical velocity measurement devices based on the correlation method. We discuss their applicability to snow flows and perform an error analysis of the observed flow velocities. Furthermore, preliminary results of velocity profile measurements in the snow flow are presented. We also describe the experimental determination of the total dynamic basal friction force exerted on the ground. An effective dynamic friction coefficient is calculated.

A new Swiss test-site for avalanche experiments has been built to study the overall dynamic behaviour of dense-flow and powder-snow avalanches and to measure avalanche impact forces along their path. An important application of the data gathered is for the verification and calibration of physical models and of computer simulation programs. For the impact studies, a wall, a pylon of circular section, a girder mast and a section of the roof of a road protection gallery are placed within the avalanche track and equipped with force transducers and pressure and strain gauges. The dynamic behaviour of the avalanches is measured by Doppler- and FMCW-radars. Snow mass balances are established by photogrammetry. The avalanches are artificially released. For many years, the Swiss Federal Institute for Snow and Avalanche Research used a test-site in the Canton Grison to study the dynamic behaviour of dense-flow avalanches. The limited number of successful experiments at this site forced the institute to examine several alternatives for an avalanche test-site. It was finally decided that a site in the Canton Valais, with several potential avalanche tracks with more than 1000 m of potential difference in altitude, was most suitable for our purpose. After several years of planning for the test-site, the civil engineering construction work started in summer 1997, followed by the installation of sensor devices and the data acquisition, transfer and storage system. In December 1997, the test-site was ready for experiments.

Velocities of mixed dense-flow/powder-snow avalanches have been measured by means of pulsed Doppler radar and by continuous wave radar at the full scale avalanche test site Vallée de la Sionne. From the radar data, we derive velocities of the saltation layer and of the powder part of the avalanche. The results obtained by the two different radar measurement techniques are compared and also are checked against the velocity data obtained by opto-electronic velocity sensors installed at different heights on the 20 m high mast in the avalanche track and against videogrammetry velocity data. We demonstrate that the measurements are consistent and discuss how information about the avalanche structure can be derived from the measurements.

Aspects of volume scattering and emission theory are discussed, taking into account a weakly scattering medium, the Born approximation, first-order renormalization, the radiative transfer method, and the matrix-doubling method. Other topics explored are related to scatterometers and probing systems, the passive microwave sensing of the atmosphere, the passive microwave sensing of the ocean, the passive microwave sensing of land, the active microwave sensing of land, and radar remote sensing applications. Attention is given to inversion techniques, atmospheric attenuation and emission, a temperature profile retrieval from ground-based observations, mapping rainfall rates, the apparent temperature of the sea, the emission behavior of bare soil surfaces, the emission behavior of vegetation canopies, the emission behavior of snow, wind-vector radar scatterometry, radar measurements of sea ice, and the back-scattering behavior of cultural vegetation canopies.

The fundamental principles of radar backscattering measurements are presented, including measurement statistics, Doppler and pulse discrimination techniques, and associated ambiguity functions. The operation of real and synthetic aperture sidelooking airborne radar systems is described, along with the internal and external calibration techniques employed in scattering measurements. Attention is given to the physical mechanisms responsible for the scattering emission behavior of homogeneous and inhomogeneous media, through a discussion of surface roughness, dielectric properties and inhomogeneity, and penetration depth. Simple semiempirical models are presented. Theoretical models involving greater mathematical sophistication are also given for extended ocean and bare soil surfaces, and the more general case of a vegetation canopy over a rough surface.

A rigorous definition of the noise figure of radio receivers is given in this paper. The definition is not limited to high-gain receivers, but can be applied to four-terminal networks in general. An analysis is made of the relationship between the noise figure of the receiver as a whole and the noise figures of its components. Mismatch relations between the components of the receiver and methods of measurements of noise figures are discussed briefly.

Frequency modulated continuous wave (FMCW) radar uses a very low
probability of intercept waveform, which is also well suited to make
good use of simple solid-state transmitters. FMCW is finding
applications in such diverse fields as naval tactical navigation radars,
smart ammunition sensors and automotive radars. The paper discusses some
features of FMCW radar which are not dealt with in much detail in the
generally available literature. In particular, it discusses the effects
of noise reflected back from the transmitter to the receiver and the
application of moving target indication to FMCW radars. Some of the
strengths and weaknesses of FMCW radar are considered. The paper
describes how the strengths are utilised in some systems and how the
weaknesses can be mitigated. It also discusses a modern implementation
of a reflected power canceller, which can be used to suppress the
leakage between the transmitter and the receiver, a well known problem
with continuous wave radars