Avalanche Release Areas Research Articles

Glide-snow avalanches: a mechanical, threshold-based release area model

Abstract. Glide-snow avalanches release at the ground–snow interface due to a loss in basal friction. They pose a threat to infrastructure because of the combination of unreliable mitigation measures, limited forecasting capabilities, and a lack of understanding of the release process. The aim of this study was to investigate the influence of spatial variability in basal friction and snowpack properties on the avalanche release area distribution and the release location. We developed a pseudo-3D, mechanical, threshold-based model that consists of many interacting snow columns on a uniform slope. Parameterizations in the model are based on our current understanding of glide-snow avalanche release. The model can reproduce the power law glide-snow avalanche release area distribution as observed on Dorfberg (Davos, Switzerland). A sensitivity analysis of the input parameters showed that the avalanche release area distribution was mostly influenced by the homogeneity (correlation length and variance) of the basal friction and whether the basal friction was reduced suddenly or in small increments. Larger release areas were modeled for a sudden decrease and a more homogeneous basal friction. The spatial variability of the snowpack parameters had little influence on the release area distribution. Extending the model to a real-world slope showed that the modeled location of avalanche releases qualitatively matched the observed locations. The model can help narrow down the length scales and timescales for field investigations. Simultaneously, it can grow in complexity with our increasing understanding of glide-snow avalanche release processes. Input parameters such as the basal friction or snowpack parameters could potentially all be connected to the liquid water content. This would allow for the use of meteorological measurements to drive the model. The model has the potential to help identify potentially dangerous conditions for large or numerous avalanches which would help improve glide-snow avalanche forecasting.

GIS-based determination of potential snow avalanche areas: A case study of Rize Province of Türkiye

Natural hazards are a part of critical issues affecting people and the environment. One of these natural hazards is snow avalanches. With the increase in the world population, it has emerged that decision-makers should take precautions against such natural hazards for population movements, construction, transportation, and tourism. Essential solution parts of this problem lay behind surveying, GIS, and spatial analysis-planning. This situation will be primarily due to the snow conditions, but certain terrain areas are susceptible. Snow avalanches' release mechanism depends on many factors, such as terrain, meteorological reports, snowpack, and other triggering parameters. Areas with certain topographical features that allow the deposition of snow masses are called avalanche-release areas. GIS helps to make decisions concerning spatial planning within avalanche release areas and finding risky zones. This study aimed to determine the potential avalanche release areas in the GIS environment in Rize, Türkiye, which was chosen as the pilot region. In the study, the detection of these avalanche areas was estimated using a mathematical equation model proposed by Hreško (1998) and determined with the help of GIS. Factors such as elevation, curvature, aspect, slope, and land cover type were used to estimate avalanche risk areas. A Model Builder workflow has also been created to automate the process stages. As a result of the study, avalanche risk areas were determined and mapped for the Rize mountainous region.

Estimation of Spatial Snowpack Properties in a Snow-Avalanche Release Area: An Extreme Case on Mt. Nodanishoji, Japan, in 2021

An extreme dry-slab snow avalanche occurred on January 10, 2021, at Mt. Nodanishoji, Gifu, Japan, during a heavy snowfall. The avalanche ran down a horizontal distance of approximately 2,800 m and damaged trees and infrastructures. This was estimated to be the second largest recorded avalanche in Japan. However, physical snowpack properties and their vertical profiles and spatial distribution, which caused the avalanche, were not addressed in the release area immediately following the avalanche, mainly due to unsafe and lousy weather conditions. Based on a snow depth distribution observed by an unmanned aerial vehicle and a numerical snowpack simulation in the avalanche release area, the spatial distributions of the mechanical snowpack stability and slab mass and their temporal evolution were estimated in this study. The procedure was validated by comparing the calculation results with the observed snowpit and spatial snow depth data. The results indicated that two heavy snowfall events, approximately 3 and 10 days before the avalanche onset, generated two different weak layers made of precipitation particles and associated slabs above the weak layers. The older weak layer was only generated on the northward slope due to its low temperature, whereas the newer layer was predominant over the avalanche release area. The procedure employed in this study is expected to be applied to other avalanche cases in the future.

Automated snow avalanche release area delineation in data-sparse, remote, and forested regions

Abstract. Potential avalanche release area (PRA) modeling is critical for generating automated avalanche terrain maps which provide low-cost, large-scale spatial representations of snow avalanche hazard for both infrastructure planning and recreational applications. Current methods are not applicable in mountainous terrain where high-resolution (≤5 m) elevation models are unavailable and do not include an efficient method to account for avalanche release in forested terrain. This research focuses on expanding an existing PRA model to better incorporate forested terrain using satellite imagery and presents a novel approach for validating the model using local expertise, thereby broadening its application to numerous mountain ranges worldwide. The study area of this research is a remote portion of the Columbia Mountains in southeastern British Columbia, Canada, which has no pre-existing high-resolution spatial datasets. Our research documents an open-source workflow to generate high-resolution digital elevation models (DEMs) and forest land cover datasets using optical satellite data processing. We validate the PRA model by collecting a polygon dataset of observed potential release areas from local guides, using a method which accounts for the uncertainty in human recollection and variability in avalanche release. The validation dataset allows us to perform a quantitative analysis of the PRA model accuracy and optimize the PRA model input parameters to the snowpack and terrain characteristics of our study area. Compared to the original PRA model our implementation of forested terrain and local optimization improved the percentage of validation polygons accurately modeled by 11.7 percentage points and reduced the number of validation polygons that were underestimated by 14.8 percentage points. Our methods demonstrate substantial improvement in the performance of the PRA model in forested terrain and provide means to generate the requisite input datasets and validation data to apply and evaluate the PRA model in vastly more mountainous regions worldwide than was previously possible.

Geospatial Analysis and Simulation of Glacial Avalanche Hazard in Hunza River Basin

Glacial avalanche hazard poses threat to human lives and damage settlements / infrastructures in alpine glaciers mountainous regions. A gigantic ice plus rock avalanche destroyed Gyari military camp in Siachen sector on April 2012 and buried 139 personals. The study focuses on geospatial analysis and simulation of Shishper glacial avalanche of Hunza basin. To simulate the potential glacial avalanche hazard to Hassan Abad settlements, an empirical process based Glacier Avalanche Model; Rapid Access Mass Movement Simulation (RAMMS) is utilized. The model encompasses avalanche release area and height for the execution of simulation. The model output of Shishper glacial avalanche resulted; a max pressure of 450 Kpa, max velocity of 40 m/s, and the max flow height of 80m, while the resulted surge extent output was 2500m. The potential hazardous Shishper glacial avalanche remains a continuous hazard to Hassan Abad of Hunza valley including Karakoram Highway and Frontier Works Organization (FWO) camp. The study has resulted in identifying the Upper Indus Bain (UIB) being more prone to glacial avalanche hazards because of host factors in general and the anthropogenic factor in particular.

Integrated grey-green management of avalanche risk: Economic and ecologic evidences from the Western Italian Alps

Avalanches represent one of the most common natural hazard in the Alps, and their risk mitigation is increasingly demanding for local administrations. The frequency of these events has also been influenced by the land use changes that affected the Alps in the past: from the over-exploitation of the first half of the XX century to the depopulation and natural reforestation of the last decades. In this study, we performed an economic and ecologic assessment of two alternative mitigation measures against avalanches: steel snow fences, the BAU scenario, and the combination of wooden snow fences gradually replaced by the protection forest, the Grey-Green Scenario. For both alternatives all costs sources that their implementation entails were evaluated, and the effects of the presence of fences on the natural forest regeneration was assessed. The results show how wooden fences are the most cost-effective solution to be placed in avalanche release areas below the treeline, due their high effectiveness coupled with lower construction costs compared to other types of measures, no dismantling costs and the contribution of the protective effect of the forest. Their adoption in the local risk management strategies, notwithstanding the shortcomings of the forest (e.g. fires, pests, …), could represent a reliable way to foster the implementation of ecosystem based disaster risk reduction solutions in mountain areas and meet the protection needs of the society in the most cost-effective way.

Simulation of Glacial Avalanche Hazards in Shyok Basin of Upper Indus

Glacial avalanche hazards can threaten lives and damage infrastructures in high altitude mountainous regions. In April 2012 a gigantic ice plus rock avalanche destroyed military camp at Gyari and killed 139 persons. Antecedent, the objectives of this study are to simulate and model Gyari camp glacial avalanche with reference to its extent, height, momentum, velocity and pressure and, to simulate and model the other potential glacial avalanche prone areas in Shyok basin. To simulate the Gyari camp glacial hazard and other potential glacial avalanche hazards, an empirical process based Glacier Avalanche Model; Rapid Access Mass Movement Simulation (RAMMS) is used. The RAMMS model encompasses the variables like avalanche release height and release area for the conduct of simulation. The model output of Gyari glacial avalanche hazard resulted from a max pressure of 2500 KPa, max velocity of 90 m/s, and the max flow height of 40 m, while the resulted output debris volume calculated was 4.3145 million m3. The calibrated agreement was found in extent and height of actual debris in comparison with RAMMS simulated output. The potential hazardous glacial avalanche prone areas of Shyok basin were simulated by RAMMS model after the model being calibrated to the actual incident of Gyari. The study has resulted in identifying the Siachen glacier conflict zone being more prone to avalanche hazards because of host factors in general and the anthropogenic factor in particular.

Geospatial probabilistic modelling for release area mapping of snow avalanches

This paper presents GIS-based mapping of snow avalanche release areas using probabilistic occurrence ratio (POR) model. The proposed method utilized the topographical and avalanche occurrence dataset. The avalanche occurrence topographical factors such as slope, aspect, curvature and elevation generated from ASTER-GDEM V2 whereas vegetation cover from Landsat 8 OLI were considered in the proposed model. An avalanche inventory map of avalanche occurrence locations was prepared for training and validation of the model. From avalanche inventory map, 80% of avalanche occurrence data for training and remaining 20% was used for validation of the model. Finally, avalanche release area map was generated using the POR model and classified into five categories based on the Jenks natural break method. The RA-index and ROC-AUC methods were used for validation of the avalanche release area map. The validation accuracy of the model is confirmed as 92.41% and 89.13% as per success and prediction rates.

Automated snow avalanche release area delineation – validation of existing algorithms and proposition of a new object-based approach for large-scale hazard indication mapping

Abstract. Snow avalanche hazard is threatening people and infrastructure in all alpine regions with seasonal or permanent snow cover around the globe. Coping with this hazard is a big challenge and during the past centuries, different strategies were developed. Today, in Switzerland, experienced avalanche engineers produce hazard maps with a very high reliability based on avalanche database information, terrain analysis, climatological data sets and numerical modeling of the flow dynamics for selected avalanche tracks that might affect settlements. However, for regions outside the considered settlement areas such area-wide hazard maps are not available mainly because of the too high cost, in Switzerland and in most mountain regions around the world. Therefore, hazard indication maps, even though they are less reliable and less detailed, are often the only spatial planning tool available. To produce meaningful and cost-effective avalanche hazard indication maps over large regions (regional to national scale), automated release area delineation has to be combined with volume estimations and state-of-the-art numerical avalanche simulations. In this paper we validate existing potential release area (PRA) delineation algorithms, published in peer-reviewed journals, that are based on digital terrain models and their derivatives such as slope angle, aspect, roughness and curvature. For validation, we apply avalanche data from three different ski resorts in the vicinity of Davos, Switzerland, where experienced ski-patrol staff have mapped most avalanches in detail for many years. After calculating the best fit input parameters for every tested algorithm, we compare their performance based on the reference data sets. Because all tested algorithms do not provide meaningful delineation between individual PRAs, we propose a new algorithm based on object-based image analysis (OBIA). In combination with an automatic procedure to estimate the average release depth (d0), defining the avalanche release volume, this algorithm enables the numerical simulation of thousands of avalanches over large regions applying the well-established avalanche dynamics model RAMMS. We demonstrate this for the region of Davos for two hazard scenarios, frequent (10–30-year return period) and extreme (100–300-year return period). This approach opens the door for large-scale avalanche hazard indication mapping in all regions where high-quality and high-resolution digital terrain models and snow data are available.

Potential slab avalanche release area identification from estimated winter terrain: a multi-scale, fuzzy logic approach

Abstract. Avalanche hazard assessment requires a very precise estimation of the release area, which still depends, to a large extent, on expert judgement of avalanche specialists. Therefore, a new algorithm for automated identification of potential avalanche release areas was developed. It overcomes some of the limitations of previous tools, which are currently not often applied in hazard mitigation practice. By introducing a multi-scale roughness parameter, fine-scale topography and its attenuation under snow influence is captured. This allows the assessment of snow influence on terrain morphology and, consequently, potential release area size and location. The integration of a wind shelter index enables the user to define release area scenarios as a function of the prevailing wind direction or single storm events. A case study illustrates the practical usefulness of this approach for the definition of release area scenarios under varying snow cover and wind conditions. A validation with historical data demonstrated an improved estimation of avalanche release areas. Our method outperforms a slope-based approach, in particular for more frequent avalanches; however, the application of the algorithm as a forecasting tool remains limited, as snowpack stability is not integrated. Future research activity should therefore focus on the coupling of the algorithm with snowpack conditions.

Linking snow depth to avalanche release area size: measurements from the Vallée de la Sionne field site

Abstract. One of the major challenges in avalanche hazard assessment is the correct estimation of avalanche release area size, which is of crucial importance to evaluate the potential danger that avalanches pose to roads, railways or infrastructure. Terrain analysis plays an important role in assessing the potential size of avalanche releases areas and is commonly based on digital terrain models (DTMs) of a snow-free summer terrain. However, a snow-covered winter terrain can significantly differ from its underlying, snow-free terrain. This may lead to different, and/or potentially larger release areas. To investigate this hypothesis, the relation between avalanche release area size, snow depth and surface roughness was investigated using avalanche observations of artificially triggered slab avalanches over a period of 15 years in a high-alpine field site. High-resolution, continuous snow depth measurements at times of avalanche release showed a decrease of mean surface roughness with increasing release area size, both for the bed surface and the snow surface before avalanche release. Further, surface roughness patterns in snow-covered winter terrain appeared to be well suited to demarcate release areas, suggesting an increase of potential release area size with greater snow depth. In this context, snow depth around terrain features that serve as potential delineation borders, such as ridges or trenches, appeared to be particularly relevant for release area size. Furthermore, snow depth measured at a nearby weather station was, to a considerable extent, related to potential release area size, as it was often representative of snow depth around those critical features where snow can accumulate over a long period before becoming susceptible to avalanche release. Snow depth – due to its link to surface roughness – could therefore serve as a highly useful variable with regard to potential release area definition for varying snow cover scenarios, as, for example, the avalanche hazard assessment for transport routes or ski resorts.

Influence of weak layer heterogeneity and slab properties on slab tensile failure propensity and avalanche release area

Abstract. Dry-snow slab avalanches are generally caused by a sequence of fracture processes, including failure initiation in a weak snow layer underlying a cohesive slab followed by crack propagation within the weak layer (WL) and tensile fracture through the slab. During past decades, theoretical and experimental work has gradually increased our knowledge of the fracture process in snow. However, our limited understanding of crack propagation and fracture arrest propensity prevents the evaluation of avalanche release sizes and thus impedes hazard assessment. To address this issue, slab tensile failure propensity is examined using a mechanically based statistical model of the slab–WL system based on the finite element method. This model accounts for WL heterogeneity, stress redistribution by slab elasticity and possible tensile failure of the slab. Two types of avalanche release are distinguished in the simulations: (1) full-slope release if the heterogeneity is not sufficient to stop crack propagation and trigger a tensile failure within the slab; (2) partial-slope release if fracture arrest and slab tensile failure occur due to the WL heterogeneity. The probability of these two release types is presented as a function of the characteristics of WL heterogeneity and the slab. One of the main outcomes is that, for realistic values of the parameters, the tensile failure propensity is mainly influenced by slab properties. Hard and thick snow slabs are more prone to wide-scale crack propagation and thus lead to larger avalanches (full-slope release). In this case, the avalanche size is mainly influenced by topographical and morphological features such as rocks, trees, slope curvature and the spatial variability of the snow depth as often claimed in the literature.

Influence of snow depth distribution on surface roughness in alpine terrain: a multi-scale approach

Abstract. In alpine terrain, the snow-covered winter surface deviates from its underlying summer terrain due to the progressive smoothing caused by snow accumulation. Terrain smoothing is believed to be an important factor in avalanche formation and avalanche dynamics, and it affects surface heat transfer, energy balance as well as snow depth distribution. To assess the effect of snow on terrain, we use an adequate roughness definition. We developed a method to quantify terrain smoothing by combining roughness calculations of snow surfaces and their corresponding underlying terrain with snow depth measurements. To this end, elevation models of winter and summer terrain in three selected alpine basins in the Swiss Alps characterized by low, medium and high terrain roughness were derived from high-resolution measurements performed by airborne and terrestrial lidar. The preliminary results in the selected basins reveal that, at basin scale, terrain smoothing depends not only on mean snow depth in the basin but also on its variability. The multi-temporal analysis over three winter seasons in one basin suggests that terrain smoothing can be modelled as a function of mean snow depth and its standard deviation using a power law. However, a relationship between terrain smoothing and snow depth was not found at pixel scale. Further, we show that snow surface roughness is to some extent persistent, even in-between winter seasons. Those persistent patterns might be very useful to improve the representation of a winter terrain without modelling of the snow cover distribution. This can for example improve avalanche release area definition and, in the long term, natural hazard management strategies.

Automated identification of potential snow avalanche release areas based on digital elevation models

Abstract. The identification of snow avalanche release areas is a very difficult task. The release mechanism of snow avalanches depends on many different terrain, meteorological, snowpack and triggering parameters and their interactions, which are very difficult to assess. In many alpine regions such as the Indian Himalaya, nearly no information on avalanche release areas exists mainly due to the very rough and poorly accessible terrain, the vast size of the region and the lack of avalanche records. However avalanche release information is urgently required for numerical simulation of avalanche events to plan mitigation measures, for hazard mapping and to secure important roads. The Rohtang tunnel access road near Manali, Himachal Pradesh, India, is such an example. By far the most reliable way to identify avalanche release areas is using historic avalanche records and field investigations accomplished by avalanche experts in the formation zones. But both methods are not feasible for this area due to the rough terrain, its vast extent and lack of time. Therefore, we develop an operational, easy-to-use automated potential release area (PRA) detection tool in Python/ArcGIS which uses high spatial resolution digital elevation models (DEMs) and forest cover information derived from airborne remote sensing instruments as input. Such instruments can acquire spatially continuous data even over inaccessible terrain and cover large areas. We validate our tool using a database of historic avalanches acquired over 56 yr in the neighborhood of Davos, Switzerland, and apply this method for the avalanche tracks along the Rohtang tunnel access road. This tool, used by avalanche experts, delivers valuable input to identify focus areas for more-detailed investigations on avalanche release areas in remote regions such as the Indian Himalaya and is a precondition for large-scale avalanche hazard mapping.

Data-driven mapping of avalanche release areas: a case study in South Tyrol, Italy

Avalanche hazard and risk mapping is of utmost importance in mountain areas in Europe and elsewhere. Advanced methods have been developed to describe several aspects of avalanche hazard assessment, such as the dynamics of snow avalanches or the intensity of snowfall to assume as a reference meteorological forcing. However, relatively little research has been conducted on the identification of potential avalanche release areas. In this paper, we present a probabilistic assessment of potential avalanche release areas in the Italian Autonomous Province of Bolzano, eastern Alps, using the Weights of Evidence and Logistic Regression methods with commonly available GIS datasets. We show that a data-driven statistical model performs better than simple, although widely adopted, screening criteria that were proposed in the past, although the complexity of observed release areas is only partly captured by the model. In the best case, the model enables predicting about 70 % of avalanches in the 20 % of area classified at highest hazard. Based on our results, we suggest that probabilistic identification of potential release areas could provide a useful aid in the screening of sites for subsequent, more detailed hazard assessment.

Wind and drifting-snow gust factor in an Alpine context

AbstarctWind-transported snow is a common phenomenon in cold windy areas, creating snowdrifts and contributing significantly to the loading of avalanche release areas. It is therefore necessary to take into account snowdrift formation both in terms of predicting and controlling drift patterns. Particularly in an Alpine context, drifting snow is a nonstationary phenomenon, which has not been taken into account in physical modeling carried out in wind tunnels or in numerical simulations. Only a few studies have been conducted to address the relation between wind gusts and drifting-snow gusts. Consequently, the present study was conducted at the Lac Blanc pass (2700ma.s.l.) experimental site in the French Alps using a snow particle counter and a cup anemometer in order to investigate drifting-snow gusts. First, it was shown that the behavior of the wind gust factor was coherent with previous studies. Then the definition of wind gust factor was extended to a drifting-snow gust factor. Sporadic drifting-snow events were removed from the analysis to avoid artificially high drifting-snow gust factors. Two trends were identified: (1) A high 1 s peak and a mean 10 min drifting-snow gust factor, greater than expected, were observed for events that exhibited a gamma distribution on the particle width histogram. The values of drifting-snow gust factors decreased with increasing gust duration. (2) Small drifting-snow gusts (i.e. smaller than or of the same order of magnitude as wind gusts) were also observed. However, in this case, they were systematically characterized by a snow particle size distribution that differed from the two-parameter gamma probability density function.

Back calculation of the In den Arelen avalanche with RAMMS: interpretation of model results

AbstractTwo- and three-dimensional avalanche dynamics models are being increasingly used in hazard-mitigation studies. These models can provide improved and more accurate results for hazard mapping than the simple one-dimensional models presently used in practice. However, two- and three-dimensional models generate an extensive amount of output data, making the interpretation of simulation results more difficult. To perform a simulation in three-dimensional terrain, numerical models require a digital elevation model, specification of avalanche release areas (spatial extent and volume), selection of solution methods, finding an adequate calculation resolution and, finally, the choice of friction parameters. In this paper, the importance and difficulty of correctly setting up and analysing the results of a numerical avalanche dynamics simulation is discussed. We apply the two-dimensional simulation program RAMMS to the 1968 extreme avalanche event In den Arelen. We show the effect of model input variations on simulation results and the dangers and complexities in their interpretation.

Evaluating the benefit of avalanche protection forest with GIS-based risk analyses—A case study in Switzerland

The protection of people, buildings and infrastructure against natural hazards is one of the key functions of mountain forests. Since the protective function strongly depends on small-scale local conditions such as terrain and stand characteristics, spatially explicit evaluation methods are necessary to provide information required for an effective and cost-efficient forest management. Risk analyses are recognized as the best method for estimating the danger from various natural hazards. Currently, however, risk-based strategies are rarely addressed in the management of protection forest. We present and discuss a risk-based approach to evaluate the protective effect of mountain forests in a spatially explicit manner to demonstrate the advantages of future risk-based protection forest management. We illustrate the approach by performing a GIS-based risk analysis in the case study area ‘Bannwald of Andermatt’ (Switzerland) for a 300-year snow avalanche event. Classifying forest structures based on aerial photographs allowed developing different forest cover scenarios and modeling potential avalanche release areas within the forest. Potential avalanche release areas above the forest and the avalanche run-out distances under five different scenarios of forest cover were calculated by using a two-dimensional avalanche simulation model. We calculated the annual collective risk for each scenario and compared the change in risk to reveal the spatial distribution of the protective effect of the forest. Resulting risks differed strongly between forest cover change scenarios. An enlargement of an existing wind-disturbed area within lower parts of the slope resulted only in a slight increase of risk. In contrast, the effect of an unforested area in the upper parts of the observed forest more than doubled the risk. These results show how a risk-based approach can help to quantify and illustrate the impact of differences in forest cover on the protective effect of mountain forests. It is a promising approach to determine the economic value of protection forests and thus provide quantitative and qualitative information for cost-efficient forest maintenance planning. With regard to the achievements of research to date, the presented approach may serve as a valuable method to support decision-making in a future protection forest management.

Effect of timber removal from windthrow slopes on the risk of snow avalanches and rockfall

The protective effect of windthrow-related wooden structure elements such as living trees, snags, stumps, boles, and mounds against rockfall and snow avalanches was studied after windthrow in a steep mountain protection forest. The study site near Disentis was a 2.49 ha uncleared windthrow caused by the storm Vivian in February 1990. From this real scenario three virtual scenarios were derived: the unstocked slope, the forested site, and the cleared windthrow site. In the scenario “unstocked slope” all timber elements were virtually removed. In the scenario “forested site” the former forest with a tree density of 451 stems ha −1 was virtually reconstructed. In the scenario “cleared windthrow” all boles, i.e. the fallen dead stems, were virtually removed. Four hundred and fifty-one wooden structure elements per hectare remained. In the scenario “uncleared windthrow” additionally 411 criss-crossed boles ha −1 were found, resulting in a total of 862 structure elements ha −1. They made the surface very rough sometimes extending several meters above the ground surface. At the level of 3 m above the ground there were still 137 wooden structure elements ha −1 in 1991. This density decreased to 109 within the first 10 years. The area of influence of all wooden structure elements was determined by buffering them with half their heights above ground and intersecting them. This “effective area” covered 44% of the total area. Compared with values from the literature, the scenario “unstocked slope” is clearly a rockfall and avalanche release area. The other three scenarios, however, have, for at least the first decade after the windthrow, enough structure elements of sufficient height to effectively prevent any release of avalanches and to stop rolling rocks. The scenario providing the best protection was the uncleared windthrow. Within a few decades, however, the decaying wood gradually loses its strength and, as a consequence, its protective effect. We therefore conclude that leaving the thrown timber on the spot can be a good option in protection forests to best ensure safety, whereas removing the fallen timber and clearing the area reduces the protective effect to a level often similar to that of the preceding stand. The protective effect of the decaying wood must be replaced by forest regeneration before it falls below a critical level. This restoration process is enhanced if numerous saplings from the previous stand (advance regeneration) are present. Otherwise, it should be speeded up by planting trees.

The influence of topographic parameters on avalanche release dimension and frequency

Avalanche hazard zones in Switzerland are defined by the frequency and the impact pressure of a potential avalanche event. Therefore, it is necessary to accurately estimate the release frequency and, related to the frequency, the release area for a specific avalanche track. In order to improve this hazard mapping procedure, a statistical analysis of avalanche releases on well-documented avalanche paths was performed. In the region of Davos, an almost complete database of avalanche events over the last 50 years exists. Using Geographic Information System (GIS) technologies in combination with Digital Elevation Models (DEMs), the recorded avalanche release areas were analysed with respect to topographic characteristics. Topographic parameters like “slope”, “confinement”, “aspect” and “distance to the next ridge” are derived from the DEM. The statistical analysis results in general rules and probability distributions for release extents as a function of the frequency and the topographic parameters. The general rules are a valuable aid for avalanche experts in cases where information about historical avalanche is lacking for a particular track. Furthermore, the probability distributions can be directly used as input for uncertainty modelling of avalanche run-out distances using Monte Carlo methods. This preliminary analysis indicates that mean slope, curvature and distance to the ridge are the most important parameters influencing the avalanche release area frequency; higher average slope angles lead to the frequent release of small avalanches.