Abstract
(Englisch)
|
The extreme avalanche winter of 1999 has caused Europe's avalanche experts to rethink existing methods to protect the population in mountainous regions. New methodologies, based on experiments, statistical analysis and modern computer simulation methods, presently under development in all European countries, must be advanced with the goal that they can be rapidly introduced into practice in order to be able to manage the extreme avalanche periods better. The aim of the project is to improve the quality of avalanche hazard mapping and the efficiency of defense structures by understanding the physical process involved in dynamics of catastrophic avalanches and their interaction with defenses structures.
Zoning land has proved to be one of the most efficient tools to protect human settlements against avalanches. Although various calculation methods have been applied over many years, there is still an urgent demand to improve the hazard quantification process. The large number of influences that must be taken into account (fracture heights, topography, vegetation)- in conjunction with all the natural uncertainties - makes it necessary to have a closer look on the sensitivity and reliability of the calculated results.
The research goals of this part of the project are:
a) to find a best fit parameter set for avalanche calculation models,
b) to describe the sensitivity and the uncertainty of the results,
c) to combine statistical and dynamical methods and
d) to develop a new approach for avalanche hazard zoning and risk management.
In the first research period, data about avalanche events were collected in order to have a good database to apply models to and to perform sensitivity and uncertainty analyses.
In the second research period, other past avalanches data were introduced into the database and the different kinds of models to study the development of an avalanche were analysed and compared.
By now, both empirical procedures including statistical/topographical and comparative models for runout distance computations, as well as dynamics models for avalanche motion simulations exist. The empirical procedures permit an assessment of runout distance only, while the more advanced dynamical models give much additional information concerning the nature of the sliding event (flow heights, velocities, impact pressures, etc.). The aim of avalanche hazard mapping is to present the spatial variation of hazard on geographical maps. The simplest strategy of avalanche hazard zoning and risk management is to avoid the presence of any human being or constructions in an endangered area. In this case, the only need of the practitioners is the knowledge of the maximum run-out distance of an avalanche which can be found by statistical methods alone. However, it is often impossible to keep the infrastructure out of endangered areas. Then the main requirement is to know the impact pressure of an avalanche event at a given location. For the reliable dimensioning of protective measures also more detailed information about the duration of the impact pressure, the shear forces, the flow height and the run-up height are required. This requires the use of physical methods. Often a combined use of statistical and physical methods is fruitful. Finally, procedures to take the protective measures into account on the hazard maps are needed.
The SLF contributed to the improvement of the database and to the uncertainty analysis. Distribution functions for the release area have been worked out to be used as input in the MonteCarlo analysis and a first try to link topographic parameters to release frequencies of avalanches has been done.
The second part of the project deals with defense structures in the runout zone. Up to now their design is mostly empirical since there are only few models that can be used for quantitative design of defense structures. Therefore, it is the aim to develop a theoretical framework about the interaction of avalanches with defense structures and to make this knowledge available to engineers.
In the first research period, SLF performed laboratory granular flow experiments in order to study the interaction between the flow and defense structures. A 9 m long laboratory chute with variable inclination was set up and optical velocity sensors were developped and tested on this chute.
Model experiments on the effectiveness of various defence structure configurations have been performed in collaboration with the Icelandic Meteorological Institute (IMO). This work also proved the validity of such model experiments on different scales of physical modelling.
The work done by the SLF in the second research period is mainly determined by the upscaling of the laboratory experiments performed in the first period onto the 20 m long and 2.5 m wide snow chute of the SLF at Weissfluhjoch. Velocity sensors were adapted to the velocity measurement of flowing snows and refined for high-resolution velocity profile measurements. The chute bottom was roughened to obtain turbulent, avalanche-like snow flow which can be scaled to the laboratory experiments via the similarity criterion of the Froude number. The effectiveness of four different defence structure configurations was tested by relating velocities of a undisturbed reference snow chute flow to the velocity of the snow flows on the chute after the interaction with the defense structures. This work again was performed in collaboration with IMO. The scalabilty of the laboratory and snow chute flows give hints to the applicability of laboratory test results to real avalanche problems.
|
