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Unit IV. Avalanches.

Focus on:

• терминологическая лексика
• видо-временные формы глагола и их употребление

 

Revision:

• mean/means/by all means/by no means
• слова-заместители that/those
• суффиксы частей речи
• употребление other/another/the other/others
• as/since/because/because of
• усилительная конструкция «it is … that (which)

 

Text Study.

I

The hazard of snow avalanches to life and property increases from year to year. It is enhanced by the general rise in population, which places more communities and structures in the hazardous areas; by the growing popularity of skiing, which attracts ever more thousands to the snowy mountainslides, and by the expanding networks of communications – highways, pipelines, power lines, electronic relay systems – whose mountain crossings must be protected. Concern about the avalanche problem is by no means new, but in recent years there has been an intensification of efforts to find effective ways of controlling hazard. Several useful techniques have been developed, and other interesting ideas are under study.

There are two basically different types of snowslide, one much more dangerous than the other. They are known respectfully as “loose snow” avalanches and “slab” avalanches.

Loose-snow slides occur frequently, but they seldom grow very large or cause much damage. It is the slab type of avalanche, which sets in motion as one massive body a large area of snow, that presents the principal menace and is the main object of control efforts.

Investigations of the predisposing conditions are focused essentially on the structure and nature of the snow itself, particularly its cohesion. This has two aspects: on the one hand, the strength of bonding between the snow crystals within a layer; on the other, the degree of bonding of one layer to another. Cohesion between the snow particles depends in large part on the age and nature of the snow crystals, which vary considerably in form. Crystals of certain compact, nonstellar types tend to become firmly cemented together. Weather conditions also play a part: strong bonding between crystals is encouraged by windpacking and by riming – the accretion of supercooled water droplets as the snow is deposited.

 

II

The most elementary defense against avalanches is to attempt to forecast their occurrence so that people and transient traffic can be warned to stay clear. Although physical studies and experience have armed us with many clues, avalanche forecasting is still an art rather than a science. The variables that determine when conditions are ripe for the triggering of a slide are numerous and complex. It is difficult to measure the mechanical properties of snow samples with any precision even in the laboratory, because the characteristics of a sample can change rapidly while it is being handled. There are, however, significant factors that can be measured in the field: the density and thickness of the snow slab, the size of the load compared with the shear strength of the substratum, certain patterns of the snow structure. With information thus obtained it is possible to predict quite accurately the probability of the occurrence of avalanches within the given area. It is not possible, however, to forecast precisely where and when a slide will take place, because of unknown variables such as the existence of creep tensions and the unpredictability of the various natural or manmade shocks (earth tremors, construction blasting or the like) that can trigger the fracture of a slab.

 

III

Today there are available a number of engineering techniques for control of avalanches, some providing passive defence (protection against actual slides), others designed to prevent their occurrence. Fundamentally there are two different ways to prevent or control avalanches: (1) modification of the terrain and (2) modification of the snow. Which includes the deliberate release of slides when and where they will do little or no harm. The first general method is expensive and requires continued maintenance, but it is reasonably permanent and offers maximum protection. The second is comparatively cheap but must be applied repeatedly, perhaps many times each winter. Modification of the terrain is usually chosen when the problem is to protect a large area or fixed installations; modification of the snow is employed most commonly for protecting highways and ski slopes.

 

Ex.1.