Snow avalanches

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Encyclopédie environnement - avalanches - couverture

Avalanches are fast snowflows on a mountain slope. By extension, these can be flows of other materials; sometimes we talk about underwater avalanches or stone avalanches. The motor of flow is gravity. The speeds reached by an avalanche cover a wide range from low speeds (a few m/s) to speeds approaching those of a free-fall body (more than 30 m/s). Flow heights are also very variable: from a few meters for dense flows to more than 100 m for diluted flows. The distances covered are also very variable: from a few tens of metres for a small flow to several kilometres for large avalanches involving volumes of several hundred thousand m3.

1. Avalanche danger in the world

Avalanches are typical flows in mountainous regions. They are observed as soon as the slope of the ground is sufficient (generally more than 30°) and the snowpack is thick enough to form a mobilizable volume of snow. Avalanches represent a threat that may seem much lower than other natural hazards such as floods or earthquakes (about 0.1% of the damage generated worldwide or in Western countries). However, given their frequency and omnipresence in mountainous areas, they pose a strong threat to all human activities (transport, tourism, industry), infrastructure and urbanised areas. In France, avalanches kill an average of 31 people each year. Most of the victims are taken away during tourist activities (mainly skiing); on rare occasions over the past three decades, avalanches have caused victims on the roads or in homes. The last disaster occurred in 1999, with 12 deaths in Chamonix. In less developed countries such as Afghanistan or Pakistan, the number of victims can exceed 100 when high altitude villages are swept by avalanches.

2. Chalenges in the study of avalanches

There are two main problems with avalanche research:

  • The first concerns the triggering of avalanches. The aim is to determine which areas may be the site of an avalanche departure for given snow and weather conditions. The triggering is most often the result of instability of the snowpack (e.g. following heavy snowfall). It can also be caused accidentally by the passage of a skier or deliberately by an explosion (in ski resorts to secure their ski domain)  when the snow cover is particularly unstable. The scientific issue therefore revolves mainly around the prediction of phenomena in the near future. Most of the western countries concerned by avalanches have set up services that deliver daily newsletters on avalanche risk during the winter season (this is one of Météo-France’s missions).
  • The second problem concerns the spread and stopping of avalanches. The aim is to determine how far avalanches can go in a given area, how often large avalanches are to be feared, and what efforts they can exert on obstacles. This is a problem of great importance for the urbanisation of mountain municipalities and the sizing of structures under the threat of avalanches. Most Western countries now have legislation that regulates construction according to the risk involved. In France, this is the purpose of risk prevention plans. While risk mapping is an important element of prevention, it is not sufficient because structures can be established in areas of moderate risk and it is necessary, in this case, to ensure their protection by civil engineering structures (reinforcement of structures, stop or deviation dams, etc.). This is the purpose of the paravalanche engineering techniques.

3. Cause of avalanches

Avalanches are often studied and classified according to their triggering mode. A better understanding of the triggering mechanisms is essential for predicting avalanche risk or for training mountain climbers.

The most common scenario observed in avalanche departures is as follows: a succession of snowfalls form a snowpack with a structure similar to a large millefeuille, each stratum of which potentially constitutes a sliding plane. During snowfall, the weight of recent snow may exceed the strength of the lower strata. The surface of the snowpack breaks in the form of a plate, hence the name plate avalanches. The exact triggering mechanisms are complex. Most current theories emphasize the role played by the interface between the snow plate and the underlying snowpack: a fragile layer often takes place there and it would be its collapse that would be the primary cause of the trigger. It is this type of avalanche that is particularly feared by skiers because the weight of a single person can be enough to initiate the breakage within the brittle layer and set a large amount of snow in motion in a few seconds. While wind promotes snow transport and snow cohesion, it is generally a factor in the instability of the snowpack in the short term (over a time scale ranging from a few hours to a few days); however, in the longer term (beyond two to three days), snow accumulation and strong cohesion work in favour of increased stabilization of the snowpack. Wind is therefore not systematically a factor of instability, and it cannot by itself explain the instability of snow cover composed of dry snow.

Not all avalanche departures necessarily occur in the form of a snow plate. When the snow is weakly cohesive, there are occasional departures: a small mass of snow moves and drags other snow masses downstream. As the avalanche progresses, it grows in width and volume. It is the snowball effect (from the stories of travellers in the Middle Ages to Tintin in Tibet, the books are rich in tasty descriptions assimilating the avalanche to a huge ball of snow).

Some avalanches occur long after a snowfall. This is the case, for example, when a snowpack suddenly gets wet due to rain or an increase in air temperature. Liquid water then appears. Since it is mainly concentrated in the contacts between snow crystals, it confers a certain cohesion to the snow under the effect of surface tension forces. Early in the season, the night cooling of the snowpack leads to the freezing of this liquid water. The freeze-thaw cycle therefore contributes to a significant increase in snow cohesion. The low frost (or even its absence) leads to an increase in the liquid water content. As the latter is in excess, it becomes an unfavourable factor, thus allowing the formation of melting or spring avalanches at the end of the season.

In some cases, liquid water reaches the ground and lubricates the interface with the snowpack, significantly increasing the sliding speed of the snowpack in some areas. The snowpack can then crack locally to the ground. If these cracks do not restore a static balance of forces, a slip avalanche can occur: the entire snowpack to the ground begins to slide and disintegrate. We’re talking about a landslide avalanche.

4. Avalanche spread

 Encyclopédie environnement - avalanches - avalanche coulante
Figure 1. Flowing avalanche (avalanche des Lanches at Peisey-Nancroix, Savoie, 25 February 1995). The photograph shows a multitude of avalanche languages, some of which came against the cottages of Les Lanches, which helps to have a scale of scale of the deposit. These languages reflect the continuous nature of the snow stream: it is not a compact mass that has descended, but a succession of “waves” that have made their way through the earlier deposits. This type of flow is reminiscent of volcanic lava flows: slow (a few m/s), thick (a few m), and very pasty. [© PGHM Bourg-Saint-Maurice]
The shape of snowflows is extraordinarily varied. It is partly conditioned by topography. The slope of the natural terrain plays a key role since gravity is the driving force behind the flow: the steeper the slope, the greater the potential speed. Variable factors such as snow consistency or snowpack structure have a significant influence on flow dynamics. Once the avalanche has started, it can continue to carry snow, especially if the slopes are steep and the snowpack is not very resistant. It can also lead to ambient air, which in turn promotes the suspension of snow and the formation of snow clouds called aerosols.

 Encyclopédie environnement - avalanches - avalanche en aerosol
Figure 2. Aerosol avalanche (Roux d’Abriès, Hautes-Alpes, January 2004). The avalanche was artificially triggered after heavy snowfall. The cold, dry snow quickly gained speed (a few tens of m/s). It has massively entrained air to form an aerosol, which is a few dozen metres high. Due to its high speed on arrival at the valley bottom, the avalanche does not spread, but goes up the opposite slope. Despite this speed, it causes little damage to trees because the snow suspended in the air has a low density (a few kg/m3), and therefore the impact pressure remains low (less than 2 kPa). Hidden from view, there is also a denser flow that follows the line of greater slope, then dies in the bed of the stream. [© Maurice Chave]
Given the great diversity of flows, it is convenient to consider two ideal shapes:

  • the flowing avalanche, a dense snow flow whose trajectory follows the relief fairly closely (see Figure 1);
  • aerosol avalanche, a diluted, high-speed flow that tends to follow the steeper line of slope (see Figure 2).

Most avalanches fall into one of these two categories. In some cases, they have characteristics specific to each of the two categories (sometimes referred to as mixed avalanches).

The velocity of the flowing avalanche is generally in a wide range of 5 to 25 m/s, sometimes 50 m/s. The density is high, between 150 and 500 kg/m3. The flow heights are generally a few meters. As they follow the terrain, it is quite simple to predict their trajectory. What is more complicated is to determine their stop rating and spread during their stop. Most flowing avalanches stop on slopes greater than 10°, but some have travelled long distances on shallow slopes (between 5° and 10°).

The aerosol avalanche has a high speed, often between 60 and 100 m/s. A pure aerosol is a cloud of suspended snow, with low average density (a few tens of kg/m3). The density is very variable in the aerosol. Indeed, under the effect of gravity, there is a higher concentration of particles towards the base of the flow. Due to its high speeds, the aerosol draws in massive amounts of ambient air, which allows it to increase its height dramatically since heights of 10 to 100 m are commonly observed. To compensate for the dilution of the aerosol under the effect of this air incorporation, the aerosol must also carry snow from the snowpack. In the absence of training, the aerosol continues to expand until it loses all strength.


The Encyclopedia of the Environment by the Association des Encyclopédies de l'Environnement et de l'Énergie (www.a3e.fr), contractually linked to the University of Grenoble Alpes and Grenoble INP, and sponsored by the French Academy of Sciences.

To cite this article: ANCEY Christophe (February 8, 2019), Snow avalanches, Encyclopedia of the Environment, Accessed October 4, 2024 [online ISSN 2555-0950] url : https://www.encyclopedie-environnement.org/en/soil/snow-avalanches/.

The articles in the Encyclopedia of the Environment are made available under the terms of the Creative Commons BY-NC-SA license, which authorizes reproduction subject to: citing the source, not making commercial use of them, sharing identical initial conditions, reproducing at each reuse or distribution the mention of this Creative Commons BY-NC-SA license.

雪崩

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Encyclopédie environnement - avalanches - couverture

  雪崩是指在山坡上因雪的快速流动导致的崩塌现象。从广义上讲,这种崩塌现象也可以由其它物质的流动造成,比如水下的浊流或石块的流动。这种流动的动力是重力。雪崩的速度范围很广,从低速度(几米/秒)到接近自由落体的速度(超过 30 米/秒)。发生流动的高度也时有不同:从密集流的几米高到稀释流的100 多米高。雪崩覆盖的距离范围也很广:从小流量的几十米到数十万立方米体积大雪崩的几千米。

1.雪崩对世界的威胁

  雪崩是山地区域典型的流动现象。只要地面坡度足够大(一般超过 30°),且雪层厚度能够形成一定可移动的雪量,雪崩就会发生。雪崩的威胁似乎远远低于其他自然灾害,如洪水或地震(约占全世界或西方国家损失总量的 0.1%)。 然而,鉴于其在山区发生的频率和无处不在的特点,雪崩对所有的人类活动(运输、旅游、工业)、基础设施和城市化地区都构成了严重的威胁。在法国,雪崩平均每年会造成 31 人死亡。大多数遇难者在旅游活动(主要是滑雪)中被雪崩夺去生命;在过去三十年中,雪崩很少会在路上或家中造成人员伤亡。上一次发生这样的灾难还是在 1999 年,共造成 12 人在法国小镇西蒙尼遇难。在阿富汗或巴基斯坦等欠发达国家,高海拔区域的村庄发生雪崩时受灾人数可能超过百人。

2.雪崩研究中的挑战

  雪崩研究主要关注两个问题:

  • 第一个问题是雪崩的触发。研究这个问题的目的是在给定的雪况和天气条件下,确定哪些地区可能是雪崩开始的地点。雪崩的触发通常是积雪不稳定的结果(例如在大雪之后)。当积雪特别不稳定时,雪崩也可能因滑雪者的通过意外形成或者由爆破(在滑雪场实施,以确保滑雪区域的安全)而 刻意造成。因此,相关科学研究主要围绕着对未来短期内雪崩现象的预测展开。大多数受雪崩影响的西方国家都在冬季提供有关雪崩风险的每日资讯服务(这是法国气象局的任务之一)。
  • 第二个问题是关于雪崩的蔓延和停止。其目的是确定雪崩在特定地区能够蔓延多远,大型雪崩频繁到什么程度应引起重视,以及为阻止雪崩蔓延还可尝试哪些措施。雪崩的蔓延和停止对于在雪崩威胁下的山区城市的城市化和建筑物规模来说是一个非常重要的问题。大部分西方国家现在都有立法,根据所涉及的风险来监管建筑。在法国,这是风险预防计划的目标。虽然风险勘测是预防雪崩的一个重要因素,但仅凭此还是不够的,因为建筑物有可能建造在中等风险的区域。在这种情况下,通过土木工程措施(加固建筑结构,建造拦截坝或偏流坝等)保护建筑物的安全是十分必要的,这也是雪崩工程技术的目的所在。

3.雪崩产生的原因

  雪崩通常是根据其触发模式来研究和分类的。更好地了解其触发机制对于雪崩风险预测或登山者培训都是至关重要的。

  雪崩发生最常见的情形是:连续的降雪形成了一个结构类似于千层酥蛋糕的积雪层,其中的每一层都有可能构成一个滑动平面。降雪期间,近期降雪的重量可能超过低层雪的承重强度。积雪层的表面会以板块的形式断裂,因此这被称为板块雪崩(plate  avalanches)。确切的雪崩触发机制很复杂。目前大多数理论强调表面雪板和底层积雪之间的界面所起的作用:脆弱的雪层往往存在于那里,而它的崩溃将会是触发雪崩的主要原因。滑雪者特别害怕的就是这种类型的雪崩,因为一个人的重量足以促使这个脆弱层破裂,并在几秒钟内使大量的雪开始移动。虽然风促进了雪的运移和雪的粘结力,但在短期内(在几小时到几天的时间范围内),它通常是造成积雪不稳定的因素;然而,从长远来看(超过两到三天),雪的累积和强粘结力有利于增加积雪的稳定性。因此,风不是一个系统性的不稳定因素,它本身并不能解释由干雪组成的积雪的不稳定性。

  并非所有的雪崩都以雪板(snow plate)的形式发生。当雪的粘结力较弱时,偶尔也会出现雪崩:一小块雪移动并将其他雪块往下拖。随着雪崩的继续,它的宽度和体积都不断增长。这就是雪球效应(从中世纪旅行者的故事到漫画《丁丁在西藏》,这些书中都有大量将雪崩类比为一个巨大雪球的有趣描述)。

  有些雪崩在降雪很久之后才会发生。比如当积雪层因为降雨或者气温上升突然变得湿润,液态的水就出现了。由于主要集中在雪晶之间的接触处,这种液态水在表面张力的作用下,使雪具有一定的粘结力。在这个季节的早期,积雪层的夜间冷却导致了液态水的冻结。因此,冻融循环促成了雪粘结力的显著增加。低霜(甚至无霜)导致了液态水含量的增加。由于液态水过量,它成为了一个不利因素,从而在季节末形成雪的融化或春季雪崩。

  在某些情况下,液态水到达地面并润滑积雪层的界面,使某些区域积雪层的滑动速度显著增加。然后积雪层会在地面局部破裂。如果这些裂缝不能恢复力的静态平衡,就会发生一种滑动雪崩:整个积雪层都开始滑动并崩解,这就是滑坡雪崩(landslide avalanche)。

4.雪崩的蔓延

环境百科全书-土壤-流动的雪崩
图1.流动的雪崩(1995 年 2 月 25 日在萨瓦省佩塞楠克鲁瓦发生的雪崩)。
这张照片呈现了大量雪崩的特有信息,一部分雪崩冲向了兰奇的村舍,这有助于我们了解雪崩冲积的规模。这些信息反映了雪流的连续性:它不是一个落下来的紧凑雪块,而是穿过早期沉积层的一连串“波浪”。这种类型的流动让人联想到火山岩浆的流动:缓慢(几米/秒)、厚重(几米),并且非常粘稠。[图片来源:圣莫里斯堡,高山警队;PGHM  Bourg-Saint-Maurice]

  雪流的形状异常多样,一定程度上受地形影响。自然地形的坡度起着关键作用,因为重力是流动的驱动力:坡度越陡,潜在速度就越大。诸如雪的粘结程度或积雪层结构等可变因素对雪流的动态变化也有很大影响。一旦雪崩开始,它可以继续裹挟更多的雪,特别是在陡峭斜坡和积雪层抵抗力不强的情况下。雪崩还会导致环境空气变化,反过来促进雪的悬浮并形成雪云,这种云被称为气溶胶。

环境百科全书-土壤-气溶胶雪崩
图2.气溶胶雪崩(上阿尔卑斯省,Roux d’Abriès,2004 年 1 月)。
这场雪崩是一场大雪后人为触发的。冰冷、干燥的雪迅速获得了很高的速度(几十米/
秒)。它夹带大量空气,形成了几十米高的气溶胶。由于到达谷底时速度非常快,雪崩并没有蔓延,反而沿着对面的斜坡上升。尽管速度很快,但它对树木造成的损害很小,因为悬浮在空气中的雪密度很低(几公斤/立方米),因此冲击压力也很小(小于 2千帕)。在图片中看不见的地方,还有一股密度更高的雪流,沿着坡度线移动,最终在河床上消逝。[图片来源:查韦·莫里斯]

  鉴于雪流的多样性,适合考虑两种理想形态:

  • 流动雪崩,一种运动轨迹与地形非常接近的密集的雪流(见图 1)。
  • 气溶胶雪崩,一种稀释的高速移动的雪流,其倾向于沿着较陡的坡度线移动(见图 2)。

  大多数雪崩属于这两类中的一类。在某些情况下,它们同时具有这两类中的一些特征(有时称为混合雪崩)。

  流动雪崩的速度大范围一般在 5 至 25  米/秒以内,有时能达到 50  米/秒。其密度很高,在 150 至 500 公斤/立方米之间。雪流高度一般在几米左右。当它们沿着地形移动,预测它们的轨迹是很简单的。复杂的是确定他们在停止过程中的停止速率和扩散的范围。大多数流动雪崩停在大于 10°的斜坡上,但有些雪崩在平缓的斜坡(5°至 10°之间)上却能够移动很远的距离。

  气溶胶雪崩的速度很快,通常在60 至100  米/秒之间。纯气溶胶是一团悬浮态的雪,其平均密度很低(几十公斤/立方米)。 气溶胶的密度变化很大。的确,在重力作用下,流向底部的颗粒物浓度更高。由于其超快的移动速度,气溶胶能够吸收大量周围环境中的空气,这使它能够大大增加自身高度,因此通常观察到的高度在 10 到 100 米之间。气溶胶一定也会从积雪层携带积雪,来抵消融合空气后对自身的稀释。在没有限制的情况下,气溶胶持续扩张,直至自身失去动能。


The Encyclopedia of the Environment by the Association des Encyclopédies de l'Environnement et de l'Énergie (www.a3e.fr), contractually linked to the University of Grenoble Alpes and Grenoble INP, and sponsored by the French Academy of Sciences.

To cite this article: ANCEY Christophe (March 6, 2024), 雪崩, Encyclopedia of the Environment, Accessed October 4, 2024 [online ISSN 2555-0950] url : https://www.encyclopedie-environnement.org/zh/sol-zh/snow-avalanches/.

The articles in the Encyclopedia of the Environment are made available under the terms of the Creative Commons BY-NC-SA license, which authorizes reproduction subject to: citing the source, not making commercial use of them, sharing identical initial conditions, reproducing at each reuse or distribution the mention of this Creative Commons BY-NC-SA license.