自然风险

Encyclopédie environnement - risques naturels - natural hazards - natural disasters

  自然灾害是地球历史上不可分割的一部分。风暴、飓风、洪水、热浪、火山爆发、地震、海啸、山体滑坡、陨石坠落……都是促使地球不断演变的自然现象。自然风险这一概念被定义为自然现象所呈现的属性,其可能对人类遗产造成有害后果,这种有害后果与人类的脆弱性密切相关。在本文中,我们回顾了与自然风险相关的基本概念,并试图通过山区重力风险的案例来阐释这一主题。最后,我们将探讨目前正在开发的旨在为自然风险管理提供保障的方法和工具。

1. 自然风险意味着什么?

  自然风险的概念由来已久。在法国,根据《保险法典》第L125-1条“…被视为自然灾害的影响,直接“不可保”的物质的损害是自然因素异常强度的决定性原因,而通常采取的防止此类损害的措施未能防止其发生或尚未采取措施…”。全球范围内的自然灾害平均每年导致近1300亿美元的经济损失,影响超过2.2亿人,其中有超过92,000人丧生。

  根据前述的定义,可以假设自然风险的概念是在人类接触自然环境时出现的。如果我们缺乏对环境的了解,就会产生以不可预见、甚至是暴力的方式表现出来的过程,并严重影响人类的项目(物质或非物质方面),由此形成风险的概念。从这一定义出发,产生了两个观点:

  • 一方面,自然风险是关于人类项目的,通过这些项目,人类与自然的关系得以体现。这些反映了一种文化状况,也反映了一种意识形态姿态,不仅定义了人类在自然界中的地位,而且还定义了人类对自然的特权。
  • 另一方面,我们强调了知识所起的基本作用。由于所有知识都是不完善的,因此风险的概念是概率性的,这就证明了引入机会的合理性。

  因此,自然风险的概念随着时间和空间的演变而不断发展。它反映了人类对自然关系的感知模式,也反映了特定社会相对于自然环境的演变程度。最后,它表达了我们的智力和技术手段的优劣,以了解管理我们环境的机制以及其中发生的现象和过程。由此可见,处理自然灾害的方式可能可能并非一成不变,并且可能在过去的几个世纪甚至近几十年中已经发生了变化。

  如今,人们普遍将风险定义为强度和自然发生事件概率的乘积。事件的强度与发生移动的物质体积有关,也与其现象的动力学有关,例如物质移动的速度。

环境百科全书-自然灾害-Montroc雪崩
图1. Montroc雪崩,1999年2月。[来源:Irstea Coll档案馆]

  特定场地对于某种现象的脆弱性表示其暴露于该现象的程度,以及该现象发生时预期的损害程度。我们讨论的是人类设备、设施或基础设施的脆弱性,但通常也适用于社会实体(河谷区域、市镇、地区等)。因此,脆弱性的概念既包括物质价值,也包括社会价值。

  例如,当滑坡阻碍交通道路时,对现象的描述(堆积程度和高度)主要依赖于岩土力学,而对社会后果的评估(交通改造、直接或间接心理影响、旅游业的影响、各种经济影响等)则更多地基于人文和社会科学。

  当某一场地容易受到已识别的危害时,这种情况就称为风险。因此,风险的概念被定义为危险与脆弱性的乘积

ALEA = INTENSITE × PROBABILITE D’OCCURRENCE

RISQUE = ALEA × VULNERABILITE

  应注意的是,风险指的是一种现象。如果这种现象是自然产生的,那么我们就称之为自然风险。

2. 阿尔卑斯地区,一个自然风险的实验室?

  很明显,自然风险的潜在范围涵盖了所有区域,不管其来源是地表(大陆或海洋)还是大气。这两种来源也可以结合在一起,因为极端的气候现象会导致大陆的混乱。因此,飓风或热带风暴将对暴露的大陆产生直接破坏性后果。

  自然风险的概念在山区得以充分体现,然而那里的矿物结构却蕴含着一种虚假的平静。以阿尔卑斯山弧为例,它通常是呈现突然性和猛烈性的典型场景。让我们提及泥石流(Grand Bornand,1987年)、熔岩流(Bourg-Saint-Maurice,1981年)、雪崩(Montroc,1999年)、落石(Isola 2000年,2015年)。从更具历史意义的角度来看,我们不能忽略1248年11月格兰尼埃山发生的崩塌,这次崩塌导致了数亿立方米的物质坍塌。通过模拟被数百万立方米岩石改变的土壤表面,这一灾难性事件的痕迹仍然存在于地面上。由于自然现象的突然性和涉及的能量,其对人类环境是十分危险的。如果我们假设重力(重力的来源)是这种能量的一个基本组成部分,那么我们就能理解山区这些事件的规模。在山区,斜坡的高度会产生相当大的势能(位于相对高度为h的质量M的势能,等于M x g x h,g是重力加速度)。

  阿尔卑斯山谷的特殊之处主要在于山丘或山脉的坡度,通常是陡峭的。这一特征是第四纪冰期(吕尔密亚时期)冰川作用遗留下的,使其形成了槽形山谷,山谷中的斜坡后来被急流所侵蚀。此外,岩石斜坡的断裂状态通常较为显著,突出了阿尔卑斯山的构造遗产。

  还应注意的是,当前山谷底部的冰碛地貌是由先前存在的冰川形成的,最终变成了一个含有大量粘土的花岗岩区域。由于粘土的力学性质对含水量极为敏感,因此土壤表层粘土的存在是一个需要特别注意的因素。尤其在经历了长时间的强降雨等足以改变土壤水力负荷的情况下,可能引发重大灾害,如滑坡或泥石流。

环境百科全书-自然灾害-脆弱性和落石
图2. 脆弱性和落石,2014年3月。[来源:Irstea Coll。存档]

  我们现在熟悉的阿尔卑斯山建筑是漫长历史(包括构造和气候等方面)的产物。如果说当前的结构看起来像是凝固在表面的稳定性之中,那只是因为时间和空间这两个截然不同的尺度是对立的。在阿尔卑斯山长期的造山运动历史中,人类活动只是一个非常短暂的插曲,其规模超过了人类的日常生活和认知的范畴。因此,为了认知和分析在该地区发生的地球物理和力学过程,我们有必要在智力上做出努力来避免这种尺度上的矛盾。地表发生的事故(如崩塌、滑坡等)最终只是让两个时间尺度不时混淆的一个简短插曲。尽管这些现象对山脉地形地貌的影响很小,但在人类尺度上的报告却并不是这样的情况:从人类、社会和经济的角度来看,生命的逝去、设施和基础设施的破坏以及通讯线路的中断都是严重的后果。

3. 风险的社会文化

  20世纪初,工业技术的迅猛发展导致发达国家社会经济发生了巨大变革。一方面,为了利用自然资源(矿产资源和水电资源),工业有时候会在并不宜居(可能面临危险)的区域建立。另一方面,由于山脉形成了自然的贸易壁垒(涉及人员和货物流通),通讯网络得到了发展并变得更加密集。

  与此同时,城市社会已经在这些山区的某些地区建立起休闲和休息用地;在这方面,冬季旅游业一直是规模大且日益增加的季节性移民背后的驱动力。

  最后,随着工业时代的兴起,城市精神逐渐渗透到了山区,从而反对与自然过于脱节的态度。事实上,人与自然的关系已有数千年的历史。在像内陆山谷这样不宜发展的地区,人们开始意识到人与环境之间存在一种并不稳定的平衡状态:这是一项需要耐心的工作,而这份耐心是由人们与环境联结在一起的日复一日的劳动所获得的。互相支持的必要性和集体行动意识使每个人都对潜在危险的来源保持关切。在这方面,认真关注自然遗产的保护成为预防自然灾害的集体努力的一部分。这一使命是每个人的责任;每一个人通过他们的经验,以及他们在与灾害要素密切接触过程中的感知,为遗产的维护和保护做出了贡献。

  新思维的引入,再加上不那么传统的生活方式和经济方式,正在严重破坏关于这个问题的数据。因此,这个问题关乎变幻莫测的自然,并不会妥协于人类的计划。此外,个人对自然遗产管理的责任日益减轻。

  最后,我们可以得出以下结论:

  • 自然风险的概念被定义为自然表现(危害)的属性,其对人类遗产的后果可能是有害的(脆弱性)。
  • 自然风险管理必须被理解为一种文明事实,通过这种文明事实,在特定时间和地点由特定社会的意识形态和社会经济标准定义的自然概念规定了人类对其环境干预的规则。

4. 现代自然风险管理

  自然风险不同于技术风险,其成因(危害性)不是(先天的)与人类以及人类建筑物有关,而是与自然原因有关。相反,技术或工业风险具有人为的根源,与某一文明的设备、基础建设或生活方式有关。化工厂爆炸产生的污染物,通过陆地或空气的扩散就是一个很好的例子。

  自然灾害可能源于重力,也可能不源于重力。重力自然灾害主要包括滑坡、崩塌、雪崩和山洪(包括泥石流)。冰川成因的风险不太常见,但必须考虑冰川下的水被排空后所带来的风险(如勃朗峰地块上的特鲁斯冰川(Glacier de Tête Rousse))。

环境百科全书-自然灾害-急流熔岩
图3. 2003年7月发生在Valgaudemar山谷(Hautes-Alpes)的急流熔岩示例[资料来源:Coll.M.Bonnefoy,Irstea]

  在非重力成因的自然风险中,我们将提到地震风险、水力风险(洪水)和风暴(暴雨和极端阵风的关联)。更边缘一点的,火山风险与全球火山仍然活跃的地区有关。地面上的破坏可能相当大,如影响留尼汪岛南部的经常性熔岩流。这些风险的管理基于两种完全互补的策略:观察(或监测)和预测。通过观察,可以更好地了解与特定现象相关的机制,更好地了解发生的条件,评估重现期;此外,观测和监测网络是收集物理数据(如运动测量、加速记录等)的有效手段,可与建模工作结合使用。预测既基于观测和监测网络,通过分析实时数据(例如洪水监测网络),也基于更多的上游工作,以了解机制,并将其集成到数值模型中,以模拟中短期现象的演变。所谓的数值模拟正是指这个广阔的领域。

环境百科全书-自然灾害-稳定性的数值模拟
图4. 雅典卫城东南角稳定性的数值模拟(采用“离散单元法”)[资料来源:C.Lambert的博士论文。Coll. F. Darve, Grenoble INP]

  数值方法的出现和强大计算手段的推广使得今天对雪崩或滑坡等复杂现象的数值模拟成为可能。然而,重要的是要记住,这些计算是基于特定现象(岩石悬崖的断裂状态、积雪的构成)数据的先验知识,受到许多不确定性的影响。这完全证明了引入概率方法以在输入数据上包含一定程度的不确定性,并处理这种不确定性对输出结果的传播。该领域今天仍在蓬勃发展,是一个非常活跃且仍然非常开放的研究领域。

 


参考资料及说明

封面图片:Aussois in Savoie,块体坠落的风险和更普遍的重力危害。[来源:© François Nicot]


环境百科全书由环境和能源百科全书协会出版 (www.a3e.fr),该协会与格勒诺布尔阿尔卑斯大学和格勒诺布尔INP有合同关系,并由法国科学院赞助。

引用这篇文章: NICOT François (2024年3月6日), 自然风险, 环境百科全书,咨询于 2024年10月4日 [在线ISSN 2555-0950]网址: https://www.encyclopedie-environnement.org/zh/sol-zh/natural-hazards/.

环境百科全书中的文章是根据知识共享BY-NC-SA许可条款提供的,该许可授权复制的条件是:引用来源,不作商业使用,共享相同的初始条件,并且在每次重复使用或分发时复制知识共享BY-NC-SA许可声明。

Natural risks

Encyclopédie environnement - risques naturels - natural hazards - natural disasters

Natural hazards are an integral part of the history of our planet. Storms, cyclones, floods, heat waves, volcanic eruptions, earthquakes, tsunamis, landslides, meteorite falls,… are all natural phenomena that contribute to the Earth’s incessant evolution. The concept of natural risk is defined as the attribute of a natural manifestation (hazard), the consequences of which with human heritage can be harmful (vulnerability). In this article, we revisit the fundamental notions attached to the concept of natural risk, trying to illustrate the subject with examples relating to gravitational risks in the mountains. We will conclude by discussing the approaches and tools currently being developed to ensure the management of natural risks.

1. What does the concept of natural risk mean?

The notion of natural risk has been around for a very long time. In France, according to article L 125-1 of the Insurance Code”… Are considered as the effects of natural disasters, direct “uninsurable” material damage having had as a determining reason the abnormal intensity of a natural agent, when the usual measures to be taken to prevent such damage have not been able to prevent its occurrence or have not been taken… ». On average per year, from 2000 to 2012, natural disasters around the world cost nearly $130 billion, affecting more than 220 million people, of whom more than 92,000 died.

In line with the previous definition, it could be assumed that the notion of natural risk emerges when the human species comes into contact with the natural environment. The lack of knowledge of an environment, which generates processes that can manifest themselves in an unforeseen, sometimes violent way, and which acutely interferes with human projects (material or immaterial), leads to the idea of risk. From this first approach, two ideas emerge:

  • On the one hand, it is about man’s projects, through which his relationship with nature is manifested. These reflect a cultural situation, but also an ideological posture, defining not only humanity’s place in nature, but also its prerogatives over it.
  • On the other hand, we have highlighted the fundamental role played by the concept of knowledge. Since all knowledge is incomplete, the notion of risk is therefore of a probabilistic nature, justifying the introduction of chance.

The notion of natural risk is therefore evolving over time and space. It reflects the type of relationship that man perceives with nature, but also the degree of evolution of a given society in relation to the natural environment. Finally, it expresses the quality or weakness of our intellectual capacities, and thus of our technological means, to understand the mechanisms that govern our environment, as well as the phenomena and processes that occur within it. It is therefore understandable that the way in which natural hazards are treated has probably not always been the same, and has probably evolved over the past centuries, or even decades.

Today, it is accepted to define risk as the product of  intensity and the probability of occurrence of a naturally occurring event. The intensity of an event can be related to the volumes of materials mobilized, as well as to the dynamics of the phenomenon (e.g. material travel speeds).

The vulnerability of a given site to a phenomenon expresses its degree of exposure to it, and the degree of damage expected in the event of the phenomenon occurring. We are talking about vulnerability for human equipment, facilities or infrastructure, but also generally for a social entity (a valley, a commune, a district, etc.). The notion of vulnerability therefore includes both physical and social values.

Encyclopédie Environnement - risques naturels - Avalanche de Montroc - avalanche
Figure 1. Montroc Avalanche, February 1999. [Source: Archive Irstea Coll.]
For example, when a landslide obstructs a communication axis, the description of the phenomenon (extent and height of deposition) relies largely on the mechanics of soils and rocks, while the assessment of societal consequences (traffic modification, direct or indirect psychological impact, impact on tourism, various economic impacts, etc.) is based more on the human and social sciences.

When a site is vulnerable to an identified hazard, it is called a risk. The notion of risk is therefore defined as the product of a hazard by a vulnerability:

Encyclopédie environnement - risques naturels -

It should be noted that a risk refers to a phenomenon. If this phenomenon is of natural origin, then we will speak of a natural risk.

2. The Alpine territory, a laboratory for natural risks?

It is quite clear that natural risks potentially extend to all territories, insofar as the origin can be terrestrial (continental or oceanic) or meteoric. These two origins can also be coupled, since extreme climatic phenomena can induce disorders affecting the continents. Thus, a cyclone or tropical storm will have direct destructive consequences on the exposed continent.

The notion of natural risk takes on its full meaning in mountain regions, where mineral architecture nevertheless suggests a deceptive calm. As an illustration, we can therefore use the example of the Alpine arc, which is often the scene of sudden and violent demonstrations. Let us mention mudflows (Grand Bornand, 1987), torrential lava (Bourg-Saint-Maurice, 1981), snow avalanches (Montroc, 1999), rockfalls (Isola 2000, 2015). From a more historical perspective, we cannot omit the collapse of Mont Granier, which occurred in November 1248, mobilizing several hundred million cubic metres of material; the imprint of this catastrophic event is still present on the ground, through the modelling of the soil surface modified by millions of cubic metres of rock.

A natural manifestation is dangerous for the human environment because of its suddenness and the energy mobilized. If we assume that gravity (the source of the force of gravity) is one of the essential components of this energy, then we understand the magnitude of these events in mountainous areas, where the height of the slopes induces considerable potential energies (the potential energy of a mass M, located at a height h in relation to a reference value, is equal to the product M x g x h, g being the acceleration of gravity).

The specificity of the Alpine valleys lies in particular in the slope of the hills or mountains, which is often steep. This aspect is inherited from the glaciations of the Quaternary era (Würmien episode), shaping the trough-shaped valleys, whose slopes were later eroded by torrential action. In addition, the fractured state of the rocky slopes is often important, highlighting the tectonic heritage of the Alps.

It should also be noted that the former presence of glaciers at the bottom of the valleys is the cause of the current moraine landscape, whose ultimate state of alteration is the formation of a granitic arena containing an often significant clay fraction. The presence of clay in the surface horizon of the soil is an element to be considered with great attention, due to the extreme sensitivity of the mechanical behaviour of clays to water content. In particular, a period of heavy rain, changing the hydraulic load in the soil, can cause major disorders such as landslides or mudslides.

Encyclopédie environnement - risques naturels - éboulement rocheux - rockfall
Figure 2. Vulnerability and rockfall, March 2014. [Source: Irstea Coll. Archive]
The alpine building we are now familiar with is the result of a long history (tectonic, climatic). If the current structure seems frozen in apparent stability, it is only because two very different scales, time and space, are opposed. Man lives only a very brief episode in the long history of the orogenesis of the Alps, whose dimensions surpass those of his daily life and appreciation. It is therefore necessary to make the intellectual effort to avoid this contrast of scale, in order to perceive and analyze the geophysical and mechanical processes that take place there. The accidents that occur on the surface (block falls, slips,…) are ultimately only brief episodes that allow the two time scales to be confused from time to time. However, if the consequences of such manifestations remain very insignificant on the general morphology of a mountain range, this is not the case when they are reported on a human scale: loss of human life, destruction of facilities and infrastructures, disruption of communication routes, are all dramatic consequences from a human, social and economic point of view.

3. Towards a social culture of risk

At the beginning of the 20th century, the industrial explosion led to a socio-economic upheaval in developed countries. On the one hand, industries have established themselves in sectors that are sometimes inhospitable (potentially exposed to the occurrence of hazards), in order to take advantage of natural resources (mining resources, and hydroelectric resources). On the other hand, as mountain ranges constitute natural barriers to trade (flows of people and goods), communication routes have developed and become more dense.

At the same time, urban society has taken over some areas of these mountain regions for leisure and rest; in this respect, winter tourism has been the driving force behind massive and increasing seasonal migration.

Finally, the urban spirit that accompanies the industrial era penetrates into mountain regions, thus opposing very disjointed postures towards nature. Indeed, man’s relationship with nature has been the completion of several millennia of history. In regions as hostile as some internal mountain valleys, people are becoming aware of the precarious balance that unites people to their environment: a work of patience, imposed by a daily labour by which people unite to their environment. The necessary solidarity and sense of collective action led to everyone being concerned about the sources of potential danger. In this context, careful attention to the maintenance of the natural heritage was part of the collective approach to protecting against natural hazards. This task was everyone’s business; everyone, through their experience, their sense of observation in close connection with the elements, contributed to the maintenance and preservation of the heritage.

The introduction of new thinking, combined with a less traditional way of life and economy, is significantly disrupting the data on the problem. It is therefore a question of containing a capricious nature, so that it does not compromise man’s plans. In addition, individuals are increasingly relieved of their responsibility for the management of natural heritage.

To conclude, we can therefore conclude that:

  • The concept of natural risk is defined as the attribute of a natural manifestation (hazard), the consequences of which on human heritage can be harmful (vulnerability).
  • Natural risk management must be understood as a civilizational fact through which the conceptualization of nature, defined at a given time and place by ideological and socio-economic criteria specific to a given society, dictates the rules for human intervention in its environment.

4. Modern natural risk management

Natural risks differ from technological risks in that the cause (hazard) is not (a priori) linked to man, to what he has built, but to a natural cause. On the contrary, technological or industrial risks have an anthropogenic origin, linked to an installation, an infrastructure, or a way of life of a given civilization. The explosion of a chemical plant with diffusion of pollutants by land or air is a very illustrative example.

Natural hazards may or may not be of gravity origin. Gravity natural hazards mainly include landslides (boulder falls, landslides, landslides), snow avalanches, and torrential floods (including mudflows). Risks of glacial origin are less frequent, although the risk of emptying subglacial water pockets must be considered (example of the Glacier de Tête Rousse, in the Mont-Blanc massif).

Encyclopedie environnement - risques naturels - modelisation-numerique - torrential lava
Figure 3. Example of torrential lava, occurring in July 2003, in the Valgaudemar valley (Hautes-Alpes). [Source: Coll. M. Bonnefoy, Irstea]
Among the natural risks of nongravity origin, we will mention the seismic risk, hydraulic risks (floods and floods), and storms (associations of heavy rainfall and extreme wind gusts). More marginally, the volcanic risk concerns areas of the globe where volcanoes are still active. Damage on the ground can be considerable (e. g. recurrent lava flows affecting the southern part of Reunion Island).

The management of these risks is based on two perfectly complementary strategies: observation or monitoring and forecasting. Observation makes it possible to better understand the mechanisms associated with a given phenomenon, to better understand the conditions of occurrence, to assess the return periods; in addition, observation and monitoring networks are effective means of collecting physical data (movement measurements, accelerated recordings, etc.) which can then be used in conjunction with modelling work. The forecast is based both on the observation and monitoring network, by analysing real-time data (e.g. flood monitoring network), and on more upstream work to understand the mechanisms and integrate them into a numerical model to simulate the evolution of phenomena in the medium and short term. It is this vast field that is referred to as numerical modelling.

Encyclopedie environnement - risques naturels - modelisation numerique - numerical modeliing
Figure 4. Numerical modelling (by the “Discrete Element Method”) of the stability of the southeast corner of the Acropolis of Athens. [Source: PhD thesis of C. Lambert, coll. F. Darve, Grenoble INP]
The advent of numerical methods and the generalization of powerful computing means make possible today the numerical simulation of complex phenomena such as avalanches or landslides. However, it is important to keep in mind that these calculations are based on a priori knowledge of data specific to the phenomenon (fractured state of a rocky escarpment, constitution of the snowpack) which is subject to many uncertainties. This fully justifies the introduction of probabilistic methods to incorporate a level of uncertainty on the input data, and to treat the propagation of this uncertainty to the output results. This field is still booming today, and is a very active and still very open field of research.

 


References and notes

Cover photo. Aussois in Savoie, risks of block falls and more generally gravitational hazards. [Source: © François Nicot]


环境百科全书由环境和能源百科全书协会出版 (www.a3e.fr),该协会与格勒诺布尔阿尔卑斯大学和格勒诺布尔INP有合同关系,并由法国科学院赞助。

引用这篇文章: NICOT François (2019年2月7日), Natural risks, 环境百科全书,咨询于 2024年10月4日 [在线ISSN 2555-0950]网址: https://www.encyclopedie-environnement.org/en/soil/natural-hazards/.

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