Floods 洪水

李不延

世界上很多地方都可能發生不同类型和不同程度的洪涝灾害。本期，我们将了解洪水的形成、影响及如何防洪。

There are few places on Earth where flooding is not a concern. Any areawhere rain falls is 1）vulnerable to floods， though rain is not the only cause.

How floods form

A flood occurs when water inundates land thats normally dry， which canhappen in a multitude of ways.

Excessive rain， a ruptured dam or 2）levee， rapid melting of snow orice can 3）overwhelm a river， spreading over the adjacent land， called afloodplain. Coastal flooding occurs when a large storm or tsunami causes thesea to surge inland.

Most floods take hours or even days to develop， giving residents time toprepare or 4）evacuate. Others generate quickly and with little warning. Socalled“flash floods” can be extremely dangerous， instantly turning a babblingbrook or even a dry wash into rushing rapids that sweep everything in theirpath downstream.

Climate change is increasing the risk of floods worldwide， particularly incoastal and low-lying areas， because ofits role in extreme weather events andrising sea level. The increase in global temperatures can contribute to hurricanes that move more slowly and dropmore rain， funneling moisture into atmospheric rivers like the ones that led toheavy rains and flooding in California in early 2019.

Meanwhile， melting glaciers and other factors are contributing to a rise insea level. More than 670 U.S. communities will face repeated flooding by theend of this century， according to a 2017 analysis; its happening in more than90 coastal communities already.

Impacts of flooding

Floods cause more than $40 billion in damage worldwide annually，according to the Organization for Economic Co-operation and Development.In the U.S.， losses average close to $8 billion a year. Death tolls haveincreased in recent decades to more than 100 people a year.

When floodwaters recede， affected areas are often blanketed in siltand mud. The water and landscape can be contaminated with hazardousmaterials such as sharp debris， pesticides， fuel， and untreated sewage.Potentially dangerous mold blooms and quickly overwhelm water-soakedstructures.

Residents of flooded areas can be left without power and clean drinkingwater， leading to outbreaks of deadly waterborne diseases like typhoid，hepatitis A， and cholera.

Flood prevention

Flooding， particularly in river floodplain， is as natural as rain and has been occurring for millions of years. Famously fertile floodplains such as theMississippi Valley， the Nile River Valley， and the Tigris-Euphrates Valley havesupported agriculture for millennia because annual flooding has left tons ofnutrient-rich silt deposits behind.

Humans have increased the risk of death and damage by increasinglybuilding homes， businesses， and infrastructure in vulnerable floodplains.

To try to mitigate the risk， many governments mandate that residentsof flood-prone areas purchase flood insurance and set constructionrequirements aimed at making buildings more flood resistant—with varyingdegrees of success.

1） vulnerable adj. 易受傷的

2） levee n. 防洪堤

3） overwhelm v. 淹没；压倒

4） evacuate v. 撤离；（从危险的地方）撤出

在地球上，鲜有地方不把洪水当回事。虽然雨水并非唯一因素，但任何有降雨的地方都可能引发洪水。

洪水如何形成当水淹没平常干燥的土地时，就会发生洪水，这可能以多种方式发生。

雨水过多，大坝或堤坝破裂，冰雪快速融化，都会淹没一条河流，甚至蔓延到邻近的土地，即泛滥平原。当一场大风暴或海啸导致海水涌入内陆时，沿海洪水一触即发。

大多数洪水需要数小时，甚至数天才能发展起来，这样居民就有时间准备和疏散。另一些则几乎没有什么预警，刻不容缓。所谓的“暴洪”极其危险，

可能瞬间将潺潺的小溪，甚至是干流变成急流，顺流而下，横扫一切。

由于在极端天气事件和海平面上升中，气候变化“功不可没”，它正在增加世界范围内发生洪水的风险，尤其是在沿海和低洼地区。随着全球气温升高，可能导致飓风移动更慢，降雨更多，将水分汇集到大气河流中，就像2019 年初导致加利福尼亚州暴雨和洪水的大气河流一样。

与此同时，冰川融化和其他因素正在导致海平面上升。根据2017 年的一项分析，到21 世纪末，超过670 个美国社区将面临反反复复的洪水，而这种情况已经发生在90 多个沿海社区。

洪水的影响

根据经济合作与发展组织的数据，洪水每年在全球造成的损失超过400 亿美元。在美国，平均每年的损失接近80 亿美元。近几十年来，死亡人数已经增加到每年100 多人。

当洪水消退时，受灾地区往往被淤泥和泥浆覆盖。水和景观可能受到有害物质的污染，如尖锐的碎片、杀虫剂、燃料和未经处理的污水。有潜在危险的霉菌大量繁殖，会迅速吞噬被水浸透的建筑物。

洪水地区的居民可能因为没有电和干净的饮用水，导致一些致命的经饮水传播的疾病暴发，如伤寒、甲型肝炎和霍乱。

防洪

尤其是在河流泛滥平原，发洪水就像下雨一样自然，已经发生了数百万年。

著名的肥沃泛滥平原，如密西西比河谷、尼罗河谷和底格里斯- 幼发拉底河流域，几千年来一直支持着农业的发展，因为每年的洪水都会留下大量营养丰富的淤泥沉积物。

人类在脆弱的泛滥平原不断建造房屋、企业和基础设施，增加了死亡和破坏的风险。

为了试图降低风险，许多政府强制洪水易发地区的居民购买洪水保险，并制定了施工要求以提高建筑物抗洪水能力，均取得了不同程度的成功。