Floods 洪水

李不延

世界上很多地方都可能發(fā)生不同類型和不同程度的洪澇災(zāi)害。本期，我們將了解洪水的形成、影響及如何防洪。

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. 淹沒；壓倒

4） evacuate v. 撤離；（從危險的地方）撤出

在地球上，鮮有地方不把洪水當回事。雖然雨水并非唯一因素，但任何有降雨的地方都可能引發(fā)洪水。

洪水如何形成當水淹沒平常干燥的土地時，就會發(fā)生洪水，這可能以多種方式發(fā)生。

雨水過多，大壩或堤壩破裂，冰雪快速融化，都會淹沒一條河流，甚至蔓延到鄰近的土地，即泛濫平原。當一場大風(fēng)暴或海嘯導(dǎo)致海水涌入內(nèi)陸時，沿海洪水一觸即發(fā)。

大多數(shù)洪水需要數(shù)小時，甚至數(shù)天才能發(fā)展起來，這樣居民就有時間準備和疏散。另一些則幾乎沒有什么預(yù)警，刻不容緩。所謂的“暴洪”極其危險，

可能瞬間將潺潺的小溪，甚至是干流變成急流，順流而下，橫掃一切。

由于在極端天氣事件和海平面上升中，氣候變化“功不可沒”，它正在增加世界范圍內(nèi)發(fā)生洪水的風(fēng)險，尤其是在沿海和低洼地區(qū)。隨著全球氣溫升高，可能導(dǎo)致颶風(fēng)移動更慢，降雨更多，將水分匯集到大氣河流中，就像2019 年初導(dǎo)致加利福尼亞州暴雨和洪水的大氣河流一樣。

與此同時，冰川融化和其他因素正在導(dǎo)致海平面上升。根據(jù)2017 年的一項分析，到21 世紀末，超過670 個美國社區(qū)將面臨反反復(fù)復(fù)的洪水，而這種情況已經(jīng)發(fā)生在90 多個沿海社區(qū)。

洪水的影響

根據(jù)經(jīng)濟合作與發(fā)展組織的數(shù)據(jù)，洪水每年在全球造成的損失超過400 億美元。在美國，平均每年的損失接近80 億美元。近幾十年來，死亡人數(shù)已經(jīng)增加到每年100 多人。

當洪水消退時，受災(zāi)地區(qū)往往被淤泥和泥漿覆蓋。水和景觀可能受到有害物質(zhì)的污染，如尖銳的碎片、殺蟲劑、燃料和未經(jīng)處理的污水。有潛在危險的霉菌大量繁殖，會迅速吞噬被水浸透的建筑物。

洪水地區(qū)的居民可能因為沒有電和干凈的飲用水，導(dǎo)致一些致命的經(jīng)飲水傳播的疾病暴發(fā)，如傷寒、甲型肝炎和霍亂。

防洪

尤其是在河流泛濫平原，發(fā)洪水就像下雨一樣自然，已經(jīng)發(fā)生了數(shù)百萬年。

著名的肥沃泛濫平原，如密西西比河谷、尼羅河谷和底格里斯- 幼發(fā)拉底河流域，幾千年來一直支持著農(nóng)業(yè)的發(fā)展，因為每年的洪水都會留下大量營養(yǎng)豐富的淤泥沉積物。

人類在脆弱的泛濫平原不斷建造房屋、企業(yè)和基礎(chǔ)設(shè)施，增加了死亡和破壞的風(fēng)險。

為了試圖降低風(fēng)險，許多政府強制洪水易發(fā)地區(qū)的居民購買洪水保險，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。