The pe­cu­liar­ity of ur­ban floods

What are ur­ban floods and why are we wit­ness­ing them?

Down to Earth - - COVER STORY -

Ur­ban­i­sa­tion af­fects dis­as­ters just as pro­foundly as dis­as­ters can af­fect ur­ban­i­sa­tion,” writes Mark Pelling, ge­og­ra­pher and cli­mate change ex­pert at King’s Col­lege, London, in his book, The Vul­ner­a­bil­ity of Ci­ties.

In re­cent times, ur­ban flood­ing has emerged as a ma­jor con­cern.As the weather gets more er­ratic and one-time rain­fall, like the one re­cently wit­nessed in Srinagar, in­creases, large con­cen­trated pop­u­la­tions in ur­ban ar­eas face in­creased risk of flood.

There are some pe­cu­liar­i­ties of flood­ing in ur­ban ar­eas. Its pri­mary rea­son is sur­face wa­ter runoff. Sur­face runoff is the ex­cess wa­ter from rain or melt­ing snow that flows over the earth’s sur­face with­out get­ting ab­sorbed. In the ur­ban land­scape, it is con­trolled and man­aged ar­ti­fi­cially through drains which flush out the runoff from the city, un­like in ru­ral set­tings where the runoff is ab­sorbed nat­u­rally by farm­lands and ponds.

Ur­ban ar­eas are char­ac­terised by im­per­vi­ous sur­faces like roads, pave­ments and build­ings. High rate of de­vel­op­ment and con­struc­tion in th­ese ar­eas has re­sulted in the loss of soft land­scape. This de­creases a city’s ca­pac­ity to ab­sorb wa­ter, mak­ing it de­pen­dent solely on the out­flow of sur­face wa­ter runoff. Un­der such cir­cum­stances, even mod­er­ate rain­fall can lead to flash floods in low-ly­ing ar­eas. Ci­ties lo­cated along a river might face an added prob­lem if the river flows at a higher level within its em­bank­ment. Guwahati, a low-ly­ing city on the bank of the Brahma­pu­tra river, faced un­prece­dented flood­ing this year.

In the past few years, flood­ing in Delhi

due to over­flow of the city’s 18 ma­jor drains has be­come a common phe­nom­e­non. Heavy rain in the Ya­muna’s up­stream in­creases its wa­ter level in Delhi, due to which the drains in the city ex­pe­ri­ence re­verse flow.

As more and more farm­lands and green ar­eas are be­ing ur­banised, the amount of sur­face area for wa­ter per­co­la­tion is get­ting re­duced. As a re­sult, all the runoff flows on the land, with­out be­ing ab­sorbed. This in­creases the chance of floods.

Another rea­son for ur­ban flood­ing is the lack of drainage sys­tem in an ur­ban area. As there is lit­tle open soil to ab­sorb wa­ter, nearly all the ex­cess rain­wa­ter needs to be trans­ported to the drainage sys­tem. High­in­ten­sity rain­fall can cause floods when a city’s drainage net­work does not have the ca­pac­ity to drain away ex­cess wa­ter in ad­e­quate time.

A 2003 study by C P Konard, pub­lished in the US Ge­o­log­i­cal Survey, shows that the streams in the ur­ban ar­eas of the US rise more quickly than those in ru­ral ar­eas dur­ing storms and have higher dis­charge. Thus, ur­ban spa­ces flood more rapidly.It also shows that de­bris from bro­ken bridges and other con­struc­tion that the streams col­lect fur­ther re­strict the wa­ter’s flow from the city, in­creas­ing its level and caus­ing floods.


The risk of in­fras­truc­tural dam­age in­creases with in­creas­ing ur­ban­i­sa­tion. A 2013 re­search pa­per by U S De, for­mer ad­di­tional di­rec­tor-gen­eral of me­te­o­rol­ogy (re­search), Pune, an­a­lysed flood­ing in four megac­i­ties of the coun­try—Delhi, Chen­nai, Kolkata and Mumbai.The pa­per demon­strated how rapid and un­con­trolled ur­ban­i­sa­tion is at the root of floods and flood-re­lated da­m­ages in th­ese ci­ties. It noted that the mech­a­nism for ur­ban flood­ing is com­plex and lo­ca­tion­spe­cific. Hence, each city needs its own flood man­age­ment prac­tices.

Encroachment is another fall­out of ur­ban­i­sa­tion. The pa­per men­tions that the num­ber of wa­ter bod­ies in Delhi has been re­duced to 600 from the orig­i­nal 800 due to encroachment. It notes that the flood­plains of the Ya­muna—home to thou­sands of il­le­gal colonies—are the most pop­u­lated parts of Delhi. High pop­u­la­tion den­sity de­mands more in­fra­struc­ture, lead­ing to en­vi­ron­men­tal degra­da­tion. By 2025, the pop­u­la­tion of trop­i­cal Asia is es­ti­mated to rise to 2.4 bil­lion. Many of the most pop­u­lated ci­ties of the world—Tokyo, Mumbai, Shang­hai, Kolkata, Jakarta, Delhi, Seoul, Manila and Dhaka—are lo­cated in Asia, three of which are in In­dia.

Kolkata has been built on wet­lands. Chen­nai, too, has seen mas­sive con­struc­tion in re­cent decades, re­duc­ing soil cover and veg­e­ta­tion. While other ci­ties can ex­pand in ad­join­ing ar­eas, Mumbai can­not due to its long coast­line.The city was built by merg­ing seven is­lands and hilly ar­eas. Nearly 60 per cent of Mumbai’s pop­u­la­tion lives in poorly built tem­po­rary set­tle­ments. Only three

High­in­ten­sity rain­fall can cause floods when a city's drainage net­work loses ca­pac­ity to drain pre­cip­i­ta­tion

out­falls (dis­charge point of a waste stream into a body of wa­ter) to the sea have flood­gates.The re­main­ing 102 out­falls in the city open di­rectly into the sea. Dur­ing high tide, the sea wa­ter en­ters the drainage sys­tem through th­ese out­falls, caus­ing floods.


The year 2005 was recorded as the hottest year of the cen­tury. In­ci­den­tally, in the same year, the worst ur­ban flood­ing was re­ported in Mumbai on July 26-27. Dur­ing those two days, the city wit­nessed an un­prece­dented 944 mm of rain­fall in 24 hours. In the same year, 10 se­vere ur­ban floods were re­ported from across the coun­try. Three-fourths of Chen­nai was in­un­dated. It af­fected more than 500,000 peo­ple.

In 2006,22 ci­ties in In­dia re­ported floods. The in­creas­ing trend of ur­ban flood­ing was car­ried into 2007, where the num­ber of af­fected ci­ties rose to 35.Ex­treme weather events have in­creased in re­cent times. “Floods and droughts will be­come more fre­quent. One pro­jec­tion shows that the in­ten­sity and the num­ber of trop­i­cal cy­clones will in­crease in the next 40-100 years,” De says.

Anil K Gupta, di­rec­tor of the Wa­dia In­sti­tute of Hi­malayan Ge­ol­ogy in Dehradun, Ut­tarak­hand, adds that the en­tire Hi­malayan range is vul­ner­a­ble be­cause of ris­ing tem­per­a­tures. “Each and ev­ery val­ley—be it Kashmir, Kedar­nath or Badri­nath—faces the threat of in­creased pre­cip­i­ta­tion,” he says.

Ac­cord­ing to the Jammu and Kashmir State Ac­tion Plan on Cli­mate Change,2013, min­i­mum tem­per­a­tures in the Hi­malayan re­gion are pro­jected to rise by 1°C-4.5°C. The re­port also says that the num­ber of rainy days in the re­gion in 2030s may in­crease by five or 10.The in­ten­sity of rain­fall is likely to in­crease by 1-2 mm per day. “What is in­creas­ing is sud­den pre­cip­i­ta­tion, which hap­pened dur­ing the re­cent Kashmir floods,” Gupta says.

A re­port by the UN In­ter­gov­ern­men­tal Panel on Cli­mate Change notes that ev­ery year there will be at least one ex­treme weather event in the Hi­malayas. “Last year, it was the Ut­tarak­hand dis­as­ter. Be­fore that we saw floods in Pak­istan and cloud­bursts in Jammu and Kashmir,” says Jatin Singh, chief ex­ec­u­tive of­fi­cer of Skymet, a pri­vate weather fore­cast company.

In the course of such events, many ur­ban ar­eas are likely to be af­fected, the way Srinagar was caught un­awares. The In­dia Me­te­o­ro­log­i­cal Depart­ment (imd) is yet to recog­nise that ex­treme weather events are a re­sult of cli­mate change. For the last four ma­jor nat­u­ral dis­as­ters in In­dia—Mumbai floods of 2005, Leh cloud­burst of 2010, Ut­tarak­hand dis­as­ter of 2013 and J&K floods of 2014—imd has cited dis­tur­bances in the wind cur­rent and mon­soon as rea­sons. It used words like “un­prece­dented”, “un­usual” and “unique”, but of­fered no ex­pla­na­tion for why th­ese events are hap­pen­ing at such high fre­quency.

In light of the com­plex­ity of ur­ban floods, we need a com­pre­hen­sive plan of ac­tion to re­duce the dam­age thus caused.

The Army and Na­tional

Dis­as­ter Re­sponse Force were not fa­mil­iar with the to­pog­ra­phy of Srinagar. This de­layed the scal­ing up of res­cue


Mumbai was plunged into chaos as the city re­ceived more than 900 mm of rain­fall

in July 2005; (Right) Flood in Chen­nai the same year af­fected more than 500,000 peo­ple



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