Brac­ing up for the big

As cli­mate change con­tin­ues to un­leash in­creas­ing amounts of rain on the city, its en­gi­neers bat­tle to keep one of the world’s best un­der­ground drainage sys­tems up to the job, writes Sylvia Chang.

China Daily (Hong Kong) - - FOCUS - PRO­VIDED TO CHINA DAILY

Mer­cury rising

With up to 3,000 mil­lime­ters (mm) of an­nual rain­fall, Hong Kong is one of the sog­gi­est cities in the Pa­cific Rim. The rains come in May, with the fogs and mists suf­fus­ing the ce­ramic tiles and win­dow frames of the city’s shoe­box homes, soak­ing the densely packed city with over 300 mm of rain for ev­ery one of the next five months.

With the rains come floods and land­slips, as the tor­rents cas­cade down the city’s steep moun­tain slopes which ac­count for 60 per­cent of the city’s nat­u­ral ter­rain.

There are ty­phoons and tor­ren­tial rains 10 or more times a year — from the Am­ber rain­fall warn­ing, with down­pours ex­ceed­ing 30 mm in an hour, up to the dan­ger zone of the Red and Black warn­ings which bring tor­rents of more than 70 mm in an hour and when only the fool­hardy ven­ture forth.

Peo­ple seem un­de­terred by the im­mi­nent dan­ger of floods that may fol­low when rain­fall seems to reach the point of mad­ness. Some ac­tu­ally wel­come the news, in recog­ni­tion of the fact that the warn­ings of im­mi­nent dan­ger in­crease the chance of an un­earned day off.

Sci­en­tists say it will get worse, with trop­i­cal del­uges be­com­ing more fre- quent and of greater in­ten­sity. The ex­perts want peo­ple to ed­u­cate them­selves about the dan­gers. Mean­while, the en­gi­neers bat­tle to keep up, adding im­prove­ments to what is ac­knowl­edged to be one of the world’s best un­der­ground drainage sys­tems. They know for sure that the city will re­main un­der im­mi­nent threat.

Con­crete roots of the city

At those times when the city seems be­lea­guered by the del­uge, a city un­der the ground is re­spond­ing at full ca­pac­ity. The city’s flood con­trol sys­tem, se­nior en­gi­neer Richard Le­ung Wah-ming ex­plained, func­tions on three ma­jor sys­tems, to en­sure that the city can stand up to the kind of sav­age rain­storm that’s likely to come once in 200 years. Le­ung is with the Land Drainage Di­vi­sion of the Drainage Ser­vices Depart­ment (DSD).

At the up­per lev­els of the city, con­crete build­ings, roads and pave­ments are heav­ily packed onto the moun­tain slopes. Stretch­ing along the slopes there are four rain tun­nels, five to seven me­ters high — large enough to run a dou­ble decker bus through. Tor­ren­tial rain on the up­land sur­face is di­verted away from low-ly­ing ar­eas down­stream. The flood­wa­ters are col­lected and put to good use in the city’s wa­ter sys­tem or are al­lowed to drain back into the sea.

At the mid-lev­els, where more of the pop­u­la­tion is con­cen­trated, three un­der­ground storm-wa­ter stor­age tanks are de­signed to open their gap­ing mouths to swal­low the rain­wa­ter. Two have been com­pleted and are op­er­a­tional. The largest, at Tai Hang Tung in West Kowloon, was ex­ca­vated to a depth of over 10 me­ters. It’s ca­pa­ble of hold­ing 100,000 cu­bic me­ters of stormwa­ter, al­most equiv­a­lent to 40 stan­dard swim­ming pools.

The un­der­ground stor­age tank be­neath the Happy Val­ley race­course is still un­der con­struc­tion. Once com­pleted in 2018, it will have a ca­pac­ity of 60,000 cu­bic me­ters. That’s enough to sop up the drainage from the en­tire 130 hectares of Wan Chai, Cause­way Bay and Happy Val­ley, even if it’s the kind of nasty down­pour that hap­pens only once in 50 years.

Le­ung of the DSD ex­plained the stor­age tanks con­tain wa­ter-level sen­sors, cul­verts and tide gauges which op­er­ate au­to­mat­i­cally ac­cord­ing to the real-time data of wa­ter flow and tidal lev­els. When the rain­fall in­ten­sity is low, the weirs of the tank are kept closed and stormwa­ter flows along the cul­vert. When it rains heav­ily, the weirs lower au­to­mat­i­cally to al­low stormwa­ter to over­flow into the tank for tem­po­rary stor­age. Af­ter the flood has passed, the weirs will be low­ered fur­ther to al­low the stored stormwa­ter to flow back, while the re­main­ing stormwa­ter will be pumped out.

At the lower lev­els of the city and in ru­ral ar­eas, drainage chan­nels are en­larged and dredge sew­ers are straight­ened, stretch­ing out like a web to­tal­ing 360 kilo­me­ters in length — equiv­a­lent to the air travel dis­tance from Wash­ing­ton, D.C. to New York.

The un­der­ground world is like a con­crete root sys­tem which main­tains the health and longevity of the city above ground. The roots of the sys­tem — sew­ers and storm-wa­ter drains — which run about 1,700 kilo­me­ters and 2,300 kilo­me­ters re­spec­tively, spread through­out the city and off­set most of the flood haz­ards. All ma­jor flood “black spots” have been neu­tral­ized, and the num­ber of dan­ger points, pos­ing dif­fer­ent de­grees of dan­ger, has been re­duced from 90 in 1995 to eight this year, ac­cord­ing to sta­tis­tics from the DSD.

The depart­ment also de­ploys standby con­tin­gency teams dur­ing se­vere weather. “They dash out to mon­i­tor, in­spect and record lo­ca­tions where flood­ing is re­ported. They dis­cuss the sit­u­a­tion and move for­ward to tackle prob­lems,” Le­ung said with ev­i­dent pride.

Ac­cel­er­at­ing cli­mate change

Even with the world-class stan­dards, Le­ung said the flood con­trol sys­tems still face se­vere chal­lenges in the face of the ever-chang­ing global cli­mate. Schol­ars, af­ter all, say we face even more ex­treme weather at much higher lev­els of in­ten­sity as cli­mate change pro­gresses.

As sta­tis­tics from the Hong Kong Ob­ser­va­tory show, rain­fall records are con­stantly bro­ken. In the pe­riod be­tween the 1880s and the late 1970s, the max­i­mum hourly pre­cip­i­ta­tion records at one time, would stand for 40 years on av­er­age. This was re­duced to less than 20 years be­tween the late 1970s and the 1990s. The records be­gan to be bro­ken ev­ery 10 years, or less, be­tween the 1990s and 2014.

“This means that cli­mate change is re­ally ac­cel­er­at­ing,” Le­ung em­pha­sized, urg­ing peo­ple of all sec­tors to face it squarely.

Ac­cord­ing to the Hong Kong cli­mate change re­port re­leased in Novem­ber 2015, the num­ber of ex­tremely wet years is ex­pected to in­crease from three be­tween 1885 and 2005 to about 12 be­tween 2006 and 2100. The es­ti­ma­tion draws on the sce­nario out­lined in the lat­est re­port by the In­ter­gov­ern­men­tal Panel on Cli­mate Change (IPCC) un­der the United Na­tions.

Le­ung said the DSD is us­ing the com­bined data from the IPCC and Hong Kong Ob­ser­va­tory to re­view Hong Kong’s flood pre­ven­tion sys­tem with re­gard to cli­mate change.

“The ca­pac­i­ties of the cur­rent rain­storm tanks and chan­nels may not be able to hold heavy rain­storms in the fu­ture. To­gether with the chang­ing land use and new ur­ban de­sign, we need to up­date the cur­rent sys­tems,” Le­ung said.

Block­age of land­slides and de­bris

On Oct 19, a Black rain­storm warn­ing was is­sued, the first ever dur­ing the month of Oc­to­ber since the warn­ing sys­tem was put in place in 1998.

Hong Kong Ob­ser­va­tory said the hourly rain­fall ex­ceeded 100 mm, far above the thresh­old of 70 mm for the Black sig­nal. The to­tal rain­fall on that day reached 223 mm, in a city with an an­nual rain­fall be­tween 1,400 and 3,000 mm, depend­ing on the district.

The down­pour struck sud­denly and in only a few ar­eas. The wind whipped, and the sea churned at the Vic­to­ria Har­bour front. Trees were up­rooted at Kennedy Town. There were floods in 14 ar­eas, in­clud­ing three which se­verely dis­rupted traf­fic and caused prop­erty losses to nearby res­i­dents.

Chen Ji, as­so­ci­ate pro­fes­sor at the Depart­ment of Civil En­gi­neer­ing at the Univer­sity of Hong Kong (HKU), ob­served the wa­ter of floods was muddy and clay-col­ored. This is his­tory re­peat­ing, he said, six years on from June 2008 when heavy rain­fall struck and caused floods with soil-laden wa­ters in sev­eral lo­ca­tions.

In­side the Hak­ing Wong open podium at HKU in June 2008, Chen re­called that he no­ticed the sump pits un­der­ground. Based on his many years of ex­pe­ri­ence, Chen un­der­stood a land­slide must have oc­curred nearby, cre­at­ing a block­age of the drainage sys­tem.

Judg­ing from th­ese two in­ci­dents and sev­eral oth­ers of less in­ten­sity, Chen said, “The most, most sig­nif­i­cant thing to con­sider for the fu­ture of flood pre­ven­tion in Hong Kong, es­pe­cially in ur­ban ar­eas, un­der the threats of cli­mate change, is how to pre­vent floods caused by land­slides and de­bris which lead to block­age of drainage sys­tems.”

The pre­lim­i­nary find­ings by the DSD con­firmed Chen’s judg­ments. Re­spond­ing to the Leg­isla­tive Coun­cil on Nov 23, Sec­re­tary for De­vel­op­ment Paul Chan Mo-po said, “A con­sid­er­able amount of tree branches and leaves were torn down by strong winds and ac­cu­mu­lated on the slopes up­stream. They were washed down sub­se­quently into the catch basins and storm-wa­ter drainage sys­tems down­stream by the se­vere rain­storm on Oct 19, re­sult­ing in block­age of the drainage sys­tems, and hence un­der­min­ing their over­all flood­reliev­ing ca­pac­i­ties.”

Fur­ther in­spec­tions and prac­ti­cal so­lu­tions on fu­ture po­ten­tial in­ci­dents are still un­der re­view af­ter two months, ac­cord­ing to Le­ung of the DSD.

“We need de­tailed data on ex­actly how much rain­fall was recorded at ex­actly what time, and we need to test if sim­i­lar cir­cum­stances could hap­pen again,” he said.

Liv­ing with floods

The cur­rent stan­dards of flood pre­ven­tion for the plan­ning and de­sign of storm-wa­ter drainage sys­tems are based on anal­y­sis of his­tor­i­cal data and thus, as sci­en­tist their lim­i­ta­tions on the fu con­trol strat­egy.

“Due to the un­cer­tainty change, or even the nat­u­ral fre­quency and in­ten­sity of hard to pre­dict. It is high that a once-in-200 year rains be­come a once-in-100 or onc event,” Chen of HKU said.

Usu­ally, rain­storms strike in sum­mer, be­tween May a ber. This year, how­ever, with ty­phoons in Oc­to­ber, it raine the nor­mally dry month.

This ab­nor­mal weath ex­plained, was due to a str rence of La Niña this year spheric phe­nom­e­non whic more trop­i­cal cy­clones an above-av­er­age rain­fall to Ho

It is pos­si­ble this phe­nom be­come more fre­quent

2000 2014 1990 2010

Com­pos­ite im­age show­ing the lo­ca­tion of the Tai Hang Tung un­der­ground flood stor­age tank.

Floods caused by heavy rain­fall struck in dif­fer­ent time pe­ri­ods at dif­fer­ent lo­ca­tions. (Left to right: Wan Chai in 1960s-1980s, She­ung Wan on June 24, 2005 and June 7, 2005, and Chai Wan on Oct 19, 2016)

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