홍콩의 38억불 짜리 우수배수터널 프로젝트 VIDEO: Hong Kong's vast $3.8 billion rain-tunnel network

Hong Kong's vast $3.8 billion rain-tunnel network

The city's complex drainage system protects Hong Kong from once-disastrous flooding caused by seasonal typhoons. But will it be strong enough to withstand the effects of climate crisis?

CNN Digital Expansion 2017. James Griffiths

Written by: James Griffiths, CNN

Hong KongPublished 26th July 2020

Below the mountains that overlook Hong Kong's financial district, drainage engineer Alex Lau is ankle-deep in water flowing into a giant tunnel almost twice the height of a double-decker bus.

Part of a ground-breaking, $3.8 billion (30 billion Hong Kong dollar) drainage network, this tunnel runs nearly the length of Hong Kong Island and has saved the city from floods that decades ago routinely cost lives and caused widespread destruction.

홍콩의 38억불 짜리 우수배수터널 프로젝트    홍콩의 금융지구가 내려다보이는 빅토리아산 아래 배수 기술자 알렉스 라우(Alex Lau)가 2층 버스의 거의 두 배 높이인 거대한 터널로 흘러들어가는 물줄기에 발목까지 차 있다. 38억 달러의 획기적인 배수망의 일부인 이 터널은 홍콩섬의 거의 전 길이에 걸쳐 있으며, 수십 년 전 끊임없이 인명이 희생되고 광범위한 파괴를 초래했던 홍수로부터 도시를 구했다. "이 터널은 홍콩 북부 지역의 강우량의 약 3분의 1을 소화하고 있다"고 그는 터널 서쪽을 바라보고 있다. "약 34개의 취수구가 있으며, 차단된 물은 모두 터널로 우회하여 바다로 운반될 것이다." 2007년부터 5년 동안 두 대의 거대한 터널 보링 머신을 파낸 이 터널은 맨해튼의 절반 길이인 10.5km(6.5마일)의 터널로 아시아에서 가장 습한 도시 중 하나인 비에 직면해 있는 이 거대도시의 시급한 문제를 해결했다. 배수 서비스 부(DSD)에 따르면 홍콩은 매년 약 2,400mm의 비가 쏟아진다. 그중 약 80%는 단 몇 달 안에 내린다. 하지만 세계에서 가장 밀도가 높은 도시들 중 하나인 이 도시의 배수터널 네트워크를 구축하는 것은 쉽지 않았다. 홍콩 섬은 이미 터널로 교차되어 도시의 지하철 시스템을 운반하고 산악 지형을 가로지르는 도로를 수용하고 있었다. 결과적으로, 홍콩 서부의 배수 터널은 도시 뒤쪽의 야산을 통과하여 수면 아래 십여 미터도 안 되는 곳에 위치한다. 하지만, 홍콩 서부의 배수 터널은 장마철마다 이 터널이 그곳에 있다는 것을 알고 있는 주민은 거의 없다. 기술자들은 물이 스며들어 그 일부가 무너지는 것을 막기 위해 터널 위쪽에 불안정한 땅을 파야 했다. 도심 밀집지역에서 주배수로 통하는 배관과 출입구 등 터널 취수구를 건설하는 데 어려움을 해결하기 위해 터널을 위로 파낸 뒤 지하망 밖으로 떨어진 자갈과 잔해를 채취해 가로수위에 너무 큰 지장을 주지 않기로 했다. 잦은 홍수 진정한 홍콩의 여름은 그 도시에 대한 모든 주민들의 사랑을 시험하는 것이 될 수 있다. 숨막힐 정도로 덥고 습한 6월부터 9월까지, 높은 온도는 태풍과 폭풍우에 맞서고, 이 기간 동안 도시는 우산을 쓸모가 없게 만드는 집중 호우를 견뎌내야 한다. 홍콩은 기상관측소가 발행해 수백만 명의 주민 휴대폰에 전송하는 황색에서 흑색까지 폭풍에 대한 색상 코드 경고 시스템을 갖추고 있다. 1967년 도입된 제도에 따라, 이 경보들은 도로 사고나 심지어 도시의 일부 산악 지역에 내린 비로 인한 산사태의 위험 때문에 불필요한 여행을 피해야 할 때를 경고한다. 회사와 학교는 시간당 70 밀리미터(2.7 인치) 이상 비가 오는 경우 문을 닫는다. 거대한 지하 터널 덕분에 미미한 물난리를 겪기도 하지만, 그것은 과거에 비하면 아무것도 아니다. 황기철 콘페이퍼 에디터 Ki Chul Hwang Conpaper editor curator

edited by kcontents

"This tunnel intercepts about one third of the rainfall for the northern Hong Kong area," Lau says, as he looks west to the point where the tunnel fades to black. "We have about 34 intakes and all the intercepted water will be diverted into the tunnel and carried all the way to the sea."

Dug with two vast tunnel-boring machines over five years, beginning in 2007, the 10.5-kilometer-long (6.5 mile) tunnel, about half the length of Manhattan, has solved a basic but pressing problem facing one of Asia's wettest cities: rain. Hong Kong receives about 2,400 millimeters it per year, according to the Drainage Services Department (DSD). And about 80% of that water falls in just a few months.

But building this network in one of the world's densest cities wasn't easy.

An entrance to the tunnel, located near the neighborhood of Tai Hang. Credit: Joshua Berlinger/CNN

Hong Kong Island was already criss-crossed by tunnels, carrying the city's subway system and accommodating roads that slice through the mountainous landscape.

Consequently, the Hong Kong West Drainage Tunnel runs through the hills behind the city, tucked less than a dozen meters below the surface -- though few of the city's residents, whose lives it protects each rainy season, ever realize it is there.

Engineers had to shore up insecure ground above the tunnel to prevent water seeping through and causing parts of it to collapse. To tackle the difficulty of building tunnel intakes -- pipes and entrances leading to the main drain -- in dense urban areas, the decided to dig upwards from the tunnels and then extract gravel and debris that fell down out of the subterranean network to avoid causing too much disruption at street level.

Frequent flooding

A true Hong Kong summer can be a test of any resident's love for the city. Stiflingly hot and humid, from June to September, high temperatures are only broken by typhoons and rainstorms, during which the city endures torrential downpours that render umbrellas useless.

On June 7, 2008, more than 145 millimeters (5.7 inches) poured down on the city in 24 hours. That's more than what Seattle, Washington receives most months. Photos from the storm show people trudging through knee deep water, and at least two people were killed in landslides caused by the torrential downpour. Credit: MIKE CLARKE/AFP/AFP via Getty Images

Hong Kong has a color-coded warning system for rainstorms, from amber to black, issued by the Observatory and sent to millions of residents' mobile phones. Building on a system introduced in 1967, the alerts warn people when to avoid unnecessary travel due to the danger of road accidents, or even landslides caused by rain in some mountainous parts of the city.

Offices and schools shut during black rain, which is defined as when more than 70 millimeters (2.7 inches) per hour. While black rain causes minor flooding -- mainly due to the impermeability of tarmac and concrete -- thanks to the giant underground tunnels, it is nothing compared to the past.

Water pours into the Happy Valley Rainwater Storage tank during a heavy storm in 2016. Credit: HVUSSS

The deadliest typhoon hit the city in September 1906, killing an estimated 15,000 people, according to Hong Kong Observatory, 5% of the city's then population of 320,000. In the 1960s and 70s, Hong Kong suffered hundreds of fatalities due to tropical cyclones. Thousands also lost their homes due to flooding, and the city had to spend millions on water damage repairs every year.

Heavy urbanization only made flooding worse, as large areas of natural ground were paved over, swapping soluble grass and mud for concrete and tarmac, which rather than absorb rain, allowed water to accumulate in puddles or, worse, cause flood. Older areas of the city were also compromised, with drainage systems built for a far smaller population and to poor standards.

Since 1995, the department has spent around $3.8 billion on various projects. That has included installing 2,400 kilometers (1,500 miles) of drains, 360 kilometers (224 miles) of river channels, four vast underground tunnels spanning 21 kilometers (13 miles), and four stormwater storage tanks. Another 11 tanks are being constructed.

The Hong Kong West Drainage Tunnel is the largest of four, and runs through the mountains above Tin Hau, in the eastern part of the island, past the busy financial districts of Admiralty and Central, to Cyberport on the western coast. At its widest, the tunnel is 7.25 meters in diameter.

It slopes gradually downward so the water is carried out by gravity, though pumps can also be engaged for heavy flow. Even on a dry day, water streams down a nearby intake from high above in the mountains. The intake looks like an artificial waterfall made of concrete steps designed to slow down the flow. Far from smelling like a drain, the water heading into the tunnel has little aroma, and looks a river.

One of the tunnel's intakes, full of rainwater. The water progresses from the intake into the tunnel, instead of inundating the city below. The entrance to the tunnel lies near the middle of the photo, under the life preserver ring. Credit: HK Drainage Services Department

Other solutions in Hong Kong are tailored to the district. So, rural villages and towns can be protected with dykes and channels dug to redirect or temporarily store the water, while heavily built-up, urban areas require more intensive engineering work.

These solutions, which direct rainwater away from vulnerable areas and eventually return it to the ocean, have helped to reduce the number of flooding blackspots from 126 in 1995, to just five today.

Richard Leung, the DSD's senior engineer, won't be drawn on whether all this has been value for money, but for context, flooding in southern China led to economic losses of more than $2.9 billion (20 billion RMB) in June alone.

Racecourse storage

One of the largest storage tanks in Hong Kong lies beneath one of its best-known landmarks: the Happy Valley Racecourse, where, before coronavirus, tens of thousands of gamblers bet on weekly horse races. Hundreds more use the grounds as an outdoor running track, football pitch, and rugby field during the rest of the week.

"You would never know this was here from the outside," Lau says inside the tank. The concrete floor is streaked with mud and dotted with small puddles. Small stones, silt and leaves are partially lit by fluorescent lights that do little to illuminate the darkness.

The chamber looks like a vast, empty underground car park. At its fullest, it can hold up to 60,000 square meters of water, or the equivalent to 24 standard swimming pools.

View full text

https://edition.cnn.com/style/article/hong-kong-tunnels-climate-crisis-intl-hnk-dst/index.html

Hong Kong's vast $3.8 billion rain-tunnel network

 kcontents