20 October 2016
Map of a stormwater tunnel from Causeway Bay to Pok Fu Lam. The facility ensures the safety of Hong Kong's key business and financial districts during typhoon and rainy season. Photo: HK Govt
Map of a stormwater tunnel from Causeway Bay to Pok Fu Lam. The facility ensures the safety of Hong Kong's key business and financial districts during typhoon and rainy season. Photo: HK Govt

How Hong Kong stays safe during typhoons and rainstorms

As Hong Kong had the No. 8 typhoon signal this week, office goers in the city got at least half a day off from work.

Many people were no doubt glad that the so-called “Li’s Force Field” didn’t play out and deprive them of a special holiday.

Tycoon Li Ka-shing may have chosen not to use his magic, or may be his powers proved insufficient, to repel Typhoon Nida, people rejoiced in light-hearted banter in online forums.

Well, the comments were in reference to an imaginary, tongue-in-cheek conspiracy theory related to Li.

According to the theory, “Superman” Li can be counted on to deploy a “force field” — a giant, invisible perimeter — to repel tropical cyclones so that businesses and stock markets won’’t be affected, to keep the city’s wheels of commerce running.

As things turned out, that didn’t happen this week.

The Observatory hoisted the No. 8 storm signal, this year’s first, at 8:40 Monday evening and didn’t pull it down until 1 pm the next day, enabling most people to stay curled up in bed and skip work.

Coming back to the real world, we should thank our stars that we live in Hong Kong.

Unlike many other parts of Asia, where natural disasters often wreak havoc on people’s lives, Hong Kong is able to see through typhoons and hurricanes with minimal damage.  

The city’s world-class infrastructure and well-oiled emergency response means that residents end up suffering only small inconveniences such as flight suspensions, cancelled events, fallen trees, etc.

We must thank the Drainage Services Department and its engineers and maintenance staff, who guarantee our peace of mind even if a major typhoon hits the territory.

Let’s also not forget that Hong Kong typically gets no less than 2,400 millimeters of rainfall per year, one of the highest among cities in the Pacific Rim, but the city now rarely sees any flooding.

Tunnels and reservoirs

“The sewer is the conscience of the city,” Victor Hugo once said.

In 2012, Hong Kong inaugurated two massive drainage tunnel systems, both built at a cost of several billion dollars.

You may not know that a 3.7-kilometer drainage tunnel in western Kowloon has been up and running since four years ago, sheltering over 500 hectares of built-up areas spanning from Lai Chi Kok, Cheung Sha Wan to Sham Shui Po with over 300,000 residents.

Flow from the rural upper catchment will be intercepted by and discharged into the Victoria Harbor via the tunnel system as a flood relief measure.

The main tunnel is below an urban reclamation area, passing through MTR’s Tsuen Wan Line, West Rail Line and the Express Rail Link. During excavation work, the front of the tunnel boring machine was pressurized to prevent underground water and soil from moving and technicians had to work under 4.2 times atmospheric pressure 45 meters below the ground.

Also, a HK$3.4 billion, 11-kilometer tunnel running from Causeway Bay to Cyberport in Pok Fu Lam with 34 intakes along the way was finished in the same year.

Part of the Stormwater Drainage Master Plan Study in Northern Hong Kong Island, the tunnel is the longest of its kind in Hong Kong, almost as long as the MTR’s Tseung Kwan O Line. With a diameter of 6-7 meters, it intercepts stormwater from Mid-Levels and discharges it directly into the sea.

Yet tunnel systems alone aren’t adequate to safeguard the city, as in some dire circumstances the maximum flow of stormwater, the surface runoff, may still overwhelm their designed capacity. So Hong Kong has thought of building reservoirs, underground.

Many horseracing fans may be unaware that the Happy Valley Racecourse has been partially excavated underneath.

Construction of a HK$1.07 billion, 60,000 cubic meter reservoir equaling the combined size of 24 standard swimming pools is moving into top gear, beneath the city’s 170-year-old racecourse.  

The entire project is scheduled for commission before the rainy season of 2018, making the area’s drainage systems able to withstand a rainstorm with an intensity of a 50-year return period.

The rationale for such a giant underground reservoir is that Happy Valley and parts of Tai Hang, one of Hong Kong’s most affluent residential neighborhoods, are particularly prone to severe flooding as the area adjoins mountainous terrain.

Residents still have vivid memories of a huge flood that submerged almost the entire district, as well as the racecourse, in June 2008 when Hong Kong was hit by one of the worst black rainstorms in history.

The Drainage Services Department figured that the racecourse, being the lowest spot in the area, could be an ideal site for a flood reservoir, given that expansion of the existing drainage systems is constrained by narrow roads and congested underground utilities.

The tank will store stormwater from upstream catchment during heavy rainstorms to reduce the peak flow through the downstream drainage systems.

There is also another such reservoir, called the Tai Hang Tung Storage Scheme, beneath the bustling Mong Kok.

The number of flooding blackspots territory-wide has now been brought down to just eight, and all the ones categorized as under severe threat have been tackled since 2010.

It’s a remarkable feat that is worth trumpeting.

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A bird’s eye view of the construction site of a 60,000 cubic meter reservoir that will lie beneath the Happy Valley racecourse. Photo: HK Govt

A boring machine that is used for construction of stormwater tunnels in Hong Kong. Photo: HK Govt

The main tunnel of the 12-kilometer drainage system from Causeway Bay to Cyberport. Photo: ARUP

A main intake of the Lai Chi Kok stormwater tunnel. Photo: HK Govt

An inside look of the Tai Hang Tung stormwater storage tank. Photo: HK Govt

EJ Insight writer

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