Bracing for worse to come: city's typhoon defences need to evolve, experts say

Rising sea levels and weather extremes make it vital city's typhoon defences evolve, experts say

PUBLISHED : Thursday, 14 August, 2014, 3:24am
UPDATED : Thursday, 14 August, 2014, 4:56am

During typhoons in Hong Kong, winds slam into skyscrapers with such force that buildings sway. Tidal surges turn Victoria Harbour into a cauldron of foaming waves. The roads of Central and Mong Kok become rivers of water. And residents lock themselves inside, listening to the echo of rain hammer on their outdoor air-conditioning units like furious pounding on typewriters.

Hong Kong has a protective system of defences to weather tropical cyclones. These defences continue to evolve, which is vitally important because experts predict the city will see even fiercer storms in the coming decades that will test engineering skills.

Rising sea levels and increasingly extreme weather present a challenge that could threaten the city's standing as an Asian financial centre and tourist hub.

The average temperature in Hong Kong is rising about 0.12 degrees Celsius every decade, with a mean 4.8 degrees rise predicted by 2100, according to the Hong Kong Observatory.

The blame, the observatory says, rests with growing levels of greenhouse gas emissions as well as the phenomenon of what's called the urban heat island - areas that are significantly warmer than their rural neighbours. Contributing factors are the city's crowded streets choked in excess motor vehicle emissions, poor ventilation between buildings and scarce vegetation.

The city's warming has triggered increased evaporation from the ocean and consequently more water in the atmosphere, Observatory forecasters say.

"If you have more vapour in the atmosphere, then under the right conditions it will come down as rain … The amount of rain will increase, and there will be flooding. People need to be aware of that," says Professor Johnny C L Chan, dean of the school of energy and environment at City University.

Climate change is likely to heighten the ferocity of future typhoons, says Hong Kong Observatory director Shun Chi-ming. This will mean higher wind speeds and heavier rainfall during typhoons. Observatory scientists predict average annual rainfall in Hong Kong will increase 11 per cent by century's end.

Warming has caused a rise in Hong Kong's sea levels averaging 2.6mm per year from 1954 to 2010 - a trend likely to continue at a rate of 2.4 to 2.7mm a year, according to the observatory.

"The three deadliest typhoons on record, namely those in 1906, 1937 and 1962, shared a common feature - an abnormal rise in sea level," says Shun.

"And the global sea level is expected to rise in future decades, which, in combination with storm surges from tropical cyclones, will increase the chance of flooding in coastal regions like Hong Kong."

If the city doesn't prepare, it risks a repeat of Hong Kong's 1962 disaster, some warn. We could get another Typhoon Wanda, Chan, who is a professor of atmospheric science, says.

Then, thousands of people were left homeless after a tidal wave rushed through Sha Tin and Tai Po, crumpling huts and sending fishing boats spiralling through the air.

But compared with the storms that hit the city more than 100 years ago, Wanda's 434 deaths were a blip.

The typhoon of 1874 was a bigger monster, claiming 2,000 lives. Many who died were fishermen who lived on their boats. In the storm, their crafts were tossed like twigs.

The Hong Kong Observatory opened in 1883. Before creating the numerical signal system, the observatory used cannon fire and explosives to warn the public of a storm's imminent arrival.

Over the next two decades, many of the seafarers who had once lived along the waterfront began trading their boat homes for dwellings on land. But their low-lying homes were vulnerable to flooding, and a 1906 typhoon smashed into these structures, killing about 16,000 - 5 per cent of Hong Kong's population at the time.

To combat the water's power, architects resorted to a simple tactic - setting buildings on stilts. Thus began the city's obsession with building skyward, and most structures constructed during the 1950s, '60s and '70s were elevated on wood or steel pilings. While building high offered protection from waves and water, deadly winds posed more challenges. To ensure the safety of the city's ever-higher skyscrapers, the Hong Kong Buildings Department started in the 1960s to track the wind-bearing capacity of all buildings.

The Code of Practice on Wind Effects in Hong Kong, first published by the Buildings Department in 1983, details how much wind pressure a structure can withstand before it will sway and shear. To the layman, the code is impenetrable, but it is required reading for engineers designing Hong Kong high-rises, which by the 1990s were reaching more than 200 metres into the sky.

Dr Alex To, a wind engineering specialist from Arup, the London-based firm that built the International Finance Centre and International Commerce Centre towers, says Hong Kong's skyscrapers may sway from the force of a typhoon, but are sturdy and safe. "The IFC and ICC are designed to sway in typhoon winds similar to other super high-rise buildings around the world. They are perfectly safe [because] the vibrations have been limited in their design to an acceptable level that will not cause a nuisance to the occupants," he says.

Stiff reinforced concrete cores that accommodate the stairs, lifts and service shafts of the IFC and ICC help to minimise vibrations, To says. In addition, Arup engineers designed connections called outriggers between the core and the large external columns of both structures, maximising the stiffness of the buildings and keeping them upright in the face of gale-force winds.

"The function of these outriggers is similar to a skier using his arms and shoulders to hold onto the ski poles, providing a better and more efficient stability system," To says.

Engineers working on the city's expansive bridges must also consider stability. When Arup constructed the 1.6 kilometre, cable-stayed Stonecutters Bridge from 2004 to 2009, workers erected a mast to gauge wind speed, providing data that was later used to determine exactly which parts of the bridge needed extra reinforcement.

"A bridge is just like a tall tower lying horizontally," To says. "A bridge's cross section is very thin when compared to that of a building, and it can vibrate in the wind just like an aerofoil."

To keep heavy rains from fraying the bridge's cables, engineers introduced what To calls a "dimpled surface texture" to the normally smooth supports.

"These indentations, which are much like the surface of a golf ball, create friction, disturbing the flow of the water and therefore limiting the vibrations on the bridge's cables," To explains.

The result is a durable bridge that suffers a minimal amount of sway in typhoon winds.

Assuming Hong Kong's skyscrapers and bridges are relatively safe, what about its streets?

Storm surges, the abnormal rise in ocean level generated by a storm far beyond the swell from tides, are the most immediate typhoon-induced threat to low-lying Hong Kong. Observatory director Shun says they "can cause sudden and serious flooding in low-lying areas close to the seashore", potentially triggering landslides.

Typhoon Wanda in 1962 triggered a storm surge that in mere minutes reached five metres high and flowed towards Tai Po, "leaving the whole town awash under 10 feet of water rushing in from the sea and carrying with it everything in its path", the South China Morning Post reported at the time.

During the development of the new towns in the late 1960s and 1970s, flood-prone areas of Tai Po were filled in three metres above sea level to prevent future tragedies. As other districts in the New Territories developed, the Hong Kong Drainage Services Department instituted a HK$10 billion flood prevention programme. Twenty-four flood pumping facilities were constructed along 55km of river, protecting hundreds of hectares of low-lying land, including Lo Wu and Sha Tin in the northern New Territories and Yuen Long in the northwest.

On Hong Kong Island and Kowloon, the Geotechnical Engineering Office examined 60,000 man-made slopes and retaining walls, reinforcing and conducting safety tests on nearly 10,000 that were found to be vulnerable to cyclones.

"In the old days a lot of the damage from rain was through landslides," says City University's Chan. "But then the government spent so much money protecting the slopes in Hong Kong that the number of landslides has dramatically decreased in the last 20 years. Even if you have a very strong storm or heavy rain, the number of landslides is quite small."

The next step was to wring excess water from the city. In 1996, the Drainage Services Department began working to build larger floodwater tunnels, costing about HK$8 billion.

The tunnels intercept water flow, known as surface run-off, from upland areas and discharge it into the sea, keeping urban areas from flooding, says Jennifer Loo, a Drainage Services Department spokeswoman.

The Hong Kong West drainage tunnel cost more than HK$3 billion. The longest drainage tunnel in the city, it stretches 11 kilometres from Tai Hang, runs under Mid-Levels and discharges in Cyberport, protecting Sheung Wan, Central, Admiralty, Wan Chai and Causeway Bay.

The Lai Chi Kok drainage tunnel and Kai Tak transfer scheme handle the other side of the city, intercepting and redistributing run-off from east to west Kowloon. To prevent overflowing, the tunnels connect to diversion chambers that direct water into "storage schemes", gigantic underground reservoirs built to temporarily store rainfall.

The Tai Hang Tung storage scheme, completed in 2004, was built to hold 100,000 cubic metres of water. It can redistribute more than 100mm of rainfall an hour - far surpassing the 70mm hourly rainfall classified by the observatory as a "black rainstorm".

Building Kowloon's storage schemes and tunnels cost the government an estimated HK$4 billion. But thanks to them, the heavy floods in Kowloon Tong that submerged the crowded streets of Mong Kok, particularly in 1997 and 1998, were greatly lessened.

While many scientists say Hong Kong is prepared for most of what nature may hurl at it, some experts have raised concerns in professional circles that the city is vulnerable and must anticipate the hurdles of a rising sea level and increased rainfall.

A 2010 assessment of the territory's weaknesses by the Environmental Protection Department notes that Hong Kong "is vulnerable to climate change because of the agglomeration of people and assets in a small area. Hong Kong's vulnerability is compounded by its dependence on imported food, water, energy and other products that are required for it to thrive".

But the city has a great ability to adapt, the report notes. Its typhoon defences must evolve, experts say, if the city is to avoid becoming a wind-lashed, storm-smashed, water-washed wreck.



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