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Typhoon Mangkhut

Super Typhoon Mangkhut: engineer warns massive storm could put structural safety of Hong Kong-Zhuhai-Macau mega bridge to test

Whether bridge can withstand destructive force of storm depends on height of waves at artificial islands, structural engineer Ngai Hok-yan says

PUBLISHED : Thursday, 13 September, 2018, 10:11pm
UPDATED : Friday, 14 September, 2018, 8:33am

The structural safety of the multibillion-dollar Hong Kong-Zhuhai-Macau bridge could be put to the test by a super typhoon predicted to be the most powerful in Hong Kong’s history, an engineering expert has warned.

That came as Super Typhoon Mangkhut edged ever closer to Hong Kong, prompting residents to take precautionary measures to safeguard their properties.

Predicted to be the most powerful storm since records began, with maximum sustained winds of up to 205km/h (127mph), the tropical cyclone is forecast to pass within 100km (62mph) of the city on Sunday morning.

Concerns had previously been raised over the structural safety of the city’s mega projects especially the bridge linking Hong Kong to Zhuhai and Macau as the main stretch was situated in the Pearl River Estuary’s Lingdingyang waters and prone to strong winds.

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The 55km (34 mile) bridge, which is expected to open this year, reportedly has a design lifespan of 120 years and is built to resist wind speeds of up to 201km/h (125mph).

In April, interlocking concrete blocks placed around the edges of an artificial island – which connects the Hong Kong bridge section to a tunnel in mainland China waters – sparked safety fears as they appeared to have drifted.

But the bridge authority later clarified the blocks, known as dolosse and which serve as protection against breaking waves, were specially designed to be submerged in a “random manner” to avoid exerting too much pressure on the undersea tunnel.

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It also stressed the location withstood the devastating Typhoon Hato last year. But Mangkhut is deemed to be stronger than Hato, which packed wind speeds of up to 185km/h (114mph).

Structural and geotechnical engineer Ngai Hok-yan said the artificial islands in mainland waters and at the Macau side would face a greater risk from Mangkhut than the one at Hong Kong’s Chek Lap Kok which was partly protected by Lantau Island.

“Whether the bridge can withstand the destructive force of Super Typhoon Mangkhut depends on the height of the waves at the artificial island,” he said.

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The expert said the dolosse could resist waves of up to four metres according to the mainland design.

“If the waves are higher than four metres, then there is a chance these protective blocks will be washed away, and very quickly. Depending on the wind direction, the waves could be as high as six metres according to American forecasts,” he said.

“Without the protection of the dolosse, the worst case scenario would see the undersea tunnel detach from the artificial island and float above the sea, and also the collapse of the island.”

Ngai added the Macau artificial island was subject to a greater risk because it faced the sea.

Hong Kong has so far committed to contributing about HK$10.7 billion (US$1.38 billion), or 43 per cent, of the main bridge’s construction cost. However, the Hong Kong government will also end up spending another HK$110 billion to build the city’s connection to the main bridge.

In a circular released on Wednesday, the Buildings Department called for property management agents to take precautionary measures to avoid potential destruction caused by Mangkhut, including inspecting roofs, basements, drains, scaffolding, slopes and retaining walls, and providing pumping equipment in areas of flooding risk and proper protection materials for windows.

“The associated heavy rain, squalls, and storm surge may pose a threat to Hong Kong, particularly the coastal and low-lying areas … From past experience, many public and building safety incidents could have been avoided if proper precautionary measures had been taken,” it said.

As for the Tsing Ma Bridge and Ting Kau Bridge leading to Hong Kong International Airport, they will be subject to traffic management in phases according to the storm’s wind speed.

When the mean speed on the Ting Kau Bridge exceeds 65km/h (40mph), it will be completely closed. The Tsing Ma Bridge will be completely closed if the wind speed exceeds 165km/h (102mph).