Storm Surge

"Pause and consider it: a wave crashing up the Shatin Valley."

It feels as though we are experiencing a typhoon nearly every two weeks at the moment. All eyes are on the approaching Super Typhoon Ragasa, which is expected to arrive on Tuesday. It is packing winds comparable to Typhoon Mangkhut in 2018, and warnings of a storm surge are already emerging. 

Hong Kong is no stranger to the destruction caused by typhoons. On 1st September 1937, an unnamed typhoon moved westward across Hong Kong Island. Wind speeds quickly reached 125 mph. The storm was small, intense, and agile. The Observatory anemometer broke, having exceeded its designed tolerance. Later assessments estimated wind speeds of up to 149 mph. 

Victoria Harbour, crowded with ships, bore the brunt of the initial impact. Vessels were pulled from their berths and driven ashore. Captains surrendered the fight to let their boats run aground. Around 600 ships are reported damaged, with hundreds of vessels sunk. 

Simultaneously, a storm surge caused water to crash through Wan Chai, Causeway Bay, and Central. Basements were flooded, as ground-floor premises were overwhelmed. Food stored in waterlogged warehouses is now unusable, as streets have turned into rivers. Only the steep terrain prevented further damage. 

As the storm moved westward, the back-end part was expected to cause much greater destruction. Around 1 am, the eye of the storm crossed over to the Mainland, bringing with it fierce winds and heavy rain. Unfortunately, the peak of the gale coincided with a high tide. The swollen waters of Mirs Bay pushed westward with significant force, sweeping through Starling Inlet, Tolo Harbour, and Tide Cove. 

Constrained by the terrain in the Tolo Channel, the surge gained momentum. It stored energy. As it struck the shoreline, that energy was released. A wave the height of two double-decker buses swept inland without mercy. The residents of Shatin and Tai Po had no warning. 

The area was considerably less populated than it is today. The Shatin Valley, a narrow floodplain, was home to a farming community. Villages and hamlets were grouped together. The Kowloon-Canton Railway follows the route it uses today.

​There are no tall buildings, no concrete channels, as the Shing Mun River winds its way to the sea. There was no racecourse, while Ma On Shan was a small community.

As the destructive wave charged inland, it destroyed everything in its path.

All along the coast from Ma On Shan to Tai Po Market, roads, bridges, and homes vanished. The railway embankment was swept away, and then the railway tracks collapsed. At Tai Po Market, 60 homes and their occupants disappeared. The flood rolled towards Tai Po Kau, over half a mile inland. It carried battered humans, cattle, pigs, dogs, ducks, and debris with it.

Estimates of the number killed range from 10,000 to 12,000. However, this was not the first such surge; similar events happened in 1874, 1906, and 1923. 

The public blamed the Observatory. Using rudimentary science, the Observatory's warnings proved insufficient. Over time, the importance of storm surge forecasting became clear. By 1975, it was vital. The growth of new residential and industrial areas on reclaimed land heightened the risks. New towns meant more people were in danger.

​Predicted Track of Super Typhoon Ragasa by Windy.com

In the 1970s, the Observatory computerised the study of surges: tide monitors, satellites, and other technology supplied data to the system. Using computer modelling leads to better predictions. The government then set up a dedicated Storm Surge Unit in January 1978.  Since 1994, the Observatory has used the SLOSH model to predict surges with afair degree of success. 

Nonetheless, predicting the surge is one thing. It doesn’t resolve the question of what you should tell people to do. What are the escape routes? Where should people shelter, and who is coordinating?

Furthermore, modern developments have reduced Tolo Harbour's capacity to absorb a tidal surge. In fact, we have increased the risk. The Plover Cove Dam, built in 1968, decreased available space. The engineers understood this, although the link to a potential tidal surge did not register with policymakers. 

Since the 1980s, development in Shatin, Ma On Shan, and Tai Po has encroached on Tolo Harbour. Reclamation and construction have reduced the area's capacity to dissipate a tidal wave. The Science Park is built on reclaimed land, as are sections of the Tolo Highway. The small wetland at Tai Po Kau, hemmed in by concrete, cannot help absorb the impact. 

With the Shing Mun River fixed by embankments, it will act to funnel any surge inland. Research indicates that a tide surge of nearly four metres is possible with the right combination of circumstances. Amplified by the limited space, the consequences are catastrophic.

Pause and consider this fictious scenario based on modeling by HKO scientists. Imagine such a wave crashing up the Shatin valley. The first to be hit is the Marine Police Base at Ma Liu Shui. Next, it would smash through the Shatin Sewage Works, grabbing everything to carry it inland. A wave of toxic sewage now surges inland.

​The Shatin Racecourse track is submerged under water. The Tate’s Cairn Highway disappears as the surge pushes through the ground floor of the Shatin Hospital.  

Shoreline properties at Siu Lek Yuen are submerged under water. Underground car parks fill with foul, stinking sludge. The surge extends to Shek Mun and presses forward. Shatin City One residents get off lightly—the raised estate is protected by its podium levels. Across the river, Wo Che and Lek Yuen Estates are less fortunate, as ground-floor access is flooded. Next come New Town Plaza and Sha Kok Estate. 

Finally, the wave barrels into the narrow streets of Tai Wai. Che Kung Temple is waterlogged. Meanwhile, power is failing as sub-stations shut down. People trapped in lifts are unreachable by the Fire Services. With roads flooded, access is impossible. The sheer number of calls overwhelms the mobile telephone system, causing it to shut down.   

Shatin Police Station comes to a halt due to a flooded compound. Ma On Shan Police Station is situated at a higher level and escapes the worst. Tin Sum Police Station is similarly protected by its elevation, although it is operating on emergency power. 

Meanwhile, similar events are happening in Tai Po. The low-lying industrial estate is flooded. The gas plant automatically shuts down as fail-safe systems activate. Parts of Tai Po Town Centre are submerged. The East Rail and Ma On Shan lines are halted. 

Tragically, residents in Providence Bay, next to the Science Park, pay a terrible price. Their waterfront homes bore the worst of the surge. The ground floor flats are flooded with water and debris. 

Across Shatin and Tai Po, people have died. Many more were injured. The retreating waters leave behind a mass of untreated sewage, human remains, and sludge. The risk of disease is high. Tolo Harbour, filled with floating cars, carcasses, and assorted debris, drains towards the sea. 

I’m sure that these days, early warnings would alert us to a surge, hopefully preventing much death and destruction. For this to happen, a well-resourced, tested, and properly executed plan must be in place. While just under one million people live in the Shatin, Ma On Shan, and Tai Po areas, not all are at risk.

​Many developments are on high ground and are therefore unlikely to be directly affected, although they may experience power outages, transportation disruptions, and water supply issues. 

Certainly, it is possible to avoid this. Constructing a surge barrier across the Tolo Channel would safeguard the area. At its narrowest point, the channel is approximately 1.5 km wide. A movable flood barrier similar to the Thames Barrier should be technically feasible and might become necessary as sea levels continue to rise. 

Meanwhile, the government requires a strong evacuation plan. Rapidly withdrawing citizens from flood-prone low-lying areas is essential.  

There is no place for complacency. It has occurred before; it will occur again.