When discussing rising sea levels, it is essential to clarify the concept of Chart Datum (CD). This is an extremely low tidal reference point, set below the average lowest tide, with sea levels typically above this benchmark. In Hong Kong, during non-storm conditions, the astronomical high tide at Chai Wan reaches approximately 2.75 metres; once the water level hits 3 metres, coastal areas begin to flood, and at 4 metres, significant damage occurs. However, most areas along Victoria Harbour are designed to a height of only about 4 metres, placing them perilously close to the risk threshold.
Historically, two major records—Typhoon Wanda in 1962 and Typhoon Mangkhut in 2018—pushed the water levels in Victoria Harbour close to 4 metres, enough to paralyse large sections of the city. Alarmingly, on both occasions, Hong Kong narrowly avoided the worst-case scenario: the surge did not coincide with the astronomical high tide. For instance, Mangkhut’s surge of approximately 2.35 metres, when added to the 2.75 metres of high tide, could have resulted in a water level of about 5.1 metres, far exceeding current defenses. Moreover, neither Wanda nor Mangkhut were the most intense super typhoons; should a stronger storm directly strike or pass within tens of kilometres south of Hong Kong during high tide, water levels would rise even higher. Such strokes of luck are unlikely to repeat.
Looking further back, a storm in 1874 pushed the water level in Victoria Harbour close to 5 metres, clearly demonstrating Hong Kong’s natural limits. Sea levels are gradually rising, and high-water events are becoming increasingly frequent; under a scenario of a 1.4-metre rise, extreme water levels of about 4.7 metres could occur once every ten years. For a city that begins flooding at 3 metres and suffers severe damage at 4 metres, this indicates that extreme events are shifting from rare occurrences to periodic threats.
In light of this outlook, Hong Kong must ultimately decide how to construct its defenses. The most straightforward approach would be to raise coastal seawalls to an average height of 6 metres, but this would come at an exorbitant cost and transform the waterfront into concrete barriers, damaging the landscape and public spaces. Another option is to build three large seawalls around the periphery of Victoria Harbour—an eastern dam connecting from Fat Tong Au to Cape Collinson, a southern dam from Mo Sing Leng to Discovery Bay, and a western dam from the airport to Castle Peak—effectively enclosing Victoria Harbour as a semi-closed inland sea. Although such port dams are extremely costly, they can simultaneously withstand storm surges and tsunamis, providing the most comprehensive hard defense. As for shipping channels, large openings could be reserved in the dams for the passage of large vessels, or container and cruise terminals could be relocated outside Victoria Harbour to create a complete defensive line.
However, before discussing any proposals, authorities should assess the casualties and economic losses that would result from storm surges or tsunamis elevating water levels to 6 metres. This would help the public understand which areas of the city would be lost in the worst-case scenario, how much infrastructure would be incapacitated, and what the financial costs would be. Without this baseline awareness, the public cannot engage in a meaningful discussion about what constitutes reasonable flood prevention standards.
Finally, climate change remains the fundamental challenge. While Hong Kong must prepare for extreme scenarios, it is even more crucial to reduce emissions and mitigate global sea-level rise. To withstand the impact of the next powerful typhoon, the city must undertake two simultaneous actions: prepare in advance and address the crisis at its source. We have already witnessed nature’s limits, and whether the city can endure the next impact depends on whether we are willing to take that step today.
