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Saturday, 28th September 2013

A narrow escape from repeating history

One of the main objectives for the "Hong Kong Observatory - Under the Same Sky 130 Years" exhibition held at the Hong Kong Museum of History this summer is to look back on the historical extreme weather phenomena in Hong Kong since its establishment in 1841, especially severe damages caused by storm surges[1] induced by typhoons (Table 1 and Figure 1). Among them, the typhoon that hit the Pearl River estuary during 22 to 23 September 1874 might be one of the reasons for establishing the Hong Kong Observatory.

One hundred and thirty-nine years later on the same date came the menace of Severe Typhoon Usagi. Even when Usagi was far away over the western Pacific, the Observatory had already expected that Usagi would move westwards through the Luzon Strait and approach the coast of Guangdong. From past records, typhoons following such track could pose severe threat to Hong Kong, especially that the storm surge brought by Usagi would probably cause flooding in low-lying areas as it came at the astronomical high tide.

Dr C.M. Cheng pointed out in his blog "An uninvited guest during Mid-Autumn Festival holiday"[2] that the impact of Usagi to Hong Kong would be quite different depending on its passage on the north side or the south side of Hong Kong. As to storm surge, if Usagi passes to the south of Hong Kong, prevailing southeasterly winds will push on the sea surface and pile up sea water over the coast. Coupled with the low pressure centre of Usagi, serious storm surge can be resulted. On the contrary, if Usagi passes to the north of Hong Kong, prevailing northwesterly winds will push offshore water back to the sea, thus counteracts the effect of the low pressure, resulting in less significant storm surge. Furthermore, whether the moving speed of Usagi would act in concert with the astronomical high tide to make an aggregate effect for a higher water level on top of the astronomical high tide also brings uncertainty in predicting the storm surge effect.

Despite the uncertainty and the advance in infrastructure of Hong Kong nowadays as compared with one or two centuries ago, Usagi did pose a severe threat to Hong Kong. In 2008, the passage of Severe Typhoon Hagupit about 180 km south-southwest of Hong Kong[3] brought severe flooding and damage at Tai O and Cheung Chau (Figures 2 and 3). The Observatory was duty bound to provide an early alert to the public and the relevant government departments to take precautionary measures as early as possible against the possible high winds and flooding for assurance of public safety.

Fortunately, Usagi passed about 80 kilometres to the north of Hong Kong and brought storm surge of only 0.5 to 1 metre to us. As the time of the occurrence of maximum storm surge was quite near to the time of astronomical high tide (Figure 4), there were some minor flooding in low-lying areas. Huge sea waves bombarding the shore at Kennedy Town and Tsim Sha Tsui were also observed.

What would happen then if Usagi took on a track of about 100 kilometres[4] southward when it was near Hong Kong (red line in Figure 5) with the time of occurrence of storm surge matching with that of the astronomical high tide? Computer storm surge simulated results (Figure 6) showed that storm surge of about 1.7 metres would occur at Quarry Bay[5] and added up with the astronomical high tide (2.2 metres), resulting in a sea level of nearly 4 metres, rather close to that caused by Wanda in 1962. If Usagi took on an even more southward track (green line in Figure 5), the simulated storm surge would be even larger. Severe storm surge was so close - we just escaped from repeating history! We have been really lucky - Hong Kong is indeed a blessed place!

H Y Mok

[1] Observatory's Blog: What is a storm surge?
[2] Observatory's Blog: An uninvited guest during mid-autumn festival holiday An uninvited guest during mid-autumn festival holiday
[4] The average 24-hour forecast track error of major tropical cyclone warning centres is about 100-150 km
[5] According to press reports( Chinese only), storm surge of up to 2.07 metres was recorded along the eastern coast of Guangdong due to Usagi

Table 1.      Records of major historical storm surges in Hong Kong

Table 1

Figure 1

Figure 1.      Tracks of the typhoons affecting Hong Kong in 1874(1), 1906, 1937 and Typhoon Wanda in 1962.
((1) track estimated by historical weather observation data and numerical model)

Figure 2

Figure 2.      Storm surge induced by Severe Typhoon Hagupit on 24 September 2008 raised
the water level at Cheung Chau to 3.5 metres, damaging houses along the shore.

Figure 3

(Source: TVB)
Figure 3.      Storm surge induced by Severe Typhoon Hagupit on 24 September 2008 caused severe flooding at Tai O.

Figure 4

Figure 4.      Storm surge and sea level (above Chart Datum) recorded by the tide gauge
at Quarry Bay during the passage of Usagi.

Figure 5

Figure 5.      The actual track of Usagi (in black) and the simulated tracks if Usagi moved about 100 kilometres (in red)
or 200 kilometres (in green) southward when it was near Hong Kong.

Figure 6

Figure 6.      Computer simulated storm surge and sea level (above Chart Datum) at the tide gauge at Quarry Bay
if Usagi took on a track of about 100 kilometres southward when it was near Hong Kong.

Last revision date: <11 Dec 2013>