The hidden danger of swells, an underestimated killer due to tropical cyclones

8 January 2019

One should still recall the widespread damages in Hong Kong brought by Mangkhut on 16 September 2018. It was fortunate that there was no fatality in Hong Kong on that day. According to the Observatory's records, there was no fatality for the past three "Signal No. 10" storms including Mangkhut. However, a recent unfortunate incident happened on 31 October 2018 when Severe Tropical Storm Yutu was located about 600 km southeast of Hong Kong and the Standby Signal No. 1 was in force. A young man was tragically drowned in the rough seas while surfing in Shek O that afternoon. According to press reports, 29 people were killed or missing when tropical cyclone warning signals were in force since 2000. About 40% of these incidents were associated with water activities. Among them, eight were engaging in swimming, surfing or diving; three were swept away while fishing; another one, an off-duty fireman, tragically lost his life while attempting to save a swimmer in rough seas.

Actually, the threats of tropical cyclone are not limited to the associated inclement weather (e.g. high winds, squally heavy rain, storm surge, etc.) when it comes close. A tropical cyclone can also inflict casualties when it is located at hundreds of kilometres away from us. Huge waves are usually whipped up by high winds associated with tropical cyclones. These waves could travel much further away from the storm and they are known as swells. Affected by the outer subsiding air of a distant tropical cyclone, local weather may be deceptively fine with light winds. In such situations, the awareness of the public on the effect of the weather would usually be lowered. As swells travel at speeds much faster than the movement of tropical cyclone, when the storm is still at a distance, the severe swells generated by it could have already reached Hong Kong. Even worse is that the wave heights would increase suddenly when these swells enter shallow water of the shore and attack the shoreline with a surprise, posing hazards to people close to the shoreline or engaging in water sports activities. Therefore, when tropical cyclone warning is in force, we should be on the alert and pay attention to the announcement and swell information provided by the Hong Kong Observatory. Members of the public should also stay away from the shoreline and avoid engaging in water sports activities.

Figure 1      Cases of death or missing during tropical cyclone passages since 2000

Figure 2      Swells can pose hazards to people close to the shoreline or engaging in water sports activities

HUI Kin-chung and CHEUNG Sze-yuen


References:
B. Y. Lee: Caught red-handed
https://www.weather.gov.hk/blog/en/archives/00000033.htm
CHAN Chik-cheung: Swells from distant typhoons
https://www.weather.gov.hk/education/article_e.htm?title=ele_00171
Hong Kong Observatory video: Tropical cyclone swells
This link will open in a new windowhttps://youtu.be/8NJUHduPnhE