▲ Swell waves
Swell waves are like predators on the savanna, crouching low as they approach before swiftly snatching their prey.
This is because they remain invisible until they reach the shore, suddenly turning into massive waves that crash down.
As major beaches across the country open sequentially to welcome vacationers, damage caused by swell waves is already occurring this summer.
According to the East Sea Regional Coast Guard Headquarters and other authorities today (June 24), 18 coastal accidents occurred along the East Coast of Gangwon and Gyeongbuk provinces alone on June 6–7, when swell waves continuously rolled in.
Among these incidents, two people were swept into the sea by swell waves while taking photos on the beach, resulting in one death.
Swells refer to "waves that are not directly generated by the wind at a specific location."
They include "wind waves generated within low-pressure systems or typhoon zones that travel to other sea areas" and "waves remaining on the sea surface after the wind has died down."
The Korea Meteorological Administration (KMA) explains swells as "a hazardous weather phenomenon where the energy of strong wind waves generated in the open sea travels to the coast, causing sudden waves that can lead to damage even when there is no wind."
Swells have a long wave period of 8 seconds or more.
This is because short-period components disappear and only long-period components remain as they propagate from the open sea to the coast, which is the primary reason why swells are so dangerous.
When long-period swells enter shallow waters, friction with the seabed reduces their wave speed and wavelength.
Since the wave period does not decrease, the wavelength shortens, compressing the energy within the swell and causing the wave height to rise.
Collision with the underwater topography is another reason why wave heights increase in shallow waters.
Unlike wind waves, swells have rounded crests and gentle slopes, making them difficult to see. However, as they reach shallow waters and their wave heights increase, people on the shore are suddenly confronted with waves as tall as houses.
The fact that swells are generated by low-pressure systems or typhoons that have moved far out to sea also adds to the danger.
This is because swells often strike when vigilance is lowered as the sea calms down after the strong winds and rough waves from low-pressure systems and typhoons have subsided.
While swells can occur in any sea, in South Korea, they primarily happen along the East Sea.
Over the 11 years leading up to 2023, swell waves were observed on the East Coast for an annual average of 97.7 days, overwhelmingly higher than the South Coast (55.4 days) and the West Coast (31.4 days).
This is because the East Sea has no islands and possesses a simple coastline.
When considering the seasons, swells frequently roll into the East Coast from autumn to spring.
During winter, a "high-in-the-west, low-in-the-east" pressure pattern forms around the Korean Peninsula. At this time, low-pressure systems developing in inland China or near Korea pass over the peninsula to the far open waters of the East Sea. They then gain strength over the relatively warm sea, blowing northeasterly winds toward Korea, which often generates swells.
However, winter is not the season when most "swell-related accidents" occur.
The period when accidents occur most frequently is summer, when more people visit the coast.
According to a paper published in the 2018 journal of the Korean Society of Coastal and Ocean Engineers ("Analysis of Swell Accident Characteristics on the East Coast and Establishment of Countermeasures"), an analysis of swell accidents on the East Coast from 2013 to 2017 showed that accidents in summer were the most frequent at 42%, followed by autumn (35%), winter (19%), and spring (4%).
If swells are expected to roll in, people should avoid going to the coast as much as possible.
In particular, one must not ignore instructions to leave the beach just because high waves are not immediately visible.
Life jackets must be worn when fishing on seashore rocks or breakwaters. When swells roll in, high waves can crash over these rocks or breakwaters, so it is better to avoid such activities altogether.
Since October last year, the KMA has been providing swell risk prediction information by dividing all sea areas around the Korean Peninsula into 2,877 small sea zones.
Swell risk prediction information can be found on the KMA Marine Meteorological Information Portal (https://marine.kma.go.kr).
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