Swells strike Korea's east coast ahead of summer vacation season

A series of sudden, large waves, known as swells, is causing dangerous incidents, including one death, along Korea's east coast, raising critical safety concerns amid the summer holiday season. 

Eighteen coastal accidents were recorded along the Gangwon and North Gyeongsang coastlines on June 6 and 7, when long-period swells hit the region, per the Korea Coast Guard on Wednesday. In one incident, two people were swept into the sea by a swell while taking photos on a beach; one of them died.

Unlike ordinary wind-driven waves, swells are not generated by local winds, and their smooth, rounded crests make them far harder to spot than the choppy profile of a wind wave. The Korea Meteorological Administration (KMA) describes them as "a hazardous weather phenomenon in which energy from strong waves formed in distant waters travels to the shore, causing waves to surge suddenly even when there is no wind."

With a wave period typically lasting eight seconds or more, the energy a swell carries is compressed as it enters shallow water and friction with the seabed slows it down, causing wave heights to spike dramatically and sending walls of water crashing ashore without warning.

Korea's east coast is especially prone to swell activity because of its relatively straight, unbroken coastline and lack of offshore islands. Between 2012 and 2023, swells were observed on an average of 97.7 days per year on the east coast — far more than the south coast of 55.4 days or the west coast of 31.4 days.

While swells occur most frequently from autumn through spring, accidents tend to peak in summer, when more people visit the shore. A 2018 paper published in the Journal of Korean Society of Coastal and Ocean Engineers analyzing east coast swell incidents from 2013 to 2017 found that 42 percent occurred in summer, followed by autumn with 35 percent, winter with 19 percent and spring with 4 percent.

Authorities advise the public to stay away from the shoreline when swell activity is forecast and to heed evacuation warnings even when the sea appears calm. The KMA also provides live swell risk forecasts through its marine weather information portal. 

BY JANG GU-SEUL [[email protected]]