Typhoon Haiyan in Hernani: Nickson Gensis’s Footage and the Surf Beat Wave

On the morning of 8 November 2013, Plan International aid worker Nickson Gensis filmed a wall of churning seawater tearing through Hernani, a fishing and farming town on the Pacific coast of Eastern Samar in the Philippines. His short clip of a house being swept away became one of the defining images of Typhoon Haiyan, and two years later it helped scientists explain why the water hit Hernani like a tsunami.

Typhoon Haiyan (Yolanda): one of the strongest storms on record

Haiyan, known in the Philippines as Yolanda, formed in the western Pacific on 3 November 2013. It made its first Philippine landfall at Guiuan, at the southern tip of Eastern Samar, at about 4:40 a.m. local time on 8 November, at peak strength. The US Joint Typhoon Warning Center estimated one-minute sustained winds of 315 km/h (195 mph), and the central pressure fell to 895 hPa.

The storm went on to cross Leyte, Cebu, Iloilo and Palawan before a final landfall in northern Vietnam. It killed about 6,300 people in the Philippines, with 5,902 deaths in Eastern Visayas alone. More than six million people were displaced and around 1.8 million were left homeless. Most of the deaths were caused by storm surge and waves. In Tacloban, a 5.2 m surge reached the second storey of the airport terminal.

The Hernani video

Gensis was sheltering with a small group on an upper floor of a boarding house near the shore when the sea came in. He kept filming while the water surged through the street below and carried away a neighbouring house. Plan International released the footage as “Eyewitness footage of Typhoon Haiyan washing house away”.

Video: Eyewitness footage of Typhoon Haiyan washing house away (Plan International).

Residents later described three to four destructive bores rolling in, each flooding the town for minutes. The water around the filmed house was estimated at about 2 m deep and moving at roughly 5 m per second, fast enough to destroy the seawall and many of the houses behind it. Once the danger passed, Gensis and his colleagues went out to assess damage in the communities Plan supported.

Surf beat: why it looked like a tsunami

Hernani sits behind a fringing coral reef about half a kilometre wide, which should have protected it. Volker Roeber and Jeremy Bricker of Tohoku University’s International Research Institute of Disaster Science used frames from Gensis’s video in a 2015 study in Nature Communications to work out what happened.

Their answer was surf beat. Storm waves travel in groups, or sets, and each group carries a long, low wave component with a wavelength of several kilometres. Offshore this is almost invisible. At Hernani, the steep seaward edge of the reef made the short storm waves break and lose their energy quickly, but the long group waves kept almost all of theirs. They crossed the reef flat and steepened into a turbulent bore at the beach, carrying energy very similar to past Pacific tsunamis.

The researchers also found that Hernani was not the worst case. If the reef had been about half as wide, the group waves could have resonated with it and grown even larger.

Lessons for coastal communities

Storm surge is the rise in sea level caused by wind pushing water ashore and by a storm’s low pressure. Disaster agencies usually map coastal flood risk with surge models, and those models leave out the behaviour of individual waves. The Hernani study showed why that matters: a town can be flooded by a tsunami-like bore even when the calculated surge looks modest.

The authors called for wave-resolving flood models and for reassessing hazard maps in reef-sheltered communities on storm-prone islands such as Okinawa and Hawaii. As Roeber put it, “Tsunami-like waves such as the one in Hernani will happen again.” Heed evacuation orders on reef coasts too: a reef offers far less protection from a major typhoon than it seems.

For a very different kind of wave, see another water-and-ice event with a misleading tsunami label, or browse weather photographs recording disruption to daily life.

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