By Dr. Carlos Primo David
“Daluyong.” According to National Artist and Chairman of the Komisyon ng Wikang Filipino (KWF) Virgilio S.Almario, this is the closest Filipino translation to what we now know as a “storm surge”.
The majority of Filipinos probably will be unfamiliar with the Filipino term, but by now everyone knows how destructive storm surges can be in the aftermath of Super Typhoon Yolanda. Storm surges, like other natural hazards present in the country—e.g., earthquakes, volcanic eruptions, floods and landslides—will surely happen again in the future. Thus, it is only appropriate that we learn about what storm surges really are: when the ocean reclaims part of the land during a typhoon.
What’s a storm surge?
A storm surge is an abnormal rise of the ocean generated by a weather disturbance such as a tropical cyclone, over and above the predicted high tide mark. The US National Weather Service says that the rise in water is mainly wind-driven and therefore, the stronger the winds brought by a typhoon, the higher the storm surge.
Hurricane Katrina, which hit the southern coast of the United States in 2005, had maximum sustained winds of 280 kph and produced 8-meter storm surges. Katrina was one of the strongest typhoons to hit land—but it’s a far cry from Yolanda’s 315 kph winds, according to the Joint Typhoon Warning Center.
Along with sustained wind speeds, another critical factor is the size of the typhoon itself. The bigger the typhoon, the longer it will have an impact on a particular area as it moves along. Size will have a direct correlation with how long the storm surges will be in effect.
Resio and Westerink (2008) discussed other factors that contribute to the height of storm surges. They say that besides the typhoon’s wind speed and size, the geometry of the coastal area may very well tell how far storm surges will go inland.
A coastal area with a very gentle slope—you know this from beaches where you can walk a hundred meters and the water will still only be up to your waist—is more prone to storm surges.
In contrast, beaches with steep ocean floors will be less prone to surges.
Resio and Westerink also said that, as waters come in towards the land, friction between the advancing water and the ocean floor underneath can slow down a storm surge. Coral reefs and rocky shores help dissipate the wave energy.
Barriers—man-made seawalls and breakwaters and natural mangroves and wetlands—also help block storm surges up to a certain extent.
Typhoon Pedring in 2011 caused storm surges to flood the Roxas Boulevard baywalk and Hotel Sofitel by up to 4 feet. The surges would have been higher, if not for the Manila Bay breakwaters and seawall.
Another factor that worsened the effect of the storm surge in Tacloban is the city’s location: Yolanda may havepushed the ocean water from San Pablo Bay, into its narrowest part San Juanico Strait, where Tacloban is located.
There might still be debate on whether daluyong is the best Filipino word for storm surge but there will definitely be no argument on the threat it poses and the need for us to do something before it happens again. — TJD, GMA News
Dr. Carlos Primo “CP” David is a geologist professor at the University of the Philippines-National Institute of Geological Sciences (NIGS). He earned his Ph.D. in Environmental Science and Geology from Stanford University and is the project leader of Climate X.
Dr. CP David is also the project leader of an NOAH component, FloodNET. The FloodNET Project comes up with computer models for the critical RBs, automate the process of data gathering, modeling and information output, and release flood forecasts.
This article was originally published in GMANews Online on November 15, 2013