MANILA, Philippines – Storm surge first burst into Filipino consciousness when Super Typhoon Yolanda (international name: Haiyan) caused water levels to rise by 15 feet in parts of Eastern Visayas, decimating houses and killing thousands.
So what is a storm surge and what factors add to its deadliness? How can communities prepare for them?
Graphics by Raffy de Guzman
WHAT IS A STORM SURGE?
WHAT CAUSES STORM SURGE?
The low atmospheric pressure created by the storm contributes around 5% of the height of a storm surge.
STORM SURGE RECORDS
The deadliest storm surge in the 21st century was brought by Cyclone Nargis. It killed more than 138,000 people in Myanmar in May 2008.
The highest storm surge ever recorded reached 48 feet (14.6 meters). It occurred in Bathurst Bay, Australia, on March 5, 1899, during the onslaught of Tropical Cyclone Mahina.
WHO'S AT RISK?
Around 47 million people in the Philippines, or 51% of the total population, live along the country's 18,000-kilometer stretch of coastline.
STORM SURGE FACTORS
The low atmospheric pressure brought about by a storm contributes minimally to the height of a storm surge. Lower pressure leads to higher storm surge.
Storm forward speed
If the coast is open, a storm that moves fast will produce a higher storm surge. But if the body of water is partly enclosed, a slow storm will produce higher storm surge.
The stronger the winds from a storm, the higher the surge.
Storm approach angle
A storm moving towards land perpendicular to the coastline is likely to produce higher storm surge than a storm moving parallel to the coastline or at an oblique angle.
Shape of coastline
A straight coastline also allows water to disperse before hitting the shore.
Slope of ocean floor
A gently sloping ocean floor leads to higher storm surge because the water piling up is blocked by the ocean floor, and has nowhere to go but up.
TOTAL WATER LEVEL
But storm surge is only one ingredient to the rise of total water level along the coast due to a storm.
Here are the other factors:
Strong rainfall brought about by a storm can cause river waters to rise. When this water flows downriver and reaches the coast, it contributes to the water level rise there.
HOW TO PREPARE FOR STORM SURGE
As much as possible, human populations should not live near coastlines. They should be moved farther inland, far from the reach of storm surge or sea level rise. The exact distance will depend on the characteristics of the coastline. Areas only a few meters above sea level are particularly vulnerable and will require greater distances for 'no build' zones.
They should communicate quickly and coordinate closely with the government bodies and weather agencies capable of predicting storm surges. Such bodies include PAGASA and Project NOAH under the Department of Science and Technology and independent group Weather Philippines.
Lea Soria, Marine Science Institute of the University of the Philippines
Earth Observatory of Singapore
Comprehensive National Fisheries Industry Development Plan 2010
National Weather Service, National Oceanic and Atmospheric Administration