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Saturday, November 27, 2021

Special Report: Ruthless wave

Storm surge risks increase as typhoons grow stronger, sea levels rise

BEFORE super typhoon Yolanda (Haiyan) struck land on Nov. 8, 2013, Project Nationwide Operational Assessment of Hazards (Noah) said 68 areas in the Philippines could be hit by a storm surge.

A storm surge is a sudden rise in the sea level above the normal high tide on the coast driven by the winds of a storm.

In Cebu, Tuburan and Carmen towns could expect surges of 3.2 meters and two meters high, respectively, the Department of Science and Technology project to undertake disaster science research said.

But after the storm, both Tuburan municipal administrator Roy Palaugon and Carmen Councilor and action officer Hermogenes Maningo told Sun.Star Cebu that no surge had hit their towns.

“Tuburan has never tried a storm surge,” said Palaugon. “I don’t know why they included it in the forecast of surge areas. Tuburan is surrounded by islands and is across Negros.”

Maningo, however, said that while Carmen was spared this time, he had heard stories that in the past, a storm surge had reached the chapel, about 150 meters from the coastline.

Project Noah executive director Dr. Mahar Lagmay said his group will complete this year a study that will better determine the country’s surge- and flood-prone areas.

Aerial shot of Bogo City after Typhoon Yolanda
WIND AND SEA. Super Typhoon Yolanda’s ferocious winds and rain walloped Bogo City’s agriculture and infrastructure, including its bus terminal near the shore. But the city didn’t get a storm surge, unlike Daanbantayan town further up north. Other parts of Cebu have been hit by storm surges in the past. With Cebu’s sea level rising, the threat of inundation to its coastal population increases. (Sun.Star Cebu file)

History lesson

In Manila, Meno Mendoza, weather forecaster at the Philippine Atmospheric, Geophysical & Astronomical Services Administration (Pagasa), cautioned Cebuanos against believing Cebu will be shielded from storm surges because it is bounded by other islands on both sides that are closer to larger bodies of water.

“Historically, Cebu has had surges. There was a historical event in Talisay,” he told Sun.Star Cebu.

An area’s vulnerability to a storm surge depends more on whether the storm hits it directly, and not whether it is bounded by other islands, he said.

Indeed, Talisay City, south of Cebu City, has seen many surges. Last October, typhoon Ramil enhanced the seasonal southwest monsoon (habagat), causing meter-high waves to damage homes in three barangays. And Ramil didn’t even make landfall. It simply passed north of Batanes on its way to Japan.

In July 2012, seven barangays—Poblacion, Biasong, Tanke, Dumlog, Pooc, Cansojong and San Roque—were hit by waves at least 1.7 meters high in some parts when the habagat was aggravated by typhoon Gener.

In 2008, waves from typhoon Frank damaged 100 houses on Talisay’s coast.

Despite these incidents, Filipinos more accustomed to seeing typhoons bring winds, floods down bald mountains, overflowing rivers and landslides didn’t really grasp the threat posed by storm surges until typhoon Yolanda brought a surge five to seven meters high one to two kilometers into Tacloban City, Leyte, killing thousands of residents.

From 1589

Two weeks after Yolanda, Project Noah revealed that storm surges had hit the country 49 times from 1589 to 2013 before Yolanda.

It said a two- to three-meter-high surge had hit Carmen, Cebu when typhoon Ruping struck on Nov. 10-14, 1990.

It also showed surges in Negros Occidental during typhoon Amy in 1951 that killed 991 people, and Panay and Boracay in 2008 during typhoon Frank (Fengshen) that killed 938 people. In Narvacan town, Ilocos Sur, typhoon Didang (Nadine) brought a 9.1 meter storm surge in 1968.

But the deadliest surges were the 7.3 meter surge on Oct. 12, 1897 that hit Samar and Leyte, killing 1,500 people; the five-meter surge that came when typhoon Nitang (Ike) struck southern and central Philippines and Negros Island on Sept. 2, 1984, killing 1,400 people; and the six-meter surge that lashed Cateel, Boston and Bangaga in Mindanao, killing more than 1,000 when typhoon Pablo (Bopha) struck on Dec. 4, 2012.

A paper written by N. Nirupama of the Disaster and Emergency Management department of the New York University and T.S. Murty of the Department of Civil Engineering in the University of Ottawa, Canada says storm surges differ from earthquake-generated tsunamis in that “tsunamis can affect hundreds to thousands of kilometers along the length of a coastline,” while storm surges, at most, “can affect a few tens of kilometers along the coast.”

After Yolanda, the Department of Environment and Natural Resources (DENR) 7 released a list of areas vulnerable to storm surges in Cebu. (See box at right.)

DENR 7 spokesman Eddie Llamedo said a coastal area will be more vulnerable to a storm surge if the area has a shallow seabed, a long beach front and no natural barriers such as mangrove areas and coastal forests.

He said port areas and piers were less vulnerable to storm surges due to the presence of deep sea beds and man-made barriers such as seawalls and wave breakers.

Deputy General Manager Yusoph Uckung of the Cebu Port Authority said Yolanda brought waves to the port area, but these did not rise above the wharf.

He felt a breakwater was not needed because “there’s Kawit Island, Shell Island, Mactan Island and Bohol (to slow down waves that may hit Cebu from the east).”

“Our harbor is between Mactan and Cebu Island,” he added, saying the National Disaster Risk Reduction and Management Council has not recommended the building of a breakwater.

Pagasa-Visayas chief Oscar Tabada says storms in Philippines intensified
STORMS INTENSIFIED. Super Typhoon Yolanda (Haiyan) was said to be the strongest storm to make landfall in world history, but Philippine Atmospheric, Geophysical and Astronomical Services Administration (Pagasa) Regional Services Division-Visayas officer-in-charge Oscar Tabada says the Philippines may get even stronger storms in the next 20 years. (Sun.Star Cebu Photo/Arni Aclao)

High density

Based on the DENR’s recent geohazard survey conducted in Cebu’s coastal areas, about 44 sitios are moderately to highly susceptible to storm surges.

Llamedo said areas were classified as highly to moderately susceptible on the basis of the density of the population living near the coast and the possible fatality count a storm surge could result in.

Based on the assessment of the DENR’s Mines and Geosciences Bureau (MGB) 7, most of the vulnerable sitios are in Bantayan Island, which hosts Bantayan, Madridejos and Sta. Fe towns.

Olango Island, east of Mactan Island, is not on the list, but a study on its vulnerability to sea level rise by the Asia-Pacific Network for Global Change Research and the DENR 7’s Amuerfino Mapalo showed that it has already been hit by storm surges.

In 1951, typhoon Amy hit during high tide, bringing a storm surge that moved 15 meters inland. In 1995, waves rushed in again with Typhoon Pepang, the study said.

After the MGB 7’s survey in the provinces of Bohol and Negros Oriental, it might resurvey all coastal areas in Central Visayas to check if more areas are vulnerable to storm surges, Llamedo said.

Relocation

With the initial results of the survey now in the hands of local government units (LGUs), Llamedo urged chief executives to act immediately to prevent the loss of lives, such as by relocating residents in coastal sitios and implementing a 20- to 40-meter “no build zone” in coastal areas.

The implementation of a “no-build zone” is required under Presidential Decree 1067 or the Water Code of the Philippines.

Under Section 51 of PD 1067, banks, rivers, streams, lakes and seashores “throughout their entire length and within a zone of three meters in urban areas, 20 meters in agricultural areas, and 40 meters in forest areas, along their margins, are subject to the easement of public use in the interest of recreation, navigation, floatage, fishing and salvage.”

Families

Task Force Palig-on of the Cebu Capitol estimates that in the 15 towns and one city in northern Cebu hit hardest by Yolanda alone, 31,000 families live in the 40-meter no-build zone.

Lapu-Lapu City said 5,000 to 10,000 households in some 28 coastal barangays live within 40 meters of its coast.

In Talisay City, public information officer and local disaster risk reduction and management officer Vince Monterde said 875 families from six barangays live within 40 meters of the coastline, by the Department of Social Welfare and Development’s count.

The Talisay City Risk Reduction Council is considering two spots in the hilly Barangay Maghaway for their relocation this year, he said.

The city will also strengthen its seawall. And this year, the area in Barangay Poblacion with no seawall will get one that will connect to the seawall of Barangay Dumlog, he added.

Robinson Jorgio, seismic observer at the Philippine Institute of Volcanology and Seismology’s Seismic Monitoring Station in Lapu-Lapu City, warns those planning a seawall, however, that if the waves overtop the wall, the water that goes in will be trapped inland or take longer to flow back out to sea.

This was the experience of Japan during the March 2011 earthquake, when the resulting tsunami overtopped its 15- to 20-meter-high seawalls.



Bad location

The Philippines is vulnerable to storm surges because it is an archipelago.

The country also sits in the western Pacific Ocean from latitude five to 20 degrees north of the equator, about the same range as western Pacific typhoon formation.

The Northwest Pacific Ocean basin is the most active of the seven basins of tropical cyclone formation, accounting for a third of the world’s storms.

Tropical cyclones are those with maximum winds of 35 kilometers per hour (kph) near the center or higher. They are called typhoons when winds are at least 119 kph.

On average, the Philippines gets 20 tropical cyclones a year, of which nine make landfall.

Only 2.27 of these cyclones cross Cebu Province or 100 kilometers of its boundaries yearly, said Pagasa Regional Services Division-Visayas officer-in-charge Oscar Tabada.

But this may change.

Storms rarely form within five degrees latitude of the equator, but typhoon Pablo (Bopha) in December 2012 formed very close to the equator at just 3.8 degrees, said the National Weather Service for the United Kingdom.

Typhoon Sendong (Washi) in December 2011 was also an unusually low-latitude storm, said the US National Aeronautics and Space Administration.

As a result, both storms hit a lower part of the Philippines, Mindanao, instead of Luzon, the usual path for storms.

In “A manifestation of climate change? A look at Typhoon Yolanda in relation to the historical tropical cyclone archive,” Carlos Primo David et al. of the National Institute of Geological Sciences said that from 1884 to 2012, some 20.2 percent of all tropical cyclones that formed in the West Pacific basin made landfall in Northern Luzon, 9.8 percent made landfall in Visayas-Mindanao, and 8.4 percent in Southern Luzon-Bicol.

But in recent years, more storms have entered Visayas-Mindanao. They cited the 1990s when 12-15 percent of the storms formed entered Visayas-Mindanao, and 2013, when 20 percent of all storms formed hit the area.

Another alarming trend is global warming driven by the rise in carbon dioxide emissions, mostly through fossil fuel burning by power plants, industries and vehicles, which warms the seas.

In their 2010 study “Tropical cyclones and climate change” published in Nature Geoscience, Thomas Knutson et al. said with sea surface temperatures in tropical cyclone formation areas rising “several tenths of a degree Celsius” in the past few decades, rainfall rates will rise 20 percent within 100 kilometers of the storm center later this century.

This will increase the risk of landslides and floods.

The country’s first storm this year wasn’t strong, with winds of just 64 kph, but it was still deadly due to rains.

By Jan. 19, torrential rains by “Agaton” had killed 40 in landslides and floods in Mindanao—even without Agaton making landfall.

From Jan. 11-15 alone, Davao Oriental, Compostela Valley, the Dinagat Islands, Lanao del Norte and the area between Surigao and Agusan del Norte had each received more rain than they usually received in a month, said the Manila Observatory team, citing data from the Tropical Rainfall Measuring Mission.

Stronger storms

A policy research paper written by Susmita Dasgupta, Benoit Laplante, Siobhan Murray and David Wheeler for the World Bank (WB) in 2009 pointed out that an increase in sea surface temperature will also intensify cyclone activity and heighten storm surges.

The paper noted an increase of more than 25 percent in storm surge-prone areas in the Philippines.

Knutson et al. project a two to 11 percent rise in the intensity of tropical cyclones by 2100.

“In the next 10-20 years, the storms will already be very strong,” agreed Pagasa’s Tabada. “The number of storms will be cut in half, but they will be strong.” (See sidebar.)

Quick melt

The increasing global temperature has caused polar ice sheets and glaciers to melt, causing sea levels to rise.

Last month, a study by an international team of glaciologists published in the Nature Climate Change journal said Antarctica’s Pine Island Glacier had receded by 10 kilometers from 1992-2011 alone. Its melting is forecast to add up to a centimeter to ocean levels in 20 years.

Sea level rise makes a storm surge higher, posing more risks to coastal communities.

“The Philippines and Myanmar are likely to lose 52.29 percent and 48.89 percent of their coastal GDP (gross domestic product), respectively,” in a once-in-a-century type of catastrophic storm surge, warned the WB policy research paper, which looked at the effects of sea level rise and storm surges on 84 developing countries.

It found that with just a 10 percent intensification of storm surges, the combined coastal land area of the 84 countries vulnerable to inundation from a disastrous storm surge would increase to 25.7 percent from the current 19.5 percent. This will directly affect about 174 million people.

Rising sea

At the present rate of global warming, the United Nations’ Intergovernmental Panel on Climate Change projects a rise in sea levels of 26 to 82 centimeters (10.4 and 32.8 inches) by 2100, up from its projection, made in 2007, of seven to 23 inches, after it noted an acceleration in the average rate of global mean sea level rise from 1.7 mm a year over the 20th century to 3.2 mm a year since 1990.

In the Philippines, the sea level has “risen by two centimeters from 1963 to 1993 based on tidal gauge readings in Manila Bay,” said the DENR’s Mapalo, citing Pagasa.

The mean sea level in Cebu rose from 1.728 meters from 1950-1969 to 1.744 meters from 1970-1989, he said.

The WB paper stressed the need for “rapid action” to protect endangered coastal populations, noting that the cities highly vulnerable to sea level rise and storm surges also belong to the lower-income group. (Tomorrow: Evac, bodega and anchor)
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