Friday, December 5, 2014

The Philippines Has a History of Catastrophic, Fast-Moving Storm Surges

When Super Typhoon Haiyan (Yolanda) slammed the Philippines last year, it generated a tsunami-like storm surge that rapidly swept over portions of southern Samar and Leyte, including the city of Tacloban. Surge levels exceeded 6.1 m (20 ft), and many people exclaimed that such a catastrophic surge had never been seen before in the Philippines.

However, if we dig into the history of the Philippines, we find numerous storm surge events that were catastrophic. The water in some of these events rushed in very rapidly, much like Haiyan's surge.


Super Typhoon Haiyan generated a tsunami-like storm surge last year near Tacloban, Leyte. The force of this sudden surge carried ships into neighborhoods. Image: Noel Celis/AFP/Getty Images

One reason the Philippines is so vulnerable to storm surges is because they take so many direct hits from powerful typhoons. In fact, the Philippines experience more strikes from power tropical cyclones than any other nation on Earth. In the 43-year period from 1948-1990, 850 tropical cyclones entered the Philippines Area of Responsibility (PAR), which averages to approximately 20 tropical cyclones per year (Soriano 1992; Bankoff 2003). During this period, 384 tropical cyclones made landfall in the Philippines, or an average of about nine tropical cyclones per year (Arafiles et al. 1984; Soriano 1992). This nation observes storm surges about four to six times per year (Arafiles et al. 1984).

The unique landscape of the Philippines, which is an archipelago, consisting of hundreds of islands, also impacts storm surge patterns. The shape and orientation of these islands enables water levels to rise rapidly and very locally, meaning one side of an island may experience a large storm surge, while the other side, sometimes just several km away, experiences no storm surge at all.

The Philippines are made up of thousands of islands, which influence storm surges into very localized patterns.
Image: George Tapan http://junvillegas.blogspot.com/2013/02/island-hopping-at-hundred-islands.html 

The Philippines has observed seven of the top 10 storm surge levels in the history of East Asia. Here is a list of these top seven water levels from the SURGEDAT database:

Rank     Height (m)        Year          Storm Name                             Location
1            9.14*                1968         Didang                                      Narvacan Ilocos Sur
2            7.30                  1897         Typhoon of Samar/Leyte          Samar and Leyte
3            7.00                  1912         Typhoon of Leyte/Cebu            Leyte and Cebu
4            6.50                  2013         Haiyan                                      Tacloban, Leyte
5            4.60                  1984         Unnamed                                  Sarangani Island
6            4.50                  1983         Unnamed                                  Near Infanta
7            4.50                  1981         Rosita                                       South of Tinambac
* Questionable accuracy

This list provides some interesting information. The highest storm surge in the Philippines history is a 9.14 m surge at Narvacan Ilocos Sur in 1968. However, this surge event is questionable, as the number was only provided by a historic map, and no text or anecdotal stories confirm destruction of a large surge that year. Also, the location of this surge event is in the northwest portion of Luzon, which means that this surge would have been produced by wrap-around winds on the backside of a typhoon. It is doubtful such winds could produce a 9-m surge.

However, what is confirmed very well in the history are the 7.3-m (24-ft) and 7.0-m (23-ft) surges that occurred in 1897 and 1912, respectively, as well  as the 6.5-m (21.3-ft) surge generated by Haiyan last year in Tacloban. These events are confirmed by several historic sources and stand as the highest credible surge events in the history of East Asia.  And the surges for all of these events peaked in the "Eastern Visayas" region of the Philippines, in the regions of Leyte and Samar.

This provides context for Hagupit, which is a large, powerful category-4 tropical cyclone, approaching the region with the top three surges in the East Asia Hall of Fame.



Super Typhoon Hagupit is a large, category-4 typhoon, approaching the most dangerous region in East Asia for storm surge. The three highest storm surges in the history of East Asia occurred just south of Hagupit's forecast track.
Image and Info: Hal Needham




History tells us a few things about these surge events. One important fact is that the flood waters can stick around for awhile. For example, the 7.3-m (24-ft) surge that struck in 1897 persisted for at least three hours (Bankoff 2003).

Perhaps even more important is the fact that surges in the Philippines can strike extremely rapidly. This is because of the complex configuration of the islands, inlets and bays. For example, when Haiyan struck last year, eyewitnesses described the storm surge like a rapidly-moving tsunami. This pattern has happened other times as well. In 1908 a storm surge struck the community of Tarol so suddenly that many people could not escape the wrath of this event. According to Arafiles and Alcances (1978, pg. 366),

It was about 4 A.M. of the 13th, when we observed that the water was rising and beginning to flood the lowland of the town, but no one gave this fact any importance as it is usual, when a typhoon passes, that the water of the river rises about a meter. But at about 6 A.M. almost suddenly the waves of the sea like mountains of water precipitated themselves upon the barrio of Tarol, destroying houses and whatever they met in their way. The level of the sea rose so rapidly that only some of the inhabitants had time to escape and save their lives.


Fascinating history. Here we have a location that frequently observes storm surges, so few people take notice or evacuate when the typhoon winds intensify. However, each typhoon track is unique from others, and apparently the track of this storm was just perfect to push a sudden surge into Tarol. The surge claimed many lives because the rapidity at which it struck. It is likely that the surge was not even that high....the highest storm surge in the SURGEDAT database for 1908 is a 3.0 m (10 ft) surge. It's just likely that this surge struck like a wall of water, inundating the community all at once.


As Super Typhoon Hagupit bears down on the Philippines, it will produce a unique storm surge pattern never seen before. Highest surges will occur in areas of strong onshore winds. Image: NOAA

Super Typhoon Hagupit threatens to devastate portions of the Philippines with strong winds, heavy rains and high storm surge. Keep in mind that storm surge is very localized, so an area with strong onshore winds could observe a 3-m surge, for example, while a location just a few km away observes a surge less than 1 m. Also, Hagupit will generate a very different inundation profile than Haiyan, so some areas that did not observe high water last year will experience the rapid onset of a high surge and destructive waves.

REFERENCES

Arafiles, C.P., and C.P. Alcances, Jr., 1978: Storm Surge Potentials of Selected Philippine Coastal Basins, available on the Web at: http://www.pagasa.dost.gov/ph/research.shtml.

Arafiles, C.P., L.A. Amadore, C.S. Doctor, and C.L. Davis, 1984: Vulnerability of the Philippines to Storm Surges, Proceedings of the Second Symposium on Tropical Cyclones in the South China Sea and Western North Pacific Ocean, Quezon City, Philippines, 11-13 December 1984, pgs 142-151.

Bankoff, G., 2003: Cultures of Disaster: Society and Natural Hazard in the Philippines, Published by RoutledgeCurzon, London, United Kingdom, 232 pp.


Soriano, B., 1992: Tropical Cyclone, Statistics in the Bicol Region, Ang Tagamasid, 20, 15 pp.


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