Sinking lands behind worsening floods

Posted at 09/30/11 8:31 PM

Calumpit residents are evacuated by truck as the flood continue to rise in several villages in Bulacan as rainwater from last Tuesday's Typhoon Pedring onslaught make their way to low-lying villages in province north of Manila. Photo by Rem Zamora for abs-cbnNEWS.com

MANILA, Philippines - Aside from global warming causing stronger cyclones and rising oceans levels, sinking lands in Metro Manila and Central Luzon are causing floods to worsen, scientists have warned.

Lands are sinking because of the natural compaction of soil and rapid withdrawal of groundwater, according to Dr. Fernando Siringan of the Marine Science Institute at the University of the Philippines Diliman.

Siringan, in a post-"Ondoy" assessment paper, said land subsidence is the least understood but very important cause of flooding.

Citing studies made by the Japan International Cooperation Agency (JICA), he said that at Manila's South Harbor, the mean sea level rose by just around 2 millimeters per year from 1902 to the early 1960s until it then started rising more than 10 times as fast, or around 2.6 centimeters per year.

Siringan said intensified groundwater withdrawal is causing land areas to sink, particularly in Metro Manila, where groundwater demand is still increasing.

Camanava and Pampanga Delta

As an example, Siringan said parts of the Camanava area consisting of Caloocan, Malabon, Navotas, and Valenzuela City sank by between 2.7  to 9.1 centimeters from 1991 to 2002.

Siringan's study -- co-authored by Gemma Narisma, Kelvin Rodolfo, Celine Vicente, and Antonia Loyzaga of the Manila Observatory and Mahar Lagmay of the National Institute of Geological Sciences in UP Diliman -- also identified insufficent drainage caused by outdated drainage structures, filling up of river channels  by sediments and garbage, encroachment of waterways, and the disappearance of about 21 kilometers of small rivers as other causes of worsening floods in Metro Manila.

Siringan and Rodolfo, who is also concurrently a professor at the Department of Earth and Environmental Sciences at the University of Illinois in Chicago, said in separate paper published in 2006 that metropolitan Manila sank by an average of 5 cm to 9 cm per year from 1991 to 2003.

Meanwhile, the Pampanga Delta area that covers parts of southwestern Pampanga and Bulacan, sank by between 3 cm to 9 cm in the same period.

No data is immediately available for the years 2004 up to the present.

Typhoons, southwest monsoon not to blame

The study said typhoons should not be immediately tagged as the cause of worsening floods in the areas.

"The southwest monsoon and typhoons annually deliver approximately 2,000 millimeters of rain to the region, but the amounts have been decreasing since 1900 (Intergovernmental Panel on Climate Change, 1995; Jose et al., 1996) and cannot be blamed for the worsening floods," the paper said.

Dr. Greg Bankoff, an associate professor in the School of Asian Studies at the University of Auckland, New Zealand, also identified sinking lands as one of the primary causes of severe flooding in northern Metro Manila and parts of Pampanga and Bulacan.

"The extent of flooding has also been considerably aggravated in recent decades by land subsidence," he said in a study published by the International Institute for Asian Studies.

"Sediments that underlie river deltas have a high water content that is 'squeezed' by the weight of succeeding deposits, a process that is greatly accelerated when groundwater is extracted faster than it can be replenished by natural recharge from rain seeping back into the ground," he explained.

Sinking land, higher sea level

"As the land around Manila Bay sinks and the level of the sea rises, flooding has become more prevalent not only in the city but also in the surrounding provinces," Bankoff said.

"Climate Risks and Adaptation in Asian Coastal Megacities: A Synthesis Report," another study published by the World Bank, said aside from groundwater pumping, geological movements related to the West Marikina Valley Fault could be causing land areas in Metro Manila and nearby provinces to sink.

"Land subsidence continues decades after the groundwater pumping stops," said the study that covers Metro Manila, Bangkok in Thailand and Ho Chi Minh City in Vietnam.

The study also cited the lack of new data on how much Metro Manila lands have sunk.

"One difficulty in Metro Manila is that the station that is supposed to measure land subsidence of Manila de Bay is located on one of the old piers of Manila Port, which itself is sinking gradually," it said.

1-in-100-year flood scenario

The study expects around 97 square kilometers of land in Metro Manila to be affected by severe floods if the metropolis is struck by a "1-in-100-year event" by 2050, up from around 82 square kilometers in 2008, if government does not step in with measures recommended by the JICA  in a flood protection master plan drafted in 1990.

It warned that if Metro Manila experiences such a "1-in-100-year flood" by 2050, more than 2.5 million people are likely to be affected in the National Capital Region.

The World Bank study identified the following areas in Metro Manila at extreme risk in such a "1-in-100-year flood" scenario: Manila, Quezon City, Pasig City, Marikina City, San Juan, Mandaluyong City, and the Camanava areas.

Metro Manila has already experienced such a "1-in-100-year flood" from tropical storm Ondoy in 2009.