Floods to persist in Bulacan after recording high subsidence rate from 2014 to 2020 — study

Floods in the province of Bulacan will continue to persist after it recorded a high subsidence rate due to natural and human-induced factors, according to a recent study by researchers from the University of the Philippines Diliman. 

Using remote sensing techniques, researchers found that municipalities of Guiguinto, Bulakan, and Balagtas in Bulacan had a maximum subsidence rate of 109 mm/year based on analysis from 2014 to 2020. 

“Land subsidence in metropolitan cities is a growing concern due to increasing population, urban development, and groundwater demand,” the study read. 

The study pointed out that deforming regions within Bulacan coincide with industrial complexes mixed with residential areas. 

“In Bulacan, factories of paper, food manufacturers, and concrete and metal supply are the major industries with the highest subsidence rates,” the researchers said. 

“As observed in our resulting InSAR images, areas with high subsidence rates also plot in residential areas in Calumpit, Malolos, Guiguinto, Balagtas, Bocaue, and Marilao [...] All of these cities have had high population growth in the past decades, and as the population continues to increase, there is also a growing demand for household water supply.” 

The researchers added the high subsidence rate leads to decrease in ground elevation that creates a basin “conducive to flooding.” 

Further, sea-level rise due to climate change worsens the flooding in coastal areas. 

“This situation is evident in Tawiran Village in Obando, Bulacan, where seaside roads are inundated during high tide, and ground floors are permanently submerged in seawater,” the study read. “As a result, residents have resorted to either elevating their homes or abandoning the ground floor altogether, opting to reside on the upper floors.” 

With the sea level rising and land subsidence, floods may worsen especially in coastal areas like Bulakan, Bulacan where an airport is expected to rise on the reclaimed area. 

“The worst affected community is Pariahan Village in Barangay Taliptip, Bulakan, Bulacan, where a major international airport is being constructed through land reclamation along Manila Bay,” the study said. 

“Taliptip was a walkable area but has become permanently flooded since the early 2000s. If not addressed, the new airport in the reclaimed area will continue to suffer threats from worsening floods and storm surges.” 

Decrease subsidence rates 

On the other hand, several cities in Metro Manila such as the CAMANAVA area (Caloocan, Malabon, Navotas, Valenzuela), and Las Piñas City have observed a decrease in their subsidence rates, from 35-40 mm/yr and 17 mm/yr, respectively, to roughly 15 mm/yr and 6 mm/yr. 

The researchers attributed it to “the groundwater restrictions set by the National Water Resources Board (NWRB) in 2004 when the problem was recognized and brought to the attention of authorities.” 

Bulacan and Metro Manila have declared a state of calamity amid the massive flooding caused by the southwest monsoon or Habagat and Super Typhoon Carina last July. 

— LA, GMA Integrated News