Sea level rise, characterized by the increasing average height of the oceans, is a significant consequence of climate change. However, another largely overlooked factor contributing to the sinking of land is land subsidence.
A concerning report from researchers at the University of the Philippines reveals that Panabo City, situated within Metro Davao, experiences the highest rate of subsidence, measuring 38 millimeters annually. This phenomenon is reportedly associated with the presence of fishponds and industrial areas in the region.
The study, published in the International Journal of Applied Earth Observation and Geoinformation, indicates that in Bunawan, land subsidence is particularly pronounced beneath a chemical manufacturing facility. Additional subsiding areas have been identified near the Davao River in Talomo and Buhangin, as well as in residential and commercial zones close to the coastline in Poblacion and Talomo.
Jolly Joyce Sulapas, Audrei Anne Ybañez, and Dr. Alfredo Mahar Francisco Lagmay from the UP Diliman College of Science, in collaboration with Kayla Milcah Marasigan and Julian Marie Bernice Grageda of the UP Resilience Institute Nationwide Operational Assessment of Hazards, conducted an analysis of land subsidence in key cities across the Philippines during the period from 2014 to 2020.
Land subsidence, characterized by the gradual sinking or abrupt collapse of the ground surface, has been a persistent concern in major urban centers throughout the Philippines. However, it has only recently gained acknowledgment as a serious threat, due to recent research and improvements in monitoring technologies.
The UP researchers employed Interferometric Synthetic Aperture Radar (InSAR), a satellite-based technology that utilizes radar to generate images of the Earth’s surface, to assess the rate of subsidence across various regions. InSAR functions similarly to capturing two photographs of the same location at different intervals and subsequently analyzing them to determine any changes in position.
“Along coastal communities and coupled with sea level rise, subsidence creates a compounding effect of intensifying coastal hazards, leading to more frequent and severe flooding events, especially during high tides or typhoons,” the UP researchers wrote.
“This is further exacerbated by local contexts of urbanization and hazard management regulations,” they noted. “Coastal areas north of Metro Manila experience moderate to high flood hazards due to sea level rise and land subsidence.”
Although there are several causes, excessive groundwater extraction has been cited as the main culprit. Data from 2003–2010 showed that Caloocan, Malabon, Navotas, Valenzuela, Manila, Rosario, San Pedro, Las Piñas, and Dasmariñas subside at rates of approximately 20 to 42 millimeters per year due to the over-extraction of groundwater and other human activities.
“Excessive groundwater extraction is a leading cause of urban land subsidence – it is projected to impact 19% of the global population by 2040,” said a feature published by World Economic Forum.
As urban areas expand, the demand for freshwater for both domestic and industrial purposes rises. In areas where surface water resources are scarce or contaminated, groundwater often serves as the main source of water.
Groundwater is found beneath the boundary known as the water table. The water table serves as the demarcation between the unsaturated zone and the saturated zone. Within the saturated zone, groundwater, along with rocks and sediments, is present, collectively referred to as an aquifer.
Rocks and sediments play a crucial role in retaining groundwater. During rainfall, water is reintroduced into the saturated zone. However, if groundwater is extracted at a rate that exceeds its natural replenishment, it may result in subsidence.
Coastal cities are particularly vulnerable to land subsidence, although cities situated inland are not exempt from facing similar difficulties. A recent analysis of 99 coastal cities revealed that 33 of them are sinking at a rate five times greater than the rise in sea levels.
In the UP study, it’s not only Metro Manila and Metro Davao which has experienced land subsidence. Consider the following findings:
Metro Cebu: Subsidence rates of up to 11 millimeters per year were observed in Naga City where a landslide occurred. Lower subsidence rates are observed near the coast. In Mandaue City, a distinct deformation is observed along the length of the Butuanon River draining towards the Mactan Channel.
Metro Iloilo: Iloilo experiences a low subsidence rate of 9 millimeters per year, concentrated in coastal districts, which poses an increasing risk of tidal flooding and groundwater contamination. The highest rates are within Mandurriao and Molo Districts and La Paz District. These districts are focal commercial, industrial, and residential hubs.
Legazpi City: Legazpi’s highest rate was 29 millimeters per year, primarily in the southeastern flank of Mayon Volcano. While volcanic activity may contribute to this, excessive groundwater extraction likely accelerates the problem.
“The high population density of cities makes the information more relevant to the public, as its impact on people is greater,” Sulapas stressed. “Cities are also more vulnerable to land subsidence because the subsurface strata, or the rocks beneath the cities, are relatively younger.”
The subsiding regions are predominantly located in industrial and commercial sectors, where extensive artificial constructions exacerbate the phenomenon of subsidence.
The UP researchers believe it is essential to monitor these areas diligently, as subsidence heightens the risk of flooding, compromises the integrity of buildings and infrastructure, and leads to significant economic repercussions. This hazard also amplifies the susceptibility of residents in coastal areas to elevated tides and storm surges, which are further intensified by climate change.
“It seems like industrial complexes require a lot of water to keep their operations running. The same can be said for residential areas,” Sulapas pointed out.
As such, the study recommends sustainable groundwater use and improved water management practices.
Excessive groundwater extraction is merely one factor contributing to this phenomenon. Rapid urban development, tectonic activity, and the natural compaction of sediments are also significant contributors to land subsidence on a global scale, the UP researchers pointed out.
