Ground subsidence, the gradual or sudden sinking of the Earth’s surface, has been a longstanding issue in major metropolitan areas across the Philippines but has only recently been recognized as a significant hazard, following more recent studies and advancements in monitoring technologies. This phenomenon is caused by natural factors like tectonic activity and sediment compaction but is also exacerbated by human activities, especially excessive groundwater extraction. In the Philippines, with densely populated metropolitan areas, subsidence amplifies the risks of flooding, infrastructure damage, and storm surges.
A recent paper by Jolly Joyce Sulapas and the Hazard Assessment Team of the UP Resilience Institute (UP RI) NOAH Center presents subsidence rates in key urban cities: Metro Manila, Metro Cebu, Metro Davao, Metro Iloilo, and Legazpi City. This study entitled “Ground subsidence in major Philippine metropolitan cities from 2014 to 2020,” published in the International Journal of Applied Earth Observation and Geoinformation (https://doi.org/10.1016/j.jag.2024.104107) in August 2024, shows important insights into the long-term effects of ground subsidence.
The research utilized Interferometric Synthetic Aperture Radar (InSAR) to monitor ground movement over time and establish a link between land subsidence and factors such as rapid urbanization, over-extraction of groundwater, and the broader impacts of climate change. It highlights how these processes, particularly unsustainable groundwater use, contribute to the worsening of subsidence, creating serious risks for infrastructure, public safety, and environmental sustainability.
The most notable findings for major cities and regions are:
- Metro Manila and Bulacan: The highest subsidence rate was recorded at a startling 109 mm/year encompassing Guiguinto, Balagtas, and Bulakan in the Bulacan Province coinciding with mostly industrial zones, as well as residential and agricultural land use.
- Metro Cebu: Subsidence rates of up to 11 mm/year were observed in Naga City where a landslide occurred. Lower subsidence rates are observed near the coast.
- Metro Davao: Panabo City has the highest subsidence rate, at 38 mm/year. Subsidence in the region is tied to fishponds and industrial zones, where water pumping has led to localized sinking.
- Metro Iloilo: Iloilo experiences a low subsidence rate of 9 mm per year, concentrated in coastal districts, which poses an increasing risk of tidal flooding and groundwater contamination.
- Legazpi City: Legazpi’s highest rate was 29 mm 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 paper advocates for better groundwater management wherein utilizing InSAR technology will help monitor the problem in the long-term. It emphasizes the importance of regulating groundwater extraction in regions showing the highest subsidence rates, such as industrial and agricultural zones and heavily populated residential areas.
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An Inquirer Feature on Ground Subsidence
Recently, Audrei Anne Ybañez, one of the researchers of UPRI NOAH HAT, was interviewed in a documentary by Inquirer (available here) to raise public awareness and shed light on the real-world implications of subsidence issues in the Philippines. The video highlights the research team’s findings and underscores the urgency of addressing land subsidence in Metro Manila and its surrounding areas. It points to communities in Hagonoy and Obando Bulacan where roads and homes are sinking, forcing residents to raise their homes or abandon submerged floors entirely.
The video also touches on the economic implications, particularly for infrastructure projects in areas prone to subsidence. The video emphasizes the need for a comprehensive approach involving policy changes, monitoring, and public awareness to combat the growing hazard. Public awareness campaigns like this are essential in bringing attention to a hazard that is often overlooked but can have devastating long-term consequences.
Moving Forward: Long-Term Solutions
The findings of this study provide a crucial baseline for future efforts to mitigate subsidence risks. Beyond regulation, the integration of real-time monitoring technologies like InSAR into urban planning processes could provide critical early warnings for at-risk communities.
Addressing subsidence is not just an environmental concern but also a pressing socioeconomic one. If left unchecked, it could lead to massive economic losses, infrastructure damage, the displacement of communities, and worse, the loss of lives. The findings of the study serve as a clarion call for action, emphasizing the importance of proactive measures to safeguard vulnerable communities from this sinking threat.
To know more about the paper in detail, visit: https://doi.org/10.1016/j.jag.2024.104107.