Manila City and other parts of Metro Manila are sinking by a few millimeters every year from natural geological processes, but human activities including excessive groundwater extraction, heavy construction, and the proliferation of concrete roads are accelerating the rate of land subsidence and making the city’s flood problem significantly worse.
Mines and Geosciences Bureau (MGB) Senior Geologist Michelle Mendoza made the statement in an interview on DZRH News program The Situation Report on Friday, September 11, explaining the phenomenon of land subsidence in Metro Manila and why some areas of the city now sit below sea level during high tide.
Mendoza said the natural component of land subsidence comes from the loose sediments that underlie much of Metro Manila, particularly Manila City, which naturally compact over time as they release water and settle under their own weight.
“Yung mga sand, silt, and mud kasi natin kusa niyang ire-release yung water na nandiyan sa kanya. Pag na-release niya po yun, yung gravity nung mga sediments ay parang mag-a-activate din, so parang naturally magiging compacted sila. Bumababa din siguro a few millimeters lang naman din yan per year yung natural na land subsidence,” the MGB senior geologist said.
Mendoza said the human-induced component is far more severe, driven primarily by the over-extraction of groundwater to meet the demands of rapid urban development, which has dropped the water table and caused clay layers underground to compact significantly.
“When it comes to human-induced land subsidence, dito na po papasok yung mga industrial and commercial demands natin. Syempre rapid ang growth and development sa buong National Capital Region, so nagkaroon din ng excessive or over-extraction ng groundwater, that’s why yung water table natin nag-drop din or bumagsak. Naging compacted yung clay layer subsurface. Yung top portion ng ground ay bumagsak din,” Mendoza said.
The weight of buildings, commercial structures, and concrete roads adds further pressure on the subsurface, compounding the rate at which the ground sinks beneath Metro Manila’s most densely developed areas.
“Idagdag pa po natin yung development, yung paggawa ng mga concrete na roads, yung tabi-tabing mga buildings, may weight kasi yan. Yung weight pa lang ng building sa isang commercial area, malaki po ang impact niyan sa pag-subside ng land natin,” Mendoza said.
Mendoza said Manila City is among the areas where researchers have documented significant millimeter-level sinking in past decades, and that other cities within Metro Manila have also experienced measurable subsidence due to groundwater over-extraction.
“Manila yung natatandaan ko po na may ilang millimeters yung binaba niya, but there are other cities din within the National Capital Region na nagkaroon ng significant percentage ng subsidence in the past decades due to over-extraction ng groundwater nga po,” she said.
The MGB senior geologist said the practical consequence of land subsidence is that it lowers the ground relative to sea level and high tide marks, meaning that areas which previously flooded only during severe storms now flood during ordinary rain events because the ground itself has dropped.
“Yung Manila mismo yung isa sa mga nakikitaan natin na mas mababa siya relative to the sea level rise and the highest tide level ng water because of land subsidence,” she said.
