The western coastal cities of Metro Manila, including Manila, Navotas, Malabon, and Valenzuela experience flooding faster than other parts of the capital region because they sit on unconsolidated materials, loose sand, silt, and mud that water penetrates easily and moves through quickly.
Mines and Geosciences Bureau (MGB) senior geologist Michelle Mendoza explained this in an interview on DZRH News program The Situation Report on Friday, September 11, breaking down the geology of Metro Manila’s most flood-prone areas in plain terms for residents and policymakers.
Mendoza said the ground beneath these cities was formed by coastal lowlands and delta plains, meaning the soil is composed of materials that have not been compacted by geological forces and therefore have spaces between particles that water can move through rapidly.
“Yung ground po natin diyan ay composed of unconsolidated materials. Ibig sabihin medyo loose yung mga materials natin, that’s why easily penetrated siya by rainwater,” the MGB senior geologist said.
Mendoza said the loose nature of the soil means water both enters and exits quickly, which sounds like an advantage but is in fact a liability, because the same ease of water movement that allows rainwater to percolate down also allows it to rise up when the water table is high.
“Water percolates easily doon sa mga sediments since hindi nga siya compacted, so mas madali pong ma-absorb ng water at mas madali ding ma-release ng water kasi nga po may spaces in between the sediments,” she said.
Mendoza said the contrast with the central ridge of Metro Manila, covering Taguig, Muntinlupa, and parts of Quezon City, is stark, as those areas sit on volcanic tuff or adobe, a resistant rock that does not absorb water easily and therefore does not flood as quickly.
The MGB senior geologist said the geological difference between the western lowlands and the central ridge is one of the most important but least understood reasons why some parts of Metro Manila flood while others nearby remain dry during the same rain event.
