In fact, new data shows alarming cases of "local sea level rise" of one meter in just 11 years. It was not an actual rise in the global level of the sea; it was a local, apparent rise caused by sinking, or subsidence of the land. This means that in a very short time, some areas actually sank by a meter. Samples of soil reaching a depth of 10 meters below the surface were also taken, indicating the possibility of the areas sinking even lower.
But to the dismay of UP experts, government agencies — particularly, the Department of Public Works and Highways — continue to carry out billion-peso flood-control projects in these areas, with complete disregard of these findings. In addition, human activities contributing to land sinking or subsidence, like extraction of groundwater, proceed at a rate that has exacerbated natural causes such as global warming.
Siringan revealed that Manila Bay has risen by one meter from 1964 to 2002, an effect he attributed to subsidence. It is now known that subsidence of this magnitude has occurred in only 11 years from 1991 to 2002, in Pampanga, Bulacan, and Camanava. With coastal plains in the area having a very gentle slope, a meter of subsidence means tidal incursion of up to 10 to 20 kilometers further inland.
Interestingly, excessive use of groundwater has been found to be the main cause of subsidence. Wells pump water from aquifers, the water-filled sandy layers underground. Over-extraction, however, causes the pressure in the aquifers to become less than the pressure in the clay layers above and beneath it. Thus, the aquifer sucks water out of the clay, compacting it and making the land surface subside. The study by Siringan and Rodolfo shows that in Manila sea level rose at the slow rate of two millimeters a year from 1902 to the early 1960s — essentially the rate of global sea-level rise. However, the rate increased sharply to 2.35 centimeters a year from 1963 to 1980, "a trend that correlates very strongly with the increase in groundwater pumpage over the same period."
But while people themselves say that their land is sinking — as apparent in their water wells’ "rising" from the ground — government agencies do not seem to care.
The Camanava megadike project implementor, the Department of Public Works and Highways (DPWH), refuses to heed the call of the UP experts for some design changes. Siringan and Rodolfo insist that the river walls and floodgates be set higher to accommodate subsidence. They oppose the project’s earth dike component that will ring a two-km by 0.6-km area in Malabon, which is already below sea level. They added that it will sit on highly compactible materials which will undergo "differential deformation" under the dike’s weight. As the dike is made up of soil, a small breach can easily enlarge, letting the floodwaters through. The UP team observed that the newly raised river wall along Malabon River was almost overtopped during recent typhoons even if the spring tide was not yet at its maximum height.
In Pampanga, Siringan and Rodolfo are opposing another DPWH-planned river improvement project. They argue that the artificial river to be dug along the Pampanga River to drain the plains is situated in a low-lying area that is sinking and gets flooded at maximum tide. This means that the river will just trap water along at least three-fourths of its length. The dredging of rivers in low-lying areas encourages the backflow of water and, ultimately, salt-water intrusion from the Bay.
Such an intrusion will poison groundwater and soils, they say. Siringan added that the building of dikes has deprived the areas around the rivers of silt (such as from Mt. Pinatubo), which could replace the elevation lost to subsidence. In addition, water in adjacent areas can no longer drain into the river.
In 2001, false-color satellite imagery revealed that there were more waterlogged areas along the diked Pampanga River, which could be attributed to the dike itself.
"We have made so many presentations to our peers in the national and international scientific and academic communities. Our reports have been published in reputable national and international scientific journals. Ironically, our findings are much better received by our scientific colleagues here and abroad than by our own government," Siringan said.
However, the UP team is encouraged by the National Water Resources Board, which has used the UP findings as basis for ordering the closure of some illegal deep wells and for convincing people of the consequences of over-extracting ground water.