Flooding in Pampanga Delta worsening

Flooding in the Pampanga Delta in Central Luzon has worsened over the years.

One of the main reasons is that the delta is sinking. Since the 1990s, for instance, the depth of floods in five Pampanga towns — Bacolor, Macabebe, Masantol, Minalin and Sasmuan — and in the City of San Fernando has been averaging 3.4 feet, reaching six feet during the typhoon season.

In the 1980s, flood depth only ranged from one to three feet.

A University of the Philippines (UP) study found that floods in the region have become more frequent, higher, longer lasting and more widespread.

The research, titled "Net Sea Level Change in the Pampanga Delta: Causes and Consequences," was conducted by a team composed of Rhodora Aparente, Corazon Lamug, Kevin Rodolfo, Nathaniel Baluda, Cristina Remotique, Fernando Siringan, Jose Vicente Balboa, Cherry Ringor and Napoleon Villanueva, staff members and students of the National Institute of Geological Science (NIGS) Geology Laboratory.

The scientific study, funded by the Department of Agriculture-Bureau of Agricultural Research (DA-BAR), delved deep into the environmental changes, causes and consequences in the Pampanga Delta.

The delta covers Pampanga, Bulacan, Bataan and Camanava (Caloocan City, Malabon City, Navotas and Valenzuela, the commonly flooded cities and towns in Metro Manila).

It was among the DA-BAR-funded projects reviewed and deemed important by an external program and management review team headed by Dr. Feliciano Calora Sr., a noted scientist.

The NIGS and University of the Philippines researchers attributed the worsening trends of flooding partly to the siltation after the 1991 eruption of Mt. Pinatubo. Floodwaters could no longer be contained by channels filled with lahar.

But trends are similar in Bulacan and Camanava, which received no Pinatubo sediments, they said.

Flooding has worsened both before the 1991 volcanic eruption and away from Pinatubo. At high tide, the Pantalan Bago in Orani, Bataan is flooded in seawater up to one-meter deep.

What is causing the worsening flooding?

Even before the 1991 Pinatubo eruption, channel capacities were decreased by encroaching houses and fishponds, increased siltation owing to deforestation, and garbage.

Other causes were a decrease in flood plain areas owing to urban sprawl, fishponds, flood control dikes, and global sea level rise and land subsidence, the least understood but the most important.

Land subsidence is the lowering of land, even large areas, owing to natural compaction of underlying soil and sediments and compaction hastened by rapid withdrawal of groundwater, which is the most important.

"River deltas subside naturally as they accumulate sediments," the researchers said.

Each deposit is compressed, its water squeezed out by the weight of succeeding deposits. If sediments stop coming, slow subsidence continues and the delta slowly drowns. Groundwater withdrawal speeds up this process 10 times or more.

How does groundwater withdrawal cause land subsidence?

Pumps extract water from aquifers (layer of sand and gravel soaked in water). If too much water is pumped out of the aquifer, the pressure is reduced in the spaces between the grains of sand and gravel. Water in the clay layer is sucked into the aquifer, causing the clay layer to shrink and the ground surface to sink.

There are simply far too many wells in Pampanga, the researchers observed. The number of wells and the volume of water they pump out daily are simply too much for a small area.

Fishponds draw large, unregulated amounts of groundwater. Current practices in aquaculture cause accumulation of excess fish feeds, which rot and pollute the water. Frequent flushing with groundwater is required.

The long-term solution to prevent too much soil subsidence and flooding is to activate existing groundwater guidelines, the researchers said.