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Metro Manila sinking; farms, fishponds blamed

Friday, February 15, 2013 06:48 AM    Views : 2804by:Manila Business Insight

Metro Manila is sinking.

Groundwater overuse is causing the plains around northern Manila Bay to subside about 10 centimeters a year, more than 10 times faster than sea level rise, a study on ground subsidence in Metro Manila shows.

The average sea level rise caused by global warming in the Philippines is about 8 millimeters a year, according to a study conducted by Dr. Fernando Siringan  and Dr. Kelvin S. Rodolfo of  the University of the Philippines (UP)  Diliman.

Siringan is a professor at the UP National Institute of Geological Sciences while Rodolfo is a consultant at Project Noah (Nationwide Operational Assessment of Hazards) at the UP National Institute of Geological Sciences and Professor Emeritus at the Department of Earth and Environmental Sciences, University of Illinois at Chicago.

Since 1997, Siringan and Rodolfo have been studying the worsening floods and tidal incursions around northern Manila Bay. They presented data from the study during a conference on the risks that coastal megacities with populations over 10 million face as a consequence of global warming and sea level rise.

The conference was organized by the Manila Observatory, Ateneo de Manila University and Canada's International Development Research Center.

 Siringan and Rodolfo reviewed how overuse of groundwater causes subsidence and looked at the  consequences and the efforts taken to control it. They looked at the physical and climatic setting of north Manila Bay's flood-prone delta plains and the population trends that might reflect the response to the slow flooding.

While the sea level may rise 1 millimeter to 3 mm. per year due to global warming, the study observes that policy makers are oblivious to or ignore the principal reason for ground subsidence in Metro Manila –  the overpumping of aquifers.

Aquifers are the  underground  layer of water-bearing permeable rock, gravel, sand or silt from which groundwater is  extracted.

Such ignorance allows the government to treat flooding as a lesser problem that can be mitigated through large infrastructural projects that are both ineffective and vulnerable to corruption, the study said.

"Money would be better spent on preventing the subsidence by reducing groundwater pumping and moderating population growth and land use, but these approaches are politically and psychologically unacceptable," it said.

And even if groundwater use is greatly reduced and enlightened land-use practices are initiated, natural subsidence in river deltas and global sea-level rise will continue to aggravate flooding, although at substantially lower rates.

Deltas

The eruption of Mt. Pinatubo in 1991 caused floods and lahars (flowing slurries of volcanic debris)  to fill channels southeast of the volcano. But long before that, and in areas that received no volcanic sediments, regional floods were worsening. They were and are blamed on upland deforestation, rapid urbanization, channel encroachment by squatters and fishponds and the dumping of garbage in estuaries.

Manila Bay is bordered by river deltas in the provinces of Bataan, Bulacan and Pampanga, and northern Manila's KAMANAVA area (Kaloocan, Malabon, Navotas and Valenzuela). The plains comprise almost 3,000 square kilometers, extending southward from Angeles City and Arayat town to the coast, which stretches from northeastern Bataan eastward and southward to KAMANAVA.

The coastal plains are so low and flat that the one-meter elevation is 10-20 kilometers inland. Thus, normal spring tides only 1.25 meters high extend many kilometers upstream, and even small rises in relative sea level translate into large inland encroachments.

The more seaward flats, which are marshy and cut by numerous tidal inlets, are occupied almost entirely by fishponds that increasingly encroach into channels and expand northward. Rice paddies still above tidal influence, with two annual crops, eventually are converted into fishponds as the sea slowly expands northward.

The southwest monsoon and typhoons annually deliver approximately 2,000 millimeters of rain to the region, but the amounts have been decreasing since 1900 and cannot be blamed for the worsening floods.

Due to an unfortunate combination of coastal configuration and seasonal wind regime, waves generated during the rainy southwest monsoon also raise tide levels by as much as 80 percent at the northern end of the bay, hindering run-off into the bay.

Unfortunately, about 70 percent of the rain arrives during the wet season between May-June and September-October. In the flood-plagued areas, surface reservoirs are too small to store enough of these waters for agriculture, fishponds and domestic needs during the dry season.

As the population grows, uncontrolled and unmonitored use of groundwater, already far too heavy, inevitably accelerates, the study observed.

Agriculture is also partly to blame for the groundwater subsidence in Metro Manila. Rice, by far the biggest crop north of Manila Bay, consumes more water than any other crop.

And high yielding rice varieties use proportionately more than traditional varieties, the study observed, adding that to mitigate drought during El Niño episodes, the Department of Agriculture provides shallow wells to farmers and aggravates subsidence.

Fishponds make enormous demands on their environments; around Manila Bay, subsidence begins when ponds are first diked and dried. This rapidly dewaters the upper few meters of sediment, which, having the greatest porosity and water content, can shrink the most.

Nutrient sources, such as chicken manure, are introduced to support algae before filling the ponds and then before every annual restocking. Rotting excess algae effectively poisons the pond water, which is flushed into the sea – deteriorating the environment of free-living species – and is replaced with great quantities of river or groundwater.

Flooding is worsened by illegal expansion of fishponds into tidal channels.

Also to blame for groundwater overuse are privately owned golf courses and swimming pools that are maintained with large, unregulated volumes  of groundwater.

However, domestic groundwater use may not cause the most subsidence. People complain that their wells stop flowing when high-volume pumps of large plantations or fishponds are active. Some of the most elusive and crucially lacking data are the rates and volumes extracted, because farm and fishpond operators do not allow their measurement.

Electromagnetic bandwidths sensitive to water show how waterlogged areas around the northern bay are evolving and expanding. Fishpond dikes at the coast continue to be raised in response to rising relative sea level, confining coastal wetlands and subdividing them into smaller ponds.

Notably, the areas along the Pasac River, which was contained with dikes after the 1991 Pinatubo eruption, are now waterlogged. Without the dikes, floods escaping from the channel would have deposited sediment on the floodplain, counteracting the elevation loss from autocompaction, and compensating in part for subsidence. ScienceNewsPhilippines

Source: http://www.malaya.com.ph/index.php/special-features/agriculture/24318-metro-manila-sinking-farms-fishponds-blamed

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