Special Features Agriculture Floods more frequent, last longer
Floods more frequent, last longer
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- Published on Friday, 15 February 2013 00:00
By A Web design Company
Sea level has been recorded at Manila’s South Harbor since 1902.
It rose around 1.3 millimeters per year (the global rate) until the early 1960s when it increased to about 2.6 centimeters per year.
`Residents have reported that between 1991 and 2002, the worst annual floods have increased in height by 0.2 to one meter in Bulacan and KAMANAVA, and by 0.3 to one meter in Bataan and Pampanga provinces. (KAMANAVA refers to Kaloocan, Malabon, Navotas and Valenzuela.)
Over the 11-year period, springtide heights have increased by 0.3 to 2 meters. These data indicate subsidence rates ranging from 1.7 8.3 centimeters per year, with more typical values of 2.5 to five centimeters per year. The highest rates were observed around fishponds.
Direct measurements at water wells that appear to be rising out of the surrounding ground in an area of more than 100 square kilometers north of Pampanga Bay confirm these rates. Currently, even moderate rains flood the more inland Pampanga municipalities, whether or not they are affected by the deposition of Pinatubo sediment. Guagua, parts of Sasmuan and Lubao are inundated for six months of the year, Minalin for almost nine months and Macabebe for three to four weeks.
Storm floods also are becoming more frequent and last longer in Bulacan and KAMANAVA.
In coastal communities, typhoons and southwest monsoons used to trigger floods that typically lasted for only around two hours, peaking during high tides. Now, spring tides may take an entire day to subside, and areas that stood above tide levels 30 years ago are frequently flooded by almost one meter.
Most coastal Bulacan barangays now experience spring tides that take between half a day and one day to subside. Roads need to be raised regularly in order to keep them navigable during spring tides and heavy rains.
It is difficult to convince local people of the role of subsidence in aggravating the flooding. For many, their ancestral homes and lots are their only assets, and they cannot conceive of leaving.
The government is too poor to resettle them elsewhere properly, and is already committed to expensive engineering solutions, which, although probably ineffective and even dangerous, the desperate flood victims are eager to believe will work.
Some people are reluctant to recognize that their own prodigal use of groundwater contributes to subsidence and the consequent flooding. Furthermore, the heavy seasonal rains leave the mistaken impression that the exceedingly abundant water must be recharging the ground below, no matter how much is withdrawn.
Part of the difficulty lies in the fact that the process is hidden; it is much easier to blame flooding on visible causes such as fishponds and slums encroaching on estuaries, and the choking of drainages by water hyacinths and garbage.
Some people have learned about the ramifications of their excessive groundwater use but, denied alternative sources, have resigned themselves to the worsening situation. Many acknowledge that free-flowing artesian wells must aggravate subsidence, but fear that temporarily closing a well might result in it drying up or make the water dirty. Others would like to take action, but do not know to whom they can turn.
Unlike an earthquake or volcanic eruption, the worsening floods are gradual, and permit temporary, stopgap solutions. Optimism is rampant during the flood-free half of the year, when people want to forget the wet and discomfort.
National and local government responses have been disappointing. Indeed, local politicians distribute pumping wells to woo and reward voters. Driven by the three- and six-year election cycles, government efforts favor short-term contingencies over efficacy, and are largely palliative, soothing the anxious public with projects that accomplish little.
In fact, none of the flood-control projects beginning in 1938 has eased flooding in Bulacan and Pampanga.
Strong attachment to place makes many people, especially the poor, eager to believe that engineering measures will free them from floods.
Coupled with the strong public clamor for engineering solutions, the great potential for illegal profit inevitably leads to large flood-control projects, regardless of their efficacy.
The engineering project that elicits the greatest concern is designed to protect KAMANAVA from both rainstorm and tidal flooding through the use of dikes, river walls, flood-control gates and pumps.
Funded with a Japanese loan of P5 billion, or about $90 million, the project includes an 8.6-kilometer polder dike, composed only of earth, to enclose and protect the Malabon and Navotas areas that are already at or below mean sea level.
A recurrence of the 1.93-meter spring tide, the highest on record would leave the polder dike with only 10 centimeters of freeboard. In Manila Bay, wave set-up can raise tide levels by as much as 80 percent.
The dike’s height of 2 meters was justified by an analysis that yielded storm-wave heights of only 1 meter. The modelers misread the bay depths charted in meters as given in fathoms (1.83 meters), and used wind speeds of only 40 kilometers per hour – those of a mere tropical depression.
The designers plan to pump floodwaters out of the polder during low tides, but sustained southerly winds can raise sea level significantly for days, rendering the structure ineffective. Even discounting storm waves, surges driven by typhoon winds can raise sea level to overtop this height. Two of these surges parked large ocean-going ships on Manila’s coastal boulevard.
The designers acknowledged that groundwater use between 1965 and 1990 has caused subsidence of 2.72 centimeters per year. That figure is already too low, yet they incorporated only 0.65 centimeters per year into the design, depreciating the implications of continuing subsidence for future maintenance. In 10 years or less, portions of the dike could subside one meter, and founder.
Quite possibly, the project exacerbates the danger posed by floods by giving the endangered people an undeserved sense of security. ScienceNewsPhilippines
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(SOURCE: “Global sea-level rise is recognized, but flooding from anthropogenic land subsidence is ignored around northern Manila Bay, Philippines” by Kelvin S. Rodolfo and Fernando P. Siringan)
