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The risk of flooding

(Article published in the May 11,2011 issue of Manila Standard Today) 

With tropical storm “Bebeng”, the second this year to enter the country and the first to hit land in town, the rains have obviously come. And, even as we hope and pray against it as we have done year after year, the floods are not too far behind.  Admittedly, Bebeng is a rarity for it is not often that we get a typhoon this early part of May; but with 11 people dead in the wake of Bebeng, can anybody deny that time it is for us to get a understanding of the flood phenomenon in order for the better to prepare for it in the days ahead?

Floods seen as risk factors are, according to the Composite Vulnerability Analysis prepared by the technical advisory mission sent to Metro Manila late ’76 by the Office of the United Nations Disaster Relief Co-coordinator (henceforth referred to as the UNDRO report) on which I heavily rely in crafting this piece, directly impact our lives depending on a flood’s combination of water level and velocity of the water flow.  Low level and quickly subsiding floods we may consider just one of the many instances of minor nuisance; but high level and slow moving floods are causes for more serious concern. 

Floods that have high water level and at the same time are slow moving result in (a) operating losses, i.e. they disrupt the run of the economic activities and the rendition of social services in the affected area, in (b) capital losses arising from the damage done to buildings and engineering works, and, most tragically, in (3) loss of lives. In Metro Manila, the more frequent floods are of the low level kind, causing on the total high operating losses, but hardly any loss in capital, much less loss in lives.  But then, in my book, the loss of a single life in a flood is one loss too many.  


Essential to being prepared for floods is the understanding of why and how they occur.  Basically, there are four reasons why we have floods. Floods may be due to heavy rainfall; or they may be the result of river bank overflow; or due to tsunamis; or, finally, due to storm surge.

Of these four categories of floods, we are familiar with the first two; the last two we hardly know.  The feared tsunamis are trains of high waves from the off-shore heading inland due to displacement of huge volumes of water there caused by the movement of the sea floor precipitated by a number of events, such as earthquakes, volcanic eruptions, and underwater explosions, including nuclear arms testings.  The movement of the water is akin to what happens when a pail hurled upward towards the burning wall of houses by residents seeking to control the damage to their community.

Storm surges, as differentiated from tsunamis, are floods of water being forced on shore by pressure of the wind above the surface of the ocean.  Water, seeking to escape the force of the wind pressing against the sea bed, goes up the sea shore trying to avoid being sandwiched by the irresistible force of the wind and the immovable object that is the floor. The movement is somewhat similar to the outflow of toothpaste when you squeeze the tube or, in the case of stronger winds, of the water rushing out of the firemen’s hoses.

The configuration of Manila Bay, with its narrow mouth and shape aided not insignificantly by Corregidor’s standing at almost the center, as if saying to tsunamis and storm surges “thou shalt not pass,” and dissipating their energies most likely explain why there are virtually no known records of tsunamis and storm surges in Metro Manila and why many of those in the City of Manila are unfamiliar with them. 

Flood due to heavy rainfall is very sudden and rapid, “flash flood” it is called, and occurs within hours after a heavy rainfall.  The rush of heavy volumes of water per unit of time has the sweeping, and cleansing, effect on our streets similar to the water being flushing down a toilet bowl.  The frequency of this flood, which I would like to also call “flush” flood, is related closely to the area’s climatic conditions, the topography of the locale, and, most important, the operating condition of the drainage system.  A good drainage system that has the capacity of getting the water out to the open sea could make frequent flash or “flush” floods a bit tolerable.

The flooding due to river bank overflow is considered as the more dangerous of the first two classes of floods.  True, the build-up time and velocity of the water current are not as, respectively, short and quick as that of a flash flood, but it is the more destructive.  It washes away crops before they are harvested, destroys homes along the river banks, and damages the furniture and fixtures within, carries them away to be rammed against other homes or deposited in the most unlikely places. 

River bank overflow is almost always caused by the condition of the locale’s topography.  Shallow river beds, for instance, are incapable of letting pass high rates of heavy flow; hence the water which, high school science says seeks its own level, behaves like motor vehicles on a very busy highway; they go to the side streets to the left and to the right, in order to by-pass the heavy traffic on the main road. 

But nature is not the only one to blame.  Often, river bank overflow is aggravated by human works and projects that reduce the outflow of water into the sea, primarily reclamation.  The shore line near the mouth of the Pasig river, for instance, has been subjected to reclamation since 1893.  The North Harbour, the South Port, the Rizal Park, most of  the computer generation do not know, are all areas reclaimed from the sea.  The site of the Mall of Asia is also reclaimed land. 

Since most of the reclaimed areas have a higher elevation than the hinterlands, the cumulative impact of these reclamation projects, past and present, is to slow down the delivery by Pasig River of the waters to the sea.  Waters, as it were waiting transport thus behave like airline passengers during peak travel season: they loiter in the sidelines and in the by-ways, until their flight is announced to be ready for boarding.

All these indicate that we cannot and must not accept, the French King’s “après moi, le deluge.”  The rains are here; it is almost certain that the floods they bring will not wait for us to go before they come.