The two most common explanations I’ve been hearing about “Ondoy’s” destructive impact are “too much rain, too soon” (that was actually the Inquirer headline) and “climate change.”

The “too much water in one day” explanation makes a lot of sense, but I worry that it feeds into fatalism. It puts us in a position like that of a child singing, “Rain, rain go away, come again another day.” Unfortunately, we can’t ask the typhoon to go away, or not to rain too hard.

Climate change? The term has become almost formulaic, blamed for everything from dengue fever to food shortages. No doubt, climate change is a serious problem, but again, like the “too much rain” explanation, it can lead to fatalism since the solutions to climate change are so long-term.

If we’re to mobilize government and the citizens to prevent more destructive floods of this kind, we need to get back to basic physics and geology and look at what can be done in the short term.

Too technical? Not if we use a simple analogy. Go out to your garden with a bucket. Put it under a faucet and turn on the tap. It quickly fills up, and overflows, right? Even after you turn off the tap, the bucket remains full.

Now take away the bucket and just let the water flow into the garden soil. Water tends to accumulate, but it tends to dissipate sideways so it doesn’t rise as quickly. And after you turn off the tap, the water level eventually subsides.

That’s what happens whenever it rains, except on a much larger scale, and in more complicated environmental terrains. When Ondoy struck last Saturday, the basement in my home quickly flooded, and stayed that way for several hours. It was cemented and had drainage holes too small to take up the sudden rush of water. Outside of the house, the garden was quickly submerged as well, but the water level quickly subsided, absorbed into the soil.

Record rainfalls

Let’s look now at Ondoy (international codename: Ketsana). Our weather people tell us that 455 mm of rain fell in 24 hours, exceeding the previous record of 334 mm on June 7, 1967.

The figures do sound staggering but it could be worse. The record for the most rainfall (at least what has been measured) occurred in 1966 on Reunion Island, near Madagascar, with 1,825 mm of rain in 24 hours. The record for the United States is Alvin, Texas, where in July 1979, there was 1,090 mm of rain in one day. Neighboring Hong Kong routinely gets heavy rain; in June 2008, during a typhoon, it got 145 mm of rain in one hour. There were flash floods, but the water subsided within a few hours.

The volume and speed of rainfall are important, but flooding is a function of where the rain falls. So let’s look at why Metro Manila is so vulnerable.

Metro Manila is mostly one large plain, attracting human settlers hundreds of years ago because of the Pasig River (the Tagalogs were, literally, the taga-ilog—of the river) and the ocean (Manila Bay). To the east, we have the Cordillera mountain range, which, when it was still thickly forested, provided Manila’s residents with a watershed area, the trees preventing waters from rushing down.

With the deforested mountain slopes, Metro Manila had a double whammy. We were getting rains directly from the skies, as well as rains rushing down from the Cordillera, without any trees and watershed areas working as natural dams.

Sadly, some of the towns to the east of Manila did act as “dams”: Pasig, Cainta, Rodriguez (formerly Montalban).

In Metro Manila itself, the old festering problem of clogged riverways contributed to the flooding. But we forget —and I want to acknowledge Ana Maria Gonzales for an insightful Internet article—that the lack of urban planning contributed as well. As the cliché goes, Metro Manila is one big concrete jungle. We cement everything, including our gardens, and all our subdivisions and buildings block the natural passageways for water, converting Metro Manila into one big potential reservoir for floodwaters.

There’s a people factor here. Metro Manila alone has 11 million people (according to the 2007 census) who need living and work space. The lack of urban planning has meant that these spaces impinge on all kinds of natural catchment and drainage areas that could prevent flooding.

I can hear people going, “Oh but look at crowded Hong Kong. They’ve managed.”

I agree, but let’s look at the finer details. Hong Kong is actually less densely populated than Metro Manila. It “only” has 7 million people in 1,054 sq km of land compared to Metro Manila’s 11 million in 636 sq km. Filipinos know Hong Kong mainly for its high-rise buildings and shopping malls and Disneyland, unaware that some 70 percent of Hong Kong’s land area consists of green space, including some splendid natural reserves. Typhoons that pass the Philippines usually move on to Hong Kong, but a combination of good urban planning and ecological awareness allows Hong Kong to take the storms with much less adverse impact.

We can take many more people (and many more millimeters of rain) in Metro Manila, if we linked urban planning to geography, to ecology, even to culture. I grow weary of commentaries that blame the poor, mainly “squatters” and their lack of garbage disposal. The middle- and high-income Manila residents should also think of our ecological “footprints” as well, meaning our impact on the environment. We generate much more garbage than the poor and are often much more cavalier about waste disposal. I’m not just talking about the McDonald’s styrofoams I see being thrown out from a BMW, but the tons of garbage that have to be sent out to landfills in the province of Rizal.

We shouldn’t just be thinking of Metro Manila. All over the world, cities are being caught off guard by huge flash floods. My friends in Jakarta texted right after Ondoy, totally empathizing because they’ve had similar problems of worsening floods. Right here at home, we should be asking why Iloilo and Cagayan de Oro cities, which didn’t have floods before, were inundated after fairly mild typhoons.

The physics lesson is simple: If rainwater can’t go downward into the earth, or can’t be dissipated sideways, it can only go up, up, up. We had floods because there was no way out for the water. And we have no way out except to think ecological.

I couldn’t help but think again of Lola O, Odette Alcantara, the environmentalist who passed away just the other week. Bear with me as I repeat Lola O’s straightforward way of describing ecological inter-relatedness: Langit, Araw, Hangin, Ako, Tubig. . .magkaugnay. The earth, the sun, the wind, I, the waters. . . we are all interconnected.

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E-mail: mtan@inquirer.com.ph