Wednesday, May 24, 2006

On flash floods

There are many types of floods, but offhand, I can think only 3 of them: still flood, flash flood, and lahar. The first is what you see in low-lying areas (like Espana ave. in Manila); the second are those fast, cascading strong waters on rivers and canals; the third is sand + water (various mixtures, could be up to 80% of sand, only 20% water), prominent on areas that experience volcanic eruption like Mt. Pinatubo.

Mt. Pinatubo's sand deposit has hardened already, so we do not see lahar flows anymore in central luzon during the rainy season. Still flood could pester us because they occur on areas where there is often high population density, so the people can hardly move and small vehicles cannot pass through them. But still flood don't bring down with them your house or your car; it's just that with heavy downpour, the drainage cannot expurgate the rain water fast enough onto the canals, so there is temporary "lake" that is being created.

What people cringe about are flash floods. You see them on tv -- strong waters cascading on rivers and bringing down with them logs and plants, houses and small structures, farm animals, even people and vehicles. As strong waters rampage down, they also redraw river banks as soft soil on river banks fall down, so the river becomes wider.

In my treehouse in the farm, whenever I get trapped there by strong rains, I just watch a nearby creek suddenly turn into a wild river. I see the flood water rising inch by inch, minutes by minutes. Occassionally, we (along with our farm caretaker) could hear some loud "bangs" from there -- they turn out to be logs and/or uprooted roots of trees being transported down by the strong waters, and when they hit some big solid rocks, that's the loud bang or noise created. Since we watch on the 2nd floor, our view of the cascading river is quite good.

And since our farm is somewhere midland, I can only imagine the damage to crops, even houses, in the lowlands, before the flash flood would exit to and be silenced by a bigger body of water, the sea.

About half to one hour after the strong downpour has stopped, the wild river goes back to an ordinary creek. The muddy and wavy waters return to colorless water. Yeah, the flash floods have wiped out most of the impurities in the riverbeds and riverbanks. The crawling vines, the dark green algae, the decaying fallen leaves, and so on. And then you realize that flash floods have some positive results after all. And those huts and weak houses that the flash floods have knocked down is nature's way of culling the weak and non-durable. Also nature's way of saying that "riverbanks are for plants and water, not for houses". Nature makes this message clearer than "government warnings".

So, if typhoons' strong winds wipe off even temporarily the heavy air pollution deposit in urban areas, flash floods also wipe off even temporarily the obstructions and impurities in riverbeds and riverbanks. And if you gather big stones for whatever construction needs -- like our various stone terraces construction in hilly and sloping parts of the farm -- the flash floods expose the big stones as they have scraped off the small stones and sand. So, gathering the big stones is easier.

But overall, flash floods are more harmful than beneficial. That is why rainwaters should seep in the soil as much as possible so that flash floods will be minimized. From my experience, building lots of stone terraces on sloping lands really help a lot in controlling and minimizing flash floods. You put lots of organic matter (dried and fallen leaves, plants, tree branches, etc.) as filling materials, you cover them with soil. When strong rains fall, the terraced soil act like huge sponges gulping in as much water as they can, and release water down as little as possible. And the growth of plants and trees on those terraced soil are also fantastic. They have lots of organic matter and trapped water deposit in their roots.

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