Thursday, August 16, 2007

Urban flooding and the cement jungle

Metro Manila and many big urban sprawls around the world are more susceptible to large-scale flash flooding than those in their counterpart rural areas. And such flooding is highlighted in broadcast media like TV, radio, newspapers and the web. Well, majority of those media centers are based in the big cities, that’s why.

But there are certain characteristics of urban areas that make them more susceptible to wide flooding than those in rural areas, on per hectare or per square kilometer comparison. I see a number of reasons for this.

One, with a few exception, big cities and urban sprawls are located in the lowlands like mouth of seas, bays and big rivers. Hence, by the simple law of gravity, all rain water runoff will pass by these big cities until it gets out onto the seas.

Two, construction and repair of drainage and canals sometimes lag behind structural developments surrounding these waterways. For instance, space (like width and depth) for canals can be sacrificed when buildings and other structures are maximizing each square meter of land that it can develop.

And three, the cement jungle. Most rain water, especially if the rain is not strong, are supposed to seep down the soil and not drain onto canals, if the land surface is not cemented. But with all those houses and buildings, schools and malls, roads and sidewalks, parking lots and other open areas that are cemented, the volume of rainwater that go into canals and drainage becomes big. And the heavier the rains, some canals and drainage can overflow, resulting in ever bigger and deeper flood.

But growing urbanization and the desire of people to get rid of mud and slippery surface when these get wet makes the cement jungle expand ever wider.

One solution to this problem is to have more open spaces and non-cemented land in cities and communities. Examples are community parks and gardens, urban forest parks, and golf courses. Many environmentalists will not like or realize this, but the open space for grass and trees of those golf courses help absorb huge volume of rain water to seep into the soil and not fall into drainage and canals. Although it's true that during the dry season, these golf courses also use huge amount of water to irrigate those grasses to keep them healthy. But it's the trade off if we want to have more open spaces that absorb rain water, not to mention recharge the underground water table.

Another solution, a minor one, is that in constructing open parking spaces, the surface should not be cemented, and use crushed stones to prevent the surface from getting muddy while allowing the rainwater to seep below the ground.

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