Flood adaptation

SKETCHES - Ana Marie Pamintuan (The Philippine Star) - November 27, 2020 - 12:00am

Looking at paintings and illustrations from the Spanish colonial period, you can see that flooding has been plaguing residents in what we now know as Metro Manila and other parts of Luzon even back in the 19th century, and surely long before that.

Maybe the monsoons and typhoons simply bring too much rain in this part of the planet, swelling mountain streams, rivers and lakes, dislodging soil and volcanic mud from the slopes, and sending them rampaging down and smashing into dwellings along their path.

But even after seeing how nature works, and regularly suffering from nature’s wrath, our ancestors continued to build their communities close to large rivers and lakes, on mountain slopes near streams, at the feet of active volcanoes, in fertile valleys surrounded by mudslide-prone mountains.

We adapt to our natural environment, risking natural calamities. We don’t leave because we are threatened by regular typhoons, floods, earthquakes, volcanic eruptions. If people did, where would Japan be?

With the information now available, it’s of course possible to ban new residential, commercial and industrial development in high-risk areas and strictly enforce easement rules in coastal communities as well as around rivers and lakes.

For properties that have sat on danger zones for decades, however, it’s clearly a challenge to get the occupants to move to safer areas. The typical reaction of those who saw their homes damaged or destroyed in the recent floods and typhoons was: it’s home, they survived and they would rebuild.

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Adaptation is the key, and increasing disaster mitigation capabilities. Earthquakes? The Japanese of yore built houses made of light materials that could be easily rebuilt (but after devastating fires during earthquakes, they began using sturdier materials). In the Netherlands, where about one-fourth of the land is below sea level, people have been reclaiming land and keeping out the North Sea through a sophisticated network of dikes, dams, storm surge barriers, sluices, locks and levees.

Powerful water pumping stations are used for subways and tunnels that pass through seas. Such pumping systems are quite costly to procure and maintain.

Structural adaptation measures are expensive, and governments need to discuss whether their economies can afford it.

Landscape architect and urban development planner Paulo Alcazaren, who was our guest on OneNews’ “The Chiefs” this week, said countries such as Singapore have started using porous (also called pervious or permeable) concrete for certain roads and curbs, and building perforated parking lots where rainwater can drain to the ground or be directed, using pipes, to landscaped areas or water detention basins.

Paulo describes such materials as “a bit more expensive” than the regular ones (up to three times more, according to some websites).

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Civil engineers have built bioretention basins for flood control. A detention basin temporarily stores stormwater runoff and then gradually discharges it. A retention basin has a permanent water pool, installed usually at the outfall of a residential or business district, from where pollutants can be filtered out.

Paulo points out that flooding in Marikina, Cagayan Valley and several other areas comes from above, not below – from the sky to the mountains and the waterways on the slopes, and on down to the rivers and lakes on the ground.

So it makes sense to minimize the water that comes down from the mountains. Adaptation includes avoiding or minimizing activities that heighten the risk of disaster occurrence. Illegal logging and destructive agricultural activities such as slash-and-burn farming on the slopes must be prevented. Many people depend on the forest for their livelihood, so sustainable agroforestry must be designed into reforestation programs.

As for mining and quarrying, which look awful and obviously destroy watersheds while the activities are ongoing, the country will have to decide if it wants such activities, and if we can afford to rely completely on imports for our raw material requirements. Some mining companies are touting post-operation restoration of areas stripped of minerals, with former sites covered with rich loam and turned into farms or ecoparks. They will need models to present to skeptics.

Where nature has given way to property development, and people refuse to relocate despite awareness of the risks and actual experience of the disasters, there are engineering interventions for adaptation and flooding mitigation.

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Paulo and several other uban planners point out that with the massive amounts of rainfall dumped all over the country every year, water harvesting should be boosted. Water detention basins can be built, from where the water can be redirected for gardening or farm irrigation, for example.

Houses or buildings can have roof catchment cisterns. The water can then be recycled for use in gardens, and for cleaning driveways and flushing toilets.

Out of necessity, water harvesting was more extensive in our country before piped water was introduced. Older houses have cisterns and deep wells.

For perennially flood-prone communities that residents refuse to abandon, Paulo is suggesting, seriously, the construction of houses and buildings on elevated platforms or reinforced stilts.

The German city of Hamburg in fact has an area regularly inundated by flooding from the Elbe. The mighty river gave rise to Hamburg but has also been the city’s bane. After an apocalyptic flood in February 1962 that submerged the Wilhelmsburg borough and killed 315 people in Hamburg alone, German engineers built a flood control system whose underlying principle is managing the surge of stormwater and redirecting it so it doesn’t paralyze the area.

Some years ago when I visited Hamburg I was amazed by the elevated pedestrian walkways and “floatable” buildings built on pontoons or piles, with windows on the lower floors that can be quickly shut and rendered watertight. Even the park where concerts can be held is amphibious.

We’re still a long way from amphibious multistory buildings and parks, but at least we’re working on bioretention basins.

“It’s a hard road to travel, but we’re taking the steps,” Paulo Alcazaren told us.

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