Trees: Protection from flash floods, landslides

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For thousands of families from Metro Manila, Calabarzon to Central Luzon, Ilocos region and up to the Cordillera Administrative Region, many may be familiar with the Biblical Noah’s Ark story.

But millions more have or are getting an experience of what it is.

To meteorologists at the Philippine Atmospheric, Geophysical and Astronomical Services Administration (Pagasa), the government’s weather agency, this is all part of climate change where a phenomenal rainfall volume in 2009’s Typhoon Ondoy (international code name Ketsana) was dropped in just one day.

To environmentalists, the floods are worsening because the people have destroyed nature’s first defense against it—the trees.

“If you have trees within and around  communities, you will not worry too much about flooding as those who have treeless areas,” ecologists at the Norwegian University of Life Sciences (NULS) and the Center for International Forestry Research (Cifor) would say.

Erratic rainfall could worsen with soil erosion and landslides, the two international environmental agencies said.

Trees prevent floods, landslides

Trees contribute immensely in preserving soil. Far reaching roots hold soil in place and fight erosion, NULS-Cifor said, adding that trees absorb and store rainwater, which reduce runoff and sediment deposit after storms.

They help the groundwater supply recharge, prevent the transport of chemicals into streams and prevent flooding.

Trees help reduce erosion by increasing filtration, holding soil particles together, and slowing wind and water flow, the research institutions elaborated.

Each mature tree, about 10 years old, absorbs in its trunk and branches some 1,500 liters to 2,000 liters of water which help keep the water table up. The trees’ roots suck water deep from under the ground to as low as 200 feet.

They hold the soil together so that erosion is prevented. They absorb rain during rainy days such that water run off is prevented from going down low-lying areas like residential places, streets and markets, NULS-Cifor ecologists explained.

No trees, no protection

The environmental group Cordillera Ecological Center (CEC) in Baguio City explained: “When there are no trees to protect the soil, gully erosion occurs because the shape of the terrain concentrates water flows over or through the land; and the soil is not cohesive enough to prevent soil loss.”

This is what happened in Rizal province, where years of quarrying, especially in Rodriguez and Montalban has resulted in gullies that brought down rainwater inundating towns up to Marikina river.

Gully erosion is best controlled by planting trees on higher ground around the gully area, CEC said.

Citing its many years of forestation experience, CEC said wherever possible, trees should be planted in conjunction with deep-rooted and fast-growing grasses, which will also use water, increase infiltration and control flow.

While slowing of water flow may not prevent gully erosion completely, it can dramatically reduce the rate of erosion.

Diversion banks can take water elsewhere, but quite often this shifts the problem to a new site. Building a dam is an effective but expensive solution, CEC bared.

Trees help hold the soil together

Although not as important as reducing flow, trees planted in and around the gully may help hold the soil together. The closer to the gully, the wetter the soil will be, and may need to be planted with different species.

To avoid scouring, plant small shrubs with flexible stems within the gully rather than trees, CEC explained, adding that it had a five-year tree-planting experience in stopping gully erosion in the town of Mankayan caused by the Lepanto mining activities in Benguet.

CEC environmental specialist Dobbels Wallang said sheet and rill erosion can also happen if a property is treeless. To prevent this from happening, trees should be planted across the slope. The distance between the rows of trees planted affects the slope’s length which, in turn, affects the rate of soil loss, he explained.

Trees and shrubs can also be planted in strips along contours to check run offs. Banks can be used to decrease slope length, but trees are much less susceptible to damage by floods than conventional banks, and have the added bonus of increasing the rate of water infiltration, he added.

Making the Agno River as an example, where dozens of Pangasinan and La Union barangays were flooded by the overflowing river, Wallang said trees can also be planted on river banks to provide bank stabilization.

While trees obviously cannot change slope steepness, they will slow down the rate of water flow when planted across the slope. Trees are most effective on steep slopes when used in conjunction with dams, contour and diversion banks for changing slope angles and increasing infiltration, he said.

The land can save communities

CEC Foresters Darwin Pandosen and George Facsoy, who have led CEC reforestation efforts in Leyte and Davao del Sur, said to fully understand the problem of flooding and landslides, one must understand the land.

“The land is like a sponge, it can only take so much, and without trees holding soil particles together, it is bound to release what it has absorbed,” they said.

And when water is released, it follows its own level as a law of physics. Therefore, trees at the higher elevation of every communities should be preserved, should be planted and low-lying canals and rivers banks’ hedged with trees, they recommended.

The foresters frown on the existing procedure of deepening canals. Instead, they said, roads and pathways should be elevated and hedged with trees.

During dry season, they said, grass, sedges, bushes and shrubs must not be cut because these act as soil cover, preventing topsoil erosion from torrential rains come rainy season.

In the Philippines trees are being felled at a rate of 300 hectares to 600 hectares annually, a cut  rate so fast, many of the trees have never been seen by the existing generation.

The country, like most developing nations, is vulnerable to global warming. The problem is made worse by the widespread destruction of forests.

The trees, once a symbol of the Philippines’s rich and proud heritage, tell its people of what they are—strong, enduring, hardy, magnificent and symbolize all the people of  the country.

All of a sudden, they are being destroyed along with generations yet to come.

The enemy: the people.

Bengwayan has a masters degree and PhD in Development Studies and Environmental Resource Management from University College Dublin, Ireland, as a European Union fellow. He is currently a fellow of Echoing Green Foundation in New York.

Image credits: Nonie Reyes

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