Monday, February 20, 2006

Danger signs of landslides

By Tonette Orejas, Inquirer

WATCHING out for signs of a landslide can save lives.

Standing objects like trees, posts, poles and fences that tilt give hints of ground movement and the possible loosening of soil and rocks, according to the Philippine Institute of Volcanology and Seismology (Phivolcs).

Cracks on mountain slopes and loose deposits indicate that these are not stable areas and are prone to bigger landslides, Phivolcs chief Renato Solidum said.

Solidum asked people to stay away from steep slopes traversed by streams and creeks that can funnel the soil and other debris downstream.

He also asked people to look out for road cuts, which indicate that the soil underneath is weak.

In the case of the killer landslides in St. Bernard town, Southern Leyte province, Solidum said heavy rainfall in the area -- 480 millimeters from Feb. 1 to 16 compared with the monthly average of 137 mm -- most likely triggered the disaster.

"There had been so much rain in the area and water added to the weight of the soil," Solidum said.

The presence in the area of a fault zone that generates earthquakes was a contributory factor, he said.

A volcano, Mt. Kabalian, lies in St. Bernard but this has been inactive and the tremors there on Friday could not have been caused by volcanic activity, according to Solidum.

Install rain gauges

Geologist Arnulfo Cabantog of the Mines and Geosciences Bureau of the department of Environment and Natural Resources earlier asked landslide-prone upland villages in the Central Luzon areas of the Sierra Madre and Zambales mountain ranges to install improvised rain gauges because very few of these instruments had been installed by the weather bureau.

Rain gauges are used to measure the amount of rainfall in a particular area. The level of water trapped in a container at certain hours can help upland villagers know if heavy rains had fallen in the area.

Cabantog said the measurements were bases to guard against landslides and flash floods.

The provinces of Aurora, Nueva Ecija, Bulacan and Tarlac in Central Luzon, and the mountainous villages in Quezon were hit by landslides and floods in November and December 2004. The disaster left more than 1,000 dead and about 5,000 families homeless.

PVC pipe

The improvised rain gauges, according to Cabantog, could use PVC pipes that are four inches in diameter. The top should be open to catch the rainfall while the bottom should be well sealed to prevent dripping.

This, he said, should be placed in an open space in a common area such as a school, barangay hall or chapel.

A ruler should come in handy to measure the water level. When the level reaches 100 mm, or 10 centimeters in less than five hours, Cabantog said this should be taken as a sign by residents to evacuate to safer ground.

Village officials should designate a person to measure the rainfall regularly and a team to share the information with the other villagers.

The Philippine Atmospheric, Geophysical and Astronomical Services Administration considers rains in an area heavy if gauges measure 7.5 mm an hour. In Japan, Cabantog said, 200 mm of rainfall was the standard warning for possible landslides.

Residents must also consider the number of days rains have drenched an area. If the mountain soil was saturated for days, as happened during the four successive typhoons in 2004, another rainfall could trigger a landslide.

Residents should also watch out for damming at the peaks of hills and mountains. A sign to watch out for is when there's little water flowing in a river even when it is raining, Cabantog said.

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