24 YEARS AFTER ORMOC: Move people out of fault lines, reforest ridges to prevent mudslides – expert

  • 3 November 2015, Tuesday

Source: InterAksyon
3 Nov 2015

MANILA - The only way to guarantee that the country would not suffer devastating flash floods and mudslides is “to move communities out of fault lines and reforest ridges to check soil erosion.”

This recommendation comes from Prof. Beatriz Jadina of the Visayas State University (VSU) in Baybay City, Leyte, who said that 6,000 people have been killed by landslides and flash floods in Leyte since 1991.

In her monograph “GIS-aided Biophysical Characterization of Southern Leyte Landscape in Relation to Landslide Occurrences” published by the Southeast Asian Regional Center for Graduate Study and Research in Agriculture (Searca) in 2013 just before the devastating magnitude-7.2 earthquake hit Bohol, Cebu, and the rest of Central Visayas and typhoon Yolanda battered Samar, Leyte, and several other provinces, Jadina already warned that communities should steer clear of the eight-kilometer danger zone along the 52 landslide points in Leyte.

Jadina said Leyte’s forest cover in 1954 was 53 percent but deforestation, logging, and mining had reduced the figure to only 14 percent in 2010.

With Southern Leyte experiencing continuous rainfall, runoff will be high and the soil will become soggy, loosening the rocks beneath and causing them to eventually roll down from mountainous areas stripped of vegetation.

On November 5, the country will remember the 1991 tragedy that killed 6,000 people in Ormoc City, many of whom never rose from their sleep as the ground shook and floodwaters swamped their houses.

Cascading murky water drowned residents days after heavy rainfall hit the area and many years after logging operations stripped the once-verdant island of much of its forest cover.

On February 17, 2006, a landslide buried Guinsaugon, St. Bernard, Southern Leyte, burying alive 1,126 people by official count.

Geologists said a cliff face of a ridge straddling the Philippine Fault collapsed in a combination rock slide-debris mass movement event, sending silt rolling down to Guinsaugon and burying houses, schools, and other structures along its path.

Jadina said like in the Ormoc tragedy, the area was hit by 10 days of successive rainfall and a minor earthquake.

She noted that in both tragedies, rainfall was estimated to be the heaviest for Ormoc and St. Bernard in 100 years.

“The highest number of landslide occurrences was observed in San Francisco, Liloan, and San Ricardo, Southern Leyte. Landslides generally occurred at the ridges near the fault line. It is strongly recommended that the unsuitable zones be planted with trees and forest cover should be restored, if possible to 1954 level to enhance stability,” Jadina said.

The highest frequency (21 percent) of landslides were observed in “Miocene-andesitic, basaltic, dacitic flows, and breccia geologic formation” that is associated with the Leyte segment of the Philippine Fault.

“It occurred at angles greater than 18° and was at the highest frequency (54 percent) at angles greater than 50° in both concave and convex slope curvatures,” Jadina explained.

Forest cover in Southern Leyte decreased from 53 percent in 1954 to 38 percent in 1992, and slid to a precipitously low 14 percent in 2010, indicating the severe loss of forest trees that can hold huge volumes of rainwater and prevent soil erosion, according to the Searca study.

Current vegetation is dominated by cultivated crops, coconut, and abaca, none of which can store water in bigger quantities.

“The province is mapped as slightly suitable (36 percent) to moderately suitable (51 percent), to suitable (13 percent) for coconut and abaca production. Slightly suitable areas are those found at higher elevation (more than 600 meters above sea level) and steep slopes (more than 50°), while those suitable are found at lower elevations and gentle slopes,” Jadina noted.