Duterte administration

Heavy rainfall, topography deadly mix in Iligan flooding

KD Suarez
Experts say body count "too low," missing persons database urgently needed to help in identifying the dead, missing, and the living

Experts say body count “too low,” missing persons database urgently needed to help in identifying the dead, missing, and the living

WASHED OUT. Communities at the banks of the Mandulog River were washed out by the heavy flood. Photo courtesy of the UP National Institute of Geological Sciences.

WASHED OUT. Communities at the banks of the Mandulog River were washed out by the heavy flood. Photo courtesy of the UP National Institute of Geological Sciences.

MANILA, Philippines – Extreme rainfall, combined with topography and communities living on a major river’s flood plain, formed a deadly mix that devastated Iligan City at the height of tropical storm Sendong.

This was part of the findings of a team of geologists from the University of the Philippines (UP) National Institute of Geological Sciences (NIGS), part of the state university’s task force that responded to the disaster.

Dr Alfredo Mahar Lagmay said that the destruction in Iligan City is “worse than the devastation in Cagayan de Oro,” and comparable to the destruction in Guinsaugon, Southern Leyte in 2006.

Rainfall

The UP NIGS team, in its assessment, said most rainfall fell over northwest Mindanao, particularly over the Iligan area.

Based on data from the Tropical Rainfall Measurement Mission (TRMM) of the US National Aeronautics and Space Administration (NASA), rainfall totals in the area were between 300-400 mm from Dec 13 to 19, 2011, compared to 200-250 mm in other areas along the storm’s path.

Most of the rain fell between 11:30 pm and 1:30 am on Dec. 16 and 17, Lagmay explained.

PATH OF DESTRUCTION. The Mandulog River became the conduit of flood, debris, logs, and mud from the mountains to Iligan City. Photo courtesy of the UP National Institute of Geological Sciences.

PATH OF DESTRUCTION. The Mandulog River became the conduit of flood, debris, logs, and mud from the mountains to Iligan City. Photo courtesy of the UP National Institute of Geological Sciences.

Iligan City, meanwhile, sits at the mouth of the Mandulog River, a roughly 50 km-long river originating from the mountains of Lanao del Sur. The river has a roughly 700 square kilometer watershed.

Tsunami-like surge

Lagmay explained that based on their assessment of the area, during the storm, water from the mountains, along with logs, trees, mud, sand and other debris from the mountains, cascaded down the river “like a tsunami.”

He said the surge of water and debris from the mountains took not just the course of the river, but spilled over and took over the flood plain, where the city was situated.

The area near the mouth of Mandulog River in Iligan City before the flood, courtesy of Google Earth.

WASHED OUT. Communities at the banks of the Mandulog River were washed out by the heavy flood. Photo courtesy of the UP National Institute of Geological Sciences.

BEFORE AND AFTER. Comparative views of the area near the mouth of Mandulog River. Top: Before the flood, courtesy of Google Earth. Bottom: After the flood, courtesy of UP National Institute of Geological Sciences.

Flood plains, he explained, usually become part of the river system during extreme events such as Sendong. They estimate the lower 35 km of the river swelled its banks and overflowed to the flood plain.

The high volume, high velocity surge of water and debris, which Lagmay called a “hyperconcentrated flood,” then took everything in its path, causing the level of death and destruction in the city.

Aerial photographs taken by the team showed entire riverside communities washed out by the flooding.

He said logging and mining contributed to the flooding, but the extremely heavy rainfall in the area is the main culprit in the flooding. Based on their observations, cut logs are only a percentage of the trees washed out during the typhoon.

LOGS. Hundreds of logs washed out from the mountains now cover the seashore in parts of Iligan City. Photo courtesy of UP National Institute of Geological Sciences.

LOGS. Hundreds of logs washed out from the mountains now cover the seashore in parts of Iligan City. Photo courtesy of UP National Institute of Geological Sciences.

“Illegal logging aggravates floods, but in this case the primary reason for the floods was the extreme rainfall event brought about by tropical storm Sendong,” he said.

Developmental issue

The NIGS team also checked out a potential resettlement site in Sta. Elena in Iligan, but the city government is still looking for other sites, he said.

They have recommended for those areas severely devastated by the floods not to be inhabited anymore, because flooding can occur at the said areas again in the future.

The disaster also brings up the issue of development, Lagmay said, since most of the high-risk areas are also places where population and urbanization is increasing.

He also cited the need for more widespread information dissemination, especially among residents of disaster-prone areas.

He added, it is very important to review where the hazards are, locate the most vulnerable communities, and plan disaster response way in advance. (See related story)

The magnitude of destruction has also brought some problems in looking for other victims, said UP vice president for public affairs Prospero de Vera. He said the body count is “too low” and the Iligan City government is set for a more intensive search for victims.

Dr Corazon de Ungria, member of the forensics team, said it is an urgent need for a missing persons database, to help in identifying the dead, missing, and the living.

UP Padayon

The University of the Philippines’ Padayon Disaster Response Team, headed by UP vice president for public affairs Prof. Prospero de Vera, was formed in response to the disaster, to help in relief and rehabilitation efforts in the city as part of their institutional mandate.

UP President Alfredo Pascual said the task force was composed of four teams – tackling health, sanitation, forensics, and geohazards – and were present at the disaster area last Dec. 27-29, 2011.

He said they chose Iligan City because help is more needed there compared to Cagayan de Oro City, and the local government there sought for their assistance.

The geohazards team, headed by Lagmay, focused on gathering geohazard data from the devastated areas, and to assess the viability of resettlement and relocation sites, Pascual said. He added the geohazards team will help the Iligan local government in post-disaster planning.

The task force was also composed of 22 doctors, engineers, scientists and other experts from different units of the state university.

The university also has linked up with the Mindanao State University-Iligan Institute of Technology (MSU-IIT) to cooperate in data generation and collection for continued monitoring and study.

Meanwhile, the other team focused on medical care, water and sanitation, and helping identify the deceased victims.

Pascual added, the teams are scheduled to submit reports to the Iligan City government, and other stakeholders on Monday, Jan. 2, 2012. – Rappler.com