What hit Barangay Andap, New Bataan, Compostela Valley (Initial Assessment)

AMF Lagmay, RN Eco, J Alconis and B Salvio

National Institute of Geological Sciences, University of the Philippines

On 4 December 2012, typhoon Pablo (international name Bopha) made landfall over the Philippine island of Mindanao, a year after tropical storm Sendong (International name Washi) devastated the same island. Hardest hit by Sendong were the cities of Iligan and Cagayan de Oro where 1268 lives and 1.6 billion pesos worth of property were lost (NDRRMC, 2011) .  Classified as a Category 5 typhoon by US meteorological experts, Pablo packed winds with average speeds of 185 km/hr and gusts reaching 210 km/hour. Typhoon Pablo's eye crossed Mindanao through Davao Oriental, Compostela Valley, Agusan del Sur, Bukidnon, and Misamis Oriental. It continued west-northwest to Negros Oriental, then to Sulu Sea, crossing Palawan before reaching the West Philippine Sea where it is now as of the time of this writing.

More than 44,437 families were evacuated in Mindanao and in the Visayas Region, Central Philippines.  Despite advanced warnings and preparations by communities against the tropical cyclone, Pablo exacted heavy damage in Mindanao with 459 deaths, 532 missing and 4.17 billion pesos worth of damage to infrastructure and agriculture. The region with the most number of recorded deaths is Compostela Valley, Mindanao, with 236 fatalities. Davao Oriental follows with 180 dead, recovered mostly from the coastal town of Baganga, where typhoon Pablo made landfall. The municipality of New Bataan in Compostela Valley,  where Barangay Andap is situated, has 115 casualties.

Barangay Andap is gone, buried in a pile of rocks the size of boulders (Figure 1).  The village was overwhelmed by a rapid downward moving mass of material that was fluid as wet cement and composed of boulders, gravel and sand.  In its wake, it left a pile of rubble called a debris flow deposit.

Figure 1. What was once the center of Barangay Andap in New Bataan, Compostela Valley province is now a new riverbed with an estimated width of nearly a kilometer stretching up to eight kilometers down to the town proper. Typhoon Pablo brought flashflood in this town on Tuesday, leaving over a hundred persons killed and hundreds still missing. Mindanews Photo by Ruby Thursday.

When debris flows happen in volcanic slopes, they are called lahars. In 2006, Typhoon Durian (international code name Reming) generated lahars (Paguican et al., 2009) that left deposits similar to those that now cover Barangay Andap. Within and on top of the deposit, there lay boulders strewn in the debris field (Figure 2).

Figure 2. Village along the footslopes of Mayon Volcano covered by lahar deposits generated by rains of typhoon Durian.

In December 1999, heavy rains spawned floods and landslides in Cordillera de la Costa, Vargas, Venezuela, a densely populated city located on an alluvial fan at the mouth of a mountain drainage, near the base of steeply-sided mountains. Debris flows overwhelmed Cordillera de la Costa, Vargas, and destroyed 700 apartment buildings and 800 houses (Figure 3).  A total of 19,000 people died from the catastrophe (Larsen et al 2001).

Figure 3. Debris flow damage in Cordillera de la Costa, Vargas, Venezuela Source: http://www.webpages.uidaho.edu/~simkat/cors220_files/lecture12.html

According to the Geology dictionary, the definition of debris flows is the rapid, downward mass movement of particles coarser than sand, often including boulders one meter or more in diameter, at a rate ranging from 2 to 40 kilometers per hour. Debris flows occur along fairly steep slopes. It can also refer to the material that descends in such a flow.

A debris flow would look something like what is shown in the diagram (Figure 4).

Figure 4. Debris flow diagram (Mathias Jacob, 2005)

Many videos of actual debris flows are found on the internet. Below is a video of one such event in Clear Creek Town, Colorado, captured and uploaded on youtube.

http://www.youtube.com/watch?v=8mKC3eID074\

Heavy rains brought by typhoon Pablo spawned floods that carried eroded gravel, sand and boulders from the mountains. Like in Cordillera de la Costa, Barangay Andap is situated at the mouth of a mountain drainage network at the base of steeply-sided slopes.  It was nested on an alluvial fan, normally found at the base of mountains where water drains. As what happened in the catastrophic debris flow event in Cordillera de la Costa, Barangay Andap was situated along the path of a debris flow spawned by intense rainfall during the onslaught of typhoon Pablo.

Figure 5. Outlined in red is the Barangay boundary of Andap Source: philgis.org barangay boundary map.

Figure 6. Location of Andap according to MGB maps and Google Earth photo pins.

Figure 7. Location of Andap and low resolution flood hazard map of Project NOAH. Misfit of the hazard map is a rendering issue.

References:

http://ndrrmc.gov.ph/attachments/article/835/UPDATE

http://www.alcwin.org/Dictionary_Of_Geology_Description-110-D.htm

Mathias Jacob, 2005 A size classification for debris flows. Engineering Geology, Volume 79, Issues 3–4, 11 July 2005, Pages 151–161

Larsen, M.C., Vásquez Conde, M.T., and Clark, R.A., 2001, Flash-flood related hazards: landslides, with examples from the December, 1999 disaster in Venezuela in E. Gruntfest and J. Handmer, eds., Coping with Flash floods, Kluwer Academic Publishers, p. 259-275.

E. M. R. Paguican, A.M.F. Lagmay, K. S. Rodolfo, R. S. Rodolfo, A.M.P. Tengonciang, M. R. Lapus, E. G. Baliatan, E. C. Obille, Jr. (2009) Extreme rainfall-induced lahars and dike breaching, 30 November 2006, Mayon Volcano, Philippines. Bulletin of Volcanology Volume 71, Number 8 845-857