Report on Pinatubo (Philippines) — September 1993
Bulletin of the Global Volcanism Network, vol. 18, no. 9 (September 1993)
Managing Editor: Edward Venzke.
Pinatubo (Philippines) Lahars produced by typhoon rains cause additional damage
Please cite this report as:
Global Volcanism Program, 1993. Report on Pinatubo (Philippines) (Venzke, E., ed.). Bulletin of the Global Volcanism Network, 18:9. Smithsonian Institution. https://doi.org/10.5479/si.GVP.BGVN199309-273083.
15.13°N, 120.35°E; summit elev. 1486 m
All times are local (unless otherwise noted)
Lahar activity that began in late June following the onset of the rainy season continued in early October. Intense rainfall triggered lahars from the slopes of Mount Pinatubo on 4-6 October 1993. Flow sensors along the O'Donnell-Tarlac River (NE) detected lahar events at 0400-1800 on 4 October. Lahars along the Pasig-Potrero (SE) and Sacobia-Bamban (E) rivers occurred from about 2130 on 4 October to 0600 on 5 October. All three drainages had a second episode of lahar activity between 1630 on 5 October and 0830 on 6 October. The first episode was generated during westward passage of a typhoon over Luzon Island towards the South China Sea; the second was triggered when the same typhoon turned back towards NE Luzon. Another typhoon crossed Luzon from the South China Sea on 7 October and caused smaller lahar events in the late afternoon (1550-1920). On 4-7 October about 145 mm of rainfall was measured in the middle of the Sacobia Pyroclastic Fan, drained by the Pasig-Potrero and Sacobia-Bamban rivers.
Along the Pasig-Potrero River, an early morning debris flow on 5 October deposited 4-5 m of sediment near Mancatian (~8 km SW of Angeles, see 18:8 for map) and buried Mancatian Bridge 2 (400 m W of the main Pasig-Potrero channel) and tens of houses. An active flow that afternoon nearly overtopped the earthen dike along the channel. Erosion caused by debris flows near Mancatian on 6 October re-exposed the bridge. Deposition occurred farther downstream during the second episode. Lahars on 6 October overtopped the earthen dike about 6 km from the bridge between the villages of Mitla and Balas, depositing 3 m of debris in some barangays (communities) E and NE of Santa Rita (9-11 km downstream of Mancatian), and burying hundreds of houses. Lahar deposits downstream of the Pasig-Potrero River were 2.5-3.0 m thick in Mitla (in-channel), 0.5-3.0 m in San Basilio, 2.0-3.0 m in Balas and San Isidro, and 1.5 m thick in San Jose.
Lahars along the Sacobia-Bamban River on 4-6 October resulted in 5 m of deposition at Macapagal village, shifting the active channel 100 m N. Scouring near the original highway in Mabalacat during the early part of the 5 October lahar flows damaged portions of the Mabalacat dike. Flows that afternoon and evening resulted in 2.0-2.5 m of in-channel deposition and eroded the approach of the road connecting Mabalacat and Bamban. Deposition along the side of the dike lessened the freeboard to 1.6 m in the stretch about 600-700 m E of the hill where the dike was anchored. Field investigations on 7 October revealed 2-3 m of deposition at a barangay in Mabalacat. Muddy to hyperconcentrated stream flows reached as far as barangay San Roque, Magalang. Earlier events on 5 October deposited 30-40 cm of debris below the San Roque Bridge; vertical clearance is now only about 1 m. Sediments up to 10 mm thick were observed in the vicinity of barangay San Roque. Strong discharge of muddy stream flow about 200 m from the San Francisco Bridge outside the S dike has partially damaged some portions of the Concepcion-Magalang Road. No significant lahars were noted along the main channel at the San Francisco Bridge.
An aerial survey of the O'Donnell River on 13 October revealed fresh deposits down to Santa Lucia, 12 km from the headwaters. Significant deposition was observed along the stretch from Crow Valley (2 km from the headwaters) down to Santa Juliana, 8 km away.
The Zambales Lahar Scientific Monitoring Group (ZLSMG) noted that heavy runoff from Mapanuepe Lake (Santo Tomas-Marella drainage, SW) began to overtop the Santo Tomas protective dike in two places on the evening of 4-5 October. Near the abandoned Western Luzon Agricultural College, the dike that had been breached on 19 August and temporarily repaired was breached again to the W on the evening of 5 October. An E overtopping caused flooding of the highway along Magsaysay and in several villages, including Barangay Magsaysay and the town of Castillejos. As of 7 October, all the heavy Mapanuepe flow was passing through the gap and fanning through most of San Marcelino, causing flooding to depths of 2-4 m. Tens of thousands of people have been isolated by damage to the National Highway, and are facing continuing threats from lahars and flash flooding, particularly in Castillejos and San Marcelino. Vehicular traffic between Castillejos and San Marcelino was badly hampered by the flooding, and was impossible from San Marcelino to San Antonio and San Narciso. The dikes N and S of the Santo Tomas River are in very precarious shape, and floodwaters and dilute lahars from the Marella and Upper Santo Tomas River threaten the municipalities of San Felipe and San Narciso.
Massive debris flows into Barangay Santa Fe, triggered by heavy rains from the afternoon of 4 October through 6 October, caused three confirmed deaths, according to the ZLSMG. At least 11 others were missing and presumed dead as of 7 October, including nine people who were working in the middle of the lahar field on a private contract to manipulate the Marella and Mapanuepe drainages.
Geologic Background. Prior to 1991 Pinatubo volcano was a relatively unknown, heavily forested lava dome complex located 100 km NW of Manila with no records of historical eruptions. The 1991 eruption, one of the world's largest of the 20th century, ejected massive amounts of tephra and produced voluminous pyroclastic flows, forming a small, 2.5-km-wide summit caldera whose floor is now covered by a lake. Caldera formation lowered the height of the summit by more than 300 m. Although the eruption caused hundreds of fatalities and major damage with severe social and economic impact, successful monitoring efforts greatly reduced the number of fatalities. Widespread lahars that redistributed products of the 1991 eruption have continued to cause severe disruption. Previous major eruptive periods, interrupted by lengthy quiescent periods, have produced pyroclastic flows and lahars that were even more extensive than in 1991.
Information Contacts: Philippine Institute of Volcanology and Seismology, Philippines; R. Alonso, K. Rodolfo, and R. Tamayo, Zambales Lahar Scientific Monitoring Group.