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Mount Pinatubo Revisited: A Study of Lahar Erosion |
| At dawn on June 15, 1991, a cataclysmic eruption began with a tremendous explosion that destroyed ten deserted villages. This eruption deposited approximately 5 to 7 km3 of volcanic fragments in pyroclastic flows on the slopes of the volcano and over neighboring towns and agricultural areas. It is this material that continues to threaten structures and lives in the area, in the form of lahars (debris flows) during heavy rainstorms. The lahars from the Mount Pinatubo volcano have been particularly damaging to the surrounding area. This set of slides shows how the disaster that began at Pinatubo in 1991 continues to threaten the population in the area. |
Lahar Terraces | |
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Lahar Terraces Photo showing the recent lahar terraces (with people standing) and the older lahar terraces resembling horizontal layers which can be observed from the valley exposures. It indicates that older lahars were far more extensive than those from the 1991 eruption. The largest eruption is thought to have occurred in the period between 30,390 +or- 890 and 35,000 years B.P. This calderagenic eruption produced extensive pyroclastic flows occupying an area five times larger than the 1991 pyroclastic flow deposits. Photo credit: Cees J. van Western, International Institute for Aerospace Survey and Earth Sciences, The Netherlands; and, Arturo S. Daag, Philippine Institute for Volcanology and Seismology |
Lahar Terraces | |
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Lahar Terraces Ancient Pinatubo lahar outcrop. The recent pyroclastic flow deposit is capping this outcrop. The incision has been quite extensive, about 100 m deep from the original level. Photo credit: Cees J. van Western, International Institute for Aerospace Survey and Earth Sciences, The Netherlands; and, Arturo S. Daag, Philippine Institute for Volcanology and Seismology |
Lahar Terraces | |
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Lahar Terraces Older (at least 400 years B.P. and covered with vegetation) and recent (1991) pyroclastic flow deposits showing very similar features such as composition and color. Photo credit: Cees J. van Western, International Institute for Aerospace Survey and Earth Sciences, The Netherlands; and, Arturo S. Daag, Philippine Institute for Volcanology and Seismology |
Undisturbed flow front | |
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Undisturbed flow front Undisturbed flow front of the secondary pyroclastic flows taken at Pasig River. The photo was taken one day following a 1993 event. Flows such as this one were triggered by a large, secondary explosion which occurred on October 6, 1993. A large, secondary pyroclastic flow was directed towards the Pasig River. This event aused the capturing of the entire upper Sacobia catchment by the Pasig River, and caused a major shift in lahar delivery from the Sacobia to the Pasig River. Photo credit: Cees J. van Western, International Institute for Aerospace Survey and Earth Sciences, The Netherlands; and, Arturo S. Daag, Philippine Institute for Volcanology and Seismology |
Flow damage to vegetation | |
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Flow damage to vegetation Areas affected by the secondary pyroclastic flows in the Abacan River, April 4, 1992. Coconut trees were burned from the heat of pyroclastic flows. Eventually the pyroclastic flow-covered zone drained into the Sacobia River. The Abacan River ceased to cause any serious lahar threats to the City of Angeles since the lahars now followed the main courses of the Sacobia and Pasig Rivers. .Photo credit: Cees J. van Western, International Institute for Aerospace Survey and Earth Sciences, The Netherlands; and, Arturo S. Daag, Philippine Institute for Volcanology and Seismology |
Lahars in river channels | |
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Lahars in river channels Lahars overfilled the channel of Sacobia River by about 10 meters. Active lahars channel meanders on relatively flat terrain, endangering the community by lateral erosion or by lahar deposition. Since the pre-eruption river valleys were completely filled up, new streams developed on top of the pyroclastic flow level with channels that were partly different from the pre-eruption situation. The Sacobia watershed is situated on the eastern slope of the volcano and drains three major rivers: the Sacobia, the Pasig, and the Abacan. The total volume of pyroclastic flows deposited in 1991 in the Sacobia catchment was 1.78 km3. It covered an area of 24 km2 and reached 15 km downstream from the crater. Erosion rates were calculated to be between 219 and 136 million m3 per year. A comparison is made with erosion estimates from other sources and a relation is established between erosion rates and volumes of lahars for the coming years. Photo credit: Cees J. van Western, International Institute for Aerospace Survey and Earth Sciences, The Netherlands; and, Arturo S. Daag, Philippine Institute for Volcanology and Seismology |
Lahars in river channels | |
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Lahars in river channels Lahar deposits along the Mancatian Porac area delivered by Pasig lahars. Affected areas due to lahar avulsions (break outs) reached more than 2 km on both sides from the previous 200 m channel. Photo credit: Cees J. van Western, International Institute for Aerospace Survey and Earth Sciences, The Netherlands; and, Arturo S. Daag, Philippine Institute for Volcanology and Seismology |
Lahars in river channels | |
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Lahars in river channels The Sacobia lahar channel along the Bamban area. The main road, which is a vital link to the major cities in the north, has often become inaccessible due to the passage of lahars. Photo credit: Cees J. van Western, International Institute for Aerospace Survey and Earth Sciences, The Netherlands; and, Arturo S. Daag, Philippine Institute for Volcanology and Seismology |
Lahars in river channels | |
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Lahars in river channels Remnants of the recent pyroclastic flows left uneroded along the valley sides. Photo taken along the Sacobia channel. Photo credit: Cees J. van Western, International Institute for Aerospace Survey and Earth Sciences, The Netherlands; and, Arturo S. Daag, Philippine Institute for Volcanology and Seismology |
Lahars in river channels | |
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Lahars in river channels View of the Sacobia lahar channel. The channel was incised by about 120 meters in this portion. Photo credit: Cees J. van Western, International Institute for Aerospace Survey and Earth Sciences, The Netherlands; and, Arturo S. Daag, Philippine Institute for Volcanology and Seismology |
Lahars in river channels | |
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Lahars in river channels Deeply incised pyroclastic flow deposit. The photo was taken along the Sacobia River. Within a few months after the eruption, the main rivers had eroded impressive gorges, up to about 35 meters. Photo credit: Cees J. van Western, International Institute for Aerospace Survey and Earth Sciences, The Netherlands; and, Arturo S. Daag, Philippine Institute for Volcanology and Seismology |
Lahars in river channels | |
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Lahars in river channels Gravelly lahar terraces from debris flows. The original level of 1991 pyroclastic flow deposits is on top of the hill showing patches of unvegetated deposit. Photo credit: Cees J. van Western, International Institute for Aerospace Survey and Earth Sciences, The Netherlands; and, Arturo S. Daag, Philippine Institute for Volcanology and Seismology |
Restraining structures | |
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Restraining structures Dike constructed along the Dolores in Mabalacat in order to protect villages from lahar encroachment. Photo credit: Cees J. van Western, International Institute for Aerospace Survey and Earth Sciences, The Netherlands; and, Arturo S. Daag, Philippine Institute for Volcanology and Seismology |
Restraining structures | |
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Restraining structures Remnant of gabions (steel wire nets filled with boulders and used especially in constructing dams and foundations) constructed to stop/weaken lahar flows. Apparently the structures were damaged and eroded due to high density of flows. The lahars even overtopped the structure. Photo credit: Cees J. van Western, International Institute for Aerospace Survey and Earth Sciences, The Netherlands; and, Arturo S. Daag, Philippine Institute for Volcanology and Seismology |
Stream capture, Causes of Lahar Erosion | |
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Stream capture, Causes of Lahar Erosion Capture point of the Abacan River and Sacobia River. The channel has been incised by about 100 meters as a result of stream erosion after capture. During the 1991 eruption, pyroclastic flow deposits buried the drainage divide between these two rivers. Photo credit: Cees J. van Western, International Institute for Aerospace Survey and Earth Sciences, The Netherlands; and, Arturo S. Daag, Philippine Institute for Volcanology and Seismology |
Stream capture, Secondary explosions | |
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Stream capture, Secondary explosions Recent secondary pyroclastic flow deposit from secondary explosions which occurred during the April 4, 1992, event. Secondary explosions are phreatic (steam) explosions occurring in the pyroclastic flow deposits. These are produced when water comes into sudden contact with the hot in-situ pyroclastic flow deposits, producing expanding steam. Two dams were currently being built along this section which were buried by this event. (Note ash accumulation on plants in foreground.) This explosion occurred between Sacobia and Abacan. The secondary explosions at the so-called "Abacan gap" produced 2-3 km of channel-confined secondary pyroclastic flow deposits and resulted in the recapture of the upper catchment by the Sacobia River. This caused large destructive lahars in the downstream section of the river which buried several villages and destroyed hundreds of hectares of agricultural land. The secondary pyroclastic flows completely buried the recently built lahar-retaining structures constructed along the Sacobia River. Photo credit: Cees J. van Western, International Institute for Aerospace Survey and Earth Sciences, The Netherlands; and, Arturo S. Daag, Philippine Institute for Volcanology and Seismology |
Damage to Structures from Lahars | |
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Damage to Structures from Lahars Damaged houses along the Abacan River channel. Several houses were carried away by lahars in 1991 due to lateral erosion. During and immediately after the eruption, lahars resulting from the passing of the typhoon Yunya did not follow the pre-eruption river valley of Sacobia, but were drained through the Abacan River valley, causing destruction in the City of Angeles. Photo taken in 1992. Photo credit: Cees J. van Western, International Institute for Aerospace Survey and Earth Sciences, The Netherlands; and, Arturo S. Daag, Philippine Institute for Volcanology and Seismology |
Damage to Structures from Lahars | |
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Damage to Structures from Lahars Church in Porac buried by avulsing lahars from the Pasig River. Thickness of lahar deposit is approximately 4 meters. A large, secondary explosion occurred on October 6, 1993, and as a result, a large, secondary pyroclastic flow was directed towards the Pasig River. The event caused the capturing of the entire upper Sacobia catchment by the Pasig River, and caused a major shift in lahar delivery from the Sacobia to the Pasig River. Photo credit: Cees J. van Western, International Institute for Aerospace Survey and Earth Sciences, The Netherlands; and, Arturo S. Daag, Philippine Institute for Volcanology and Seismology |
Damage to Structures from Lahars | |
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Damage to Structures from Lahars The aftermath of the 1991 eruption at Clark Air Base. Prophetically, the last movie shown at the Clark Air Base Cinema prior to the major eruption of Mount Pinatubo was Last Days of Pompeii. Photo credit: Cees J. van Western, International Institute for Aerospace Survey and Earth Sciences, The Netherlands; and, Arturo S. Daag, Philippine Institute for Volcanology and Seismology |
Damage to Structures from Lahars | |
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Damage to Structures from Lahars Houses buried by lahars in Mancatian Porac. The lahar deposit is approximately 3 meters. Photo credit: Cees J. van Western, International Institute for Aerospace Survey and Earth Sciences, The Netherlands; and, Arturo S. Daag, Philippine Institute for Volcanology and Seismology |