Immediate and Long-Term Hazards from Lahars and Excess Sedimentation in Rivers Draining Mt
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IMMEDIATE AND LONG-TERM HAZARDS FROM LAHARS AND EXCESS SEDIMENTATION IN RIVERS DRAINING MT. PINATUBO, PHILIPPINES By Thomas C. Pierson and Richard J. Janda, U.S. Geological Survey, and Jesse V. Umbal and Arturo S. Daag, Philippine Institute of Volcanology and Seismology U.S. GEOLOGICAL SURVEY Water-Resources Investigations Report 92-4039 Prepared in cooperation with the PHILIPPINE INSTITUTE OF VOLCANOLOGY AND SEISMOLOGY and the U.S. AGENCY FOR INTERNATIONAL DEVELOPMENT Vancouver, Washington 1992 U.S. DEPARTMENT OF THE INTERIOR MANUEL LUJAN JR., Secretaiy U.S. GEOLOGICAL SURVEY DALLAS L. PECK, Director For additional information Copies of this report can be write to: purchased from: Thomas C. Pierson U.S. Geological Survey David A. Johnston Cascades Books and Open-File Reports Volcano Observatory Federal Center, Box 25425 5400 MacArthur Boulevard Denver, CO 80225, USA Vancouver, WA 98661, USA Dr. Raymundo Punongbayan, Director Philippine Institute of Volcanology and Seismology Hizon Building, 6th Floor 29 Quezon Ave. Quezon City, Philippines ii CONTENTS Page Abstract .................................................................. 1 Introduction ................................................................ 2 Purpose and scope ....................................................... 2 Physical setting .......................................................... 3 Climate ............................................................ 3 Topography and surface drainage ......................................... 3 Geology ............................................................ 5 Methods of investigation and hazard-map preparation .............................. 6 Data collection ....................................................... 6 The hazard map ...................................................... 6 Zones at high and moderate potential hazard ............................. 6 Zones at low potential hazard ........................................ 7 Geographic names .................................................... 7 Acknowledgments ........................................................ 7 The 1991 eruptions ........................................................... 7 Chronology of events ...................................................... 7 Types and extents of eruptive deposits ......................................... 8 Ongoing phreatic activity .................................................. 10 Lahars associated with 1991 volcanic activity ....................................... 10 Lahar frequency ........................................................ 11 Lahar magnitude ........................................................ 11 Lahar source areas ...................................................... 12 Channel response and lahar deposition ........................................ 12 Conveyance reaches between pyroclastic valley fills and alluvial fans ............... 12 Alluvial fan reaches .................................................. 13 Predicting future lahar hazards ................................................. 15 Predicting future sediment delivery to low-lying areas ............................. 15 Persistence of excess sediment problems ....................................... 16 Future sediment accumulation volumes at Mt. Pinatubo ........................... 18 First method ....................................................... 18 Second method ..................................................... 21 Considerations for hazard mitigation ............................................. 21 Types of hazards from lahars ............................................... 21 Strategies for mitigating lahar hazards ........................................ 24 Engineering approaches to mitigation ..................................... 25 Sediment-retention basins .......................................... 25 Lahar channelization .............................................. 26 Special considerations for individual drainage systems ............................. 26 North- and east-side drainages .......................................... 26 Tarlac River .................................................... 26 Bamban River ................................................... 27 Abacan River ................................................... 28 Pasig-Potrero River ............................................... 28 Gumain River ................................................... 29 West-side drainages .................................................. 30 Santo Tomas River ............................................... 30 Maloma River ................................................... 31 Bucao River .................................................... 31 Conclusions ............................................................... 32 References cited ............................................................ 33 m ILLUSTRATIONS Page Plate 1. Potential hazard zones from lahars and excess sedimentation around Mt. Pinatubo ...................................... In pocket Figure 1. Map showing pre-eruption drainage pattern at Mt. Pinatubo ................ 4 2. Map showing location of summit caldera and deposits of pyroclastic flows, lahars, and airfall tephra ............................. 9 3. Sequential cross sections of selected channels draining Mt. Pinatubo ......... 14 4. Annual rates of sediment accumulation in lowland areas around Mt. Galunggung and Mount St. Helens volcanoes after eruptions ........... 16 5. Exponential decay curves for sediment accumulation rates at Mt. Galunggung, Mount St. Helens, and Mt. Pinatubo (estimated). ......... 17 6. Schematic diagram of transportation and distribution of pyroclastic material produced by the June 1991 eruptions of Mt. Pinatubo ............. 23 TABLES Page Table 1. Estimated volumes of pyroclastic-flow deposits in the major drainage systems at Mt. Pinatubo .................................. 19 2. Estimated range (by method 1) in eroded sediment volumes to be delivered to lowlands at the base of Mt. Pinatubo ................................ 20 3. Estimated average annual volumes of sediment to be eroded from all slopes of Mt. Pinatubo over the 10 years after the June, 1991 eruptions ............................................ 22 4. Estimated range (by method 2) in eroded sediment volumes to be delivered to lowlands and the base of Mt. Pinatubo .............................. 22 IV CONVERSION FACTORS Multiply To obtain meter (m) 3.281 foot (ft) kilometer (km) 0.6214 mile (mi) centimeter (cm) 0.3937 inch (in) cubic meter (m3) 1.308 cubic yard (yd3) square kilometer (km2) 0.3861 square mile (mi2) cubic meter/second (m3/s) 35.31 cubic foot/second (cfs) gram/cubic centimeter 62.43 pound/cubic foot (g/cm3) (Ib/ft3) megagram (Mg) 2,205 pound (Ib) Temperature in degrees Fahrenheit (°F) as follows: °F = 1.8 x °C + 32 IMMEDIATE AND LONG-TERM HAZARDS FROM LAHARS AND EXCESS SEDIMENTATION IN RIVERS DRAINING MT. PINATUBO, PHILIPPINES By Thomas C. Pierson and Richard J. Janda, U.S. Geological Survey and Jesse V. Umbal and Arturo S. Daag, Philippine Institute of Volcanology and Seismology ABSTRACT The June 15, 1991 eruption of Mt. Pinatubo, one of the largest eruptions in the world this century, deposited 5-7 cubic kilometers of pumiceous pyroclastic-flow deposits and about 0.2 cubic kilometers of tephra-fall deposits on the slopes of the volcano. Decreases in infiltration capacity and evapotranspiration caused by the eruption and its deposits have increased the rate and magnitude of surface runoff production, and the new deposits provide a vast supply of highly credible sediment. Lahars (debris-flow and hyperconcentrated-flow flood surges) generated by rain falling on credible pyroclastic-flow deposits and by lake outbreaks are a serious hazard to areas surrounding the volcano as far out as 50-60 kilometers. Proximal zones of potential hazard can be affected by direct impact of lahars, bank collapse from lateral channel migration, backflooding due to blockage of tributary valleys, and inundation by lahar deposits or fluvial sediments. In addition to these effects, distal zones may experience siltation of canals, drains, fish ponds, and coastal zones. Most bridges in the potential hazard zones have already been destroyed, and many roads have been directly inundated or backflooded. It is likely that 40 to 50 percent of the newly deposited pyroclastic materials, together with a volume of remobilized older deposits (equivalent to roughly 10 percent of the new material) will be eroded before sediment-delivery rates return to near normal. This translates to a total of 1.2 to 3.6 billion cubic meters of sediment that could wash down onto low-lying areas within the next decade, the expected period of abnormally high sediment yields. The sediment-delivery rate is likely to be highest in 1991 or 1992 and then decrease exponentially. About three quarters of this total will wash down rivers on the west side of the volcano, where rainfall is heavier. Some of this sediment will reach the sea, but much will be deposited in wide alluvial valleys. About one quarter of the total will wash down east-side rivers and be deposited on broad alluvial fans. As of mid-September 1991, about 150 square kilometers of land had been buried under several meters of coarse deposits, much of the deposits covering prime agricultural land. A new lahar hazard map of the study area shows (1) areas having had active lahar erosion or deposition as of September 1, 1991, (2) areas of high hazard from future lahar