Modelling the Erosion of Pyroclastic Flow Deposits and the Occurrences of Lahars at Mt. Pinatubo, Philippines Arturo Santos Daag ITC Dissertation number 104 This research was carried out at the International Institute for Geo- information Science and Earth Observation (ITC), P.O. Box, 7500 AA, Enschede, The Netherlands, http://www.itc.nl CIP-Data Koninklijke Bibliotheek, Den Haag © Daag, Arturo Santos ISBN 90-6164-218-3 Modelling the Erosion of Pyroclastic Flow Deposits and the Occurrences of Lahars at Mt. Pinatubo, Philippines Modellering van Erosie in Pyroclastic Flow Afzettingen en het Voorkomen van Lahars in Mount Pinatubo, Filippijnen. (Met een samenvatting in het Nederlands) PROEFSCHRIFT ter verkrijging van de graad van doctor aan de Universiteit Utrecht op gezag van de Rector Magnificus, Prof. Dr. W.H. Gispen ingevolge het besluit van het College voor Promoties in het openbaar te verdedigen op donderdag 13 November 2003 des middags te 12.45 door Arturo Santos Daag Geboren op 14 Augustus 1962 in San Miguel, Betis, Guagua, Pampanga, Filippijnen Promotoren: Prof. Dr. J.H.J. Terwindt Utrecht University, Faculty of Geographical Sciences Prof. J.J. Nossin International Institute for Geo-Information Science and Earth Observation Co-promotoren: Dr. Th. W.J. van Asch Utrecht University, Faculty of Geographical Sciences Dr. C.J. van Westen International Institute for Geo-Information Science and Earth Observation Acknowledgements This page is dedicated to several persons and organizations that one way or another assisted in taking this thesis to its completion. I would like to take this opportunity to express my warm gratitude to the following: To the Netherlands Government for providing financial support through the Netherlands Fellowship Program (NFP), Netherlands Ministry for Development Cooperation (DGIS), Netherlands Ministry of Education, Culture and Science (OCW) and the International Institute for Geo- Information Science and Earth Observation (ITC). To the Philippine Institute of Volcanology and Seismology (PHIVOLCS) under Department of Science and Technology (DOST), my organization, for the support to work on this thesis. To my supervisors at ITC, Professor Dr. Jan J. Nossin who accepted to be my promotor and who has been very instrumental in conceptualizing this thesis. He has been very cool on his advises and very patient in much editorial work. To Dr. Cees van Westen, my co-promotor, who has done a major contribution in editing this manuscript. His scientific advises have improved this thesis. To my promotor at Utrecht University, Professor Dr. Joost Terwindt whose comments helped to improve the scientific standard of this thesis. To my external co-promotor, Dr. Theo van Asch, who shared his expertise in dynamic modeling of lahars. To Dr. Raymundo S. Punongbayan, director of our organization PHIVOLCS, who has been very supportive to undertake the PhD. program. Several staff members of ITC have been helpful, among them are: Mr. Wim Feringa for drafting some of the geomorphological maps; Ir. E. Bergsma for lending me the portable rainfall simulator; Mrs. B. Geerdink and Mrs. R. Allessie for arranging all my visa extensions and travel documents. On a festive side, I have spent several intermittent years in Enschede since 1989, first during my post-graduate course to MSc. then later to the PhD. I have witnessed four world cup events, attended numerous salsa parties, and witnessed one firework-explosion disaster in Enschede. I express my gratitude to my Dutch friends, Bas and Melinda, and several other Filipino students from several batches with whom we shared several fiestas. i To my late father who always reminded to strive hard and to my mother who gives immaculate care. To my family which I owe for many years, for being away to the point that my son and daughter were born while I was in Holland. ii Table of Contents Acknowledgements .....................................................................................i Table of Contents ......................................................................................iii Chapter 1: Introduction ..............................................................................1 1.1 Introduction............................................................................1 1.2 Problem definition related to volcanic hazards.........................3 1.3 Current understanding of lahar and associated processes.......5 1.4 Research issues......................................................................6 1.5 Objectives of the research .......................................................9 1.6 Research methodology.............................................................9 1.7 Test area ...............................................................................11 1.8 Thesis chapter organization ..................................................11 Chapter 2 : Lahar Types and Processes ..................................................13 2.1 Introduction..........................................................................13 2.2 Sources of lahars and scale of erosion on volcanoes ..............13 2.2.1 Sources of lahars ..................................................................13 2.2.2 Scale of erosion on volcanoes ................................................14 2.3 Lahar triggering mechanism..................................................16 2.3.1 Rain-triggered lahars ............................................................16 2.3.1.1 Rain-triggered lahars at Mt. Pinatubo....................................16 2.3.2 Lake breakout-related lahars ................................................17 2.3.2.1 Mapanuepe Lake...................................................................18 2.3.2.2 Pasig Lake.............................................................................19 2.3.3 Lahars triggered by snowmelt................................................20 2.3.4 Landslides leading to lahars..................................................21 2.4 Type and characteristics of lahars.........................................21 2.4.1 Debris flow............................................................................21 2.4.2 Hyperconcentrated flow.........................................................23 2.5 Lahar sedimentology .............................................................23 2.5.1 Active lahars at Mt. Pinatubo ................................................23 2.5.2 Lahar depositional mechanism..............................................25 2.5.3 Stratigraphy of Mt. Pinatubo lahars ......................................26 2.5.3.1 Debris flow deposits ..............................................................26 2.5.3.2 Hyperconcentrated flow deposits ...........................................27 2.6 Some characteristics of Mt. Pinatubo ....................................29 2.6.1 Rheology and characteristics.................................................29 2.6.1.1 Hot versus cold lahars...........................................................29 2.6.2 Lahar frequency and magnitude............................................30 2.7 Summary ..............................................................................32 Chapter 3 : Pinatubo Area........................................................................33 3.1 Introduction..........................................................................33 3.2 Physiography ........................................................................33 3.2.1 Erosional highland terrains...................................................34 3.3 Geology .................................................................................37 3.3.1 Ancestral Pinatubo................................................................39 iii 3.3.2 Modern Pinatubo..................................................................39 3.4 Climate .................................................................................40 3.4.1 Regional climate....................................................................40 3.4.2 Local/macro climate .............................................................42 3.5 River systems........................................................................44 3.6 Pre-eruption land-use situation.............................................46 3.7 Study area............................................................................47 Chapter 4 : Monitoring the 1991 Eruption and Post-eruption Using Remote Sensing Images ........................................................................................49 4.1 Introduction..........................................................................49 4.2 Eruption chronology and deposits .........................................49 4.2.1 Tephra from the climactic eruption .......................................50 4.2.2 Pyroclastic flows....................................................................51 4.2.3 Secondary pyroclastic flows and other related events ............54 4.3 Monitoring changes...............................................................56 4.3.1 Changes due to deposition and erosion .................................57 4.3.2 Land cover changes...............................................................59 4.3.3 Vegetation changes ...............................................................64 4.4 The Sacobia-Pasig-Abacan catchment ...................................68 4.4.1 Pre-1991 eruption geomorphology.........................................68 4.4.2 Post-eruption geomorphology