FIRE and MUD Contents

Two Years of Lahars on the Western Flank of Mount Pinatubo: Initiation, Flow Processes, Deposits, and Attendant Geomorphic and Hydraulic Changes

By Kelvin S. Rodolfo,¹ Jesse V. Umbal,^{1 2} Rosalito A. Alonso,¹ Cristina T. Remotigue,³ Ma. Lynn Paladio-Melosantos,² Jerry H. G. Salvador,⁴ Digna Evangelista,⁴ and Yvonne Miller⁵

¹Department of Geological Sciences M/C 186, University of Illinois at Chicago, 801 W. Taylor St., Chicago IL 60607-7059.

²Philippine Institute of Volcanology and Seismology.

³National Institute of Geological Sciences, University of the Philippines, Diliman, Quezon City, Philippines.

⁴Mines and Geosciences Bureau, Department of Environment and Natural Resources, North Avenue, Diliman, Quezon City, Philippines.

⁵Department of Mineralogy, University of Geneva, 13 Rue de Maraichers, 1211 Geneva 4, Switzerland.

ABSTRACT

Two years after the eruption of Mount Pinatubo, lahars continued to occur along major drainages of the volcano. In Zambales Province on the western flank, pyroclastic debris is funneled as lahars through the Bucao and Santo Tomas River systems, and to a limited extent, along the Maloma River. Lahars at Pinatubo are initiated in a variety of ways. Primary, hot flows are triggered by rain with threshold values of 0.3 to 0.4 millimeters per minute sustained over periods >30 minutes. Others result from mass failures, induced by steam explosions, of partially water-saturated pyroclastic materials blocking upper tributary channels in the pyroclastic fans. Hot lahars display transitional flow types: precursory and waning stage hyperconcentrated streamflow, and debris-flow phases that coincide with peak discharges ranging from 200 to >1,000 cubic meters per second. Interactions of primary lahar channels with tributaries from outside Pinatubo, and variable discharge from contributing tributaries, result in multiple peak discharges and complex flow behavior. Primary lahars aggrade with sufficient rapidity (about 7 to 20 meters per year) to block tributaries that joined the lahar channels from watersheds outside the volcano, forming lahar-dammed lakes. Breakouts from these lakes generate cold, hyperconcentrated lahars.

All laharic deposits consist of poorly sorted pumiceous sand with very little coarse and fine material. The total sediment transported by Zambales drainages during 1991-92 was about 8.8x10⁸ cubic meters, several orders of magnitude greater than observed at other volcanoes. The lahars have filled channels almost to capacity, so future flows are expected to avulse out of the present lahar field along new routes.

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INTRODUCTION

The 1991 eruption of Mount Pinatubo deposited an estimated 5 to 6 km³ of pyroclastic materials on the slopes of the volcano (W.E. Scott and others, this volume). Roughly two-thirds was emplaced on the western slopes that are situated entirely in Zambales Province, where it buried the preexisting topography except for a few isolated peaks and ridges (fig. 1). In 1991 and 1992 some of this metastably perched pyroclastic material was reworked as landslides and secondary pyroclastic flows that were induced in part by steam explosions on the still-hot pyroclastic fans. Much more debris, however, was remobilized into lahars by intense rains during the 1991 and 1992 monsoon seasons from June to October, and the major drainages were aggraded rapidly (Janda and others, 1991; Punongbayan and others, 1991; Rodolfo, 1991; Rodolfo and Umbal, 1992; Pierson and others, this volume; K.M. Scott and others, this volume). The new blanket of fine ash and the destruction of vegetative cover reduced soil infiltration capacity and evaporation and thus greatly enhanced sediment movement (Pierson and others, 1992; Major and others, this volume). Sediment yield during these two rainy season was high, about 1.3x10⁹ m³, several orders of magnitude greater than those previously observed at other volcanoes (Vessel and Davies, 1981; Swanson and others, 1982; Lehre and others, 1983).

Figure 1. Distribution of pyroclastic fans and major drainages on the western flank of Mount Pinatubo.

Sediments eroded from the western pyroclastic fans were funneled principally into the Bucao and Santo Tomas River systems (Rodolfo, 1991; Pierson and others, 1992; Rodolfo and Umbal, 1992). Lahars of the 1991 and 1992 monsoon season rapidly filled these channels, and some flows overtopped channel banks and encroached adjacent inhabited areas and agricultural fields. Considering the vast quantity of fresh pyroclastic debris, lahars and excess sedimentation may continue for 5 to 10 years or longer (Pierson and others, 1992; Rodolfo and Umbal, 1992).

Similar rapid filling in western drainages outside the volcano watersheds, mainly from remobilized tephra, caused channel shifting and flooding of adjacent areas, especially in 1991; however, erodible pyroclastic materials on the slopes of these non-Pinatubo drainages is limited, so these adverse effects are expected to be a diminishing and short-term problem. With adequate maintenance, these channels should return to their preeruption hydraulic conditions within a few years.

In this report we describe the processes of initiation and transport of lahars on the western sector of Mount Pinatubo and the response of the rivers, both within and outside the western Pinatubo watershed, to the excess sedimentation resulting from the 1991 Mount Pinatubo eruptions. Data for this study were gathered during the 1991 and 1992 monsoon seasons.

TERMINOLOGY

The lahar is one of the most destructive phenomena associated with composite volcanoes. In this report, following the lead of a recent gathering of volcaniclastic sedimentologists (Smith and Fritz, 1989, p. 375), lahar means "...a rapidly flowing mixture of rock debris and water (other than normal stream flow) from a volcano. A lahar is an event; but it can refer to one or more discrete processes [such as debris flow and hyperconcentrated streamflow], but does not refer to a deposit." Lahars exhibit complex flow behavior, changing in character from debris flow to hyperconcentrated streamflow, and vice versa (Pierson and Scott, 1985; Rodolfo and Arguden, 1991; Smith and Lowe, 1991). Debris flows are non-Newtonian fluids having water contents generally less than 25 percent by weight, and they move as fairly coherent masses in generally laminar fashion (Pierson, 1986; Pierson and Costa, 1987). Hyperconcentrated streamflows, as originally defined by Beverage and Culbertson (1964), are stream flows having sediment concentrations between 40 and 80 percent by weight. They move in fluid fashion (Pierson and Costa, 1987), even though Kang and Zhang (1980) have reported that they possess measurable yield strengths (generally less than 400 dyn/cm²). It is not unusual for a single lahar to involve more than one debris flow phase, with transitional as well as precursor and waning-stage hyperconcentrated-streamflow phases. Given appropriate circumstances, either flow type can erode or deposit along any reach of its channel, and so the morphologic and sedimentologic effects of lahars can be very complex.

GEOLOGIC AND TOPOGRAPHIC SETTING

Mount Pinatubo, an andesite-dacite dome complex and stratovolcano, lies at the northern end of a 50-km-long Pliocene to Holocene calcalkaline volcanic belt related to eastward subduction of the South China Sea plate at the Manila Trench along western Luzon Island (de Boer and others, 1980; Wolfe and Self, 1983). The oldest Pinatubo rocks, isolated ridges of andesitic lava flows and pyroclastic deposits exposed on the middle and lower slopes of the volcano, are remnants of an "ancestral" Pleistocene stratovolcano (Delfin, 1983; Newhall and others, this volume). Silicic "modern" eruptions emplaced extensive ash- and pumice-flow deposits in all sectors of the volcano during at least six episodes since >35 ka (Newhall and others, this volume). On the western side of the volcano, pyroclastic flows from pre-1991 eruptions extended more than 17 km from the volcano, and major pyroclastic aprons, distributed within radial distances of 10 to 15 km, impinged upon ridges of the Zambales Ophiolite complex that had been uplifted in late Miocene time.

Prior to the eruption, the summit of the volcano stood at 1,745 m in altitude. The topography of the upper western flank of the volcano was characterized by steep to moderate slopes, deeply incised by a dense, radial network of drainages (table 1). Valleys at the upper slopes were deep, narrow, and U-shaped and were steep and steplike in longitudinal profiles (fig. 2). Most interfluve slopes ranged from 10° near the summit to about 2° downslope, but were steepest in the southern sector, where slopes were 28° within a kilometer from the summit and decreased to 8° near the base of the edifice. Along their lower reaches the valleys presented gently sloping, concave-upward profiles with broad, generally shallow banks flanked by multileveled river terraces, each approximately 5 to 10 m high. Most of the upper drainages were buried by pyroclastic flows during the climactic June 15, 1991, eruption, and subsequent lahars have aggraded the lower reaches almost to the level of the highest terraces.

Figure 2. Longitudinal profiles of major drainages on the western flank of Mount Pinatubo. A, Major tributaries of the Bucao River system. B. Major tributaries of the Santo Tomas River system, the Maloma River, and non-Pinatubo drainages.

Table 1. Watershed areas and estimated volumes of pyroclastic deposits of major drainages on the western flank of Mount Pinatubo and the contiguous Zambales Range.

PINATUBO DRAINAGES

In Zambales, the principal channels affected by lahars were those that tap the new pyroclastic-flow deposits. These are the Bucao River system that drains the west and northwest sectors of the volcano, the Santo Tomas River system, including the Marella River, that drains the southwest sector, and, to a much lesser extent, the Maloma River. The Bucao and Santo Tomas Rivers are expected to continue experiencing destructive lahars for several years. In contrast, due to its limited access to new pyroclastic-flow deposits, the Maloma River did not have lahars as frequently as the other two but may become more active in the future if it beheads and captures part of the watershed area of the Marella River (Pierson and others, 1992; Rodolfo and Umbal, 1992).

THE BUCAO RIVER SYSTEM

The Bucao River system, a broad watershed with an area of 659 km², drains the west and northwest sector of Mount Pinatubo and surrounding ultramafic terrain. Its Pinatubo portion, with an area of 270 km², is by far the largest catchment basin on the volcano. The southern part of the Bucao watershed was drained by nine unnamed Pinatubo tributaries of the upper Balin Baquero River, many of which were buried in pyroclastic flows during the eruption. Immediately downstream and to the north of these tributaries, the Balin Baquero River is joined by its largest Pinatubo tributary, the Maraunot River, which alone drained an area of 42 km² prior to the eruption. The upper Bucao River, which has a catchment area of 128 km² situated to the north, joins the Balin Baquero River 26 km west-northwest of the former summit.

Several large rivers and many small creeks draining the surrounding ophiolitic Zambales Mountains join the main lahar avenues. The largest of these non-Pinatubo tributaries, the Balintawak-Cabatuan River, with a watershed area of 175 km² northwest of Pinatubo, joins the Bucao River 3 km above the Balin Baquero junction, in the vicinity of the former Barangay (village) Poonbato. Normal streamflow from this river dilutes the lahars from Pinatubo drainages and is a major agent in redistributing channelized sediments along the lower reaches of the Bucao River during the dry seasons.

The Bucao Valley is narrowest (only a kilometer wide) immediately below the Balin Baquero junction, at the site of the former flood-plain sitio (hamlet) of Malumboy (now buried in lahar deposits). It widens to about 3 km, including lahar and fluvial terraces that flank both sides of the valley, downstream to the Baquilan River junction 9 km from the South China Sea. Along the Malumboy-Baquilan reach, the Bucao valley is confined between the steep flanks of the ophiolitic Zambales Mountains. The Baquilan River, with a 60-km² watershed north of the Bucao River, is its second largest tributary from outside the Pinatubo drainage system. It joins the Bucao River about 5 km downstream of the Balin Baquero confluence, where it further dilutes the flows. Downstream from the Baquilan confluence, the alluvial plain of the Bucao River widens to the north and is shared by other coastal rivers, but the Bucao River braids over the 1- to 2-km-wide southern portion of the plain and reaches the coast south of the Municipality of Botolan. This town has an aggregate population of 35,752, including the inhabitants of 22 barangays that survived the pyroclastic flows of 1991 and the subsequent lahars. In all, eight other barangays of Botolan no longer exist. Poonbato, Malumboy, and parts of two other upland barangays were destroyed by lahars, the rest by pyroclastic flows.

THE MALOMA RIVER SYSTEM

The watershed of the Maloma River and its tributaries, 153 km² in area, drains almost exclusively ultramafic terrain. About 100 m wide along its middle reaches, the flood plain broadens downstream to about 500 m at its junction, 8 km from the coast, with the Gorongorong-Kakilingan River. This largest tributary has a 42-km² watershed south of the trunk stream. The Gorongorong-Kakilingan River and other streams in the vicinity that are incised into ultramafic rocks are characterized by narrow, V-shaped valleys and sharp channel bends imposed by the structural fabric of the bedrock, mainly northwest-trending normal faults and fractures related to the Iba fracture zone (de Boer and others, 1980). In 1991, these bends became temporary impoundment sites for remobilized tephra-fall deposits eroded from its slopes. The broad, flat Maloma valley, old (pre-1991) pumiceous fluvial and debris-flow deposits exposed in its channel walls, and the continuing headward reestablishment of its tributaries up to the lower slopes of the volcano indicate that the river once drained larger portions of the southwest flank of the volcano, until a major, pre-1991 explosive eruption modified the terrain and diverted most flow away from the Maloma River.

THE SANTO TOMAS RIVER SYSTEM

The primary lahar avenue of the Santo Tomas River System, which has an aggregate watershed area of 232 km² above 50 m in altitude, is the Marella River, which drains a 31-km² area of the southwest slopes of Mount Pinatubo (fig. 1; table 1). About 19 km from the preeruption summit, the Marella River was joined by the Mapanuepe River, with a watershed of 88 km², forming the Santo Tomas River. Partially contained by a narrow bedrock constriction at the junction, the lower Marella River acted as a natural debris basin for the 1991 and 1992 lahars, which aggraded rapidly and blocked the Mapanuepe River, forming Mapanuepe Lake.

Below the junction, the Santo Tomas River flows along the northern margin of a broad, 235-km² alluvial plain that is populated by some 140,000 inhabitants in five municipalities, each with numerous barangays and sitios. The southern edge of the plain is marked by the irregular course of the Pamatawan River as it follows the northern bases of the ophiolitic southern Zambales Mountains. Along this margin, several small, isolated hills trending west from Castillejos have peak altitudes of 80 to 143 m; otherwise, local relief is under 5 m, due mainly to a deranged system of numerous shallow, small creeks that drain the greater, southern portion of the plain, which is underlain by the deposits of pre-1991 lahars and normal streamfloods. The preeruption channel morphology of the Santo Tomas River System and the interplay between Marella lahars and Mapanuepe River discharge are discussed in detail elsewhere in this volume (Umbal and Rodolfo).

NON-PINATUBO DRAINAGES

The coastal slopes of the Zambales Range between the Bucao and Maloma River systems are drained by the Tanguay and Kileng Rivers (table 1), neither of which extends eastward into the new pyroclastic-flow deposits on Pinatubo. Along these rivers, especially along the Tanguay River, mobilization of fresh tephra-fall deposits caused heavy siltation and flooding in 1991 and, to a much lesser extent, in 1992. These problems should lessen because the  watersheds have lost most of their tephra-fall accumulations; however, the accumulated silt should continue to cause serious flooding.

CLIMATIC SETTING

The annual climate of central Luzon is characterized by distinct dry and wet seasons. On the western side of Mount Pinatubo, the dry season lasts from November to April, and the wet season occurs from May or June to October. About 70 to 80 percent of the 3,800 to 3,900 mm of annual rainfall in Zambales is delivered by the Southwest Monsoon, which normally begins in June and lasts until September (fig. 3; Huke, 1963; Pierson and others, 1992). During this season, average daily rainfall is about 24 mm but may exceed 150 mm. August is the wettest month, when daily precipitation averages 36 mm but may exceed 180 mm. However, the greatest 24-h rainfall recorded in the region was 442 mm, on May 19, 1966, delivered by a typhoon prior to the onset of the monsoon season (unpub. data, U.S. Navy). Much of the annual rain is brought by three or four typhoons, which may occur in any given month but typically occur from July to November and are most frequent in August. Orographic uplift causes more monsoonal rain to fall on the volcano than on the lowlands (fig. 3), and more rain falls on the western than on the eastern slopes (Rodolfo, 1991; Rodolfo and Umbal, 1992). Short, intense rainstorms are common, particularly during late afternoons when the diurnal wind shift brings warm, moist, marine air inland.

Figure 3. Monthly average rainfall in Zambales based on data from the Philippine Atmospheric, Geophysical, and Astronomical Services Administration at Iba (1956-85), the U.S. Navy Meteorological Station at Cubi Point (1955-87), and Dizon Mines (1977-90).

DISTRIBUTION AND VOLUME OF PYROCLASTIC MATERIALS ON THE WESTERN FLANK

About two-thirds of the estimated 5 to 6 km³ of pyroclastic materials produced during the 1991 eruption was emplaced on the western slopes of Mount Pinatubo (table 1). The largest fan of pyroclastic-flow deposits, in the northwest sector, is drained by the Bucao and Maloma River systems, and a smaller fan in the southwest sector is drained by the Marella River (fig. 1). These deposits, in some areas more than 150 m thick, buried most preeruption drainages. At their farthest extents, the June 15, 1991, pyroclastic flows on the west flank of the volcano traveled more than 17 km from the summit.

Of the roughly 1 km³ of tephra fall deposited on land, about 75 percent was emplaced along a west-southwest-trending axis by prevailing winds (Nichols and others, 1991; W.E. Scott and others, 1991; Paladio-Melosantos and others, this volume; Oswalt and others, this volume). In Zambales, these deposits ranged in thickness from 5 to 35 cm. After two rainy seasons, an estimated 75 to 80 percent of the tephra emplaced on moderate to steep mountain slopes had been eroded.

CHRONOLOGY AND FREQUENCY OF SYNERUPTIVE AND POSTERUPTIVE LAHARS

The first lahar on the western sector occurred along the Maraunot River on the afternoon of June 12, 1991, triggered by a local downpour on tephra-fall deposits that had been emplaced starting on May 16 and pyroclastic-flow deposits that had been emplaced on the morning of June 12. Intense rains, from the passage of Typhoon Yunya and from eruption-induced convective storm cells, triggered lahars of varying magnitude that affected all major rivers on the volcano during the initial paroxysmal explosions on the early morning of June 15 (Janda and others, 1991; Pierson and others, 1992; Major and others, this volume). Along the Marella-Santo Tomas River, stratigraphic sections showed three successive layers of debris-flow deposits of predominantly dense old lithic fragments, including boulders more than a meter in diameter, between the June 12-14 and the June 15 tephra falls. Each debris-flow unit was separated by a thin layer of silt and clay and graded laterally downstream into hyperconcentrated streamflow and normal streamflow deposits. In most sections, the uppermost debris-flow unit was capped by a centimeter-thick laminated layer of silt and fine sand. We interpret the June 15 lahar event along the Marella-Santo Tomas River as comprising three pulses or surges of debris flows that became diluted downstream to hyperconcentrated streamflows and normal streamflows, as has occurred on other volcanoes (Pierson and Scott, 1985; Rodolfo and Arguden, 1991; Smith and Lowe, 1991). The laminated silt and sand layer on top of the uppermost debris-flow unit may correspond to overbank deposits from a normal flood or hyperconcentrated-streamflow event that may have resulted from the sudden release of waters from a tributary, most likely the Mapanuepe River, that was temporarily dammed by lahars from the Marella River. These series of flow events destroyed the bridge between Barangays Santa Fe and San Rafael, east of San Marcelino municipality (fig. 1).

In the month following the climactic eruption, rainfall from orographic uplift of the prevailing easterly tradewinds was restricted mainly to the eastern side of the volcano. On the west side during this period, not as much rain fell, and the new pyroclastic fans still had no organized systems of valleys and tributaries. Consequently, runoff was not quickly delivered and concentrated, and only a few cold, hyperconcentrated flows occurred along the Bucao and Santo Tomas River systems (Rodolfo, 1991; Rodolfo and Umbal, 1992; Umbal and Rodolfo, this volume). Small hot lahars did not occur in Zambales until July 26.

On the afternoon of August 5, 1991, the first major hot lahars aggraded the Santo Tomas channel just below the Marella-Mapanuepe junction with more than 5 m of debris and overtopped channel banks at Nagpare, a sitio of Botolan municipality along the lower reaches of the Bucao River, leaving pumiceous debris-flow deposits more than a meter thick. As the southwest monsoon season intensified, lahars became more frequent in both rivers.

A total of 73 lahars were documented along the Santo Tomas River in 1991, not including nocturnal events recorded by the BUGZ flow sensor but not verified on the ground (Umbal and Rodolfo, this volume). These consisted of 50 hot lahars, from the Marella River, and 23 cold lahars, of which 18 resulted from lake breakouts from Mapanuepe Lake and 1 from a dammed tributary from Mount Cuadrado (fig. 4). We documented 15 lahars along the Bucao River until August 17, when floodwaters eroded sections of the National Road at the southern approach to Tanguay bridge, and lahars buried the barangay road at Nagpare, Botolan, cutting off access to our Malumboy and Poonbato stations for the duration of the 1991 rainy season. Seven of these early lahars were cold and eight were hot. If individual lahars along each of the numerous Bucao tributaries could have been tabulated, this number doubtless would have been significantly higher than that of the Marella River. We completed a permanent station at Malumboy, immediately downstream of the Balin Baquero confluence, prior to the onset of the 1992 monsoons, and continuously monitored the Bucao River lahars in 1992; however, it was not possible to document the details of lahar activity along the inaccessible upper Bucao and Balin Baquero Rivers.

Figure 4. Monthly lahar frequencies in the Santo Tomas River in 1991 and 1992. Monthly rainfall is the long-term average at Dizon Mines (1977-1990), the rainfall monitoring station closest to the volcano.

In 1992, fewer lahars occurred on both the Santo Tomas and the Bucao Rivers; however, most of the flows were considerably larger and were mostly debris flows, whereas the 1991 events were mostly hyperconcentrated flows. At the Santo Tomas River we documented a total of 47 lahars, of which 39 were hot and 8 were cold hyperconcentrated flows resulting from lake breakouts. Two of these lake breakouts originated from a dammed tributary at Mount Cuadrado, whereas the rest originated from Mapanuepe Lake. Two of the events were induced by excavation of an artificial channel through the debris dam.

At the Bucao River in 1992, we observed 41 lahars originating either from the upper Bucao or Balin Baquero tributaries. Barangay Poonbato was buried in 17 m of sediment by several major debris flows, including two documented events on August 20 and September 10 that respectively left deposits 3 m and 4.5 m thick. For most of the season, lahar flow below the Balintawak junction was generally restricted to a principal channel 10 to 20 m south of the north bank that varied in width from 20 to 100 m before dissipating into a broad, braided surface a kilometer downstream of our Malumboy watchpoint. Cool Balintawak-Cabatuan discharge dominated the northern portion of the channel, and lahars from the upper Bucao and Balin Baquero Rivers generally were restricted to the southern portion; consequently, flow temperatures rarely exceeded 37°C where we could measure them at Malumboy on the north bank.

In the area of the Bucao-Balin Baquero confluence, and extending downstream to Malumboy, lahars from the latest pre-1991 eruption had formerly left a broad, vegetated alluvial fan (fig. 5A), into which was incised a central Bucao channel bounded along the south bank by two major and two lesser terraces, each 5 to 10 m high (fig. 6A). The channel was completely filled in by 1991 lahar deposits, which restored and smoothed the fan surface above the base of the uppermost terrace (figs. 5B, 6B). Thus, the 1992 lahars from the Balin Baquero River generally fanned out widely over the smooth, restored fan surface (figs. 5C, 6C) into shallow (<1 m), hyperconcentrated sheet flows. These merged laterally northward with the Bucao-Balintawak flows along 3 km of fan margin and generally remained restricted to the southern portion of the principal flow channel. On September 5, however, a major debris flow from the Balin Baquero River raised the middle of the fan and the central portions of the lahar avenue at Malumboy by overtopping both margins of its principal channel. Most subsequent Balin Baquero lahars followed the new channel, where they remained segregated from Bucao flows before coalescing with them more than a kilometer downstream of Malumboy.

Figure 5. Eastward views of the Bucao-Balin Baquero lahar fan. A, June 1991, before major lahars. Note the lahar terraces and the vegetation cover. Mount Pinatubo is in the background. The Balin Baquero River discharges onto the fan in the right portion of the frame, to the right of the prominent, vegetated terrace in the distance. B, January 1992. C, September 1992, after the lahar season.
Figure 5A-B
Figure 5C

Figure 6. Southward view from Malumboy watchpoint of the central portion of the Bucao-Balin Baquero lahar fan. A, June 1991, before major lahars. Note the 5-m-high terraces along the south bank. The light color of the mountains is due to fresh ash fall. B, Same view, January 1992. C, Same view, September 1992, after the lahar season.
Figure 6A
Figure 6B-C

We have little data regarding the 1991 and 1992 lahars of the Maloma River. Eyewitness accounts suggest that most 1991 lahars were cold, hyperconcentrated flows resulting from remobilized tephra-fall deposits. Possible exceptions were the September 2 and 14 lahar events, which were described as steaming and were debris flows, judging from their deposits along the middle reach of the channel. In contrast, most 1992 lahars were hot, and abundant charred wood and charcoal in their deposits indicate that the upper tributaries of the Maloma had eroded into the new pyroclastic fan.

LAHAR TRIGGERING MECHANISMS

Pinatubo lahars were triggered mainly by intense rainfall brought about by monsoon rains and passing typhoons. If the telemetered data from the BUGZ station of the U.S. Geological Survey-Philippine Institute of Volcanology and Seismology (USGS-PHIVOLCS) rain-gauge network are representative of rainfall over the Marella pyroclastic fan, intensity threshold values for initiation of Marella lahars were 0.3 to 0.4 mm/min, sustained over periods lasting longer than 30 min (Pinatubo Lahar Hazard Taskforce, 1991, memorandum dated August 28, 1991; Regalado and Tan, 1992). Tuñgol and Regalado (this volume) cite a similar threshold of 0.3 mm/min over periods lasting longer than 30 minutes for lahars along the Sacobia River on the east side of the volcano. These rainfall intensities are below normal maxima for monsoonal rains and typhoons in this region. They are significantly lower than the threshold defined by Caine (1980) from a worldwide compilation of both volcanic and nonvolcanic debris flows, and by Rodolfo and Arguden (1991) for laharic debris flows of Mayon Volcano. This discrepancy is due in part to the fact that the Pinatubo lahars include not only debris flows but also hyperconcentrated flows, which are triggered by rainfall of lower intensities and durations. In any case, the abundant, erodible volcaniclastic sediment was easily and frequently mobilized by the typhoons and monsoonal rains of 1991 and 1992, in a remarkably rapid response to the drastic geomorphologic changes wrought by the Pinatubo eruption on the watersheds and on the hydraulic characteristics of the river channels.

Lahars were also triggered by lake breakouts from non-Pinatubo tributaries blocked by pyroclastic-flow deposits and aggrading lahar channels, especially at the Marella-Mapanuepe junction of the Santo Tomas River system, as described in detail by Umbal and Rodolfo (this volume; also see Umbal and others, 1991; Rodolfo and Umbal, 1992; Pierson and others, 1992; K.M. Scott and others, this volume, Tuñgol and Regalado, this volume). Lake breakouts were initiated by erosion of the blockage by overtopping lake waters that generated normal streamfloods, sometimes with hyperconcentrated-streamflow stages of short duration. At times, rapid aggradation by heavy lahar flow from the upper Bucao blocked the Balintawak River, but the blockage could not last for more than a few days before it was overcome by large volumes of accumulated Balintawak waters that eventually broke out and remobilized the channel deposits. Occasionally, heavy lahar activity along the Balin Baquero also dammed the combined Bucao-Balintawak discharge for short periods and generated streamfloods and cold lahars upon breakout. The longest lasting episode commenced with damming on August 29. The dam was partially breached 2 days later, but limited blockage persisted until September 4.

In some instances, lahars may have been triggered by steam explosions that generated strong ground vibrations, which induced failure of partially water-saturated pyroclastic materials, particularly near the vicinity of pyroclastic-flow- and lahar-dammed tributaries. One such event was the June 19, 1992, Marella lahar, as evidenced by (1) the lack of triggering rainfall, (2) a strong steam explosion minutes prior to the lahar, and (3) flow-sensor seismic records showing a large initial amplitude followed by a gradual decline in high-frequency signals. This event, consisting of a precursory hyperconcentrated flow followed immediately by a debris flow with a lobate front several centimeters high consisting mainly of pebble- to cobble-sized fragments, left a proximal hummocky deposit (fig. 7). Alternate explanations are that a lake breakout may have triggered the explosion, or, conversely, that the explosion triggered the lake breakout.

Figure 7. Hummocky topography of the proximal portion of the June 19, 1992, lahar along the middle reach of the Marella channel, approximately 13 km away from the volcano. Most likely, a steam explosion caused a landslide, which was transformed downstream into a lahar. The size and frequency of hummocks decrease downstream. For scale, note the meterstick in the center of the photograph.

LAHAR FLOW CHARACTERISTICS

The majority of 1991 lahars observed at our watchpoints on the Bucao and Santo Tomas Rivers were hyperconcentrated flows, although larger events had distinct debris-flow phases. During 1992, however, most lahars, particularly those along the Marella channel, were dominated by debris-flow phases.

The pattern of flow successions of most Marella hot lahars was from initial muddy streamflow, to hyperconcentrated flow, to debris flow, and finally to waning-stage hyperconcentrated flow. Transitions between phases were characterized by changes in flow behavior, flow temperature, and flow depth. Sediment concentrations in sampled lahars increased correlatively during the transitions from hyperconcentrated to debris flow. Corresponding to these transitions, hydrographs showed measured discharge increasing to single peaks, followed by gradual declines back to normal streamflow conditions (fig. 8A).

Figure 8. A, Hydrograph of the June 28, 1992, lahar along the Santo Tomas River showing the typical sharp increase in discharge followed by a gradual decline. Peak discharge in the hydrograph coincided with the transition from hyperconcentrated streamflow to debris flow. B, Hydrograph of the July 29, 1992, lahar at the Santo Tomas River. The second peak at the tapering end of the graph coincides with the arrival of discharge from the Mapanuepe River, which had been temporarily blocked by Marella lahars.

Variable contributions by non-Pinatubo tributaries resulted in multiple peak discharges (fig. 8B), and complex flow behavior. In some instances, multiple peaks may have resulted from (1) variable or nonsynchronous rainfall on the catchment basins of individual tributaries and (2) repeated damming at channel bends and constrictions by lahars, and breaching of these dams by subsequent lahar or lake-breakout flow, as frequently occurred at the bedrock constriction of the Marella-Mapanuepe junction. Minor pulses in discharge may also have resulted from internally driven flow instabilities (Davies and others, 1991).

Discharge and lahar-generation patterns were especially complicated in the Bucao River system. It is the only system on Pinatubo in which two major lahar avenues merge, the Balin Baquero River, which drains the largest pyroclastic fan on the volcano, and the upper Bucao river, which has a Pinatubo watershed area comparable in size to that of the Marella River. Furthermore, the Balintawak-Cabatuan watershed contributes enormous volumes of normal streamflow to the Bucao River from outside the volcano's drainage network, and its hydrographs are complicated because this large area does not receive its rainfall uniformly over time. Mixing of Balintawak-Cabatuan muddy streamflow with lahars from the upper Bucao reaches was minimal because discharge from both rivers was strong. This lack of mixing resulted in two contrasting flow types, muddy, reddish, lateritic streamflow from the Balintawak-Cabatuan River and gray, mainly hyperconcentrated lahars from the upper Bucao River (or from the combined laharic discharge of the Bucao and Balin Baquero rivers), flowing in the same channel and remaining distinct and immiscible for several kilometers downstream from the confluences.

When strong rain was localized on the upper Bucao watershed, the resulting lahars could temporarily block the Balintawak-Cabatuan River for up to 2 days, and subsequent rupture of the blockage by accumulated water behind it caused cold lahars. Similarly, rainfall localized on the Balin Baquero watershed at times generated lahars that blocked the combined Bucao-Balintawak discharge and caused hot lahars to reach our Malumboy watchpoint, and subsequent rupture of blockage delivered peak discharges at times when no rain was falling on the volcano.

Along the Zambales lahar avenues, pulses or surges of muddy streamflows were precursors of incoming lahars (fig. 9). These surges had minimal sediment concentrations, generally less than 15 percent by volume, and flow temperatures a few degrees higher than ambient temperature, which normally was 24°C. The typical 2.5- to 3.5-m/s velocities of these surges enabled them to overtake and override the surface of normal streamflows, which had velocities below 1 m/s.

Figure 9. Precursory muddy streamflow surge, 25 cm high, overtaking and overriding the preceding flow. Photograph was taken along the Santo Tomas River near the PLHT-ZLSMG watchpoint at Dalanaoan, San Marcelino. The stage marker is 8 m high.

A gradual increase in flow depth and increased surge frequency signaled the onset of hyperconcentrated flow, which we designated as such primarily from its visual aspect and erosive behavior: increased turbulence, continuous migration across the width of the channel accompanied by strong lateral erosion at typical rates of 0.5 to 3 m/min, and the formation of standing waves as high as 2 to 3 m and 15 to 20 m long (fig. 10), in phase with antidune bedforms of lesser height. The wave dimensions are useful tools for indirectly measuring the depths and velocities of these dangerous flows. Empirically, we observed that flow depths were about 1.5 to 2 times the wave height. Flow velocities U, which theoretically equate to wavelength L according to the equation U² = gL/2 p that governs gravity waves (Kennedy, 1969), including atmospheric and deep-water ocean waves, agreed well with velocities measured by natural and artificially introduced floats.

Figure 10. Standing waves in hyperconcentrated streamflow in the Santo Tomas River near the PLHT-ZLSMG watchpoint at Dalanaoan, San Marcelino, on August 14, 1991. A, Amplitudes of 1.5 m and wavelengths of 6 m before the waves. B, Waves breaking during transition to debris flow.

Bulk densities of hyperconcentrated-streamflow phases of three Santo Tomas lahars systematically sampled in June and July 1992 ranged between 1.20 and 1.35 g/cm³. Sediment concentrations were low in comparison to the arbitrary 40 to 80 weight-percent values of Beverage and Culbertson (1964), ranging between 32 and 49 weight percent and averaging about 40 percent. Corresponding percentages by volume were 18 to 31 percent with an average of 24 percent. These low values may reflect the fact that virtually all of these materials were pumice fragments with dry densities of 0.9 to 1.4 g/cm³.

As a hyperconcentrated lahar progressed in intensity and discharge, its standing waves started to break, roaring continuously like ocean surf, and the flow became very turbulent and erosive. The flow no longer cut sideways so strongly; instead, it cut downward into the channel bed. The material eroded from the bed became part of the lahar, making it more dense and energetic, so it could flow faster. We regard this as a positive-feedback phenomenon: because the flow contained more and more solid volcanic material, it pressed more and more heavily on the channel bed, and the increased weight and speed of the lahar allowed it to incise the channel more deeply. Velocities at this stage ranged from 3 to 6 m/s, and flow samples showed increased flow densities, on the order of 1.10 to 1.25 g/cm³. At the Marella-Santo Tomas confluence, measured flow temperatures also showed systematic increases during this stage, from 22°C to 33°C.

The end of the transition from the hyperconcentrated-flow phase to the debris-flow phase was manifested by a gradual dampening of turbulence to a smooth slurrylike consistency of the flow surface. Discharge increased, commonly accompanied by a rise in flow level; however, during several episodes along the Bucao River, the flow stage at the north bank outside the principal channel instead dropped by 1 or 2 dm, and the channel narrowed because its deepening increased its capacity so it could accommodate all of the flow. The onset of a debris-flow phase, with the exception of the June 19, 1992, lahar event, lacked bouldery fronts typical of debris flows elsewhere (Johnson, 1970; Okuda and others, 1980; Pierson, 1981; Johnson and Rodine, 1984; Pierson, 1986; Umbal, 1986; Ledda and Rodolfo, 1990; Ledda, 1991), probably because boulders are not abundant in the source area.

The reduction of turbulence in the debris flows was probably due to increased sediment content, which constituted as much as 85 percent by volume. Measured flow densities, however, were typically between 1.4 and 1.5 g/cm³ and never greater than 1.70 g/cm³--less than the typical 2.0 g/cm³ observed in debris flows elsewhere (Costa, 1984). This is because the pumiceous solid phases floating in the sampled flow surfaces had typical dry densities of 1.1 to 1.5 g/cm³, some values being even less than 1 g/cm³. By weight, sediment contents of 10 debris flows systematically sampled in June and July 1992 were 57 to 91 percent and averaged 69 percent. Corresponding volume-percent values were 39 to 86 percent and averaged 57 percent.

Average debris-flow temperatures were slightly above 45°C, but some were as high as 70°C, and earlier flows in 1991 may have been even hotter. Large, fragile masses of lahar deposits scoured from the channels, some as big as 5 m across, were commonly observed bobbing in the debris flows. Many meter-sized pieces were transported intact for distances of more than 3 km despite flow velocities in excess of 6 m/s at slopes of less than 1°, a feat attesting to the laminarity of the flows. Debris-flow phases, corresponding to peak-flow discharges, lasted on the average between 15 to 30 minutes, but major events lasted up to an hour.

Increased turbulence and gradual drops in flow temperature and stage characterized waning stages of flow. Strong vertical and lateral scouring of the channel accompanied this hyperconcentrated waning flow (the "lahar runout" of Scott, 1988). Nickpoints and other irregularities on the channel floor served as loci for headward erosion, and they migrated upstream at rates of several decimeters per minute (fig. 11). Remnants of these waning-stage, headward-eroding nickpoints are near-vertical faces with more than a meter of drop in channel floor levels.

Figure 11. Headward-eroding nickpoints of a waning-stage hyperconcentrated streamflow along the lower reach of the Marella channel. The stage marks on the tree are each 0.1 m; the vertical drop between the upstream and downstream channel floor at this stage is about 1.5 m. The nickpoints migrated upstream at rates of 0.33 to 0.50 m/min for 45 minutes.

MORPHOLOGY AND SEDIMENTOLOGY OF THE DEPOSITS

Debris-flow deposits at Pinatubo are commonly lobate. Each lobe has a convex-upward cross-sectional profile with a narrow U-shaped channel incision at the center of the lobe (fig. 12). Slump features are common along the inner banks of these channel incisions. Individual lobes vary in width from less than a meter to as much as 200 to 400 m, and thicknesses range from 5 to 8 m. Some of the small lobes exhibit steep, narrow levees along their margins, where most of the coarser particles congregated. Near the source area, where debris-flow deposits predominate, the surface topography is generally irregular and gently undulating. At the distal portion of the lahar field, where debris flows had been progressively diluted to hyperconcentrated flows and normal streamflows, surface topography is generally smooth and flat. Thin, indurated crusts, littered with pebble- to cobble-sized pumice fragments, armored and non-armored earthballs incorporated from eroded older deposits, and charred and uncharred plant debris, mantle the surfaces of most debris-flow deposits. Successive fillings and undercuttings of the active channel commonly formed several lahar-terrace levels.

Figure 12. Cross-sectional profile across a debris-flow lobe.

Deposits of laharic debris flows and hyperconcentrated flows at Pinatubo are poorly sorted and sand rich with minor coarse and clay-sized components (fig. 13). The low densities of pumice fragments buoyed them toward the upper portion of debris flows, so inversely graded beds resulted. Hyperconcentrated-flow deposits generally lack flow structures except for crude stratification with lenses of coarse-grained, clast-supported pumice. Water-escape structures are common in debris-flow deposits, and in hyperconcentrated-flow beds as well.

Figure 13. Representative outcrops of recent laharic deposits showing interlayered debris flow (unstratified) and hyperconcentrated streamflow (stratified) deposits. In A, handle of shovel is 0.5 m long. In B, pencil is 15 cm long; note the thinly laminated deposits of hyperconcentrated, waning-stage hot lahar sandwiched between two debris-flow deposits at the top of the section.

In contrast, most beds resulting from lake breakouts are better sorted, fines-depleted and stratified streamflood deposits (fig. 14), commonly with crossbedding, although poorly stratified hyperconcentrated-flow beds are also present. Well-sorted, clast-supported accumulations of pumice fragments are common along portions of the channel where flows overtopped channel banks. Fine-grained deposits accumulated locally in stagnant areas such as abandoned channels and interlobe margins but are thickest within the boundaries of aggrading lahar channels and lahar-dammed lakes. Sediment components of the breakout streamflow deposits are mostly juvenile pumice fragments, with minor lithic fragments, mostly andesite, dacite, and gabbro, that were incorporated, together with old pumice fragments, from lateral and vertical channel scouring.

Figure 14. Typical deposits from Mapanuepe Lake breakouts. Note meter scale in A. Shovel in B is 0.7 m long.

INITIAL DRAINAGE DEVELOPMENT ON THE PYROCLASTIC WATERSHEDS

A ubiquitous layer of fine tephra fall initially mantled the surfaces of the pyroclastic fans, where it reduced water infiltration and increased the surface runoff, which first flowed as sheetwash and later developed into dense, pinnate rill networks (fig. 15). These incipient, deranged, intricate rill networks followed local topographic lows and left temporary accumulations in local depressions such as steam explosion craters and interlobe contacts between the deposits of individual pyroclastic flows. The covering of flat areas on the pyroclastic fans by coalescing small debris-flow tongues and lobes emanating from these incipient rill networks indicates that rill-gully flows were already sediment-enriched even at this stage. The early period of drainage development on the pyroclastic fan was one of constant micropiracy between various rills and channels, and the abandonment of the less efficient ones, generally those with low gradients. Small lakes and ponds had been formed where pyroclastic-flow deposits blocked tributaries from adjacent older terrain. The sudden release of water from these ponds facilitated channel incision on the pyroclastic fan and may have been a factor in determining which among the incipient channels would develop into main tributaries.

Figure 15. Incipient drainages on the Marella pyroclastic fan a month after the eruption.

Two months after the eruption, the networks of rills incised into the surfaces of the pyroclastic fans were feeding semipermanent, youthful, headward-eroding stream channels. Most of these incipient dominant streams developed along the margin of the pyroclastic fan where it abuts pre-1991 eruption topography; the streams then converged into the remnant trunk channels of previous major tributaries below altitudes of about 300 m. By the end of the 1991 rainy season, deep, narrow tributaries, about 15 to 70 m wide and less than 20 m deep, constituted the primary conduits for lahars funneled toward the Bucao and Santo Tomas Rivers. For a majority of these tributaries, the depth of vertical dissection was still well above those of pyroclastic-flow aggradation, although a few tributaries, particularly those within the Maraunot and upper Bucao watershed area, had eroded below the pre-1991 surface level by the end of the 1991 monsoon season (C.G. Newhall, oral commun., 1991). By the end of the 1992 rainy season, pyroclastic fans were densely dissected by deep, narrow channels (fig. 16), but some new upper tributary channels had widened enough to develop meanders. The channels that had formed along the contacts of pyroclastic fans with the older terrain adjacent to Pinatubo tended to erode laterally into the less resistant pyroclastic materials, thus shifting into the pyroclastic fields.

Figure 16. Dense, narrow rills and gully systems incised into the loose pyroclastic materials of the Marella pyroclastic fan after two monsoon seasons.

Fine ash winnowed from pyroclastic deposits by steam explosions replenished the primary ash deposits that had been eroded from the surface of the pyroclastic field. This mantle of fine materials minimized infiltration of rainwater into the pyroclastic materials and maintained relatively high rates of surface runoff on the pyroclastic fan.

CHANNEL RESPONSES TO LAHARS

PINATUBO DRAINAGES

The overall trend for channels draining the western pyroclastic fans has been dominated by erosion along the upper reaches, aggradation and erosion by laterally migrating channels along the middle reaches, and continuous aggradation along the lower reaches (fig. 17). Deposition and erosion, however, can occur locally along any reach, in response to changes in channel constrictions and bends, and in response to local changes in slope, as at migrating nickpoints. The boundaries of these zones change with each major flow event and are also sporadically altered by mass failures on the pyroclastic fans. For the Marella channel and the upper main tributaries of the Bucao River, the middle, transitional zones lie between 180 and 300 m in altitude.

Figure 17. A, Longitudinal profile of the Maraunot-Bucao River showing the level of pyroclastic-flow and lahar aggradation along the channel stretch after two monsoon seasons following the June 1991 eruption. B, Longitudinal profile of the Marella-Santo Tomas River showing the level of pyroclastic-flow and lahar aggradation along the channel stretch two monsoon seasons after the June 1991 eruption. Note very limited changes in stream level at the lower reach of both rivers.

Figure 18. Rate of aggradation at the PLHT/ZLSMG watchpoint at Dalanaoan, San Marcelino, after two monsoon seasons. The pre-1991 lahar profile is hachured, and the pre-1992 lahar profile (bold date) is shown by a heavy line. Floodwaters escaping from Mapanuepe Lake periodically scoured the channel (dashed profiles). The channel was finally abandoned after the lahar on September 23, 1992, and a new channel was incised to the west, near Mount Bagang.

Figure 18 documents about 20 m/yr of aggradation at the constricted portion of the Marella-Mapanuepe River junction during the last two monsoon season. The overall trend has been of continuous channel fill temporarily interrupted by episodes of scour during lake breakouts, mainly from the lahar-dammed Mapanuepe Lake. Repeated plugging led to the abandonment of the channel and the formation of a new channel midway between the old channel and Mount Bagang, an isolated ultramafic block, during the early part of August 1992.

The large width of the Bucao River and the soft, quick condition of the lahar deposits prevented us from conducting a similar detailed, repeated channel cross-sectional survey after each flow event. Qualitatively, however, the overall trend has been one of continued aggradation on the depositional fan between the Bucao-Balintawak junction and Malumboy, except along the north bank, where scour by normal Balintawak discharge maintains the principal channel and minimizes aggradation. Since the eruption, the 1.5-km-wide reach at Malumboy has aggraded at average rates of 7 m/yr. At the National Highway bridge across the Bucao River, 2.5 km from the coast, aggradation rates were 1.75 m/yr.

Downstream along the lower channel reaches, aggradation systematically decreased in response to changes in stream gradient and channel morphology. Where gradients are fairly uniform, thicknesses of lahar deposits do not vary much, changing dramatically only at major breaks in the stream profiles. Aggradation rates were relatively high upstream of channel constrictions, where repeated channel plugging caused flows to deposit. Downstream, flows spread in the wider channel, where their thickness, basal shear stress, and competence all decreased, so deposition began. Debris flows were deposited en masse, in contrast to more fluid hyperconcentrated flows and normal streamflows, which spread out and traveled farther downstream, carrying decreasing, finer-grained loads because the coarser sediments were selectively deposited en route. In consequence, the rivers experienced relatively rapid aggradation within the region of active debris-flow deposition and progressively less aggradation downstream beyond this point.

A combination of the foregoing factors may account for the disparate accumulation of volcanic debris along the Santo Tomas River below the Marella-Mapanuepe junction (fig. 17B). Gradually decreasing gradient and channel widening promoted debris-flow deposition at rates of more than 7 m/yr above 30 m in altitude. Channel infilling dropped dramatically below this altitude, tapering to thicknesses of 2 to 3 m/yr at 20 m in altitude. The low density and fine grain size of channel deposits along this lower segment results in continued sediment movement even during low flow conditions. The inflection in the stream profile at 50 m in altitude was smoothed by aggradation in 1991 and 1992, but the break in slope at 20 m altitude still persisted at the end of that period. Part of the channel condition along the lower reach may have resulted from human intervention, primarily dredging in order to maintain channel capacity.

At the Maloma River, a concrete dike extending 400 m upstream from the National Highway bridge was breached by floodwaters that reoccupied an old meander route that had been abandoned when the dike was constructed. By the end of the 1992 monsoon season, aggradation at the bridge was about 3 m above the preeruption stream level. A sharp curvature of the channel a kilometer upstream from the Maloma bridge may direct future flows to cut a route joining it to Anonang Creek. Aggradation of the Maloma River forced its Gorongorong-Kakilingan tributary to shift its channel course and flood an estimated 0.6-km² area south of the Maloma channel margin.

Only minor lahars, consisting principally of remobilized tephra fall, flowed along the Maloma River in 1991, because only about 5 km² of new pyroclastic deposits on the Pinatubo slopes are drained by the head of the northeasternmost Maloma tributary. However, continued headward erosion of this tributary into the pyroclastic field caused lahars to increase in frequency and extend farther downstream in 1992, where they delivered charcoal that local inhabitants began to harvest at the National Highway bridge in September.

LAHAR-DAMMED TRIBUTARIES

The rapid aggradation along the Bucao and Santo Tomas Rivers (about 20 m/yr at our Dalanaoan watchpoint and 7 m/yr at our Malumboy watchpoint) has blocked the mouths of non-Pinatubo tributaries and has caused ponds and lakes to form. The largest of these lahar-dammed lakes is Mapanuepe Lake in the southwest sector of the volcano, at the Marella-Mapanuepe confluence (fig.19). By the end of the 1992 monsoon season, the lake, with an average depth of 25 m, covered an area of about 8 km². The evolution of this lake is discussed in detail in a separate study by Umbal and Rodolfo (this volume). In 1992 another lahar-dammed lake of moderate size was situated upstream of the Marella-Mapanuepe junction at the confluence of the Mount Cuadrado tributary to the Marella River. Along the Bucao River system, the largest lahar-dammed lake was formed at the confluence of the Balintawak-Cabatuan River with the upper Bucao River. At its maximum extent, the area of this lake was about 4 km². With an average depth of only about 15 m, it was shallower than Mapanuepe Lake.

Figure 19. A view of Mapanuepe Lake, southwest sector of Mount Pinatubo. The lake was formed when the Mapanuepe River was blocked by rapidly aggrading Marella lahar deposits (light-colored deltaic margin in middle ground). The rising lake waters had already submerged three barangays at the time of this photograph.

Waters accumulating behind these natural dams eventually rose to levels sufficient to overtop and erode the lahar blockages. Intervals between damming and initial incision of a blockage depended on the height and stability of the blockage, and the rate of lake-level rise, which was dependent on the size of the river watershed and lake area. Lake breakouts from Mapanuepe Lake were able to remobilize more than a million cubic meters of the deposits of previous lahars in less than 2 days, after which discharge from the lake would slow down or stop altogether. Continued aggradation along the Bucao and Santo Tomas Rivers has prevented complete draining of the lakes, which have steadily increased in size, keeping pace with the rising channel-floor levels at the river junctions. These lakes along the margins of the lahar channels will survive as long as lahars of significant size continue to occur, possibly for more than 5 years.

NON-PINATUBO DRAINAGES

Even before the eruption, areas adjacent to the Maloma, Tanguay, and Kileng Rivers were lined with concrete and earthen flood-prevention structures to contain the perennial monsoonal flooding. After the eruption, flooding was exacerbated by rapid siltation of the channels from tephra-fall deposits eroded from the river watersheds. Siltation was so severe along the Tanguay and Kileng Rivers that the channel shifted toward the southern margin of the flood plain. Aggradation of the Tanguay River at the National Highway exceeded 3 m by the end of August 1991, reaching the level of the bridge. Overtopping floodwaters repeatedly eroded the southern approach of the bridge and swept away several hastily constructed bridges. This same section of the National Highway was again rendered impassable by floodwaters for much of the 1992 monsoon season.

ESTIMATED VOLUME OF REMOBILIZED SEDIMENTS

The amount of sediment transported during a single lahar is extremely variable, depending primarily on the intensity and duration of rainfall on the watershed area and upon the resulting relative dominance of debris flow and hyperconcentrated flow. Average discharge by individual lahars, reconstructed from field assessments, was on the order of 200 to 300 m³/s, but values exceeding 1,000 m³/s were not uncommon. Small- to moderate-sized lahars remobilized between 2x10⁵ to 10⁶ m³ of sediment; larger events lasting up to 4 h may have transported about ten times this amount.

Approximately 8.8x10⁸ m³ of sediments were remobilized by lahars along the Bucao, Santo Tomas, and Maloma Rivers during the 1991 and 1992 monsoon seasons (table 2), not including the sediments that reached the ocean. The aggregate area affected by these lahars is about 112 km², much of which was prime agricultural land. The sediment contributions by the major rivers are compared to the extrapolated amount of materials remaining to be mobilized in table 3. The total annual sediment yield by Pinatubo drainages for 1991 and 1992 is within the range of values predicted by Pierson and others (1992). An estimated 2.5x10⁷ m³ of sediments, mainly tephra-fall deposits, were eroded from river watersheds outside the Pinatubo drainage basin and deposited downstream, where they affected an area of roughly 6.5 km² (table 4). This amount, after correction was made for the estimated proportion of older materials, comprised 75 to 80 percent of the 1991 tephra fall emplaced within the river catchment basins (table 4). Most of the remaining tephra-fall deposits are situated on stable, flat or gently sloping terrain. Those remaining on steep to moderate slopes are already sheltered by rejuvenated vegetation that intercepts rains and minimizes raindrop impact. Thus, potential erodible deposits in the watersheds outside Mount Pinatubo already have been largely depleted.

Table 2. Area affected and amounts of sediment remobilized by lahars along major drainages on the western flank of Mount Pinatubo during the 1991 and 1992 monsoon seasons.

Table 3. Predicted and actual quantities of sediment remobilized during the 1991 and 1992 lahar seasons.

[Pyroclastic-flow volumes are the low estimates of Pierson and others (1992). Lahar volumes are from the PHIVOLCS Lahar Monitoring Team (1992) and the Zambales Lahar Scientific Monitoring Group (1992). Volumes are in millions of cubic meters]

* Estimated to be 10 percent of the volume of pyroclastic flow that could be eroded.

** The volume of remobilized sediments exceeded forecasts.

Table 4. Amounts and distributions of 1991 tephra-fall deposits and amounts subsequently remobilized by 1991 and 1992 monsoon rains in the Kileng and Tanguay watersheds.

DISCUSSION AND CONCLUSIONS: PROSPECTS FOR THE FUTURE

For almost all lahar-affected channels, including the Bucao River, sediment yields were lower in 1992 than in 1991. The only exception is the Santo Tomas River, which had a slight increase in sediment yield in 1992. Whether this apparent overall decrease indicates that sediment delivery rates by lahars are starting to taper is still an open question (Janda and others, this volume). Several lines of evidence suggest that this may not be the case.

Fewer lahars occurred in 1992 than in 1991; however, most were debris flows that on average transported more sediments per event than lahars in 1991. This increase in sediment transport may have been due to rainfall delivered by fewer but heavier storms in 1992. Another factor may be the continued enhancement of drainage integration that delivered materials more efficiently to the main channels.

In 1991 and 1992, the Pacific-wide weather condition termed the El Nino-Southern Oscillation (ENSO) caused the southwest monsoon rains to arrive late and to be unusually mild. The 1992 monsoon season was relatively short, and its monthly rainfalls were less than both the 1991 and the normal long-term values for the region. This ENSO episode was continuing in 1993, so the outlook for the timing and severity of the succeeding monsoon seasons was uncertain.

The worst-case scenario for lahars still had not happened by 1992. The largest, most destructive lahars would be triggered if a major typhoon were to approach Pinatubo, as last happened June 15, 1991. If no exceptionally heavy and protracted rainfall were to occur for several more years, it is possible, but not likely, that the reestablishment of vegetation cover would prevent such a worst case from happening at all.

In 1991 and 1992, lahars were confined mainly to the pre-1991-eruption river channels. By the end of the 1992 lahar season, however, these channels largely had been filled, so future lahars were expected to seek paths through previously unaffected areas. Prediction of the future avenues on the coastal plain of the Bucao River is relatively straightforward, owing to its well-defined topographic containments. The low river gradient west of the Baquilan confluence would probably not permit extensive debris flows near the coast, so the southern part of the plain was expected to continue aggrading slowly with the deposits of hyperconcentrated lahars and normal streamflows. Without engineering intervention, this portion should eventually aggrade sufficiently to cause flows to encroach on the southern outskirts of Botolan. A strong dike, anchored to the bedrock ridge west of Baquilan and properly footed to withstand the lateral erosion characteristic of hyperconcentrated lahars, could be extended to the coast south of Botolan to save that municipality from slow burial. The success of such a structure would be enhanced by periodic dredging

of the river from its mouth to the National Highway to facilitate disposal in the ocean of sediment that would otherwise contribute to aggradation in the channel and on the plain. A prominent submarine canyon funnels Bucao sediment discharge westward into deep water. A recent hydrographic survey by the Mines and Geosciences Bureau (A. Bravo and E. Domingo, oral commun., 1993) verified that the nearshore gradient of the canyon axis is about 110 m/km and that the canyon head extends shoreward to the vicinity of the southern part of the river mouth. Its presence should facilitate the disposal of dredging spoil.

The Maloma headwaters, which barely reached the new pyroclastic fan after the eruption, have already extended headward to a significant degree and may become a more active lahar conduit in the future, with serious consequences for the more than 1,000 inhabitants of Barangay Maloma. Increased lahar activity would enhance flooding and affect several sitios on the Maloma River flood plain. Detailed surveillance of the continuing headward extension of the Maloma headwaters, and the degree to which it might capture runoff from the Marella watershed, is also necessary in view of the consequences for Barangay Maloma and for the towns and villages of the Santo Tomas flood plain as well.

Along the Santo Tomas River, the record of aggradation since 1991 is clear (fig. 20). In 1992, the zone of maximum deposition, dominated by debris flows, had extended 3 km farther downstream than in 1991, to the vicinity of Barangay San Rafael. Greater aggradation rates in proximal compared to distal reaches cause downstream increases in gradient over time and possibly contribute to the downstream increases in debris-flow runout, a trend that may continue.

Figure 20. Longitudinal profiles of the Santo Tomas River below 250 m in altitude, showing the extent of aggradation by the various lahar types. The dashed arrows denote possible downstream extensions of each flow type in the future.

In late 1992, a dike along the southern bank of the Santo Tomas River was being planned to protect the municipalities of Castillejos, San Marcelino, and San Narciso with a dike, anchored to the east on the edge of the ophiolitic mountains west of Barangay San Rafael and extending to the coast (F. Soriquez, Department of Public Works and Highways, written commun., 1993). Such a dike would have provided an excellent test of the following hypothesis:

The eastern end of this structure will be most severely tested by laharic debris flows from the Marella River, and it must be built higher than the sum of (1) debris flows themselves (as thick as 9 m); (2) expectable run-up (when flows encounter a barrier, their inertia causes them to rise against gravity g by a vertical distance h that depends on flow velocity U according to Chow's 1959 equation U³ = 2gh). A debris flow with the velocity of 7 m/s, which is typical for this area, will run up about 2.5 m vertically against a dike); and (3) continued aggradation of up to 15 m/yr. The structure must also be built to withstand impact forces of 10 to 1,000 t/m² (Pierson and others, 1992), the buoyant lift due to the high flow densities of lahars, and lateral and vertical erosion by waning-stage hyperconcentrated flows that can undermine dike footings to undetermined depths.

Unfortunately, the dike was not built to such specifications and was breached and overtopped at both its eastern and western ends in 1993.

Future lahar paths through the Santo Tomas River sector are much more difficult to predict than they are for the Bucao alluvial plain. The numerous pre-1991 eruption creeks incised into the southern portion of the Santo Tomas alluvial plain are all potential avenues for avulsing lahars, especially creeks from the southern margin of the 1992 lahar field in the vicinities of Barangay San Rafael, and due north of San Marcelino.

ACKNOWLEDGMENTS

We thank all our fellow members of the Pinatubo Lahar Hazards Taskforce and Zambales Lahar Scientific Monitoring Group, especially M.B. Angeles, R.A. Arboleda, M. Bumanlag, R. Giron, V. Jalique, N. Lacadin, R.U. Solidum, R. Tamayo, R. Tan, J. Tor, and R. Viran, and field drivers F. Bunuan, S. Fragante, E. Gutierrez, P. Mabasa, R. Manuel, B. Rizaldo, R. Unating, and, especially, R. Rivera, who also performed as videocamera operator, electronics and electrical specialist, general handyman, cook, and morale booster at Malumboy. Director J.B. Muyco and Chief Geologist Edwin Domingo of the Philippine Mines and Geosciences Bureau, Director B. Austria and Professor E. Tamesis of the Philippine National Institute of Geological Sciences, Director R.S. Punongbayan of PHIVOLCS, Mr. F. Salonga of the Philippine Shipyard and Engineering Corporation, Undersecretary M.H. Pablo of the Philippine Department of Public Works and Highways, Mayor R.J. Gordon and the City of Olongapo, Botolan Mayor S. Deloso, Congresswoman K.H. Gordon, Gen. P. Dumlao, Dr. E.M. Sacris of Dizon Mines, the U.S. Navy's Cubi Point weather station, and the U.S. Marine Air and Ground Task Force provided invaluable logistic support. PHIVOLCS and USGS personnel of the Pinatubo Volcano Observatory were generous in sharing information. We also would like to express our appreciation to J.J. Major, G.V. Middleton, C.G. Newhall, and R.S. Punongbayan for their critical comments and suggestion for improving the manuscript. The research was supported by National Science Foundation SGER Grant EAR-9116724 and EAR Grant 9205132.

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