FIRE and MUD Contents
1U.S. Geological Survey.
2Philippine Institute of Volcanology and Seismology.
Monsoon and typhoon rains in the first rainy season following the June 15, 1991, eruption of Mount Pinatubo generated more than 200 hot (~50 °C) lahars in drainage basins on the east side of the volcano. Channelized lahars having peak discharges on the order of 100 to 1,000 cubic meters per second typically were noncohesive pumiceous debris flows, some of which transformed to hyperconcentrated flows prior to final deposition. Nearly all of the sediment in the lahar deposits (>90 percent) during this first rainy season was eroded from the thick pyroclastic-flow deposits filling valleys in the upper reaches of the watersheds. Lahar deposition occurred primarily on low-gradient, coalescing alluvial fans 15 to 50 kilometers downstream from the caldera at the base of the volcano, where deposit thicknesses generally ranged from 0.5 to 5 meters (mean thicknesses about 1.5 to 2 meters). Total depositional volume on the east-side alluvial fans in 1991 was about 0.38 cubic kilometers, which is almost one-third of the potential contributing volume from the source pyroclastic sediments. Sediment yields in 1991 were on the order of 1 million cubic meters per square kilometer per year for three of the five east-side drainages, nearly an order of magnitude greater than the maximum sediment yield computed following the May 18, 1980, eruption at Mount St. Helens, Washington, U.S.A.
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Mount Pinatubo volcano, situated at approximately 15° N. lat and about 100 km northwest of Manila in central Luzon, Philippines, stood 1,745 m above sea level prior to June 15, 1991. Its deeply dissected, forested flanks were drained by eight major river systems, five of which flowed eastward: the O'Donnell-Tarlac, Sacobia-Bamban, Abacan, Pasig-Potrero, and Porac-Gumain systems (fig. 1). The steep upper slopes of the volcano above about 1,000 m in altitude were deeply dissected, having slopes as steep as 60° to 70° and channel slopes ranging from about 7° to 20°. On the lower flanks of the mountain, from about 1,000 m down to 200 to 300 m in altitude, channel slopes flattened out to about 1° (gradient 0.01-0.02 m/m), and down to this point, streams had been deeply to moderately incised in older pyroclastic deposits. Heavily populated and intensively farmed alluvial fans and alluvial plains with gradients from about 0.02 to less than 0.0002 m/m extended beyond the volcano's lower flanks. On this alluvial apron, all of the rivers had been (at least in part) artificially straightened and constrained by earthen dikes.
Figure 1. Preeruption drainage map of Mount Pinatubo and vicinity, showing main rivers affected by the June 1991 eruptions.
Mount Pinatubo's climate is controlled by the easterly flow of the North Pacific trade winds and the Northeast Monsoon for most of the year, but from May through October a southwesterly flow of moist air, the Southwest Monsoon, dominates. These months generally are the rainiest of the year; August is usually the wettest month. Although major tropical storms (typhoons) may also strike during this season, it is more common for typhoons to pass to the north of Luzon. These typhoons exert a strong pull on the southwesterly flow and cause heavier than normal rainfall in the Mount Pinatubo area (Maj. W. Nichols, U.S. Air Force, oral commun., 1993).
Monsoonal rainfall is highly variable in space and time, and mountains about 20 km to the west and about 40 km to the southwest of Mount Pinatubo act as partial orographic barriers to complicate rainfall patterns further. Clark Air Base weather radar indicated that storm cells vary in size but are typically on the order of 10 km in diameter. The monsoon rains on the east side of Mount Pinatubo usually begin with heavy showers and thunderstorms for about 15 to 45 min, followed by light to moderate more continuous rainfall for up to 10 h (Maj. W. Nichols, U.S. Air Force, oral commun., 1993).
Numerous debris flows and hyperconcentrated flows, collectively termed lahars (Smith and Lowe, 1991), were triggered during and following the climactic explosive eruption of Mount Pinatubo on June 15, which erupted a total bulk volume of 8.4 to 10.4 km3. The eruption removed part of the mountain's summit and left a caldera about 2.5 km in diameter. It deposited abundant, loose volcaniclastic debris that would later be the primary source sediment for lahars--5 to 6 km3 of pumiceous pyroclastic-flow deposits in the heads of valleys draining the volcano and about 0.2 km3 of tephra on the volcano's flanks (out of the 0.7 km3 that fell on land) (W.E. Scott and others, 1991; this volume; Pierson and others, 1992; Paladio-Melosantos and others, this volume). About one quarter of this erupted material was deposited on the east flank of the volcano (fig. 2).
Figure 2. Distribution of 1991 volcanic deposits (thick, pumiceous pyroclastic-flow deposits; thin, pyroclastic-surge deposits; and June 15 coarse tephra-fall deposits) acting as primary lahar source sediments in east-side watersheds at Mount Pinatubo (after W.E. Scott and others, this volume; Paladio-Melosantos and others, this volume). Tephra that fell on slopes close to the caldera was largely swept away and mixed with subsequent flow and surge deposits. Distribution of thin, fine-grained ash layers that fell before or after the main June 15 eruption is not shown (see W.E. Scott and others, this volume). Locations of rain gages and watershed boundaries are also indicated.
During the 1991 monsoon season, lahars eroded and transported large volumes of 1991 eruption deposits within all the major watersheds draining the volcano. These lahars were triggered by (1) monsoonal rainstorms, sometimes enhanced by greater than normal southwesterly air flow during the passage of typhoons farther to the north, (2) volcanically induced convective rainstorms over localized heat sources (main vent and thick pyroclastic-flow deposits), and (3) breakouts from debris-dammed lakes (Rodolfo, 1991; Punongbayan and others, 1991; Umbal and others, 1991; Pierson and others, 1992; Oswalt and others, this volume; Rodolfo and others, this volume; K.M. Scott and others, this volume). By the end of October 1991 lahars had buried hundreds of square kilometers of agricultural land, caused extensive damage to homes, roads, and bridges in lowland areas surrounding the volcano, and displaced tens of thousands of people (Pierson and others, 1992; Rodolfo and others, this volume).
Lahars are discrete flow events involving highly concentrated mixtures of volcaniclastic sediment and water (Smith and Lowe, 1991) that can be triggered during or after volcanic eruptions by a number of mechanisms (Neall, 1976; Major and Newhall, 1989). Two rheologically distinct types of sediment-water flow, debris flow and hyperconcentrated flow (see Pierson and Scott, 1985; Smith, 1986; Pierson and Costa, 1987), occur as lahars at Mount Pinatubo. Differentiation of lahar types was based on field observations and evidence of relative yield strength and viscosity (see Pierson and Costa, 1987).
Debris flows are liquefied slurries of poorly sorted sediment (particles ranging from clay to usually the largest clasts available, including boulders) that (1) have the consistency of wet concrete, (2) possess high viscosity and a finite yield strength (a non-Newtonian "plastic" fluid), and (3) usually exhibit laminar flow, often with higher flow velocities than for similarly scaled water flows (Costa, 1984; Johnson, 1984; Pierson and Costa, 1987). Sediment concentrations are so high that sediment and water move en masse, and particles have great difficulty segregating by size or settling out of suspension. Fluid bulk densities of debris-flow slurries typically range from about 1.8 to 2.3 g/cm3 for grains with typical "lithic" particle densities of 2.4 to 2.7 g/cm3, giving volumetric sediment concentrations in the range of 50 to 75 percent, depending on grain-size distribution. The 1991 Mount Pinatubo deposits involved high proportions of lower density pumice, but volumetric sediment concentrations were similar. Debris-flow deposits characteristically exhibit massive internal structure (lack of internal stratification), very poor to extremely poor sorting, dense packing (often highly consolidated, even in fresh deposits), inverse to normal grading, and support of coarse clasts within a finer grained matrix.
Hyperconcentrated flows are dense suspensions of sediment in water, but concentrations are low enough for coarser sediment particles to be able to settle out of suspension when flow velocities decrease. These flows appear more viscous than normal-concentration streamflow, somewhat resembling dirty motor oil; flow is characteristically turbulent, but some turbulence is dampened by the higher fluid viscosity (Beverage and Culbertson, 1964; Pierson and Scott, 1985). Sediment concentrations typically range from about 20 to 60 percent by volume (depending on grain-size distribution), overlapping in range with debris flows (Pierson and Costa, 1987). Hyperconcentrated sediment-water mixtures possess a low yield strength (Hampton, 1972; Kang and Zhang, 1980), but normal-density gravel is not carried in suspension as it is in debris flows. Hyperconcentrated-flow deposits are characterized by (1) coarse sand to fine gravel textures, (2) poor sorting, (3) horizontal bedding (often very faint and thicker than typical fluvial laminae), (4) absence of crossbedding, and (5) intrastratal occurrence of small gravel lenses or outsized gravel clasts. All of these features suggest rapid aggradational deposition from suspension or traction (Pierson and Scott, 1985; Smith, 1986).
All of the 1991 lahars had raised temperatures from inclusion of hot pyroclastic-flow deposits. The maximum temperature measured in moving flows was about 60°C, and some fresh lahar deposits were as hot as 80°C. Recent hot lahars have also been described at Mayon volcano (Philippines) by Arguden and Rodolfo (1990), where effects of the raised temperatures were noted in flow behavior and deposit characteristics. Without "cold" lahars of similar composition at Mount Pinatubo for comparison, there was no way to determine whether slurry temperature significantly altered debris-flow rheology or sedimentology.
The hydrologic response of volcanic landscapes following explosive eruptions in tropical or semitropical regions (areas of potentially heavy rainfall) has been studied and described for several cases in Southeast Asia and Central America (Schmidt, 1934; Segerstrom, 1950, 1960, 1966; Ulate and Corrales, 1966; Waldron, 1967; Ollier and Brown, 1971; Kuenzi and others, 1979; Smart, 1981; Hamidi, 1989; Rodolfo, 1989; Rodolfo and others, 1989; Arguden and Rodolfo, 1990; Rodolfo and Arguden, 1991). Except for a general description of long-term sedimentation following the 1902 Santa María (Guatemala) eruption (Kuenzi and others, 1979), the hydrologic response to large eruptions (>=10 km3 of ejecta) had not been documented prior to this volume. This study, together with companion papers from this volume (Arboleda and Martinez; Major and others; Marcial and others; Martinez and others; Rodolfo and others; K.M. Scott and others; Tuñgol and Regalado; Umbal and Rodolfo), examines part of the hydrologic response of Mount Pinatubo following the eruption of June 15, 1991. Better understanding of the type, distribution, magnitude, and frequency of hydrologic processes at Mount Pinatubo is required for effective long-term hazard prediction (see Pierson and others, 1992).
This paper focuses on lahars triggered in eastward- and southeastward-draining watersheds after the main sequence of eruptions in June and covers the period from mid-July (when recording instruments were installed) to October 1991. The gap from June 15 to July 18 is not covered, but relatively few lahars were noted during this period. Data were collected on lahar occurrence in the Sacobia, Abacan, and Gumain Rivers, where radiotelemetered acoustic flow sensors (Hadley and LaHusen, 1991, 1995; Marcial and others, this volume) were deployed. Flow data were received and stored on computers at the Pinatubo Volcano Observatory (PVO), located in the base operations building at Clark Air Base, 25 km east of the volcano. The Pasig-Potrero River was an active contributor of lahars (Arboleda and Martinez, this volume), but continuous monitoring was not carried out there. Little information was available on lahar occurrence in the O'Donnell-Tarlac basin during the 1991 monsoon season, but lahar activity there appeared to be relatively minor.
Lahar sediment at Mount Pinatubo came from five distinct sources in 1991: (1) coarse tephra dropped from the eruption columns of the mid-June eruptions, (2) pyroclastic-flow deposits, (3) fine-grained tephra from ash-cloud deposits, phreatic explosions, and eruptive events postdating June 15, (4) 1991 lahar deposits, and (5) unconsolidated volcaniclastic deposits predating June 15. The spatial relations between these source materials and processes responsible for sediment mobilization are schematically depicted in figure 3 and discussed below. Grain-size distributions of representative samples of the different units are shown in figure 4.
Figure 3. Schematic representation of spatial relations between different types of volcanic deposits acting as lahar source sediments in upper watersheds on the east flank of Mount Pinatubo.
Figure 4. Grain-size distribution of representative samples of lahar source sediments from the east side of Mount Pinatubo. Pyroclastic-flow deposits sampled and analyzed by W.E. Scott and others (this volume). Hot lahar and fine tephra samples collected along Sacobia River at west end of Clark Air Base.
Coarse tephra (predominantly coarse ash to lapilli) fell on all surfaces on the east flank of Mount Pinatubo during the June 15 eruption, achieving accumulations of about 10 to 50 cm (fig. 2). On the broad, thick valley fills it is mostly interbedded with the pumiceous pyroclastic-flow deposits (W.E. Scott and others, this volume). On valley sides and upland surfaces not swept by the pyroclastic flows, it accumulated as a single, relatively porous layer (Paladio-Melosantos and others, this volume).
The coarse tephra was capped on the east side of the volcano by thinly bedded fine tephra (fine sandy to silty ash) that originated from frequent ventings from the caldera, phreatic explosions, and secondary pyroclastic-flow ash clouds. It was carried predominantly northeastward by prevailing winds (W.E. Scott and others, this volume). The deposits contained abundant accretionary lapilli and were as thick as 1 m. Owing to its grain-size distribution (fig. 4) and relatively loose packing, this layer of fine deposits absorbed water during the rainy season up to nearly the point of saturation and then could liquefy when disturbed--by being walked on or presumably by earthquakes. Overflights by helicopters in the upper Sacobia watershed revealed relatively flat areas where lateral-spread mass movements had mobilized this layer (W.E. Scott, U.S. Geological Survey, oral commun., 1993).
Two typical hydrologic effects of such tephra mantles are (1) the formation of incipient crusts on tephra deposits, which decrease the infiltration capacity of the ground surface, and (2) the destruction or burial of vegetation. The hydrologic response of ash-covered upland slopes is altered so that proportionately more rainfall runs off as surface flow and that it runs off more quickly (Segerstrom, 1950; Waldron, 1967; Collins and Dunne, 1986; Leavesley and others, 1989). The prodigious volumes of loose, erodible sediment widely distributed over the landscape acted both as a sediment source for lahars and as a runoff-enhancing infiltration barrier that also helped to generate lahars.
Erosion from steep tephra-covered hillslopes in the watersheds occurred as rill and gully erosion (fig. 5), with some minor shallow landsliding. Integrated rill and gully networks were well established by the middle of the monsoon season. By October, runoff from monsoonal rainfall had extensively dissected the tephra mantle with rills tens of centimeters wide, many of which had cut down to the preeruption ground surface. These rills extended fully from ridge crests to bases of slopes. Roughly 20 to 50 percent of these deposits had been eroded by the end of the first monsoon season, on the basis of visual estimates.
Figure 5. Rill erosion in coarse tephra at ridge crest 9 km southeast of summit caldera. Slope here is about 20°; headcut in center of photograph is about 20 cm wide. Tephra is about 40 cm thick at this site.
Pyroclastic-flow deposits extended as far as 16 km from the vent in east-side drainages and caused profound changes to the landscape by filling entire valleys and changing steep, deeply dissected terrain to broad, gently sloping (up to about 3°) plains. Total accumulated volume in eastward-draining watersheds was about 1.4 km3 (1.4x109 m3), with most of that accumulating in the heads of the Sacobia and Pasig River valleys. Total area covered was about 34 km2 (W.E. Scott and others, this volume). Mean total thickness of the deposits, by drainage basin, ranged from 15 to 48 m, with maximum localized thickness in the upper Sacobia valley in excess of 200 m (W.E. Scott and others, this volume). The great thickness of these deposits has caused diversions of surface drainage and the blockage of tributary valleys, resulting in the formation of unstable lakes, including several in the Sacobia basin.
Massive, very poorly sorted pumice layers tens of meters thick are the predominant facies composing most of the thick valley fills. Thinner layers of pumice-fall and ash-cloud deposits are interbedded within the main valley fills, and lithic breccias overlie the pumiceous deposits near the vent (W.E. Scott and others, this volume). The pumiceous deposits are predominantly sand (fig. 4) and have little gravel coarser than cobble size. Fine ash (finer than 0.0625 mm) makes up from a few percent up to a maximum of 18 percent of the deposit by weight (W.E. Scott and others, this volume).
The pyroclastic flows were very hot when emplaced, and heat retained in the deposits throughout 1991 profoundly affected subsequent geomorphic processes. The preeruption magma temperature at Mount Pinatubo was estimated to be 800±20°C (Rutherford and Devine, 1991), and the emplacement temperature of the pyroclastic flows was probably not much below this. Temperatures around 300°C were measured 30 cm below the deposit surface shortly after deposition (R. Hoblitt, U.S. Geological Survey, oral commun., 1991). During the 1991 monsoon season and continuing through the 1992 and 1993 seasons, numerous secondary phreatic explosions occurred almost daily, particularly during or shortly after rainstorms, due to rapid vaporization of water trapped beneath the hot pyroclastic valley fills (see Rowley and others, 1981; Moyer and Swanson, 1987). Explosions commonly sent ash eruption columns 3 to 4 km high, sometimes higher, and left explosion craters up to hundreds of meters in diameter. This activity regularly destabilized the valley-fill deposits during the rainy season and at times remobilized them as secondary pyroclastic flows (Torres and others, this volume). Erosion of the valley fills was facilitated by this phreatic activity through dilation of pyroclastic deposits (making them more vulnerable to erosion), disruption of surface crusting, and transport of loose debris directly into active channels.
Erosion of pyroclastic-flow valley-fill deposits has been by both bank erosion and vertical incision. In the narrow, vertically walled gullies and channels developed on the fill surfaces (fig. 6), sections of banks (typically 1 to 10 m3) topple or slide into the channels when undercut by lahars. Videotapes and direct observations of various types of flows in the lower part of the Sacobia valley fill showed that moderate to high discharges of hyperconcentrated flow or dilute debris flow--flows with fully developed turbulence--were most effective in erosively undercutting banks. On channel bends where lahars continuously impinged on vertical banks, lateral channel shifts of tens of meters per day were common. Bank erosion supplies large volumes of sediment directly to the lahars and results in progressive increases in lahar discharge with distance downstream. Collapse of hot pyroclastic-flow deposits into a passing flow causes especially vigorous steaming (fig. 7). Rapid (hourly to daily) fluctuations of channel bed levels (up to about 10 m) due to alternating bed incision and aggradation were observed during the 1991 rainy season in the lower valley fill in the Sacobia River valley. Bank erosion was particularly active during channel aggradation.
Figure 6. Erosion of thick, pumiceous pyroclastic-flow deposits filling the Sacobia River valley in early September 1991. A, Oblique aerial view of channel development approximately 7 km downstream from caldera rim. View looking south. White circular area in upper left center is hot, dry pumice ejecta from a phreatic explosion. B, Steep unstable bank of channel about 9 km downstream from the caldera rim. Bank collapse has exposed light-colored, dry, still-hot deposits (lower right); bank is probably 4 to 5 m high. Note the relative thinness (tens of centimeters) of the wetted zone on top of the pyroclastic-flow deposits. Dark hill (left center) is crest of ridge from preexisting topography.
Figure 7. Steam generated by collapse of very hot bank of pyroclastic-flow deposit into lahar in Sacobia River at west end of Clark Air Base, August 24, 1991. View looking downstream. Bank is 5 to 6 m high.
Repeated cutting and filling of channels by lahars on the surface of the pyroclastic-flow deposits resulted in multiple periods of lahar deposition during the aggradational phases. This resulted in a gradual replacement of pyroclastic-flow deposits with lahar deposits, particularly in the downstream portions of the valley fills. By October 1991, a large proportion (estimated 30 to 50 percent) of the distal Sacobia valley-fill pyroclastic-flow deposits (those adjacent to the west end of Clark Air Base) had been replaced by lahar deposits. Subsequent erosion of these deposits resulted in remobilization of deposits previously eroded from farther upstream. Bank collapse in the cooler lahar deposits was accomplished by sapping (erosion by outward-seeping ground water) at the foot of banks, as well as by undercutting from passing lahars. Lahar deposits in the valley-fill channels had the same general appearance as the primary pyroclastic-flow deposits, but minor differences included (1) lower temperatures (50-80°C at the distal end of the Sacobia valley fill); (2) slightly higher content of silt- and clay-sized particles (fig. 4) and consequently more cohesion; (3) slightly darker, browner coloration; and, if the deposits were fresh, (4) the presence of interstitial water.
Four automatic tipping-bucket rain gages (designated PI2Z, MSAC, FNGZ, and QADZ) were situated on divides of eastward-draining watersheds on Mount Pinatubo, no more than 9 km from the nearest other gage, during at least part of the 1991 monsoon season; a fifth, manual gage (CAB) was located farther east at Clark Air Base (fig. 2; table 1). The four automatic gages telemetered realtime rainfall data to PVO, while the CAB gage was read at 6-h intervals by U.S. Air Force meteorologists. These five rain gages covered a combined catchment area of about 300 km2 in terrain that is steep and rugged, and where precipitation is significantly influenced by orographic effects.
Measured daily rainfall amounts in 1991 at CAB were about average for the monsoon season at Mount Pinatubo (Maj. W. Nichols, U.S. Air Force, oral commun., 1991; U.S. Air Force, Environmental Technical Applications Center, Scott Air Base, unpub. report, 1988). August was the wettest month (fig. 8). A total of 60 rainstorms were recorded by the automatic gages during the period from July 18 to October 31, 1991, not all of which triggered lahars; a storm is here defined as a more or less continuous period of rainfall bounded by dry intervals of at least 6 h. Storms were both localized convective storms and more regional storms, usually related to the passage of a typhoon weather system nearby. Most storms in 1991 seemed to be the localized type, seldom delivering more than 100 mm of rain or having durations in excess of 10 to 15 h. Larger, apparently more regional storms typically delivered more than 200 mm of rain and had durations well in excess of 24 h (table 2).
Figure 8. Monthly rainfall totals for the 1991 monsoon season on the east side of Mount Pinatubo. Asterisked totals are incomplete for the month because of gage or telemetry malfunction. The PI2Z total for the last part of July was proportionally extrapolated to a full month on the basis of the partial-to-complete ratio at the CAB gage.
Table 1. Site characteristics of rain gages on the east flank of Mount Pinatubo during the period July 18-October 31, 1991.
Spatial distribution of total storm rainfall was highly variable at the five east-side rain gages in 1991, with consistent differences of at least several hundred percent between some of the gages (fig. 9). Similar disparities occur in storm durations and intensities (table 2). Heaviest rainfall was usually recorded at the QADZ gage (69 percent of storms), probably because of its higher altitude and the southwesterly tracks of most of the storms. Heaviest rainfall totals were recorded 21 percent of the time at PI2Z and 10 percent of the time at FNGZ. Of the storms analyzed, the MSAC rain gage never had the highest rainfall totals of the four automatic gages, which is likely a reflection of its low altitude and the rain-shadow effect of the ridge on which the QADZ gage is located.
Figure 9. Comparison of total storm rainfall recorded at the four automatic rain gages and CAB for representative storms of different durations. Each of these storms triggered lahars.
Table 2. Characteristics of rainstorms that triggered lahars on the east side of Mount Pinatubo (rain gages PI2Z, MSAC, FNGZ, and QADZ) for period July 18 -October 31, 1991.
Fifty-two of the 60 recorded storms triggered lahars in east-side watersheds in 1991 (table 2). Eighty-five percent of the lahar-triggering storms had durations less than 20 h (most less than 10 h) and are considered to be localized convective storms. This localization is evident in the spatial distribution of rainfall as reflected in differences in storm totals shown in table 2. In 23 percent of the cases, lahars were triggered when no rain at all was recorded at one of the 4 automatic gages, and no rain was recorded at two of the four gages for 12 percent of the lahar-triggering storms (table 2). Given the typically small cell diameter for convective storms (about 10 km), which is close to the average distance separating adjacent gages (9.3 km), it is unlikely that many of the rainstorm characteristics measured at a single rain gage would be representative of the rainfall that actually triggered lahars. Therefore, rainfall totals and intensities recorded at east-side rain gages must be considered to be only minimum values for any given storm.
In order to test whether rainfall-intensity thresholds for triggering lahars could be defined by using data from all four automatic gages, the highest rainfall intensities (both average storm intensity and maximum rainfall-burst intensity) from among the four rain gages were used to construct rainfall intensity-duration plots (fig. 10) for the 1991 rainstorms (see Caine, 1980; Rodolfo and Arguden, 1991; Rodolfo and others, this volume; Tuñgol and Regalado, this volume). Storms were differentiated as those causing no lahars, those causing at least one minor lahar, and those causing at least one major lahar (see table 2 for definitions of major and minor lahars). Although the big storms clearly triggered major lahars, major lahars were also triggered by seemingly small, low-intensity storms. These anomalies can be explained if it is assumed that the "small" storms are really just the edges of bigger but localized, more intense convective storms that have slipped in between the rain gages. The upper limit of storm intensities not triggering lahars would be a more valid basis for definition of a lahar-trigger threshold, but not enough nonlahar storms were recorded in 1991 to do that. Either a denser rain-gage network or a longer record is required to characterize lahar-triggering rainfall accurately in these upper basins. It also follows that rainfall data from any one telemetered rain gage would be unreliable for forecasting lahars for the purposes of warning downstream communities; the use of all four gages together improves reliability somewhat.
Figure 10. Intensity-duration plots for 1991 rainstorms; data points for each storm are the highest intensity values from the PI2Z, MSAC, FNGZ, or QADZ rain gage. Note that the different storm types do not readily fall into separate fields. A, Average storm intensity versus duration. B, maximum burst intensity versus duration (bursts typically 0.1- to 1-h duration).
"Volcanic thunderstorms" were a phenomenon that occurred throughout the 1991 monsoon season, locally augmenting the normally occurring rainfall in the vicinity of Mount Pinatubo and triggering lahars. U.S. Navy and Air Force meteorologists repeatedly observed (on weather radar) a spatial and temporal association between the formation of intense convective storm cells and localized volcanic heat sources, such as pyroclastic- flow deposits and the eruptive vent (Pierson and others, 1992; Oswalt and others, this volume). Thus, some lahar-triggering rainstorms can be generated directly by volcanic activity if atmospheric conditions are favorable.
Most of the lahars occurring between July and October 1991 on the east side of Mount Pinatubo appear to have been directly triggered by rainfall, although the connection between measured rainfall and lahar magnitude or frequency is not clear (figs. 10, 11). Outbreaks of lakes temporarily dammed by pyroclastic-flow or lahar deposits occurred during or following periods of heavy rain and apparently augmented rainfall runoff in triggering lahars on at least three occasions: July 25 on the Sacobia River (W.E. Scott and others, this volume), August 21 on the Sacobia River (Pierson and others, 1992), and September 7 on the Pasig-Potrero River (Pierson and others, 1992; K.M. Scott and others, this volume). In the last case, the lahar caused fatalities. Major lahars occurred on several other occasions when little or no rainfall was recorded (table 2), but, as previously noted, these may have been due to undetected localized storms.
Figure 11. Comparison of maximum daily rainfall (rain gage indicated) with daily lahar totals from the LSAC, ABAC, and GUMA flow sensors (see figs. 2 and 17 for locations) for most of the 1991 monsoon season. Flows counted as separate lahars if peaks separated by at least 1 h and acoustic intensities (at 10-100 Hz) greater than 1,200 acoustic units (for LSAC and ABAC) or 800 units (for GUMA).
Acoustic flow sensors (Bautista and others, 1991; Hadley and LaHusen, 1991, 1995; Marcial and others, this volume) were installed along river channels to record ground vibrations produced by passing lahars. Locations of two flow sensors along the Sacobia River (USAC and LSAC, situated 8.7 km apart), one along the Abacan River (ABAC) at the point of stream capture from the upper Sacobia basin, and one along the Gumain River (GUMA) are shown in figure 2. Acoustic signals were transmitted back to PVO via a ground-based radiotelemetry system. When plotted against time, signal intensity plots resemble hydrographs (fig. 12). Between July 18 and October 28, a total of 219 lahars were recorded. Eyewitnesses noted the occurrence of additional lahars between June 15 and July 18, before the flow sensors began operating, and some small lahars probably occurred in November.
Figure 12. Rain-gage and acoustic flow-sensor data on same time base for a part of August 1991, showing the spatially varied nature of the rainfall and the uncertain relation between lahars and measured rainfall (some lahars apparently beginning before rainfall begins).
Direct sampling of lahars during the monsoon season was limited, but videotapes and observations of numerous flows in the Sacobia and Abacan Rivers indicate that lahars ranged from turbulent, erosive hyperconcentrated flows (fig. 13) to viscous, usually laminar debris flows (fig. 14). A sample of muddy streamflow from the Sacobia River collected between lahars has finer sediment and is much less concentrated (table 3).
Figure 13. Example of hyperconcentrated flow occurring in the Sacobia River about 20 km downstream from the caldera on September 26, 1993; flow is from left to right. Flow depth is 45 to 50 cm; width is about 30 m. Surface velocity is varying between 2.5 and 3.1 m/s, and fluid temperature is ambient. Sediment concentration of the flowing mixture is 39 percent sediment by weight (23 percent by volume). This flow was actively eroding laterally as the picture was taken; sections of the bank in the foreground were toppling into the flow every few minutes.
Figure 14. Example of debris flow occurring in the Sacobia River at two adjacent sites about 16 km downstream from the caldera on August 20, 1991. Flow is estimated to be about 2 m deep and 15 m wide; surface velocity varied between 4.8 and 5.9 m/s (at site in photograph A), and fluid temperature was approximately 50 to 60°C. On the basis of these flow parameters, flow should be supercritical. Sediment concentration of the fluid mixture was 85 percent sediment by weight (70 percent by volume). The flow in photograph A (immediately upstream of photograph B) appeared to be flowing laminarly (toward the camera) and appeared to have a "rigid plug" in the center part of the flow. The flow in photograph B (moving left to right) erupted in violently turbulent standing waves (about 1 to 2 m high) where the channel is slightly more constricted and slightly steeper than in A. Such turbulence was very seldom observed in debris flows having such high sediment concentrations, although nonbreaking standing waves were not uncommon.
Table 3. Physical properties of lahar dip samples from the August 20 Sacobia River lahar and the September 4 Abacan River lahar compared to a low-flow sample (September 8, Bamban River).
Dip samples were collected directly from the surface of an apparently laminarly flowing debris flow in the Sacobia River on August 20 and from the surface of a very turbulent lahar, which had the appearance of a dilute debris flow, in the Abacan River on September 4. The samples are representative of the sampled flows, because very little material moving in the channels was coarser than fine gravel. Both flows were intensely steaming hot lahars with temperatures of about 50-60°C, but no unusual aspects of flow behavior were observed that might be attributable to temperature. The Sacobia slurry samples were composed predominantly of sand-size particles with gravel-fraction particle densities ranging from 0.96 to 1.47 g/cm3, owing to the high pumice content (table 3). Sample bulk densities of 1.66 to 2.02 g/cm3 correspond to sediment concentration fractions of 0.73 to 0.85 by weight (0.55 to 0.70 by volume). Although the Abacan River lahar was more turbulent than the Sacobia flow (for approximately equivalent distances downstream from head of basin), sediment concentrations were not much less. The two Abacan samples from the west end of Clark Air Base both had sediment concentration fractions of 0.72 by weight; slightly different average particle densities changed the volume fractions to 0.51 and 0.55 (table 3). A sample from the same flow collected about 9 km downstream 30 min later had a sediment concentration fraction of only 0.44 by weight. In general, the lahars became more dilute with distance from source, and transformations to hyperconcentrated flow commonly occurred as lahars reached or flowed out onto the alluvial fans.
Extremely pumice-rich debris-flow depositional units within 1991 lahar deposits in the east-side drainages confirm evidence from surface dip samples that the valley-fill pumiceous pyroclastic-flow deposits were the dominant sediment source for the 1991 lahars. Lithic-rich deposits (having particles of normal density) were rare and were limited to relatively thin sandy layers deposited by hyperconcentrated flows or normal streamflow (fig. 15). This is in contrast to field observations of 1992 and 1993 lahar deposits, which more typically showed a marked differentiation between lithic-rich and pumice-rich debris-flow facies within single lahar depositional units; this differentiation suggests density stratification or segregation within flows and a mixed source area for later lahars (1991 pumiceous deposits and older lithic-rich volcaniclastic deposits).
Figure 15. Still-warm, unstratified debris-flow deposit from the August 20-21 lahars at the San Francisco bridge, which is about 41 km downstream from the caldera on the Bamban River. Photograph taken on August 23. The debris-flow deposit is extremely pumice rich and is sandwiched between two thin, darker, lithic-rich sand units deposited during less concentrated flow. The shovel grip is 14 cm wide.
Grain-size distributions of the dip samples were quite similar to the distributions of the pyroclastic-flow source deposits upstream (fig. 16). Distribution similarities were especially close for grain sizes finer than medium sand; gravel fractions fluctuated more. All the lahar samples were very poorly sorted, as is typical for debris-flow samples (table 3). Most of the samples are positively skewed (excess fine material) and leptokurtic (tails of distribution more poorly sorted than central portion).
Figure 16. Cumulative grain-size distribution curves for samples of flowing lahars on August 20 (Sacobia River) and September 4 (Abacan River), and one sample of low-discharge "normal" flow on September 8 (Bamban River at San Francisco bridge).
Three acoustic flow sensors, the LSAC, ABAC, and GUMA stations (fig. 2) detected passage of 219 lahars down the Sacobia, Abacan, and Gumain Rivers through the 1991 monsoon season. They were in operation for 105, 92, and 55 days, respectively, between July 18 and October 31, and they recorded 93, 95, and 31 lahars each. A fourth flow sensor (USAC) was located in the upper part of the Sacobia basin. These totals break down to average daily rates of 0.9, 1.0, and 0.6 lahars per day, respectively. Lahars also occurred in the O'Donnell-Tarlac and Pasig-Potrero Rivers but could not be documented on a continuous basis. During August, the occurrence of three to five lahars per day in a channel were common (fig. 12); the maximum recorded in a single day was 7. The lower rate in the Gumain River could have been due to either instrument insensitivity (the flow sensor was located farther from the channel than at either of the other two sites) or the relatively smaller supply of source sediment in that basin (Pierson and others, 1992).
Most of the posteruption lahars occurring after the typhoon that accompanied the June 15, 1991, eruption (Major and others, this volume) were triggered by localized monsoonal rainstorms, although some were triggered by more regional storms, and several were augmented by lake breakouts. The 1991 lahars from east-side drainages (contributing catchment areas of approximately 50 to 250 km2) generally were of moderate magnitude compared with other lahars worldwide, particularly those generated by rapid snowmelt (Pierson, 1995). Typical posteruption lahars were estimated to be 2 to 3 m deep, 20 to 50 m wide, and moving at 4 to 8 m/s (surface velocity, in channels up to 10 km upstream of fan heads) with peak discharges in the range of about 200 to 1,200 m3/s. A few lahars in late July were as deep as 5 m and as fast as 11 m/s, with peak discharges possibly as large as 5,000 m3/s. Average celerity (measured peak to peak) of four sharply peaked lahars between the USAC and LSAC stations (8.7 km apart) varied between 3.6 and 12.1 m/s. The acoustic intensity of the lahars on the flow sensor records had not been calibrated to actual discharge in 1991.
The 1991 lahars commonly were multipeaked and had flow durations of 2 to 4 h. If a right triangle is taken as a simple model for hydrograph shape, the range of common discharges above would result in an approximate average lahar volume per event (water and sediment) of between 0.7x106 and 8.6x106 m3. Because debris-flow hydrograph recessional limbs typically show an exponential rather than a linear decrease in discharge (Weir, 1982), the above values probably overestimate lahar volumes somewhat; 0.5x106 to 5x106 m3 might be a more realistic range.
During the 1991 monsoon season, phenomenally large volumes of sediment were transported out of the upper east-side watersheds at Mount Pinatubo, primarily by lahars, and deposited (1) in and adjacent to valley-confined stream channels downstream from major pyroclastic valley fills, (2) on the low-gradient coalescing alluvial fans at the foot of the volcano, and (3) slightly beyond the fans on very flat alluvial plains and in coastal marshes (fig. 17). Most of the deposition occurred on the distal ends of the fans, with thinner fine-grained deposits extending out in channels onto the alluvial plain. Individual depositional units of debris flows, found predominantly in the channels upstream of fans and on the proximal parts of the fans, were generally 1 to 2 m thick. No crusting or discoloration was observed that might be attributed to slurry temperature, as has been reported elsewhere (Arguden and Rodolfo, 1990). Aggradation by deposition in confined valleys, which was as much as 25 m, and accompanying geomorphic changes have been reported by Punongbayan and others (1991) and by K.M. Scott and others (this volume).
Figure 17. Pattern of lahar deposition primarily on alluvial fans on the east side of Mount Pinatubo following the 1991 monsoon season, relative to the distribution of pyroclastic source sediment (thick, pumiceous pyroclastic-flow deposits and thin pyroclastic surge deposits). Average thickness of deposits on fans is approximately 2 m. Alluvial fans and alluvial plains are defined on the basis of topography and drainage patterns.
Alluvial fan surfaces, primarily from about 25 to 45 km away from the caldera, were the east-side areas most seriously affected by lahar deposition during the 1991 monsoon season (figs. 17, 18). Sediment deposition on the east-side fans generally ranged from 0.5 to 5 m in thickness and appeared to average about 2 m. This aggradation caused rivers to avulse out of existing channels (including engineered channels) and to spread the deposition in broad, braided channels over large areas of countryside (fig. 18). In general, deposition occurred primarily on the distal ends of fans during the early part of the monsoon season. Later in the season, areas progressively farther upstream became inundated, although some of the down-fan depositional areas expanded as well.
Figure 18. Lahar deposition on alluvial fans on the east side of Mount Pinatubo. A, Oblique aerial view (looking west, upstream) of lahar deposition along Bamban River about 35 km northeast of caldera rim on August 15, 1991. Left-bank channel levee was breached on June 15. B, Lahar deposition on either side of levee-confined Pasig-Potrero River, about 34 km southeast of the caldera rim on August 23, 1991. View is looking southwest, across the fan.
Deposits of the 1991 lahars, including those of the syneruption events (Major and others, this volume) covered more than 200 km2, most of it prime agricultural land (table 4). Many more tens of square kilometers were inundated by backflooding that was caused by the blockage of preeruption stream channels and canals by the aggrading lahar deposits (fig. 19).
Figure 19. Backflooding and transient lake caused by the damming of the Marimla River by lahar deposits along the main Sacobia-Bamban channel, August 23, 1991. That main channel visible in distance. View is looking east. Breakout of a former lake here augmented the lahar on August 21 that destroyed the Bamban bridge.
Lahar deposition on farmland and in towns has forced tens of thousands of residents into refugee evacuation centers. Lahars buried or otherwise adversely affected 111 barangays (villages) and 3 separate military installations on the east side of the volcano. In addition, bank erosion from lateral migration of the Abacan River channel destroyed hundreds of homes and businesses around Angeles City (fig. 17). Most major highway bridges on all sides of the mountain had been destroyed by the end of 1991. The Sacobia-Bamban channel aggraded about 20 m in just over 2 months (E. de la Cruz, Philippine Institute of Volcanology and Seismology, oral commun., 1991), and the Highway 3 (MacArthur Highway) bridge crossing the channel was destroyed by a lahar on August 21 (fig. 20).
Figure 20. Site of former Highway 3 (MacArthur Highway) bridge at Bamban town (concrete bridge piers visible in channel) and remaining spans of railway bridge across Sacobia-Bamban River on August 23, 1991. View is upstream, looking southwest. The highway bridge was destroyed by a lahar on August 21; the railway lost one bridge span (far right of photograph) to lahars on June 15 and long segments of earthen embankment (upper right-center and upper left of photograph) on August 21.
The total volume of volcaniclastic sediment deposited in 1991 in the east-side basins is about 0.38 km3 (3.8x108 m3), which is equivalent to about 30 percent of the total volume of the 1991 pyroclastic source deposits in the upper basins. Approximate depositional volumes and sediment yields for the eastward-draining basins are shown for basins individually in table 4. Volumes are derived from contour maps of deposit thicknesses based on aerial photography, preeruption topography (maps with 10- and 20-m contour intervals), and field measurements or estimates of thickness. Given the imprecise nature of preeruption map information and the local variability of deposit thickness, error bars of at least ±20 percent should be attached to these volumes. The computed volumes of sediment eroded from east-side watersheds in 1991 are at the upper limit or exceed the range of volumes of eroded sediment that were predicted by Pierson and others (1992) (table 4).
Annual total sediment yields, computed on the basis of 1 year's data, are on the order of 106 m3/km2 y for three of the basins (table 4). These values are extraordinarily high--about an order of magnitude higher than those computed for Mount St. Helens following the May 18, 1980, eruption (Janda and others, 1984). Such large amounts of sediment being transported have had significant effects on channel morphology (K.M. Scott and others, this volume).
Table 4. Sediment volumes delivered to depositional areas (primarily alluvial fans) and sediment yields for east-side drainages at Mount Pinatubo, June-October 1991, based on areas of deposits and measured or estimated deposit thicknesses contoured over the depositional areas.
In the first rainy season following the June 15, 1991, eruption of Mount Pinatubo, seasonally typical monsoonal rainfall on the east side of the volcano generated hundreds of hot lahars. The primary sediment sources for the lahars in each watershed were the thick pyroclastic-flow deposits that filled the heads of the valleys tens to hundreds of meters deep. Lahar deposition occurred mainly on the apron of coalescing alluvial fans surrounding the volcano. Deposition began early in the season at the downstream ends of the fans and progressed up-fan during the later part of the 1991 monsoon season.
Extraordinarily large volumes of volcaniclastic sediment have been transported from the flanks of the volcano to the surrounding alluvial fans and plains. After only one rainy season, the lahar deposits covered more than 200 km2 with several meters of coarse volcaniclastic sediment. Total lahar-deposit volume on the east-side fans at the end of 1991 was more than one-third of a cubic kilometer, which is equivalent to about 30 percent of the pyroclastic-flow valley-fill deposits (primary source sediments) in the heads of the drainage basins. Sediment yields for three of the watersheds, on the order of 106 m3/km2 y, are about an order of magnitude higher than yields computed for the Toutle River at Mount St. Helens following the 1980 eruption there, which had been among the highest ever computed for moderately sized drainage basins.
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