1Philippine Institute of Volcanology and Seismology.
2Michigan Technological University, Houghton, MI 49931.
Hot lahars occurred in the Sacobia-Bamban River system from June to September 1992. Intensified monsoon rains during the third week of August through the first week of September 1992 initiated the largest debris flows. Approximately 0.7x108 cubic meters of 1992 lahar debris, added to 1.0x108 cubic meters of debris from 1991 lahars, covered 90 square kilometers by the end of 1992.
Field observations of active lahars showed severalfold attenuation between Mactan (16 kilometers from the summit) and Maskup (24 kilometers from the summit), especially in those flows which had peak discharge of <200 cubic meters per second at Mactan. These smaller lahars also traveled about half the speed of lahars with discharge rates >1,000 cubic meters per second. Some flows transformed from debris flows to hyperconcentrated streamflows and muddy streamflows between the two observation posts.
Channel aggradation, most pronounced in the middle and upper parts of the Sacobia-Bamban fan, caused numerous avulsions and thus severe damage to homes and agricultural lands in the towns of Bamban and Mabalacat; the town of Concepcion, badly hit in 1991, was largely spared by upstream avulsions in 1992.
Note to readers: Figures open in separate windows. To return to the text, close the figure's window or bring the text window to the front.
The June 1991 eruptions of Mount Pinatubo deposited an estimated 5 to 7x109 m3 of pyroclastic materials in the major valleys radiating from the volcano's vent (Daligdig and others, 1992; W.E. Scott and others, this volume). These unconsolidated, loosely compacted pyroclastic-flow deposits are highly erodible and are the primary source of Pinatubo lahars.
About 6x108 m3 of these pyroclastic-flow deposits were emplaced on the eastern slopes of Mount Pinatubo, forming the Sacobia pyroclastic fan (fig. 1) (W.E. Scott and others, this volume). The deposit separates into two main lobes downslope, a southeast lobe drained by the Pasig-Potrero River and a northeast lobe that, initially, was drained by both the Abacan and Sacobia Rivers. A secondary pyroclastic flow on April 4, 1992 (Torres and others, this volume) eroded a narrow divide between the Sacobia and Abacan rivers, allowing the Sacobia to capture all flow from the northeast lobe of the Sacobia pyroclastic fan.
Figure 1. Distribution of the 1991 pyroclastic flow and lahar deposits on the east flank of Mount Pinatubo and locations of lahar observation posts and remote instruments.
From December to May of each year, the climate of Mount Pinatubo is dominated by the northeast monsoon and is relatively dry. Much greater precipitation occurs from June to September with the onset of the southwest monsoon. The heaviest rains of all are brought by typhoons that pass over or northeast of the Pinatubo area during these months and October. Typhoons passing northeast of Pinatubo increase rainfall by enhancing prevailing southwest monsoonal air flow.
Heavy precipitation and runoff erode material from the upper slopes, especially by downcutting and widening of gullies on the pyroclastic valley fill. Mixtures of water and sediment, with temperature ranging from 38° to 59° C and 40 to 90 percent sediment (by weight) flowed as far as 40 to 60 km from their source, burying agricultural lands and communities. This paper summarizes observations of active lahars, lahar channels, and lahar damage along the Sacobia-Bamban River during the 1992 rainy season.
From the volcano's summit down to 1,000 m in elevation, the preeruption channel of the Sacobia River occupied a deep, narrow, V-shaped canyon, and the channel gradient decreased from 30° to 10°. From 1,000 m down to about 200 m in elevation, the channel slope decreased to between 6° and 1°. Along this reach it cuts into 600- to 3,000-year- old nonwelded pyroclastic-flow deposits (Newhall and others, this volume; Umbal and Rodolfo, this volume). From 200 m (opposite the Mactan gate of Clark Air Base) to 100 m in elevation (8 km downstream), the average slope decreases to 0.62° (see fig. 2), and the channel becomes box shaped. In this reach the channel is at least 500 m wide and erodes into old lahar terraces and well-indurated fluvial deposits. At 100 m in elevation, it reaches a 30-m-wide constriction (Maskup), below which the Sacobia channel is joined by two tributaries, Sapang Kawayan and the Marimla River, and the resulting river is known as the Bamban River. The pre-1992 channel depth at Maskup was 7 m.
Figure 2. Longitudinal profile of the Sacobia-Bamban River, starting at 350 m in elevation, 12 km from the summit of Mount Pinatubo, and extending down to less than 50 m in elevation. Preeruption profile is shown by the lower solid line; 1991 lahar deposit by the dashed line; 1992 lahar deposit by the upper solid line. Note that vertical exaggeration is 5x and that only the lower, gently-sloping reaches of the Sacobia are shown.
The first lahar along Sacobia-Bamban channel occurred on June 14, 1991, and larger lahars occurred during the climactic eruption of June 15; these covered the broad floor of the Sacobia channel upstream from the town of Bamban (Major and others, this volume). More devastating lahars occurred during periods of intense monsoonal rainfall a few months after the climactic eruption (Pierson and others, this volume).
On the basis of data from acoustic flow sensors (Hadley and Lahusen, 1991), lahars in the Sacobia and Abacan occurred (on average) once every 1.2 days, and 3 to 5 lahar events per day were common during heavy rains in August (Pierson and others, 1992).
Initial channel response near the fanhead was aggradational, with deposition of 3 m at Mactan (figs. 1, 2). However, this initial phase of deposition was followed by rapid downcutting from late August to early September. Downstream, in the 8-km-long, 500- to 800-m-wide channel from Mactan to the constriction at Maskup (figs. 1, 2), deposition was at least 5 to 8 m and perhaps as much as 15 m (C.G. Newhall, written commun., 1992). Still farther downstream, at the Bamban Bridge, initial channel response in July was scouring, but, by late August, 20 m of aggradation had occurred at the Bamban Bridge (Pierson and others, this volume; K.M. Scott and others, this volume). The 1991 lahar deposits covered a total area of about 80 km2 of agricultural lands and villages (Pierson and others, 1992) (fig. 3A).
Sapang Kawayan, the Marimla River, and a smaller tributary of the Sacobia just downstream from Maskup (fig. 3A), were dammed by aggradation in the Sacobia. Lakes formed behind these natural dams. Sediment that had dammed the Marimla River was breached on August 21, 1991, and the outbreak flood contributed to the lahars that destroyed the Bamban River Bridge along the MacArthur Highway (fig. 3A) (Pierson and others, 1992).
Figure 3. A, Lahar-affected areas along the Sacobia-Bamban River in 1991. Major settlements and significant points along the river are numbered. The two main roads connecting Manila with northern Luzon, the MacArthur Highway and Magalang-Concepcion Road, were cut and threatened, respectively, by major lahar avulsions. B, Distribution of 1992 lahar deposits superimposed on 1991 lahar deposits. Early 1992 lahar deposition was on and north of 1991 deposits; deposition in late August and September was on and south of 1991 deposits. Cross-sectional profiles are illustrated on figure 8.
During the 1992 rainy season, two observation posts were established along the Sacobia channel to monitor discharge and flow characteristics. The first post (Mactan) is about 0.5 km downstream from the Mactan Gate of Clark Air Base and about 16 km from Pinatubo's summit. Located at the south bank of the channel at about 240 m in elevation, the post overlooks a 60-m-wide constriction in the channel. The second observation post (Maskup), is located at another constriction at 100 m in elevation, about 8 km downstream from Mactan and 4 km upstream from the town of Bamban. The Maskup post is also located on the south bank of the channel and overlooks a 25- to 30-m-wide constriction.
Observations were by the PHIVOLCS Lahar Monitoring Team, often under trying conditions of rainfall, steam and secondary explosions, ashfall, and darkness. Estimated flow velocities are surface velocities; flow depths were estimated by measuring visible channel depth before and during each event and then subtracting the latter from the former. Such estimates of flow depth are necessarily uncertain; actual depths could have been greater (if the flow was scouring) or less (if the flow was depositing). Channel widths were usually taken to be the width of the freely flowing surface or, if that could not be seen through the steam, the width of the channel as measured before each event.
Information presented in this paper about 1992 rainfall came from a lone rain gauge installed near the divide between the northeast and southeast lobes of the Sacobia pyroclastic fan, and instrumental records of 1992 lahars came from an acoustic flow sensor that was installed in 1991 about 2 km upstream from Mactan (fig. 1) (Hadley and LaHusen, 1991; Marcial and others, this volume). An alarm automatically sounded at our Pinatubo Volcano Observatory on Clark Air Base when preset thresholds of rainfall or flow were exceeded.
Lahars along the Sacobia-Bamban channel were triggered by three different types of rainfall: (1) ordinary, short-duration, localized rain showers on the upper slopes of the volcano; (2) heavy rains brought about by a passing typhoon; and (3) heavy precipitation lasting for days, brought by the southwest monsoon.
During June, lahars were infrequent (on average, one every 6 days). Rain showers of at least 6 to 8 mm in 40 min generated hot lahars, with peak discharges of up to 75 m3/s and temperatures of 38° to 59° C. Rainfall and lahars began to increase in July (fig. 4A,B), but lahars were still relatively small, with peak discharges of about 150 m3/s. All lahars were channel confined, and none resulted in any serious damage. However, on several occasions, they resulted in 1 to 2 m of aggradation or scouring of the channel, depending on the rheological character of the flow.
Figure 4. Daily rainfall and number of lahar events for A, June; B, July, C, August, and D, September based on the Sacobia flow sensor and rain gauge. The peak of monsoon rains occurred from the end of the second week through the last week of August. Lahars became more frequent starting on the last week of July and progressively increased and peaked in the third and fourth weeks of August and the first week of September. The August 28-30 and September 4-5 lahar events were the most devastating for 1992. High rainfall and lahars on June 27, July 11, and July 20 reflect the passage of typhoons.
Small flows with discharges of 100 m3/s or less at the Mactan observation point generally did not reach the downstream observation post at Maskup. Often, observers looking 2 to 3 km upstream from Maskup could see steam rising from hot lahars that approached, but never reached, Maskup. Channel gradient decreases and channel width increases between Mactan and Maskup, so many small lahars deposited their sediment before reaching Maskup.
Small lahars that did reach Maskup showed a marked decrease in flow magnitude and sediment concentration relative to Mactan. A sample of hot debris flow taken at Mactan at 2350 on August 16 contained 77.5 percent (by weight) sediment content. By the time this event reached Maskup, it was only muddy streamflow, 0.2 m deep and with less than 10 percent by weight sediment. As with flows that failed to reach Maskup, most sediment was deposited upstream from Maskup. Small lahars of this type also occurred on June 28 and August 3.
Typhoons passed near Pinatubo on June 27, July 11, and July 20. Resulting lahars had peak discharges of 60 to 250 m3/s at Mactan. The debris flows observed at Mactan were sustained and reached the Maskup observation post, though their peak discharge had decreased by at least 50 percent and surface velocities had decreased from 4 m/s to 2.5 m/s.
At 1838 on July 20, a 4-m-deep debris flow with 4 m/s velocity and a peak discharge of 250 m3/s was observed at Mactan. Its temperature ranged from 55° to 62° C. The same flow at Maskup (at 1915) was 1 m deep, moving at 2.5 m/s, and had a peak discharge of about 75 m3/s. Cumulative rainfall during this event was small: only 24 mm in 11 hours.
In general, typhoon-generated lahars during the early part of the 1992 season were relatively small and channel confined. Most flow events were short, ranging from 3 to 7 hours, on the basis of flow sensor records.
Toward the end of the second week of August, lahars began to occur at a rate of one to two per day. Most were still small, and those which were debris flows at Mactan had usually transformed into hyperconcentrated streamflows or muddy streamflows by the time they reached the Maskup observation post. From the third week of August until the first week of September, as typhoons passed over the north end of Luzon, monsoonal rains intensified and triggered debris flows with discharges at Mactan ranging from 600 to 1,100 m3/s. These debris flows avulsed near Mabalacat and Bamban, resulting in casualties and damage to properties in several barangays that had been only minimally affected by the 1991 lahars. Some areas that were not affected in 1991 were buried by 3 to 4 m of lahar deposits in 1992 (fig. 3B).
On August 20, five lahar events were recorded by the acoustic flow sensor. The initial flow started as early as 0515 and lasted almost 2 hours. Four more distinct events followed until 2300, each lasting an average of 2 hours. The biggest debris flow, which occurred between 1845 and 2100, avulsed on the south bank of the channel near Maskup. Farmland along the channel was partially buried by a thin layer of debris-flow deposits. This was the first lahar event for 1992 that overtopped the channel bank. Within the channel at Maskup, 2- to 3-m-thick lahar deposits were emplaced, leaving only 1 to 2 m of freeboard.
On August 26, a typhoon that passed over the northern portion of Luzon induced continuous heavy monsoonal rains in the Pinatubo area. Soon after, small to moderate-sized hot lahars (still less than 300 m3/s), with temperatures of 50° to 60° C, started cascading down the major drainages of Mount Pinatubo.
From 0940 to 1815 on August 27, at least six debris-flow and hyperconcentrated streamflow events were observed at the Mactan post. The first event, which occurred at 0940 with peak discharge of about 240 m3/s and a surface velocity of 5.5 m/s, was the largest flow for the day. The rest of the peak discharges ranged from 100 to 200 m3/s. The lahars were still mainly channel confined and no significant damage was reported.
Three big multipulse debris flows were generated by continuous rains on August 28. One pulse, passing Mactan at 1313 with a discharge of 760 m3/s, passed Maskup at 1350 with a discharge of 390 m3/s. Hot debris flows almost overtopped the south bank terrace of the constriction, and freeboard virtually vanished.
The first debris flow to hit the town of Bamban started at about 2300 on August 28 and continued until 0330 on August 29. The Bamban channel had been deepened during the early part of 1992, prior to the onset of the rainy season, when the Department of Public Works and Highways (DPWH) built 4-m-high protective earth levees on both sides of the channel. During the early morning lahar event of August 29, compounded by breaching of the lahar-dammed Marimla Lake, the north levee (dike) of the Bamban River was breached, and parts of Bamban town were buried beneath 2 to 3 m of sediment (fig. 3B). Beyond the area of direct deposition, flooding reached depths of 0.5 to 1 m.
The Bamban channel was completely filled, so when another debris flow arrived at around 1000, it overtopped dikes on the southern side of the channel as well. Lahars avulsed into Barangay Pag-asa (fig. 3B) and spread toward the east-southeast. By 1030, most houses along this path were either partially to totally buried by lahars (fig. 3B).
About the same time, another debris flow passed the Mactan post with an estimated peak discharge of 700 to 850 m3/s and surface velocity of 8.5 m/s. By the time it reached the Maskup observation post at 1113, the flow had a maximum discharge of 250 m3/s and surface velocity of 4.6 m/s. It spread directly toward Barangay Pag-asa and buried houses up to their roof levels (2 to 3 m deep). A 3-km stretch of the MacArthur Highway, which connects the towns of Mabalacat and Bamban, was buried with 3 m of lahar deposit. Throughout the duration of these flows, there was a noticeable absence of lahars at San Francisco Bridge, only 9 km downstream from Maskup (fig. 3B), because breaches and avulsions were already routing flow outside the engineered channel. Portions of the Magalang-Concepcion Road were also flooded by water overflowing from heavily silted irrigation canals.
From 1300 to 1400 on August 29, hot debris flows continued to bury some barangays of Bamban and Mabalacat. By this time, the field monitoring group had already abandoned its watchpoint at Maskup, because lahars had inundated portions of the access road to the watchpoint. At 1325, even larger debris flows (peak discharge of 1,100 m3/s) passed Mactan and, within an hour, buried Barangay Tabun (fig. 3B) beneath 2 to 3 m of deposit.
Lahars consisting mostly of hyperconcentrated streamflow continued to pass Maskup along the Sacobia channel on August 30-31. The events of August 31 appeared on flow sensor records as intermittent peaks against a high background, but, by this time, field observation and discharge measurements at Maskup had been temporarily discontinued because of the high risk of working in the area. Also, the access road to the post had been closed due to the avulsion and deposition of lahars in Barangays Pag-asa and Dolores.
Starting on September 1, rains poured down anew on the slopes of Mount Pinatubo. On September 4, a series of multipulse lahars was recorded by the Sacobia flow sensor. The debris flow that reached the Maskup observation post at 1030 overtopped both the north and south river banks and deposited about 2 to 3 m of hot debris. Our newly built observation post was buried to that depth, as were several houses along the northeast perimeter wall of Clark Air Base (fig. 3B). At about 1100, part of this wall collapsed from the lateral pressure of the lahar debris. Houses in Dolores, a barangay of Mabalacat immediately northeast of the Clark Air Base wall, were buried in 2- to 4-m-thick lahar deposits (fig. 3B). Across and 2 km beyond the MacArthur Highway, the advancing lahar spread thinly over additional houses and agricultural lands. By September 5, flows waned and stopped. Thereafter, small, mostly hyperconcentrated streamflows continued intermittently through the end of September. The latter flows were relatively insignificant and had no damaging effect on downstream communities.
1991 lahars left about 1.0x108 m3 of deposits on approximately 80 km2 of alluvial fan and plain of the Sacobia-Bamban River, much in distal areas near Concepcion, Tarlac. Lahars during 1992 buried many areas near Bamban anew, and lateral spillover raised the area of severe damage to about 90 km2. Along the 8-km stretch of channel between Mactan and Maskup, aggradation averaged 6 m, and the volume of 1992 lahar deposits was at least 7x107 m3 (PHIVOLCS, unpub. data, 1992).
The Bamban Bridge on the MacArthur Highway (fig. 3A) was a vital link in the major transportation route between south and north Luzon and connected the towns of Mabalacat, Pampanga, and Bamban, Tarlac. After the original bridge was washed away during a major lahar event on August 21, 1991, DPWH dredged this portion of the Bamban channel and maintained two temporary bridges to keep the MacArthur Highway open. But, by August 29, 1992, lahar deposition and scouring of bridge approaches made the new bridges impassable, and lahars of September 4 buried the new bridges beneath 3 to 5 m of fresh deposit.
The three Sacobia tributaries that were blocked in 1991 were impounded again by 1992 lahar deposits. Then, when the lahar dam of Marimla Lake was breached on the early morning of August 28, 1992, floodwaters flowed into San Nicolas Creek (fig. 3A) and thence into lowlying portions of Bamban. The town was submerged in at least half a meter of floodwater for several days. Continuous outflow from Marimla lake undercut and destroyed a 3-km stretch of concrete pavement of the MacArthur Highway in Bamban.
Table 1. Summary of discharge measurements taken from active lahar flows from two observation posts along the Sacobia River.
|
Date |
Mactan |
|
Maskup |
Percent attenuation of discharge (M-m)/M x 100 |
Lag time at peak flow (min) |
Total rain (mm) |
Front velocity (m/s) |
||
|---|---|---|---|---|---|---|---|---|---|
|
Peak discharge (M) (m3/s) |
Surface velocity (m/s) |
Peak discharge (m) (m3/s) |
Surface velocity (m/s) |
||||||
|
July 20 |
260 |
4 |
|
75 |
2.5 |
71 |
38 |
20.9 |
3.5 |
|
August 3 |
80 |
5 |
|
2 |
1.5 |
98 |
45 |
10.8 |
3.0 |
|
August 16 |
150 |
3 |
|
2 |
1.5? |
99 |
50 |
8.0 |
2.7 |
|
August 27 |
220 |
5.5 |
|
110 |
6 |
49 |
44 |
18.8 |
3.0 |
|
August 28 |
760 |
8.5 |
|
390 |
11 |
49 |
37 |
32.9 |
3.6 |
|
August 29 |
760 |
8.5 |
|
260 |
5 |
66 |
32 |
42.4 |
4.2 |
|
|
950 |
8 |
|
370 |
5 |
61 |
21 |
29.1 |
6.4 |
Figures 4A-D and 5 compare lahar occurrence to daily and monthly rainfall from May to September of 1992, as recorded by the Sacobia flow sensor and rain gauge. The frequency of lahar events increased from June to the early part of August and peaked during the latter part of August when rains were heaviest. From visual observations of dark clouds, we think that all of the 1992 lahars were triggered by rainfall, even though four events in early June had no significant recorded rainfall (fig. 4A). Because only one rain gauge was present, in the middle Sacobia watershed, some localized rains farther upslope or far from the rain gauge may not have been recorded.
Figure 5. Monthly rainfall and number of lahar events for the period May to September 1992. September rainfall is incomplete due to failure of the instrument on September 3.
More detailed correlation of rainfall and lahar generation (Tuñgol and Regalado, this volume) suggests that a rainfall rate of about 9 mm in 30 min is the minimum amount required to generate lahars in the 53-m2 catchment of the upper Sacobia River.
We caught the flow front of only one flow at both observation posts, on June 28. The flow was small (peak discharge at Mactan was <200 m3/s), and its front took 55 min to travel 8 km from Mactan to Maskup. Peaks of flows with peak discharge <200 m3/s took as much as 50 min to travel that distance, while peaks of larger flows (200 to 1,200 m3/s) traveled that distance in 20-30 min.
The largest discharge we were able to estimate was 1,100 m3/s, at Mactan on August 29. Judging from flow sensor records (Tuñgol and Regalado, this volume), discharge during flows of August 31 might have been greater.
Table 1 and figure 6 show discharge measurements that can be compared from Mactan to Maskup. Peak discharges at Mactan are followed by peak discharge at Maskup 20 to 50 min later. The change, or percent attenuation, in peak discharge at these two points is [{M-m}/M] x 100, where M is peak discharge at Mactan and m is peak discharge at Maskup. Most lahars attenuated over this reach by 30 to 50 percent, by upstream sedimentation and probably also by longitudinal stretching of peak flow.
Figure 6. Pairs of hydrographs of selected lahars, as estimated at the Mactan and Maskup observation posts. Note downstream attenuation and traveltimes of 20 to 50 min from Mactan to Maskup.
Figure 7 shows apparent relationships of rainfall and peak discharge to percent attenuation and the traveltime ("lag time") of peak flow between Mactan and Maskup. Although our data are sparse, it appears that percent attenuation is less when there was more rain in the lahar source area and a greater peak discharge (fig. 7A,C). It also appears that flows associated with more rain, and larger flows, traveled faster than smaller ones (fig. 7B,D).
Figure 7. Relationships between rainfall, peak discharge, percent attenuation, and the traveltime of peak flow between Mactan and Maskup. A, Percentage attenuation of peak discharge versus rainfall during the flow. B, Traveltime ("lag time") of peak flow versus rainfall. C, Peak discharge at Mactan versus percentage attenuation. D, Peak discharge at Mactan versus "lag time."
Channel cross sections were established in early July at four locations along Sacobia-Bamban River and resurveyed after each major lahar event to monitor deposition and changes in channel morphology (fig. 8). Reconnaissance field inspections were also conducted along the river after each significant event to estimate changes in channel bed elevations. At Macapagal Village, 5 to 6 km upstream from Maskup (fig. 1), up to 10 m of deposition occurred between August 12 and October 10, consistent with the observation that lahars attenuated markedly between Mactan and Maskup. At Maskup, alternating aggradation and scouring took place between July 2 and August 26, with net aggradation of about 3 m (added to 2 m from June) (fig. 8, section B-B'). Minor lahar avulsion on August 20 almost buried the reference benchmark. Because of this, the Maskup line was transferred about 10 m downstream from its original location after the August 26 survey. Another meter of aggradation occurred from August 26-29 (fig. 8, section C-C'). Because benchmarks on both the north and south banks were buried during the September 4-5 lahars, future surveys cannot be tied to those of 1992. Total 1992 aggradation at Maskup was >10 m.
Figure 8. Cross-sectional profiles across the Sacobia-Bamban River showing changes in channel bed elevations due to aggradation or scouring. Dates of the 1992 channel surveys are indicated. Locations of the sections are shown in figure 3B. Cross section C-C' was established 10 m downstream from B-B' after a major lahar avulsion on August 20. Figure 3B shows location of cross section lines.
Surveys of a cross section adjacent to the new Bamban Bridge (fig. 8, section D-D') showed about 3.5 m of deposition from August 19 to September 1. During the same period, no significant aggradation took place at the San Francisco Bridge (fig. 8, section E-E') because major deposition and avulsions occurred 6 km short of the latter bridge. Continued dredging of the channel near that bridge from August 13 until the last survey on September 18 appears as 1.5 m of apparent scouring.
At both Mactan and Maskup, channels also experienced repeated episodes of incision, channel widening, and backfilling in a matter of minutes--much more than is apparent from figure 8. Net fill of 3 to 4 m at Maskup occurred by tens of episodes of alternating fill and scour. Hyperconcentrated flows were especially effective agents of lateral erosion, while debris flows tended to deepen channels before they slowed and deposited their sediment (Pierson and Scott, 1985; Punongbayan and others, 1992).
Most lahars from early June until the second week of August were channel confined. Their damage was limited to minor lateral erosion of the protective earth dikes built by the DPWH along Bamban channel. However, these early, small-magnitude lahars gradually aggraded and decreased the carrying capacity of downstream channels. Then, when relatively large lahars (debris flows) of 1,000 to 1,200 m3/s started flowing down the Sacobia River on August 29, discharges far exceeded channel capacity. Major avulsions (channel overflow) occurred at constrictions in the Sacobia channel, for example, at Maskup and in channel bends at Barangays Pag-asa and Dolores (fig. 3A,B). These avulsions, farther upstream than those of 1991, focused lahar deposition between Mactan, Maskup, and Bamban rather than downstream. Relatively little deposition occurred in the town of Concepcion, as feared after serious 1991 impacts, but occurred instead near the towns of Bamban and Mabalacat.
Intense monsoon rains and rain from typhoons were the primary causes of lahars of the Sacobia-Bamban River in 1992. Approximately 7x107 m3 of lahar sediment was deposited during 1992 in addition to 1x108 m3 in 1991. By the end of 1992, 90 km2 of Sacobia alluvial plain and fan was covered with lahar deposits. The focus of lahar deposition in 1992 was on the upper part of the Sacobia alluvial fan, 20 to 30 km from Mount Pinatubo's summit.
Flows attenuated from the head of the fan (Mactan) to another observation point 8 km downstream (Maskup). A number of flows also transformed from debris flows to hyperconcentrated streamflows and muddy streamflows over that same reach. Complex alternation of deposition and scouring ended with net deposition along most of the fan, locally as great as 10 m.
Because of this continued channel filling, the communities along Sacobia-Bamban River that were identified in the lahar hazards map of PHIVOLCS (1992) are still vulnerable to lahar encroachment and heavy siltation.
We thank our colleagues for dedicated participation in the 1992 lahar monitoring activity: N.M. Tuñgol, T.M. Regalado, B.S. Tubianosa, A.S. Daag, R.C. Torres, M.L.O. Paladio, R.B. Quiambao, E.G. dela Cruz, A.L. Pataray, R.G. Garduque, R.M. Macaspac, and C.G. Newhall and family. We also thank the PVO-CAB staff, headed by Mr. J. Sincioco with I. Eto, M. Isada, and A. Ramos, for the assistance that they have extended for the group. Flow sensor and rain gauge data were provided by PHIVOLCS' Volcano Monitoring and Eruption Prediction Division. Air support was provided by the Philippine Air Force. Special thanks go to Director R.S. Punongbayan, R.U. Solidum, J.A. Daligdig, and R. Dinicola for their valuable suggestions.
Daligdig, J.A., Besana, G.M., and Punongbayan, R.S., 1992, Overview and impacts of the 1991 Mt. Pinatubo eruptions: Proceedings of the 4th Annual Geological Convention, 1991: Geological Society of the Philippines, p. 29-55.
Hadley, K.C., and Lahusen, R., 1991, Deployment of an acoustic flow sensor monitor system and examples of its application at Mt. Pinatubo, Philippines [abs.], Eos, Transactions, American Geophysical Union, v. 72, no. 44, p. 67.
Major, J.J., Janda, R.J., and Daag, A.S., this volume, Watershed disturbance and lahars on the east side of Mount Pinatubo during the mid-June 1991 eruptions.
Newhall, C.G., Daag, A.S., Delfin, F.G., Jr., Hoblitt, R.P., McGeehin, J., Pallister, J.S., Regalado, M.T.M., Rubin, M., Tamayo, R.A., Jr., Tubianosa, B., and Umbal, J.V., this volume, Eruptive history of Mount Pinatubo.
PHIVOLCS, 1992, Pinatubo lahar hazards map: Manila, National Economic Development Authority, Manila, 1 sheet at 1:200,000; 4 sheets, quadrants 1-4, at 1:100,000.
Pierson, T.C., and Scott, K.S., 1985, Downstream dilution of a lahar: Transition from debris flows to hyperconcentrated streamflow: Water Resources Research, v. 21, no. 10, p. 1151-1524.
Pierson, T.C., Janda, R.J., Umbal, J.V., and Daag, A.S., 1992, Immediate and long-term hazards from lahars and excess sedimentation in rivers draining Mount Pinatubo, Philippines: U.S. Geological Survey Water-Resources Investigations Report 92-4039, 37 p.
Punongbayan, R.S., Umbal, J., Torres, R., Daag, A.S., Solidum, R., Delos Reyes, P., Rodolfo, K.S., and Newhall, C.G., 1992, A technical primer on Pinatubo lahars: Quezon City, PHIVOLCS, 20 p.
Scott, K.M., Janda, R.J., de la Cruz, E., Gabinete, E., Eto, I., Isada, M., Sexon, M., and Hadley, K.C., this volume, Channel and sedimentation responses to large volumes of 1991 volcanic deposits on the east flank of Mount Pinatubo.
Scott, W.E., Hoblitt, R.P., Torres, R.C., Self, S, Martinez, M.L., and Nillos, T., Jr., this volume, Pyroclastic flows of the June 15, 1991, climactic eruption of Mount Pinatubo.
Torres, R.C., Self, S., and Martinez, M.L., this volume, Secondary pyroclastic flows from the June 15, 1991, ignimbrite of Mount Pinatubo.
Tuñgol, N.M., and Regalado, M.T.M., this volume, Rainfall, acoustic flow monitor records, and observed lahars of the Sacobia River in 1992.
Umbal, J.V., and Rodolfo, K.S., this volume, The 1991 lahars of southwestern Mount Pinatubo and evolution of the lahar-dammed Mapanuepe Lake.
PHIVOLCS | University of Washington Press | U.S.Geological Survey
This page is <https://pubs.usgs.gov/pinatubo/martinez/>
Contact: Chris Newhall
Last updated 06.11.99