1Philippine Institute of Volcanology and Seismology.
Localized rainshowers, monsoonal rains, rains from tropical cyclones, and release of impounded water generated 62 lahars in the Pasig-Potrero River System during the 1992 rainy season. Most were triggered by rainfall exceeding the power function (of intensity and duration) I = 0.7D-0.37. Following a secondary pyroclastic flow on July 13, 1992, lahars from the upper part of the Pasig-Potrero watershed were temporarily diverted into an impounded lake, and lahars reaching downstream areas were small and generally erosive. When heavy rainfall and erosion on August 29, 1992, breached the impoundment and reestablished direct drainage of the upper part of the watershed, major lahars (peak discharge, 1,400 cubic meters per second) filled downstream channels, avulsed, and buried nearby villages and farmland. About 38x106 cubic meters of sediment was delivered downstream during the 1992 lahar season.
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The 1991 eruption of Mount Pinatubo deposited about 1x109 m3 of pyroclastic-flow materials on its eastern slopes, of which about 3x108 m3 is in the Pasig-Potrero watershed (W.E. Scott and others, this volume; Pierson and others, 1992). About 40 percent of this, or roughly 1.2x108 m3, is expected to be washed down as lahars over the next few years (Pierson and others, 1992).
In 1991, lahars in the Pasig-Potrero River started to occur during and shortly after the climactic eruption on June 15 (Major and others, this volume). The most destructive lahar occurred on September 7, 1991; it buried parts of the town of Bacolor in Pampanga with 1 to 3 m of volcanic debris. This lahar was associated with the breakout of a transient debris-dammed lake in the headwaters of the river. By the end of the 1991 lahar (rainy) season, an estimated 50x106 m3 of volcanic materials had been deposited on the alluvial fan of the Pasig-Potrero River.
The Pasig-Potrero River continued to be an active lahar channel in 1992, conveying sediments to its lower and middle reaches and damaging previously unaffected areas. This paper describes events that occurred along the Pasig-Potrero River during the 1992 lahar season.
Visual observation and monitoring at the Mancatian watchpoint (fig. 1) by the Philippine Institute of Volcanology and Seismology's (PHIVOLCS') eastside lahar monitoring team started on July 1, 1992. Unpublished reports that were written by the team after each major lahar event were a primary reference during preparation of this paper. Information about events prior to this date, and other events that escaped monitoring, is from interviews, post-event field surveys, and data supplied by the Philippine National Police (PNP) Watchpoint Center (WPC). WPC data were obtained through the two-way radio linking the Pinatubo Volcano Observatory at Clark Air Base with the PNP.
Figure 1. Lahar monitoring network of the Pasig-Potrero River system. The open circle marks the location of the rain gauge (RG), and the open square marks the location of the acoustic flow sensor (FS). The open triangle near Mancatian Bridge was the PHIVOLCS watchpoint (now covered). Red areas are the 1991 pyroclastic-flow deposits.
The 1992 lahars in the Pasig-Potrero River were triggered by (1) localized rainshowers before the rainy season, (2) monsoonal rains and thunderstorms during the rainy season, (3) extended monsoonal rains ("siyam-siyam") heightened by a tropical cyclone, and (4) release of impounded water.
A localized afternoon rainshower on April 11 produced the first 1992 lahar of the Pasig-Potrero River. This and all subsequent lahars up to the onset of the rainy season in early June were triggered by localized rainshowers, most of which occurred in the late afternoon or early evening. Almost all of the flows were small, distinct, single-pulse, muddy to hyperconcentrated streamflow events lasting for a few minutes and resulting in channel scour along the middle and lower reaches of the channel. Discharge ranged from 2 to 20 m3/s. The May 10 event was a small, hyperconcentrated streamflow that was sustained for 1-1/2 h by 41 mm of rainfall during a period of 2 h.
During the early part of the 1992 rainy season (early to late June), before the southwest monsoon intensified, short-duration monsoonal rains triggered lahars from 0.2 to 0.5 m deep and 5 to 20 m wide with peak discharges ranging from 2 to 50 m3/s. These were mostly hot, hyperconcentrated streamflows and were predominantly erosional at the Mancatian watchpoint, located about 10 km downstream from the toe of the nearest 1991 pyroclastic-flow deposit (fig. 1).
The first cyclone to hit the country in 1992, tropical depression Asyang, did not intensify the southwest monsoon. It produced 60 mm of rainfall that generated only a small, hyperconcentrated streamflow. On July 11, Typhoon Konsing spawned 80 mm of rain that triggered a pumice-rich, multipulsed, hot, hyperconcentrated streamflow and debris flow. The flow reeroded and carried 4-m-long chunks of freshly deposited lahar terrace. The peak flow, with a depth of at least 2.5 m, was laminar with an average velocity of 10 m/s and a discharge of at least 700 m3/s. Samples taken during the waning stage of the peak flow yielded concentrations of 42 to 52 percent sediment by weight.
Intensified southwest monsoon winds, bringing 83 mm of rain on July 13, triggered a moderate-sized (50 to 150 m3/s), multipulsed, hot, hyperconcentrated streamflow and also caused secondary explosions and a secondary pyroclastic flow from still-hot 1991 pyroclastic-flow deposits in the headwaters of the Pasig-Potrero River (Torres and others, this volume). The secondary pyroclastic flow partially filled the Pasig-Potrero channel from the foot of Mount Dorst down to 250 m in altitude and blocked a tributary at the foot of Mount Cutuno (fig. 2). Damming of this tributary led to formation of a small transient lake at that location.
Figure 2. July 13, 1992 secondary pyroclastic-flow deposit (stippled). Areas enclosed by the dotted lines are the 1991 primary pyroclastic-flow deposits. The striped area is where the 1992 transient lake was formed.
In spite of the heavy rains spawned by tropical depression Ditang on July 20, no lahar was observed at the Mancatian watchpoint, and nothing was registered on the Pasig-Potrero acoustic flow monitor (AFM). These rains might have triggered a lahar in the uppermost headwaters of the Pasig-Potrero, but any such flow failed to reach Mancatian, because it would have been blocked by or dissipated on deposits of the July 13 secondary pyroclastic flow. Continuous rain eventually created new rills and gulleys on the fresh deposits of the secondary pyroclastic flow, reintegrating drainage, so that monsoonal rains induced by the tail end of tropical depression Ditang the next day brought a small, hot debris flow past the Mancatian watchpoint. Flow width ranged from 3 to 18 m, while flow depth ranged from 0.5 to 1.7 m; peak discharge was 58 m3/s. The sediment concentration in one sample was 68 percent by weight. Estimated temperature was 40-50°C.
The southwest monsoon was strongest from the last week of July to the last week of August, but lahars generated during this period remained abnormally minor. Lahars produced from July 27 to August 28 were small, with flow width ranging from 2 to 25 m, flow depth from 0.2 to 1.5 m, and velocities from 2 to 5 m/s. Discharge ranged from 4 to 50 m3/s. These lahars were erosional in the middle reaches, past Mancatian. Flows were not steaming and were only lukewarm to the touch (30-40°C). Apparently, lahars that reached the Mancatian watchpoint were formed only from the distal portion of the July 13 secondary pyroclastic flow deposit; flows from higher in the watershed were temporarily diverted into the small lake near Mount Cutuno during late July or early August.
A slightly larger lahar occurred on August 4, triggered by a sudden heavy thunderstorm over the alluvial fan itself, downslope from the rain gauge. Its peak flow at Mancatian was 3 m deep, 15 m wide, and had a velocity of 4 m/s. Rain that began with tropical depression Gloring on August 17 and lasted up to August 22 produced generally small, hyperconcentrated streamflows.
Continuous heavy rain in the Pasig-Potrero watershed, starting on August 27, initially triggered only small muddy to hyperconcentrated streamflows. However, at about noon of August 29, a major debris flow reached Mancatian. The debris flow was hot, laminar, pumice-rich, and carried chunks of collapsed new lahar terraces as much as 5 m long. Flow width ranged from 30 to 45 m, flow depth from 2 to 4 m, and velocity from 4 to 10 m/s. Peak discharge was estimated to be about 1,400 m3/s. The flow was uniform, maintaining its laminar character for about 9 h. This lahar filled much of the Pasig-Potrero channel with sediment, first in the leveed reach below Mancatian and then upstream even above Mancatian.
The August 29 lahar incised the deposit of the July 13 secondary pyroclastic flow and thereby cleared the channel to convey lahars from the headwaters of the watershed. Subsequent lahars triggered by monsoonal rains, sometimes strengthened by passing weak tropical cyclones, were small- to moderate-sized hot, hyperconcentrated streamflow or debris flow. Typically, these lahars after August 29 were 0.3 to 1.5 m deep, 5 to 15 m wide, and had velocities of 2 to 4 m/s.
The last significant lahar event in 1992 occurred on October 26, during the passage of Typhoon Paring. This was a relatively small, hot, hyperconcentrated streamflow that resulted in minor scouring of the channel bed along the alluvial fan.
In 1992, 62 lahars were observed or detected by the Pasig-Potrero acoustic flow monitor. From April until early July, lahars occurred at the rate of 1 every 5 days. From July 18 to August 28, when the southwest monsoon was at its strongest, there was a lahar, on average, every 1.6 days. Some days during this period had 2 distinct lahars. Probably, more lahars would have been observed during this period were it not for the channel blockage by the July 13 secondary pyroclastic-flow deposit and later diversion of flow into the temporary lake. After a channel was reestablished through the secondary pyroclastic-flow deposit on August 29, and when the southwest monsoon had considerably weakened, the occurrence rate was one every 3 days.
Table 1 is a list of rainfall-triggered lahars at the Pasig-Potrero River from May 10, 1992, to August 28, 1992. Ideally, we would have determined the amount of rainfall needed to trigger lahars on the basis of events occurring before the secondary pyroclastic flow of July 13. However, because the number of data points preceding July 13 is small, we have considered events both before and after the secondary pyroclastic flow. The results is a higher overall threshold than probably would have been estimated had the secondary pyroclastic flow not occurred.
The method of data treatment by Rodolfo and Arguden (1991) was adopted. Rainfall events selected were those preceded by a pause of at least 1 h and immediately followed by a flow as recorded by the flow sensor. Only those flows with AFM amplitudes of 100 units were considered (see related work by Tuñgol and Regalado, this volume).
A power curve fitting routine (fig. 3) gives the resulting equation for the threshold rainfall for the 1992 Pasig-Potrero lahars:
I = 0.7D-0.37
where I = rainfall intensity in millimeters per minute, and
D = duration in minutes.
Lahars postdating August 29 were not considered because the Pasig-Potrero acoustic flow monitor failed during the morning of August 30.
Figure 3. Threshold rainfall curve for 1992 Pasig-Potrero lahars. Rainfall indicated by shaded rectangles generated lahar events before July 13; rainfall indicated by shaded triangles produced lahar events from July 21 to August 28, 1992. Open rectangles indicate rainfall that (apparently) did not trigger any lahar. The curve represents the maximum rainfall that does not trigger lahars, rather than the minimum (apparent) rainfall that does trigger lahars (Tuñgol and Regalado, this volume).
Table 1. List of lahar events from May 10 to August 28, 1992. Lahars from July 21 to August 27 are believed to have originated in the lower part of the Pasig-Potrero watershed, below most of the July 13 secondary pyroclastic-flow deposit.
|
Date |
Rainfall intensity (mm/min) |
Rainfall duration (min) |
|---|---|---|
|
May 10 |
0.82 |
36 |
|
May 20 |
.68 |
31 |
|
June 19 |
.39 |
11 |
|
June 27 |
.11 |
222 |
|
July 11 |
.16 |
168 |
|
July 13 |
1.28 |
47 |
|
July 21 |
.25 |
47 |
|
July 27 |
.25 |
42 |
|
July 28 |
.76 |
40 |
|
August 4 |
.22 |
22 |
|
August 18 |
.13 |
60 |
|
August 19 |
.25 |
13 |
|
August 26 |
.25 |
40 |
|
August 27 |
.68 |
13 |
|
August 28 |
.17 |
474 |
Rainfall on July 26 is above our threshold curve yet did not result in a lahar (fig. 3). Either that rainfall was localized within a small area near the rain gauge or, as we suspect, any lahar that had been generated was trapped by temporary diversion of drainage into the small lake near Mount Cutuno. The first streams that were reestablished through the July 13 secondary-pyroclastic-flow deposit drained into the lake (C. Newhall, written commun., 1993). Interestingly, the threshold for triggering lahars in the Pasig-Potrero was similar to that in the neighboring Sacobia drainage (Tuñgol and Regalado, this volume), despite temporary disruption of drainage in the Pasig-Potrero by the secondary pyroclastic flow of July 13.
Muddy to hyperconcentrated streamflows from April to June 1992 were erosional, so scour resulted along the upper and middle reaches of the channel. Erosion of the channel floor was noticeable at the Mancatian watchpoint and at other observation points along the middle and lower reaches of the channel. Significant aggradation occurred during lahars of Typhoon Konsing of July 11, but the deposition was confined within manmade levees between Mancatian and the more distant Santa Barbara bridge. Lahars after the July 13 secondary pyroclastic flow were predominantly erosional throughout this reach, although there were instances of thalweg filling by as much as 0.5 m in some portions of the channel near the Santa Barbara bridge (fig. 1). On August 29, a debris flow generated during the early stage of the event began aggrading the channel about a kilometer downstream of the Santa Barbara bridge. Then, aggradation migrated progressively upstream within the channel and completely filled the levee-bounded channel up to the location of the Mancatian bridge. Because the channel was full, lahars eventually avulsed on both sides of the channel, inundating sugarcane fields and burying parts of Barangay Mitla under 1 to 2 m of debris (fig. 4). Succeeding lahars flowed along the southwest side of the Pasig-Potrero River, following the Sapang Matua and Quiratac Creeks and further aggrading the Mitla area by at least another meter. The September 4 lahar flowed further downstream toward the barangays of Balas and San Isidro (fig. 4). By September 8, lahars that passed through the Patutero Creek overflowed at Barangay San Juan, Guagua, covering some rice fields with a thin veneer of lahar deposits (fig. 4). By September 9, the 1992 lahars in the Pasig-Potrero River had affected approximately 9 km2 of prime agricultural and residential land.
Figure 4. The 1992 lahar deposits of the Pasig-Potrero River system. This map shows the sequence of lahar deposition in the alluvial-fan section of the Pasig-Potrero River system from August 29 to September 8, 1992. Note that the complete areas covered by the September 4-5 and September 8 events are not shown; only those areas which were covered for the first time during those events are shown. Thus, for example, the September 8 event covered areas with coarse stippling and a substantial part of the area that had been covered by earlier lahars.
Debris flows of September 21 and 25 aggraded the channel bed near Mancatian to the level of the surrounding areas (A.S. Daag, oral commun., 1992). Near Mitla, lahars breached a portion of the diked channel from the outside, so that by the waning stage of the September 25 lahar, a portion of the flow reentered the original channel. The October 21 lahar flowed through the main channel, and by October 26 downcutting and channel scouring had resumed.
Muddy and hyperconcentrated streamflows from August 13 to 28 were small, lasted no more than 52 min, and correlated well with rain that fell on the Pasig-Potrero watershed during this period. On August 29, a major lahar in Pasig-Potrero River began suddenly. Heavy rain had been falling for 2 days before the event, but only sediment-laden streamflow had been reaching Mancatian. About 1200 on August 29, without any significant increase in rainfall, a major debris flow began. After an initial surge of at least 1,400 m3/s, the discharge fluctuated (without noticeable pulses) between 500 to 700 m3/s. This continuous, rapid, uniform flow lasted for about 9 h (fig. 5) as laminar flow, except for short-lived (up to 3 min), localized episodes of turbulence marked by antidunes.
Figure 5. Lahar events in the Pasig-Potrero River on August 29, 1992, as recorded by the acoustic flow monitor. Flows associated with the lake discharge were detected at 1219.
Even when the rainfall intensity changed, there was no corresponding change in the discharge or the character of the flow as observed from the Mancatian watchpoint. From 1552 to 1753, the rainfall intensity increased to 5 mm/min from a previous average of 0.08 mm/min. However, discharge fluctuated only between 580 and 700 m3/s, as noted above. Apparently, the effect of rainfall was being buffered or swamped by some other factor.
During an aerial survey on August 13, the lake at the foot of Mount Cutuno occupied two-thirds of the tributary valley and covered an area of approximately 3x105 m2 (fig. 6A). When next seen on September 7, the area of the lake had been reduced to about 7x104 m2 (fig. 6B). During the latter survey, the lake was draining out through a narrow bedrock notch (fig. 6C), which probably controlled discharge from the lake. The amount of water that was released from the lake (on the order of 106 m3 of water) is roughly comparable to the volume of water that would have been needed to mobilize discharge of 600 m3/s of lahar for 9 h (between 106 and 107 m3 of water), especially if one adds in water available in the channel from rains of that day.
Figure 6. The transient lake at the foot of Mount Cutuno before and after the August 29 lahar. A, View (from the north) of the incipiently dissected secondary pyroclastic flow, August 6, 1992, showing diversion of drainage from the Pasig-Potrero watershed diverted around the tip of a ridge and into the transient lake. B, The transient lake on September 7, 1992, as seen from the northeast. By that date, much of the lake had drained through the gully visible along the left side of this photograph. C, The transient lake on September 7, showing the small notch between a bedrock knob and the canyon wall, through which lake water was draining. View is from the southwest.
Figure 6AB
Figure 6C
The above observations suggest that the lake started discharging on August 29. The releasing water quickly bulked up to form a major debris flow. Because flow was controlled by a bedrock lip, it was sustained for 9 h; without the bedrock lip, flow might have been much more catastrophic than it was.
By the end of the 1992 lahar season, about 38x106 m3 of old and new pyroclastic materials had been delivered to the lowlands along the Pasig-Potrero River. The main bulk of this was carried during the August 29 lahar, the biggest lahar in the Pasig-Potrero River for this year. The volume of 1991 and 1992 lahar deposits, combined, is about 88x106 m3.
Despite these voluminous lahars, there is still a great volume of potential lahar materials left at the headwaters of Pasig-Potrero River. With some lahars triggered by only 5 mm of rain in 20 min, we expect that the Pasig-Potrero River will have lahar problems for years to come. Judging from events in 1992, small and highly diluted lahars will be generated during the early stages of the next rainy season, scouring and laterally eroding the channel. The initial channel response will be to convey lahars all the way down to distal reaches of the alluvial fan. Then, subsequent big and sustained lahars will aggrade and reduce channel capacity, with the effect starting in the lower reaches and moving headward. Recessive flows from lahars and small diluted flows will resume scouring and might restore part of the channel conveyance capacity for the lahars in the next rainy season, though not necessarily within the original, engineered channels. This pattern will probably be repeated in the next several years, possibly accompanied by events such as large secondary explosions or secondary pyroclastic flows, and lake drainage, such as occurred in 1992.
With heavy deposition from 1992 lahars on the upper parts of the Pasig-Potrero alluvial fan, the potential for channel avulsion near and above Mancatian increases. If the lahars escape the channel to the northeast, the barangays of Pasig and Manibaug are threatened. If the lahars avulse to the southwest, then the lahars will inundate the floodplains southwest of the channel before impacting the sand dike that separates the floodplains of Pasig-Potrero River from those of the Porac River. Breaching or overtopping of the sand dike will depend on the volume of source materials remaining and the channel morphology at that time; in the event of such a breach, the Pasig-Potrero River could be diverted into the Porac River, threatening Porac town proper as well as areas downstream. Hazard will be highest during periods of heavy rain, but lahar watchers must also be on guard for sudden release of any impounded water, even on rainless days.
Editor's note added in proof (August 1995): In October 1993, the Pasig-Potrero River captured the uppermost watershed of the Sacobia River. Lahar activity on the Pasig-Potrero in late 1993, all of 1994, and to date in 1995 has been similar to that of 1992, and substantially higher than it would have been had the channel capture not occurred. In 1994, lahar deposits dammed the same tributary near Mount Cutuno that was dammed in 1991 and 1992 (fig. 2), and breakout of that lake in September 1994 generated lahars that buried several barangays north and west of the original channel, including San Antonio, Parulog, and San Vicente, Bacolor.
The authors thank the entire Eastside Lahar Monitoring Team for their support, especially in the data gathering phase. Geodetic surveys by Elmer Gabinete and the Ground Deformation Team, data processing and curve fitting by Ishmael Narag, data retrieval and computer work by Theresa Regalado, material and manpower support from the PVO-CAB staff, photographic documentation by D.V. Javier and A.S. Daag, information from the PNP-WPC police watchpoints, logistical support by the Clark Air Base Command, and reviews of this paper by C.G. Newhall, K.M. Scott, and M.T. Dolan are highly appreciated.
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.
Pierson, T.C., 1992, Rainfall-triggered lahars at Mt. Pinatubo, Philippines, following the June 1991 eruption: Landslide News, no. 6, p. 6-9.
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 the rivers draining Mount Pinatubo, Philippines: U.S. Geological Survey Water-Resources Investigations Report 92-4039. 35 p.
Rodolfo, K.S., and Arguden, A.T., 1991, Rain lahar generation and sediment-delivery systems at Mayon Volcano, Philippines, in Fisher, R.V., and Smith, G.A., eds., Sedimentation in volcanic settings: SEPM Special Publication no. 45, p. 71-88.
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.
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