Organization: University of Illinois at Chicago
Date: Wednesday, 2 Sep 1992 16:47:13 CDT
From: <U25720@uicvm.uic.edu>
Newsgroups: soc.culture.filipino
Subject:    Lahars at Pinatubo: firsthand accounts

To kababayans and lahar/volcano watchers:
I am transmitting an unedited report of Professor Kelvin Rodolfo, an
authority on lahars, who is monitoring Pinatubo. Much of the report is
self-explanatory, and will help visualize what lahars are and what they
can do. Kelvin, whose dad was a UP professor and mother a
russian-american, was born in Zambales and did his undergrad at UP
Diliman before doing graduate studies in the U.S. Volcano/lahar
monitoring is hard, dangerous work, but we can be proud that Filipino
women and men are at the forefront.
Comments are welcome.


Carlo "Caloy" Arcilla                    * * * *
Dept. of Geological Sciences          xx    Civilization exists by
University of Illinois at Chicago   xxxxxxx  geologic consent ...
U25720@UICVM.BITNET              xxxxxxxxxxx  subject to
U25720@UICVM.UIC.EDU           xxxxxxxxxxxxxxxx change
                                           without notice -- Will Durant
*********************************



Started 23 July 1992
Malumboy, Botlan, Zambales
Philippines

      Recent, well publicized resurgence in low-
frequency seismicity at Pinatubo Volcano signifies
imminent eruption, most likely of the 12 June 1991
magnitude.  This prospect is far less disastrous than
the paroxysmal events of 16 June 1991, but overshadows
the face that this year's lahars ("mudflows") promise to
be more devastating than last year's particularly from
the volcano's western slopes in coastal Zambales
province.

      We have been in Zambales since June 5, studying
lahars that cascade down Pinatubo's western slopes.  My
graduate student Jesse Umbal, my local employee Rosalito
(Boy) Alonso and I are now members of an informal multi-
agency group of geologists that includes three other
veterans of last year's lahars:  Cris Remotique of the
National Institute of Geological Sciences (University of
the Philippines), Marnette Bumanlag of the Mines and
Geosciences Bureau (Department of Environment and
Natural Resources), and Jed Paladio of the Philippine
Institute of Volcanology and Seismology (PHIVOLCS).
Jess is in charge of our overall Zambales effort, and of
our main group on the Santo Tomas river system that
drains the entire southwestern sector of the volcano,
and is gathering his second season's lahar data for his
thesis.  This letter is written not as an official
statement of our group, but for the information of my
friends in the Philippines and abroad.

      Jess's group occupies a primary watchpoint at
Sitio [village] Dalanaoan immediately below the junction
at which the Marella River, which drains a major
pyroclastic field, and the Mapanuepe, which drains a
sizeable watershed outside the volcano join to form the
Santo Tomas River.

      The Santo Tomas sector has been drastically
affected by three typhoons that preceded the tardy
Southwest Monsoon, especially by the last two:  Konching
on 11 July and Ditang on 20-21 July.  Konching's lahars
included debris flows that badly damaged a complex of
channel-, spur-, and ring-dikes that were started only
in late June and were still being constructed, out of
bulldozed, unreinforced lahar deposits.  Tragically, a
van containing a family was trapped in this complex; two
small children and an old man were swept away.  The body
of one of the children was recovered the next morning on
the beaches of Botolan, 28 kilometers up the coast,
carried there by the strong northward littoral drift
characteristic of the coast and season.

      Typhoon Ditang, which ended two days ago, was even
more intense, and "totalled" the dike complex, left 5-
meter thick lahar deposits, and carved a deep channel
along the south bank of the Santo Tomas River, behind
the dike meant to protect Barangay San Rafael, which was
already badly damaged last year.  After a relative lull
in late July and early August, the arrival of the
Southwest Monsoon rains has continued to aggrade the
Santo Tomas river system, to the extent that the highest
lahar-terraces left after the last eruption has already
been reached, rendering our Dalanaoan watchpoint
temporarily inutile.


      The northwestern, Bucao River lahar system was
badly neglected after August last year.  We were forced
then to stop our observations at Sitio Malumboy, on the
north (right) bank of the Bucao River ten kilometers up
from the coast and the town of Botolan, because lahars
began to bury the highway from the village of Baquilan
three kilometers downstream of Malumboy, and Malumboy
itself, under 6 meters of pumiceous sand, pebbles, and
small boulders.  These deposits were treacherously soft
and could not be crossed on foot until several months
into the dry season, and no path along higher ground
existed.

      Today, only the peaks of the Malumboy schoolhouse
walls poke less thana meter above the surface of the
lahar avenue.  The galvanized steel roof was salvaged
months ago.  Someone has used the east wall-peak as the
side of a charcoaling pit.  The long, broad Bucao lahar
plain, three kilometers across at its widest, is
littered with partly buried, tree fragments that are
partially or wholly carbonized and tattered, having been
exhumed from the vast pyroclastic fans and transported
by lahars.  Many Aetas of all ages gather the charcoal
for sale; small family groups collect the partly
carbonized wood and complete its charring in pits dug
into or above the lahar field.  The acrid smoke, mixed
with steam in wet weather, is pervasive.  Delivered by
rubber-wheeled carabao carts to the roads's end at Sitio
Baquilan, a sackful of charcoal brings 30 pesos -- about
US $1.25, a good price, in this calamitous economy.

      As of last week, mechanized vehicles can no longer
cross the lahar field from Baquilan to Malumboy and
points east, but between the almost-daily rains the two-
wheeled carts continually plod upstream and return
loaded with, besides charcoal, crude lumber, wild
bananas, and sundry other forest products.  A few Aetas
are always in view, wading along the northern part of
the channel, through braids of sediment-laden water that
are never less than knee-deep in one braid or the other.
Some carry palmwood bows and feathered cane arrows
steel-pointed variably for birds and larger game such as
deer and boar.  Others carry fishing poles, for tilapia
are thriving surprisingly well in the many small lakes
that formed wherever a stream draining the  mountains
other than Pinatubo was invaded and partially dammed by
last year's lahars riding high on their own rapidly
aggrading avenues.

      To assure ourselves access to, and exit from
Malumboy, we have had a trail cut for us along the
valley wall well above the lahar avenue, by hiring Aetas
who have been resettled near Botolan but who frequently
hunt, fish and forage at the site of their village of
Magisguis upstream of us that was devastated by
pyroclastic flows last year.  The trail is seven
kilometers long because it must cut two kilometers
upstream to cross Malumboy Creek, which now passes
through a deep lahar-dammed lake where it joins the
Bucao.  The quality of the trail improves with every
major lahar episode, for the resettled people need food
and the modest income from charcoal and timber salvaged
from the lahars.

      Now, early in the early in the rainy season,
cloudburst lahars frequently bury the schoolhouse peaks
that mark Malumboy, but re-expose them a day or so
later.  The three-kilometer stretch of buried road
between Baquilan and Malumboy re-emerges a short
distance to the east.  Upstream along this road a
kilometer above the schoolhouse, we have documented the
lahars of the three typhoons from the concrete shell of
last year's watchpoint while weatherproofing it.  The 4m
x 9m bunker was being built for us early last August but
was never finished.  Its corrugated steel roof was
liberated during the dry season.  Thanks to a U.S.
National Science Foundation grant, augmented by a
liberal grant from Mayor Richard Gordon and at the City
of Olongapo to defray unexpected local expenses, and

thanks to generous carabao-cart convoys of building
materials from Botolan's Mayor Sally Deloso, we boast
storm-proof video windows, screening, s small kitchen,
rudimentary lavatory, and a separate room for two, all
in steel-reinforced, concrete-faced hollowblock.  Our
first benefactor, Congresswoman Katherine Gordon, has
blessed us with a diesel generator with which to charge
our electrical and electronic gear, and to enjoy
electric light and radio a few hours day.  We can house
five fairly comfortably, and could take in two more in
a pinch.

      We chose this site last year because the lahars
from the upper Bucao and Maraunot -- Balin Baquero river
systems conjoin here, after collectively draining more
than half of the Pinatubo flanks underlain by an
estimated 4.8 cubic kilometers of new pyroclastic
deposits.  No lahar enroute to Botolan proper (pop.
35752) can pass this point unobserved.

      This year, our Malumboy party includes Jed
Paladio, Noel Lacadin of the Central Luzon office of the
Mines and Geosciences Bureau, Department of Environment
and Natural Resources (MGB-DENR Region III), Digna
Evangelista of the central MGB office in Manila, and our
driver, Ramoncito (Monching) Rivera, also from MGB-
Manila.  At Malumboy, Monching has little driving to do,
having to park his 4x4 truck in Baquilan, but he
performs yeoman service as videocamera operator,
electronics and electricity specialist, general
handyman, cook and morale booster.  Jess Umbal, Boy
Alonso and Cris Remotique also join us on alternating
weeks.

      As we did last year, we collaborate with police
and army personnel assigned to watch for lahars and warn
Botolan of their coming.  Apart from this participation
in hazard mitigation, our scientific missions this year
are:  to define the configuration of the lahar surface
in order to estimate the volume of pyroclastic material
brought down by last year's lahars, and to set a
baseline with which to evaluate the net results of this
year's activity at season's end; to monitor, by large-
scale mapping and video photography, the continuing
evolution of large fans of lahar deposits in the Bucao
channel.  Hans Schaal, a professor of Art at the
University of Illinois at Chicago trained and assisted
us in taking baseline footage, and himself superbly
documented the first typhoon lahars at Dalanaoan.  These
prime data are in his keeping at UIC.  We also study the
behavior of the flows and are soliciting support from
the Department of Public Works and Highways (DPWH) in
gathering data that could possibly be of use in attempts
to channelize the lahars out to sea as debris flows.
Otherwise, the fate of Botolan appears to be certain:
slow burial by hyperconcentrated lahars over the next
several years.

      Here, below the Bucao -- Balin Baquero junction,
the master lahar avenue is at its narrowest, only a
kilometer across in contrast to its more typical widths
of two to three kilometers.  Usually, it has been more
dry than wet, the light gray surface coursed by several
separate shallow flows a few tens of meters wide.  Our
post stands 14 meters above the channel floor - a safe
height; Jess has calculated that Balin Baquero lahars
approaching us head-on would have to flow at speeds of
6 km/h to run up and reach us.  We have built on a
precipitous riverbank, cut into a litter of ultramatic
boulders (some quite huge) held together in a strong
matrix of lateritic clays, that has so far been
impervious to lateral cutting by lahars.  Thus, we are
not quite as insane as the military watchers, in their
own watchpoint across the road and 47 meters above us,
believe.  When it is not raining, as is becoming rare,
we map the lahar field and quantify the fields of view
of our video camera with planetable and GPS.  In times
of flow, we tape the flows and measure their velocities
by various techniques, including the clocking of large
objects flowing past.

      The east-west orientation for the valley funnels
most airflow,and so the winds are dominantly easterly at
this time -- modified Northeast Tradewinds.  (During
recent, initial incursions of South Indian air, they
have been westerly.)  The Bucao River system does not
receive the full brunt of the monsoonal rains.  Looking
southwest toward the coast over which the clouds pour,
our view is blocked by tall mountains of the coastal
Zambales Range, which forcibly lift the moist air coming
in from the South China Sea, causing heavy rain on the
coast, so what we receive is already partly dehydrated.
At such times, though, enough rain falls on Pinatubo's
pyroclastic-laden northwestern slope to generate many
lahars.  These are largely the more dilute
"hyperconcentrated" variety -- what we are locally
calling "lahar na malabnaw", which consist of only about
half to three fourths volcanic debris.

      Laharic debris flows ("lahar na malapot"),
containing no more than about a quarter to as little as
15 percent water by weight, are rarer because they are
diluted by normal stream flow from several large rivers
and many small creeks draining the surrounding
ophiolitic Zambales Mountains and joining the main lahar
avenues.  Thus, lahars descending the upper Bucao are
met by clean Kabatoan-Balintawak waters from a 160-km2
watershed to the northeast, and are diluted down to
normal, although sediment-laden, stream flow along our
bank.  Coming northwestward toward us from the greatest
of Pinatubo's pyroclastic fans by far, the flow from the
Balin Baquero River splays out widely over a broad fan
of its won making, at low stages in many braids that
successively meet the reach of the Bucao River from a
kilometer above, to a kilometer below our site, diluted
down to shallow hyperconcentrated flow by discharge from
many small creeks that drain the southern valley slope.
In times of the strongest flow the entire fan is covered
by lahar a few centimeters to a half meter deep.

      Along a half-kilometer reach of the Bucao
extending upstream and downs stream of our post, the
collective flow from all these rivers,brought to our
north bank by the gradient of the Balin Baquero fan,
gather into a single channel that is typically 30 and
rarely more than 100 meters wide.  Discharge from Umaya
Creek, entering the southern Balin Baquero flow across
the river from us, dilutes it into "malabnaw" lahars
that also run transversely west and north toward us a
across the fan.  In this channel, the concentrated and
constricted discharge remobilize the pumiceous sediment
into lahars that persist even on dry days when the
separate flows upstream are small.  At such times flow
is typically 10 to 15 km/h, marked by standing waves
("antidunes") five to 12 meters long and one to two
meters high.  When the lahars are best developed by
heavy rains on Pinatubo, flow at our bank remains normal
stream flow, reddened by the lateritic soils of the
Balintawak River, but the channel is widened to the
south by gray lahar flow.  They are initially dilute,
with standing waves at least 1 meters long and three to
four meters high.  When most of the weaves are breaking,
the flows are transitional to debris flows which, when
fully developed, look deceptively quiet because of their
laminarity, but shoot along the channel at speeds of up
to 30 km/h and can be catastrophic if they overtop their
banks an overrun settlements.  Below this reach,
however, the lahar avenue widens again, as debris flows
splay out over another fan, into separate
hyperconcentrated streamflows, their speeds greatly
reduced by the increased basal friction.

      The village of Poonbato, four kilometers upstream
of us, was already badly damaged by lahars last year.
Much of the damage was wreaked by hyperconcentrated
flows, which can laterally erode the river banks at
rates of a meter per minute or greater.  Much of the
hillside cemetery is eroded away, but we have
occasionally paused in our velocity measurements to
watch an exhumed coffin briskly ride over the standing
waves on its way to the sea.  At times, skeletal parts
or an entire corpse require the investigations of the
police.

      We believe that the channel at this site is
sufficiently constrained by its geomorphic setting to be
stable for several years.  It is an ideal site for
cooperative research with the Department of Public Works
and Highways, to study the lahars in order to channel
them properly.



1830H 16 August 1992

      The last three weeks were unseasonably dry, with
only occasional lahars triggered by afternoon and
evening cloudbursts, but our first serious southwest
monsoonal episode of the season is about to commence.
Yesterday's satellite weather infrared photos,taken at
0500H Local Time (2100 UMT) show a low pressure system
that maya be developing into a typhoon.  It is far to
the east, centered at about 150N, 1300E, and headed on
a west northwest course toward the northern tip of
Luzon, and thus may deliver no-direct precipitation to
Pinatubo.  However, the photos also show a large mass of
cold cloudtops over most of the South China Sea, its
northeastern edges already flirting with the central
Luzon by the westerly wakes of typhoons; this one began
to dump rain on Malumboy two afternoons ago.

      Yesterday, the intruding mass delivered moderate
precipitation to the northwestern sector of the volcano.
Last evening, the clouds were still thin enough at times
to deliver watery light from the full Moon, and today
the Sun broke through for several periods long enough
for us to recharge our solar lanterns.  The rains
brought by the backwash of Typhoon Gloring began to
arrive in full force in mid afternoon.  Since then it
has rained almost constantly, with blustery intervals of
up to an hour.

      Pinatubo has been shrouded in white cloud and rain
much of the day, but the darkest clouds in view have
passed over the Balintawak-Kabatuan watershed.  The
runoff from that river has dwarfed the laharic discharge
from the Upper Bucao, and also the easternmost Balin
Baquero braid, which slips down that river's fan into
the Bucao a kilometer upstream from here.  This pattern
was established during the three typhoons of July, and
only "normal" (though very dirty) streamflow reaches our
bank.

      Tonight the clouds are dark and heavy, but some
moonlight still manages dimly to illuminate the scene.
By 9 PM the gusts had strengthened, but without too much
rain.  At O;30 PM the stage below us was at 17.5 meters
above sea level (masl), and flow was still less than a
meter deep and dominated by Balintawak-diluted normal
flow.  At 10 PM, the north-bank channel downstream of
the observation post was about 40 to 50 meters wide,
scarcely more than a meter deep, and transitional
between normal and hyperconcentrated.  Starting at
2340H, the first heavy rain lasts until 0115H on August
17.



Monday 17 August

      The rain fell almost throughout the entire day,
with heavy bursts lasting from a few minutes to an hour.
By 0845H, the lahar channel along which virtually all
the Upper Bucao-Balintawak - Balin Baquero discharge was
concentrated 30-35 meters wide at its narrowest,
measured 300 meters downstream of our post.  The
southern half of the channel, fed by main, wide braids
of shallow hyper-concentrated Balin Baquero flow
extending down across its fan in a broad splay, was
characterized by antidunes 12 to 20 m long.  The largest
were a little less than a meter high, and thus we
estimated flow depth at approximately 1.5m.  Their
velocities were 4.4 - 5.4 m/s.k  Occasionally, the
antidunes disappeared, replaced by shooting flow with
velocities, measured at 0905H, of 6.0 to 11.1 m/s.  The
flow along the north bank was slower, normal "dirty"
streamflow, dominated by dilute Balintawak discharge.

      By noon, the stage below our observation post was
17.5 meters above sea level (masl), its normal height.
Two hours later, the level dropped 0.1 m over a 15
minute interval, possibly signifying that channelized
debris flows were increasing the overall channel
capacity by eroding their bed.  At 1509H the channel
downstream of our post had widened to about 40 m, due
primarily to expanded lahar flow along the southern part
of the channel.  The water remained "normal streamflow"
along the north bank, but along the south half
hyperconcentrated flow with velocities of 3.1 - 3.7 m/s
are marked by antidunes 7 to 9 m long, and 1 m high,
breaking occasionally.

      At 1725H intense rain and strong wind arrived from
the northwest.  The first lahar surges began to flow
down the Balin Baquero 20 minutes later.  An hour later,
the flow along the constricted channel was still
primarily hyper-concentrated, with velocities at 2.2 to
4.6 m/s occasionally replaced by laminar debris flow.
We noted that the antidunes, which frequently broke,
were marked by bands, less than a meter wide, oriented
parallel to flow.  We attribute the banding to
variations in suspended sediment concentration due to
lateral assimilation by the main flow of successive
downstream contributions from the fan where its margin
meets the main channel.

      Moderate to occasionally heavy rain continued
through the night, the stage rising and falling one or
two decimeters over hourly periods, in response to
spates of intense rain falling over various parts of the
total watershed upstream of us.



Tuesday 18 August

      At 0840H, the stage was still stable at 17.6 masl.
Flow in the channel downstream of the across from
culverts had widened to about 100 m wide, characterized
mainly by hyperconcentrated flow along its southern
portion.  Several half-hour episodes of fairly intense
rain occurred between 0800 and 1000H./  The stage
remained fairly steady, reading 17.7 masl at 1010H, but
rose abruptly to 18.0 masl at 1050H and to 18.1 at
1105H, when numerous shallow surges began descending the
Balin Baquero fan.  At the constricted channel,
velocities measured 6.7 m/s in hyperconcentrated
streamflows with breaking antidunes about 30 m long
along its southern part.  The stage continued to
fluctuate fairly rapidly, for example,from 17.9 masl at
1130H to 18.0 masl at 1134H.

      River traffic, bamboo rafts and lone or multiple
rides of truckwheel inner tubes, is surprisingly
frequent.  Today, the flow is a bit too fierce for the
familiar sight of several young men and boys riding a
prize log downstream to where it can be beached from the
shallower flows and cut into timber by chainsaw.
      Rain and lahar flow began to taper off in the
afternoon and evening.



Wednesday-Friday, 19-21 August

      Rainfall had fallen sufficiently by the morning to
permit planetable mapping, as we continued to establish
our baseline of the 1991 lahar deposits, interrupted
only by occasional showers.  The schoolhouse roof-peaks
are now buried; quite possibly, they will remain so.

      On the evening of the 21st, however, we received
word from the PNP-AFP lahar watchers at the Delta 10
station that they were to investigate reports that the
part of Poonbato still remaining after the 1991 lahar
season had been buried by lahar deposits on the night of
20-21 August.  If verified, these flows were either
caused by localized intense rain on the Bucao watershed
above Poonbato or, more likely, by the sudden breaching
of a lahar dammed tributary.



Saturday, 22 August

      At 0730H, I accompanied the PNP-AFP investigating
team to Poonbato, after crossing the swift, waist-deep
Bucao River with some difficulty.  "Magsaysay Island",
formerly a hill at the northwestern approach top
Poonbato, has been badly eroded and buried in new,
laharic debris-flow deposits to a level only one meter
above the floor of the skeletal remains of the PHIVOLCS
forward observation post that was abandoned in June
1991.

      Up to 5 m of laharic debris-flow deposits have
buried most of what remained of the town prior to this
monsoonal event.  Many houses have simply disappeared.
In the town plaza, the basketball court is covered, and
only the tip of the concrete PNP outpost is unburied.
A half meter of the roof of the health center in front
of the Catholic church is also emergent; the church
itself, with its brightly muralled front, is half
buried.  The Aglipayan church is buried precisely to the
eaves of the steeply pitched room that had survived last
year's paroxysmal ashfall.  A fair prominent, 20-meter
wide braid of the Bucao River in waning, thigh-deep,
normal-streamflow now runs through the town, incised two
meters into the new debrites.

      By 0930H the policemen completed their census of
the remaining Poonbato populace:  136, including at
least 70 children, most to weak to ford the Bucao River,
which the party forded at noon.  The weather in the
afternoon at Malumboy permitted our continued mapping.

      At this juncture, we still do not know whether the
debris flows were merely the result of local intense
rains, or were caused by temporary damming of an upper
Bucao tributary and a subsequent breakout of the
resulting lake.  We hope to answer this question by
detailed ground mapping and helicopter overviews of the
site.

Friday, 28 August

      The last six days have become increasingly rainy,
each allowing only one or two hours of field mapping, it
at all.  Now we are experiencing another full-blown
lahar episode, triggered by monsoonal rains aggravated
by two passing storms (Huaning and Isang).  This is our
last entry, for Jed Paladio will try to leave for
Baquilan and Manila tomorrow, taking the diskette of
this report with her for printout and distribution.  It
appears that the rains will continue for some days.

      We have been experiencing gale-force winds
accompanied by intense rain for most of the day.  During
the height of the storm at about 1500H the new latrine
and kitchen of the police-army watchpoint above us,
built out of hollow block only three weeks ago, was
literally blown away in pieces.  Our own post, and even
our tarpaulin-thatch video station three hundred meters
down the road have not been badly damaged.  There are
advantages to our low position.....

      The pattern of rainfall and lahar flow established
earlier this season remains in force.  All day, the
river flow has been dominated by reddish Balintawak
discharge along our bank it appears that the Balin
Baquero watershed on the volcano has received much less
rain.  Flow has   accordingly been "normal" streamflow
and dilute hyperconcentrated lahars.  Average river
stage at our post has been 17.7 masl, with frequent,
rapid rises and falls of up to a half meter, brought on
by varying peak rainfalls on the various watershed.  At
1740H, the first real lahars from the Balin Baquero
began to descend in gray surges only a few decimeters
high, but in two sets of successive kilometer-long
crescents, down the northeastern and portions of the
fan.  At dusk, flow in the main channel appeared to be
thickening into hyperconcentrated flow marked by
occasionally breaking standing waves.

      Now, at 2230H, the wind has died down considerably
and the rain only comes in short burst.  River stage
below us continues to fluctuate, up to 18.1 masl from
17.7 masl an hour ago.  We will need to look further
into the role of laharic transitions in such rises and
falls; at our downstream station, the main channel is
still about 80 meters wide, but only a meter or so deep,
judging from standing-wave heights, and is waning-stage,
dilute hyperconcentrated flow in visual character.

      Thus, it would appear that the precipitation
episode is over.  The few remaining people of Poonbato,
and the charcoal and timber foragers, however, need to
be wary of strong lahars generated by lake breakouts.

It appears that we may have seen our last of the
Malumboy schoolhouse....

Kelvin S. Rodolfo
Professor of Geological Sciences
University of Illinois at Chicago