monitoring of geomorphological lahar

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Monitoring of GeomorphologicalConsequences of Lahar Deposition 1991-1996

in the Santo Tomas Basin, West Luzon,Philippines

Jan J. Nossin

1. Introduction and objectives

Prior to its 1991 eruption, Mt. Pinatubo was considered dormant from its repose period for thepast four centuries (Tayag, et al., 1991). Recognition of the volcanic unrest at Mt. Pinatubo began whensteam explosions occurred on April 2, 1991.

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Fig 1. 3-D diagram of Pinatubo area (Daag & van Westen, 1996)

The century's greatest volcanic eruption, the 1991 eruption of Mt. Pinatubo, has accumulated

approximately 7.0 km³of pyroclastic flow deposits on its slopes (Daligdig, et. al., 1991). Mostly daciticin composition, about 1.3 k m³were deposited along the Marella-Sto. Tomas Watershed (Rodolfo, et.al., 1993). Fine particles of ash were deposited around a 50-km radius, on the south, southeast and



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southwest of the volcano (Fig 2). Approximately, pyroclastic flows and ash deposits cover more than4000 km² ; that includes the eight river basins draining the volcano. The ashes ranged from a few centimeters thickness at 40-50 km from the volcano to roughly half a meter near the crater.

Fig 2 Deposits from the 1991 eruption of Mt. Pinatubo (Daag & van Westen, 1996.)

Pyroclastic flow deposits around the crater area reach much greater thickness and fill up whole valleys to overtop the divides (Figs 3, 4 ). At the advent of the rainy season, these soft andunconsolidated fine grained materials are easily eroded and flow down at high speed formingmixtures of volcaniclastic materials and water at various rates that inundate and bring morphologicchanges along its route. This phenomenon is evident in the Marella-Sto. Tomas River system on the

southwest flank of the volcano, which has been singled out for this study.

The eruption and its aftermath have been recorded in great detail in a large number of papers,compiled in “Fire and Mud”, edited by C.G. Newhall and R.S. Punongbayan (1996). This work serves asstandard reference.

The present paper deals with the application of remotely sensed data including aerial photos tothe monitoring of geomorphic changes brought about by the dynamism of lahar in the Marella-SantoTomas riverbasin.



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Previous Research and Studies

From the time Mt. Pinatubo showed restlessness, a great many studies have been conductedranging from geology to hazard assessment, mostly initiated by the Philippine Institute of Volcanology and Seismology.

Prior to this, a detailed geomorphologic study in the Sto. Tomas Plain has been carried out by Javelosa (1985, ) with emphasis on the effects of neotectonics. This study offers basic information onthe pre-eruption geomorphology of the Santo Tomas basin. In a later study, (Javelosa 1994) couldcompare this with the morphology immediately after the eruption.

Nossin and Javelosa (1996) proposed a first risk-assessment approach in lahar-stricken areasaround the Pinatubo.

Calomarde (1997) made an extensive study of the geomorphic changes in the Marella-SantoTomas system after the eruption; much of his work is used for this paper.

Available data

Along with field observations, the following sources of information were available:

• aerial photographs* 1976 pre-eruption vertical photos ( scale 1: 18,000 )* 1991 post-eruption vertical photos ( scale 1: 15,000 )* 1992 post-eruption vertical photos ( scale 1: 25,000 )

• satellite imageries* SPOT-XS ( 4 Feb 1988, 2 Apr 1988, 18 Dec 1991, 25 Apr 1995 )* SPOT-PAN ( 12 Feb 1995, 14 Mar 1995 )* LANDSAT-TM ( 26 Jan 1992 )

• maps* topographic maps (1:50,000)* geologic map* land use map

The most visible output of this research is the updated temporal morphology of Sto. Tomas-Marella area, and the change detection map of Marella-Sto.Tomas basin . The development of laharinfilling of the basin from 1991-96 is monitored for areal coverage, thickness and morphologicalconsequences.

Location of the Study Area

The study area is situated on the western side of Zambales Province, Western Luzon,Philippines The area of interest is situated along the western flank of Mt. Pinatubo (fig.2). Theuppermost headwater comes at approximately 20 kms. from the volcano's crater. The headwater areais drained by the deeply incised and moderately narrow-channeled Marella River. At the southernmostend of Marella River, Sto. Tomas River is formed at the confluence of Marella and Mapanuepe Rivers.Bedload and channel materials of these rivers are transported in a west-southwest direction towardsthe South China Sea.

The Sto. Tomas plain is roughly 26 km. wide east to west and 13 km. wide north to south

(Javelosa, 1985).



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Rainfall

Climate in Central Luzon is characterized by well defined wet and dry seasons. Considering the west side of Mt. Pinatubo, the dry season lasts from November to April and the rainy season from May or June to October. About 70 to 80 % of the 3,800 to 3,900 mm of annual rainfall in Zambales area isconveyed by the Southwest Monsoon. In the rainy season, the average daily rainfall is about 24 mm

but can exceed 150 mm. August, being the rainiest month, delivers a daily precipitation average of 36mm to 180 mm. The greatest 24-hour rainfall recorded was 442 mm (May 19, 1966) on the onset of atyphoon prior to the monsoon season. Much of the annual rain is brought by three or four typhoons.

Geomorphologic Setting

The Marella-Sto. Tomas River system is the main drainage system in southwest Zambales.

The Sto. Tomas Plain, forming part of the geotectonically active Zambales Range, isconspicuously surrounded by polygenetic landforms (Javelosa, R., 1985). The area is dominated by morphostructural diversities of the topography. The mountains that evolved from pre-Quaternary

movements are made of the Cretaceous-Paleogene ultramafic complex (peridotite/gabbro). Theuplifted peridotites are characterized by relatively high relief and mountainous terrain. The gabbrosare associated with fault-designated crest lines and rejuvenated gully heads.

A tectonic dike complex of pre-Quaternary age is closely associated with the igneous complex(diabase, basalt and undifferentiated volcanics). Morphologically, the slightly elevated sheeted dikecomplex strikingly occupies most of the hill zone and rolling topography. The moderately elevated

basalts and the strongly elevated assemblages of undifferentiated volcanics and sedimentary rocks arecharacteristically hilly to mountainous.

The interesting aspect in the geomorphic development of Sto. Tomas Plain is in the alluvial-

filled graben resulting from Quaternary movement. The accreted alluvial deposits include terraces(relict of an incised alluvial fan), distributary bars/islands, and distributary channel areas. The lattercan be considered abandoned distributaries of Sto. Tomas River.The pre-eruption terrace morphology shows pronounced deformations and mismatches, ascribed toneotectonism (Javelosa, 1985, 1994).

In the coastal plain, the notable features which are possible evidence of tectonic movementsinclude the uplifted and truncated beach ridges and swales complex, and the blocked drainage of theplain. Even the main rivers (So Tomas and Pamatawan) have great problems maintaining their outlets.

An inherent -but not too imaginary- hazard is that flooding of the Santo Tomas Plain may increase as aresult of this blocked drainage.

The 1991 Cataclysmic Eruption

The climactic eruption of Mt. Pinatubo of 1991 is the beginning of volcanic upheaval in this west side of the Philippine Mobile Belt ( Nossin et al., 1992). Its repose period of approximately 400 years is relatively short (Phivolcs, 1991). The renewed activity of Mt. Pinatubo started on 2 April 1991and climaxed in the Plinian calderagenic eruption on June 15-16 (Newhall & Punongbayan, 1996).Considered as one of the largest volcanic eruptions of the 20th century, as measured by the volume of its product, approximately 7 km³of pyroclastic deposits were distributed over the major watershedssurrounding the Pinatubo; the Santo Tomas- Marella system holds approximately 1.3 km³of them.Daag (1994) described the 1991 deposits:



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Ash and tephra:

Any volcanic materials of fine particles ranging from 0.004 mm (fine ash) to 2-64 mm (coarselapilli) ejected during the course of eruption and transported through the air are termed as tephra orashfall (Ollier, 1990). Normally, coarse particles are deposited much closer to the crater whereas thefiner ones are carried through the air and dispersed further from its main source as influenced by thedrifting wind.

The syn-eruption typhoon caused the ash to be wet and heavy. As the accumulation increased,roofs of many houses collapsed. Agricultural lands and utility facilities were made useless andunoperable by the heavy downpour and accumulation of ashfall.

The poor cohesion and loose character of the deposit made it more erodible on areas wheregradient was high and slopes were relatively long. Such cases occurred when right after the eruption, aheavy rainfall eroded and washed down much of the deposit.

Pyroclastic Flow deposits

The high emplacement temperature of some 800°C of the pyroclastic flow deposits causesabsulute dryness and absolute absence of cohesion, rendering it highly vulnerable to erosion andtransport by rainwater.

Controlled by the topography, the bulk of the deposit flowed towards the north-northeast where the gradient is extremely high, and to the south-southwest (Marella river Valley) where slope isrelatively long. The enormous bulk of these deposits has completely filled the upstream valleys tothicknesses of 100 metres or more, obliterating the drainage system and overtopping divides( Fig 3).

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Fig. 3 Pyroclastic flow fil l-up of valley system just after eruption; upper Sacobia valley,Mt.Pinatubo in background (from Daag, 1994).



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Fig. 4 Pyroclastic flow field after three years of erosion (1994), upper Marella.

Lahars

Volcanic mudflows (lahars) of great destructive capacity started down the slopes simultaneous

with the emplacement of the pyroclastic flow deposits, as the eruption took place in the rainy seasonand coincided with the passage of a typhoon.

The mobilized pyroclastic flow deposits fill up major drainage lines emanating from the volcano, with lahar material. Going westward, the Sto. Tomas riverbed has been completely transformed into lahar channels, from the pyroclastic flow source area at the middle Marella valley along the triple junction of Marella-Mapanuepe-Sto. Tomas River down the Sto Somas valley to thecoastal areas. A lahar stream broke out southwestward in the direction of Castillejos, without reachingthis town. The pre-eruption Santo Tomas riverbed in its downstream tract was lined by dikes which

were not able to withstand the lahar and gave way in several places.

The Sto. Tomas - Marella River System

Mt. Pinatubo typically exhibits radial drainage patterns originating from the proximity of thecrater. Downstream, they are profoundly modified by structural controls.

In the Zambales area, the lahar channels are those that tap the pyroclastic flow deposits. Along the southwest flank of Mt. Pinatubo, the Marella-Sto. Tomas River system is the main drainagesystem. Stretching at about 43 kms long from the summit, Marella River trends southwestward then

joins the Mapanuepe River , forming the Sto. Tomas River.

Marella has a watershed area of 92 km2 above 50m in altitude draining the southwest slopeof Mt. Pinatubo.



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Pre-eruption morphology of the Marella river is braided in nature, and the channel is lined by terraces with straight, probably fault-controlled edges. After the eruption of Mt. Pinatubo, the channelhas aggraded rapidly.

The eruption in 1991 and subsequent rapid erosion and degradation of pyroclastic flows during

the rainy seasons, has made the Marella River down to the junction of Mapanuepe River, and all the way along the Santo Tomas Rivers, a natural debris basin for lahar deposits.

Fig.5 Lahar deposits of Marella valley j oining with Santo Tomas valley.

Below the junction, Sto. Tomas River flows along the northern margin of a broad alluvial plainmade of fluvial/ fluviovolcanic deposits of the Santo Tomas River, and underlain by mostly ancientlahar deposits.

The present (i.e. post-eruption) riverbed of Sto. Tomas is higher than the floodplain, thusoverbank spills and channel avulsions are now common along its entire length.

The Sto. Tomas Plain is about 13 km wide north to south and 26 km wide west to east(Javelosa, 1985; Fig. 2, 6)

Factors That Trigger Lahar Processes

Lahars, particularly on the west of Pinatubo are basically rain lahars, triggered mainly by intense rainfall brought about by monsoon rains and passing typhoons. Rainfall intensity over Marella, with threshold value of 0.3 to 0.4 mm per minute for over 30 minute period, initiates lahar flows.Daag (1994, 2000) makes intensive study of the threshold conditions for lahar initiation.

Lahars can also be triggered by lake breakouts. This happened from non-Pinatubo tributaries

blocked by pyroclastic flow deposits and aggrading lahar channels, particularly at the Marella-Mapanuepe junction of Sto. Tomas River System. Lake breakouts were initiated by erosion of the



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blockage by overtopping lake waters that generated normal streamfloods, sometimes withhyperconcentrated streamflow stages of short duration ( Rodolfo , 1993).

Lahar Extent and Distribution

The extent and distribution of lahardeposits within the specified study area is delineated basedon the created color composites of the available georeferenced images for a particular year. Based onlahars’ flow shape and context, the created FCC images were used as backdrop images to screen-digitize the feature of interest using ILWIS Version 1.4.

1991 Lahar Extent

The first rapid in-flow of lahars along the Sto. Tomas-Marella River Valley on June 15, 1991immediately spread out laterally overflowing from the pre-eruption channel (Fig 6). The bridgeconnecting Sta. Fe with Castillejos was totally wrecked after the passage of the devastating lahar. The1991 rainy season brought down 185 M.m³debris ( Pierson, 1993 ) spread at 41.57 km². Aggradationalong the stretch of Sto. Tomas -Marella averaged from 1 to 24 metres with maximum of 30 to 40 m at

the upstream proximal section of Marella.

In the low-lying areas near the coast, the breaching of dike in both banks partially blanketedthe nearby villages. burying them in > 1 m thick sediments.

Lateral erosion and deposition of lahars destroyed a substantial area of agricultural lands. Although dikes existed along stretched of the Santo Tomas river, they were not really a match for theforce of the lahars and a large lahar flow continued in the southwesterly direction towards the town of Castillejos. From the Dec 1991 SPOT image it is clear that a substantial area was covered by lahardeposits, which however did not quite reach Castillejos.

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Santo Tomas basin, west Luzon, Dec. 1991

Fig. 6 Santo Tomas basin, Dec. 1991; SPOT XS color composite. Note lake Mapanuepe blocked by

Marella lahar deposits, and lahar directed to the SW threatehing Castillejos town.



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Fig. 7 Lahar deposits in Marella valley, Feb. 1992 In background, PF deposits (Marella field) andMt. Pinatubo

1995 Lahar Extent

The situation in 1995 shows the accumulated lahar deposits along the Sto. Tomas-Marella (Fig

8 ). Lahars have encroached upon the northern and southern bank sectors of Sto. Tomas immensely affecting the entire villages of. Sta. Fe and San Rafael. In the southern part of. San Rafael, lahar flowsextended ~3.0 km farther south towards San Marcelino; the encroachment took place in 1993 after adike rupture and overtopping.

The combined energy of lahars from Marella and floodwater from the lake, have ruptured anewly constructed dike and buried several villages and the agricultural college . Lahar deposit alongthe Sto. Tomas-Marella for the year 1995 reached 125 M.m³( Rodolfo et al, 1993 ) extending to anarea of 52.22 km² .



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Santo Tomas basin, west Luzon, April 1995

Fig. 8 Santo Tomas valley, 1995, SPOT XS colour composite. Note effects of dike and theaccumulation of lahar deposits in midstream of Santo Tomas valley.

2. Lahar Change Detection

Change detection as it relates to remote sensing data is based on the assumption that there aremeasurable radiometric differences between successive dates corresponding to changes on the ground.In this paper, change is used and defined as ..." alteration, not only of the surface components, but thelandscape itself ..." ( Milne, A.K., 1992). Therefore, image enhancement for change detection can bedefined as finding out or identifying the alteration and then intensifying that observed alteration sothat the spatial distribution and amount of actual change can be accurately described and measured.

Methodology

Visual interpretation, and three-date color composite overlaying techniques were used. Theuse of medium scale aerial photographs ( 1:18,000 to 1:25,000 ) and 1:50,000 topographic map proveda vital aid in interpretation of the satellite images. The image processing was done in an ILWIS system(version 1.4.1) (Calomarde, 1997).

In general, steps for the analysis include:

1. Choice of the satellite images to be used2. Subsetting of the large image data3. Radiometric and geometric corrections4. Merging of images to the study area5. Application of change detection techniques

6. Initial output of result, field data verification, evaluation7. Final output



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For this analysis, pre-eruption and post-eruption images are used:

SPOT - XS ( 1988, stereopair ), SPOT - XS ( 1991 ), LANDSAT - TM ( 1992 ), SPOT - PAN ( 1995,stereopair), SPOT - XS ( 1995 ).

Since the changes that occurred are abrupt and cover a wide area, features for ground controlpoint identification (GCP) are difficult especially in places largely affected by tephra, pyroclastic flowsand lahars. To make the search for the GCP's easier, it was best deemed to subset a larger area. Then,after applying radiometric and geometric corrections , a subset of the study area was made.

Geometric Registration of data using ground control points (GCP)

By applying geometric transformation to the image of each band, the correction for thegeometric errors in the image can be made.

The post-eruption three-band SPOT-XS (1995) was geometrically corrected to providepositional accuracy and allow proper registration with other data layers in GIS. A map to imageregistration was done wherein SPOT-XS (1995) is registered to a 1:50,000 topographic map usinggrid/metric coordinates (50 UTM).

With SPOT-XS (1995) as the master map, an image to image transformation is then done wherein SPOT-PAN and LANDSAT-TM bands are the slave maps and co-registered with its 20m x20m resolution . This procedure resulted in only one registration and resampling of images, and allimages are registered to a common base to give spatial compatibility .

The change detection techniques applied/used include:

Visual Interpretation using Color Composite

Each of the four datasets were individually enhanced and filtered to produce images whereinits appearance are useful for visual interpretation. The datasets are linearly stretched to accentuatethe contrast and edges of the various landcover, particularly the main interest: lahar.

Stereoscopic interpretation o f SPOT imagery.

For the pre-eruption situation, stereo-SPOT imagery is available, of February and April, 1988, with sufficient sidelook difference.

Stereoviewing of one of these combined with the Dec 1991 image shows a loss of stereoscopy inall areas affected by PF deposits.

Merging of Images as Color Composite

A single band of the three image data sets of different dates is utilized. For visualinterpretation, the combination of NIR bands is used because of high contrast of water with land.(Taoet al., 1993). Likewise, vegetation's response is high compared to land. Thus, in each of the NIR bands,

vegetation and land are white whereas pure water is black. The selected NIR bands are those from1988 SPOT-XS ( date 1)- Blue , 1991 SPOT-XS ( date 2 )- Red and 1995 SPOT-XS ( date 3 )- Green.

As combined, it resulted to a false color composite of multi-temporal space (Fig 9).



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Black or white ( achromatic color ) signifies no change whereas the chromatic colors indicate change.This method of three-date color composite is an immediate way to identify change or equate wherelarge scale changes occur.

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Tb3851; b3/88 B, b3/95 G, b3/91 R

Fig.9 Multitemporal color composite of SPOT Band 3 : 1988 blue, 1991 red, 1995 green.

Image Interpretation

The three-date color composites present major areas of change and no change. The black and white color indicate no change as shown by some portions of the vegetated areas. Chromatic colorssuch as blue, yellow, cyan, magenta, red and green denote areas of change. Change from 1988 to 1991is indicated by red and cyan. Red shows the highly silted tailings pond used by Dizon Mines, and thehypercontrated streamflow lahar channels along the Sto. Tomas River. Red also shows the water areasin 1991 that are now replaced by laharic deposits. The adjoining edge of the major debris flows presenta viable example. Cyan shows the additional land area and vegetation cover that were submerged dueto the growth of the lake. Likewise, it is manifested by the watered agricultural area on the lowlands,some vegetation areas under shadow. Cyan along the edge of the lake represents the growth andexpansion from 1988 to 1995.

Blue, green, yellow and magenta represent areas that changed twice from 1988 to 1995. Blue istypically represented by land or vegetated areas submerged under water. These are the landssubmerged by Mapanuepe Lake in the 1991 rainy season. Green areas show scarce vegetation or baresoil to healthy cogonal grasses and back to scarce vegetation or bare soil. Both green and yellow represent areas with abrupt changes from 1988 to 1995. These changes are represented by the leadingedge of the major lahar debris flows along the Sto. Tomas River in 1991 and the thick ash deposits onthe scarcely vegetated Mt. Pimmayong. Magenta could be displayed by vegetation in which reflectanceresponse possibly returned to a high value. Other possibilities of magenta could be regrowth areasalong the slopes and lowlands, and the pyroclastic deposits on the upper reaches of Marella River.



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Compression to binary images

Another way of identifying change is, to select and area or feature of change with its DigitalNumber (DN) value, and then stretch the image between this value minus one, and this value itself.

The result is a black and white image, showing the selected feature, and all those that have the

same or higher DN values, in white, and all those with lower DN, in black.

This can be repeated on images of other years, selecting the same features and stretching theimage in the same way to black and white.

Three such images, georeferenced, can then be overlaid and assigned red, green and bluecolors.

The result is a clear picture of only eight colors: red, green, blue, yellow, magenta, cyan, black and white. (fig. 10) As there are no half-tones in the component images, the resulting image has only

these eight colours. The cut-off values have to be chosen with care, based on numerous pixel read-out values, as otherwise these images could be misleading. Care has to be taken that the area or feature of change, in each of the component images, is selected in the same manner, and that the lower cut-off

value is just below the DN value of this feature. It is to be noted that by this method, only the changesin that particular selected feature can be monitored with confidence.

In the present case, the lahar area is quite extensive and therefore changes show up clearly.The method is described by Nossin (1992 a , 1992 b, 2000).

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Tom2851 b2/88 B, b2/95 G, b2/91 R. Binary

Fig. 10 ‘Binary’ colour composite of SPOT band 2, 1988 B, 1991 R, 1995 G



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Lahar Thickness

Thickness data for the generation of lahar thickness maps are provided by PHIVOLCS. From themeasured section lines along the Sto. Tomas-Marella River Valley, points are gathered, carefully

plotted and digitized as point file data. The TIN method is applied in areas where data is needed. Another method used is by connecting lines from points of the same elevation. The lines are thendigitized, rasterized and interpolated, creating a DEM. The base map used for lahar thickness mapgeneration is at scale of 1:25,000.

1991 Lahar Thickness

Lahars begin to deposit loads at the mid-upper section where a 500 m high hill blocks a south-southwest flowing tributary of Marella river( Fig 11). At that constricted section, lahar thickness isestimated to be >40m. As lahar flows by gravity, it spreads laterally and fills-up deep channels of thepre-eruption drainages. At the Marella river, the average thickness is estimated at 21m to 30 m as itapproaches the confluence site. Portions of the deposits flow towards the east-southeast in theupstream direction of Mapanuepe River. Apparently, the almost leveled configuration of the riverconcentrates an estimated 10 m to 20 m thick lahar deposits at its mouth. Thus, the river is dammedand the valley is transformed into a lake.

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Fig. 11 Thickness of lahar deposits, 1991 (from Calomarde, 1997).

From this confluence site, the narrow channel towards the Sto. Tomas River impedes thedebris flows. The lateral dispersion of lahars as they enter the Sto. Tomas main river, distributes thesediments at an average of 11m to 20 m thick at the center, thinning out at 10 m to 5 m, as it nears theedge of the hilly terrain north and south of the bank. Further downstream along Sto. Tomas River, thechannel bed is up to 5 m higher than its original elevation. At the southern bank, lahars of 1 m to 3 mhave invaded a village and township, as the dike gave way. At the northern bank, lahar deposit of 1m to2 m destroying agricultural lands, is observed.



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1995 Lahar Thickness

Lahar deposits have thickened at the mid-upper section of Marella river by >10 m from the

1991 level (Fig 12). This indicates that voluminous amounts of loosely compacted pyroclastic flowshad been eroded upstream in the middle upper footslope of the volcano. Scour and fill processes work uninhibited as lahars cascade along the pre-eruption channels. However, as the channels overflowed,lahars generate their own path, creating secondary active lahar channels.

From the confluence site of Maralla and Mapanuepe rivers,, the constricted zone widens tothe extent of 500m with an average deposit thickness of 30 to 40 m.

461 4

S a n N a r c i s o

San Antonio

S a n M a r c e l i n o

Castille jos

Bgy. San Ra fae l

Bgy. S ta . Fe

D i z o n

ROAD

D I K E

1 1 - 2 m 1 9 - 2 5 m

2 - 5 m 2 5 - 3 0 m

5 - 9 m 3 0 - 3 4 m

9 - 1 9 m 3 4 - 3 9 m

CLASSIFIED 1995 LAHAR THICKNESS MAP

4 0 - 4 5 m

> 4 5 mN

0 4 k

R . I . C a l o m a rd e

Fig 12. Thickness of lahar deposits, 1995 (from Calomarde, 1997)



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1991 1995 ChangeThickness (m) Area (km 2) Area% Area (km 2) Area% Area%

1-2 12.42 25 10.54 20 -5

3-5 17.36 35 15.67 15 -20

6-9 8.96 18 22.84 14 -4

10-19 6.48 13 27.4 13 0

20-25 1.69 4 31.48 12 +8

26-30 0.78 2 36.73 12 +10

31-34 1.23 3 33.81 9 +6

35-39 0.06 0.1 14.06 3 +2.9

>40 0.12 0.3 3.9 0.8 +0.5

Table 1 Lahar deposit thickness 1991-95, changes along Sto Tomas-Marella River.

The lahar delta into Mapanuepe Lake has prograded in area by 27% from its 1991configuration with an estimated thickness increase of 3 to 5 metres.

The dikes confine the lahar distribution pattern within the Sto. Tomas River bed. Lateraldistribution of lahars over the Santo Tomas plain is minimized if not totally prevented, by theconstruction or reinforcement of the dikes.

In the period just after the eruption the existing dikes could not withstand the first laharonrush. Emergency measures to strengthen the dike with the lahar deposits were at times not effective

due to lack of time before the next monsoon and because of lack of coheasion within the lahar materialused for the dike.

Although dikes are built to prevent the entry of lahars in the low-lying areas, breaching hasoccurred in several sections, in the period between 1991 and 1995, and lahars have spread laterally.Thus, several villages situated on the edge of the river have been totally buried in > 3m thick of finesediments.

Since there has been a decrease in sediment transport and the dike could be made of sufficientstrength, no significant breaching of the dike has occurred after 1995. The solid armoured dike wasstrong enough to sustain the lahars.

Following the west-northwest flow of Sto. Tomas River, lahars have increased the channelelevation to about 10m to 15m above its original level.

In 1996 the dike has been reinforced and armoured with concrete. It looks reasonably strongnow, but new lahars will further fill in the basin between the dikes and with time, the danger of overtopping will increase.

From the lahar deposition and the geomorphic configuration of the So Tomas plain it can now be deduced that the pre-eruption terraces as described by Javelosa (1985), can be attributed to ancient

lahars; the higher ones can be ascribed to the Maraunot eruptive period; dating of charcoal in the lahardeposit at a site northeast of Castillejos gives an age between 2800 and 3000 radiocarbon years beforepresent (Newhall et al, 1996). Embedded lower terraces near the present confluence site of Marella and



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Mapanuepe rivers have been ascribed to the more recent Buag eruptive period with an age locally of 760 ± 60 14C years BP.

Breaching of Mapanuepe Lake

Although the lahar flow waned along Marella, continued run-off in Mapanuepe watershedraised the water level in the lake until it overtopped and eroded the debris dam. Breaching of thedammed- lake, first took place on October 12, 1991 when overtopping of the lahars by lake waterstarted. Initial overtopping and breaching always occurred at the lowest, southern margin of the

blockage

The lake is never drained totally. After each breakout, lahars normally plug the breach anew.Thus, after every rainy season, the lake expands until such state that the inflow is balanced by theoutflow thereby maintaining its level (Figs 6, 8 , 13, 14) .

The cutting in late 1991 of an artificial outlet northwest of the lake through the northwesternhills has kept the water level constant. But with the southwesterly flow direction of the lahar, asnoticed in late 1995, the sealing of the outlet is no longer a remote possibility ( PHIVOLCS Update,1995).

Fig 13 Mapanuepe lake dammed by Marella lahars, 1994. Marella PF field in background.



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Fig: 14 Mapanuepe dam deposits exposed after first draining of the lake, Feb. 1992 Mt.Pinatubo inbackground.



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3. References

Calomarde, 1997, Lahar, An attribute of change in the morphology of Sto Tomas – Marella River; Mt.

Pinatubo, Zambales, Luzon, Philippines, MSc thesis, ITC, Enschede, the Netherlands

Daag A.S. 1994 Geomorphic Development and Erosion of the Mt. Pinatubo 1991 Pyroclastic Flows inthe Sacobia Watershed, Philippines: a Case Study using Remote Sensing and GIS, 106 p. ITCMSc thesis.

Daag, A.S., 2000 (in prep.) A study of lahar triggering factors at Mt. Pinatubo; Ph.D thesisDaligdig A., Besana G.M and Punongbayan R.S., 1991 Overview and Impacts of the 1991 Mt. Pinatubo

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