Lo REPORT ON

THE SEDTENT ASPECTS OF THE

PROPOSED MARIKINA RIVER hLTI-PURPOSE PROJECT

By

Charles W. Thomas#, Consulting Hydraulic Engineer

anila, Philippines

May 1954 F OREW ORD

The studies covered in this report were carried

out under the joint PHILCUSA-FOA aid program.

As will be noted the report is more detailed and

lengthy than is ordinarily prepared to cover a sedi­

mentation investigation. This was intentional and

was premised on two basic objectives. The first, and

primary objective, to give considered advice on the

immediate question at hand, namely, the sediment as­ pects of the proposed Marikina River tIulti-Purpose

Project. The second, to impart a better understanding

of the overall sediment studies to those who will use

the report and may be required in the conduct of their

duties to make similar investigations in the future. THE SEDI4ENT ASPECTS OF THE PROPOSED MARIKINA RIVER MULTI-PURPOSS PROJECT

INTRODUCTION

The Marikina river system drains the southern extremities of

the Sierra Madre Cordillera and enters the Central Izon Plain at

Montalban about 15 miles northeast of Manila, Figure 1. The drain­

age area is roughly triangular in shape, Figure 2, with the base

extending in a north-south direction and laying along one of the

main ranges of the Sierra Madre. The apex is at the damsite. The

base of the triangle is roughly twice the height. The southern

tributaries rise along the divide between this basin and the drain­

age systems flowing to the south and entering Laguna de Bay. The

northern tributaries drain the area south of the Angat Basin. The

western boundary is the high north-south trending ridge of Binango­

nan limestone and extensions of this rfdge, one extending in a

northeasterly direction and the other in a southeasterly direction.

The damsite is in a gorge where the Marikina passes through the

limestone ridge. This narrow gorge has almost vertical walls ex­

tending about 1,000 ft. above the stream, Figure 3. Wawa Dam,

abandoned at present, is a low development in this gorge. A second­

ary gorge near Montalban marks the point of entrance of the river

onto the Plain. Fran this point the river flows west and south some twenty miles to Pasig, Rizal where, in a confluence with the Napin­

dan and Patoros Rivers, the Pasig is formed. LE6ENO.

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AERIAL VIEW LOOKING DIXNSTREAM AT MAIN DAM

SITE ON MARIKINA RIVER - AFTERBAY DAM SITE IN BACKGROUND. 'N rUt -~ 4-' '4'.

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The objectives to be gained by construction of the dam are to

provide stored waters for:- (1) power devolopnent; (2) irrigation;

(3) municipal and industrial water supp]y and (4) flood control. The power produced is prinarily intended to meet peaking require­ ments in the load center of the Luzon grid which is in and around

Manila. Irrigation benefits will be provided for presantly devel­ oped lands in the river valley below the dam and areas extending southward to Laguna de Bay. Supplemental water will be provided to meet the increasing needs for municipal and industrial use in

Greater Manila. Allocation of flood storage capacity in the reser­ voir will alleviate the flood problem in the vally below the dam and in Manila and its environs.

It.is not intended thr.t this report is either complete or final as it has been prepcared in connection with the planning of the project. The meager available data have been used to formulate an estimate of the sediment aspucts as thQy now exist or may be­ come apparent in the future. Certain conclusions and recomenda­ tions are made to indicate additional data needed and observations to be made as work on the project moves forward through the various phases to include operational status..

DESCRIPTION OF DRAINAGE ARUA

Information regarding the drainage basin is very scanty and in some instances that available is not reconcilable. Reliable topographic maps do not exist and no geologic maps have been made.

Ground reconnaissance is not advisable because roads and trails - 6'­

are lacking and due to unsettled social conditions, security is not

adequate. Personal air reconnaissance has been limited to one brief

sortie. The following cozmmcnt hr.vo bean gathered from the available meager information.

Extent. The drainage btsin above the damsite covers an area of 297 sq. kms. (115 sq. miles). With a maxmnum watersurface elevation

of 195 meters (project datum, which is 10.47 meters, 34.3 feet, below moan sea level), the reservoir will occupy 22 sq. km. (8.5 sq. miles)

or approximately 7A percent of the drainage area.

General Characteristics. Shuets; Antipolo, No. 3454-1, first edition AS I; Aount Irid, No. 3455-II, second edition SWPA I; and

Mount 'iracatouhuol, No. 3455-1, sucond ddition SWPA I of War Depart­ ment, Corps of igineers, U.S. Army maps Luzon, Philippine Islands together with iniormation gained on brief xrial and ground recon­ naissance were used to formulate thu following description. The con­ tributing watershed above tho storage reservoir is relatively small and may be considered as a single unit. The Harikina river is formed by the confluence of the Boso-Boso and Montalban (Tpyabasan) rivers. Approxi-mately, the south one-third of the area is drained by the Boso-Boso river system and the north two-thirds by the

Montalban (Tayabasan) river and tributaries. There is one small right bank tributary that empties directly into the Marikina a short distance above the dpasite. This strean drains a limited area to the north and erst, the greater part of which will be in­ cludod in the reservoir'.

10 - 7 ­

ueo.Logy. There are no geologic maps of the watershed area nd

no personal ground reconnaissance was made, with the exception of

one trip along the river to a point a little over one mile above

the drmsite. These couments are based primarily on unplublishud

notes of Mr. Earl N. Irving, Geologist, United States Geological

Survey and a brief air reconnaissance. The high Sierra forming the

eastern boundary of the drainage basin is probably composed of the

same basement complex as that of the general area. This bedrock is

metamorphosed andesites, basalts -nd some sedimcnts and is locally

intruded by diorite, gabbro and serpentine. This material appears

to occupy approxiatoly one third of the basin. The other two thirds,

lying to the west, is primrrily volcanics and are composed of besalts,

andesitos, agglomerates and tuffs. Along a short portion of the western boundary, and at the dnmsite, the bedrock is limestone.

Some recent alluvial deposits existing along the streama above the dansite will be included in the reservoir. There may be a high terrace of alluvial material occurying a small area on the head waters of the Boso-Boso river. The air reconnaissance revealed several bare points in the high areas which may be local igneous intrusions.

Geography and Cover. The watershed is young, geologically speaking. The topography above the dainsite is mountainous. The land forms in the western and southern portions are eroded to a isomewhat rounded shape. The eastern and northern parts are rugged and steep slopes are predominant. In this area the valleys are generally narrow and have steep sides. The eastern boundtry of

%I1 - 6­

the basin is constituted by a series of peaks that form one range of

the Sierra Madre. These peaks range in elevation from about 2,000

feet up to 4,750 fct, the apex of Mount Irid, Figures 1 and 2. The

elevation of the river bed at the damsite, where the stream passes

through the gorge on the west boundary of the basin, is approximately

200 ft. above sea level. There are a number of peaks in the basin

that rise above 2,000 ft. in elevation, Since the basin is approxi­

mately 18 miles long, in a north-south direction, and 8 miles wide at

the apex of the trianglo, the resultant stream slopes are quite steep,

especially at the higher elevations. Slopes of the streams near the

head waters exceed 1,000 feet per mile in awny places and in some

instances are as much as 2,000 feet. The topographic map of the re­

servoir area propared by National Power Corporation, Figure 5, shows

that the Marikina avora.ges about 32 feet per :Aile in the reach from

the damsite to its origin at the confluonce of the Boso-Boso and

Montalban (Tayabasan). From this point to the upper and of the ro­

servoir, the Boso-Boso averages about 37. fet per mile and the

Montalban about 53 feet. Slopus in the Boso-Boso and its tributaries

between the reservoir and the h..adwators are generally flatter than

those pertaining in the Montalban (Tayabasan) syston.

Erosional processes appear to be moderately ,ctive throughout the watershed. Such activity is arrested by the vogetal cover,

consisting of grass, brush and trees, that protects the major portion of the lnnd surface. Two rather extensive cleared areas in the upper

Boso-Boso Basin are probrble contributors of fine sediments. There

are also some bare areas in the Montalban basin but from the air - 9­

those appuer to be sound rock. Deep canyons in this bnsin undoubtedly

contribute heavier sediments. At thu time of the air reconnaissaneo,

April 30, 1954, consid'erable activity in the way of clearing, burning

Pnd planting was noted, Figure 4. Miany of the slopes had recently

boen denuded to the tops of the ridges, particularly from the damsite

to perhaps 2 or 3 kilometers above the confluence forming the Mtrikina.

Just how much this destruction of lnd cover extends above reservoir.

level could not readily oo determined but no doubt it is considerable.

Very slight evidence of slides was noted in the area. There was

some active head and bank cutting in the gullies and canyons that would

contribute to sediment production. Some evidence of shout erosion was

noted during the ground reccnnaissance above the damsite. The aroas

in which this was z.ost dominant had been denuded of vegetation by

burning and/or clearing. Thu planted cross are not terraced and will erode. considerably.

HYDROGRAPHIC AND IMToROMGIC DATA

The climate in the area is tropical. The mean annual rainfall

is almost 82 inches, based on 83 years of record at Manila. Essential-

Ly, all of this rainfall occurs in the seven "wet" months, June to

December. During the typhoon of August 1952, over 20 inches of per­

cipitation is ? .'I to havo fallen in less than 18 hours. At least

one storm with an intensity of over 15 inches in 24 hours has been

experienced during the period of rainfall record at Manila. Experi­ ence curves derived from Philippine streams show that the 115 sq. miles drainage bnsin will produce, in extreme cases, a run-off of FIGURE 4.

aERIAL VIM OF PORTION OF IARIKIlA WATERSHED

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about.135,000 cfs. and frequently as much as 30,000 cfs. The hydro­

graphic studies show tha't by utilizing recorded streamflow records

and supplementing these with rainfall records, the watershed will

produce an average annual yield of 435,000 acre feet or 3,780 acre * feet per square mile of drainage area. Being in a typhoon belt,

large floods can be expected.

PROPOSZD DAM AND RSSERVOIR

Present plans for the multi-purpose project include a dam raising

from river bed elevation of 43 m. to approxinatily 195 m. (project

datum). The maximum reservoir elevation will be about 195 m. and at

this level will have a storage capacity of some 940 million cubic

meters (762,000 ad.I) The top of the flood pool will be at elevation

193.7 m. (Storage 746,000 a.f.); top of power pool, 181.0 m. (storage

535,000 a.f.);,top of dead stornge 155.0 (storage 260,000 a.f.) The.

shape of the reservoir is shown in Figure 5.

FIELD DATA GATHERED

Two series of suspended sediment samples have been taken in the

Marikina river. The National Power Corporation took eight samples near the proposed damsite in 1953. The Bureau of Public Works took a series of samples during 1953 and 1954 at a point on the river some distance downstream.

The letter transmitting the results of the analyses of the first samples to the General Manager of the National Power Corporation, states that, "the sanples were trken at about 73.0 meters upstream of Wawa Dam, at m4dstream, by depth integration method". The width u.a. ~--

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of the river at the sampling station was about 215 feet. "The water

sediment samples were analyzed in nccordnnce with the Method Used in

Measurement and Analysis of Sediment Load in Streams." A summary of

results of this series cf samples is given in Table 1. The discharge

given in this table is the flow at the time the sample was taken as

determined from the stage-discharge relationship of the stream as it passes over Wawa Dam.

The report of the suspended sediment sampling done by the Bureau

of Public ilorks states that the samples wore taken at the downstream

side of the Vargas Bridge over the Harikina River in Pasig, Rizal. 6 (This point is sme 22 river miles downstram from the proposed dam

and is only about 3/4 of a mile upstream from the confluence with

the Pasig River.) A total of 48 samnrles were taken during the period

Arril 24, 1953 tc 'Karch 19, 1954. Of the 4l days when samples were taken, stream flow was indicated on only 12 days while the remaining

36 dqys negligible or no discharge prevailed. The report states,

"The negligible water discharge is due partly to the effect of the tides, partly to the defects of the instrument (current meter) and to a certain degree may also be caused by the backwater offect of the discharge coming from Laguna Lake." The samples wore taken with a D-49 semrler developed by the U.S. Inter-Agency Committec on Sedi­ mentation. The dorth integration method was used and samples were taken at nine verticals in the ipproximately 395 foot wide stream.

A summary of the date obtained from the 12 samples taken on days when flow was indicated is given in Table 2. The discharges given in this table were taken from the report and are based on a TABLE 1

ANALYSIS OF SUSPENDED SEDIMANT SAhPL!3 FROh KARIKINA RIVER AT WAWA DAM

Sam-: SAMPLE OBTAINED : SAND : SILT CLAY -* : Total Flow of ple : Date : : Cone. : Density : % Conc. : Density : % Cone. ? Density:: Conc.by : Marikina No. r 1953 : Hour : by Wt. : lbs/cf : by Wt. : lbs/cf i by W. : lbs/cf : Wt. PPM CFS 1 Oct 1 0830 0.000893 0.004692 0.000572 62 734 2 13 0930 .000367 .002252 .000203 28 434 3 28 0930 .027925 48.6 .025556 38.6 .002933 23.2 564 4060 4 28 1530 .003198 .015898 35.6 .003108 25.5- 222 4837 5 29 0700- .010228 .006446 .000515 172 435 6 30 0930 .000229 .002095 .000372 27 690 7 Dec 2 0930 .001505 .003011 .000419 49 4767 8 6 1400. .001383 .002119 .000822 43 459

Total U.045728 0.062069 74.2 0.008944 48.7 Average 0.005716 48.6 0.007759 3-7.1 0.001118 .24.4 146

Average sample is 39.2% sand, 53.1% silt and 7.7%clay. (Sand size 1 to 1/20 m, Silt size 1/20 to 1/500 mm, clay size 1/500 mm and below.) Overall initial average density 48.6 x 0.392 19.05 37.1 x 0.531 = 19.70 24.4 x 0.077 1.88 40.31bs/cubic foot or 43.560 x 40.63 = 2000 21.78 x 40.63 = 1291.2 Tons/Acre foot

NOTE: These amples were taken by National.Power Corporation at about 73 meters upstream of Wawa Dam at midatreEm.

I ? - 15 -

TABLE 2

ANALYSIS OF SUSPENDED SEDDMIJT SAMPLES FROM MARIKINA RIVER AT VARGAS BRIE IN PASIG *RIZAL

Sample OBTAINED Total Cone. Flow of : Sample :0 Date Marikina : by Weight No. 1953 Hour PPM S CFS 1 June 5 1000 203.* 2655 2 26 1030 224.1 186 3 July 24 1325 115*7 373 4 Aug. 7 1225 323.8 3634 5 13 3145 376.3 4103 6 20 1015 350.0 4138 7 21 1030 877.4 22070 8 Sept 22 1050 206.6 8202

9 24 1400 428.0 16580 10 25 1235 86.5 6341 11 Oct 28 1400 889.0 12070 12 Dec. 1 1310 100,4 2070

Data taken on samples 1 and 12 show an average of 17.3%qagd and 72.7%silt and clay. (All material 1/16 mm and over con­ sidered as sand and that less than 1/16 mm as silt and clay). NOTE:- These samples were taken by the Bureau of Public Works

at the downstream edge of Vargas Bridge across the Marikina in Pasig, Rizal. - 16 ­

a gage hoight-discharge relationship obtained from the gagekeeper's

records at the Pasig-viarikinn Junction strff gage.

In addition to the quntitative data enumerated above, a ground

reconnaissance was made of the stream and immediate environs from a

point about 1 mile above the damsite to the confluence with the Pasig.

This reconnaissance included observation of the Puray River, a right

bank tributary, along its course from the confluence with the Hiarikina

at Montalba-n to a point about 1/2 mile upstrra and short reaches of

the left bank tributarics near their mouths. The air reconnaissance

previsously mentioned permitted obsurvAtion of the watershed and the

stream system from the dasito to the confluence with the Pasig.

ANA LYS.!,S OF 2QUANTITATIVE DLTA

The suspended se.diment swarles obtainred at 'dawn Dam were analyzed

in the laboratory for determination of concentration of the sample

and size and density of the sediments contained. The results are given in Table I. The sampling nnd analyses were carried out in accordance with recognized practice and the results should be quite dependable. The samles were taken in the low ranges of discharges, hence, it would be expected that the relationship between discharge and sediment load would be quite erratic. The dearth of samples, the relatively short period covered by the san-ling, and the narrow range of discharge variation will tend tD indicate that the data would probably not yield reliable results when '.nalyzed by statis­ tical methods. Such methods are promised on the mathematical theory of probability Pnd require an adequate samrle of the data that is to be analyzed and are further based on the assumrtion that the sample - 17 ­

selected is unbiased.

Nevertheless, in the absence of other information, these data

were used in an attemip-t to establish the transrort qualities of the

stream by the 113w-duration sediment rating-curve method. The flow

duraticn data assembled by NFC for hydrologic studies for the Marikina Project were used in this study.

The d?,ta derived from samling were quite inadequate for estab­

lishing the sediment carrying characteristics of the stream at the

damsite rrimarily because of the small number of smples taken. A

review of the conditions under which the samples were taken at the

downstream station showed that at this site and at the upstreanm site,

the slopes of the strea::in the immaediate vicinity of both sam.pling

stations are- si:.:ilr; that conditions pertaining in the watershed areas might be considered as seawhat thu samne althzu-:h the area at the lower

site is almost double that at the uprer site (boing 567 and 297 sq.

las. respectively) and the drainage area between the two sites has

flatter slores and different land use. However, it was felt that some correlation of the two sots of data might be rossible. The data de­ rived from both sap1ling stations were lrocessed and are shown on

Figure 6. The correlation was better than anticipated and the study was completed by matching the flow-duration curve, Table 3 and

Figure 7, with the sediment-ratinG curve, adding the estimated bed load, and aprlying the results with suitable cz-efficients to the watershed. This study showed that the watershed was Froducing about

0.8 acre foot of sediment per square mile of drainage area rer year and that the reservoir would have a very long life to serve the purpose ;rsr., ...... * - 0-.cV 4* £- 1 -­ *i.I . .4,--

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TABIE .3

MEAN FIDW DURATION OF MARIKINA RIVER AT WAWA DAM IN CUBIC FST PER SECOND

Number . Mean . Number Mean Number S number of Discharge: of . Discharge of S Discharge S Day s : e. f. 6.: Days c. . s. :sDys 5 0. f. B. 1 13,690 65 950 240 83 2 9,636 70 877 250 72 3 7,866 80 753 260 65 4 7,025 90 670 270 58 5 6,470 100 587 280 53 6 5,817 110 513 290 49 8 4,777 120 453 300 45 10 4,114 130 400 310 42 15 3,252 140 360 320 37 20 2,648 150 320 325 36 25 2,194 160 285 330 34 30 1,900 170 -248 335 33 35 1,650 180 217 340 31 40 1,480 190 191 345 30 45 1,324 200 166 350 28 50 1,210 210 137 355 27 55 1,092 220 112 360 26 60 1,018 230 95 365 25

NOTE:- Original data compiled by National Power Corporation from recorded measurements made by Metropolitan Water District at Wawa Dam for the period 1913 - 1929 (17 years). we.. g~~ft*O% TO 5 SC *0 %l DI i

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for which it was intended.

The results of the sunpling by the Bureau of Public Works were

analyzed by that croncy utilizing the mean sediment concentration

and moan water discharge for tho duration of a flood flow. This

analysis showed that the exrected annual silt production per equare

mile of drainage area would be about 0,42 acre foot of suspended

sediments or a total yield of about 0.45 acre foot. Thus the life

of the reservoir would be longer than that obtained from the previous

analysis. The results are modified by a conclusion in the report

that, "The accuracy of the total annual sediment outflow, as stated

before, would not be authoritatively reliable in view of the infre­

quent sanpling rrocedure followed".

Adequate facturl information was Aot tvailable to rermit a cal­

culation of the amount of bed load thatt might be moved by the stream

at either of the two suspended load sampling stv.tions. In each of the above analysos the bed load was assumed to be 10Q of the indicated suspended load. Experience tablos show that if the suspended load concentration is low (1,000 ppn. or less by weight), and contain a small amount of sand; and the bed materials are primarily gravel and rock; the bed load will average about 5%of the total load by weight.

At Wawa Dam the samples show that the suspended load contains approx­ imately 40% sand and those at Pasig about 17%. Therefore, it was assumed that a reasonabl value for thu bed load movement would be obtained by taking 1C of the indicated suspended load. The hydrologic studies made in connection with the pl1nning of the Marikina project show that the averago annual flow yield from - 22 ­ the watershed area will be approximately 3,780 acre feat per square mile of drainno arca. The total storage capacity provided in the reservoir will be 6,625 acre foot per square mile of drainage area.

Such rates are well beyond the range of experience curves developed from data obtained on the sedimentation of existing reservoirs.

However, extrapolation indicates that the sediment yield should be higher than th.t indicated by the above two analysis but would not be of a magnitude to threaten the economic life of the reservoir.

QUALITATIVE DATA

The ground reconnaissance and the brief aerial observation por­ mitted gathering of certain qualitative data. The stream bed above

Wawa Dani follows the normal patturn in that thu size gradation of the bed material progressively increases from the dam upstream. At the upper limit of the reconnaissance (about 1 mile above the dam) the material was faikly well graded, consisting of sand and gravels and some cobbles. The largest material observed was about 8-inch size. Very few of these were present on the bar examined but.several of six-inch size were noted. It was riot determined whether this was the upper limits of the backwater from the dam but was probably in the vicinity.

Alluvial cones in the stream bed were observed at the confluence of the Boso-Boso and iontl.1ban (Tayabrsan) rivers and at the mouth of the unnamed right bank tributaiy of the Mlarikina just upstream from the dam. The orientation of these cones suggests that the heavier

sediments may be coming from the north part of the watershed. The

general characteristics of the watershed tend to support this possi­ bility. - 24 ­

area is imnediately downstream from the outlet of,the sluiceway

through the dm. It is reported that the reservoir was sluiced

periodically during the time that it was in operation (1909 to 1940)

which would account for the abrasion of the mortar and stones. The

reservoir is extensively silted at present especially in the upper

portion. If large amounts of material had been moving with the flow

over the dam in the past yeairs, it is not probable that the overflow

section would be in its present condition. The shape of the dam

might cause the material to go into suspension over the crest but if

this condition portdins the reverse curve and short apron should be

subject to some scour from the more dense materials.

The foreign rateripls present in the gorge below the dam may

also be a result of the sluicing orerations in the past, or they may

have been carried over the right end of the dwi as a result of a com­ bination of flow conditions g:enerated by the share of the dam and the approach.

From the gorge downstreamt the confluence with the Pasig the

Marikina river is on an average grade of approximately 4J feet per mile. Meanders are frequent and some braiding is apparent at low flows. There is much bank cutting as the river reworks the alluvial deposits in the rlain. The bed is composed of sand, gravel and cob­ bles derived from cherts, quartzites and metavolcanics, Figure 8.

The stream crosses a series of natural controls formed by outcrops of Guadalupe Tuff. The controls form a series of pools in the stream­ bed. These pools trap the bed material and are one of the principal sources of the sand and gravels being removed bv hand onerations and FIGURE 8.

CHANNEL OF MARIKINA RIVER NEAR SAN MATEO. F-.

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i - -~ - 26 ­ and transferred to the Manila area. At a point in the Marikina about opposite San Mateo, both upstream and downstream from the confluence of the Ampid, much gravel is being extracted by band methods. No material over 411 was noted in this reach. The material is being re­ moved from a pocket in the stream bed created by a natural control at the site of the old crossing of the 48-inch water line from U1awa

Dam. Other similar natural controls in the river are located near

San Jose, near Mangahan and in Pasig. There may be others.

The Puray river in its lowor roaches traverses a plain that may have been formed by the erosion of a conglomerate area. In this reach the stream meanders and there is much bank cutting into deposits of sand and gravel with very little overburden, Figure 9. The material is well graded and boulders as large as 24 inches may be found in the streanbed at the confluence with the iderikia2. At this confluence the there is a small cone extending into/flood channol of the Marikina.

Another natural rhenoraenon which has an influence on the sedi­ ment problem is the fact that there are, at or near the mouth of the

Ampid, the Nangka, and perhaps other tributaries, natural controls formed of a hard clVy that is part of the recent alluviums. These controls of relatively stable material reduce the degradation of the channel but are conducive to bank cutting as the stream channel mean­ ders above the control.

-DISCUSSION OF OBSELRVATION

It does not appear judicious, because of the lack of adequate data# to carry the study to a point of predicting the actual useful life of the reservoir and the position that the sediments will assume. FIGURE 9.

BANKS OF TIE PURAY RIVER CHANNEL NEAR

CONFIENCE lITH MARIKINA-GRADED SAND

AND GRAVEL DEPOSITS .7-.-. .T'

. I ~.. .- ,, - 28 -

However, some general coments are in order. The conditions pertain­

ing in the watershed in the rast and the samrles taken when these

conditions existed indicate a low rate of sediment yield.

It must be borne in mind that the samples were taken before the

destruction of the vegetcl soil cover began. At p'resent, considerabie

area has been denuded by burning and clearing and the indications are

that this area will be expanded appreciably in the interim period

before completion of the dan. Burnint and clearing in the reservoir

area below high water level will be an asset to the rroject. The

vegetation, ;articularly the trees, that exists in the reservoir at

the time of fillin, will die. hs this dead vegetation is loosened

from the ground it will be carried toward the dam and will greatly

aggravate operational Trocedures. Hence, remioval of this vegetation

prior to filling will reduce the debris problem at the dam. On the other hand, any burning rnd clearing in the draiange basin above maximum reservoir will certainly increase the sediment yield. Exten­ sive destruction of the cover and increased agricultural use will change the sediment producing characteristics of the watershed to the extend that a complete re-evaluation of the sediment aspects would be imperative.

Assuming that the radical changes mentioned above do not occur, the presently indicated rate of yield of sediment and the nature of this material do not roint toward rapid reduction in reservoir capacity.

This sediment will accumulate at different ratcs in the portions of the reservoir allocnted for the various purposes in the multi-purpose reservoir. Therefore, the position that the deposits assume is worthy of mention. - 29 -

All reservoirs fill with the top of the deposited sediment as­ suming more or less of a slope and not with a level surface. If it filled with a level surface, the sediment would first fill that volu below the level of the outlets and would cause no trouble as the ef­ fective capacity of the reservoir would not be reduced. Because the sediment does deposit on a slope there is some reduction of effectiv capacity at all levels in the reservoir, beginning concurrently with the start of storage of water. Since in any reservoir some slope of the stream is necessary to transport the sediment from the upper end to the lower end of the reservoir it follows that there may be con­ siderable deposit of matcrial in the valley above the reservoir at levels above that of the maximum water surface.

The location .ofi* sediment deposits in reservoirs is influenced by several factors. Among these are:

(1) The amount and size distribution of the sediment load.

(2) The amount and fluctuation of the stream discharge.

(3) The method in which the reservoir is operated (whether

continuously full, nearly full or frequently empty; wide

fluctuations of levels over a short period or a long

period of time).

(4) The shape of the reservoir and the slope of the original stream valley.

(5) The size and location of reservoir outlets.

Past experience indicates that sediment deposition in the lower portion of the reservoir (that normally allocated to dead storage) will result; If the water level is usually at low elevations (es­ - 30 ­ pecially at the time of floods), sedimentation concentration is low and consists of a large portion of find silt and clay sizes, a re­ servoir formed in a valley having a short length and relatively steep slopes on the floor and small outlets at the dam located somewhat above the original streambed.

Combinations of factors conducive to sediment deposition in the upper portion of the reservoir are: Heavy sediment concentrations in the flow constituted principral.ly of sand size or larger, water levels usually retained at high elevations, large outlets set low in the reservoir, constrictions in the reservoir at intermediate locations,

.nd the presence of abundant vegetation at the head of the reservoir.

Variations of these combinations leadl to intermediate locations of the sediments in the reservoir area.

The proposed Marikina reservoir will hve a moderate seasonal drawdown; is short and the original valley floor has a steer slope; and the samples taken at the damsite indicate a relatively low rate of sediment transport with the material consisting of about 60% silt and clay sizes; the power take, through which the larger volume of water will be released, is small in relation to the inflow to the reservoir and is located considerably above streambed level; and there is considerable vegetation in thu valleys at the head of the reservoir inlets. Consideration of those factors indicates that the sediment will be distributed throughout the reservoir, with somea above maximum water level and in all probability will not be carried in its entitety into the 260,000 acre foot volume rrovided for dead storage. N 31 ­

'In connection with the discussion of the position that the sedi*.

ments will assume in the roservoirp mention should be made of another

phonomonon that can influence both 'he rate of deposit and the final

position. Density currents have recently been the subject of consi­

derable study as a possible means of reducing the rate of filling of

reservoirs. Under certain combinations of conditions, stratified flow

through a reservoir will form. The more dense, silt-laden water may,

upon entering the reservoir, flow in a layer under the less dense

clear.wnter. This flow may continue throughout the length of the

storage basin and reach the dam. If suitable outlets are provided,

this sediment bearing flow can be released through the dam before the

material can settle in the reservoir area. If outlets do not exist

the position of the sediments will be altered by the formation of the

density currents. Sufficient howledge of the phenomenon is not yet

available to enable n- accurate appraisal of its influence.

The low-level outlet included in the proposed plans for Marikina

Dan may prove to be a valuable asset at a later date should a combina­ tion of conditions in the reservoir be conducive to the formation of

of density currents. The existence of these currents must be recog­ nized and proper operation of the appurtenances at the dam must be

followed if sediment removal is to be accomplished.

DISCUSSION OF THE SEDIMENT POBLES

When a dam is built in a stream, the forces of nature are thromn into unbalance. This is particularly true of the established transport regimen of the stream. It is difficult to predict the exact physical changes that may be effected but, in general, an aggradation of the - 32 ­

original streambed upstream from the dam end a degradation below can

be expected as the altered flow pattern adjusts the slopes to conform

to the now flow regimen. The problems incident to aggradation upstroam

have been discussed in terms of reservoir sedimentation.

The rolease from the reservoir of water that has been almost

entirely relieved of its silt load results in a condition such that the silt carrying caracity may exceed the load. If bed materials are

available and the slopes of the streambedp which may have been altered

during construction period, are proper, the stream will pick up its

load again. The resulting degradation my precipitate operational

troubles. In the present proposals for Marikina Dam there is some

difference in maximum design water surface level of the afterbay

storage and the maximum design water surface level at the draft tubes

of the power plant. Unless controls of a permnnent nature are in­

cluded in the tailrace canal, degradation might progress to a point

where the draft tubes would be at least partially unwatered. The

afterbay dam will certainly provide a control that will place a limit

on any degradation of the stream immediately below the main dam.

The watershed draining directly into the afterbay storage is

small and will produce limited quantities of sediment.

Wawa creek onpties into this storage area. In its present con­

dition the sediment yield from this source is not great. However, building of roads, opening of quarriae and other activities connected with construction of the main dam may greatly acclerate the sediment yield. Should adverse cond tions develop, a debris dam could be con­

structed at an existing site near the confluonce of this stream and

qAO - 33 ­ he Makikiha,

The proposed operational program will result in releases of water from the main dam to the afterbay that are quite free of sediments the major portion of the time. During flood flows, the primary reservoir will trap approximately 95% of the sediment transported by the stream above. Therefore, sediment problems of a serious nature are not ex­ pected in connection with the diversion of the municipal-industrial water or that for irrigation and no special devices for excluding or extracting sediment load need be included in the diversion structures other than those features normally used in good design.

The plans proposed for the afterbay dam and appurtenances include sluiceways in connection with the irrigation and municipl-industrial water intakes. It is possible that these sluices may not be essential.

However, the cost of including them during initial construction will be considerably less than for delayed construction, should operational difficulties or development of density flows in the storage reservoir and subsequent changes in the operational plans indicate the need for such structures.

Operation of the project contemplates minimum releases to the river downstream from the afterbay dam except during periods of high

run-off. With such flow conditions prevailing, the degradation of the river bed below the afterbay dam would progress very slowly, if at all, during the major portion of the year. The large flows will greatly accelerate degradation activity. There are, however, two factors that will act to retard degradation. The first is the fact that there now exists in the river bed certain natural controls in -34 ­

the form of resistant materials. The river crosses an exposed ledge

of Guadalure Tuff about 1/2 kilometer in extent, ur and down river,

at a point just below the confluence of the Ampid. This location is

near Barrio Ampid in San iMateo and is approximately 4j river miles

downstream from the afterbay damsite. The Guadalupe Tuff formation

is quite erosion resistant and lowering of the existing control at

this location will be very slow. There are controls of the same mate­

rial, but less extensive in length, at points downstream; (1) near

San Jose, (2) just downstream from the proposed flood channel loca­

tion and (3) in Pasig, Rizal. There may be others.

The second natural phenomenon that will act to retard degradation

is the uncontrolled Puray River that enters the Marikna as a right

bank tributary approximately one-half mile below the afterbay dam.

Examination of this stremn indicates that during the rainy season it

carries a sizeable bed load of sand, ;ravcl, and cobbles derived from

bank cutting in the alluvial plain. Sinco the banks contain consi­

derable fines, and a soil overburden, considerable suspended load is

probably carried also. This material, transported into the Marikina,

will provide at least a portion of the required load for the releases

at the dam. Because of the greatly reduced flow in the Marikina and

the uncontrolled flow in the Puray, it is possible that on aggradation

problem may develop at the confluence. In later years, it may be ne­

cessary to remove some material from the Marikina in order to maintain

a channel and provide protection from floods and bank cutting for

Montalban. If the material deposited in this area should prove to be of suitable gradation, establishment of aggregate processing plants - 35 *

might aid in the removal.

Similar problems may develop at the mouths of the Mongo, the

Nangka and the kmpid but examination of the beds of these strenms

ruvoals smaller materials that requiro lessor velocities for trans­

port and the Marikina may be able to move the loads presented.

Judging from the present condition of (1) the bed of the Puray

and (2) Wasp Dam, it arpears that the Puray is the major source of

the aggregates being removed by hand operations from the bed of the

iarikina at various locations from Montalban to Pasig and that closro

of the Marikina at 'vTowa Dam would not seriously reduce this aggregate

gathering activity. The series of pools in the Marikina between the

above mentioned controls should refill each rainy season with mate­

rials produced by the uncontrolled Puray.

The operation and maintenance costs on irrigation systems are

normally increased by sediment problems in n amount proportionate to

the magnitude of those problems. .rosion and doposition of materials

may occur at various places in the system, particularly near the head­

works, near major structures, at points where ston or waste run-off

enters the system and where the conveyances pass from one soil type

to another of quite different characteristics in a short distance.

The conditions at the diversion of the proposed irrigation system

have been discussed and are such that a silt load will not be intro­ duced into the system at that point. Therefore, no special devices

for excluding or extracting materials are necessary. Transported mate­ rials in the system must be produced within the system or carried into it at places other than the point of diversion from the river. The - 36 ­ grades on the canals are relatively flat and the alinement does not cross apparent areas of unstable material. Hence the probabilitvy of degradation and bank cutting is rather remote. The system contains a minimum of major structures where resulting flow conditions will produce bank cutting and erosion. Such trouble spots are usually found below chutes and drops, and transition sections.

The plans propose to take a minimum of stor water run-off into the conveyance system but rather to pass natural water courses of any consequence over or under the canals. At points where storm water may enter the systen debris and silt will also enter. Since this entrance is expected to occur primarily in the rainy season, the material will rer.ch a certain degree of consolidation by the time the canals are put into full operation at the beginning of the dry season and removal by draglines or similar mechanical devices will be advisable.

With a nominal maintenance program no major sediment problems are expected to develop in the irrigation grstem. Those which do develop should be local in nature.

The land slopes are quite flat in the irrigible area. Enlarge­ ment and degrading of drains and natural water courses serving to evacuate excess water resulting from waste or stonms should not occur if reasonable measures are taken to care for these excesses.

The operating program for the Marikina irrigation system proposes flow in the canals the greater part of the year.

The flow will be below normal capacity a considerable portion of this time and the average velocities will be less than the design values. - 37 -

This condition is conducive to aquatic vegetal growth, particu­

larly in warm climates. The problems created by moss, algae, etc.,

in canals are further aggravnted by the deposition of sediments in the contaminated areas. The presence of the water weeds in the flow

section changes the velocity distribution pattern and causes a result­ ant eneral reduction in forward velocity. Transported material in the flow then tends to settle out more rapidly. The resulting depo­

sition is a combination of sediments and vegetal growth and is difficult to remove from the canals and laterals. it program for elimination or

reduction of aquatic growths will probably be necessary.in the early operational phase to alleviate possible sediment problens precipitated by their preseise.

If the entrance to the rroposed flood channel near Rosario is

located on the outside of the river bend as planned and the centerline

of the channel is alined with, or near, the centerline of the approach­

ing stream there should be no shoaling problems develop that will tend to reduce the carrying capacity of the flood channel. The flat slopes

along the line of this channel and the backwater produced by Laguna de

Bay will combine to alleviate the possibility of degradation. In con­ nection with this flood channel# it may be possible that the natural

control in the Marikina, which may have produced the existing meander,

could be improved by artificial means and an increase in the amount of flow diverted to Laguna de Bay could be accomplished.

ADDITIONAL STUD5.1 The small number of suspended load samples taken at the proposed dansite is not adequate for complete analysis of the reservoir sedi­ - 38 ­

mentation problem. The location of the sampling station and the

techniques applied are quite satisfactory. -a program providing for

periodic sampling of the suspended load should be initiated at an

early date and continued until closure of the stream is accomaplished.

If this program yields the results that might normally be expected,

the data gathered should cover a fair ranE;e of discharges and sediment

loads, and a more complete analysis of the sediment aspects will be

possible. The additional data will also serve to indicate whether or

not the alterations to which the vegetal cover in the drainage basin

is presently being subjected will seriously change the sediment yields indicated by previous samples.

The sampling technique enployed by National Power Corporation could be improved by the use of an integrating samler such as has been developed by the U.S. Inter-Agency Conuttee on Sedimentation.

Although the samrler utilized for the previous studies has been em­ ployed in a number of locations and the results are accepted as satisfactory, use of a more recently developed sampler will tend toward results of a higher order of accuracy and furthermore will yield data that is more nearly comparable to those collected elsewhere,

Since accurate. determination of the sediment carrying capacity of a stream is very difficult, any standardization of equipment and methods that may be accomplished will add to the overall significance of the results. The sampling program should not be delayed or curtailed because of unavailability of the improved sampler but steps to acquire sv--ch equipment should be taken in order to enhpgue the value of later sudies, - 39 -

To determine the extent of channel aggradation and degradation

after construction of a dam, it is highly desirable to obtain channel

conditions above and below the site prior to commencement of construc­

tion activities. The first step in this rrocedure is to establish

sedimentation ranges and reservoir survey markers in accordance with

established practices both above end below the damsite, After com­

pletion of the dam, reriodic resurveys will provide factual date for:

possible litigation; required future improvements; operational purposes (especially regarding capacity relationship); relating effects due to

changes in stream regimen c= structures and any proposed upstream or

downstream improvement; and to provide information that will aid in

future planning. In the case of the Marikina Project, establishment

of the sediment ranges and reservoir, markers and resurveys will provide

data for studying some of the possible sedit.ontation problems previously mentioned.

When the project moves into the operational phase, a program

should be initiated to determine certain physical parameters in the

reservoir. The data derived from periodic sampling and analysis of

resarvoir water to determine turbidity, and possibly salinity, with

respect to depth and location in the reservoir together with tempera­ tures prevailing at the roints of sampling can be usod as a basis for developing the most advantageous procedure in operating the appur­ tenances at the dam. The existence and order of magnitude of density flows in the reservoir can only be determined from data derived by initiation in such a program. - 40 a-

COORDIETION OF STUDIAS

Exp-erienco has shown that activu coordination of the activities of organizations involved in streaun developnent will in most instances, permit gathering a maximum of data with a minimum budget contribution from each organization. Development of the Marikina involves a number of organization, some of which my be private. Any coordination of effort toward combining sediment data grthering that may be accomplised should certainly be beneficial. The nature of the infonnation needed will in some instances preclude a joint effort toward obtaining it but if the end results are made available to the other organizations they may be helped in the solution of their problems. - 41-

CONCIUSIONS

The following is a summary of the conclusions previously indicated in this report. They are a result of a study of the quantitative sediment data gathered by National Power Corporation and the Bureau of Public Works; perusal of the plans evolved for the development of the Marikina River Multi-Purpose Project by the Marikina Project

Coordinating Committee of the National Economic Council; a brief low level airplane flight over the drainage basin and river valley below the damsite; ground reconnaissance of the river from the dam­ site to its confluence with the Pasig and a very limited portion of the reservoir area above the damsite; and a review of available reports of conditions in the watershed and stream valley below the damsite with particular reference to published and unpublished notes of Mr. Earl i. Irving on the geology of the area.

(1) The dearth of available data precludes a detailed analysis

of the sedimentation aspects of the proposed Marikina Multi-

Purpose River Project and negates the possibility of

drawing extensive conclusions. This is particularly true

in regard to suspended load transport of the stream.

(2) The data upon which the study was based show no single

factor or combination of factors presently existing which

indicate that any serious sediment problem will develop

during the economic life of the project and probably for

a considerable period thereafter. - 42 ­

(3) More information regarding the sediment transport Oapacity

of the Marikina at the damsite is needed. This may be ob­

tained from a continuing program of periodic suspended

load samples taken at the damsite and a laboratory analysis

of the materials contained in these samples. Pertinent

data for continuous successful operation of the project

and for future planning can be gained by establishment

-f sedimentation ranges above and below the damsite and

a -ystematic program of observations on these ranges.

During the operatural phase a program of observations

should be carried on in the reservoir to determine the

existence of density flows and provide data for modifica­

tion of the operating schedule.

(4) The natural factors predominant in the watershed area combine to make erosion only moderately active and there­

fore it is not an extensive sediment producer. The area

has enjoyed a period of relief from man made alterations

of natural processes. Recent termination of this period

of relief and the current burning and clearing of vegetal

cover and conversion of the land to agricultural use

without benefit of terraces will certainly increase the

sediment yiel. The amount of increase will be dependent

upon the extent to which the vegetal cover is destroyed.

(5) No special measures are recommended for inclusion at this

time to protect the storage reservoir from silt deposits - 43 ­

other than watershed management.. The entering sediment

load will be relatively light. The nature of the materials

carried by the stream and factors pertinent to deposition

combine to indicate that the sediments will be deposited

at all levels in the reservoir to include some in the stream

valleys above. The large amount of dead storage provided

should amply protect installations at the dam..

(6) Tt size of the reservoir may be conducive to the formation

of density flows. Should such flows develop, a portion of

the entering sediment might be passed through the low level

outlet at the dam by following a suitable operating schedule

based on observations in the reservoir to determine the

existence of the density flows.

(7) No degradation problems are anticipated below the main dam that cannot be coped with by following good design and

construction practices,

(8) The watershed draining directly into the afterbay storage

is small and will produce limited quantities of sediment

provided roads and construction activity do not change

present conditions extensively. The proposed operational

program will result in releases of water from the main dam

to the afterbay that are quite free of sediments the major­

portion of the time. During flood flows the primary re­

servoir will trap approximately 95% of the sediment

transported by the stream above. Therefore, sediment - 44­ problems are not expected in connection with the diversion

of the municipal-industrial water or that for irrigation

and no special devices for excluding or extracting sedimeq$

load need be included in the diversion strictures. This

does not preclude the necessity for the sluceways to

relieve local deposits and to lower the pool for routine operational maintenance.

(9) It is difficult to predict the exact physical changes

that will occur in the stream channel downstream from the

afterbay dam to th confluence with the Pasig. A degra­

dation may be expected but such action will be retarded

by the nature of the bed materials and the existence of

a series of natural controls of erosion resistant material

that cross the river bed. hElimination or reduction of the

flood peaks conbined with the program of minimum releases

from the afterbay dam and the transport of materialp by

the uncontrolled Puray River into the Marikina may re­

sult in an aggradation and channel changes at that point.

This could create operational difficulties and necessitate

channel improvement measures. Similar problems may develop

at the confluence with the Mango, Nangka and Ampid but the

saterials carried by these streams are of smaller diameter

and require lesser velocities for transport. It is probable

that flows from the uncontrolled Puray will be able to move

the loads presented by the three smaller streams. - 45 -..

(10) The livelihood of those engaged in the Production by hand

methods of sand and gravel from the bed of the Marikina

from Montalban to Pasig should not be jeopardized by

construction of the project. The primary source of re­

plenishment of these materials from year to year is the

Puray River which will remain uncontrolled under the present plan.

(11) Elimination or radical reduction of peak flood flows

should decrease the sediment yield in the Valley below

the dam by alleviating sudden major channel changes and

scouring of overflowed riparian lands. Any curtailment

in loss of these valley lands will be a direct benefit.

Any indirect benefits that may accrue due to the decreased

sediment yield should be directly reflected in the cost of

channel dredging in the Pasig and at its mouth.

(12) The entrance to the proposed flood channel from the

Marikina River to Laguna de Bay is located on the outside

of a bend in the river. The alinement of the initial

reach of the flood channel with respect to the river

channel can be made such that no major shoaling or cutting

should occur. The grade of the channel in flat and back­

water produced by Laguna de Bay will restrict velocities

to a point wbere degrading of the channel in the soil

.types traversed will not be a.major problem..

(3} Bocause of the favorable conditions existing at the after­

bay dam, the irrigation coniwdyance system.proposed for the - 46 ­

project will not be charged with a silt load at the .point

of diversion. Problems attendant to non-silting non­

scouring canal sections can be met with careful design

and construction practices. The proposed grades are

relatively flat, generally about 0.0004, and the aline­

ment does not cross apparent areas of unstable material,

The networks will contain a minimum of major structures

that are likely to produce scour and bank cutting. It is

proposed to take a minimum of storm water run-off and its

attendant debris load into the conveyance system but

rather to pass the natural water courses of any consequence

over or under the canals. With a nominal maintenance pro­

gram, no major sediment problems are expected in the

irrigation system. Those that do develop should be loopl

in nature and may occur where canals cross changes in soil

types, bplow drops and other structures, at places where

storm water run-off enters a conveyance, or where flow

velocities are retarded by aquatic vegetal growth.

(14) The land slopes are quite flat in the irrigible area.

Enlargement and degrading of drains and natural water

courses serving to evacuate excess water resulting from

operational waste c' storms shoUld not occur if reasonable

measures are taken to care for those excesses6

(15) Should construction of the project be delayed and con

current extensive changes in land use and cover or should - 47 ­

major construction activities occur, a re-evaluation of the sediment aspects would be in order.

RECOMMENDATIONS

As a result of the study of the sediment aspects throughout the project and the conclusions reached from that study, the follow­ ing is recommended:

(1) That a watershed management program be initiated without delay and that the area be kept under close surveillance in order to restrict clearing and burning of vegetal ground cover and conversion of land to agricultural use without terracing to areas below rcservoir high water level. (2) That the low levol outlet through the primary dam and the sluiceways at the afterbay cdam be retained in the plans for initial construction but that any special construction for retaining, excluding or extracting sediments be de­ ferred until the need is determined to exist.

(3) That a program of periodic suspended load sampling be con­ tinued at Waawa Dam until closure of the stream and that analysis of these samples be made to determine concentra­ tion, and particle size and density of the material. That, if practicalj an integrating sampler be obtained for use in these studies but in no event should the program be delayed until this is accomplished.

(4) That the data derived from the sampling program be processed at the termination of the study and the results - 46 -'

be used to better analyze the sedimentation aspects in the primary reservoir.

(5) Thpt sediment rangus be established and markers provided,

both upstream and downstream from the dam site, prior to

cormiencement of construction activities and that a program

of observations on these ranges be formulated.

(6) That in the operational phase a program be initiated to

obtain data on the existence of density currents in the reservoir.

7) That a resurvey of the sediment aspects of the project

should be made if there are radical changes in land use

and cover in the area and an appreci&ble delay in the start of construction.

C.e ,1 June 7P 1954