AQNCY FOR INi'KRNATIONAL DEVELOPMCNT FOR AID USE ONLY W.AIHIN TON. 0. . C.205 3 BIBLIOGRAPHIC INPUT SHEET -a
A. PRIMARY 1. SUBJECT Science and technology TCOO-0000-G750 C'LASSIt FICATION * Applications--Thailand SlAAY 2. TITLE AND SUBTITLE Salinity intrusion and flood control investigation for Bicol RiverBatn
3. AUTHORIS) Ackermann,N.L.; Tingsanchali,Tawatchai; Sahagun,V.A.
4. DOCUMENT DATE s. NMBER OF PAGES 6. ARC NUMBER 1975 T ft..277Q ARC 7. REFERENCE ORGANIZATION NAME AND ADDRESS
AIT
a. SUPPLEMENTARY NOTES (Sponsoring Organization, PubliIherap Availabllity)
(InATT research rpt.no.51)
9. ABSTRACT
10. CONTROL NUMBER IIl PRICE OF DOCUMENT
12. DESCRIPTORS 13. PROJECT NUMBER Bicol River' Thailand Flood,control 14. CONTRACT NUMBER Hydrology AID-492-834 Salt water intrusion Is. TYPE OF DOCUMENT
AID 890.1 (4.741 AslanAAlT Institute of Technology Bangkok Thailand
research report. 'No. 51
SALINITY INTRUSION AND FLOOD CONTROL INVESTIGATION FOR BICOL RIVER BASIN
by
Norbert L. Ackermann Tawatchal Tingsanchali Virgillo A. Sahagun
In cooperation with Conducted for BICOL RIVER BASIN COUNCIL US/AID Philippines as represented by Contract No. AID 492-834 National Hydraulic Research Center National Irrigation Administration Bureau of Pu *Ilc Works
Bangkok, Thailand May 1975 SALINITY INTRUSION AND FLOOD CONTROL INVESTIGATION FOR BICOL RIVER BASIN
by
Norbert L. Adcormann Profssor of Water Resources Engineering
Tawatchai Tinganchali Assstant Remrch Professor
Virgillo A. Shagun Senior Research Associate
DIVISION OF WATER RESOURCES ENGINEERING ASIAN INSTITUTE OF TECHNOLOGY
Bangkok, Thailan May 1975 In a previous study oonduted Iby the Aeian: Insitute of Tec Ihino ogy, the effeot of varioue.food controt alternatives on food Zeve.s throughout the Bicol River Basin were deterined. This infovation is provi-ed fn Volumes I and II of the, AXT Reaearch Report No. 48. BicoZ River Basin Flood Contro Inveatigation, (7). Xn that repor-jten flood controZ measures were desoribed in term of their ability to provide effective flood reZief for 'design storme having IS3 and 50year return pqrOd. Other ombinations of food control meaeures in corunotion with various irrigationwithdrawal rates were later identified as worthy offurher investgation. In the present audy those flood contro measures ".denttf[ed as deserving frther, invetigation are analyxed in
tems of their-effectivene in reduroi water surface.ZeveZ in the Bicol and Stpocot Rivers. Effeots of impementing various flood oontroZ measures on saZinity intrusion into these -river is aso:studied as weZ as-,the effects of various proposed fZood relief measurea on lon term' drainage in the upper reaches of the Bico River. TABE OF CONTENS
CHAPTER" TITLE PAGE
Preface i ITabe of Contents ii List of Figures iv List of Tables vi
INTRODUCTION 1 1.1 Location and Description 1 1.2 Flood Control Measures 3 1.3 Flood Drainage from Upper Reaches of Bicol River 4' 1.4 Salinity Intrusion 5, II EFFECT OF FLOOD CONTROL MEASURES ON RIVER STAGES 6 2.1 Description of Basin Modifications 6 2.2 Effect of Basin Modifications on River Stages 8: III LONG TERM DRAINAGE IN UPPER REACHES OF BICOL RIVER 21 IV SALINITY INTRUSION 26 4.1 Introduction 26 4.2 Basis of Mathematical Model of Salinity Distribution 28 - Salt Balance Equation, 28 - Hydrodynamic Equations 30 - Coupling of Salt Balance and Hydrodynamic Equations 31 - Finite Difference Scheme 32, 4.3 Construction and Investigation of Hydrodynamic a -linity Models tor Bicol and Sipo ivers 32 - Model Construvn 33 - Boundary Conditions 34 - Initial Conditions 38 - Quasi-Steady Computation 39 - Model Study 39 4.4 Model Application 45 - Tidal and Hydrological Conditions 45 - Roughness Coefficients and Diffusion Constants 45, - Basin Nodifications - Present System with No Other Modification.s,46 TAXLE OF CNTENhS:1ACMrriIED)
CHAPTER TITLE, PAGE, - Present System with Cutoff no.3 46. - Present System with Withdrawal at. Bicol-Ragay Diversion 47 - Present system with Cutoff no.3 and. Withdrawal at Bicol-Ragay Diversion" 48 4.5 Discussions 54
V SUMMARY AND CONCLUSIONS 60 Flood Peak Reduction 60 Long Term Drainage 60 Salinity Intrusion 63 REFERENCES 65 -LIST OF FIGURES
4FIGURE TITLE PAGE
11 The Bicol River Basin 2 2.1 Changes in Water Surface Levels at Lake.Bato'due to Flood Control Measures 11 2.2 Changes in Water Surface Levels at Ombao due to Flood Control Measures 12 2.3 Changes in Water Surface Levels at Naga Cityrdue. to Flood Control Measures 13 2.4 Changes in Water Surface Levels at Libmanan-due to Flood Control Measures 14 2.5 Discharge Hydrograph for Sipocot-San Miguel Diversion 15 2.6 'Changes in Water Surface Levels at Libmanan due to Flood Control Measures 16 2.7 Changes in Water Surface Levels at Naga City due to Flood Control Measures 17 2.8 Changes in Water Surface Levels at Ombao due 'to Flood Control Measures 18 2.9 Peak Water Surface Profiles along Bicol and Sipocot 'Rivers for 13 Year Flood 19 20( Peak Water Surface Levels at Libmanan, Naga City, Ombao and Lake Bato for Flood Control Measures for 13 Year Flood 20 3.1 ! Water Levels at Libmanan, Naga City, Ombao and Lake Bato Affected by Channel Deepening 23 3.2 Water Levels at Nodes 16 through 19 as Affected by Channel Deepening 24 3.3 Water Levels at Node 20 and SI Affected by Channel Deepening 25 4.41 Node and Branch Schematization for Bicol and Sipocot Rivers. Bicol River Basin 35 4.2 Constant Fresh Water Inflow to Nodes, Bicol and Sipocot Rivers 36 4.3. Water Level and Salinity Boundary Conditions and .Corresponding River Discharge at Bicol River Month for Existing River System (No Flood Control Scheme) 37 LIST OF 'FIGUES (cONTIuED)
CHAPTER TITLE -PAGE 404 Discharge, Water Level and Salinity at Libmanan for!'Existing River System (No Flood Control Scheme) 41 4.5 Discharge, Water Level and Salinity at Cuyapi for Existing River System (No Flood Control Scheme) 42 4.6 Discharge, Water Level and Salinity at Naga City for Existing River System (No Flood Control Scheme) 43 4.7 41aximum and Minimum Salinity Distributions along Sipocot and Bicol Rivers for Existing River System (No Flood Control Scheme) 44 4.8 Discharge, Water Level and Salinity at Libmanan for Various Flood Control Measures 49 4.9 Discharge, Water Level and Salinity at Cuyapi :for Various Flood Control leasures 50 4.10 Discharge, Water Level and Salinity at Naga City for Various Flood Control Measures 51 4.11 Salt 'Water Intrusion along Bicol River and Maximum' Possible Salinity at Naga City 52 4.12 Salt Water Intrusion along Sipocot River and Maximum Possible Salinity at Libmanan 53 4.,13 Maximum Salinity Distribution along Sipocot and .Bicol Rivers for Various Flood Control Schemes 58 4.14 Minimum Salinity Distribution along Sipocot and Bicol Rivers for Various Flood Control Schemes 59
- V, LIST OF TABLiS
TABLEi TITLE PAGE-
2.1 Description of Flood Control Systems 7
2*2 1Effectiveness of the Various Flood Control Measures in Chanfina Flood Peak Level (13-year Flood) 9 2.3 Effectiveness of the Various.Flood Control Measures irA Changing the 11-day Average River- Stage Following the Flood Peak (13-year Flood) 10
Summary Table I 61
Summarv Table II 62
Summary Table III 64 I. INTRODUCTION,,"
1.,1 Location and Description The Bicol River Basin is located in the lower portion of the Island of Luzon extending from 1220451 to 123040' east longitude and from 130001 to 14000' north latitude. Situated about 400 km. from the city of Manila , the basin embraces the central portion of the Province of Camarines Sur, the northern portion of the Province of Albay and a small portion of th( Province of Camarines: Norte. The basin as shown in Fig.l.l is an elongated flat plain b6rdered by a chain of mountains and volcanoes at the eastern side and highlands and low hills on the western side.. It-covers-an area of 3120 sq.km. and extends in northwest-southeast direction approximately 135.km. with!a maximum width of 35 km. The basin's principal waterway is the. Bicol River which originates at Lake Bato and meanders generally in a northwestwardly direction-until it meets " the Sipocot River coming from the north. From the confluence of these !two rivers, the Bicol widens considerably as it travels in a northeast directibn to its outlet at San Miguel Bay. the main tributaries of the: Bicol River are the Quinali, San Agustiin, Irraya and Pawili Rivers, while tnose or the Sipocot River from the north are the -ulantuna, Culacling !and Yabu Rivers. The three lakes in the.basin - Lakes Baao,'., Buh, .and Bato, a provide tributary,; flow into ,the Bicol'River. The scenic Mayon Volcano is also w ithin the basin.*. 2
W+ool .'. .' . ' A.-..
1I PULANTUNA RESE R.~ .. * ......
MANI AN MIGUE BAY.
G:Itulf .G CITY LOCATION MAP % ""'",oPill|;
I ego
Basin Boundary "O AraS e t Flodn
Proposed Flood Control 5 0 a 10 15 20OKW8.
Fig.1.1:, Thl icol Rver Basin 1.2 Fl',od .,Control Measures Six modifications to the river system in the Bicol River Basin were proposed as measures which would serve to reduce river stages during periods of flood. The suggested modifications shown in Fig.ll are (U) Pulantuna Reservoir, (2) Sipocot Diversion, (3)Tidal Barrier, (4) Cutoff Channel no.3, (5)Bicol Diversion, and (6)Channel Deepening, The effectiveness of the following 10 systems of basin modifications designed to reduce river stages were reported by Ackermann, et al: (1975). (i) Pulantuna Reservoir (i) Sipocot San Miguel Diversion (iii) Tidal Barrier (iv) Cutoff no.3 (v) Bicol Ragay Diversion (vi) Channel DeeDeninr (vii) Tidal Barrier + Pulantuna Reservoii (viii) Bicol-Ragay Diversion + Pulantuna Reservoir
+ Pulantuna Reservoir (x) Bicol-Ragay Diversion + Tidal Barrier + Sipocot-San Miguel Diversion + Pulantuna Reservoir The (1) Sipocot Diversion + Cutoff no.3 (2): Sipocot Diversion + Bicol Diversion (3) Sipocot Diversion + Cutoff no.3 +!Bicol"Diversion (4)"Cut0ff no. 3+ Bicol'Diversion (5) Cutoff no.3 + Channel Deepening, (6) Cutoff no.3 + Bicol Diversionl+ Channel!Deepening -4 (7) Bicol Diversion + Channel.Deepening '(8) Sipocot Diversion withdrawing a controlled: , discharge of 32 CMS. 1.3 Flood Drainag:e fromUpper Reaches of Bicol Itiver The river basin in the vicinity or Laxe jjato proviaes. a .arge. voLume of storage which is effective in reducing flood peaks in the lower reaches of the Bicol River during rainfalls associated with typhoon storms., ,.considerable time is required however to,drain this water from the LaKe Bato regions Surrounding agricultural land ,isinundated for many weeks after the occurrence of a major storm in the basin,. If the Bicol: River is deepened in the upper .reaches of the basin, the flood'waters would be drained more rapidly in the Lake Bato region, and hence promote better land drainage. A structure controlling the dis charg.e from Lake Batowould be required however if the River were deepened, in order to prevent the dowastream peak river stages to be furtheri increased durine periods of maximum flood discharge. In,the present study, the levels in the-Bicol River were studied ror a period of 4 days following flood peaKs for the roiLowing Dasin modifications: () Channel Deepening +.Controi Structure at Lake Bato (2), Channel Deepening + Bicol-Ragay Diversion + control Strutue at Lake Bato. -5 1.4 Salinity Intrusion The proposed flood control meatiures woUd have:aneffectonthe hvdro ,,dynamics of the river system during? periods of low flov. These changes could have adverse errects on the,salinity distribution in the basin. Of principal concern is the effects,on salinity distribution which would' result from-the construction of Cutoff no. 3 and from fresh water withdrawal from the Bicol River at the location of the proposed Bicol-Ragay:Diversion. In the present study, these effects are investigated. -6- II EFFECT OF FLOOD CONTROL MEASURES ON RIVERSTAGES 2.1 Description of Basin Modifications The effect of various flood control measures on Peak river stages produced during the 13 year design storm were investigated. The storm having this frequency of reoccurrence had the same rainfall intinsity as that produced by Typhoon Sening in October of 1970. The-total 2 day basin rainfall for the 13 year design storm was 360 mm. Eleven various basin configurations were investigated. Theleffects of these various alternatives in producing changes in the river stages are shown in Figs.2.1 through 2.12. These alternatives are referred to for purposes of convenience as Systems A. B and C and Systems 1 through 9. System A The basin modification referred tobas System A consists only of a deepened portion of the Bicol River nearLake Bato as showniin Fig.l.1. The elevation of this deepened portion is l meter.above.MSL. System B System B is similar to System'A except that the deepened elevation of the river is 2 meters above MSL. Systems 5, 6 and7 which. shall be described below include the deeDened river channels'to the 2 meter level referred to as System B. System C System C is similar to System A except that the deepened elevation of the river is 3 meters above MSL. Systems 1 through 9 and A through C are described in Table 2.1. The "x" marks represent those basin modificationsincluded in a particular -7- Table 2; 1 - Description of Flood Control Systems. Basin Modification .- - Channel Deepeningto: - - - - - 1 meter above MSL x - , ------ 2 meters above.MSL x x x x 3 meters above MSL I,x' i - i~- - I I I - I,. - - - Sipocot Diversion x x:, x Cutoff no.3 x Ix x x x Bicol Diversion - x x x x x Controlled withdrawal of 32 CMS at Sipocot Diversion XI The effect on flood levels caused by a constant withdrawal rate of 32 CMS at the Sipocot Diversion was also considered. This condition,is included in Systems 8and 9. 2,*2 Effect,of Basin Modifications on:River Stages .An,analysis was.first made 'ofthe effects of deepening the'upper •'reaches ofthe Bicol River as described by Systems AB and C. The various. levels,of-channel deepening produced different river stages at different locations, in,the.river basin. This."information is provided-in Fig .2i90 The intermediate bed elevation of 2 meters above MSL was used in the sub sequent analysis of Systems 5, 6 and 7.: An estimate of the response of Systems 5, 6 and 7 to channels deepened to the 3 meter and 1 meter levels can be estimated by observing the relative effects of channel 8 -,deepening to different elevations at Lake, Bato and Ombao. Figs.2,1 thrugh 2.4 show the effect of Systems.1 through 7 on river stages at Lake Bato, Ombao, Naga City, and Libmanan. In each figure the upper diagram, called the Reference Stage Hydrograph, shows the stage hydrograph for a 13 year storm 'in the unmodified river basin. The lower diagram describes the daily change in river stage from conditions described in the Reference Hydrograph which result from the7?basin modifications. Schemes 8 and 9 include consideration. of conditions which would exist if irrigation withdrawals: are restricted ito the Sipocot "Diversion of only 32 CMS0 Under uncontrolled conditions at the location of the diversion, a much larger discharge would normally pass through the diversion channel into San Miguel Bay. Fig.2.5 provides a comparison between the flows through the uncontrolled diversion for the 13 and 50 year flood and the controlled flow of 32 CMS. The:32 CMS represents only approximately 13%, of the maximum discharge that would otherwise pass through the,'diverslon channel. It would therefore be expected that. the effect of the',32 CMS withdrawal rate would be,small on reducifng the flood peaks throughout the basin. Figs. 2.6 through 2.8 show, the: change in the river stages at Libmanan, Naga City and Ombao which would result from river basin modifications previously described as Systems 8 and 9. The information is presented for floods having a recurrence period of 13 and 50 years. Fig. 2.9 shows the water surface profile along the Bicol :and Sipocot Rivers during the .period of flood peak for Systems 8 and 9.. This figre shows the negligible effect that: the controlled,discharge of :32 CMS has on the peak water levels. 'Fig.2.i0 describes the effect of Systems A, B,. u and 1 through 9 on the change in peak river stages in the event of a 13 year storm. ... Tables 2.2 and 2.3 provide a summary of information regarding flood peak reduction and changes in the, lday average of .river stages .for all.of the various flood control systems investigated. Table 2.2 - Effectiveness of the Various Flood Control Measures in Changing Flood,Peak Level. (13-year Flood) Reduction or Increase in Flood Peak Combination of Flood Control Level (cm) Measures Lake Bato Ombao Naga City Libmanan 1. Sipocot diversion + Cutoff no.3 -2 -24 -20 -44, 2. Sipocot diversion + Bicol -3 -64 -17 - - 64 diversion 3. Sipocot diversion + Cutoff no.3 + Bicol diversion -2 -75 -10 .-67 4. Cutoffno.3 + Bicol diversion -2 -75r' l0 -4 5. Cutoff no.3 + Channel "4 1 +11 - 11 deepening (2 m) -46 -14 +11 -1 6. Cutoff no.3 + Bicol diversion +,Channel deepening (2 mn)-6-94 7."deepening Bicol diversion + Channel :: (2 m) ' W"-. 551 :;6-16 : ) ,, *8. 32 CMS at Sipocot diversion -2 -9 -8 -12 9. 32 CMS at Sipocot diversion-+ -3 -64 -712 Bicol diversion 10 Table 2,3- Effectiveness of the Various Flood Control Measures in Changing the ll-day Average River Stage Following the Flood Peak (13-year Flood) Reduction or Increase in the 11-day, Combination of Flood Control Average River Stage following the Measures Flood Peak (cm) Lake Bato Ombao JNaga City Libmanan. 1. Sipocot diversion + Cutoff no.3 -2 -18 -72 -15',. 2. Sipocot diversion + Bico- 7 - diversion -23 3. Sipocot diversion + Cutoff no.3 0 -61 -109 -17 + Bicol diversion 4. Cutoff no.3 + Bicol diversion 0'. -61 -109 -1 5. Cutoff no.3 + Channel -58 +7 -62 0 deepening (2 m) 6. Cutoff no.3 + Bicol diversion' -51 -37 106 -1 + Channel deepening (2 m) 7. Bicol diversion + Channel 50 - 31 -66 -, deepening (2 m) .50 8. 32 CMS at Sipocot diversion 0 0 2 -U. 9. 32 CMS at Sipocot diversion.+. 0 -4 7 -13 Bicol diversion /-13 YEAR FLOOD LAKE BATO S10.0 -- I 20 9.0, Lake bank elevation.-. P 7/ R/ Q w ./P . &0 REFERENCESTAGE HYDROGRAPH 20 "FOR NO FLOOD CONTROL SCHEME 7. 0 2 4. 6 1 8 10 12 : 14 +20(i.. . . .' a r -40 9 E0 - 6,-n-o 00 *= 0:.;u f Sipoco o diversion + Cneloffpninno.3 ®Cut off no. 3 + Bicol diversion o ® Cut off no.3 +I-Channel deepening ©Cut off no.3 + Bicol diversion + Channel deepening ... Bicol diversion+ Channel deepening, Fig."2.1 Changes in Water Surface Levels at Lake Bato due to- Flood Control Measures - 12- OMBAO ' 8.0 .... " ' ' " " r13 YEAR FLOOD " " "' S 7.0- O "-River bank elevotior 6.0 REFERENCE STAGE HYDROGRAPH * FOR NO FLOOD CONTROL SCHEME' 0 F +20-2 4 ----6 ,Days 0 -12 14 :' .- I00 - Sipocot diversion + Cut off no.3 , .. i ".> :, ' 2 , Sipocot diversion -I Bicol diversion' r 4.r '. Sipocot diversion + Cut off no.3 + Bicol diversion •, " ° .~ ."li!'~~ Ctofn.3 + Channel deepening ."."': . , ..: ii", ;!.(:i i6 Cut off no. 3 + Bicoi diversion + Channel deepening "1 1 ' = ' :i. 'iii "i( Boo diversion + Channel deepening ," " - " Fi.2.2 : Changes in Water Surface Levels at Ombao due to • Flood Control Measures NAGA CITY; io / - - EERNC ST- HYRORH 13 YEAR FLOOD 2. a~ 10- - - REFERENCE STAGE HYDROGRAPH 0 River ban.eevaio FO7i 'NO FLOOD CONTROL SCHEME 0 o,-. e\ake~vto - O . ii #"R ,j 100 12 C 40-- 0 601 C 'D 160 Legqend. . ',0 Sipocot diversion + Cut off no. 3 2 Sipocot diversion+ Bicol diversion 3 Sipocot diversion+ Cut off no. 3 + Bicol diversion 4 Cut off no.3 +Bicol diversion 5 Cut off no.3 + Channel deepening 6 Cut off no.3 + Bicol diversion + Channel deepening 7 Bicol diversion + Channel deepening Fig. 2.3 Changes in Water Surface Levels at Naga City due to Flood Control Measures -14- LINMANAN 3.0 -13 YEAR ILOOD o 2.0 -- River bank r /elevation *~1.0, S0 REFERENCE STAGE HYDROGRAPH -- FOR NO FLOOD CONTROL SCHEME " " S0 P~ " 4 6 8 10 12 14 Days ," 0 0 -20 c-40- E-0 -60, In'- QSipocot diversion + Cut of f no.3 .19( Sipocot diversion + Bicol diversion '--2 d:'Sipocot(XJ diVersion + Cutoff no.3 +Blcol diversion ® ) Cut off no.3 +Bicol diversion 0: Cut off no. 3 + Channel deepening . = Cut off no. 3 + Bicol diversion + Channel deepening' ® Bicol diversion + Channel deepening Fig. 2.4 Changes is Water Surface Levels at Libmanan due to Flood Control Measures L 280 -I-- . 15w - 1260 -R-- - FLO-OD-- YEA 05 [t I Of .- 13 EA FO 3 O ~~ 4 5. 6 91 l * I .f 1. Y. AR F O f __I h; I - - - 20 - - _1_ Wihdawlot of.. - - ...... -- - ... .- . .- . ..- - J32CMS.(Systems 8and 9) . • 0 ' I ; 2 -3 4 , 8 . .6 7 8 9 I0 II; 12 13 14 Fig. 2.5 :Dlscharge Hydroaraoh for Soot b-san. Miguel Divrsion. - 16 - LIBMANAN, 50 EAR L- 0 REFERENCE $SIAGEHYDROGRAPH FOR I .0 . -.... R W - - - - 0 3 4 . S 6 7 8 1 12' 13 -10-14 0 ~Days 1 o REERNC S,,,oE<,,OGRA.oR o 6 7)2(32CMS) 0 IvIr 1 3 1 -40 Sipocot di ion (32 CMS 13 YEAR FLOOD -... S-801 -41Dayts 3 a . a.a a .... 1 2a a a a a a 8.. ;c . -.. 9 Slpocot dtversion(32CMS'+OicA dihvmIion 4 - - Sipocot diversion (32CMS) -L 50 YEAR FLOOD a a a I a a Fig. 2.6 Changes in Water Surface Levels at Ubmanon due to Flood Control Measures - 17 - NASA CITY U) .- :, j 25------ to.0 13,YEAR FLOOD .~ -5OYEAR FLOOD -0 " I I ' I" " ,,,. : I. '' I" -I . A I , , '//------"" 0 I 2 3 4 5 6 7 8 9 110 1 12 13 14 -~Days 2C - 8 SIpoct diversion (32 CkiS), -I O Q-..-. . . 2 ..-- - - ,-. .. -:: : I I a a - .a . sipocot diversion (32 CMS) + Bicol diWrso -60 ------Lo 77J, - 13,YEAR FLOOD, I 2 3 4 5 6 7 a 9 10 II 121 14 -~ Days S20 a...." ' ., 8~. s Si"oc1. Ivdrsson (32 CMS) -10 ' I _ 9 Sipocot diversion (32 A ) + Bicol7 dii"on J"o I I . - i -a a. -60 -- - ,dvrin(2C S - .... . a 50Y1AR FLOOD - , " F 2.7 Ch.g.s In Water Surface Levels at Naga city due to Flood Control Measures 1.8 7..8- I OMBAO Mm I LAY . . I 50 NREFERENCE RLrb CONTROLSTGHYDIOG'RAPH MEASURESNFD FOR FLOWS5- -'-'''!"1'"- WITH ..-.-"' """" 4.O I 121 -2D A .c- i --- . "0' 1 2 . ,3 : 4 5 .6 :::7 8 ,;9 10 11 1 '1 Days -"i h' ' ; .i.i: ' * ''-- '* Sip:Dcot c Iv sIm(32"V! '- MS) " ". '. ' , ...... : - Sipocot diversion 11 CMS) . , .. [:.. ~ ~~~~~+lcol diverion' :,'. .'- :..-,, 2C -- -~ Days -- 1 FLO CONTR MEASURES Sipocot vent (3C 0 1 2 3 4 5 6 7 8 9 10 11. 12 13 14 'li:-,40, ~+ Bico diversion ,. Iocotdiverion-,n C. -32 -M 1 1 . 2 1 513rYEAR Fu T(3IM - I I J - 10 02.8 Change inWater Surfaceylevels at Ombo due to Food Control Measures - .9 11 - ,------I 4.I cr. 0 I i 2 0 w. "ISW SAM~ll'W U0140"13 - 20 02 40)(a)()® ®b®® ® ®® 3.8 0 0 0 -4 - - 2 3.6 -- 3.4 - 44 1N0 -64 -67 3.-3.2 +4+4+4 "il 3301-10 -10 8 -20_, *2.8 is-17 • z~e .A C r -40 +!10 80- K4 -9 - -2 -242 . 7.6 .9 -64 -62 -64 -A0 10.0 9.- - 0 * 9.6 -40~ 9.4 -s_AI8 -48-46 -43 -44 1 -60.+ LAK~E BkrO (a) ' (c) 2 3 () 5 Alternative depths of TSipocot dk(32CMS channel deepening + Idiv.(2 S Sipocot diI.+ Cut off dvCt o -Bicol divSipocot + Channel div. deepening(32CMS Sipocot div.,+ Bicol div. .toff+Biool div.+Ctwnnel deepening' Sipocot div.* Cut off+Bicol div.- L -ut off + Channel deepenind* L-cut off + Bicol div. Fig.2.10 Peak Water Surface Levels at Ubmnmnf,Nagp CityOmboo a;d Lake Boto for Flood Control Measures for 13 Year Flood 1117 LONG, TERM' DRAINAGE,.INl UPPER REACHES, OF BICOL RIVER The post flood period in the upper reaches ofthe Bicol River Basin is characterized by areas of inundated or swampy land in which the water remains stagnated as a result of poor drainage conditions. By deepening the upper reaches of the Bicol River between kilometers 69 to 84 better drainage would exist as a result of the lowering of the river water surface elevations. In the present studys the long term water levels in the river were studied for conditions where the 13 year 2-day design storm was followed by a surface runoff which corresponded to an average base flow condition in the watershed. The relationship between the base flow and the corresponding watershed area is Qb= 0.063 A where b = base flow in cubic meters per second A = catcnment area in square kilometers. In the region or interest between Ombao and Lake Bato, the river water ,levelswere simulated for a 42 day period following the 13 year storm. In order that the deepened channel would not cause increased flood peaks downstream, a control structure was considered to exist at the exit from Lake Bato. This control structure was operated in a manner that, during the pre-flood peak period, a downstream discharge would be provided similar to that which would have occurred in the ummodified or undeepened river basin with no control structure. Immediately after the period of downstream peak water levels was reached, the control structure was, fully opened to permit Lake Bato to discharge through the deepened - 22 channel downstream with no losses or flow obstruction from the control structure. As shown in Figs.3.1 through 3.3, the gate is shown to open on day 14. Two conditions were investigated. One where the control structure existed with a deepened channel (System B) and the other where t.he control structure and deepened channel existed as well as the"Bicol- Ragay Diversion. "The downstream boundary condition formed by the ocean level was fixed at mean sea level. A separate analysis revealed the7 fact that this simplification only produced noticable variations at Libmanan from those where astronomical tides were included. This Simplification enabled larger time increments to be adopted when computing flood levels for the lonj term stage hydrograph in the basin. Figs.3.1, 3.2 and 3.3 show the stage hydrograph at Libmanan, Naga City and eight stations between Ombao and Lake Bato. The18.75 km. reach between Node 18 and Lake Bato experienced a significant reduction in water surface elevation. This: reduction would enable the surface drainage in the Lake Bato and Lake Baao area to be accelerated if adequate drainage channels to the river exist or could be provided. The results also indicate that the effect of the Bicol-Ragay Diversion does not maerially affect the river stages in tne deepened portion of the Bicol ,-River. - 23 day t200 25 30 35 4.0 445 3.0 .... LIBMANAN" " " •L eg end " - I. " r.,,-, * : . --.-- ico diersoGatoe opened on day 14 2.0 Present system(no bsin modifiton) 1.0 .' :icol -!-X- diversi-----+GGteopened 14on day 0 River bank Rie •!::*, " NA A CITY... 02. 0 , 81.0 0.0 7.04.0 River bank O"... 83.0 8.0. .0 01.0 09.0., ..... SATO t u si I I si I a i I 5i I IS I " 5 i0 15 20 25 30 35 40 45 day.-.- Fig. 3.1 WalW LeVels at Llbmnnon,Nago CltyOmbao and Lake Boto as Affected by Channel Deepening. 24 day 5 0 15 20 25 30 35 40 45 50 55 GO 0NOM 16 Legend -Present system (no basin modIfIcatlon) I.0 - Gae opened on day 54 70 ..... Bof diveruion+ Gat opened on day,4 SO E 0 zIo 0 F10-River ba l No 17 da 85.0 'P.0 6.0 5 10 15 20 25 30 35 40. 45 50 55' day Fig. a2 Water Levls aOf Nodes rothro4 19 as Affected by Channel Depening. . 25 day ,5 Bleb ++ ++ ++25'- ....30+. 35 40g 45d5 DeeE 2e Preg.nt system (no boelnn migm. "~ ~F ~ ~ ~ ~~~~~~~GFt++ + + ' i- opened on1 day 14 .0 5 10 aS 20 25 30 35 40 45 50 5 fig.r3banek eel tNd 0 n t.Dmig Afctdb hne E ga0 s" - ...: 1&0 ': . ... . +? i 10:D .+ 1 5 20 5 +++ 3 5 "++ .145++ +" 1501+ J '+5 d3 55• - 36g;3. % lr'Levwls at Noe20 : Sto "D'rnl-Oa$ Affect''b :: nl. l:eepering,. - 26 S.1 •Introduction During the period of low flow, salinity intrusion into San Miguel Bay pollutes the lower portions of Bicol River. The intrusion is caused by the effects of convection and longitudinal diffusion. Convection of salinity is produced by the movement of sea water which carries the salt water into the river. Diffusion of salinity is produced by the existence of the density gradient of the salt and fresh water mixture. The extent of the salinity intrusion depends mainly upon the fresh water flow in the river and the geometry of the river channel. Changing the river geometry (i.e. the construction of diversion canals or cutoffs) would produce a change in the salinity distribution Along the river. Diversion canals and cutoffs are usually built for the purpose of irrigation as well as flood control. However, a careful consideration should be carried out to clarify the effect of such struc tures on the changes of the salinity distribution especially during the period of low flow. Cutoffs increase the discharge capacity of the river but allow a greater amount of sea water to enter the river if the cutoffs are located within the tidal reach. Diversion canals divert a portion of the fresh water from the river and reduce the river discharge to the sea. A reduction in fresh water flow in the river channel would result in an increase in the salinity distribution along the river. In the present study, a mathematical model of the river system describing the salinity distribution throughout the Bicol River'Basin -s developed. The effects of salinity intrusion during periods ofi low - 27 flow will be studied for the followin bain ofigatin.. (i)Present system with no other modifications (ii)Present system plus Cutoff no.3 (iii) Present system Plus irrigation withdrawal of 50% of Bicol River discharge through Bicol-Ragay Diversion.- (A withdrawal of 50% of Bicol River discharge is estimated as 30 CMS during the periods of low flow.) (iv) Present system plus Cutoff no3 plus 50%sof B~col Rver dscharai withdrawal through Bicol-Ragay Diversion. The continuity and momentum equations are used to describe he hydro dynamic or flow conditions in the river system. The salinity distribution along the rivers is computed from the salt balance equations. A base •flow identical to that described in Part III (Chapter on Long Term Drainagi is consideredas the source of fresh water supplied to the rivers. This base flow is computed from the analysis of low flow data of previous hydrologic records. A sinusoidal water surface fluctuation at the BDicol ,River mouth is assumed as the downstream boundary condition. The maximum ocean salinity is assumed at the river mouth during the period of flood tide. During the ebb phase of the tide, the salinity variation at the river mouth is computed from the amount of salt within the river basin.' The analySis is expected to provide-qualitative results which could be used to determine the relative effects of flood control measures on the, salinity istribution' in the river basin. The mathematical model is translated into.FORTRAN IV 6omputer program. Salinity distributions are. computed both in the Bicol and - 28 Sipocot Rivers. "The important locations where the variation of salinity is presented are at Libmanan, Cuyapi and Naga City. These locations are significantly influenced by salinity intrusion during the periods of low flow. 4.2, Basis for Mathematical. Model of Salinity Distributiom Salt Balance Equation The node and branch computational scheme developed by Meijer, Vrengdenhil and De Vries (1965) is used to construct the mathematical model describing the salinity distribution in the Bicol River. From the continuity of salt mass, the salt balance equation for, a node in a river system can be.written as E.(QS, E[QSIou , (AE -) + E(AEPLOtd + Convection Diffusion In Eq. (4.l) V is the volume of the mixturpe in ,the .node, S is the average weight of .salt per unit volume" of the. mixturd in.'.the node, '.is the average weight of salt per unit volume of the mixture in the upstream node, Q is the branch discharge.in or,out from the node, - A is the average cross sectional area of the branch, E is the diffusion ,-zamzter of the branch, L is the length of the branch, N is the rate of salt-mass added to the node and .t is the time. In the present study, N is considered as zero. Eq.(4-1) implies that 'the rate of change of salt.weiht content in the node is equal to,the summation of the salt mass transport produced by convection and diffusion into and out:from the node.* In the derivation ,ofEq(4.), the following assumptions are made: a) Salinity distribution over the cross section-is uniform. b) Salinity within the node is the averagesalinity in the river portion defined as the node vicinity. c) The salinity of the salt water mixture leaving izthe node,is equal to the salinity defined for. that node. d) salinity The gradient along the branch joining the two nodes is equal to the ratio between the difference in the salinities of the two nodes and the length of the branch. e) There is no dissipation of pollutant in the flow system. The salt balance equation which is'derived however is usable for-the node and branch scheatizatilon in which the length of the branch and the time step is not excessive. Considerable error may occur if the scale of the schematization is disregarded. The diffusion parameter, E, for the mass transport in an open channel described in Eq.(4.2) has been suggested by Fiwher (1970) to be a function of the diffusionconstant, k, the hydraulic mean depth, R, and theiflow velocity, Vs as follows: E kR5/6 lVI (4.02) Substituting Eq.(4.2) into Eq.(4.) and,siplifylg, the salt balance equation can be written as .d(VS) 5/6 "AS 5/6 dS 1(Qs' )in E(kR- IQ ,En(kR iQJ") NA -. 30 . Eq.(4I.3) can be simplified and rewritten as d/6 dt-v (E(QS' 1S .- E(Qs), - Z(ka dSl + E(kaR..5/6 Il dLoutdS I in out R5 PQj~in out -S(EQ nEQo~t (44) Eq. (4.4) expresses the time variation.of salinity within the node. The total derivative is used in the derivation since the salinity S of the node which is fixed in space is the function of time alone. Hydrodynamic Equations The hydrodynamic equations which are used to describe the unsteady free surface flow in open channels are the continuity and momentum equations. The equations are described in Volume II of the AIT Research Report No.48 byAckermann, et al (1975) on the determination of the effectiveness of flood control measures on the flow in the Bicol River Basin. The equations are derived to handle the dynamic as well as storage effects in the main channel and berms. The continuity equation expressed for a node becomes .dH EQin . Qout dt :F Themomentum equation for a branch is expressed as , [ Q in- out node 1-(EQ in out node 2 MQ, dt A F , + F -' 1+ 2(A l 12 A L AL 2 1 L H2 - 1 ) 3 3 [ E K.] 2 E PiQ~v iV where' M = i=1(4) PQV 2 1 3 Ki K 1 AARiA (I. - 31 - 3 3 S E Q.; A E A .9. P:1 i11 in kqs.(4.5) and (4.6), H is the stage, Q is the discharge, F is the surface area of the node, M is the momentum correction factor, T s the topwidth, is A the cross sectional area, L is the length of the branch, g isthe gravitational acceleration, K is the conveyance,, V is the velocity, n is the roughness coefficient, R is the hydraulic mean depth. The subscripts 1 and 2 respectively refer to the upstream and downstream nodes. Eqs.(4.5) and (4.6) neglect the effect of density gradient when it is applied within the tidal reach. However, such effects were studied and found to produce no significant error on the comutation-, of the water levels and discharges within the tidal reach. Coupling of Salt Balance and Hydrodynamic Equations In order to determine the salinity distribution along Bicol and Sipocot Rivers, the water levels, discharges and salinities have to be solved simultaneously. Under sim!pl2fied conditions (e.g. when the effect of the density gradient of the salt 'water mixture is neglected) the density gradient term in the momentum equation is not included. Obtaining the discharges and. water levels from the continuity and momentum equations, the salinity distributions along Bicol and Sipocot Rivers are then solved independently from the salt balance equation. No information on the salinity distri bution is required when the water levels and discharges are recomputed the for next time interval from the continuity and the momentum.equations,. - 32 - Finite Difference Scheme An exact analytical solution of the hydrodynamic and salinity equations is impossible. One possibility is to obtain the solution through the application of numerical integration. The continuity, momentum and salt balance equations are expressed in finite difference form. The finite difference schemes which are used to express the derivatives with respect to time can be written as + it . "Of] +0 oft+tJ In Eq.(4.l0), f(J,t) = where J is the unknown vector representing either stage, discharge or salinity. The weighting factor. " 6, chosen was suggested by Vreugdenhil (1968) as 0.55. Eq.(4.l0) is then substituted into the continuity, momentum and salt balance equations which are then linearized by assuming that the multiple products of the incremental changes of the stage, discharge or the salinity are small. Obtaining the set of linear finite difference equations, the incremental changes in the stage and the discharge are first solved from the continuity and momentum equations. Subatituting the computed values of stages and discharges into the finite difference salt balance equation the incremental changes in the salinity at various nodes assigned to the Bicol and Sipocot Rivers are obtained. 4,3 Construction and Investigation of Hydrodynami4and, salinity Models for Bicol and Sipocot Rivers It is required to study the iffects of Cutoff no. 3aid the Bicol- Ragay Diversion on the salinityS ' "distributon ' " ' " .... along " the;he;""' Bicolco-!"" and. SioAolS" nocot"' "' - -33 - Rivers during the period of low flow. The model which was previously constructed could describe the hydrodynamics of the river system under low flow conditions. The salinity model is constructed to describe the salinity distribution in the region for whatever hydrodynamic conditions exist. Lack of:,low flow salinity data for calibration prevented an accurate evaluation of the " diffusion constants in the salinity model. A range of assumed values of the diffusion constants were therefore used. A study is made of possible errors produced by incorrect assumptions of values used for the diffusion coefficients. Model Construction The mathematical model for the salinity intrusion consists of both the hydrodynamic model and the salinity model. The two models are operated in conjunction. The 'solution field of thi model covers the region of the Bicol and Sipocot Rivers from Lake Bat( and Napolidan to the river mouth at San Miguel Bay. The rivers, are schematized into nodes and branches as' indicated in Fig.. '. Twenty three nodes and twenty six branches are involved in the schematization for the mathematical model.,: The geometry of river channel and9 cutoffs are included in the models. The effect of the irrigation withdrawal of 30 CMHSfrom the Bicol- Ragay diversion is determined by substructing that amount of discharge' from the Bicol River at the location of the diversion. The continuity and the salt balance equation is written for every node. The momentum equations are specified for every branch. The output of the hydrodynamic model describes the time variation of the water level at every node and the discharge at every branch. The salinity model provides the discritized, - 34 salinity distribution at the nodes. Boundary Conditions The investigation is made during the low flow period. The boundary conditions assigned to the mathematical model are selected as those which could produce the low flow condition. Two types of the boundary conditions are involved in the hydodynamic models. The fresh water inflow into the Bicol and Sipocot Rivers during dry seasons (i.e. base flow) and the sea level oscillation at the Bicol River mouth at San Miguel Bay. The base flow equation which describe the amount of fresh water discharge to the rivers iat the various node points can be expressed as %-o.~06s: ° 87c~ =b 0.063 AcI CMS (4.*11) In Eq.(4.l), b is the base flow discharge into each node while A is the catchment area in square kilometers astigned to eacn node. The base flows are computed for each node and are kept unchanged throughout the period of investigation. Fig.42 shows the base flows which are computed for various nodes assigned to the Bicol and Sipocot water courses. The water level oscillation at thq Bicol River mouth, during -the period of low flow, is described by a sinusoidal wave having the amplitude of one meter with respect to mean sea level with the period, of twelve hours. No storm surges are considered during this :1w flow period. This downstream boundary condition is shown in Fig.4.3,' For the salinity model, only the salinity boundary conditi,3n at the river mouth is required. The salinity at the upstream end of the river reach (i.e. at Lake Bato of BicoL.River and Napolidan of the Sipocot River) are set as zero. The salinity boindapy conditinn at _ 35 - Sabana a CYP No" CityWatrhed Rooon ,manon IrrigatIon WithdIrawalB Lk Ombao Buhl) Fio.!4.1 Niod-d Branch Schenmtization for i, i nd Sipo~tRve. Bicol River Basin. 36 0 30 S 114 14, 12 12 F~.2 ~hVartfw~Citi Nd. io iSoctR rs 4 4 1o 3 4- 6 76 9 1 It 12 1314-151617 1S 21 223 , N e l:ai' . - 37 - BICOL RIVER MOUTHSAN" MIGUEL, BAY EXISTING RIVER SYSTEM Disft'hoe lwards . -5006 _ '. "5 ~'0 0 ,*"0 Tk, .. .-.% ~ .0 24 I Tidal Cycle o246 810 12 Hour 30 2 - 4 _2-kr , 0f nd6 10 1_2 HOUr 28-0.5 0- i, 24 .,w .. eh ri 0/ Syt . ... rt l od n o S hem 4.3;iWOW ,mw:,- +- Fk=0,3 awdrndltO m anL mpal" n Rie lkw a k ie ou o xsin •( No+ Floo Ca o Sh, , , -Ssa - 38; the river mouth at San Miguel Bay is tzeated in two parts depending upon whether.the tidal flow is in the ebb or flood phase. The boundary salinity needed to be'specified only during the flood portion of the tidal cycle. The boundary salinityduring the ebb phase is computed in thelmodel assuming that the convection of salt out of the estuary is determined by the salinity distribution in the estuary. During the flood tide, the boundary ialinityat this location has the value of the ocean salinity, So . Salinities calculated during the ebb phase will always be less than S because of the dilution due to fresh water leaving the estuary. The minimum salinity occurs at the end of the ebb phase. To avoid an abrupt: shock in the numerical scheme, it is assumed that the salinity returns to the ocean salinity S in a transition period equal to 33% of the-tidal period. The salinity variation within that transition period is shown in Fig.4.3. The ocean salinity used in the present study is assumed to have the value of 32 ppt. Initial Conditions The computations in the hydrodynamic and salinity models require information regarding theinitial flow and," salinity conditions at the start of the computation. For the hydrody.namic model, the initial steady state flow condition is used at the start of the computation. The computation of the initial steady flow condition is performed keeping the water level at the river mouth at mean sea level. A reasonable water surface profile is then assumed along the riVer reach. The computations are repeated until the computed water surface profile converges to steady state values. - 39 - In the salinity model, the initial salinity distribution alongthe river reach'is assumed to be zero everywhere except at the river mouth, where the maximum ocean salinity is 32 ppt. Quasi-Steady Computation In the present study, the computed time variations of the stages, discharges and salinities in the Bicol and,, Sipocot Rivers are repeated periodically from one tidal cycleto the next. This occurs since the water level oscillation at the river mouth is periodic and the fresh water inflow to the river is kept constant. It was found that the computed flow conditions and the salinity repeat themselves after the computation has been advanced for twenty tidal: cycles. Since the phenomenon is repeated, the computational results are therefore presented only for a single tidal cycle. Model Study Due to the lack of, salinity data, the calibration for the diffusion constants in the salinity model is impossible. From the study by many investigators, it was found that the diffusion constants k in the case of open channel flow can vary within the range 1/6 of 0 to 60 m The values of k of 0,,30 and 60 are therefore assumed in the salinity model. The hydrographs of the stages, discharges and the salinities for the values of k equal to 0, 30 and 60:are presented at Libmanan, Cuyapi and Naga City as shown in Figs.4.4, 4.5 and 4.6. From the computed results, it was found that the diffusion constant k produces only slight changes in the salinity hydrographs at those locations. The effect of the diffusion on the salinity apparently increases in the upstream direction., However, the magnitude of the chanue seems to be practicallv insip-nificant both in terms of the salinity,,variation and the length of intrusion. Figs.4.4, 4.5 and 4.6 show the salinity distributions along the Bicol and Sipocot Rivers for various values of the diffusion constant, k. in the present Istudy, the value of the diffusion constant, k, is 'taken as 30 throuhout the neaiod of study. This value of the diffusion constant is a common value applicable to the study of salinity intrusion in open channels. The profile of the maximum and minimum salinities are presented in Fig.'4.7 for various values of the diffusion constant. k, -41- LIBMANAN, SIPOCOT RIVER EXISTING RIVER SYSTEM 150------ o. .s.. s". -D tow... -100-. JSKI ....-- 0 2 4 6 8 10, 12 Hou 0 I TgdL Cycl 1 .0 - - - -- , 0 2 4 .6 a 10 12 Hour 0* 1 TWd Ccl ------ 6 4. Dscaga"g ,W t r Leel and SalInity at Libninan eftnrcr River, System (No Flood ontrol Scheme) - 42 - CUYAPI t MCOL RIVER EXISTING RIVER SYSEM 160.0 -F 000 - I I - - - - r:-: - - 1500 -- --- ," - -r - 0 2 4 6 8 10 12 Hour 0 V2 I Tidal Cycle 1.0 0.5 - - -, *0 1.5 - 'I 0 2 4 6 i 10 12 Hor 0 I Tldii Cycie 24 -...... -. - ,.-.. .-. -. . ..- go -- .. --.... - . _ t ~ ~~~,. . 'k60 "''~~~~ J <", '" ~~30.ir Fig 4.5 DiSWlawi,!ibs Lwel and SQailty at Cuyopi for Edgil River Sy*tim No Flood Control Scheme ) - 43 CIT,NAGA Y B...L R.V EXISTING ''VER 8IE ISO- - 100------ * 50'- - -- 0 0 2 4 6 8 10 12 Hour 0oV I Tidal Cyclf 1.5 - - - ,, , -1.0 oe- - - 0 2 4 6 8 10 12 Hour 0 .. .. Tidal cy. . 8 -.. - 4 0.6 >% 4 % -- i02 -- 0 q Fig. 4.6 "be Rv.e m (Lev ItCd Fow :system: (NO"'; FFlood Con:trdlSonf 26. -.SIPOCOT RIVER -- BICOL RIVER - 20 Is Legend: - ::: k 30 14: (A) 12 6 .4 21 0 :0 50 0 30 .2 0 0 10 20 30 40 50O 6 - Distance Along Sipocot nd Bcol RimfrsKm. - Fio: 4.7 Mafrjman Mkimum Saliiy DistributonsAln Sipocotr and Bico Riveirsj(or Wsting Rkw System fNo' Fia CtmtmIni w. -:545 4.4 •Model Application idal and Hydrological Conditions, ."The effect of the flood control measures on the salinity distributions along the Bicol and Sipocot Rivers is determined under specified tidal and hydrologic conditions, A sinusoidal water surface fluctuation with the amplitude of 1 meter and the period of 12 hours is specified as the boundary condition at the river mouth. No rainfall is assumed during the period of 'investigation. Fresh water inflows to the rivers are from the base flow within the basin, catchment. The base flow equation is obtained through an analysis of the low flow records. Roughness Coefficients and DiffusionConstants The flowconditlon and the salinity distribution along th 'rivers are dictated by the rough ness coefficients and diffusion constants. The roughness coefficients for the river channels and the cutoffs are the same as those determined from the previous flood control study (1). The diffusion constan;s tare taken as 30. The roughness coefficients and the diffusion constants are kept unchanged throughout the period ,of investigation. Basin Modifications The effects of salinity distribution during the period of low flow are studied for the following basin configurations. a) Present system with no other modifications b) Present system plus Cutoff no.3 C) Present system plus 30 CHS withdrawal into the Bicol- Ragay Diversion d) Present system plus Cutoff no.3 plus 30 CMS withdrawal.into .the .Bicol-Ragay Diversion. - 46 - The effectiveness of these basin modifications on the flou condition and the salinity distribution are computed and presented"..in this section. Present System with No Other Modifications From the given tidal, hydraulic, hydrologic and salinity conditions, the flow condition and the salinity distribution are computed when the present river basin is unmodified. The computed results are used as the reference in determining the effect of the vaiious flood control measures selected. Flow con ditions: at Libmanan, Cuyapi and Naga City are shown in Figs.4.8, 4.9 and 4.10. For these flow conditions, the corresponding salinity-time variations fora tidalcycle at the same locations are presented in the same figures. The maximum salinities at Libmanan, Cuyapi'and Naga City are found to be 9 ppt, 21.6.ppt and 0.8 ppt respectively. The lengths of salinity intrusion are found toibe 45 kolometers in the Bicol River and.43 kilometers'in the Sipocot Rivbr, as measured from the Bicol River mouth. This is shown in the summary material in Figs.4.ll and ,4.12 Present System with Cutoff no.3 For this system, the geometry of Cutoff no.3 is included in the basin configuration. The flow condi-' tion and the salinity distribution :along the Bicol and Sipocot Rivers are computed and compared with the system with no bas'in modification. :Nosignificant changes in the flow condition at Libmanan and Cuyapi. could be observed.; However, at NagaCity, lower discharge and peak .stage during high tide were observed. Increases in the salinities could ..be observed everywhere both in the-Bic6l and Sipocot-Rivers. An increase - 4i7 in the length of salinity intrusion was also found to occur in the Bicol River. 'IromFigs.4.8, 4.9 and',4.10, it was observed 'thatthe maximum salinities at Libmanan, Cuyapi and Naga City increase respectively from 9 to 9.4, 21.6 to 22 and 0.8 to 2.6,ppt. From Fig.4.11, the length of salinity intrusion in the Bicol River increased from 45 kilometers to 50 kilometers. No significant change in the length of,intrusion could be observed in the Sipocot River, The average increase in salinities over a tidal cycle at Libmanan, Cuyapi and.Naga City were. 0.3, 0.3 and' 1.5 ppt respectively. Present System with Withdrawal at Bicol-Ragay Diversion The' geometry of the diversion is not included in the basin configuration in this case study. The effect of the diversion is simulated by subtracting 25 and 50 percent of river discharge from the Bicol River when the river system has no modifications. Withdrawing 50% of the low flow corresponds to a discharge of, 30 CMS. At the location of the withdrawal the flow was approximately constant since the Bicol-Ragay Diversion point is .situated far upstream and almost beyond the range of tidal effects., At that'location the fresh water flow was approximately 60 CMS.'C'omparisons between the computed flow condition and salinity distribution under the condition with and without the irrigation withdrawal at the Bicol-Ragay Diversion are made. No significant changes in the flow condition at Libmanan and Cuyapi could be observed when theleffect of the diversion. is included as shown in Figs.4.8 and 4.9. Considerable decreases in the discharge and stage however occur'at Naga City which is located down stream from the diversion locaition. The results are shown,-in Fig.4.10. - 48 - The maximum salinities at Libmanan, Cuyapi and Naga City when the 50% discharge diversion is included are found to increase to 10.7, 24,2 and 4.2 ppt (i.e. by the amount of 1.7, 2.6 and 3.4 ppt respectively). The average increases in salinities over a tidal cycle at those correspond ing locations are found to be 1.5, 3.6 and 2.4 ppt. The length of intru sion along the Bicol River is increased from 45 to 50 kilometers when the 50% .diversion is included and from 45 to 47 kilometers for a 25% discharge diversion. No increase in the intrusion length in the Sipocot River could be observed. This is shown in Figs.4.ll and 4.12. Present System with Cutoff no.3 and Withdrawal at Bicol-Ragay Diversion An investigation is made of the combined effects of Cutoff no.3 and withdrawal at the Bicol-Ragay Diversion on the flow condition and salinity distribution along the rivers. The simulation is performed by including the geometry of Cutoff no.3 into the basin configuration. The diversion effect on the flow is simulated by subtracting 50 percent or 30 CMS of the Bicol River discharge at the point of diversion. From Figs.4.8 and 4.9, the salinities and the flow conditions at Libmanan and Cuyapi are practically unchanged when the cutoff and diversion effects ,are included. The results shown in Fig.4.10 indicate that at Naga City there exists a reduction in the discharge and peak stage from those which occurred when the basin had no modifications. Considerable effects caused by Cutoff no.3 and the withdrawal at the location of the Bicol- Ragay Diversion are observed at Naga City by the increase in its salinity. Figs.4.8, 4.9 and 4.10 show that the maximum salinities at Libmanan, Cuyapi and Naga City are increased respectively from 9 to 10.9 ppt, .LIBMANAN ', rSIPOCOT RIVER 200 - - - - - f., ' ~ ~"Legend: 0 - No Flood ControlSCh . -- =- Cut Off No. 3 - .50 ---- Bicol DrslWo(50 % DNraion I Divrsion) .Cut Off No.3 0 2 4 6 80 '12 Hour o 1t...... d~ I IS S0.5 a a a a aa a a a 0 2 4 6 a 10 12 HrL 0 I TIdd Cycle 'i ,i o r 6 2 - - 1 . : : 10 c.K 1 - - 0--b 0.9 a. 6 ' - Fig. 4.8 D.ihar Velr Levl andSant atL r ,bma Flood Control Meaur". ,. 50 - CUYAPI. AIMOL RIVER / 1000 .... r U), /(Legend: 0 - F 'bodControl Scheme -. Cu cOff No.33 . , -- oIDision(0% DI I -W610 Dkersin(5O% Divrsion), 0 2 6 8 "0 12 Hrw 1.8 o I' TkI Cycle .j 1.0 ------ .. r -j. 0 , - 4) 2 Hour k03 24. . *o , " 22 - N - ' a_20 -/ c •/ ,__.*. 14 --- 12 ------ Fig. 4.9 Discarg ,Water Level and Salinity at CuyaPl far Vaiou C.ntmi Masums. •Fod NAGA CITY, BICOL RIVER 150 i " :lie 50 ,*/ eed 0 - 0 No Flowd Contro[Scsrrm S - Cut 0-*- acol ffNo.3 Dhwrsion) Divrsion (50' -100 OB BieDimsion (50 % DIrsfon)+ Cut Off No. 3 L50 I I 0 2 4 6 8 0 i2 o 0 'I Tidal, Ccc 1.0 - - -0-. , 1, - - - .. ... L-0.5 1 -10 2 4 8) 12 Hour 10 -yP2 I T-nO 0 8~~~ -- -30 ~ . 0C 6 ...... FAg. 4.10 Discharge , Water L*evel and 'Salroy at Naga' Cfly ' %A*16t Flood Control Measure. .52- SCHEME UCOL lm -Nongk 0' Nom ., 30. Nomk 60U Cut Off No3pk 30 Slool Dlwvr'on W%Divesion or 15 -m CMS.),9k 30 BOWl Dhw~skon** (50 % D.wslon or 30 CMS Jk a 30 Cut Off No + BIcol . lvsrlon .k "30 Cut Off -7 ,3.oe Lsnfith of Intrusin ,Xm. Moxinwm Possile Fg',S4 o WOWt 8tuay.I R.... anW Mai.. "sue Salwty at boa CitY. SCHEMEsowRIE...... i " I SPOC)T RIVER i: Nonek * 0 - - m ....- onMk,', 0 -w- -.- A.kotJ.60 "II Cut Off No.3#k t30 m -- - Siool iverslon (25% Diversion or II i i CMS), k 30 Bicol Diverione (50% Diversion or I i 30 CMS Ik a 30 Cut Off'No3.+ f- Diversion, k's 30 IL Cut Off No63tBcol DNlverrnku 30.' m .0 12 0 8 Length of lruslon, Km. MiImum Possible Salinity at " "" "' r: " L oIbmnan PPT Figo . 4.1z, .lt Wler hnrumIon Along Socot: Rir. .d .. bibe -Solliny ot Libmanon. - 54 " 21.6 to 24.5 ppt and 0.8 to 7.Opp. The average increases inthe salinities over a tidal cycle at those respective locations are approxi mately 1.6, 2 and 5.6 ppt. The length of salinity intrusion is increased from 45 to 54 kilometers along the Bicol River. No significant increase in the length of salinity intrusion-occurs along the Sipocot River. The lengths of intrusion along the Bicol and Sipocot Rivers are shown in Figs4,ll and 4.12. Similarly for the case oi a 215 d1scnarge 1iversion and Cutoff no.3, it was found that the intrusion length along the Bicol River was increased from 45 to 50 kilometers. No significant changes occurred along the Sipocot River. 4.5 Discussions The salinity distribution and flow condition along the Bicol and Sipocot Rivers are computed from tha continuity, momentum and salt balance equations. The first two equations form the so-called "hydro dynamic model" while the last equation is used in the "salinity model". In the hydrodynamic model, the roughness coefficients are those found from previous-investigations (1). The roughness coefficients are assumed unchanged regardless of the changes in the flow conditions in the rivers. The salinity model L&jAPtalibrated since no available salinity data existed during the period of the given low flow used in the present study. The importance of the calibration for the salinity model is to determine the proper value of the diffusion-constants. These constants influence the salinity distribution along the river reach. However, from a detailed -55 study .of the behavior of the salinity model, it was found that the diffusion effect is only of minor importance in determining the longitudinal salinity distribution when compared with that produced by the effects of convection. The diffusion constants which are used in the present study are I . taken as 30 m /6 In general, the diffusion constants could vary within the range of 0 to 60 mI . The value of 30 is taken since it is commonly used when the diffusion of salt water along open channels is studied. Regardless of whether the salinity model is calibrated or not, the error in the computed salinity distribution caused by using inaccurate diffusion constants is expected to be insignificant. From the results of the present study, it was found that the flood control measures consisting of Cutoff no.3, the Bicol-Ragay Diversion withdrawal and the combination of these two schemes do not produce any significant changes in salinities and their lengths of intrusion along the Sipocot River. The maximum salinities at Libmanan and Cuyapi when those flood control schemes are included in the basin are approxi mately 10 and 20 ppt respectively. The length of salinity intrusion along the Sipocot River is approximately 45 kilometers from the mouth of the Bicol River. The location which corresponds to this length of intrusion is within the vicinity of the town of Sabang, Considerable increases in salinities and their lengths of intrusion however could be observed along the Bicol River. The presence of Cutoff no.3 together with the Bicol-Ragay Diversion withdrawal produces an increase in the length of salinity intrusion bv 10 kilometers (ie. - 56 an increase from 45 to 55 kilometers asmeasured from'San Miguel Bay. The approximate location of the 55 kilometers distance from San Miguel Bay along the Bicol River corresponds to the location immediately downstream from the town of Ombao. It was discovered that the maximum salinity at 400% and 600% for the Naga City is increased approximately by 300%, following schemes: (i) Cutoff no.3 (ii)Ragay Diversion withdrawal :iii) Cutoff no.3 and the diversion. The increase in maximum as well as minimum salinities and the salinitj profile along the Bicol and Ragay Rivers are shown in Figs.4.13 and 4.14 under various combinations of flood control schemes. From the analysis of-the results, an upstream withdrawal near the Bicol Diversion of 30 CMS would produce a greater length of intrusion and a greater increase in the salinities along the Bicol River than those produced by Cutoff no.3 alone. The removal of fresh water from the Bicol River allows salt water from San Miguel Bay to intrude upstre-an into the Bicol River since one could figuratively say that there then exists less fresh water to "push the salt water out". On the other hand, the cutoff shortens the river course and increases the channel conveyance capacity. The amount of fresh water passage to the sea is more or less the same. The cutoff however enables more of the flood tide flow carrying saline waters to enter the basin. Therefore, changes in salinities within he river portion where the cutoff is located should be expected. The results obtained from the study are expected to provide only approximations of the effects of flood control measures on the, salinity - 57 distribution along ,the Bicol and Sipoc6t Rivers. The study is made for the period of low flow when the salinity condition is expected to be critical. No information of salinity distribution along the Bicol-Ragay, Diversion is provided since the geometry of the diversion is not included in the study. During the flood period, fresh water inflow from the Bicol Basin watersheds to the Bicol and Ainecnt Rivar ai Anrmous. Intrusion of salt water from the sea therefore has less significance than that which occurs during the.low flow period. Adverse effects produced by flood control measures on the protection of salinity intrusion are expected to be insignificant as the amount of fresh water flow in the river channels increases. 32 k =30. 3D 28__ _ -SIPCOT- RIVER -81COL RIVER 26 ______24______Legend : Cut OfW aIs V___.. c to4 4 4C 0 4i 20 .0 0.0 t2D 30 40- 50 wC Disance Along SiOWt and Bicol Rivers , Kmi Fla. 4613 Mcwir 'Solkt Distibutio Along S(0600t. and. 8lM P Riversfor Various Flood CnilShms 32 30 2= k =30 -IPOCOT RIVER - 26 IOBICOL. RIVERIE - 26 - No Flood Conol Shsme 20--- Cut OffNo3 -- -Bioi Di e n (50 % ...... Diersion) C, DWsi(50%Dw )+ Cut Off -.. 3 16 . . t 14 to 12 _ _ 1 10 " 7 .\ \*...... 6/ a: 2 0) 9'"-.. 0 0 3 40 30 0 0 0ID nd. 30 4Do0 Distance Aio~ng SiPOCOt an6d Bicol IRvers, KiTL Pl.4.14 PANkvji SaOMY. DiWtibutlo Along : SlpOCo# and BciRff o -Vriu FloodCnrlW .60- V, SUMMARY AND CONCLUSIONS Three types of,problems associated with flood plain management have been investigated; flood,peak reduction long term basin drainage and salinity intrusion. Numerous basin modifications have been proposed and an analysis was performed of their effects with regard to the three types of flood plain management problems described above. Flood Peak Reduction The changes in flood peak level and the 11-day average river stage following the flood peak which would occur as a result of a :13 year flood are described in Tables 2,2 and 2.3. The additional information provided in the AIT Research Report No.48, Bicol River Basin Flood Control Investigation, have been added to the data in Tables 2.2 and 2.3 and is presented in the summary tables. The single most effective flood control measure for reducing water Levels along the Bicol River is the Bicol-Ragay Diversion. The host iffect-ive combination of-,nasures is the Bicol-Ragay Diversion and 'utoff no. 3. song Term Drainage Figs.3.1 throu', 3,3 show that by deepening the Bicol River to a evel of 2 meters aboveMSL from Km. 69 to 84 the water levels in the Lake Bato Region for post flIood"peak periods is significantly lowered. Reductions of the ord-" of 1 to 2 meters is typical during a period of approximately one month. The reduced level ofthewater surface in thi -,61 - SUMMARY TABLE I Effectiveness of the Various Flood Control Measures in Changing Flood Peak Level (13-year Flood) Reduction or Increase in Flood Peak Combination of Flood Control :. ~Mea s u r e s •. Level (cm) MeasuresLakeBato Ombao Naga City Libmanan .. 1. Sipocot diversion + Cutoff no.3 -2 -24 -20 -44 2. Sipocot diversiondivrson-3 + Bicol -6464 -7.6 -17, .. -64 ,: diversion 3. Sipocot diversion + Cutoff no.3 -2 -75 -10 -67 + Bicol diversion ,__,..-._ 4. Cutoff no.3 + Bicol diversion -2 -75 -10 -4 5. Cutoff no.3 + Channel deepening -14 +11 -11 ( 2 m ) . -r :-, 6. Cutoff no.3 + Bicol diversion -61-43 '9, -4 + Channel deepening (2 m) -43 -69 7. Bicol diversion + Channel deepening (2 m) -51 -16 -4 8. 32 CMS at Sipocot diversion -2 -9 -8 -12 9. 32 CMS at Sipocot diversion + Bicol diversion -64 -1.-12, +--o ivrin ------ 10. Pulantuna reservoir* 0 0 +21' -29' 11. Sipocot-San Miguel diversion* -2 -9 -9 -64 12. Tidal barrier* -2 -9 -28 -25 13. Cutoff no.3* -2 -24 +8 -10 14. Bicol-Ragay diversion* -2 -59 -39- +22-1 15. Channel deepening* -49 ~+2 +8 +1 16. Tidal barrier + Pulantuna -2 -9 -28 -48 reservoir* 17. Bic-ol-Ragay diversion + -2 -59 -39 -4 Pulantuna reservoir* 18. Tidal barrier + Bicol-Ragay . - 64 diversion + Pulantuna reservoir* - -64 -41 -5 19. Tidal barrier + Bicol-Ragay diversion + Sipocot-San Miguel -3 44, -41 -66 diversion + Pulantuna reservoir* * Data from AIT Research Report No.48. - 62 - SUMMARY TABLE II Effectiveness of the Various Flood Control Measures in Changing the Ul-day Average River Stage Following the Flood Peak (13-year Flood) Reduction or Increase in the Ul-day Average River Stage Following the Combination of Flood Control Flood Peak (cm) ______Measures F Lake Bato Ombao Naga City Libmanan 1. Sipocot diversion + Cutoff no.3 -2 -18 -72 -15 2. Sipocot diversion + Bicol diversion -i -'9 -73 -23 3. Sipocot diversion + Cutoff no.3 0 -61 -109 -17 + Bicol diversion 0 ,1...61 109.. 4. Cutoff no.3 + Bicol diversion 0 -61 -109 , 5. Cutoff no.3 + Channel deepening -5-, -62 0 (2n) -58___ +7 -62 00 6. Cutoff no.3 + Bicol diversion -51 -37 -106 -i + Channel deepening (2 m) .... - -_...._. __.''_ 7. Bicol diversion + Channel - -31 ., deepening (2 m) _ _3.1 -6:__.6 8. 32 CMS at Sipocot diversion 0 0 2 -11 9. 32 CMS at Sipocot diversion " 0 -4 -77 -13 + Bicol diversion 0-9 .- 7-. 10. Pulantuna reservoir* 0 0. 0 +2 11. Sipocot-San Miguel diversion* -1 -1 +2 -16 12. Tidal barrier* -1 0 -1 -9 13. Cutoff no.3*' -2 -16 -70- 0 14. Bicol-Ragay diversion* 0 -50 -74 +1 15. Channel deepening* -60 +17 +10 16. Tidal barrier + Pulantuna -1 -6 reservoir* 17. Bicol-Ragay diversion + Pulantuna 0 -50 -74 01 reservoir* 18. Tidal barrier + Bicol-Ragay -i -0 - 5' diversion + Pulantuna reservoir* -5-1 19. Tidal barrier + Bicol-Ragay diversion + Sipocot-San Miguel -1 -50 -89 -39 diversion + Fulantuna reservoir* ._. __",____., ______,_ * Data from AIT Research Report No.48. -63 river should promote lad drainage in that portion of the basin. lin order to prevent an excessive dropping of the low water level in Lake Bato.during low flow period, a control structure is, required, to regulate the Lake's discharge. Salinity Intrusion From :the results of the study, it was ,found that the proposed, flood', control measures do not significantly alter the pattern of salinity distribution and its length of intrusion along the Sipocot River. However, the construction of Cutoff no.3 and fresh water withdrawal from the river both increase the local salinities and the lengths of intrusion along the Bicol River. The lengths of intrusion along the Bicol and Sipucot Rivers and the maximum salinities at Libmanan and Naga City under various conditions of flood control measures are presented in Summary Table III. From the summary of the results, it was discovered that'Cutoff no.3. produced less increase in the local salinities and length of intrusion along the Bicol River than that produced by fresh water removai -of 30 CMS at the location of the Bicoi-Ragay Diversion. The salinity is found to extend along the Bicol River' as far as the town,of Ombao. Te extremity of the salinitv intrusion alon the Siaocot Riveiis found to be'within the vicinity of abang City. -64 . ,SUMMARY TABLE III Length of Intrusion Maximum Salinity Flod Control Schemes km ______Bicol River SipocotLRiver Naga City Libmanan ,No Basin Modification' 45 43 0.8 9 Cutoff no.3 50 " 43 2.6 9.4 Bicol-Ragay diversion of 30 CMS (50% of river, 5043 .4,2 10.7 diversion) Cutoff no.3 and Bicol . diversion of 30 CMS 55 43 7.0 10.9 (50% of river diversion) -65 .REFERENCES (1) ACKERMANN, N.L., PINKAYAN,.S., SHI-IGAI, H., TINGSANCHALI, T. and SAHAGUN, V.A. (1975), Biool River Basin Flood Control Investigation, Volume I, Research Report Summary, Research Report No.48, Asian Institute of Technology, January. (2)AUmxmNN. N.L., PINKAYAN. S., SHI-IGAI, H., TINGSANCHALI, T. and SAHAGUN, V.A. (1975), Bicol River Basin rlood Control Investigation, Volume II, Research Report No.48, Asian Institute of Technoloiy. January. (3) BERKHOFF, J.C.W. 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