USAID Davao Gulf Model

TABLE OF CONTENTS

Introduction..........................................................................1

Methods ...............................................................................2

ResultsandDiscussion ..........................................................4 Simulatedsolidwastedispersal............................................ 9 SedimenttransportfromDavaoRiver................................ 11

SummaryandConclusions ..................................................14

References ..........................................................................15

LIST OF TABLES

Table1. SummaryofwaterqualitymonitoringdatafromDavaoRiver) ................................................... 9

Table2. Annualmeandischarge,sedimentconcentrationandsedimentloadfromDavaoRiverfromTanandGuanzon ..................... 12

LIST OF FIGURES

Figure1. DavaoGulfcoursegridmodelgrid ......................... 2

Figure2. DavaoGulfhighresolutionnestedgrid .................. 2

Figure3. WeatherdatafromDavaoCityfor2008 ................ 3

Figure4. EstimatesofDavaoRiverdischargefromgaugemeasurements ............................................. 3

Figure5. JanuaryandAugustaveragesurfacecirculation ............................................................... 4

Figure6. DavaoGulfAprilaveragesurfacecirculation ......... 4

Figure7. AveragedsurfacecurrentvelocityfieldforJanuary .................................................................... 5

Figure8. AveragedsurfacecurrentfieldforAugust.............. 5

DEVELOPMENTOFADAVAOGULFHYDRODYNAMICMODELINRELATIONTOPOLLUTIONDISPERSAL i

Figure9. AveragedsurfacecurrentfieldforApril ................. 6

Figure10. MeansurfacesalinityduringJanuaryformodelrunincorporatingfreshwaterdischargefromDavaoRiver .................................................... 7

Figure11. MeansurfacesalinityduringAugustformodelrunincorporatingfreshwaterdischargefromDavaoRiver .................................................... 7

Figure12. SurfaceconcentrationofaconservativetracerdischargedfromDavaoRiverduringJanuary .................................................................... 8

Figure13. SurfaceconcentrationofaconservativetracerdischargedfromDavaoRiverduringAugust ..................................................................... 8

Figure14. Linesource(solidblackline)ofsimulatedfloatingsolidwaste. .............................................. 10

Figure15. DispersalofsolidwastefromthelinesourceduringJanuaryafter5hours,12hoursand3days. ...................................................................... 10

Figure16. DispersalofsolidwastefromthelinesourceduringJanuaryafter12hoursand4days ............ 11

Figure17. EstimatesofDavaoRiversedimentload .............. 11

Figure18. Sedimentaccumulation(incm)atthebottomusingJanuaryconditionswithaDavaoRiversedimentloadof10kg1 ....................................... 13

Figure19. Sedimentaccumulation(incm)atthebottomusingAugustconditionwithaDavaoRiversedimentloadof20kg1........................................................... 13

ii THEPHILIPPINEENVIRONMENTALGOVERNANCEPROJECT

CesarVillanoyby

DEVELOPMENTOFADAVAOGULFHYDRODYNAMICMODELINRELATIONTO

POLLUTIONDISPERSAL

INTRODUCTION DavaoGulf isoneof thebiggestembayments in thecountrywith a total surface area of about 6,600km2 and depths ofmorethan2000mnearthemouth.Thetotalcoastline lengthis about 500km and is bounded by four different provinces,namely;CompostelaValley,DavaodelNorte,DavaodelSur,and Davao Oriental. The large catchment of the rivers thatempty into thegulf implies that riverine inputs into thegulfmay carry with it significant amounts of pollutants andsedimentsfromthedifferentactivitiesconductedupstreamoftherivers.Inaddition,theregionalcenter,DavaoCityalsoliesalong Davao Gulf and may contribute to problems of solidwaste and sewage discharges into theGulf. The purpose ofthis study is to develop a hydrodynamic model for thenorthernDavaoGulfarea, todescribe thesurfacecirculationin general terms and to demonstrate the potential fate ofpollutantsreleasedatdifferentlocationsaroundthegulf.Thisis an initial step in establishing the linkages between thewatershedandthecoastalwatersofDavaoGulf.Modelssuchasthesearealsousefulinassessingthevulnerabilityofcoastalsystemstopollutionandonlongertimescales,climatechange.Itshouldbenotedhoweverthatthissimplifiedmodelhasnotbeen validated with field data. Although the data used toforcethemodelarebasedonthebestavailable information,actualfielddataforvalidationpurposesarenotavailable.

DEVELOPMENTOFADAVAOGULFHYDRODYNAMICMODELINRELATIONTOPOLLUTIONDISPERSAL 1

METHODS

Hydrodynamic flow patterns for Davao Gulfwere modeled using a nesting scheme. Aninitial coarseresolution model, with a gridresolution of 4.5km encompassing the entiregulf,wasused to generate the currents for a1.2km resolution model, which spanned thenorthernmost part up to the area south ofSamal Island (Figure1).Bathymetricdatawasobtainedbydigitizingbathymetriccharts.Twotypes of forcing were used for the coarseresolution model, namely large scale windforcingwiththeuseoftheNavyLayeredOceanModel(NLOM)modeloutput,andtides,whichweremodeledusingDELFT3DFLOW.

Figure1.DavaoGulfcoarse

Runs for the two types of forcingwere doneseparately,and the resultswere combined togenerate a timeseries of boundary currentswhich were applied to the higher resolutionmodel Figure2), which was also run usingDELFT3DFLOW. This model also includedsurfacewind forcingwithdataobtained fromthe Davao, Philippines local weather station(source:http://www.underground.com).

gridmodelgrid.

Discharge from Davao River was alsoconsidered, with volumetric flow rates, flowvelocities, and sediment input data obtainedfrom localmonitoring efforts. Figure 4 showsestimatesofDavaoRiverdischargedata fromgaugedata.Althoughtheestimateshavedatagaps, it is fairly obvious that there is a slightseasonal signal with higher discharge on thelatter half of the year. In 2003, highestdischargewasobservedduring the southwestmonsoon but in 2004, the highest discharges

wereobservedduringthestartofthenortheastmonsoon.Theaverage discharge for the entire measurement period was61m3s1and thisvaluewasused in thehydrodynamicmodelruns.

Figure2.DavaoGulfhighresolutionnestedgrid.

2 THEPHILIPPINEENVIRONMENTALGOVERNANCEPROJECT

Nodischargedatawasavailablefortheotherriversandwerenotconsequently included inthemodelruns.

Figure 3. Weather data from Davao City for 2008(source:www.weatherunderground.com).

Withtheresultingmodeloutputforthehighresolution model, three types of dispersalexperiments were conducted. For the firsttype, theDavaoRiveroutflowwasusedasasource point for a conservative tracer inDELFT3DFLOW. The riverwasmodeled as acontinuous sourceof the tracerwith a fixedconcentration,andtheevolutionofthetracerplume over time can then be observed andquantified. The second type involvedextracting the hydrodynamic currents fromtheDELFT3DmodelforuseintheGNOMEoilspillmodel. Thisparticletrackingmodelwasusedtosimulatethedispersaloffloatingsolidwaste. Although the river may be animportant source of solid waste, it is alsolikely thatnonpoint sourcesexistalong thecitycoast.Asaresult,aline source was used in thesimulations extending almostacross thewhole coastline lengthofDavaoCity.In the third type of dispersalexperiment, the DELFT3DFLOWmodelwas configured to use theDavaoRiveroutflowasasourceofsediment input into the gulf.Monitoring datawas averaged todetermine the sedimentconcentration for the modelingexperiment, which set the riveroutflowasa fixedconcentration,continuousreleasepointofsediments. Default sediment parameters (settling velocity,noncohesive nature of sediment, etc.) were used, andbackgrounderosionandsedimentationrateswereminimizedto isolatetheeffectoftheriversource.Withthismodel,thedispersalof the sedimentplumecanbeobservedover time,and theareasofconsiderablesedimentaccumulationcanbeidentified.

Figure 4. Estimates of Davao River discharge from gaugemeasurements(datasource:DPWHXI).


RESULTS AND DISCUSSION

The gulfwide seasonalsurface circulation ofDavao Gulf is shown inFigure 5. The surfacecirculation typicallycontains significant dailyvariations due to thedailyboundary forcingaswell as tidal forcing. Theplots shown in Figure 5and Figure 6 representthe 15day averagedsurface velocities, andthus represent theresidual flow if the tidesare removed. DuringJanuary, the prevailingnortheast monsoon

drivesapredominantly southward flow throughoutmostoftheGulfwithslightnorthwardflowalongthewesternsideofSamal. Net flow around Samal is counterclockwise. DuringAugustwhenthesouthwestmonsoonisblowing,thesurfacecirculation is still predominantly to the south albeit a littleweaker.Largerdifferencesoccur in the southernportionoftheGulf close to theopening to the Sulawesi Sea.A largerrecirculation gyre forms near the mouth during August.During weak wind conditions (e.g., April simulation), thecirculationinthenorthernhalfofthegulfisveryweakwithastrongerrecirculationinthesouthernhalf.

Figure5.January(left)andAugust(right)average

Thevelocityplots inFigure7 toFigure9 show the residualsurface currents using the nested model. Note that muchmoredetailinthecirculationpatternsareevidentbecauseofthehigherresolution.ThereismorevariabilityintheflowintheeastofSamalIslandwherenorthwardflowisevidentforJanuaryandApril.Forbothmonsoons(JanuaryfornortheastmonsoonandAugust for theSouthwestMonsoon), thenetfloweastofSamalIslandissouthward.Surfaceflowalongthesouthernboundaryof themodel isnorthward in theeasternandwesternsideswithsouthward inbetween.Thestrongest

surfacecirculation.

Figure6.surface

DavaoGulfAprilaveragecirculation.


Figure7. AveragedsurfacecurrentvelocityfieldforJanuary.

flow occurs during August andtheweakestduringApril.Notethatthesefiguresarebasedonaveraged daily velocity fieldsfor the month and thatsignificantdaytodayvariationscanbeobserved in themodel.Animation files of surfacecirculation are included in thisreport to show the degree ofspatialand temporalvariabilityofthesurfacecurrentfield.Netflow through the narrowchannel between Davao cityandSamalIslandappearstobenorthward for all the threemonthsconsidered.Davao River is one of thegreatest contributors offreshwater into the Gulf. Thedischarge of Davao River canrange from20120m3s1 (Figure4)anddrainsareaof1700km2.Theaveragedischarge isabout6070m3s1. Along with thefreshwater discharge, DavaoRiver also carries with itsediments eroded from thewatershed aswell as potentialpollutants released to itswaters as it traverses thewatershedtothecoast.

Figure8.AveragedsurfacecurrentfieldforAugust


It is easier to visualize howpollutants from the rivers spreadout intoDavaoGulfby lookingatdistributions of surface salinity.Salinity in the ocean is aconservative tracer and hencetheir distribution and transportare based solely on advection bycurrents and mixing with watersof different salinity. Concentration of conservati pollutantsoriginating from the rivers followclosely the surface salinitydistribution.Compareforinstancethe salinitydistributions in Figure10 and Figure 11 with thedistributions of conservativepollutantsreleasedfromtheriversin Figure 12 and Figure 13.Dissolved pollutants contained in

riverwatersmovetowardsthewesterncoastofSamal Islandduring the southwest monsoon while during the northeastmonsoon, the river discharge can spread both along thewestern coastof Samal and along the coast southofDavaoCity.Animationsofthefreshwaterplumeshowthattracesofriverwatermixedwithgulfwatersareevenadvectedtowardsthe eastern part of Samal Island from the south and thenmovetowardsthenorth.

s ve

Table1presentssomewaterqualitydataofDavaoRiverfromtheDepartmentofEnvironmentandNaturalResourcesMonitoringProgram.

Figure9.AveragedsurfacecurrentfieldforApril.


Figure10.MeansurfacesalinityduringJanuaryformodelrunincorporatingfreshwaterdischargefromDavaoriver.

Figure11.MeansurfacesalinityduringAugustformodelrunincorporatingfreshwaterdischargefromDavaoriver.


Figure 12. Surface concentration of a conservative tracer discharged from Davao River duringJanuary.

Figure13.SurfaceconcentrationofaconservativetracerdischargedfromDavaoRiverduringAugust.


Table 1. Summary ofwater qualitymonitoring data fromDavao River(source:EMBDENRXI).

Simulated solid waste dispersal For simulating the dispersal of floating solid waste, aLagrangianmodelwhichtracksnonweatheringfloatingvirtualparticlesasitiscarriedbythecurrentswasused.Alinesourcewasusedextendingalong theurbanized coastofDavaoCity(Figure14)asnowelldefinedpointsourcecanbe identified.ThesurfacecurrentswereobtainedfromtheDELFT3Dmodeldescribedintheprecedingsection.TheareasmostpronetoreceivingsolidwastefromthecoastofDavaoCity isthewesterncoastofSamal Islandforalmostall seasons (Figure 15). During the southwest monsoon, asignificantpartofparticlesreleasedalong thecoastalsoendupintheareanorthofthenarrowestchannelbetweenDavaoCityandSamalIslandandevenbeyond(Figure16).


Figure 14. Line source (solid black line) of simulated floating solidwaste.

Figure15.DispersalofsolidwastefromthelinesourceduringJanuaryafter5hours(topleft),12hrs(topright)and3days(bottom).


Figure16.DispersalofsolidwastefromthelinesourceduringJanuaryafter12hours(left)and4days(right).

Sediment transport from Davao River DavaoRiverdrainsabout1700km2ofwatershedandtogetherwiththerestoftheneighboringrivers,providesthesedimentsupply for the gulf. Estimates of the sediment supply fromDavaoRiverareshowninFigure17.Themaximumestimateisalmost .07ppms1orabout70kgs1but theaveragevalue iscloser to 10kgs1. This is similarto the estimates in Tan andGuanzon (undated) of 510kgs1(Table 2). Using the aboveestimates for the sediment loadof Davvao River, a cohesivesediment transport model alsousingDELFT3Dwasusedandthesediment accumulation at thebottom after 30 days duringJanuaryandAugustareshowninFigure 18 and Figure 19,respectively. The sediment loadused for the August simulationwas double that of the Januarysimulation.Notethatthehighestsediment accumulation (6x103cm for January and 15x103 cm for August) was limited towithin 23 km along the coast adjacent to the rivermouth.Tracesofsedimentdepositioncanbetraceduptothewesterncoast of Samal Islandwith accumulation of up to 2x103 cmoveraperiodof30days.

Figure 17. Estimates of Davao River sediment load (data source:DPWHXI).


Hodgson (1989)suggestedthatsedimentationreefs from15200mg cm2 day1 can be damaging to reefs. Tolerance tosedimentationhowever,isvariabledependingonthespecies.The sediment loadused in the simulationsarebasedon theaverageof10kgs1andthemaximumsedimentaccumulationshown in Figure 18 and Figure 19 is 0.015cm accumulationoverth modeltim o about22days(allowingforsedimentmodel spinup of 1 week). This value translates to asedimentationrateof1.8mgcm

e e f

h r

byHodgson (1989)which can impact coraleefs.

Table2. e,sedimentconcentrationandsedimentloadfromDavaoRiverfromTanandGu

Y Discharge(m3s SedimentConc(kgm SedimentLoad(kgs

2day1andissignificantlylessthan the suggested sedimentation ratesdetrimental to coralreefs.On theotherhand, thehighestDavaoRiver sedimentload measured isabout7times ighe thatthevalueused inthe model (Figure 17) and can conceivably result insedimentation rates increasing to almost the lower limit ofrange suggestedr

Annualmeandischarganzon(undated).

ear 1) 3) 1)2001 274 0.038 10.42003 198 0.027 5.3


Figure18.Sedimentaccumulation(incm)atthebottomusingJanuaryconditionswithaDavaoRiversedimentloadof10kgs1.

Figure19.Sedimentaccumulation(incm)atthebottomusingAugustconditionswithaDavaoRiversedimentloadof20kgs1.


SUMMARY AND CONCLUSIONS AhydrodynamicmodelforDavaogulfusinganestedDELFT3Dmodel approach was developed and was used to drive thedispersalof3differenttypesofpollutants.The firstdispersalmodel simulates the dispersal of a conservative dissolvedpollutant. The second model simulates the dispersal offloating solid waste and the third model is a cohesivesediment transport model. All models show the potentialimpactareasofwastecomingfromDavaoRiverandalongthecoastofDavaoCity(forsolidwastedispersal).Themostlikelycoastal areas which can be affected by pollutants releasedalong theabovementioned sourcesare thewestern coastofSamalIsland,alongthesoutherncoastofDavaoCityandalongthe narrow gap between Samal Island and the mainlandextendingafewkilometersnorthwardalongbothsidesoftheChannel. It is apparent that materials released with riverdischarge can be advected some diatance from the rivermouth.InfacttracesofriverbornematerialinthemodelcanbetracedmovingnorthwardsalongtheeasternsideofSamalIsland. Perhaps, the important point shown by themodel isthatriversystemsandhowitismixedwithgulfwatersneedtobe considered in hydrodynamic and pollutant dispersal aspollutantadvectioncanbeinfluencedbyriverplumebehavior.DavaoGulfcontainsseveralmoreriverswerenot included inthis study simply because no baseline data on discharge,sedimentloadsandwaterqualitydatawereavailable.


REFERENCES HodgsonG.1989.TheeffectsofsedimentationonIndoPacific

reefcorals.PhDDissertation.UniversityofHawaii.Prantilla EB, Martinez C. (undated). Heavy metals

contaminationinDavaoregion.Tan EC, Guanzon J. (undated). Davao River sediment

quantification: A comparative analysis. UIC ResearchJournal.


List of TablesList of FiguresIntroduction Methods Results and DiscussionSimulated solid waste dispersalSediment transport from Davao River

Summary and Conclusions References

Documents

USAID Davao Gulf Model