"SURIPIN TIlE

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ESfUARINE HYDRAULICS IN A SHALLOW DELTAIC ESTUARY WITU REFERENCE TO TIlE KALI GARANG ESfUARY

SEMARANG,INDONESIA

BY

"SURIPIN,

AThesis SubmiUedto the School or GraduateStudies in PartinlFulfilment of the Requirements for

the Degreeof Masterof Engineering

FACULTY OF ENGlNEERING AND APPLIED SCIENCE l\tEMORIAL UNIVERSITY OF NEWFOUNDLAND

AUGUST, 1992

ST,JOHN'S NEWFOUNDLAI'l) CANADA

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Abstract

Most oftheIndonesianriversarecharacterizedbytheformationof deltas at the mouths.The estuariesareusuallyshallowand thelongitudin al bed slope lo w.

Theriver s debouchinto an ocean with low tomoderatemarine energy.Therivernow fluctuates seasonally inaccordance with themonsoonseason.Duringthe wetseason, from Novemberto April.floods mayoccurduetothe intenserainfall.The riverflow becomes qune small during thehOI.dry seasons.

Naturally ,meriver sbring a lot ofsedimentswhichoriginatefrom volcano ashes and erosion of the catchmentbasin.Mo~sedimentsflowdownto the estuaries and near-shore areasduringthehighriver flowsoftherainy season.Inmost cases,manyhydra ulics structures havebeenbuillalongthe riversystems. Theyarc intended (orsedimentcontrol,flow regulation andfloodcontrol.

The fluctuationof river flow is consideredto be a majorfactor in determiningestuarinecirculationpatterns and deltaformation.Typically,thecirc ulation pattern shiftsfromthat ofahighly stratifiedestuary during thehigh riverflow ofthe rainy seasontothat ofawell-mix edestuary duringthe lowriver flowof the dry season. Furthermore ,the quantityandvariationofdischarge in associa tionwith wave effects results in variousdeltageometriesranging from river-dominated towa ve-

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dominated delLaS.River-dominateddeltas arecharacterizedbyhighlyirregularlind protruding shorelinesandbysediment barsdepositedparaJlel to thedirection ofriver now.wave-dominated dellaSarecharacterizedbystraightshorelinesw'ith sediment barriers deposited paralleltothe shorelines.

Tounderstandthecontributionofrivernow and marineforcestot"e behaviourof estuariesand the geometryof deltasinIndonesia,theKali GarangEstuary hasbeen chosen as a subjectofstudy.The Estuaryconsistsof two~ranches:Kali Scmarang,andtheWestChannel. Tidal excursion,salinity distributionand circulation patterns are used torepresent the estuarinebehaviour. TheDischargeEffectiveness Indexisused toevaluate therelativecontributionof river flow versusmarineforces10 the development of deltas.Fieldobservations ofwaterlevel,salinity,temperature, currents,and sedimentswere made duringthedryseasonof 1991. Existingdatasets related tothe study wert also collated.

The study showslIlattheKali Garang Estuaryexperiencesasmall-diumal tide-range(- 65 em).In themanmade WestChannelthetidepropagates uptheestuary asfar as theweir.Thetidal rangedecreases landward andthefreshwaterdischarge flowsseaward overthesalinewaterthatintrudes landwardbelow it.IntheKali Scmarang,the tidepropagates asfar as its mid length.Fresh watermixesdirectlywith seawater,anda weak verticalstratificationisexperienced.The saline watermovesup and down theestuaryduetotheflood and ebbtides.

Thecoastwhere theestuarydischargesto theseaissubjectto a very moderatewave climate.Maximum wavepoweroccursinphase withmaximum river

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dischargeand this resunsinthe formationofa wave-oomlnaiecdella. The highriver sedimentduringtherainy season is spreadout by wavesgeneratedbythe west monsoon.This material is thenredistributedalong the shorelinebywavesgeneratedby theeast monsoonduringthe low freshwater flowof thedryseason.

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Acknowledgements

Iwishtoexpress mygratitude 10Dr. I.H. AllenandDr.I.I,Sharp, my Supervisors,bywhomthesuperv.,.onof this thesis wascarriedout. Without their guidance, adviceand encouragement this studywould not have been accomplished.

IamgreatlyindebtedtoDr. C.A.Sharpe,the Associate Dean of the SchoolofGraduate Studiesand10Dr.U.Sharp,theAssociate Dean of Facultyof Eilt:ineering and AppliedScience for theopportunity tostudyat MemorialUniversity of Newfoundland.

Myappreciationisdue toDr.Leonard M.Lye forhis invaluablehelp andsuggestion.I wouldalso like 10say special thank you tomyparents fortheir blessing and love,andalso tomy wifeandmydaughter for theirlove, patienceand encouragement.

Appreciation isalso duetothestaff ofMemorial University of Newfoundland,HuntsmanMarine Science Centre,DiponegoroUniversity, colleagues and friendsfortheir helpand information.

..s..JLIl..l..e August, 1992

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4.1.6 TheKali Garang Delta .. . ... ... . 87 4.2 ExistingDataand General Analysis.•. . .... .... . .. 92 4.2.1 HydrologicalData •.... . ..• .. 92 4.2.2 FreshWater Discharge. ... .•. •. .. 95 4.2.3TidalData... ..• . ... .. .• .•. 100 4.2.4 Windand WaveData ... .. ... . •. •.. • .. 109

FieldObservationandAnalysis 113

5.1 Water LevelMeasurtments .•..• . •... 113 5.1.1 TidalCurves and Longitudinal Water Profiles It7 5.1.2 Estimation ofTidal Volumein the West Channel .... 121 5.l.3 Discussionon Tidal Propagation in theWest Channel .. i23 5.2 Measurement of Salinity,Temperature, and

Currentsin theWest Channel ... •. . •..•. .. ... ... 125 5.2.1 LongitudinalSalinity Distribution•. .. . ... .. 127 5.2.2 Densityand Circulation ... . . •.. .• ••....•.. 130 5.2.3 Salinity and VelocityVariation... .. . ... 133 5.2.4 Lengthof SalineWedge .. .... . . ... . . ..•... 135

5.2.5 Discussion 137

S.3 WaterAnalyses 142

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5.4 Float Tests ...•. ... . .. . ... . . ... 144 5.4.1 Currentsin the West Channel •.. .. 14S

5.4.2CurrentsintheKaliSemarang 147

5.5 Sediment Analysis .• 149

5.5.t ParticleSizeAnalysis . 149

5.5.2 Measures of SizeDistribution 153

5.5.3Discussion.... . . 154

6 Sediment Budget and DeltaFormation 6.1 WaveEnergy ...•. ..•.... ....

6.2 RJver Sediment Load .

6.2.1 Computation of Water Profiles..

6.2.2 Computations ofSedimentsRates 6.2.3 AnnualSedimentDischarge••.... . 6.3 Sedimentadon at the Kali Semara ng.. . . . 6.4 Longshore Transport .

6.5 Sediment Budget

6.6 Delta Fonnation .

6.7 Discussion .

Discussionon Water Pollutionin Estuaries Conclusions

References

155 155 159 160 161 165 169 171 173 176

In

181 188 193 Appendix A

Appendix B Appendix C Appendix D Appendix E .Appendix F Appendix G Appendix H

DataSheetsofFieldObservations OffshoreWave EnergyFlux NearshoreWave EnergyFlux . WaterProliles..•.. EstimationofSuspendedLoad Rate Estimationof BedloadRate LongshoreTransportRate Functionofd/L".• • . . ..

vii

198 217 222 227 233 234 235 239

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List of Figures

Fig. 1.1 LocationMapof Indonesia.•.. . .• .... ... ...•.

1.2 Annual DischargeofSuspended Sediment from the Drainage Basinof theWorld . . ..•... ••... Fig. 2.1 Classification of EstuariesBasedon Circulation II

2.2 AveragingRainfall(a) ThiessenPolygonand (c)tsohyetal Method ..•. ..•...•... . •...• .... ... IS 2.3 Intensity-duration-frequency(IDF) CurvesforCitiesof

Jakartaand Surabaya •.•..•• •• ••.•••••.•••• 18 2.4 Examples ofDifferentTidalTypes(a) SemidiumaI,

(b) Mixed,dominantdiurnal,and ee)full diurnaltype .. 23 2.5 Example of RecordedTideinSemarang•.. .. ..•.•. 24 2.6 WaveEnteringChannelof FiniteLengthwith

ReflectingEnd .•..• •... . .. •.. . . •. ..•. . 27 2.7 Schematic Chartof Saline Wedge..•. . .•.. •... .• 36 2.8 Relation betweenF(T,)r. )and(rj.,J ... ... •. . 48 2.9 Schematicof SedimentBudgetinEstuary•... .... .. 57 2.10 ComponentsofaDelta Plain •..••.••••... .... S9 2.11 Major FactorsControlon aDelta Formation.•.. .... 61 2.12 TheClassification ofVarious DeltaMorphologiesBased

on theRelativeStrengthof River, Tidal,and Wave Processes ..•.•..•. . ..• . . . .•• . .•... . . • . 64 Fig. 3,I Location Mapof the Mahakam Delta and The Brantas

Delta•.... . . .•• . .• ... . .•...•. . 66 3.2 Components ofRiverSystemand Major Processes

Controlin EachPart. . •. ..• •.•. • . •.. ..•.... 67 3.3 GeneralizedDirection of the West MonsoonOver

Indonesia,ApriltoNovember ••. .• .... •.•..•• 71 3.4 GeneralizedDirection of the East Monsoon Over

Indonesia,Decemberto March ••... . •....• ..•• 72

3.5 The Mahakam River Delta,Kalimantan 76

3.6 The Brantas River Delta,Java ....•... • . •• .. • . 78 fig. 4.1 LocationMap oftheKali Garang River System 81

4.2 Sketch Chart of the KaliGarangEstuary and

the SimonganWeir..•.... ... . •...•... 83 viii

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4.3 Long Sectionofthe KaliSemarang 86

4.4 PlanViewofthe West Channel 88

4.5 TheKaliQarangDelta 9\

4.6 DailyRainfall VariationDuring1990. 94

4.7 MonthlyRainfall and Evaporation 94

4.8 Variation of Average Daily Flowofthe Kali

GarangatPanjangan,1989 .. •.. . . 96 4.9 Design ChartforEstimationof OverlandTime

ofFlow .... . . .. .•..•... 99 4.10 Intensity-dura tion-frequency(IDF)Curves

~~arang 100

4.11 LayOulofthe Semarang Harbour 102

4.12 Tide inSemarang Harbour ,July1991 103 4. 13 DailyVariation andDistributionof TidalRanges in

SemarangHarbour ,1990 107

4. (4 DistributionofWater Elevation in SemarangHarbour, 1990 .. . . .•.•• ..,.. . . 108 Fig. 5.1 Sketch Chartof the Kali Garang Estuary Showing

ObservationStations.• •,.. . . • . . . . 114 5,2 Tida l CurvesinSemarangHarbour and ThreeStations

AlongtheWest Channel .... . . . .. ..•. . 116 5.3 TidalCurvesinSemarangHarbour andTwoStations

AlongtheKaliSemarang 116

SA Water Profiles attheWest Channel. tl9

5.5 WaterProfiles at theKali Semarang 120

5.6 Elevation-area eEl-A) Curves at Five Stations

of the West Channel ., ....•... . . , . . . 122 5.7 SalinityDistributionin the West Channel 128 5.8 DensityandCirculationinthe West Channel 131 5.9 Typical NetMovementatSTA.03.WestChannel 134

5,10 Formof saline wedge 136

5.1t LongitudinalSalinityProlileofthe Kali Semarang 143 5.12 Tidal Currents atSTA.03 , West Channel 145 5.13 Comparison between measured andcomputedof tida l

currentsatSTA.03 ,WestChannd 147

5.14 TidalCurren tsatSTA.05,KaJiSemarang 148 5.15 FrequencyDistributionofSediment , .. , 152 Fig. 6.1 Bathymetric Chart oftheKaJi Garang Estuary and

Surrou ndingArea 156

6.2 Distribution of Monthly Wave PowerClimate, 1989 158

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6.3 E1evation-hydr.ilulicradius(EI·R)Curves at

Five StationsAlong the West Channel.. . • • . ••.•• 160 6.4 WaterProfiles AlongtheWestChannel 162

6.5 RelationBetweenQf andQ..andOfand~ 164

6.6 SedimentLoad atEvery Section Along the West Channel .. . ....• .••...• • •.... ... .• • . •. 168 6.7 SchematicChartof theSedimentBudget in the

KaliGarangEstuary ... •. • .. .•. . .. . ... 175 6.8 Discharge/WaveClimate ontheKaliGarangDelta.. 179 6.9 Processes the Development oftheKali Garang

Delta.. •...•...•.. ..•• .... . ISO flg. 1. 1 DOandBOD Curves oftheWestChannel ••...• .. 183 7.2 DOandBODCurves of the KaliSemarang •••.•.• 184

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List of Tables

Tab. 2.1 Runoff Coefficient,C 19

2.2 TheMainTidalForcesConstituents 21 2.3 Wentworth'sModifiedScale... 41 2.4 Mean AnnualSummaries ofDischarge/WavePower

Climateand Attenuation Ratiosof SevenDeltas 63 Tab. 4.1 ThePresentFeatureofthe Kali Semarang 8'

4.2 Hydrological Data,1990 1).1

4.3 MonthlyFreshwaterFlowof theKaliGarangat PanjanganfromApril1986toDecember 1989 96

4.4 Maximum Rainfall 98

4.5 Runoffofthe KaliGarang andIts tributaries 98

4.6 Tidal ConstituentsinSemarang 101

4.7 AnalysisofTidalDatainSemarangHarbour.

May\"10July31",1990 HM

4.8 WindData(Frequency),1989 110

4.9 Wind Data(Percentage), 1989 III

4.10 Wave Climate, 1989 112

Tab. 5.1 Survey CrossSectionsinthe WestChannel 114 5.2 SurveyCross Sectionsin the Kali Semarang 115 5.3 Water SurfaceGradient attheWestChannel 118 5.4 Water Surface GradientattheKali Semarang..... . 12 1 5.5 Tidal Volumein theWestChannel

.

" 123 5.6 Theshapeof salinewedge

...

¹³⁵

5.7 EstuarineNumber,S"oftheWest Channel 138 5.8 Valuesof~KMRatiosoftheWest Channel

. "

¹³⁹

5.9 WaterAnalysisontheKaliSemarang 14) 5.10 WaterAnalysisontheWest Channel " 144

5.11 Grain Size Analysis 150

5.12 DescriptiveCharacter of Sediment

. . . .. . .

151 5.13 QualitativeDescription of Sediment

. .. .

153 Tab. 6.1 Wave Energy Flux and WaveAttenuation ofthe

Kali OarangRiver Delta,1989

.... ... .. .

158

6.2 CharacteristicofSediment 161

6.3 Relation BetweenQrandQ"andQIandQ~ 165 ,I

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6.4 River SedimentLoadinthe West Channel, 1987 166 6.5 RiverSedimentLoadin the West Channel, 1988 166 6.6 RiverSedimentLoadinthe WestChannel,1989 167 6.7 SedimentationAlong theKali Semarang.... 169 6.8 LongshoreTransport •... ... •.. .. 172

6.9 Distributionof Longshore Transport 173

6.10 Discharge EfrectivenessIndex or theKali

GarangEstuary,Based onData in1989.. In 6.1I Mean Annual Summariesof Discharge/Wave Power

ClimateAttenuationof Seven Deltas and of theKali GarangDelta.. . . .. .. . . .. .. ... 181 Tab. 7.1 SaturationValues for DO andBOD in ppmatDifferent

Temperature and SalinityUnder AtmosphericPressure 185 Tab. A.I-I Tidal Record in theWest Channel, June 26,1991 198 A. I·2 Tidal Record in theWest Channel,July04, 199 1 199 A.I-J TidalRecordintheWestChannel, July18,1991.. 200 A.I-4TidalRecordin theWestChannel,July 26. 1991.... 201 A.2-1 Tidal Recordin theKali Semarang,July 04,1991 202 A.2-2 TidalRecordin the Kali Semarang,July 18,1991 203 A.2-JTidal Record in the Kali Semarang, July 20. 1991 204 Tab. A.J·I LongitudinalSurveyData.June IS, 1991(flood) 205 A.3-2 LongitudinalSurveyData, June15,1991 (ebb) .•.... 206 A.J-JLongitudinalSurveyData.June 19,1991(flood) .... 207 A.3-4 Longitudinal SurveyData,June 26,1991 (ebb) 208 Tab. A.4-1 SectionalSurvey Data,July 04,1991(I12) 209 A.4-2SectionalSurvey Data,July 04,1991(212) 210 A.4-3Sectional SurveyData,July 18, 1991(112) 21l A.4-4Sectional Survey Data,July18,1991(212) 212 Tab . A.5-1 FloalTestin theWest Channel,July 19,1991 213 A.5-2FloatTest in theWest Channel,July 25, 1991 214 Tab. A.6-1Float Testin the Kali Semarang,July20,1991 215 A.6-2 FloatTestintheKali Semarang, July 26,1991 216 Tab. B.I Angle Between Wind Direction and Normal of

Shoreline...•.•. .. ... ..• • ..•. • •.. 217 B.2 Determination of Wave GroupCelerity... . .. .• •. 217 8.3 OffshoreWaveEnergy FluxforT=2.5seconds 218

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B.4 Offshore WaveEnergy Fluxfor T ""3.5seconds ... 218 8.5 OffshoreWaveEnergyFlux forT ""4.5 seconds •. . 219 8.6 OffshoreWaveEnergyFlux forT""5.5seconds..• . 219 B.7 Offshore WaveEnergyFlux forT

=

6.5 seconds 220 B.8 OffshoreWave EnergyFlux for T ""7.5 seconds 220 B.9 MonthlyDistribution ofOffshoreWave EnergyFlux .. 221 B.IO MonthlyDistributionof LongshoreComponentof

Offshore WaveEnergyFlux 221

Tab. C. [ Angle Between Wind Direction and Normal of Shoreline ...• . . . •... . •... . . 222 C.2 Determination of Wave GroupCelerity ....•. . .•. 222 C.3 NearshoreWave EnergyFlux for T=2.5 seconds .. 223 C.4 NearshoreWaveEnergyFlux forT=3.5 seconds.., 223 C.S NearshoreWaveEnergyFluxfor T

=

4.5seconds .. 224 C.6 NearshoreWaveEnergy Flux forT

=

5.5seconds.. 224 C.7 Nearshore Wave Energy FluxforT

=

6.5 seconds 225 C.S Nearshore Wave EnergyFlux for T

=

7.5seconds.. , 225 C.9 MonthlyDistributionofNearshoreWave EnergyPtux 226 C.IO MonthlyDistributionof LongshoreComponent of

NearshoreWaveEnergy Flux 226

Tab. D.l WaterProfiles,Initial Water Elevation+42em. 227 D.2 WaterProfiles.Initial Water Elevation+74 em . 229 D.3 Water Profiles.InitialWaterElevation+106cm 231

Tab. E EstimationofSuspended Load Rate 233

Tab. F Estimation of Bedload Rate• . .. . 234

Tab. G.l Determinationof F(a J .. . .. .. • .. 235 G.2 Determination of Shoaling Coefficient(h/HG ' ) 235 G.3 LongshoreTransport forT

=

^2.5^seconds ^.^{.. .} ²³⁶

G.4 Longshore Transport forT

=

3.5seconds. •. . .. • •. 236 G.5 LongshoreTransport for T=4.5seconds . . . . .. 237 G.6 LongshoreTransport for T=5.5seconds ..• . .. . ., 237 G.7 LongshoreTransportforT=6.5 seconds 238 G.8 LongshoreTransport for T=7.5seconds. ...•. 238 Tab. H Functionofd/L."

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239

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List of Symbols

Symbol Description Dimens ions

A. Basinarea U

A Crosssectionarea Ll

A Dimensionallongslloretransportconstant Ll'f/M

Ad Advancingwedge constant

II, SurfaceArea L2

(J Tidalamplitude L

B Channelbreadth L

C Chezycoefficient L ,n,T

C Concentration ofsediment MILl

C Wavecelerity UT

CK Wavegroup celerity UT

C. Dimensionalbedload coefficient L'/MT

Cs Storagecoefficient

D Depth L

d Diameterof particle L

d$Q Mediandiameter L

E WaveEnergy Mrr^l

e Porosity

F Ratioofthesum ofthe amplitudesofthetwo maindiurnal constituents(K ,and 01)to thesum of the amplitudesof the two mainsemi diurnalconstituents(M1andSJ

F Functionalconstant LWfl'.

F Functionalrelation F' DensimetricFroudenumber / Frequency

K Accelerationdue togravity UTI

II waterdepth L

II Wave height L

H~ Waveheight at breaker position L

III Waveheig htafter undergoing frictionalattenuation L H, Significantwaveheight obtained fromgagerecord L

H~ Wave heightatdeep water L

H.... Root mean squareof waveheight L

H. Significantwaveheight L

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H' Wa veheight after refraction L

h, Waterdepthfrom bottomto interfacelayer L

h' Water depth fromsurfaceto interface layer L

1 Rainfall intensity lJf

I. Gradientenergy

1₀ Discharge effectiveness Index LT'

i Subscrip t indexnumber j Subscript index number

K Flow ratio;the ratio ofthevolumeoffreshwaterpertidal cycle tothe volume of salt waterinthefloodtide K. Refraction.coefficientforwaves

K, Shoalingcoefficient for waves

K,It) Shoalingcoefficientfor wavesevaluatedatthebreakerposition

K"" Shoal ingcoefficientfor waves evaluatedatthewavegage

K. Longitudinaldispersioncoefficient L'rr

k Wavenumber L Wavelength L. Length ofadvancingwedge

I Length

M Meandiameter M. Median diameter M. MeanPhidiameter m Constant N Numberofstation.

Manningcoefficient

P Wave power MUTI

P, Longshore ecmpoeentofwave power MlJf^l

Q Discharge Llff

Q Longshoretransport sediment L'ff

Qj Freshwaterdischarge L¹

rr

Q, Grosslongshoretransport L'

~ Longsho re transportmovesto the leftside L'

Q. Netlongshore transport 1.¹

Q, Peakdischarge Urr

Q. Longshoretransportmovesto therightside U

q Dischargeperunitwidth LJ/LT

qfJ Bedloadrete U/LT

Qs Suspended sedimentrate L'/LT

R Hydraulic radius L

R.." Avera gerainfalldepth L

R. Re ynoldsnumber S Sali nity S. Estuarinenumber

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S, Salinityat initialpoint (1'"0)

T Returnperiod T

T Tidalperiod T

T, Tidalrange L

t Time T

"

Timeof concentration T

U Meantidalcurrent ut

velcchyin1di rection

ur

u, Velocity offresh water

ur

uA Densimetrie veloc ity

ur

u. Shear velocity

ur

V, Tidalvolume L'

v Velocityinydirection

ur

w Velocityin z direction

ur

x Linear measure L

Linearmeasure L

-,

^Anglebetween shoreline and wavecrest in deep water

-,

Angle between shorelineand wavecrest inbreakerzone

-,

^Skewness

> Specific weight M/LlT

; Increment '

,

Heightof waveprofile L

,

Kinematic viscosityof water L'rr

P Massdefuily MIL'

PJ Mass densityof fresh water MIL'

..

Average densityof freshand salinewaters MIL'

P, Mass densityof sedimentmaterial MIL'

P. Mass densityofsaline water ^MILl

·

Standarddeviation

·

Specific:density

·

Wave frequency

r Shear stress MILT'

'.

Bedshearstress MILT'

"

^Criticalshear stress M/LT¹

'"

^Latitude

~, Sediment transportparameter

~" ^The16'" percentile L

4>JIJ The 50'"percentile L

~u The84"percentile L

xvi

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Chapter 1 Introduction

Indonesiaisanarchipelagocomposedof live majorislands;Java.

Sumatra,Kalimantan(Borneo),Sulawesi(Celebes),andIrianJaya(New Guinea); and thousandsofsmallislands (Figure1.1),The regionextendsalong theequator(06U50' N to llu60'Sand 92"25'Eto 141"30'E),Indonesia ts boundedbytwocontinents, Asiain the Northand Australia in theSouth,and by two oceans,Indianin the Westand Pacilie intheEast.

Geologically.mostislandswere formedduringthe Mio-pleoceneor later periods accompanyinga seriesofvolcanic activity.Therefore, the islandsnrc characterizedby eolian and sedimentaryformationsof NeogeneTertiary together with andesitic, and basaltic rocks ofvolcanic origin.The eolianand sedimentaryrocks of theMia-plioceneare mostly derived from andesilic and basaltic rocks. Weathering processeshavechanged therocks to fertile,softsoil,Thelandmasses are covered mainlyby tropical forest.

Present-day formationofthe Indonesian Islands occurredoverthe last 15,000yearsduringthe Flandriantransgressionwhichendedapproximately5000 years ago,Increasing sea levels,of approximately100metersduringthe Pleistocene IceAge,

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~

-

⁰

i:'

'.'

,

0

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inundatedlowlandsand separatedIndonesia fromtheAsianandAustraliancontinents.

Java,Sumatra,and Kalimanta nwereseparatedfromAsia.IrianJayawasseparated fromAustralia,while Sulawesiremainedthesame lU before.

During the subsequentriseinsealevel.theriversystemshavefedahigh rate ofsediment continuou slytothe lower reaches and havebuiltathick~..quenceof deltaicsedimentswhich developed seawardsfromthe mouth.

Asmost rive r system inIndonesiadebouch intoanocean with smallto moderatemarine forces,tidal action andwaveeffects. river-dominateddeltas are usuallydeveloped.Thesedeltas are characteri zedbythebifurcationofchannels that formthrou gh thedeposited sediment.The riverflow is usually distributed betweenthe variousbranches.However,insomecaseswherethe marine forcesbecome significant, the delta geometri esare alteredto eitherintermediate river-tideorriver-wave deltas, oroccasio nallyto a tide-dominat.edor wave-dominated delta.

Climaticconditions are dominatedbythe tropicalmonsoons.Thecast monsoon usually lasts fro mMay to Octoberand the west monsoo n from November to April.The west monsoonbrings considerablerainwhichcausesheavyfloodingallover the country.

The soil,geology,and climaticconditions mentioned abovestrongly influence thecharacteris ticsoftheriver systems. On a worldscale,Indonesianriver basinspro videahigh volumeofnatural sediment materials(Figure1.2).Fresh water flowsfluctuate seasonally.Theflow is highduringthe rainyseasonandlowduring the dryseason.

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~---:--~~~.~

"

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General problemsrelatedto the characteristicsoftheriver systems mentioned above areflood, drought,erosion,and sedimentation.High riverflows duringthe rainyseasoninundatelowlyingbasins whichusuallyconsistof farmlandand denselypopulatedareas.In contrast, the lowflow during the dry season provides insufficient water foragriculture andhuman use.

In response topublic demandandprotection, the Governmentof Indonesiahas taken some counte rmeasuresagainsttheseproblems.These are focused on riverbasin and river courseimprovements.The firstis aimed at reducingerosion, and thelatterproposes to regulateriver flowand increaseriverchannelcapacityin orderto discharge floods.

As thesemeasures have been introduced,somesignificant resultshave beenobtained.Flood damage has beenreduced while shortagesof waterduring thetlry seasonhave been minimized.Moreover, the increasingavailabilityofriver waterduring the dry seasonhas resultedinanincrease of the annualrice yield.

On the otherhand,modificationof thenatural cycle ofriversby construction of dams, training walls,bank protectionand weirshas altered thenatural movementofwaterandsediment.New areas ofdeposition anderosion together with areas of potentialflooding are created . Thisis particularly true in the estuariesof many Indonesianrivers. Many of theseestuaries arc deltaicinnature.

This thesis will studythe parametersinvolvedin waterand sediment movement using a smaller Indone sian Deltaic Estuary as a practical example. The estuary chosenistheKali Garang Estuary which islocated in Scmarang,Central Java.

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Theestuary consistsortwobranch es:theKaliSemarangandthemanmade West Channel.ThisestuaryprovidesagoodiIIustntionorthegeneralcharacteristicsor Indonesian estuaries,particularlythoselocatedinJava Island.

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Chapter 2 Theoretical Background

2.1 Introduction

An estuary is defined asa'transitionzone where freshwater fromland drainage mixeswithand dilutes saline seawater.As such, the charact eristicsof any estuari es are affected by the river systemcharacteri stics and by the areaof the ocean intowhich the estuarydischarges.

The main factors,which play a crucial role in determining the characteristicsof an estuary,are freshwaterriverdischarge,tidalaction, density difference between freshand sea water,the estuarygeometry, sediment transport, influenceof winds and waves and, in somelarge estuaries,Ccriollseffects.

Tidal action can displaceconsiderable volumesof water withinthe estuary. In a shallowwater estuary,tidal motioncan be modifiedconsiderably.Range maybe amplifiedor diminisheddepending onthe topography.Thc shoreline and nature of the bottomcan alsostronglyaffect tidal currentsby obstructingor constrictingwater flow.thereby alteringthe circulation pattern. Furthermore,the level or energy containedintidal currents affects sediment transport.These currentscan playamajor

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rolein buildingtidalflats anddeltas.

Therateof fresh waterflow affectsthewaterlevel andsalt water penetration. Densityofestuarinewater isfunction ofsalinity,temperature,and sedimentload.The densitydifferenceinducedby interminglingofthe two different watersdrives water circulation and mixing processes.

Strong winds thatblowalonganestuary may cause an increase or decrease in waterlevel upto severalcentimetres.These windswillalso influencethe mixing processesintheupperlayers of water.

Oceanwaves reworkriver sediments brought seawards byriver discharge. The waves maythentransport sediment back intothe near-shoreareas, moving them inshore or alongshore.Wavesalso provide marinesedimentsforthe lower reaches of the estuary.These processesplayaconsiderable role in developmentand formation ofriverdeltas.

Additional ly.in awide, deep estuary , Corioliseffects playaconsiderable rolein modifying the circulationpatterns.IntheNorthern hemisphere.the effectofthis force isto shiftthe seaward flowing freshwater and thelandward flowing seawater layers to theright inthedirectionofflow.Inthe Southern hemispherethis force deflectsthese water masses tothe left.

2.2 Classification of Estuaries and Their Relations to Sedimentatio n

Scientists haveattempted to classifyestuariesbased on a number of

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different scientificor professionalviewpoints.Some of the common classificationsthat relate closely tosedimentation arebased on, tidalrange,sediment sources or water circulationpatterns.

Tidalrange.

DavisandHayes (quoted inKennlsh , 1986) developeda relationship betweentidal range (fR) and estuarinetype.They identifiedestuariesin termsof microtidal ,mesotidal,and macrotidal syste ms. Microtidalestuarieshave asmall tidal range (fRoS.2 m).In such casestides playaless dominantroleinreworking the river sediments than dowaves.These estuariesare usuallycharacterizedby sandysediment riverdeltas or bywavebuiltstructures or spitsalignedwith thebeach.Many Indonesianestuariesareinthisgroup.Tidal actionaffects circulationslgnificanu yonly duringthe dry season.

Mesotidal estuariesexperience an intermediate tidalrange (2In <TR

<4 m). In thistype,the distribution of sedimentdepositionisprimarilyinfluencedby tidalcurrents. As a result,tidaldeltasand meandering tidalchannels develop. Siltilnd clay materialscollect on tidal flats and in salt marshes.

Macro tidal estuarieshave a hightidal range (TR2:. 4 m).Tidalcurrents completelydominatethe distribution of sedimentswithintheestuaryand delta area.The estuariesare usually broad mouthed and funnelshaped . Sandy sediments are deposited in the central portion ofthe estuaryand fine sedimentsaccumulate on broad,tidal flats

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or saltmarshes.

Sedimentsoorct5.

Thesedimentsources ofanestuaryprovide anotherpossible methodof classification. Based onit,Rusnak(quotedinLauff,1961,p.180) classifiedestuaries into posit ivefilledandinversefilled. In the firstcategory, the sedimentsbeing depositedormovingthrough theestuary are dominated by river born sediments.

Marine source sedimentsdominant in the second case.AllIndonesianestuariesfallinto the category of positive-filledbasins, i.eestuaries in~hichriversediments are more importantthan marine sediments.

WaterCirculationPatino.

Classificationbasedon thewatercirculatio n is the mostwidely used system. This systemclassifiesestuariesbasedonthe variation ofriverdischarg e and tidal range. Cameron and Pritchard (1963)describedtherelationshipbetweenfresh waterdischargeand tidal action.andclassifiedestuariesintothree classes orordersin accordance with the strengthof riverdischargerelativeto the tidal action.The classes are termed:(I)salt wedge estuary,whichishighlystra tified ; (2) partially mixed estuary,which is moderately stratified; (3)verticallyhomogeneous estuary, witha lateral salinitygradient;and(4) sectionally homogeneousestuary,withalongitudinal

10

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salinitygradient.In the higl\lystratifiedestuary,riverdischarge dominatesstrongly over a smalltidalaction.Theless densefresh waternoatson top of denserseawater ina layer thatthinsin the seaward direction.The seawaterintrudesintothe estuaryin a wedgeshapeUtatgetsthinneras it moveslandward (Figure2.la.).Sedimentary materialin astratified estuaryaccumulates at thelimit orpenceanooof the sail wedge.

Suspendedmaterialenteringthesaltwedgewouldbebrought tothelimitof penetration by tidaland/or densitycurrents.

. , .

...

;---r~-_r-_r-_,-_,

b). . . . .

-=t ~ ~" /.. / ^I ^.)

~

--.1._ - ' -_

r >

---'

ⁱ ^"'-

·, ....·

,... ..Ma1

...WJ

J

rn

:

"

--; ^. ^... ^-

figure2.1 ClassificationofEstuariesBasedon Water Circulation,a)l/ighly Stratified. b) Paniallymixed.c) Well mixed (AfterPrucnard,/955).

Thepresence of tidal motionwithinthe estuary enhancesvertical

11

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turbulentmixing.Mixing increases as tidal action increases.When the estuaryis completelyhomogeneous, thevertical salinitygradientdisappears.Althoughlateral and horizontalsalinitygradients remain, the estuaryis knownas verticallyhomogeneous (Figure2.lc .) .In an estuary whichhas a sufficientlysmallratio of width to depth ,the lateral salinitygradient can be broken bylateral frictionalforcesandtheonlysalinity gradient retained isthelongitudinal one. The estuaryis then called sectionally homogeneous.Suspended materialin the well-mixed estuary accumulates overa much longer lengthof channelthanin a moderately stratifiedestuaryand is confined mainly to the limit of penetrationof salt water.

Betweenthe highlystratified andwell-mixed estuaries,thereis a partially mixed estuary (Figurez.tb.).Here,the tidal forceis aboutequalto the fresh water flow in contributi ngto estuarinecirculation.Material carried in suspensionisdeposited overthe limitsof thesalineintrusionwhichmayvary over several kilometres asthe interfacemoves up and down due tothe tide.

2.3 FreshWaterDischarge

Fresh water flow playsanimportant rolein determiningthe salt water distribution,circulationpattern, and sediment transport.Forthis reason,it isvery important to ascertain thefresh waterflow enteringthe estuary at any time.Basically, there are two methods of estimating the freshwaterflow:direct measurement of flow or the use of empirical formulaerelating rainfall to run-off.

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The first isthemoreaccurate butit is amuch moreexpenslv...

undertaking andasaresult datais sparse.In Indonesiathe othermethod isortenused in practice accompanied,perhaps,by occasionalcalibrations.The followingsectionwill dealwith the relationshipbetween rainfall andrunoff, togett\(rwithother Iactcrsrelalf,,'t.I toit.

2.3.1 Rainfall

Rainfallisquantitativelydescribedintermsof depth{millirnctres)and duration (minutesorhours)(Roberson,eraI.,1988).Depth is thetotalamountofmin to fallina given time.Intensity isthe rate of rainfallperunit time(rnm/minuh:sor mm/hours).The rainfall varies signincantlyfromplace to place,evenwithin afew square kilometres.Itvaries.notonlyin intensity, butalsoin time andduration.

A'reragerainfall.

Becauserainfallvaries spatially, the data ofa singlestationmaybe significantly differentfromthatofanother station withinthe samccatchmentarea.

Whenseveral stationshave recordedrainfallina given catchmentarea,all datashould be considered in determiningthe average depthof rainfalloverthebasin.The average amount canbecalculatedbyusingthe followingmethods,

13

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StationAnnieMelhod.

'Thedataof allstationsaresimple averaged as

n.u

whereR..,.isaverage rainfall depth overthe basin,~is rainfall measured in stationi, andN isnumber ofstations.

Thiessenpolygonmethod.

The rainfallmeasuredat eachstationisassumed.10berepresentativeonly ofthearea closest10it.Theportionof thedrainage area foreachstationisdetennined by drawingthe Thiessen polygon(Figure2.2a).Theaveragerainfallis computedfrom

(2.2)

where Ai is thearearepresentedbystationi.

I'

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•• Thl....n Polygon

~: "

•

1.554 RIlIIll lldl p\llln mm

. f.

b.laohyelalmethod

Figure 2.2 Averaging rainfall(a) Thiessen Polygon Method,and (b) Isohyctal Method

Isohyetalmethod.

Theaveragerainfall isdetermined basedonthe isohyetal Ilnes(i.e., lines of equal rainfalldepth) which are constructedon the map of the catchmentarea (Figure 2.2b).The average rainfallis

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(2.3)

where Rjis the rainfall on isohyetj,~is area betweenisohyetj andj+J.andmis the numberofintervalsbetweenisohyets.

Intensity-duration-frequencycurves (IDF).

Naturally.the rainfall intensityhas an inversecorrelationwith the duration;thehighertheintensity,theshorter thedura tion,and viceversa.The maximumin ariver basin flood maynotbecaused bythe heaviest storm in a short durationbutbytherainfallwith a durationas long as the time of concentration,t.The timeofconcentrationisdefinedas thetime requiredfor surfacerun-off to travelfrom the mostremotepart of the basinto thepointat whichthe flood istobe estimated.

The occurrence ofa givenrainfallintensity isirregular.For these reasons,rainfall intensity-duration-frequencycurves (IDF) are constructed to estimate themaximumrun-offfer a certainreturn period. The curve can be constructedbyusing the followingsteps

I. The maximumrainfalleachyearis selectedfor each considered duration.

2.The selecteddata are arranged orderly (rom the highestto thelowestdata.

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3. The return periodis determinedusingWeibu!l'sformula;T ::: (n+I )/m, whereTis average returnperiodinyears,nisnumber of yearsof record.

and misrankof storm.

4. Therainfallintensities foreach return period consideredarethenplottedin a table of plotting foranIDFcurve.

Typicalrainfallintensity-duration-frequencycurves(IOF)areshown in Figure2.3.This shows the curves for thecities ofSurabaya andJakarta(lndoncsia).

2.3.2 Run-orr

Surfacerun-offisaffected by numeroushydrological factorssuchasrainfall intensityand duration,geologicstructure, relief, plant cover ofcatchment area, and shape of basin.Analyticalmethods forcomputingrunoff cannotyetrelateallthose factorsin a rigorousfashion.However,many empirical methods have been developed whichmay give satisfactory results.

The mostcommonlyused isthe so-called themodified rational method.This is

(2.4)

whereQ,ispeakdischargein ml/s,C is a run-off coefficient,C. is a storage coefficient, I is themean rainfallintensityinmm/hour,and A is thedrainageareain km'.

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.0

120 180 240 300 360 Rainfall dur a ti o n(min utell)

Figure2.3 Intensity-duration-frequency(IOF) Curvesfor the Cities ofJakartaand Surabaya.(DepartmtnJ of PublicWorks,Indonesia.1961)

Run-off coefficient(C).

The run-offcoefficient C is adimensionless constantthatdependson texture, permeabilityand landuseof the area. Table2.1lists valuesof C recommendedfor the modified rational method.

Storag e(oefficie nt(CJ.

Thevalue of C, depends onthe drainage area.Thelarg erthe area thelarger theeffectof storagetends to be. The magnitude of C.canbeestimatedbythe following

18

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relation

(2.51

wheret,

=

t,+t",t,is the time requiredthesurface run-offfrom basintonearest channeland~is the time needed to travelfromthe first channelto thepoint considered.

Tab le2.1 Run-offCoefficients,C (Source Department of Public Work.

Indonesia 1982)

Type of Area Run-off

Coefficient,C

Urban business 0.95

Commercialoffice 0.70

Residentialdevelopment

Scarce housing 0.50

Medium housing 0.65

Denselyhousing 0.75

Suburbanresidential 0.40

Apartments 0.70

Schooland mosques 0.80

Industrial 0.80-0.90

Parks,cemeteries 0.25

Rail-roadyards 0.35

Unimprovedgrassland 0.35

Rainrallintensity,I.

Rainfallintensity,l,dependsont,and IDF curve (Figure 2.3).

19

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Forthe purposeof calculationof salinity,circulationpatternand sediment movement, thedirect measured riverdischargeis usedinthisstudy,

2.4 TidesandTidal Propagation inEstuaries

Tidesare therise:andfall of waterlevel generated byresponse tothe attractiveforces of the moon and sun,and bytherotationof the earth-moonsystem aboutits commoncentreof gravity.Theearth-moonsystemrevolvesarounditscentre of masswitha periodof 27.3days. The combinedgravitationaleffectbetweenthesun andthemoonisstrongestwhentheyarein alignmentwith theearth.Resulting tidesare calledspring tides.Spring tides occurevery 14days, justafter the full ornew moon.

Conversely,when the sun and themoon are at right anglesto theearth,theresulting tide is at aminimum.Thetides are thenknownasneap tidesand the moonissaidto be in quadrature.

Althoughgravitationalforcesareresponsible forthe tides,manyirregularities inthe tidalcyclecannotbe explainedon thisbasisalone.Therearemanyother factors influencing,modifyingand controllingthetides whichare initiated by gravitational forces.Someof these modifying factors are variable.Others are constant. The most noticeableofthe variableinfluencesarc atmosphericpressureand wind action.

The constantfactors result ina number of different tidalconstituents.

Doodson(1920)listedsome390differentpartial-rideconstituentswhichareall related tothe sun-earth,moon-earth,orcombined interactions.Table2.2lists the major tide-

20

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force constituents,their periodsand theircontributions10 tidesrelativeto the principal component M1•Thebasicstrengthsof thecomponentslistedin Table2.2arc northe same for all placeson thesurface of the earth.Theymay varystronglyfromonearea to anotherdependingon thelocal basingeometry. In Semarang-ladoneslaforexample, thedominant componentis the Luni-solardiurnal(K1) ,althoughthis force contributes only58.4%of theprincipal component M1•

Table22 The Main TideforceConstituents(Source'Beannun1989) Period, Coefficient Name of Constituents Symbol solarhours ratio,

M1"",IOO Semi-diurnal

Principailunar M, 12.42 100.0

Principal solar S, 12.00 46.6

Largerlunar elliptic N, 12.66 19.2

Luni-solarsemi-diurnal K, 11.97 12.7

Diurnal

Luni-solardiurnal K, 23.93 58.4

Principal lunar-diumai 0, 25.82 41.5

Principal solar diurnal P, 24.07 19.4

Largerlunarelliptic Q, 26.87 7.9

Long-Period

Lunar fortnightly M, 327.86 17.2

Lunar monthly M. 661.30 9.1

Centrifugal accelerationandboundaryirregularities modify theeffects of gravity.Water particle centrifugalaccelerationis inducedby the earth'srotation and

21

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causes moving waterin the Northern hemisphereto deflectto the right. Inthe Southern hemisphere the centrifugalactionturnsmoving particlesto the left.This phenomenon is known as the Ccriolis effect.

Finally, the boundary irregularities causeinnumerable reflectionsoftidal propagation.These boundaries alsointroducefrictionanddissipate the tidal energy .

2.4. 1TidalTypes

The principal basic tidescanbeclassified by using a ratio(Fl,of thesum of the amplitudes ofthe twomain diurnalconstituents(K.and 01)tothe sum of the amplitudes of the twomain semi-diurnalconstitue nts (M2and S1)(An Open Univers ity Course Team,1986),thus

_ AmplitudeK1+Amplitude01 F - AmplitudeM~+AmplitudeS~

(2.6)

A ratioFof0 - 0.25implies a semi-diurnaltype;a ratioFof0.25 -1.50, a mixed-predominantlysemi-diurnaltype;a ratioFof1.50 -3.00, a mixed- predominantlydiurnal:and a ratio ofFgreaterthan3.00refers to a diurnaltide.

The term semidiumaltideis usedfor atidalcurve havingtwo high waters (HW) and twolow waters (LW) of aboutequal heightduringa tidal day (24hOUfS50 minutes), see Figure 2.4a.A diurnal tide hasonlyonehig h and low water during the sameperiod(Figure2.4c).Ina mixedtidaltypethere are sometimesone high and one lowwater and sometimes twohig h andtwo low watersduring a tidal day(Figure2.4b).

22

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Figure2.4 Examplesof DifferentTidalTypes,(a)Semi-d iurnal,(b) Mixed, DominantDiurnal,and(c) FullDiurnal Type

The basic patternsidentifiedbyfactor,F,canbemodifiedby localeffects, particularly the local effectsof harmonicsof the partial tides.Figure2.5 shows areal tide as recordedin the field.

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Figure2.S Examples of RecordedTide inSemarang Harbour,Indonesia(F

=

1.67, DominantComponent isK1)

2.4.2TidalPropaeatlon in EStuaries

Tidal energy in estuaries isderivedfromlocaloceantidesratherfromthan thedirectaction of astronomical forces.As ocean tidespropagateintoa shallow water estuary, theirprofiledependsinpanon the basindepth.When thewater depth inthe estuarydoesnot appreciablyexceedthe tidalamplitude,thetide wave ispropagated as a shallow waterwave.HencethespeedC is given by

(2.7)

wherehisthewater depth;g is acceleration due togravity;and7Jistheheight ofthe water surface above mean tidallevel. As the water depthdecreases, 'l/hincreases.

Hence,thetidal wave speeddecreases. Consequently, thetidalcrest(HW) propagates fasterthanthe trough (LW).Furthermore,there isanasymmetryin the tidal cycle,

24

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with a relativelylong time intervalbetween high waterand thesucceedinglow water.

and a shorterinterval between low water and the nexthigh tide.

Tidesina riverchannelare alsomodifiedby varying degrees of interference betweenthe wavewhieh entersthe estuary and reflection from the boundariesor from theestuaryhead,frictionaldissipation,and decreasing channelcrosssection (Kemish, 1986).Dependingonthe geometry and associated resonance characteristics of the channel,the tidesmay behave as eitherprogressive waves or as standingwaves.

Progressive waves usually occur in a wide and long (assumedof infinitelength) estuary wherethe enteringwaveheight is reducedsignificantlyby bottom friction.Waves reflected byboundariesaresmaller than enteringwaves.The important point in thi$

typeofw~veis thatthecurrentsare in phasewith the tide;i.e .•the maximum currents occurnearthelime ofhigh andlow water.Minimum currents occur nearmid-tide.

In an unrestrictedestuary, where wave attenuationis smallandthe amplitude andperiod of the enteringwave equalthose of the reflected wave, a standing wave is experienced.Here , the tidal levels andvelocitiesareout ofphase by 90 degrees.The maximumamplitude occursat the head ofIhe estuaryand at anrinodes that arclocated at~ultipledistances ofonehalf of the wavelength(U2)fromthehead.Nodalpoints of zero amplitudeand of maximumvelocity exist atU4and atodd multiples thereof.

A well known example of thistype is theBayofFundy (Canada)which hasalength of aboutonequarterofthewavelength.Themaximum tidalrange attheheadissome 15 m and themaximum tidal currentsoccurat about mid-tide .

2S

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In addition to the effects of the geometric factors discussed above,estuaries have special variableinfluences on tidal level. These are large freshwater inflow at the head,wind actions and atmospheric pressure. These three factors maysignificantly affect the waterlevel.

2.4.3 TIdal Wave Entering A Channel Closed atOne End

This case may be dealtwith as one complete reflection of the entering progressive wave from the closed end. Thus two waves ofequalamplitudea and period T are superimposed, one travelling in the positive and one in the negativex direction.

In accordance with the definition on Figure 2.6 (Ippen and Harleman, 1966),then

'I ..a e ccs c e coskx (2.8)

and

u"2-ECsinatsinkx

(2.9)

where h·rr

=

kC,

k ^: 2.../L, T tidal period (seconds), L ^: wave length (m), and C wave celerity(m/s).

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x-c

j

_ - / - - _ ( - 1 1 )

x-·j

r I

__________________________ _ _ : ::::::;)o ~_S_I._

2~ I I

".t:: l==========:

Li4·l

LJ' ---~---J

Figure2.6 WaveEnteringChannel of Finite Length with ReflectingEnd.

Highwater isobtained,therefore,

atthe closedend:

=

0

Thusinequauon2.8with t

=

0 'I"""",

=

20 01the openend : =~l Thusin equarion2.8with t

=

0

'I,.,...

=

2a cos k/

The ratio of the maximumtide at theclosed end to tide at the entrance is

(2.10)

This ratio is infiniteifkl

=

T/2forllL

=

1/4 .Note that equation 2.10applies only for onetime.

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2.S Waves in Shallow Water

Waves aredisturbances on the surfaceof a fluid.Theyusually occur on the interfacebetween the air and thewater body (surfacewaves), but may also occur on theinterfacebetweentwo differentwatermasses (internal waves). Inestuaries, wavesmay affectsedimentmovementdirectlyor indirectly.Inthis section,wavemeans awind generated wave. Thesizeofa windgeneratedwave is controlled bythree variables:(I)thefetch,which is thedistancethe windblowsover thewater surfacein a constantdirection,(2)the duration with whichthewindblows in onedirection,and (3)thewindspeed.Increasingthese threefactorsresults inincreasedwave heights.

Theimportant variablesinwave observation arewave height,wave period,and wavedirection.The wave height and period mayberecordedbyusing a wavegage recorder. Since the wave directionsare difficult tomeasurethey areusually assumedto havethesame direction asthe direction of the generatingwinds.In engineeringapplicationsthemeasuredwaveheightistransferredinto a significantwave height.Munk(1944)(quotedinU.S.ArmyCERC,1984,p. 3-2) defined significant wavehight(H.)as theaverageheightof the one-third highestwaves.

2.5.1 Wave Energy and WavePower

The processes associated with waveenergyhaveasignificant effect on thedistribution of river sediments, onthe rateof longshore transportand onthe

28

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formation ofdeltas. Forthese, itisimportantto ascertainbolhlhequantityand distributionofthewavemergy atthedeltazone.

'Theenergycontainedwithinawaveoccurs intwo(arms;kinetic and potential.TbeIotalenergy(E)ina waveper unitcrestwidthis given by(U.S.Army CERC, 1984,p.2-63)

u.m

whereE is inloule/m²,pis the densityoftheseawaterinkg/.rr,gis the acceleration due to gravityinm/ s²,andHisthewave heightin metre.

Waveenergyflux istherateatwhichenergy istransmittedinthe direction ofwave propagation acrossaverticalplaneperpendicular to the directiono(

thewave advance andCJttendingover theentiredepth.Theenergyflux perunit lenglh ofwavecrest is

(1. 1l)

wherePis the energyflull.thatisfrequentlycalledthewavepower (Nls

=

J/s·m),and Cgisthegroupvelocity of thewave (m/s).Thiscanbecalculated(rom (Wiegel.1964) (1.131

In deep water (diL>0.5),thegroup wave velocityisequalto one half ofwave velocity,and in shallow water(diL<0.05) the group velocityis equal tothe wave

29

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velocityand proportionalto the root-squareofthe depthd,thus

(2.14)

(2.15) For an oblique wave,the wavecrest makesan angle, orwiththe shoreline.In that casethe wave powerbecomes

(2.16)

and the longshorecomponent(P,)is givenby

(2.17)

2.5.2 EnergyDissipationand Wave Attenuation

Whenwavestravel fromdeep water to shallowwater,wave energywill bedissipated.Basedon theoryand fieldmeasurements, Bretschneiderand Reid(1954) have.derivedequationsto estimatethe reductionof wave heightdue to bottomfriction.

The wave height,Hftafter undergoingfrictionalattenuationover a distancexis given by

HI. H'

(fHn$ .X.,)

K,.T'

30

(2.18)

(53)

whereH'is me waveheightafter refraction,fis the friction coefficient «(or low roughnessboucm,egosiltand clay the value is 0.02,Wrightand Coleman,1973),T isthe wave period,K,Is theshoalingcoefficient,and~isgivenby

• •

~(

^{_ _}^K^{_. _} ^)'

3g sinh2.( ~)

Theshoaling coefficientK.is calculatedfrom therelation

(2. 19)

(2.20)

whered isthewaterdepth(m),andLis the wavelength (m),Histhewave height(m) atthedepthd,andH: isthewaveheight in deep water(m)ifthe wave isnot refracted.

Theenergyflux inthe near-shorecanbeobtainedby substitutingHI into theenergy flux equation10replaceH.

The annualwave powercanbeobtainedfromtheexpression

1%.21)

whereHis a numberofwave-classCharacteristics.jreferstotheparticularset ofwave characteristics,andtisthepercentagefrequency of thewave setj.

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2.6 Salinity Distributi on and CirculationPatterninEstua r ies

Estuarinewaterconsists of a combinationoffreshwaterand seawater.

Freshwateroriginatesfromland drainage,and saltwaterintrudesfrom the ocean. The saltpenetratesup the estuary by advection and diffusion.The salinityin estuarine water variesgraduallyin space andtime.Itis differentbetween lower andupperparts.It changes seasonally, daily,and throughouta tidalcycle.

The salinity intrusion and circulationpatternin estuaries arefunctions of freshwaterflow,tidalaction,theestuary geometry, windinfluence, Coriolis effects, andthe densitydifferencebetween fres handsalt water.Patterns are directedbythe strengthofeachfactor relative to the others.Asthe significanceof one factoris often dependentontherelative weaknessof one or more of the others,ameasureofthe interactionbetween the differentfactors is extremely difficult to obtain.Hencethe determinationofthesalinitydistributionand circulation patterns in estuaries are complexproblems.

2.6.1 Salinity Distribution in well-mbed Estuaries

Salinitydistributionandestuarine circulationpatternsare non linear.

Thereare a number ofimportantindependentorinterdependentvariablesthat vary spatiallyand temporally.Quantitative treatment of bothphenomena canbe represented theoreticallyby threewellknown laws of mechanics;conservationof momentum,

32

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conservationof mass, and continuity.

Inawell mixedestuary,salt transportcanbedescribed byaconservation ofmass or byan edvection-dlspersionequation(HelderandRuardij ,1982).Consider a Cartesian system withcoordinates;x, y,andz, where x andyarethe horizontaland sectionalcoordinates,respectively,andzistheverticalcoordinatemeasureddownwards from themeanwater surface.The three-dimensionaladvection-dispersionequationcan bewritten as

(2.221

where

(2.231

andwhereSis salinity,K is the dispersioncoefficient,gisthe acceleration dueto gravity,1istimeandu,v and ware the x,y,and zcomponentof velocity.

Innarrow, well-mixedestuariesthe majorspatialvariability occursalong the estuaryaxis and transportof saltcanbetreatedasaone-dimensionalprocess.Aone dimensional treatmentmay also provide a very uscful result forestimatingthe cross- sectionallyaveragedquantityof salinityintrusioninapartlymixedestuary(Uclcsand Stephens,1990).

Considerariver channel with freshwaterdischarge0,and widthU.

Salinity is nearlyconstantin the ocean and approacheszero attheupstream. Equation 2.23canberewrittenas

JJ

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(2.24 )

for stationaryconditions,where6S!lJl

=

O.equatioa2.24becomes

(2.25)

(2.26) where11is the depth of theestuary.

ForQ,constant,asingle integration ofequation 2.26gives

(2.m

whereS.isthe salinityat x ""x..

FOC'a givenvalue ofK~.lhelongitudinal salinitydistributioncanbe predicted.

Dlspersio Dc:oeflidenLs(K.,)

For a one-dimensionalmethod canbeestimatedfrom the longitudinal distributionof salinity.theestuarinecross-sectional areaand theriver flow(Helder and Ruardij,1982).Therefore,thelongitudinaldispersioncoefficientcanbeestimatedby

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integratingand rearrangingequation2.26to

K... 0("5.

• dS

A"" dX ^{(1.1' )}

where K~islongitudinaldispersion.coefficient(mt/sec), Q,.is freshwaterflow (ml/sec),S~is salinity(ppt),A~iscross-sectionalarea(m~•.1:isdistance(m) along lhe longitudinalaxis ofestuary,

Salinitydistributionsalong the channelfordifferent conditions, eitherof fresh water flow ortideorof both.can be estimated fromknowledge ofthe dispersion coefficientin eachcrosssection.

2.6.2 Salinity Dis1ribution InASalt Wed geEstUllry

Keulegan (1966)statedthattheintrusion ofsaltina riveropeningtoa seawithlow tidalactionisaffectedbytheupstream movement ofa definableand limitedsalinelayer underlying the freshwater.Thelengthofanadvancing wedge(LJ isa.functionof thedensimetricReynoldsnumber(Rt ),densimetric velocity(uJ .and the velocityoffresh wateropposingthe advancingwedge(u,),seeFigure2.7,The relationis givenas

(1.19)

wherethe Reynoldsnumber,R.is quantifiedby 35

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(2.30)

and thedensimetric velocity,ueis expressed as

u•=

~

^.A£..gHP.

(2.31)

and whereD.pis the density differencebetweenfreshand saline water(kg/m^l),Pmis the average densityof the twowaters (kg/m'), andHis the depth of the river(m) .The value of the constant,A~,varieswithH/Band withR.and the value of constant,m, withR,only (Keulegan,1966).

RIver \/

Fr"'l h wllt. r

! ' f-- L~

r---

^lo

!

s••

H

Figur e2.7 SchematicChart of SalineWedge

The vertical salinitydistributionin thesaltwedgeestuary dependson the denstmetric Froudenumber,F,.This is givenby

F . _ " - r .fQiii1

36

(2.32)

(59)

whereuis the mean velocity ofthe upper layer(mls),h'is the depth of the density interface(m),gistheacceleration duetogravity(mts) ,andthe specific density,(1,is given by

(2. 33)

wherePrandP.are the freshwaterand seawaterdensities respectively(kg/rrr), Asthe valueofFr'increasesthemixingprocessbecomesmore intense, and the salinityoftheupper layerincreases.FarmerandMorgan(1953)foundthat if Fr '>I.Oaninternalwave isformed.Furtherincreasesin velocitycausethiswaveto breakandstart interfacialmixing.Hencethelocalverticalsalinitygradientdecreases.

2.6.3 Circulation Patterns

Circulationpatternsin an estuary arelargely controlled bythe strength ofthe river flow relative to thestrength of tidalcurrentsin combinationwiththe geometricalconfigurationof the estuary.Circulationpatternsrange fromthose experiencedin salt wedgesto thoseofwell mixed estuaries (see section 2.2).

The competingeffects of fresh water discharge andtidal currentsmay bequantifiedthroughthe estuarinestratificationnumber,S•.This canbeused to roughlypredict the circulationpattern intheestuary. Pickard (1975) attempted to quantifyan estuarinenumber,S" inhis formula

37

(60)

(2.341

where Uisthemeantidalcurrent,Bis thewidthofestuary,gis gravity,andOris fresh waterdischarge.Asmall value ofS.wouldleadtoastratifiedestuary,and alarge valuewould beassociatedwitha well mixed esreary. Thetransitionoccursinthe rangeO.OJto0.3.Themean current, U,is relatedtothe tidalprism, that is!hevolume of waterenteringtheestuary orpassing agiven crosssectiononthefloodtide.The mean tidal curren tcanbeestimated from thefollowing relation

u-

²^{Tr4 •} ^{.. 2}^V.,.

BH T AT

(%.35)

whereUisgiveninmis,Titistidalrange(m),A.issurface area(m²) ,BandlJarc widthand depth of channel.respectively(rn),VTistidalvolumeem'),Ais crosssection area(m!),andTistidal period(seconds).

Ina different way,Simo ns(1%9)(quo tedin Silvester, 1974) proposed quantifyingliltdegreeof mixingby aflow ratioeK),whichistheratioofthevolume offreshwaterper tidalcycletothevolumeof saltwater in therloodtide.

Then for:

K~0.7 theestuaryishighlystratified , 0.7>K>0.1 the estuaryispartiallymixed.and K~0.1 the estuary iswell mixed.

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Basedon eithertheestuarinenumber(SJor theflow ratio(K),estuaries may changetheirclassificationfrom stratifiedtowell mixedduetothereductionof^the fresh waterflow andchangefromwellmixedto stratified byincreasinginfreshwater inflow.Similarchangesmayresult fromlhe increaseand/or decreasein tidal ranges from neap to springand/orvice versa.

2.7 Sedimentation and the Movementof Sediments

The mainsourcesofnaturalsedimententering an estuary are the catchment area of theriversystem (river-borne),theocean(marine sources), and a smallfractionwhichoriginatesin the estuary frombiologically andchemicallyin situ precipitatedmaterials.

In thecomplexity of anestuarine environment,sedimentusually experiences a variety of physical,biological, andchemical processesthat go on independently orin combination.The sedimentspass through repeated cyclesof erosion, transportation, deposition,andre-suspensionpriorto being deposited permanently orbeing lost to offshoreareas.

2.7.1 CharacteretlesofSediment

Sedimentsin estuaries can range insize from gravelsandcoarse sands toclayand flocculatingsilt.The sedimentsconsist ofthe products of theweathering,

39

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disintegration,and decomposition of pre-existing rocks and sediments and therefore containmany oftherockforming minerals.Theyalso includeorganicmatterand solid materialsbiologicallyandchemically precipitatedfrom waterswithinthe basin.

Sedimentscanbe describedin termsof their chemical or physicalproperties.

Thissectiondeals with the physical properties of particleswhich are closely related tothe sedimentmovement;l.e.,thegrainsize distribution of sediment material .Knowledgeof the grain sizedistributionisimpo rtant in determiningthe origin and the mannerof sediment movement. Thesize of sedimentarygrainsis usually expressedin either a metric or logarithmic scale, in which acontinuous range of sedimentsizes issubdividedinto classes or grades.There are two commonsizescales used today:the UnifiedSoil Classification(USC) and the Wentworth'smodified scales.

These two scales havedifferenllimitsof the class sizes.The USC boundaries correspond10theUSStandard Sieve Size.The Wentworthclassification differentiates betweensedimentsusinglimits whicharealways expressed in theform of1:'mm, whereIIcan be any integer,i.e 2,4, 6,8 etc.,or 0,5,0.125,0.062,0.031,etc.The Wentworthscale was modifiedbyKrumbein(1936)bytransformingthe millimetre scalesto a phi unitscale based onthe definition

(2.36)

where thediameter,d,is inmillimetre.The Wentworth and Phi unitscalearegiven in Table 2.3 .

40

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Table1.3 Clnsification of sedimentaccording tosize(Wentllo'onh's modifiedscale)

Nomenclature Diameter

Millimetres Phi(4)) Verylarge boulders 4,000 -2,000 -12 -II

Large boulders 2,000 1,000 -II ·10

Mediumboulders 1,000 500 -10 .9

Smallboulders 500 250 -9

- .

Largedebris 250 130

a

^{• 7}

Smalldebris 130 64 7 -6

Verycoarsegravel 64 32 -6 -5

Coarsegravel 32 16 -5 -4

Mediumgravel 16 8 -4 -3

Fine gravd

•

⁴ ³ ^-²

Veryfine gravel 4 2 - 2 1

Verycoarse sand 2.000 1.000 1 0

Coarse sand 1.000 - 0.500 0 1

Medium sand 0.500 -0.250 1 2

Finesand 0.250 • 0.125 2 3

Veryfinesand 0.12.5 •0.062 3 4

Coarsesilt 0.062 - 0.03 1 4 5

Mediumsill 0.031 •0.016 5 6

Finesilt 0.016 - 0.00' 7

a

Veryfinesilt 0.00' -0.004

a

9

Coarseclay 0.004 -0.002 9 10

Mediumclay 0.002 -0.001 10 II

Fine clay 0.001 -0.0005 II 12

Very fine clay 0.0005-0.0002 12 13

Naturally, sincethe finersedimentgrainsare moremobilethan the coarser ones,a largeamount ofinformationconcerningmovementcanbeobtainedfrom astudy ofthe grain size anddistribution in differentsizefractions.From the distributiondata, thegrainsizeaccumulation curvecanbedescribed.Funhermure, the

41