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RIVER DELTASRIVER DELTAS

Depositional processes Depositional processes --Course Coastal MorphodynamicsCourse Coastal Morphodynamics

GEO3GEO3--4306; 4306; lecture lecture 44

Global sediment supply to coastal seas and oceans; location of major river deltas

Nile Delta, Egypt Solo Delta, Java, Indonesia

Morphological-sedimentological units of a delta Relation between delta area and river discharge

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Growth patterns of deltas Main Main issues of issues of DeltasDeltas

1. 1. River outflow River outflow modelsmodelsRiverRiver--dominated processesdominated processes

2. 2. Morphology riverMorphology river--dominated deltasdominated deltas3. Impact of Marine 3. Impact of Marine ProcessesProcesses

Tidal processesTidal processesWaveWave--driven processesdriven processes

4. 4. ProcessProcess--based classification based classification of of deltasdeltas5. 5. ReworkingReworking and and redistribution redistribution of of deltaic deltaic depositsdeposits –– degeneration degeneration of delta of delta systemssystems

RiverRiver--dominated processesdominated processes

Low energy Low energy wave and wave and tidal tidal regimeregimeOutflowing effluentsOutflowing effluentsProcessesProcesses; ; relative role relative role of:of:–– Bed Bed frictionfriction–– Flow inertiaFlow inertia–– Buoyancy Buoyancy of of outflowing outflowing water water massmass

Factors depend on:Factors depend on:–– Inlet geometry and basin depthInlet geometry and basin depth–– Outflow velocities/river dischargeOutflow velocities/river discharge–– Degree of tidal mixing in estuaryDegree of tidal mixing in estuary

RiverRiver--dominated outflow dominated outflow modelsmodels

Fully Fully turbulent jetturbulent jet–– High High outflow velocitiesoutflow velocities–– Homopycnal conditions Homopycnal conditions ((no density no density

effectseffects))–– ““deepdeep”” water water basinbasin

FrictionFriction--dominated dominated ((planeplane) ) turbulent jet; turbulent jet; basin morphology basin morphology –– Shallow basin Shallow basin –– Strong bed frictionStrong bed friction

Model 1: Fully Turbulent Jet Flow patterns Fully Turbulent Jet

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Model 2:

Plane Turbulent Jet

Mouth bar development Inertia dominated system

Mouth Bar development friction-dominated system

Example Wolga Delta – Caspian Sea

Buoyant Buoyant jets and plumesjets and plumes

Relatively fresh Relatively fresh water in saline water in saline basinbasin; ; salt wedge salt wedge in in estuaryestuary;;High High outflow velocities outflow velocities -- inertiainertiaStratified conditionsStratified conditionsDevelopment Development of jets and of jets and buoyant buoyant plumes and plumes and frontsfrontsROFIROFI’’ss: : Regions Regions of of Fresh Fresh Water Water InfluenceInfluence

Stratification and entrainment in river mouth

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Flow pattern Buoyant jets and plumes Cross-sectional flow patterns in plumes

Flow patterns and internal waves: mixing

Plumes and frontal zones

River Plume Red River delta, Vietnam

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Solo Delta, Java, Indonesia

Growth pattern Solo Delta, East Java Mouth bar development in river mouth

Plume observations Plume observations and and modellingmodelling

Salinity Salinity and and temperature profilestemperature profilesFlow Flow field and field and concentration concentration of of suspended suspended matter (SPM)matter (SPM)Analytical Analytical models models outflow outflow and and depositiondeposition3D 3D numerical modellingnumerical modelling

Indonesian delta systems

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Discharge regime rivers in humid tropical zone

Stratified River plumes

Analytical modelling

-Flow field

-SPM conc.

-Deposition rates

Predictions flow field and plume tickness

Deposition rates from plume MahakamMahakam Delta Delta

1992 1999

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Mahakam river plume in Strait Makassar

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Ba Lat delta, Red River Delta, Vietnam

River discharge

Discharge• 3600 m3/s• Sediment: 100 106 ton/year

Wet season• High discharge• 80% sediment transport

Dry season• Low discharge

Longshore flow

Cross-shore flow

Sediment concentration

Salinity

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Hyperpycnal Hyperpycnal jets and plumesjets and plumes

River River water water density larger than density larger than seawater densityseawater density–– Large suspended load Large suspended load (>200.000 (>200.000 ppmppm))–– HighHigh--salinity values arid regionssalinity values arid regions

GravityGravity--driven underflow driven underflow in in basin basin ((turbidityturbidity--currentcurrent))

Hyperpycnal plume Yellow River, China

Impact marine Impact marine processesprocesses

General influenceGeneral influence: : mixing mixing of water of water massesmasses, , exchange exchange of of momentum momentum and and redistributionredistribution of sedimentof sedimentTidal ProcessesTidal Processes–– Strong vertical Strong vertical and horizontal and horizontal mixingmixing–– BiBi--directional directional transporttransport

WaveWave--driven processesdriven processes–– Decrease in flow velocityDecrease in flow velocity–– Lateral expansion and refraction of outflowLateral expansion and refraction of outflow–– Increase in mixing (wave breaking)Increase in mixing (wave breaking)–– Longshore Longshore and crossand cross--shore redistribution of shore redistribution of

sedimentsediment

Tidal processes in river mouth; elongate mouth bars

Wave-dominated processes river mouthWave-dominated mouth bar morphology

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Cyclic model for delta development, Ba Lat, Vietnam

Tidal regime Gulf of Tonkin

4. Process-based classification

of River Deltas (Based on Galloway, 1975)

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Example 1: Rhône Delta, France Example 2: Ganges-Brahmaputra Delta, Bangladesh

Example 3: Porong Delta, Java, Indonesia Example 4: Mississippi Delta, USA

Example 5: Donau Delta

5. 5. ReworkingReworking and and redistribution redistribution of of deltaic depositsdeltaic deposits

Channel shifting Channel shifting and and migrationmigrationReworking waves Reworking waves and and currentscurrentsPostPost--depositional depositional and and subaqueous subaqueous mass movementsmass movements

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Channel migration Yellow River delta Channel shifting Channel shifting and and migration migration --originorigin

Upstream development Upstream development of of bifurcationsbifurcationsBreaching levees Breaching levees > > crevassescrevassesAdvantage hydraulic gradient Advantage hydraulic gradient ((slopeslope))Formation Formation of sediment (mud) of sediment (mud) plugs plugs –– obstaclesobstacles

Delta lobes Mississippi Delta since 7000 yrs.

Cyclic processes

Development of Isle Dernieres –Louisiana, USA

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Hurricane Andrew 1992

Reworking by waves Reworking by waves and and currentscurrents

WaveWave--driven driven and and tidal flow tidal flow processes processes ((ebbebb//floodflood//tidal tidal asymmetryasymmetry))Residual flow patterns Residual flow patterns ((trade trade windswinds, , densitydensity--driven flowdriven flow))Nearshore Nearshore zonezoneOffshore delta front and Offshore delta front and prodeltaprodelta

Residual flow field – Ba Lat delta/Red River (Delft 3D model) Net bedload Transport – Delft3D model

a) Effect of tidal asymmetry on bedload transport

b) Effect of residual flow on bedload transport

c) Residual flow required to balance a)

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Initial erosion (red) and deposition (blue)

a) Effect of tidal asymmetry

b) Effect of residual flow

c) Comparison data 1949-2000

Subaqueous mass movementsSubaqueous mass movements

Lack Lack of of consolidation consolidation sedimentsedimentHigh High amounts amounts of of silt silt and and clay clay ((pore pore pressurepressure))High High deposition ratesdeposition ratesRelatively steep slopesRelatively steep slopesTriggersTriggers: : waveswaves, , earthquakesearthquakes, , tsunamistsunamis

Rotational slumps and slides

Seismic records of slumps