Main issues of Deltas

Transcription

1 Global sediment supply to coastal seas and oceans; location of major river deltas RIVER DELTAS Depositional processes - Course Coastal Morphodynamics GEO3-436; lecture 4 Nile Delta, Egypt Solo Delta, Java, Indonesia Morphological-sedimentological units of a delta Relation between delta area and river discharge 1

2 Growth patterns of deltas Main issues of Deltas 1. River outflow models River-dominated processes 2. Morphology river-dominated deltas 3. Impact of Marine Processes Tidal processes Wave-driven processes 4. Process-based classification of deltas 5. Reworking and redistribution of deltaic deposits degeneration of delta systems River-dominated processes Low energy wave and tidal regime Outflowing effluents Processes; relative role of: Bed friction Flow inertia Buoyancy of outflowing water mass Factors depend on: Inlet geometry and basin depth Outflow velocities/river discharge Degree of tidal mixing in estuary River-dominated outflow models Fully turbulent jet High outflow velocities Homopycnal conditions (no density effects) deep water basin Friction-dominated (plane) turbulent jet; basin morphology Shallow basin Strong bed friction Model 1: Fully Turbulent Jet Flow patterns Fully Turbulent Jet 2

3 Model 2: Plane Turbulent Jet Mouth bar development Inertia dominated system Mouth Bar development frictiondominated system Example Wolga Delta Caspian Sea Buoyant jets and plumes Stratification and entrainment in river mouth Relatively fresh water in saline basin; salt wedge in estuary; High outflow velocities - inertia Stratified conditions Development of jets and buoyant plumes and fronts ROFI s: Regions of Fresh Water Influence 3

4 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 4

5 Solo Delta, Java, Indonesia Growth pattern Solo Delta, East Java Mouth bar development in river mouth Plume observations and modelling Indonesian delta systems Salinity and temperature profiles Flow field and concentration of suspended matter (SPM) Analytical models outflow and deposition 3D numerical modelling 5

6 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 Mahakam Delta

7 Mahakam river plume in Strait Makassar h [m] h [m] h [m] Ba Lat delta, Red River Delta, Vietnam : 1: 14: 18: 6: 1: 14: 18: 6: 1: 14: 18: Salinity [PSU] SSC [mg/l] Temperature [deg] River discharge Discharge 36 m 3 /s Sediment: ton/year Wet season High discharge 8% sediment transport Dry season Low discharge Longshore flow Cross-shore flow Sediment concentration Salinity 7

8 Hyperpycnal jets and plumes Hyperpycnal plume Yellow River, China River water density larger than seawater density Large suspended load (>2. ppm) High-salinity values arid regions Gravity-driven underflow in basin (turbidity-current) Impact marine processes General influence: mixing of water masses, exchange of momentum and redistribution of sediment Tidal Processes Strong vertical and horizontal mixing Bi-directional transport Wave-driven processes Decrease in flow velocity Lateral expansion and refraction of outflow Increase in mixing (wave breaking) Longshore and cross-shore shore redistribution of sediment Tidal processes in river mouth; elongate mouth bars Wave-dominated processes river mouth Wave-dominated mouth bar morphology 8

9 Cyclic model for delta development, Ba Lat, Vietnam Tidal regime Gulf of Tonkin 4. Process-based classification of River Deltas (Based on Galloway, 1975) 9

10 Example 1: Rhône Delta, France Example 2: Ganges-Brahmaputra Delta, Bangladesh Example 3: Porong Delta, Java, Indonesia Example 4: Mississippi Delta, USA 5. Reworking and redistribution of deltaic deposits Channel shifting and migration Reworking waves and currents Post-depositional and subaqueous mass movements Example 5: Donau Delta 1

11 Channel migration Yellow River delta Channel shifting and migration - origin Upstream development of bifurcations Breaching levees > crevasses Advantage hydraulic gradient (slope) Formation of sediment (mud) plugs obstacles Delta lobes Mississippi Delta since 7 yrs. Cyclic processes Development of Isle Dernieres Louisiana, USA 11

12 Hurricane Andrew 1992 Reworking by waves and currents Wave-driven and tidal flow processes (ebb/flood/tidal asymmetry) Residual flow patterns (trade winds, density-driven driven flow) Nearshore zone Offshore delta front and prodelta 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) 12

13 Initial erosion (red) and deposition (blue) a) Effect of tidal asymmetry b) Effect of residual flow c) Comparison data Subaqueous mass movements Lack of consolidation sediment High amounts of silt and clay (pore pressure) High deposition rates Relatively steep slopes Triggers: waves, earthquakes, tsunamis Rotational slumps and slides Seismic records of slumps 13