Transcript C2 Delta Morphology.ppt

Morphology of Deltas
James P.M. Syvitski, CSDMS Executive Director, syvitski@colorado.edu
A deltas’s morphodynamics is controlled by boundary conditions and forcing
factors: (1) sediment supply; (2) accommodation space; (3) river & coastal energy;
and (4) density differences between effluent and receiving waters.
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Database:
A=drainage area; R=basin relief; Qav=discharge; Y=sediment yield;
L=river length; Dgrd=delta gradient; CN=no. of distributary channels;
Ti=spring (K1+M2) tidal range; Wa=maximum monthly wave height;
T=basin temperature; P=precipitation; Rcf=Qav/A=runoff;
Qmx=monthly maximum discharge; Qs+b=total sediment discharge;
Cs=suspended sediment concentration; Qs=suspended load;
Qb=bedload; AD=delta area;, Cw=width of distributary channels;
Rw=bank-full river width; Dsh=depth reached by subaqueous delta;
Vs=volume of sediment delivery per thousand years;
Pr=11QDgrd =proxy of river power;
Pm=Wa2+Ti2 =proxy of marine power;
=Qs+b/Pm=ratio of sediment supply to marine dispersal;
Vs/AD Dsh =ratio of sediment supply to sediment retention;
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Seasonal satellite images were processed—LANDSAT7:30m MODIS:250m, SPOT:10m,
airphotos:1m, IKONOS:1m. Pre and post dam data were used. Discharge data came from the
WMO Global Runoff Data Center (GRDC), or the GRDC-Water Balance Model (Syvitski et
al., 2005), or government archives. Sediment load data came from long-term national surveys
(Syvitski et al., 2005) and from a modified Bagnold equation for bedload:
rgQavb Dgrd eb
Qb =
rs - r
tan f
rs
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when u ³ ucr
AD=0.0056Qav1.28Qs+b0.41
CN=0.038Qmx0.75Pm-0.6
Dgrd=0.00360.3Cs0.5Qav-0.33
TCW=Qmx1.1e0.3Tie-0.77Wa
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Distributary channel widths form variants on lognormal
distributions: Brownian or Gaussian statistics would be
reasonable for use in numerical models
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Based on just 25 deltas, the size of bed-material load (Dmm) is a
simple power-law relationship of a river's length (L):
Dmm = 106 L-0.9
Downstream fining is a well-established relationship in river
hydrology (van Niekerk et al., 1992; Vogel et al., 1992). Fine-grained
(muddy) deltas (e.g. Amazon, Mekong, Orinoco) are thus feed by
long rivers. Chemical vs physical weathering dominance may also
play an important role.
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Sediment supply to sediment retention ratio
Vs/ADDSh
•6Vs is the sediment volume (km3) delivered to the delta in
6kyr, a time when global sea level stabilized and deltas began to
form (e.g. ART model: Syvitski et al., 2005, Science)
•Dsh is water depth (m) reached by the submerged delta
determined from shelf profiles and provides a measure of the
accommodation space;
•AD is the delta area (km).
•25 deltas  and are considered in equilibrium
•18 deltas  and either had a larger Qs+b in part of its 6Kyr
(paraglacial: Lena; or wetter: Nile), or have an effective
dispersal system (Amazon, Ganges, Yangtze)
•10 deltas  and either had a smaller Qs+b in part of its
6Kyr (prior to human disturbance: Waipaoa; or drier: Eel), or
had more than one delta (Yellow), or had natural dams
(Colorado, TX).
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River Dominated
Type Delta:
Low Marine Energy
1 km
Volga: Pm/Pr=0.2, Qmx=22,400 m3/s, Qmx/Q=2.7, CN=100, =0.64,
TCW:RW=1.7. Leads to feathering of distributary channels
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Mississippi
River
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Mississippi Delta (pre-1960):
Pm/Pr=0.1, Qmx=30,500 m3/s,
Qmx/Q=2.7, CN=71, =31
TCW:RW=10,
Growth by a hierarchy of channel
splitting & abandonment
Type Delta:
Low Marine Energy
High Sediment Supply
1 km
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Lena Delta: polar 17°C, Pm/Pr=0.2,
Qm=74,400m3/s,
Qmx/Q=4.6, CN=115,
=0.3, TCW:RW=17,
Growth by mouth bars
and overflow channels
Type Delta:
Low Marine Energy
High Discharge Variability
1 km
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Type Delta:
High Sediment Supply
Low Discharge Variability
Orinoco Delta: tropical-wet, Pm/Pr=0.4, Qmx= 58,800 m3/s, Qmx/Q=1.7,
CN=27, =1.1, TCW:RW=16. Drainage channels coalesce into tidal estuaries
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River
Tigris-Euphrates Delta
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Tide Dominated
Tides
Fly Delta
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Type Delta:
High Marine Energy (Tides)
Low Discharge Variability
Fly Delta: tropical-wet,
Pm/Pr=2.5,
Qmx= 3,500 m3/s,
Qmx/Q=1.4, CN=5, =0.1,
TCW:RW=37. Growth by tidal
current divergence
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Tides
Yangtze (Chang Jiang
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Irrawaddy Myanmar
Tides
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Colorado US/Mexico
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Amazon
River Tides
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Indus
Tides
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Wave Dominated
Type Delta:
No Tides
Low Discharge Variability
Danube Delta: Pm/Pr=0.4, Qmx= 8900 m3/s, Qmx/Q=1.4,
CN=9, =1, TCW:RW=4. Controlled by river & longshore
transport
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Wave
Nile
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Wave - River
Vistula River Delta
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Wave
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Godavari
Wave
Orange River
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Brazos, Tx
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Wave
Danube
Wave
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Krishna Delta
Wave
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Wave
Rhone
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Brahmani Delta
Wave
Mahanadi Delta
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Wave - Tides
Limpopo
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Mixed Energy
or Complex
Ganges/Brahmaputra
India
Pakistan
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Tides River
Wave River
Mekong
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Type Delta:
Balanced Energy
(River vs.Tide vs. Wave)
High Discharge Variability
Copper Delta: proglacial, Pm/Pr=0.6, Qmx= 5,400 m3/s,
Qmx/Q=4.4, CN=5, =0.1, TCW:RW=8. Growth by river-tidal-wave influence
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Type Delta:
Balanced Energy
(River vs. Tide)
Niger Delta: tropical-wet, Pm/Pr=1.2, Qmx= 12,000 m3/s, Qmx/Q=2, CN=15,
=0.1 TCW:RW=8.5. Drainage channels coalesce into tidal estuaries
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Niger Delta
Wave TIdes
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Geology
Congo (Zaire)
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Geology
Pearl (Xijiang)
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Red
Wave
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Small River Delta
&
Bayhead Deltas
Eel Delta: high tides (3m) & waves (3.5m), Qmx= 750 m3/s,
Qmx/Q=3.2, CN=1. Controlled by longshore transport
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Wave
Klamuth
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Colorado TX: low tides
(0.7m), no waves in
lagoon,
1.5m waves onto barrier,
Qmx= 326 m3/s,
Qmx/Q=2.3, =0.02.
Lagoon filling delta
Pm/Pr<0.1
Pm/Pr=2.8
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the supplyto-dispersal
ratio assigns
small deltas to
the dispersal
end of the
spectrum
Paraná
River - Geology
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Homathko
Klinaklini
River - Tides
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Wave - River
Waipaoa
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Wave
Eel
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Arno
River-Wave
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Chao Phraya
Tides
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Uplifting Deltas
Sea Level Fall Deltas
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Forced regression, clinoform emerges, local depocenters are filled in.
Rapid local fluvial incision, razor-blade effect in zone of highest uplift
Grain size: yellow = sand, blue = clay
SedFlux simulation
Age
5
Distance (km)
30
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10m
Fjords are one of the only coastal basins that host clinoforms that develop
during falling sea level. Fjords offer a high preservation potential, given
their basin characteristics, and provide a modern-day opportunity to study
basin fill during a falling sea level.
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Turbidity currents go through an
period of acceleration marked by
seafloor erosion.
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Periglacial Deltas
Wave
Yukon Delta
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Ice
Yana River Delta
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Ice
Mackenzie Delta
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Indigirka
Ice
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River - Tide
Pechora
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Ice
Kolyma
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River - Ice
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Conclusion:
• Only a few environmental factors are defining in delta
morphodynamics (L, Qav, Qmx, Qs+b,  Cs, Ti, Wa, Pm, Pr) and can
be used to understand/predict AD, CN, Dgrd, & TCW.
•
The simple ternary delta/estuary diagram is inadequate to capture
the true range in morphology, and suffers from scaling: all small
deltas except those protected by bays or inlets, cannot compete
with the typical power of marine waves, longshore currents, or
tidal currents: & giant deltas are poorly represented.
•
Type deltas include those influenced by low marine energy,
polar/desert deltas, temperate rainforest & tropical deltas, deltas
dominated by wave and/or tidal energy
•
Human engineering controls the growth and evolution of a
growing number of deltas.
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