Deltas and Environmental PerspectiveDeltas and Environmental Perspective James P.M. Syvitski...
Transcript of Deltas and Environmental PerspectiveDeltas and Environmental Perspective James P.M. Syvitski...
-
Deltas and Environmental PerspectiveJames P.M. Syvitski
Environmental Computation and Imaging Facility, INSTAAR, CU-Boulder
Dynamics and Vulnerability of River Delta Systems, Boulder CO, 2007
With the great help of A. Kettner, I. Overeem, E. Hutton, Y. Saito
-
Defining a delta’s area:
1) the seawardprograding landarea that hasaccumulated since 6kyr, when global sealevel stabilizedwithin a few m ofthe present level(Amorosi and Miller,2001).
Problem:i) Relative sea level
height is globallyvariable (i.e. SL didnot stabilizeeverywhere at thesame time);
ii) shifting riverchannels maydisconnect deltaiclobes.
4300-2000 yr BP
2000-1000 yr BP
1000-150 yr BP
Since 1855
Dynamics and Vulnerability of River Delta Systems, Boulder CO, 2007
-
Defining a delta’s area:2) the seaward area of a river valley
after the main stem of a riversplits into distributary channels(Syvitski and Saito, 2007);
Problem: The split (or hinge) point canchange over time
Dynamics and Vulnerability of River Delta Systems, Boulder CO, 2007
-
Defining a delta’s area:3) the area of a river valley underlain
by Holocene marine sediments (Kuboet al., 2006);
Problem: While likely the most exactdefinition, this is a very difficultassessment, involving the collectionand dating of boreholes (cores), andgeophysical profiling over anexpansive area; &/or time consumingnumerical modeling.
Dynamics and Vulnerability of River Delta Systems, Boulder CO, 2007
-
Defining a delta’s area:4) accumulated river sediment that has
variably been subjected to fluvial, waveand tidal influences (Overeem et al.,2005);
Dynamics and Vulnerability of River Delta Systems, Boulder CO, 2007
Problem:No time is specified. Is it Holocene
or Pleistocene, or is it in the lastfew hundred years?
-
Defining a delta’s area:
5) the area drained by riverdistributary channels that are underthe influence of tides.
Problem: Is it saltwater intrusion? Or isit tidal damming?
Dynamics and Vulnerability of River Delta Systems, Boulder CO, 2007
1200
km
-
Dynamics and Vulnerability of River Delta Systems, Boulder CO, 2007
Sediment delivery to a delta is a function ofdrainage basin characteristics:1) Basin area2) Precipitation/runoff/water use: river discharge3) Basin relief4) Basin temperature5) Basin lithology6) Extent of glaciers7) Human landuse: de- / re-forestation, mining,agricultural practices, urbanization, channelhardening, use of stop-banks8) Level of impoundments: number, placement andsize of reservoirs
-
Dynamics and Vulnerability of River Delta Systems, Boulder CO, 2007
Relative Sea level for Deltas
Natural RSL rates = Eustatic Rate + Isostatic Rate + Compaction = (3.0 mm/y) + (0.3 to 3.8 mm/y) + (0.7 to 2.2 mm/y) = 4.0 to 7.8 mm/y
Accelerated compaction = petroleum & groundwater mininge.g. Po in the 1950’s: >60 mm/y — down to 20 mm/y after controls
Yangtze: 28 mm/y — down to 3 mm/y after controlsNiger (today): 25 to 125 mm/yChao Phraya (today): 50 - 100 mm/y
CAPUTO et al.,
1970
1958-1962
60 mm/y
40 mm/y
BONDESAN &
SIMEONI, 19831967-1974
20 mm/y
10 mm/y
-
End-Member Deltas
Dynamics and Vulnerability of River Delta Systems, Boulder CO, 2007
Low Marine Energy: Pm:Pr 0.2
Volga: channel feathering
Mississippi: channel splitting
-
End-Member Deltas
Dynamics and Vulnerability of River Delta Systems, Boulder CO, 2007
Polar
Lena: sea ice, permafrost,thermokarst, short seasonaldischarge (Qmx:Qav > 3)
Tropical
Niger: intense convective rainfall
Orinoco: intense rainfall leads to runoff channelsthat influence the pathway and connectivity ofthe distributary channels
-
End-Member Deltas
Dynamics and Vulnerability of River Delta Systems, Boulder CO, 2007
Desert
Nile: Low Runoff (Q:A 2.6; widemouth to river width TCw:Rw = 36
High WaveEel: high Pm:Pr ; few river mouths