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Resilient deltasResilient deltas a case-study report on the Volgaa case-study report on the Volga
and Ganges delta systemsand Ganges delta systems
Dr. Irina Overeem 1,2)
1) CSDMS, INSTAAR, University of Colorado, CO, Boulder, USA
2) Delft University of Technology, The Netherlands
Thanks to co-authors: S.B.Kroonenberg, T.Veldkamp, and S. Goodbred.
OutlineOutline
Introduction: extreme rates of sea-level?Introduction: extreme rates of sea-level?
Case-study: Volga delta over last centuryCase-study: Volga delta over last century
Case-study: Ganges delta at Early HoloceneCase-study: Ganges delta at Early Holocene
Conceptual framework: correlated change in sea-Conceptual framework: correlated change in sea-
level and sediment supply.level and sediment supply.
What are extreme sea-level changes?What are extreme sea-level changes?
Observed 20Observed 20thth century sea-level change century sea-level change 1.7 mm/yr.m/yr.
IPCC forecastsIPCC forecasts1)1) for global sea-level change rates for global sea-level change ratesuntil 2099 vary from ~3-4 mm/yr depending onuntil 2099 vary from ~3-4 mm/yr depending onscenarios.scenarios.
Volga delta response to last century sea-levelVolga delta response to last century sea-levelchange (100 mm/yr)change (100 mm/yr)
Ganges delta response to Early Holocene sea-levelGanges delta response to Early Holocene sea-levelchange (20 mm/yr)change (20 mm/yr)
1) Meehl, et al, 2007
Volga river drains 1,5 millionkm2 and has wide fluvial–dominated delta
250 km
Volga river and Caspian SeaVolga river and Caspian Sea
Caspian Sea level isCaspian Sea level iscontrolled by Volgacontrolled by Volgadischarge, which in turndischarge, which in turndepends on precipitation.depends on precipitation.ArpeArpe et al.,( 2000) showed et al.,( 2000) showedcorrelation with ENSO.correlation with ENSO.
Caspian Sea wentthrough a 3-m sea-levelcycle over the lastcentury.
(Source: Mahachkala gauge)
Delta progradation over last century
(After Alekseevskiy, Aibulatov,Chistov, 1999), based oncombination of old maps andsurface topography.
Sea-level fall - low sediment supplySea-level fall - low sediment supply
A2
E14E17 DamchiklakeA15
E3
E5
1935 1951 1981 1989
just after highstand regressionprogradation just afterlowstand transgression,aggradation
Damchiklake Damchiklake
4 km
• Relative stable coast 1850–1909 ‘highstand’
• Rapid progradation 1909-1927 due toemergence, s.l. fall 0.6 m
• Slow progradation 1927-1938, despiteadditional 1.2m s.l. fall because of reducedsediment supply.
• Rapid progradation 1935-1951, emergence and skeletonof channels
• 1951-1981 lowstand, channelnetwork fills.
• 1981- 1990 coastline is stabledespite 1,5 m sea level rise.
AquaTellUs numerical model
Fin (x)
Fero (x)
Fdepo (x) Fout (x)
stroomafwaarts
x x + 1x - 1
Fin (0)
erosion
sedimentation
Erosive flux depends onslope (S) and riverdischarge (Q)
Sedimentations flux dependson concentration (F) andwater velocity (u).DOWNSTREAM
Model simulation resultsModel simulation results
Drowning of delta brings muds on top of sands
(Overeem et al., 2003)
Discontinuoussedimentation,large depocentershifts
Ganges delta
Indian monsoon dominates GangesIndian monsoon dominates Ganges
0
10000
20000
30000
40000
50000
60000
1 2 3 4 5 6 7 8 9 10 11 12
Q i
n m
3/s
ec
Ganges River (Farakka station 1949-1973)(Source: BBC weather, 2005)
30
31
32
33
34
35
0 2 4 6 8 10 12 14
time in ka
se
a s
urf
ac
e s
ali
nit
y
(pp
m)
LongtermLongterm Indian Monsoon Indian Monsoon
Indian monsoon proxy: salinity in the Bengal basin (e.g.Kudrass et al., 2001). Indicates that paleo river dischargeswere higher.
11 – 7 ka BP, exceptionally high Q
Sediment supply high at Early HoloceneSediment supply high at Early Holocene
Volume/Time of sediment stored in GangesVolume/Time of sediment stored in Ganges
delta sequence tracks monsoonal recorddelta sequence tracks monsoonal record
Isopach map of Ganges-Brahmaputrasediments deposited in Bengal basinsince 11 kBP.
Sediment volume is 8.5 1012 m3, nearly60% of which was stored from 11-7kBP. Implies 2.2 times higher flux atEarly Holocene!
(Goodbred et al., 1999, 2000)
SedfluxSedflux StratigraphicStratigraphic Model Model
3DSedFlux
INPUT(t)
sea level(t), bathymetry (t-0)
Q, Qs, Qb (HydroTrend)
PROCESSES
River: avulsion, floodplain SR
Marine: delta plume, storms
Basin: compaction, subsidence
OUTPUT (x,z,t)
- 3D-geometry
- grain size, age
Details and Equations: Three-dimensional modeling of deltasOvereem et al., 2005. SEPM Spec Publ.
NEWEST Version: Hutton, Syvitski (in press 2007), Computers and Geosciences.
Present-day Monsoon Conditions Increased Monsoon Conditions
Brahmaputra Brahmaputra
Ganges Ganges
SedFlux simulation resultsSedFlux simulation results
500 year experiments (SLR = 2 m per 100yrs)
X-section at
30 km
distance 80 km
SedFlux simulations of rapid sea-levelSedFlux simulations of rapid sea-levelrise for a merging GB systemrise for a merging GB system
distance 80km
-25
-20
-15
-10
0 50 100 150 200 250
simulation time
sea level
Sea-level rise 2 m/100 years
Present-day Monsoon Conditions
SedFlux simulations of rapid sea-levelSedFlux simulations of rapid sea-levelrise for a merged GB systemrise for a merged GB system
Y-section at 20 km; away from last depocenter
Y-section at 50km;
centered through last
depocenter
SedFlux simulations of rapid sea-levelSedFlux simulations of rapid sea-levelrise for a merging GB systemrise for a merging GB system
Present-day Monsoon Conditions
Different Subsidence Rates
‘low subsidence’
4 mm/year uniformly over entire grid
‘high subsidence’
7.5 mm/year locally in ‘graben’
distance 80 km
distance 80 km
-0.008
-0.006
-0.004
-0.002
0
su
bsid
en
ce in
mm
/year
uniform subsidence
graben subsidence
Concept: correlated controlsConcept: correlated controls
Implication: deltasystem is more resilientto rising sea-level,because it is able tobuild rapidly.
Implication: change isamplified, i.e. deltasystem is rapidlyprograding due toemergence.
DiscussionDiscussion
Interacting forces can be explored with numericalInteracting forces can be explored with numerical
modeling. Several processes/modules need furthermodeling. Several processes/modules need further
research, notably supply mechanisms, and floodplainresearch, notably supply mechanisms, and floodplain
sedimentation.sedimentation.
Deltas that have high sediment supply rates may beDeltas that have high sediment supply rates may be
more resilient to sea-level change.more resilient to sea-level change.
A worst-case scenario?