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?