Sep 26, 2014 - Relative Sea Level Rise (RSLR) in the Bengal Delta: 1. Allison and Kepple, 2001 (1â4 mm/y); Stanley and Hait, 2000. (â¤5 mm/y). 2. Allison et ...
9/26/2014
Assessing the rate of subsidence in the Bengal Delta Maminul Haque Sarker1, Jakia Akter1, Md. Munsur Rahman2, Robert Nicholls3 and Steve Darby3 1Center
for Environmental and Geographic Information Services (CEGIS), Bangladesh 2Bangladesh University of Engineering and Technology (BUET), Bangladesh 3University of Southampton, UK
There are several studies, researches and opinions on the rate of Relative Sea Level Rise (RSLR) in the Bengal Delta: 1. Allison and Kepple, 2001 (1–4 mm/y); Stanley and Hait, 2000 (≤5 mm/y) 2. Allison et al., 2003; Brammer, 2014 (1.3−7.1 mm/y) 3. Higgins et. al., 2014; (0 to >10 mm/ y, Dhaka) 4. Mikhailov and Dotsenko (2004) – 10-20 mm/y 5. Ericson et al. (2006) – 25 mm/y 6. Syvitski et al. (2009) – 0.8 to 18 mm/y) 7. Patheick (2012) – 20 mm/y 8. DU + Colorado University, US, Subsidence ~ 10 m/y RSLR ~ Global Sea Level Rise (GSLR) + Subsidence Subsidence compaction, Isostacy, tectonics
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Implication • The results of such research have long-term implications for Bangladesh • It has become crucial for local people, local investors, decision makers and also the development partners in making decision • To find out whether the inhabitants will be encouraged to migrate to other places from the disaster prone delta area or invest huge money to rescue the delta.
Rakas lake
Manasarowar
Geo-morphological settings
China
Nepal
Bhutan
India Every year these rivers carry: -one trillion m3 of water -one billion tones of sediment
Bangladesh
More sediment makes the rivers and esturies more dynamic
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Delta development process
The Ganges and the Jamuna entered through three corridors 1. Rajmahal Hill and Barind Tract
Barind Tract
Madhupur Tract Rajmahal Hill
2. Barind and Madhupur Tracts 3. Madhupur and Maghalaya Hills
Thickness of sedimentation during the last 11,000 years
Based on Goodbred and Khuel (2000)
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Topography
Terrain cross-profile
Tidal plain are very much flat. There are depressions between floodplain and tidal plain. Gopalganj-Khulna depressions did not receive sediment either by upstream rivers or by the tidal rivers from downstream.
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Regional Sea Level Rise (RSLR) RSLR = GSLR + SUBN + M Sea Level Rise
Subsidence
RSLR
=
Regional Sea Level Rise
GSLR
=
Global Sea Level Rise
SUBN
=
Subsidence due to Natural Compaction
M
=
Crustal Vertical Movement
In this presentation I will present the different methods that could be applied to assess the RSLR 1. Changes in river courses and landforms 2. Carbon dating data 3. Changes in tidal water level 4. Archaeological monuments We have presented an approach to assess the Relative Sea Level Rise using the Archaeological monuments
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High rate of subsidence in any area or region alters the landform and river courses We investigated these using historical maps and recent satellite images
Major James Rennels surveyed in the 1760s
Back swamp
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Locations of back swamps and main course of the river in the southwest did not change significantly over the last 250 years.
Locations of back swamps and course of the rivers in the southwest did not change in particular direction over the last 250 years, which negates a high rate of subsidence locally or regionally.
No significant change in landforms and river courses during last 240 years indicates no major disturbances
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Carbon dating data also may indicate the rate of RSLR/ Subsidence
Carbon dating
10
Depth (m)
8
6 4 2 0
0
2000
4000
6000
Age (yr) Based on Allison et al (2003)
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RSLR (cm/yr)
Carbon dating 0,8 0,7 0,6 0,5 0,4 0,3 0,2 0,1 0 0
2000
4000
6000
Age (yr)
For recent data, carbon dating generally shows much uncertainties (Stanley, 2001)
Based on Allison et al (2003)
Ground water mining due to high rate of extraction may cause very high rate of subsidence
We analyzed more than 100 ground water level data
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Level (mPWD)
3
Bagerhat districet BHO001
2
BHO002 BHO003 BHO004
1
BHO005 BHO006 BHO007
0 1970
1975
1980
1985
1990
1995
2000
2005
Rate of change in GWL
60 50 40 Count
Ground water mining is very negligible, indicating very insignificant role for high subsidence
30 20 10 0
