Download Sarker et al, Assessing the rate of subsidence in the Bengal Delta

Assessing the rate of subsidence in the Bengal Delta
Maminul Haque Sarker1, Jakia Akter1, Md. Munsur Rahman2, Robert Nicholls3 and Steve Darby3
1
Center for Environmental and Geographic Information Services (CEGIS), Bangladesh
Bangladesh University of Engineering and Technology (BUET), Bangladesh
3
University of Southampton, UK
2
High rate of Relative Sea Level Rise (RSLR) is a common phenomenon in the deltas, due to compaction of
deltaic deposits, tectonics and isostasy. During the last couple of years, several researches and studies
have been carried out, for assessing the RSLR of the Bengal Delta. According to those researches the rate
of RSLR in this delta varies from 10 to 25 mmy-1 where subsidence is the main contributor. The results
coming out from those researches indicate a frightening future of the Bengal Delta, which may have
pronounced implications on the local people, decision makers and developing partners to invest for any
types of development or adaptive works against climate change. This article presents the findings of the
research which has assessed the rate of subsidence of this delta through adapting different approaches.
We tried to check the historical changes in landform or river courses during the last 250 years to assess
the existence of any high rate of local or regional scale of subsidence. We analyzed the ground water level
changes in the tidal plains due to extraction and assessed the contribution of it in subsidence. We also
analyzed the tidal water level data for assessing decade scale rate of subsidence and the carbon dating
data from secondary sources to assess the subsidence in millennium scale. We developed a methodology
for assessing subsidence in century-scale using archeological monuments in the tidal plains.
Digital Elevation Model based on surveys in 1950s and tidal gauge data were used to relate the average
level of tidal plains with the tidal variations, when tidal movement was free to enter into the tidal plains.
Moreover, relation between the level of tidal plains, homestead platforms and plinths are established
using the spot level surveys data. Archaeological monuments in the tidal plains, other than active delta
area, were used for assessing the subsidence. Based on the analyses, it appears that the rate of subsidence
in a particular location can fairly be estimated from the difference in levels between plinth of the
monuments and recently constructed structures dividing the age of the archaeological monument.
We did not find any significant amount of ground water mining in the tidal plains, different analyses show
that the rate of subsidence in different time-scale vary from 1 to 3 mmy-1, which is far below the recently
published rate of subsidence in the Bangladesh tidal plains.
Key words: Bangladesh delta, subsidence, relative sea level rise, archeological monument, tidal plain,
carbon dating, ground water level.