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International Archives of the Photogrammetry, Remote Sensing and Spatial Information Sciences, Volume XXXIX-B8, 2012
XXII ISPRS Congress, 25 August — 01 September 2012, Melbourne, Australia
TIME-SERIES ANALYSIS OF COASTAL EROSION IN THE SUNDARBANS
MANGROVE
M. Mahmudur Rahman
Bangladesh Space Research & Remote Sensing Organization (SPARRSO), Agargaon, Sher-E-Bangla Nagar, Dhaka-
1207, Bangladesh - Mahmud_Rahman 2861 @yahoo.com, mahmud@sparrso.gov.bd
Commission VIII, WG VIII/7
KEY WORDS: Mangroves, Sundarbans, Erosion, Climate Change
ABSTRACT:
Mangrove forests are fragile coastal ecosystems and could be one of the most vulnerable ecosystems to global climate change and
sea-level rise. These forests are formed in the fringe of land and ocean and characterized by the regular inundation of tidal water.
Because of the changes in sea-level and dynamic energy system in the transition zone between land and sea due to climate change,
erosion in different coastal zones of the world could be accelerated. The objective of this study is to find out the nature and pattern of
erosion that can threaten mangrove forest ecosystems. The study area is located in Sundarbans mangrove, the largest continuous
mangrove forest in the world. The study utilized time-series data of Landsat Multi-spectral Scanner (MSS), Thematic Mapper (TM)
and Enhanced Thematic Mapper Plus (ETM+) during 1970s to 2010s. Time-series change analysis was done along the selected
transect lines. The erosion rates in the Sundarbans Mangrove are variable and it is very difficult to get a conclusive result from the
analysis of those points whether the erosion rate has been accelerated in the recent past. The average rates of erosion for the eastern
and western parts are 14 m/year and 15 m/year respectively obtained form the ten selected transect lines. It is unclear that how much
coastal erosion is linked to the global warming and sea-level rise or whether any other associated factors such as geological and
anthropogenic induced land subsidence, changes in sediment supply or other local factors are driving these changes. Further studies
should be conducted in different mangrove ecosystems of the world to explore whether similar patterns of coastal erosion are visible
there.
1. BACKGROUND
The threat of global climate change is the consequence of
industrial activities that includes accelerating rate of CO,
emission. The experimental results of Manua Loa has reported
a rapid increase in the mixing ratio of CO, concentration in the
atmosphere rising from 315 ppm in 1960 to around 390 ppm in
2010 (Tans 2010). Since the industrial revolutions, CO,
concentrations in our atmosphere increased in faster rate, and
are now 30% greater than pre-industrial time (Reay and Grace
2007). The rapid increase in CO, emissions observed during
the last 250 years is assumed to continue for several
forthcoming decades and likely to create many adverse impacts
in our living system.
The consequences of climate change are in various dimensions
like changes in arctic temperatures and ice, widespread changes
in precipitation amounts, ocean salinity, wind patterns and
aspects of extreme weather including droughts, heavy
precipitation, heat waves and the intensity of tropical cyclones,
shoreline changes and inundation of low-lying areas (Adapted
from IPCC 2007). Sea-level has been rising 1.7-1.8 mm/year
over the last century and the rate has increased to 3 mm/year in
the last decade (Church et al. 2004, Holgate and Woodworth
2004, Church and White 2006, Bindoff et al. 2007) though
there is an absence of more accurate regional scenarios (i.e. for
Bay of Bengal). Sea level rise over the last 100-150 years is
probably contributing to coastal erosion in many places of the
world (Rosenzweig et al. 2007).
Mangrove vegetation is threatened due to coastal erosion,
declined freshwater flow from the hinterland and the increased
level of salinity in many deltas. Mangrove ecosystem is
characterized by the regular inundation of tidal water. The
fresh-water flow from the inland and the tidal waves from the
sea play a crucial role in the formation and the dynamics of this
fragile ecosystem. This forest is the most vulnerable to global
climate change and sea-level rise in addition to other
anthropogenic and natural causes.
Sea-level rise is currently measured at 3.24 mm per year
(UNEP 2004) that can threaten to unbalance the Sundarbans
mangrove forest water forces. Although the change in waterline
is very small, but changes in a dynamic energy system can be a
severe threat, which already bring periodic catastrophes in the
region. Various studies have already found that Sundarbans
forest has been lost 85 km? over 20 years and 200 km? of land
over 70 years (UNEP 2004). The objective of this study is to
quantify the nature and pattern of coastal erosion in the
Sundarbans mangrove, the largest continuous mangrove forest
in the world.
2. MATERIALS AND METHODS
2.1 Study Area and Data-sets
Sundarbans forest is located in the southern extremity of the
Ganges River Delta, extended about 80 km north of the Bay of
Bengal and bounded by the Baleshar River on the east and by
the Hoogly River on the west. Approximately two-thirds of the
forests lie in Bangladesh, the remaining one-third in India.
The ground within the forest is extremely flat, and elevation
more than 1.5 m above mean high tide level is seldom noticed.
The predominant tree species in the forest are Sundri (Heritiera
fomes) and Gewa (Excoecaria agallocha). Other species
include Passur (Xylocarpus moluccensis), Kankra (Bruguiera
