Proceedings of Coastal Engineering, JSCE, Vol.3, 2012, November

MORPHOLOGICAL CHANGES ALONGBANGLADESH COAST DERIVED FROMSATELLITE IMAGES
Anwar Md. Shibly1and Satoshi Takewaka2
1

Dept. of System and Information Engineering, University of Tsukuba, shibly_98@yahoo.com
Dept. of System and Information Engineering, University of Tsukuba, takewaka@kz.tsukuba.ac.jp

2Being a low laying deltaic country, the morphological changes of the coastal zones of Bangladesh
become a major social, economic and environmental concern. In this study, images of Landsat satellite
during 1989-2010 are analyzed to detect the variation of shoreline positions. The western-central coasthas been divided into different segments and changes have been observed in the south face. Shorelinechange rates have been analyzed for two period 1989-2000 and 2000-2010. Every segment has been
facing more changes in the previous decade 1989-2000 as compared to the recent decade 2000-2010.
Segments covered with mangrove are facing more changes compared to flat sandy beach which iscontradicting with general consensus that mangrove stabilize the land.
Key Words: Shoreline, Bangladesh Coast, Erosion, Accretion, Satellite Image.

1. INTRODUCTION

2. STUDY AREA

Being a low laying deltaic country, themorphology of Bangladesh coastal zone is quite
unstable and changing time to time due to erosion
and accretion. The coast is known as a zone ofvulnerabilities besides its distinctive importance fornatural resources and ecosystems. Natural threats,such as erosion, water logging, increase in waterand soil salinity, risks from climate change like sea-level rise, cyclone etc., have adversely affected themorphology of coastal zone and slowed down thepace of social and economic developments in thisregion (CZPo, 2005; Barua et al., 2010). Thechanging features of the coastal zones ofBangladesh become a major social, economic andenvironmental concern.
The objective of the study is i) to detect theshoreline position from satellite images, ii) toprovide a regional overview of coastal accretionand erosion along the western-central Bangladesh
coast over two decade, specially focus on the areas
with mangrove forest, iii) to assess whether
mangrove dominated shoreline has different
behavior from others, and iv) to identify therelationship between the morphological changes,
geomorphological as well as oceanographicalfactors.

The southernmost part of Bangladesh is borderedby about 710 km long coast line of Bay of Bengal
which covers 19 districts out of 64 districts of thecountry (CZPo, 2005). Bangladesh coastal zone canbe divided into three regions namely western,central and eastern coastal region. Figure 1 shows(a) main river system, and (b) the study area,
western-central coastal zone of Bangladesh. To
understand and discuss local situation, study area
has been divided into different segments as A, B, C,Fig.1 (a) Bangladesh map showing main rivers system.
‫ڧ‬Wave hind cast data location.ܼ
ܼ Barrage.

- 41 -Table 1 Satellite data and tide level at the images
acquisition date measured at Hiron point station.

Fig.1 (b) Western coastal zone of Bangladesh and Segment
A, B, C, D, F and G. Base image: Landsat 4, Band1, 2 and 3, 1989/1/12.‫ۼ‬‫ۼ‬Tide station.

D, E, F and G. The dark portions in Fig.1 (b)represent the land covered with mangrove forest
and bright portion denotes agricultural land or bareland without mangrove forest.
Soil characteristics of the western coastal zone
are silty loams or alluvium (Islam, 2003). Tworivers named Sibsha and Pasur diverted fromGanges flow through this area and falling into Bayof Bengal. The area lies at 0.9 to 2.1 m above meansea level (Iftekhar & Islam, 2004). Mangrove
supports feeding and breeding grounds for fish andshrimps species, enriching the area in bio-diversity.
Segments A, B, C and D are located in this region
and covered with mangrove forest popularly known
as Sundarban based on the name of Sundari trees.
The central zone receives a large volume ofdischarge from the Ganges-Bhrahmputra-Meghna(GBM) river system, and the land is formed fromsilty deposition (Allison et al., 2003). Because of
the sediment supply and strong river current, themorphology of the zone is very dynamic anderosion and accretion rates in the area are very high.
Segments E, F and G located at border ofwestern-central region. Kuakata (G) is an attractive
sandy beach located in this region and famous fortourism. The Baleswar River is falling into Bay ofBengal at western side of the segment G and theeastern side of the segment is dominated by theGBM system.
Eastern coastal zone are dominated bysubmerged sands and mudflats (Islam, 2001) andhas a long sandy beach of 145 km extending from
Cox’s Bazar to Tecknaf. Except the Eastern zone,
all parts of the coastal zones are plain land withextensive river networks and accreted land.

Sl.
No.

Satellite Name

Acquisition
DateTide level(mm)1Landsat 4

TM

1989/1/12-780

2Landsat 5

TM

1993/12/17-340

3

Landsat 5

TM

1995/11/21

N/A4Landsat 7 ETM+

1999/11/8

1100

5Landsat 7 ETM+

2000/11/26

850

6Landsat 7 ETM+

2001/11/29

1080

7Landsat 7 ETM+

2002/12/18

530

8Landsat 7 ETM+

2003/11/19-240

9Landsat 7 ETM+

2004/12/7-570

10Landsat 7 ETM+

2006/12/29-1030

11Landsat 7 ETM+

2007/11/30

620

12Landsat 7 ETM+

2008/10/31

790

13Landsat 7 ETM+

2009/12/21-20

14Landsat 7 ETM+

2010/12/24

530

Fig.2 Shoreline positions of segment F and G in 1989
and 2010.

3. DATA PROCESSING
The mid and near infrared spectral bands ofsatellite images have strong reflectance by soil andvegetation and absorbance by water, which makepossible to separate the land from water and can beused to detect the shoreline position (Kuleli, 2010).
Alesheikh, et al. (2007) showed that the mid
infrared band 5 of Landsat TM and ETM+ issuitable for extracting the land-water interface. In
this study, 14 images from the Landsat USGS
archive (http://glovis.usgs.gov/) have been analyzed
to detect the variation of shoreline positions asshown in Table 1. Most of the images have beenacquired in dry season, November and December.
Hourly tide level data from 1989-2010 has beencollected from BIWTA (Bangladesh Inland Water
Transport Authority) for nearby tide station Hiron- 42 -Point. Tide levels at the image acquisition times areshown in Table 1 and it shows that the tide level
difference is approximately 2 m.
In the first step, geo-registration was done toadjust the geographic position of the images. Thenedge detection was applied to the distribution ofpixels intensities of band 5 to locate the shoreline
position. At each segment, shorelines have been
detected in the south face of the land at a spacing of30 m alongshore from west to east. Finally to refine
the result of detected shoreline position, manual
inspection has been carried out to clear theerroneous data.
Figure 2 shows the detected shorelines positionof segment F and G in the observation year 1989
and 2010. The diagram shows that at most of theplaces, the shoreline position retreated landward
except at a small portion of eastern part of 90 º12`E
where the shoreline moved seaward.
Shoreline changes have been detected alongtransection line in N-S direction. Rate of shorelinevariation is estimated by linear regression ofshoreline positions at every transect. Figure 3
shows an example of shoreline position variation atFig.3 Variation of shoreline position at (a) T-106, Segment F, and (b) T-100, segment G. Negative position means landwards
position with respect to position of 1989.

Fig.4 Shoreline positions of 1989 and 2010. Band 5 of 2010/12/24 acquisition is displayed as base image. Over view of 1989
and 2010 images are shown in bottom right panel. Oblique strips of the upper images are due to the Landsat 7malfunction.

- 43 -two locations of segment F and G with the rate ofchanges. Vertical axis shows relative position of theshoreline with respect to the shoreline position of1989. Figure 3 shows that the erosion rate has been
reduced after 2000 compared to the previous years.
4. RESULTS
4.1 Shoreline Changes
Figure 4 shows shoreline positions for 1989 and2010 of different segments and band 5 overview
images in the right bottom panel.
Shoreline of segment A, B, C and D, covered
with mangrove, are affected by erosion that pushed
the shoreline back from its early position. Theshoreline of zones Ac1, Ac2 and Ac3 areexperiencing no changes of shoreline during theperiod of 1989-2010. One island Is2 appears ineastern front of segment A in 2010. Land areas ofisland Is1 in the western front side of segment A
and island Is3 at the eastern front of segment D
increased by approximately 30%.
Shorelines of segment E and F, having small
mangrove portion in the tip, are continuously
retreating landward. Segment E is experiencing
largest changes of its shoreline positions comparedto other segments. Segment G, sandy beach, is also
losing land except the portion Ac4 at eastern sidewhich gained land.
From Fig.4, it can concluded that most of theshoreline positions in 1989 were more seaward
compared to those of in 2010.

4.2 Shoreline Change Rate
Figure 5 shows the distribution of the shorelinechange rate for two periods: 1989-2000 and2000-2010. Among the two periods, former period
was experiencing higher change rate of shoreline
positions. It is approximately three times larger thanthe latter period. Average rate of changes for twodecade has been tabulated in the bottom row of theFig.5.
Most of the area was eroded, except someportions (Ac1, Ac2, Ac3, and Ac4) that areexperiencing accretion in the recent decade
2000-2010. The average rate of change over thewhole period of segment covered with mangrove (A,B, C, D) is -18 m/yr whereas the flat sandy beach(G) is -9 m/yr. This result is contradicting with thegeneral consensus that mangrove forest is
stabilizing the beach and protecting shore erosion.
Maximum average change rate -72 m/yr is observed
at segment E where the tip of the land is also
covered with mangrove. Segments E and F arecontinuously losing land in both decades and noaccumulations have been observed in these twosegments.

5. DISCUSSION
It has been observed from Fig.5 the mangrove
portions A, B, C, and D are facing more changes
compared to sandy open beach, segment G. Tworivers Sibsha and Pasur flow through this area andfalling into Bay of Bengal, which receive water

Fig.5 Distribution of shoreline change rates of the two periods: 1989-2000 and 2000-2010. Negative rate corresponds toerosion. The average rate of changes of the segments is shown in the bottom row.

- 44 -

from Ganges. The reduction of Ganges water afterthe construction of Farakka Barrage in India at
1975 in the upstream, the fresh water discharge
through Pasur and Sibsha has been decreased and at
the same time salinity increased dramatically atwestern coastal zones (Islam and Gnauck, 2008).
This may impact coastal ecosystems and reductionof sediment supply by the rivers. The possible
effects of increased salinity on the ecosystem of theSundarban are the dying of tops of Sundari trees,
retrogression of forest types, slow forest growth,and reduced productivity of forest sites (MPO1986). These may lead to coastal retreat andweakening of mangrove forest.
Wave hind cast data from 1989-2010 have been
analyzed to find out the effect of wave action overerosion and accretion along the coast. Hind castwave data, significant wave height, wave period,
and wave direction have been taken from theECMWF ERA-Interim archive (http://dataportal.ecmwf.int /data/d/interim_daily/). The ERA
wave dataset provides data for 00h, 06h, 12h and
18h UTC each day on a 1.5º by 1.5º longitude-latitude grid, covering the whole globe.
Figure 6 shows significant wave height and wavedirection for the period 1989-2010 at a grid 90º E,
21º N, which is the closest data to segment E, F and
G (See Fig.1(a)). The 90 degree wave indicates thewaves coming from the eastern and 180 degreefrom the southern. Incident waves from thesouth-south-west are dominating along the coast.
The prevailing waves may yield sediment transport
from west to east which supports the fact thatshoreline change rate of the west side of every
segment is facing larger changes compared to theeast side. Further, the accumulation zones Ac1 andAc2 are in the western face of the segments A and C,
and are in the down-wave of the prevailing waves
sheltered behind island Is1 and segment B,
respectively.
The comparison of wave height for the two
decade didn’t show any significant difference. So it
cannot be said that only wave action is responsible
for higher rate of erosion in the period 1989-2000.

6. CONCLUSION
Satellite remote sensing data have been analyzed
to identify the morphological changes of western-central coastal zone of Bangladesh. The regional
overview of shoreline changes rate analysis over thepast decade shows that erosion was the dominant in
the whole area. In recent decade, at some portion ofshorelines, accretion has been observed. Shorelinechange rate at mangrove portion (Segment A, B, Cand D) was larger compared to sandy open beachFig.6 Wave height and wave direction at (90º E, 21º N).
Solid line represents center of gravity of thedistribution.

Kuakata (Segment G). Also shoreline change rate of
every segment is higher in the western face rather
than eastern faces. The roles of river discharge andsouth-south-west waves have been discussed toinfer the mechanism of shoreline dynamics of thearea.

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(Received June 15, 2012)- 45 -