Accordingto Departmentof Environmental Protection State of Florida,A wetland is a land area thatis saturated with water, either permanently or seasonally, such that it takeson the characteristics of a distinct ecosystem 1. According to Khan et al, 2wetland holds water for a significant duration sufficient to support organismadapted to life in inundated or saturated soil condition and consists of widevariety of types ranging from lakes, rivers and coastal forest to deep-waterpaddy fields and ponds. According to American Societyof Civil Engineers (ASCE), a retention basin is used to manage storm water runoff toprevent flooding and downstream erosion, and improve water quality in an adjacentriver, stream and lake or bay 3Anycity is surrounded by different types of wetlands. These wetlands aresubdivided into several categories. Two categories of wetland has been consideredin this study: 1)Open water body comprises marshy and peaty inundated (during significant partof the year) low-lying areas of the Balu floodplains.2)Inland water body includes the lakes, ponds and connecting canals of differentwater bodies in the city area.
3.2. Data and Software Used:Inorder to compare the wetland change of the Eastern Dhaka, data were collected bydigitizing the Google Earth and analyzing Landsat TM image.
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Google Earth is a virtualglobe, map and geographical information program that wasoriginally called EarthViewer 3D created by Keyhole, Inc. and now owned byGoogle Inc. It maps the Earth bythe superimposition of images obtained from satelliteimagery, aerial photography and geographic informationsystem (GIS) onto a 3D globe. Most land areas are covered insatellite imagery with a resolution of about 15 m per pixel in Google Earth.Projected coordinate system of the Google Earth data is UTM (Universal TransverseMercator).For this study four years (2004, 2008, 2012 and 2016) Google EarthPro data was used. Data was acquired as KML file from Google Earth which was convertedinto Bangladesh Transverse Mercator (BTM) projection system using ArcGISsoftware.
The Landsat program isthe longest-running enterprise for acquisition of satelliteimagery of Earth. LandsatThematic Mapper (TM) was sent on the space March 1, 1984 and wasactive until June 5, 2013 4 .Images consist of seven spectral bands with aspatial resolution of 30 meters. The Landsat TM image was collected from theLandsat Look Viewer sponsored by United States Geological Survey (USGS) 5.Inthis study one images of October 22, 1996 was downloaded in TIFF (Tagged ImageFormat File) format.
Projected coordinate systems of this image was WGS84(World Geodetic System 84) which was converted into Bangladesh TransverseMercator (BTM) projection system using ArcGIS software.Thestudy has been carried out under the framework of Geographic Information System(GIS). The Image Processing tasks have been carried out. GIS analysis has beencarried out using Arc Map 10.2.2 software (ESRI, Redlands, California, USA).Excelwas used to analysis vector data. Image processing software Picasa was used toprocess the image and analysis the data.
Table 1: Acquisition of data and sourcelist3.3. Data Processing and Analysis:Dataprocessing and analysis scheme used for the study is illustrated using a flowchart as in the following figure: Figure: Flow chart for Analysis of Landsat Image.TheLandsat TM image was collected in .tiff (Tagged Image Format File) whichprovides geo-referenced image in WGS84 Projected Coordinate System. Two imagesfrom 22 October 1996 was used to cover study area. The geo-referenced twophotos were mosaic to obtain the image of the whole study area. Then thematicdata layers were generated from the geo-referenced images using on-screendigitization technique .
Wetlands were identified and redrawn using editor toolsin ArcMap. Polygon option was used as we needed area data for comparison. Datalayers were generated in ArcInfo vector format using the digitization tools ofArc Map 10.2.2. In case of Google Earth Data Retrieval, wetlands were digitizedon Google earth as polygon and imported as .KML file.
Which was converted intolayer using conversion tool of ArcMap. Then it was exported as Shape file (.shp).Resolutionof Google Earth images were very fine that no mosaic of images needed.
Thenvector analysis was run as before. Area was measured in square meters which isdefault in BTM Projected Coordinate System. Figure:Flow chart of acquiring data from Google Earth. References: 1. Department ofEnvironmental Protection State of Florida www.
dep.state.fl.us/evergladesforever/about/glossary.htm# 2.M. S. Khan, E.
Huq, A. A.Rahman, S. Huq, S. M. A.Rashid, and H. Ahmed, etal.
, “Wetlands of Bangladesh,”Bangladesh Centre forAdvanced Studies (BCAS)in association with NatureConservation Movement (NACOM), Dhaka, Bangladesh, 1994. 3. American Society of CivilEngineers ASCE Manual and Report onEngineering Practice No. 87. 1998. ISBN 1-57278-039-8. Chapter 5. 4.
N.M. “The LANDSATTutorial Workbook: Basics of Satellite Remote Sensing”NASA Reference Publication1078. NASA.
Retrieved 20 September 2011. 5.landsatlook.usgs.
gov/viewer.html Study Area:The Eastern Dhaka is our study area which is located between longitude90?24.78′ E & 90?29.37′ E and latitudebetween 23?54.98’N & 23?42.62’N.
EasternDhaka about 55,410,395 square meters of retention ponds were found which hasnow at 2016 came down to about 3,819,937 square meters; which is about 7% ofthe earlier. A total of 51,590,458 square meters of wetlands are lost in thistime . The physical geography of Bangladesh isvaried and has an area characterized by two distinctive features: a broad deltaic plain subject to frequent flooding, and a small hilly region crossed by swiftly flowing rivers.Two major river Turag andBuriganga are flowing across the hole city . Balur river , Tongi khal ,Joydevpur , Savar areas are beside the Eastern Dhaka.
This part of Dhaka is highly densed and this causes high ranged of infrastructuredevelopment that is not well planned. That unplanned structure decrease the wetland in drasticalrate which is so much dangerous for entire Dhaka not only for the eastern part. Itis observed that some 60% of the eastern Dhaka regularly goes under water everyyear between June and October due to lack of flood protection. Within our totalstudy region (1996-2016), Bangladesh faces floods in almost every year. But themajor flood s occurs in 1998 and 2004.
In this continuation, 2008 also facedflood which lasted for July- November. The flood water took a huge time to getaway from the low lands. But the other side’s condition is so much scaring. Ittook