Flood is a common feature in rapidly urbanizing Dhaka city and its surrounding areas. In this research, evaluation of flood risk of Greater Dhaka in Bangladesh has been developed by using an integrated approach of GIS and remote sensing. The objective of the study is to measure the flooding risk based on the satellite data and geomorphological land classification map under the land use/land cover change from 1995 to 2015 related with the urbanization of Dhaka city. Comparing with each landform, land cover unit and historical rainfall data the flooding return period has been calculated. Terrace, natural levee and back swamp has been divided into three sub categories. Especially the built-up zone which is closer to the river channel, former river course and the back swamps are mostly vulnerable to flood inundation. This study revealed that, 70% of Greater Dhaka district within moderate to very high hazard zone, especially surrounding city like Manikganj Sadar Upazila areas. It is expected that, this study could contribute to effective flood forecasting, relief and emergency management for future flood event.
Because of unique geographic location, Bangladesh is one of the most disaster prone country in the world [
Dhaka is the largest and densely populated area in Bangladesh. Dhaka has become one of the fastest-growing cities in the world, primarily driven by explosive population growth. The city’s population was 0.41 million in 1951 and 0.71 million in 1961. By 1974, it had risen to 2.06 million, averaging an annual growth rate of 11.15% between 1961 and 1974 [
Due to specific geographical location of Dhaka city, flood is the common natural hazard [
the people. In total, the 1988 flood affected 4.55 million people [
There are several researches and maps on flood in Bangladesh has been done before at the local and government level. Ashraf M. Dewan (2007) described about estimating flood hazard in Greater Dhaka district zone by using remote sensing and GIS techniques, which essential traces the flood hazard management strategies in greater Dhaka city [
M. Masood (2012) explained about the vulnerability and risk of mid-eastern Dhaka by using DEM and 1D hydrodynamic model, presented the flood risk and vulnerability from DEM data of mid-eastern part of Dhaka [
From the above discussion it is very clear to understand that, no research has been done before in Greater Dhaka district zone regarding to the evaluation of flooding risk on the basis of geomorphological land classification map. Considering this situation, the objective of the study is to evaluation of flooding riskbased on the satellite data and geomorphological land classification map under the land use/land cover change from 1995 to 2015. Specifically, the purpose of the study is to evaluation of flooding risk using satellite data and geomorphological land classification map concerning with the big flooding event (1988, 1998 and 2004) and with urbanization of greater Dhaka district zone.
The study area chosen for this research is the greater Dhaka district of Bangladesh (
The data for generating flood risk map of the study area has been collected from. The U.S. Army Topographic sheet (Scale 1:250,000) of 1955 [
Photographic elements and field knowledge was utilized to delineate various land use/land cover categories such as agriculture land use, built up area, water surface, bare land and vegetation cover. Satellite data was interpreted using photographic and geotechnical elements besides field knowledge about the study area. For GIS and remote sensing data analysis, a time-series of Landsat Thematic Mapper (TM) and Enhanced Thematic Mapper Plus (ETM+) images were used to derive land use/cover maps of the study area were used (
Area under each category of land use/land cover was calculated and computed in the area (km2) as well as in percentage. A comparative analysis of the land use/land cover maps was attempted to find out the changes during 1995-2015 period, by superimposing the two maps. The maps were then overlaid on DEM to know the correlation between elevation and land use/land cover (
Satellite Name | Path and Row | Bands | Date of Acquisition | Special Resolution |
---|---|---|---|---|
Landsat-ETM+ | 137 and 44 | 4, 3, 2 | 24th Nov, 1995 | 30 meters |
Landsat-TM | 137 and 44 | 4, 3, 2 | 12th Oct, 2015 | 30 meters |
No. | Categories of Data | Period | Publish Place | |
1 | Digital elevation model (30 m resolution) | October 26, 2011 | USGS | |
2 | Flood historical data | 2005 | Bangladesh Meteorological Department, 2005 | |
3 | Topographic map | 1955 | U.S. Army Map Service |
Cross section profile (
Unplanned urban expansion is one of the most important factors intensifying flood hazards in Greater Dhaka district zone. From the period of 1995 to 2015 (
Land use/land cover categories | Land use/land cover (1995) | Land use/land cover (2015) | Difference (1995-2015) | Elevation range (in meters) | |||
---|---|---|---|---|---|---|---|
Area (km2) | % | Area (km2) | % | Area (km2) | % | ||
Agriculture land use | 1121 | 43 | 895 | 34 | −226 | −9 | <3 |
Vegetation cover | 308 | 12 | 198 | 8 | −110 | −4 | 3-5 |
Water body | 352 | 14 | 239 | 9 | −113 | −5 | <1.5 |
Built up area | 716 | 27 | 1213 | 47 | 497 | 20 | 1 - 6 |
Bare land | 108 | 4 | 59 | 2 | −49 | −2 | 1 - 4 |
Total land | 2605 | 100 | 2605 | 100 |
land use (−9%) is found in the study area because most of the built up zone has been developed around the capital city Dhaka and at the same time the transportation network has been developed too.
The built up zone is composed of residential land use, commercial land use and industrial land use. In 1995, the built up zone was 27% and expanded to 47% in 2015, the significant change had occurred because of convert agricultural infrastructure into urban infrastructure in the urban fringe zone. Commercial and industrial land use changes have been observed also with the growth of the area. The accelerated industrialization and urbanization following economic reforms and population increases have greatly affected land cover change through the increase of built up areas. The net bare land area had decreased 2% because with the increase of population, the demand of food had increased too. As a result, the bare has been converted to both agriculture land use and urban land use. The vegetation cover was 12% in 1995 and dropped to 8% in 2015 .The decrease of vegetation cover was a result of the construction of residential, commercial and industrial zone to promote urban development.
The remarkable change has been occurred in the urban areas (increased 20%), where residences are developed because of expansion of the urban area around the Dhaka city, are extremely vulnerable to flooding. A superimposed image of Digital Elevation Model (DEM) and land cover map was prepared (
By comparing land use/land cover change maps, geomorphological land classification map with DEM data following results are obtained.
1) From 1995 to 2015 major land use/land cover changes in agricultural land occurred in low lying areas where is elevation ranging below 3 m. Increase in built up areas is due to shrinkage of agriculture land use and mostly transformation into residential and commercial activity. Moreover this area is located into the low lying floodplain zone.
2) Changes in built up area (20%) have occurred in almost all elevation range between 1 m and 6 m. Agricultural land use associated with high elevation range converted into bare land, and at the same time low elevated agricultural land is converted to build up zone to meet the demand for housing to accommodate growth in population.
3) Vegetation cover has been increased substantially in low elevation range between 2 and 4 m, at the periphery of dried up water body and almost all low lying floodplain areas.
Flooding due to rainfall is also a severe problem for certain city areas that may be inundated for several days, mainly due to drainage congestion. The main reason for the 1998 flood was excessive rainfall over the catchments area of the Ganges-Brahmaputra Meghna (GBM) river basin [
amount of precipitation during flooding year (1988, 1998 and 2004) were considerably higher than average rainfall. For example, the rainfall was 250 mm percent higher in 1988 compare to its normal condition. Thus, the runoff generated by rainfall could not flow out to the surrounding rivers since the water level of the river stage was also at peak. The accumulated runoff in low lying areas pushed long inundation and remained stagnant until the water level of the river stage receded. During 1988, 1998 and 2004 flood events, rainfall statistics shows that in June, rainfall was tremendously longer, for other three monsoon months it was terribly larger in 1998 event. For example, 552 mm rainfall was recorded for the month of August in 1998 which was 176 mm bigger than normal rainfall, meanwhile in 1988 it was only 169 mm which was even less than the normal.
Flood Return Period Calculation from Yearly Precipitation DataIn this study, to calculate the return period of floods from yearly precipitation data the Hazen method has been applied [
This procedure was done using the above equation, for the sample size of 62 years, by assigning ranges in ascending order, the precipitations, and the probabilities of occurrences and return periods for each year. These calculations are shown in
where:
Fa = Probability of occurrence (%)
n = Rank of each event
y = Total number of events
100/Fa = Return period
Year | Annual Precipitation (mm) | Rank | Probability (Fa) | Return Period | Year | Annual Precipitation (mm) | Rank | Probability (Fa) | Return Period |
---|---|---|---|---|---|---|---|---|---|
1984 | 252.3 | 1 | 0.8 | 126 | 1963 | 164.3 | 33 | 51.6 | 2 |
2007 | 240.4 | 2 | 2.4 | 42 | 1953 | 161.2 | 34 | 53.2 | 2 |
1991 | 237.5 | 3 | 4.0 | 25 | 2009 | 160.9 | 35 | 54.8 | 2 |
1993 | 230.6 | 4 | 5.6 | 18 | 1997 | 160.1 | 36 | 56.3 | 2 |
2005 | 219.8 | 5 | 7.1 | 14 | 2006 | 159.9 | 37 | 57.9 | 2 |
1986 | 208.3 | 6 | 8.7 | 11 | 1968 | 158.3 | 38 | 59.5 | 2 |
1988 | 206.8 | 7 | 10.3 | 10 | 1974 | 157.9 | 39 | 61.1 | 2 |
1973 | 205.3 | 8 | 11.9 | 8 | 1966 | 155.4 | 40 | 62.7 | 2 |
1959 | 204.4 | 9 | 13.5 | 7 | 1981 | 155.4 | 41 | 64.3 | 2 |
1971 | 201.3 | 10 | 15.1 | 7 | 1977 | 155.1 | 42 | 65.9 | 2 |
1983 | 199 | 11 | 16.7 | 6 | 1979 | 153.1 | 43 | 67.5 | 1 |
2008 | 198.8 | 12 | 18.3 | 5 | 1960 | 152.8 | 44 | 69.0 | 1 |
1999 | 197.8 | 13 | 19.8 | 5 | 1972 | 150.7 | 45 | 70.6 | 1 |
2004 | 195.6 | 14 | 21.4 | 5 | 1982 | 150.4 | 46 | 72.2 | 1 |
1964 | 194.3 | 15 | 23.0 | 4 | 2002 | 149.1 | 47 | 73.8 | 1 |
1998 | 192.7 | 16 | 24.6 | 4 | 1962 | 148.8 | 48 | 75.4 | 1 |
1954 | 189.5 | 17 | 26.2 | 4 | 2011 | 148.0 | 49 | 77.0 | 1 |
1978 | 187.6 | 18 | 27.8 | 4 | 1995 | 145.9 | 50 | 78.6 | 1 |
1976 | 186.5 | 19 | 29.4 | 3 | 2003 | 141.1 | 51 | 80.2 | 1 |
1980 | 184.8 | 20 | 31.0 | 3 | 2001 | 140.4 | 52 | 81.7 | 1 |
1956 | 183.4 | 21 | 32.5 | 3 | 2014 | 140.2 | 53 | 83.3 | 1 |
2000 | 182.8 | 22 | 34.1 | 3 | 1989 | 135.6 | 54 | 84.9 | 1 |
1987 | 182.3 | 23 | 35.7 | 3 | 2013 | 133.8 | 55 | 86.5 | 1 |
2015 | 181.0 | 24 | 37.3 | 3 | 1955 | 132.5 | 56 | 88.1 | 1 |
1961 | 180.8 | 25 | 38.9 | 3 | 1957 | 129.5 | 57 | 89.7 | 1 |
---|---|---|---|---|---|---|---|---|---|
1975 | 178.8 | 26 | 40.5 | 2 | 1969 | 128.3 | 58 | 91.3 | 1 |
1965 | 176.4 | 27 | 42.1 | 2 | 1994 | 128.3 | 59 | 92.9 | 1 |
1990 | 175.3 | 28 | 43.7 | 2 | 2010 | 126.9 | 60 | 94.4 | 1 |
1967 | 171.1 | 29 | 45.2 | 2 | 2012 | 110.7 | 61 | 96.0 | 1 |
1985 | 171.1 | 30 | 46.8 | 2 | 1958 | 104.8 | 62 | 97.6 | 1 |
1996 | 170.3 | 31 | 48.4 | 2 | 1992 | 96.6 | 63 | 99.2 | 1 |
1970 | 166.3 | 32 | 50.0 | 2 |
By using Hazen plotting position, the above graph (
After the liberation war in 1971, the urban area has been expanding rapidly in the study area. In recent decades, the development is more rapid then the previous. In recent years, rapid urbanization is mainly taken place in low lying areas around and within the city which serve as back swamp and flood plain zone and submerged during flooding season. Every year because of monsoon rainfall, Greater Dhaka district zone has been facing a serious drainage congestion which is one of the important factor to flood problems in Dhaka city. Due to the unplanned development of Dhaka city and filling of natural channels, it becomes very difficult for the artificial system to carry out vast amount of flood waters to the surrounding river.
In
course, 2) River bed, 3) Sand spit, 4) Back swamp (High, Medium and lower), 5) Terrace (High, Medium and lower), 6) Natural levee (High, Medium and lower), 7) Flood plain, 8) Marshy land and 9) Point bar. The criterion of the landform classification has been done according to the difference of altitude from the river.
Terrace has been developed in the upper eastern part of the study area. According to the geological evolution of Bangladesh, terraces were formed in the Quaternary Period (Pleistocene Epoch). The northern part of Dhaka city is located in the terrace zone. Based on the elevation range, terrace has been divided into three types such as higher, middle and lower. The higher terrace has an elevation higher than 5.5 m, are mostly located at the upper part of the study area and covered with commercial activity purpose. The middle one has an elevation between 4 m and 5.5 m. Lower terrace has an elevation between 2.5 m and 4.0 m located in the lower part of Dhaka city are never influenced by the normal flooding condition. These zones are also not influenced by the river flood condition. Within the Dhaka city due to poor drainage condition rainfall flood had occurred in the big rainfall event.
The natural levee has been developed around river courses due to the deposition process during monsoon period and especially in the tremendous flooding year (e.g. 1988, 1998 and 2004). In the study area, according to the elevation the natural levee has been divided into three type’s such as higher, middle and lower. The higher natural levee has an elevation between 5 m and 6 m, are mostly located at the upper part of the study area and covered with human settlements and commercial activity purpose. The middle one has an elevation between 4 m and 5 m works as a natural embankment during normal flood but submerged during extraordinary flood condition. Lower natural levee has an elevation between 2.5 and 4 m located in the lower part of the study area are submerged at the normal flooding condition.
Back swamps are also divided into three types as following; higher, middle and lower back swamps. Back swamps are located between the natural levees in the Manikganj Sadar zone. In the tremendous flooding year (e.g. 1988, 1998 and 2004), the back swamp has been long inundated. At the higher natural levee zone, the depth of inundation is higher compare with the lower natural levee zone. Severe flooding damages have occurred in this zone. The period of inundation is more than three months. The higher back swamp has an elevation between 1.5 m and 2 m, are mostly located at the upper part of the study area around the higher natural levee zone and which is covered with human settlements and commercial activity purpose. The middle one has an elevation between 0.5 m and 1.5 m and lower back swamp has an elevation of below 0.5 m located in the southern part of the study area are submerged at the normal flooding condition. At the higher back swamp zone, flood return period is much longer comparatively to the other moderate back swamp and lower back swamp zone.
The former river channel is usually channel without water bed. The former river course are located in the flood plain zone and especially in Buriganga River and Kaliganga river area and flooded in normal flood condition, which is flooded almost every year in the normal flood and rainfall condition.
In this paper to evaluate the risk of flood during 1995 to 2015 in Greater Dhaka district of Bangladesh, land-cover, elevation data, topographic map and geomorphic unit were overlaid on each other. The study demonstrates an effective way to modify the collected DEM so that it represents the current topography, which is very helpful to identify the various land cover and land form units. The objective of geomorphological land classification map is to provide information related to flood inundation risk on the basis of various landform units.
To find out the relationship between land cover and land form unit we have compared each other and the results of this paper are as follows:
Urban development of the Dhaka city and its surroundings was quite rapid during 1995 to 2015. The Urban areas have spread into lowland area such as flood plains and back swamps from 1995 to 2015. This is clearly reflected in the relationship between the urbanization area and the landforms. The results of this research revealed the relationship between land use/land cover change and the geomorphological changes indicate that the built-up areas have expanded on vulnerable landforms with respect to floods. Moreover annual rainfall is another important factor which is closely related to the flood return period regarding to different geomorphologic land form units.
From the topographic map and the land cover map, there are a higher amount of settlement and built-up zones are located in the low lying high hazard zones. Moreover the number of settlements and commercial activities are increasing in the recent decades (20%) over the low lying agriculture land, which putting an extra pressure not only on Dhaka city [
From the geomorphologic land classification map, the northern part of Dhaka city is located in the terrace zone. Based on the elevation range, terrace has been divided into three types such as higher, middle and lower. The higher terrace has an elevation higher than 5.5 m, mostly located at the upper part of the study area and covered with commercial activity purpose. The natural levee lies between 2 m and 5 m. It has been divided into 3 types too. Sometimes it works as a natural embankment during normal flood but submerges during extraordinary flood condition. Lower natural levee has an elevation between 2.5 and 4.0 m located in the lower part of the study area submerged at the normal flooding condition. Back swamp has been divided into 3 types: the higher back swamp has an elevation between 1.5 m and 2 m, are mostly located in the upper part of the study area around the higher natural levee zone and covered with human settlements and commercial activity purpose. The middle one has an elevation between 0.5 m and 1.5 m and lower back swamp has an elevation of below 0.5 m located in the southern part of the study area submerged at the normal flooding condition. In the back swamp zone, the period of inundation is more than three months. At the higher back swamp zone, flood return period is much longer comparatively to the other moderate back swamp and lower back swamp zone.
Geomorphological land form unit represents the current scenario of the study area. The map provides helpful information about flood risk zone and should be useful in assigning priority for the development of higher flood risk areas. Furthermore, this type of study will provide the updated information about geomorphic land form which is related to flood protection measure such as construction and development of infrastructure and preparedness for future flood event.
We are grateful to acknowledge the support of Bangladesh Bureau of Statistics (BBS), Dhaka University Library, Local Government Engineering Department (LGED), Survey of Bangladesh (SoB), Bangladesh Water Development Board (BWDB) and Bangladesh Meteorological Department (BMD) for providing us valuable materials and necessary assistance. We are also very much thankful to the city corporation authorities of both Manikgonj and Dhaka city for their cooperation in this research. We sincerely remember their assistance and valuable suggestions in conducting this research.
Sayed, M.B. and Haruyama, S. (2016) Evaluation of Flooding Risk in Greater Dhaka District Using Satellite Data and Geomorphological Land Classification Map. Journal of Geoscience and Environment Protection, 4, 110-127. http://dx.doi.org/10.4236/gep.2016.49009