GIS Analysis of the Detailed Area Plan and Eastern Bypass in Dhaka City using Digital Elevation Model Data and Natural Drainage Network
Md. Khalequzzaman, Ph.D., Dept. of Geology & Physics, Lock Haven University, Lock Haven, PA 17745, U.S.A.,
Frequency and magnitude of major flooding have significantly increased over the last few decades in Bangladesh in general and in Dhaka City in particular. Analysis of the historic records of the 1998 flood reveals that areas within Dhaka Metropolitan Development Plan (DMDP) that have elevations less than 10 m were flooded. The flooded area accounted for about 60% of the total area (1528 sq.km). In order to protect the city from flooding and water-logging, the Government of Bangladesh (GOB) has built an embankment along much of the western boundary of the city on the left bank of the Turag/Buriganga River. As a part of flood control measures for the Dhaka City, the GOB is actively considering to build a 30 km-long embankment, known as Eastern Bypass Road Multipurpose Project, along the right bank of the Balu River, extending from Tongi Bridge to Demra, to protect the eastern half of the city.
This study analyzed the digital elevation models (DEM) using ArcGIS software to assess the feasibility of the proposed Eastern Bypass Project on hydrologic, environmental, and ecological grounds. Calculations of flood-flow that will emanate from major rainfall events following the construction of embankments indicate that there will be a net increase in the water-level of the Balu River at Demra by as much as 36 cm, which will create a barrier to discharge of surface run-off through canals in the embankment area. Thus, continuous pumping of water accumulated within the embankments will be required. According to the plan, several retention ponds and regulators will be created to store additional run-off, and three pump stations to pump out standing waters. Hydrologic analysis shows that the proposed retention ponds and pump stations will not be adequate to handle surface run-off that will be generated by a single rainfall that is typical of the areas during the Monsoon season. Based on GIS analysis using DEM, this author argues that flood risk and water-logging can be mitigated through implementation of best management practices (BMPs) in urbanization that will include identification of natural flow accumulation and water-logging areas and by facilitating natural functioning of drainage network and by incorporating basin development factor. In order to implement the detailed area plan proposed in the DMDP, it is imperative that all abandoned channels and flood-flow zones be recovered and set aside for designated land use.
The frequency and magnitude of major flooding have significantly increased over the last few decades in Bangladesh in general and in Dhaka City in particular (Islam, 1999; Islam, 2005; Shahjanhan, 1998; Elahi, 1992). For example, all floods inundating more than 30% of the country (total area of Bangladesh is 144,000 km2) occurred after 1974. Four floods that exceed this threshold occurred in 1974, 1987, 1988, 1998, and 2004, all in the last 30 years, averaging one in every 6 years. It is imminent that the government of Bangladesh seeks options for flood risk and damage reduction. The situation in Dhaka City is much worse in the sense that any major rainfall causes serious water-logging. In order to protect the city from flooding and water-logging, the GOB has built an embankment along much of the western boundary of the city on the left bank of the Turag/Buriganga River. Since the eastern parts of the city are not “protected” by any such embankment, the GOB has decided to build an embankment along the right bank of the Balu River, which is known as Eastern Bypass Road Multipurpose Project . The proposed project is designed to protect the city from floodwater of the Balu River covering Tongi Bridge to Demra road (Figure 1). The importance of the proposed Eastern Bypass and a plan for its implementation are outlined in the detailed area plan (DAP) of the DMDP. Total area of the region affected by the project is 124 sq.km. A 30 km flood protection dam will be built from Tongi railway bridge to Kanchpur Bridge under the project, that is expected to cost an estimated 24,760 million Taka (approximately, $358 million at 2006 rate). Sluice-gates and drainage system will cost 10,000 million Taka, embankment 8,000 million Taka, and the bypass road 7,000 million Taka (The Daily Star, October 4, 2004). Before we analyze the feasibility of this proposed flood control embankment cum eastern bypass road multi-purpose project, it is important to analyze the underlying causes of increased flooding in Dhaka City and elsewhere in Bangladesh.
Figure 1: Map of the proposed Dhaka Metropolitan Development Plan showing the location of western embankment (solid line), proposed Eastern Bypass Road Multipurpose Project (dotted line), pump stations (hexagons with dot). The dark areas have surface elevations that are less than 10 m and were flooded during the flood of 1998. This map is adapted from RAJUK (2010) report and Halcrow (2005).
Major Causes of Increased Floods in Dhaka and Elsewhere:
• Siltation of Riverbeds
– Deforestation in the catchment areas of a river causes surface run-off and sediment loads in rivers to increase, which in turn fill up the rivers and reduces water carrying capacity during floods.
– Encroachment of rivers also reduces the size of the rivers and their carrying capacity.
• Inadequate sedimentation on floodplain
– If flood water does not spread over an active flood plain, such as the Balu River floodplain, then the elevation of floodplain starts to decline over time, allowing more flooding depth to occur.
• Compaction and subsidence of land
– Overdraft of groundwater is responsible for gradual compaction and subsidence of floodplain areas, such as the case in Dhaka City over the last few decades. If this trend continues, then Dhaka City’s elevation will be reduced over time, allowing more flood depths.
• Change in Land use in Watershed Areas
– Unplanned urbanization (horizontal development of houses, building of roads and parking lots) to accommodate growing population increases impervious surface in a watershed, resulting in additional surface run-off to nearby rivers and canals, increased flooding, and less infiltration through the soil.
• Climate Change
– Global warming is now an accepted phenomenon by the scientific community, which is changing the climate dynamics.
Sea level rise in the Bay of Bengal has been rising for the last several thousand years; however, the rate of sea level rise has accelerated during the last few decades at an average rate of 7 mm/year, which is higher the world average of about 3-4 mm/year for the same time period (Khalequzzaman, 1994).
• Human Interference in Natural Processes
– Embankments, polders, barrages, occupying floodplains, impede the natural flow of flood water. This is the case for the western embankment in Dhaka City.
Other factors Affecting Flooding:
• Increase in rainfall
• For example, there was a total of 341 mm of rain in 24 hours on September 2004, which broke all records since 1953. Dhaka City received a total of 575 mm of continuous rain over a period of 5 days during the same time period.
• Decrease in water carrying capacity
• Siltation of riverbeds and encroachment by land grabbers fill up the rivers, which in turn reduces the carrying capacity of canal and rivers. This is certainly the case in and around Dhaka City, where about 21 historic canals are now abandoned.
A Critical Analysis of the Feasibility of Dhaka Integrated Flood Control Embankment cum Eastern Bypass Road Multipurpose Project:
The following comments are based on information provided in Updating/Upgrading the Feasibility of Dhaka Integrated Flood Control Embankment cum Eastern Bypass Road Multipurpose Project – Interim Report submitted by Halcrow Group Limited, UK, in November 2005 to Bangladesh Water Development Board. It should be mentioned here that, since the copy of the report is in black and while print, not all figures, pictures, and illustrations came out legible. In addition, information provided in this report is always not adequate to make sound analysis and judgment. Therefore, the following is a preliminary assessment of the proposal from geological, environmental, ecological, technical and socio-economic perspectives of the author. The views expressed in the following section are of the author only, and not of the institution with which he is affiliated.
Geological Merit of the Proposal:
The basic premise of the proposed Eastern Bypass Road Multipurpose Project is based on a cordon approach, which advocates flood control by keeping water from the river from reaching the flood plain through engineering means. The analysis of frequency, magnitude, and intensity of the major floods that occurred in Bangladesh over the last few decades clearly indicate that cordon approach is not a solution to the flooding problems. Despite building of 5,595 km of embankment and polders, and 1,695 flood regulating structures, both the extent and amount of damage caused by floods have increased in Bangladesh over the last 50 years (Khalequzzaman, 1994 and 2000). The cordon approach is also based on the paradigm that floods are public nuisance and need to be eliminated. The cordon approach is not based on a understanding of hydrodynamics of rivers and the environmental/ecological role that rivers play in creating floodplains and sustaining unique ecosystems in various fluvial environments. Flooding is a natural phenomenon, which have been occurring for the time immemorial. It is through repetitive flooding of floodplains that most of what is now Bangladesh was formed. Almost 80% of Bangladesh consists of floodplain and coastal plain. Elimination of floods from floodplain will lead to reduction of elevations of lands and fertility of soils. The natural processes should not be, or cannot be, “controlled”, rather humans need to learn to live in harmony with nature. The Wetland Conservation Act of 2000 mandates protection of flood flow zones. Should the Eastern Bypass Road Multipurpose Project be built, it will be a violation of this act.
The picture below (Figure 2) shows the right bank of Balu River, where the proposed Eastern Bypass Road Multipurpose Project will be implemented (The Daily Star, 2004). The area to be protected is located on active floodplain, wetland, and flood-flow zone as outlined in the DMDP.
Figure 2: Photograph showing the area on the right bank of Balu river where the proposed Eastern Bypass Road Multipurpose Project is planned.
Hydrologic Analysis of the Proposed Project:
The proposed project will not be able to eliminate floods from Dhaka City as proposed as we now argue. The plan calls for building embankments and a bypass road inside the embankment along the right bank of Balu River. It also calls for dividing the eastern 124 km2 of the city into three major compartments, each of which will be drained using a combination of existing/re-excavated canals, sluice gates, and pump stations. There will several retention ponds, totaling an area of 1895 hectors, to hold excess surface run-off during floods. The modeling of flow simulation following the embankments show that there will be a net increase in the river level in Balu River at Demra by as much as 36 cm if the right embankment is constructed. Apparently, the flow simulations are done using historic rainfall data. This increase in river height will create a barrier to discharges by the canals that are located with the embanked area, thereby requiring continuous pumping of water that will be logged behind the embankment. There will be three pump stations with maximum capacity of 54.6 cumecs (1927 cusecs) of water. On the one hand, maintenance cost of this embankment could be prohibitive for the generations to come; on the other hand, the people of Dhaka City will be left with this permanent burden of pumping water during rainy seasons. The effectiveness of the proposed pumps, retention ponds, and draining canals as a flood control measures are questionable at best.
This author calculated the amount of surface run-off, total volume of surface flow, required depths of retention ponds, time required to pump the surface run-off that will be generated from one major rainfall. For these calculations, the author used recorded rainfall amount of 341 mm during 24 hour period on September 15, 2004. The calculations were done for all nine sub-catchments with three compartments (DC 1-3) located in the eastern half of the city. These sub-catchment areas include Jamir Khal, Boali Khal, Begunbari Khal, and Dholai-Monda Khal. It was found out that 341 mm of rain is capable of generating a total of 54.8 million m3 of surface flow in these three compartments (Figure 3).
Figure 3: Graph showing volume of run-off generated by one rainfall event (341 mm in 24 hours on September 15, 2004) in different compartmental areas within the proposed Eastern Bypass Road Multipurpose Project.
This volume of water will require an increase in the height of the retention ponds (total surface area of 1895 hectors or 18.95 sq.km), ranging from 0.8 (2.9 feet) m to 2.94 m (about 9 feet), which will be practically impossible to attain during the rainy season, because ponds themselves will be full to the brink during such a torrential rainfall event (Figure 4).
Figure 4: Graph showing additional depths required in the retention ponds to accommodate the run-off generated by one rainfall event similar to the one on September 15, 2004. Note that two retention ponds (RP 5-1&2) that fall within the sub-catchment boundary of Boali Khal, and retention ponds (RP 7-2&3) that fall within the subcatchment of Dholai Khal-Monda Khal were combined in the above figure.
In addition, it will take a total of 280 hours (more than 11 days) to pump out this stated volume of surface run-off by the three proposed pump stations with maximum capacity of 54.6 m3/s. Calculations of surface water flow emanating from this rainfall event showed that the peak discharge in the canals within the city will be in the range of 191 cusec as sheet flow to Gulshan-Banani lake from the west and 2621 cusec in the Dholai Khal (Figure 5). Most of these canals are completely filled and are not capable of carrying these peak discharges.
Figure 5: Graph showing the amount of peak discharge generated by one rainfall event similar to the one on September 15, 2004 in the outlet points of the existing canals.
From the above calculations and analysis, it is very obvious that the proposed Eastern Bypass Road Multipurpose Project will not be able to “control” flood that may result from recorded rainfall of 341 mm or greater during 24 hour period. Instead, such rainfall will create waterlog within the embanked areas that will persist for weeks, provided there is no more new rainfall during this time period, which is not a realistic assumption during rainy season. For example, the Dhaka City received a total of 575 mm of rain over five day period when 341 mm of rain fell during 24 hours. The embankment will create a false sense of security and will encourage developer to move onto the flood flow zone and wetlands with tax-payers money. In addition, the embankment will constrict floodwater within the narrow confines of Balu River, thereby causing increased flood velocity and discharge at locations downstream. In other words, the intensity of flood will increase on the east side of Balu river across from the embankment and below Demra in the Narayanganj area.
Lessons from Home and Abroad:
Embankments as flood control measures create a false sense of security among the residents living within embanked areas. Embankments have limited success both in Bangladesh and in other countries. Earthen embankments can easily breach and can be damaged by riverbank erosion. Most of the embankments in Bangladesh have experienced breaching and erosion more than once since their completion (Rahman and Chowdhury, 1998). For instance, breaching of the Gumti embankment at Etbarpur during the 1999 flood caused substantial damage to the environment and property. The effectiveness of embankments is being questioned in other countries as well. Flood control embankments along the Mississippi River are constructed using superior engineering designs and are maintained regularly by the US Army Corps of Engineers. During the floods of 1973, 1984, and 1993 these embankments and other embankments maintained by state governments in Illinois breached at many places and proved to be ineffective as flood control measures. During the 1993 flood, some 1,082 of 1,576 levees on the Upper Mississippi and Missouri River basins were either overtopped or failed. The damage caused by Hurricane Katrina in 2005 within the embanked areas of New Orleans, Louisiana, USA, is well documented.
What are the alternatives?
Floods are natural phenomena and don’t need to be eliminated. We need to find ways to live in harmony with the nature. We need to facilitate flooding in the floodplains by moving away from active floodplain, such as the ones on either side of Balu River and Buriganga River around Dhaka City.
We also need to find options for flood risk and damage reduction by exercising best management practices (BMPs) in agriculture, construction, and forestry to minimize the adverse effects that increase intensity and magnitude of flooding (Khalequzzaman, 2000). The BMPs can include all activities and projects that will increase water carrying capacity of natural drainage system, increase land elevations, increase infiltration of groundwater, decrease surface run-off, decrease impervious areas, and reduce changes in global climate.
In order to make the natural drainage system capable of carrying flood run-off, the carrying capacity of the canals within Dhaka City and of the Balu River will have be increased substantially by re-excavating and reviving them. Urbanization of catchment areas increases surface runoff, requiring additional drainage capacity of existing canals and rivers, which can be achieved by enlarging the size and depth of drainage canals and rivers. The capacity of storm sewer and sewage lines will have to be adjusted to accommodate additional discharge created by the growing population and increased run-off due to urbanization. This author has done calculations to determine the amount of enlargement necessary for the existing canals and rivers in and around Dhaka City, which is shown in the Figure 6.