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., mkhalequ@lhup.edu
Abstract:
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.
Introduction:
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 runoff 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).
Dholai KhalMonda Khal
Segunbagicha
Khal
Begunbari
Khal
DC-3A sheet
flow to G-B
lake
Jamir Khal
DC-2B Trib.
To Jamir Khal
DC-2A sheet
flow
Nali
River/Boali
Khal
12
10
8
6
4
2
0
DC-2A
Excavated
Canal
Million ft^3
Volume of Run-off Generated by One Rain Event in
Different Catchment Areas in Dhaka City
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).
DholaiManda
Khal: RP
Begunbari
Khal: RP
7-1
Jamir
Khal: RP
6
3
2.5
2
1.5
1
0.5
0
Boali
Khal: RP
5-1 & 2
Meters
Depth of Proposed Retention Ponds Needed to
Accommodate Run-off from Rain Event
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.
Dholai KhalMonda Khal
Segunbagicha
Khal
Begunbari
Khal
DC-3A sheet
flow to G-B
lake
Jamir Khal
DC-2B Trib.
To Jamir Khal
DC-2A sheet
flow
Nali
River/Boali
Khal
3000
2500
2000
1500
1000
500
0
DC-2A
Excavated
Canal
Cusec
Calculation of Peak Discharge Using Rational Equation
Following One Rain Event
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.
Nat. X-sec (ft^2)
Dholai KhalMonda Khal
Segunbagicha
Khal
Begunbari
Khal
DC-3A sheet
flow to G-B
lake
Jamir Khal
DC-2B Trib.
To Jamir Khal
DC-2A sheet
flow
Nali
River/Boali
Khal
500
400
300
200
100
0
DC-2A
Excavated
Canal
Area (Ft^2)
Natural vs. Enlarged Cross Section of Drainage Canals
Enlarged. X-sec (ft^2)
Figure 6: Graph showing a comparison between cross sectional areas of natural
canals with catchment areas similar to those of the existing canals in Dhaka City
and the enlarged cross sectional areas necessary in view of urbanization in the
catchment areas of those canals. The actual cross sectional areas of the existing
canals are not known for this study. Note that the amount of urbanization in each
of the sub-catchment areas of the existing canals in Dhaka City was taken to be
between 20% and 50%. The natural vs. enlarged cross sectional areas of the
Balu River (not shown in the graph) are 871 and 1223 ft 2, respectively.
A GIS analysis has been done for the Dhaka City Metropolitan Plan area on the
basis of digital elevation model (DEM) data from Shuttle Radar Topography
Mission (SRTM) to delineate natural flow accumulation areas and drainage
network. A 500 meter buffer zone on either sides of the natural drainage network
indicated that a substantial portion of the proposed residential zone at Sadarghat,
Rampura, Badda, Basundhara, Katasur, Sabujbag, and north of Tongi area fall
within the buffer zones, which are prone to water-logging and flooding (Figure
7). Many of the natural drainage network areas are abandoned channels that are
already encroached
Figure 7. Map showing landuse zones as proposed in the detailed area plan
(DAP) by PAJUK (2010). Note that a substantial part of the proposed residential
and industrial zones fall within the drainage buffer zones (stripped areas). This
map is adopted from RAJUK (2010) and is georeferenced using WGS-1984
coordinate system.
Lastly, the table below shows a series of BMPs that can be practices in
Bangladesh in order to reduce flood risks and damage (Khalequzzaman, 2000).
Table 1. List of BMPs and their effects on mitigation of flooding.
BMPs
Dredging rivers and streams
Re-excavation of abandoned channels,
ponds, and lakes
Dispersal of dredged/excavated
sediments on land
Least Impact Development
Establishment of vegetated buffer zone
along rivers and streams
Silt fence around construction sites
Sediment and run-off detention ponds
in construction sites
Removal of western embankments
Efficient storm sewer system in cities
Planned urbanization and compact
township
Watershed-scale landuse planning
Reforestation programs
Good governance, self-reliance, and
implementation of environmental acts
Integrated regional water resources
development plan for GangesBrahmaputra-Meghna Basin in Indian
Subcontinent
Expected effects on mitigation of
flooding
Increase carrying capacity of drainage
Increase carrying capacity of drainage;
reduce run-off
Increase elevations of earthen roads
and village platforms
Reduce soil erosion and run-off
Reduce soil erosion, run-off, and bank
erosion
Reduce soil erosion and run-off
Reduce soil erosion and run-off
Increase land elevations by flood
inundation
Increasing carrying capacity and
reduce water-logging in cities
Reduce impervious surface and run-off
Reduce run-off, soil erosion, and
impervious surface; sustainable
economic developments
Reduce run-off and soil erosion
Implementation of BMPs to mitigate
flooding problems; sustainability in
economy and environment
Flood/drought control; optimal uses of
natural resources in the region;
sustainable environment and
development
Conclusions:
The detailed area plan (DAP) proposed in the DMDP is a step in the right
direction. However, the proposed Eastern Bypass Road Multipurpose Project as
a measure of flood control is not based on hydrologic principles, and will not be
able to reduce flood risks and water-logging in Dhaka City. This project is likely
to create a false sense of security among the residents living on the floodplain
and flood flow zones of the Balu River. The embankments will not only
intensify water-logging within the embanked areas, it will likely to increase the
magnitude of flooding in areas outside of the embankment including at
Purbachal area and in downstream locations. Water-logging and flooding
propensity cannot be reduced without facilitating proper functioning of natural
drainage network and enlarging the drainage network in accordance with basin
development factor. Alternative approaches to flood-risk and damage reduction
will have to include best management practices that will help reduce surface runoff, increase infiltration (EPA, 2000), and increase carrying capacity of natural
drainage systems in Dhaka City and in catchment areas of the Balu and
Buriganga Rivers. The natural drainage network of the DMDP area need to be
recovered, excavated, and enlarged to account for basin development factor to
ensure proper draining of run-off. It is suggested that the proposed Eastern
Bypass be built as circular highway project with many bridges that will not
impede natural functioning of flood-flow zone. Limited number of pump station
may have to be installed in areas that act as a flow accumulation points, such as
the Begunbari canal (east of Rampura and Sabujbag). Some of the proposed
residential zones in the southern parts of the DAP are located in areas of natural
flow accumulation that need to be avoided for future development. GIS analysis
of the DEM data indicates that the boundary proposed in the DMDP can be
expanded to the north of Tongi and Savar in the future.
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