MORPHOLOGICAL CHARACTERISTICS OF THE RIVER
MEGHNA: A COLLOBORATIVE STUDY
M. MOZZAMMEL HOQUE
M. MONSUR RAHMAN
M. ASAD HUSSAIN
Institute of Water and Flood Management, Bangladesh University of Engineering and
Technology, Dhaka Bangladesh
Giving due importance on the control of natural disaster such as floods and river bank
erosion in Bangladesh a morphological study was initiated through a cooperation
between Bangladesh and Japan to evaluate the morphological process of the river and to
recommend the possible preventive measure to control bank erosion. For this study a
reach of the river Meghna at the close vicinity of a bridge popularly known as Japan
Bangladesh Friendship Bridge is selected in such a way that the river reach represents the
common morphological problems of the major river system of Bangladesh. For
measuring river water level the staff and the automatic water level gauges are installed in
the selected reach. For measuring bed level and scour around the piers and revetment of
the bridge echo sounder coupled with DGPS and GPS have been used. Other measuring
instruments used are recording current meter, sediment sampler and turbidity meter. In
this collaborative study the flow characteristics, mode of sediment transport, extent of
scour holes around the bridge piers and abutment, nature of migration of the sand bar
upstream of the bridge, the extent and nature of bank erosion upstream and down stream
of the bridge have been evaluated.
INTRODUCTION
In Bangladesh, natural hazards such as floods and river bank erosion cause many people
land less and homeless. Enormous discharge of the main rivers systems, the
Brahmaputra-Ganges-Meghna and their innumerable tributaries and distributaries make
the rivers unstable, especially during the monsoon. Heavy rains associated with snow
melts of the south and south-east slopes of the Himalayan Mountains cause devastating
floods. One of the severe hazards associated with floods is the stream channel changes
and bank erosion in the major rivers. The agricultural and industrial activities are greatly
impaired by the failure of embankments, bridges and other river structures due to shifting
of river courses and due to heavy on-rush of flood flows. To control the river channel
changes and bank erosion the quantitative evaluation of the river channel process and the
impact of the river improvement works on the morphological characteristics of the river
are essential. The need of such study has become even stronger after Bangladesh
experienced the consecutive devastating floods of 1987 and 1988(UNDP, 1989; World
Bank, 1989).
Giving due importance on the flood prevention and control in Bangladesh the Japan
Government has agreed to cooperate with the Bangladesh Government and the joint study
Project was initiated in 1989 to conduct the study by Bangladesh University of
Engineering and Technology of Bangladesh and the Kyoto University and the Tsukuba
University of Japan with the major objectives of (a) transfer of technology, (b)
development of research facility, and (c) perform research on the floods and related
issues. The study of morphological behaviors of the river Meghna is one of the four
topics of Japan Bangladesh Joint Study Project. The main and ultimate purpose of this
topic is to understand the process of erosion and deposition and thus evaluate the
morphological process of the rivers and to recommend the possible preventive measure of
control the riverbank erosion.
STUDY AREA
Considering the size and ease of handling the data collection process, from the river
system of Bangladesh, the upper Meghna River has been selected for the present study as
shown in Figure 1.
Figure 1. Location of the study area
The Meghna is one of the major rivers in Bangladesh having a total length of 820 km
of which 420 km flows through Bangladesh. It is an alluvial meandering river. The study
reach is situated in the vicinity of the Meghna Bridge ( 23 0 36.162 N , 90 0 36.991 E )
near Dhaka City, Bangladesh. In this river a reach about 1 km down stream of the
Meghna Bridge and about 10 km upstream of the bridge has been selected. The location
of he Bridge with the plan form of the selected river is shown in Figure 2.
L7
L10
L4
0
R10
L2
R7
R4
R2
Scale:
0
bridge
1000 meter
LD2
RD2
Figure 2. Plan form of the study area with bridge location
The river reach selected represents possibly all the elements of morphological
processes, such as erosion and accretion, sand bar formation, bifurcation, scouring along
the bank and around the hydraulic structure, and tidal characteristics. Further, due to the
construction of the Meghna Bridge, possibly some significant impacts of the bridge
structures have occurred in the selected area. Therefore, the problem in the selected area
is a representative one of the general morphological problem that a river system in
Bangladesh may encounter.
RESULTS
In this Study Project, many field instruments were installed in the Upper Meghna River
and many measuring equipments have been added to the laboratory of the Institute of
Water and Flood Management of Bangladesh University Engineering and Technology
(Sawai and Toda 2004). The relevant technology in using these equipments for filed data
collection and in processing those collected data have been transferred successfully by
the Japanese counterparts to the Project staff. Details of the technology transfer may be
seen else where (sawai and Toda, 2004). By using the developed facilities of the project
data in several areas such as bed form, bank erosion, scouring around the bridge piers,
etc., data have been collected/measured.
The data collected from the river reach are analyzed in the form of general
morphology of the area, scour along the bridge abutment, and scour around the bridge
piers, river bank shifting, and the flow characteristics.
Morphology of the area
The general bed topography of the selected study reach is shown in Figure 3. The reach
has deep scour hole on the left bank forming deeper channel along the outer bend and
deposition on the right bank forming shallow deposited sand bar along the inner bend. A
sand bar exists upstream of the bridge. This sand bar has taken the present form after
construction of the bridge and possibly provides a positive flow deviation towards the left
abutment and as a result deep scour holes are forming there. The details of the general
morphology of the study reach are found elsewhere (Rhaman, et al, 2004a; Rahman, et
al, 2004b).
Figure 3. General morphology of the study river reach
Scour along the abutment
The bed contour near the left bank abutment in August 2000 is shown in Figure 4. It can
be seen that the deep scour hole developed due to the abutment is approaching close to
the scour hole developed by the piers.
2500
2400
10
2200
5
E (m)
2100
2000
A
1900
1800
A1
1700
Reduced Water Level (m)
2300
Water Level
0
-5
-10
-15
-20
-25
-30
1600
0
1500
7000 7100 7200 7300 7400 7500 7600
100
200
300
400
500
600
700
800
900
Distance from Left Bank (m)
N (m)
Figure 4. Bed contour near
the left abutment
Figure 5. Cross section along A-A’
It was estimated from the plan form at the bridge site that the lateral length of the
abutment was approximately 90 m and the approach flow depth was 10m. The initial bed
level was at about –7m and the maximum scoured level is -28.5m, msl. Therefore, the
maximum scour depth below the original bed level was 21.5m. The water depth on the
sand bar varies between 2-5 m. The detailed bed level along the section A-A’(Figure 3)
from the left bank as measured in August 2000 is shown in Figure 5. The site was free
from the influence of channel curvature, as the bridge was located at the cross over region
of the meandering plan form. Therefore, the scour due to the effect of curvature is not
significant. The details of the scour along the abutment of the bridge of the study reach
are found elsewhere (Hinikidani and Kajikawa, Y., 2004; Rhaman, et al, 2004c;
Rahman,et al, 2004d).
Scour around the piers
The piers of the bridge are hexagonal in shape with length and width of each of the piers
is 11m and 3.2m, respectively. Figure 6 shows the bed level along the cross section at the
bridge before and after construction of the bridge. The post construction cross section
was measured during August 2000. It is observed that the scour is formed on the left bank
from pier 11 to pier 6 and deposition has taken place on the right bank from the pier 6 to
pier 3. The deepest scour has taken place around piers 8, 9, and 10. The lateral profiles of
the bridge section at the initial stage and on August 24, 2000 show that initially, 50cm
thick boulder pitching was constructed around piers for scour protection. It is clear that
there is no boulder pitching around the pier, rather scour hole has been developed around
them. There are piles up to about 40m below the pile cap above which piers were
constructed. Due to the deep scour hole, some portion of the piles below pier no.10, no.9
and no.8 have been exposed. The maximum scour depth below the original bed level was
14.5m. On the other hand, deposition took place around the pier no.3, no.4, and no.5
towards the right bank of the river. The details of the scour around the bridge piers of the
study reach are found elsewhere (Hinokidani, O.; Kajikawa, Y., 2004; Rhaman, et al,
2004c; Rahman, et al., 2004d).
15
10
Reduced Level (m)
Water Level at August 24,2000
5
P2
Initial Bed Level
0
P10
-5
P9
P8
P7
P6
P5
-10
P4
P3
700
800
August 24,2000
-15
-20
-25
0
100
200
300
400
500
600
Distance from Left Abutment (m)
Figure 6. River bed level at bridge section
900
1000
River bank shifting
To see the bank shifting of the selected river reach with time observations have been
made several times using GPS along the bank line. But from the field observation, it is
found that the inner bank line (right bank: X = 4000 to 5000 meter) is being eroded as
shown in Figure 7. But, the measured velocity vectors show the opposite behavior. The
details of bank shifting at the river reach studied are found elsewhere (Rahman et al,
2004e)
2800
Flow
Erosion
bridge
April 03, 2001
Y (m)
2750
February 12, 2002
2700
March 11, 2002
2650
2600
4000
Figure 7: River bank shifting
4200
4400
4600
4800
5000
X (m)
Figure 7. River bank shifting
Flow characteristics
At different sections of the study area velocity are measured using a 2-D electromagnetic
current meter, and data are transferred to a note-pad computer through ACM210-D Data
Processing software. The measurements are undertaken along the bridge section for 12
positions, along the abutment section, section between bridge and abutment, the other 4
main sections and 3 more sections between L2-R2 and L4-R4 (Figure 2) for 8/9
positions each. The velocity measurements for each longitudinal profile of the section are
performed at 1 m vertical depth interval along the total depth. At each location velocity
data are collected for 10 seconds which gives 10 sets of velocity data both magnitude
and direction. These data are then processed and analyzed to observe the trends and
pattern of bank erosion and also these results are used for the investigation of scouring,
especially local scouring, formation of sand bar at upstream side of the bridge relation
between depth and velocity, unit discharge and total discharge. A velocity vector model
is being prepared to investigate the relation between the sediment material (suspended as
well as bed material), size distribution and velocity distribution along the channel reach.
Also through these observations, aggradation and degradation process are also
investigated.
Velocity vectors in different horizontal planes have been presented in Figure 8. It can
be seen that along the left bank from L4R4 to L2R2, the vectors are smaller as compared
with vectors along the right bank. Then according to both the theoretical and empirical
investigation, bank erosion should occur towards the left bank. But actually erosion is
taking place towards the left bank of the concerned reach. The reason for the
contradiction is not clear. However, other forces such as wave, wind etc. may play
dominant role on the bank erosion instead of velocity. This is the remaining problem in
the field and need to be solved in future. The details of flow characteristics of the study
5000
5000
4000
4000
East Distance in meter
East Distance in meter
reach are found elsewhere (Rahman, et al, 2004a; Rahman, et al 2004b; Islam, et al,
2004).
3000
2000
3000
2000
1000
1000
= 100 cm/sec
= 100 cm/sec
0
0
0
1000
2000
3000
4000
5000
6000
7000
0
8000
1000
2000
5000
5000
4000
4000
East Distance in meter
East Distance in meter
3000
4000
5000
6000
7000
8000
6000
7000
8000
North Distance in meter
Velocity vector at 4m Depth
North Distance in meter
Velocity vector at surface
3000
2000
1000
3000
2000
1000
= 100 cm/sec
= 100 cm/sec
0
0
0
1000
2000
3000
4000
5000
North Distance in meter
Velocity vector at 1m Depth
6000
7000
8000
0
1000
2000
3000
4000
5000
North Distance in meter
Velocity vector at 5m Depth
Figure 8. Velocity vectors at four different locations
CONCLUSIONS
Based on the result of the collaborative study the following general conclusion may be
made:
1. The collaboration between Japan and Bangladesh has been found successful in
technology transfer and research capability development in the area of river
morphology of the Institute of Water and Flood Management of Bangladesh
University of Engineering and Technology.
2. Implementation of the Meghna Bridge on the river Meghna has significant impact on
the morphology at close vicinity of the bridge, especially at the upstream.
3. During post construction period a sand bar has been developed on the upstream of
the bride, heavy scour has been occurred on the left abutment and on the left side
piers and deposition has been taken place on the right side of the piers
4. The sand bar upstream of the Meghna Bridge is a non-migrating point bar and its
height has already reached at almost equilibrium. The sand bar is the major
contributing agent for the outer bank erosion or bends development. The left bank
close to the upstream termination of the guide bank will face severe erosion unless
some protective measures are provided.
REFERENCES
[1] Hoque, M. M., “Morphological Behavior of River Meghna with particular Emphasis
on the post Meghna Bri dge period: A Background Paper”, Japan Bangladesh Joint
Study Project on Floods, Phase II, Final Report, (2004).
[2] Hinokidani, O. and Kajikawa,Y., “Numerical Simulation of 3-dimensional Flow
around an Abutment”,Japan Bangladesh Joint Study Project on Floods: Phase II:
Final Report, (2004).
[3] Islam, G.M.T., Rahman, M. M. , Hussain, M. A., , Haque, M. A. , Hoque,
Hinokidani, O., and Takebayshi, H., “Effects of sand bar on sediment distribution in
an alluvial river”, Japan Bangladesh Joint Study Project on Floods: Phase II: Final
Report, (2004).
[4] Rahman, M. M. , Hussain, M. A., Islam, G.M.T., Haque, M. A. , Hoque, M. M.,
“Hydro-morphological characteristics around the Meghna bridge site in the Meghna
river”, Japan Bangladesh Joint Study Project on Floods: Phase II: Final Report,
(2004a).
[5] Rahman, M. M. , Hussain, M. A., Islam, G.M.T., Haque, M. A. , Hoque, M. M.,
Faisal, A. I. M., Rana, M. S., Takebayshi, H. and Hinokidani, O., “Measurement of
Flow and Bed Topography”, Japan Bangladesh Joint Study Project on Floods: Phase
II: Final Report, (2004b).
[6] Rahman, M.M., Haque, M.A., Hoque, M.M., Sarker, M.F.H. and Mamun, S.A.,
“Local Scouring at bridge Site in Natural River”, Japan Bangladesh Joint Study
Project on Floods: Phase II: Final Report, (2004c).
[7] Rahman, M. M. , Haque, M. A., “Local scour at sloped-wall spur-Dike-like
structures in alluvial rivers”, Japan Bangladesh Joint Study Project on Floods: Phase
II: Final Report, (2004d).
[8] Rahman, M. M., Haque, M. A. , Hoque, M. M., “Applicability of the bend
development theory in natural alluvial river”, Japan Bangladesh Joint Study Project
on Floods: Phase II: Final Report, (2004e).
[9] Sawai, K. and Toda, K. , “Details of Technical Transfer”, Japan Bangladesh Joint
Study Project on Floods: Phase II: Final Report, (2004).
[10] UNDP, “Bangladesh Flood Policy Study”, Final Report (1989).
[11] World Bank, “Bangladesh Action Plan for Flood Control”, Document of the World
Bank (1989).