京 都 大 学 防 災 研 究 所 年 報 第 54 号 Ｂ 平 成 23 年 6 月
Annuals of Disas. Prev. Res. Inst., Kyoto Univ., No.54B, 2011

Hydraulic and Morphological Consequences of Bank Protection Measures
along the Jamuna River, Bangladesh

Hao ZHANG, Hajime NAKAGAWA, Yasuyuki BABA, Kenji KAWAIKE,
Md. Munsur RAHMAN* and Mohammad Nazim UDDIN*** Institute of Water and Flood Management, Bangladesh University of Engineering and Technology** Department of Civil Engineering, Dhaka University of Engineering and Technology
Synopsis
Bank erosion is a severe problem in the rivers of Bangladesh. Besides theimportant scientific and engineering relevance, bank erosion also exerts significant
social and economic impacts in this southern Asian country. This paper describes
typical bank protection measures as well as their hydraulic and morphologicalconsequences in the lower part of the Brahmaputra River (named the Jamuna River in
Bangladesh). Based on a series of field investigation results, attempts have been made toclarify the mechanisms of bank erosion along this large alluvial river. Moreover, theperformances of existing bank protection measures are evaluated and possible solutions
for further enhancement are proposed. Special attention is paid to a historied andindigenous river training structure: Bandal. The recurrent use of Bandal-like structures
is suggested for bank protection and channel stabilization of the braided Jamuna River.
Keywords: Brahmaputra/Jamuna River, bank protection, channel stabilization, field
investigation, Bandal, spur dyke

1.

Introduction

The southern Asian country: Bangladesh is oneof the most populated nations in the world. The
land of Bangladesh is covered by a complex river
network system consisting of three major rivers: theGanges, the Brahmaputra and the Meghna, together
with their numerous tributaries and distributaries. A
brief overview of the rivers in Bangladesh isreferred to Oka (2004). One important hydrologicalaspect of rivers in Bangladesh is that 92% of thedrainage basin of these rivers lies outside of thecountry (FAP21, 2001). Moreover, the sediment
within Bangladeshi rivers consists primarily of finesands and silts with little clay matrix (Coleman,1969). Therefore, river banks are highly susceptible
to erosion when the flow conditions change. In thepast several decades, frequent flood and continuous
erosion have consumed large areas of floodplains,
made thousands of people homeless and destroyed ahuge amount of infrastructures. The problems
related to the Brahmaputra River are particularly
gigantic, and this river is selected as the target forthe current study.
The Brahmaputra River is named the Jamuna
River within the border of Bangladesh. It originatesfrom the Kailas Range of the Himalayas and flows
across China, India and Bangladesh. The river
meets with some major rivers in its lower part: theTeesta, the Ganges and the Meghna and supplies
sediment into one of the world’s largest deltas
before finally entering the Bay of Bengal. The
location and the catchment area of theBrahmaputra/Jamuna River are shown in Fig.1.

― 477 ―

ChinaNepal
BhutanBrahmaputra R.

India
Ganges R.

Jamuna R.

India
Catchment of Brahmaputra/Jamuna RiverRiverInternational border

Bangladesh

Myanmar

Bay of Bengal

Fig. 1 Location and catchment of the Brahmaputra/Jamuna River

As is known, the Brahmaputra/Jamuna Riverranks in the top group of the earth’s large rivers interms of both water and sediment discharges. In thereach within Bangladesh, the river wanders with adistance of approximately 240km and has a mean
bankfull width of some 11km. The average annual
flow discharge of the Jamuna River is around
20,200m3/s and sediment transported through theriver is about 590 million tons per year at the
Bahadurabad gauging station (Sarker et al., 2003).
The annual hydrograph of the Jamuna River ischaracterized by low flows during the winter dry
season and high flows during the summer time due
to snowmelt in the Himalayas and heavy local
rainfall in the monsoon. The river is braided withmeta-stable islands and nodal reaches, mobile
sandbars, shifting anabranches and severe bankerosion (Thorne et al, 1993). Systematic analysis oftime-series of dry season satellite images and withthe supplement of available historical maps andaerial photographs, CEGIS reported that the Jamuna
River shows a persistent trend of westward
migration (CEGIS, 2004, 2005, 2006 and 2007).
Analysis indicates that the centerline of the river
has moved an average of 4.3km towards west since
year 1830 with a maximum westward movement of
13km at its northern end. In recent years, theJamuna River is migrating westward at an averageof 75m per year. Moreover, the river is widening at

an average rate of 145m/year, accompanied by theannual creation of 1960 hectares of char land fromsediment trapped within the channel. As one of themost crowded countries in the world and the
majority of the population being still wholly
dependent upon land holdings, the bank erosion andmigration of the river have significant impacts on
the development of the society and economy.
According to combined analyses of population datawith satellite images, it is found that during theperiod of last 10 years, an average of almost 46,000
people were relocated every year due to bankerosion along the Jamuna River (IWFM, 2008).
In order to protect lives and properties fromfrequent flooding, an earth embankment was built
during the later 1950s and mid 1960s along the westbank of the Jamuna River, extending for some
220km. The embankment is generally known as theBrahmaputra Right Embankment (BRE). The
on-going bank erosion by the river, however, has
led to breaches of the BRE with attendant crop loss
and damage to buildings and infrastructures as wellas successive costly retirements of the BRE overthe past several decades (Halcrow et al, 1994).
Improvement of the performance of the BRE is
therefore an important part of flood protection andmeasures against bank erosion. In order to seek
long-term strategy for the protection of the BRE,
the Government of Bangladesh has commissioned aseries of studies since 1990. A master plan was― 478 ―

formulated and construction of bank protection
structures at different probable locations was
suggested to save priority areas. From the mid
1990s, construction of major structures was started
and there are now around 28 major bank protection
structures along the BRE. In general, thesestructures may be categorized into two kinds: one
kind is intended to mainly strengthen the resistanceof the bank to be protected with insignificant
interference on the river flow, e.g. revetments. The
other kind is aimed to decrease the hydraulicimpacts directly in front of the protected area, e.g.
spur dykes.
It has to be mentioned that most of the existingbank protection structures along the BRE were
designed using extrapolation methods based on
research results and experiences of small rivers inother places such as some European countries. The
resulted structures were generally very huge in bothsize and cost corresponding to the scale of the
Jamuna River. Although positive roles of some of
the bank protection structures have been reported
after an overall performance assessment based on
available literatures, field investigations anddiscussions with engineers and the stakeholders(BWDB, 1999), the applicability and sustainabilityof these structures were still questionable for large
Bangladeshi rivers. The fact is that most of themexperienced frequent damages or failures after theircompletion. On the other hand, a locally developed
structure: Bandal, deserves special attention. A
Bandal structure may be simply described as avertical screen mounted on a frame. The main
construction materials are bamboo mats with abundle of bamboo sticks. Bandal structures arequite cheap since bamboos are locally available andinexpensive labors are easily employable on site
without any special training. When thesediment-laden flow approaches the Bandal, flow
separation occurs: low sediment-concentrated flowat the upper layer is diverted to the main channel
and is accelerated, resulting in bed degradation in
the main channel, while high sediment-concentrated
flow at the lower layer passes through the Bandal
and deposits behind it due to velocity reduction
there. Bandal structures may be physicallyconsidered as combined structures of spur dykes
and pile dykes (Zhang et al., 2010). They have beensuccessfully used in the India subcontinent
historically. Unfortunately, their working principles
are not well clarified and design guidelines are notavailable yet due to shortage of scientific evidences.
In this paper, the hydraulic and morphologicalconsequences of typical bank protection measuresin selected river reach are investigated. Based on
the investigation results, the performances of thesemeasures are evaluated. The mechanism of bankerosion in large alluvial rivers is discussed,
improvement methods for existing bank protection
structures are suggested and probable cost-effective
solutions for the training of the Jamuna River areproposed.
2.

Study sites and research methods

Referring the master plan report of theBrahmaputra River training studies of the
Bangladesh government and based on the first-handknowledge and experiences from field visits, theSirajganj
Hardpoint(revetment)
andtheBetil/Enayetpur spur dykes were selected as thetarget structures to investigate the existing bankprotection measures. These two sites ranked in the
top group of the priority areas to be protected
designated by the government. Bandal structuresnear the Jamuna Bridge at the Randhunibari market
are also included in the study as a reference site.
Field investigations to the three sites were made in2008, 2009 and 2010. The locations of them aresketched in Fig.2.

No.8

No.7

No.6

No.5
Fig. 2 Location of study sites (Left to Right: Studysites, the Jamuna River and Bangladesh)― 479 ―

Locaal boat
G
GPSL
LaptopPlastic boat
A
ADCP

Phooto1 Field surrvey on the Jaamuna River

1
100
Percentage finer (%)

This study is conducted primaarily on the basisbof literatuure review, fieldfimeasuremments, discusssing
with locaal people andd collecting informationif
from
BWDB (Bangladesh Water Deveelopment Boaard),
CEGIS (Center(
for EnvironmentalEl and Geograaphic
Informatiion Servicess) and BUE
ET (Bangladesh
withUniversitty of Engineeering and Technology)Tthe aid of our collaaborators in Bangladesh. The
authors havehconductted a series of researches on
spur dykees over the paast several yeears (e.g. Zhanng etal., 2006, Zhang et al.., 2007, Zhanng and Nakagawa,2008 annd Zhang ett al., 2009), the knowleedge
acquired from those studies are partiallypextennded
to this reesearch if necessary. Moreover, prelimiinary
experimeental and num
merical resultss from the autthors’
research group on Bandal-like structures are also
used as supplement informationit understandd thetofundamenntals underlyiing the physiccal phenomenna.
As a field-based appplied researcch, field tripss andfield meaasurement sysstems are of greatgimportaance.
In this sttudy, a measuuring systemm consisting ofo an
ADCP (AAcoustic Dopppler Currennt Profiler) annd a
GPS (Gloobal Positionning System) is used to reecord
the 3D velocityvvecttors in the river. The AD
DCP(1200kHz, WH-ADCP
P Rio Grandee by Teledynee RD
Instruments) was moounted downw
ward beneathh thewater surrface on a parrticularly desiigned plastic boat.
And the plasticpboat wasw dragged byb a local woooden
boat duriing measurem
ments. The AD
DCP and the GPS
are connnected with a laptop installediwitth areal-timee data visualization and data analyzzing/post-proccessing progrram. A snapshot of the fieldmeasuremment is shown in Phhoto1, with thespecificaations of the measuringm
sysstem.

80
60
40
20
0
10

1000
1000
(μm
m)
Fig.3
F
Sieve annalysis resultt of sediment samples
Before filed measuremennts, basic informationon the sedimentt properties and hydrological andmorrphological characteristics
cs related to the study
areaa is collected..
The Jamunaa River is ccharacterized by finesediimentary envvironment witth highly moobile beds
and banks. The sediment is uniformly grraded andhas very low strengthsandd transport resistance.
r
Meaan sediment sizessof the rriver vary from 220um
to 165um (FAP
P21, 2001), showing inssignificant
channges from thetupstreamm to the dow
wnstream.
Seddiment samples taken fromm the floodp
plain near
the Sirajganj Toown were anaalyzed during
g the firstfield
d trip as shown in Fig.3. It is found obviously
thatt the sedimennt is much finner than that in typicalJapaanese or Euroopean rivers. The mean diiameter is
181um, having a geometric deeviation of 1.36.
On the otherr hand, the ddaily discharrge at the
upsttream Bahaddurabad gaugging station and themonnumented suurveyed crooss-sections (Section
No.5-No.8 as deppicted in Fig..2) near the thhree study
sites were obtainned from BWD
DB are shown in Fig.4
and Fig.5. The hydrograph at the Bah
hadurabad
gauging station shown in Fiig.4 ranges fromfyear
1998 to year 20006 temporallyy. The low flo
ow seasonand the high flow
w period are eevidently disttinguishedfromm year to yeaar. The river ggenerally peaaks during
the period of Juuly and Auguust. Peak disscharge isfoun
nd to be generally arround 60,00
00cumecs,
exhibiting insignificant chhanges oveer years.
Extrreme flood dischargedoccurred in 1998 and thecounntry sufferedd one of itts worse eveer floods
betw
ween July andd September iin that year.

― 480 ―

Discharge (m3/s)

100000
80000
60000
40000
20000
0
1998/01/01 1999/05/16 2000/09/27 2002/02/09 2003/06/24 2004/11/05 2006/03/20

Elevation (m)Date (Year/Month/Date)
Fig.4 Daily discharge at the upstream Bahadurabad gauging station
16
14
12
10
8
6
4
2
0
-2
-4
-6

Section No.8

Elevation (m)

0

2000

4000

6000

8000

10000

12000

14000

16000

Distance from the left side milestone (m)

16
14
12
10
8
6
4
2
0
-2
-4
-6

Section No.7
0

2000

4000

6000

8000

10000

12000

Elevation (m)Distance from the left side milestone (m)
16
14
12
10
8
6
4
2
0
-2
-4
-6

Section No.6
0

2000

4000

6000

8000

10000

12000

14000

16000

Elevation (m)Distance from the left side milestone (m)
16
14
12
10
8
6
4
2
0
-2
-4
-6

Section No.5
0

2000

4000

6000

8000

10000

12000

14000

Distance from the left side milestone (m)Fig.5 Temporal variation of typical cross-sectional profiles near study sites― 481 ―

16000

The changes of the bed elevation at four typicalcross-sections in Fig.5 demonstrate the highly
active nature of the mobile bed of the Jamuna Riverin the past several years. The locations of thesections are depicted in Fig.2. Section No.8 is in thenear upstream of the Sirajganj Hardpoint. Section
No.7 is very close to the termination of the
Sirajganj Hardpoint. Section No.6 is in the neardownstream of the Jamuna Bridge and the
Randhunibari Bandal sites and Section No.5 is very
close to the Betil and Enayetpur spur dykes. The
river is obviously composed of numerous channels
and these channels frequently shift year to year. In
the river, there are all kinds of sandbars and islands
and some of which migrate frequently. With thebackground information on the sediment properties
and flow discharges, the morphological changes
shown in Fig.5 might not be so much surprising.
Nevertheless, the difficulties lying in seeking
suitable solutions to manage the morphologicalprocesses really come to researchers and engineers
as a surprise. Without a detailed understanding on
the mechanisms involved and more quantitative
field data, decisions cannot be made confidentiallyand measures cannot be taken appropriately.
3.

Revetments

3.1 Description of the Sirajganj Hardpoint
The old established town: Sirajganj is located incentral Bangladesh, about 8km upstream of the
Jamuna Bridge and about 110km northwest of thecapital city: Dhaka. According to analyses results ofsatellite images, Sirajganj is observed to have thehighest erosion rate in the catchment area (CEGIS,
2007). The erosion of the bankline since the 1950s
has resulted in parts of the historic town fronting
directly onto the river bank (Halcrow et al., 1994).
In order to protect the town, revetment works werelaunched by the Bangladesh Government in 1998,
known as Sirajganj Hardpoint. The hardpoint
intends to control the shape of the BRE with thelowest level of intervention to the river flow. The
hardpoint was constructed with cement concrete
cubic blocks and consisted of a straight portion
along the BRE together with a round upstream
termination. The basics of the Sirajganj Hardpoint
are listed in Table1 and a photo is shown in Photo2.

Table 1 Basics of the Sirajganj Hardpoint
Length of revetment reach

2.55kmCrest level over PWD
High flood level over PWD

16.75m15.75m

Low water level over PWD

6.80m

Apron setting level over PWD-4.20mDesign scour level over PWD-13.25m

Thickness of apron

1.93mSide slope
Size of cement concrete block

1:3.5
55cm & 85cm

Completion of the hardpoint

1998*PWD: Public Work Datum, 0.457m below the seal level

Photo 2 Sirajganji Harpoint
The Sirajganj Hardpoint is exposed to the main
anabranch channel of the Jamuna River, receiving
year-round hydraulic attacks. The hardpoint,
especially the area near the upstream termination,
has been almost damaged almost every year since2007. Velocity measurements were conductedduring March 22-23, 2008 and March 19, 2009. In
addition, the bathymetry survey data around the
upstream termination was collected from the branch
office of BWDB.
3.2 Bed change around upstream termination
The monthly change of the bed elevation aroundthe upstream termination of the Sirajganj Hardpointfrom January 2008 to February 2009 was plotted in
Fig.6. The plot series provided valuable informationon the bed evolution characteristics in the proximityof the termination. Basically, the bed morphology
around the termination was dominated by a scour
hole and the hole changed its shape as well as its
bottom elevations almost all the time even in thedry season. The similarities in the scour geometry
and the fluctuations in the maximum scour depth
from January to the beginning of July indicated thatthe transport of bed forms played an important rolein the bed evolution during the low flow season.

― 482 ―

471000

471100471200471300471400471500

-22

-2
-10
-6

-18

-26

-18

-14

-14

707200-18707100

-22
2

-18

N

-14

707000-14

January 10, 2008

May 16, 2008
-14

-22
-18

-18
-22

-14
-10

-26
-18
-14

February 21, 2008

June 19, 2008
-14

-18

-22
-14

-18

-22

-18

-14
-18

March 9, 2008

-22

July 2, 2008
-18

-14
-18

-22

-22
-14
-22

-30

-18

-26

-14

-22
-18

April 11, 2008

August 16, 2008

Fig.6 Bed level change at termination (Unit: m)― 483 ―

471000

471100471200471300471400471500

-18
-22

-10
707200

-26

-18

-14

-18

707100-14-30707000

-22

N

-18

September 20, 2008

January 11, 2009
-10

-18

-14

-14
-18

-22
-26

-14
-6

-10

-14

-18

October 8, 2008
-18

-14
-10

February 4, 2009
Fig.6 Bed level change at termination (Ctd.)

-18

-14

-18

-18

November 1, 2008
-14

-22

-18

-10
-22
-26
-18

November 20, 2008
Fig.6 Bed level change at termination (Ctd.)

The scour hole exhibited significant changes
during the flood as shown in the plots in August andin September. In particular, the maximum scour
depth was dramatically increased and the locationof the deepest point also shifted a bit to thedownstream of the termination. After the flood, thescour hole area was refilled with sediment and the
maximum scour hole depth returned to thepre-flood level as obviously shown from the plots
after October. Although the 2008 flood enlarged thescour hole, the final scour hole in January of 2009
was even much smaller and shallower than that in
January of 2008. It has to be mentioned that thetermination partially failed in September of 2008
during the flood. The severe scour at the toe of thetermination resulted in stability problems of thetermination, i.e. the sliding of the apron and thesand materials beneath it took place. The
observations suggested that the mitigation of thescour due to flood and the enhancement of thetermination against scour were probably the keys toensure the effectiveness of the Sirajganj Hardpoint.

― 484 ―

North (m)North (m)East (m)Fig.7 Horizontal velocity 0.86m beneath the water
surface in 2008

East (m)Fig.8 Horizontal velocity 0.86m beneath the water
surface in 2009
Vertical (m)

0
-10
-20
-30

Vortex2.0m/sVortex-40
707100707200707300707400707500

North (m)Fig.9 Vertical velocity vectors at section P in 2008
0

Vertical (m)3.3 Flow field at the Sirajganj Hardpoint
Although the hardpoint did not exert direct
influence on the flow field, it altered the flow field
through the morphological changes around it. The
typical flow velocity patterns in year 2008 and year
2009 were plotted from Fig.7 to Fig.10 andcompared with each other.
The horizontal velocity vectors near the water
surface were shown in Fig.7 and Fig.8, together
with the sketches of major sandbars based on field
investigations and satellite images. In 2008, theapproach flow channel was diverted into two partsby two sandbars. The flow attacked the terminationof the hardpoint and resulted in two circulation
flows in front of and behind the termination,
respectively. In 2009, the west part of the big
sandbar on the east was washed away and the widthof the approach flow channel was significantly
enlarged. On the other hand, a small sandbar
appeared in the approach flow channel just in frontof the termination. These changes affect a lot on theflow structure near the termination. The twocirculations remained in 2009, but became much
weaker than those in 2008. The flow in 2009 was
further divided into two parts after passing the
hardpoint due to the detachment of the downstream
sandbar from the embankment. If one took a look at
the velocity field in the vertical direction passing
the termination in Section P as shown in both Fig.8
and Fig.9, the differences in the flow structure
could be evidently distinguished. A vortex occupied
the scour hole in front of the termination in both
years, but the one in 2008 was much stronger thanthat in 2009. As this vortex was the major engine
for the local scour around the termination, it wasthen easily understandable that the scour depth in2009 was smaller than that in 2008 as have been
confirmed by the bathymetry data in Fig.6.
The observations indicated that the movement ofsandbars significantly influenced the flow structure
and the bed morphology around the hardpoint.
Although the problem related to the hardpoint was avery local scale one, understanding the migration
characteristics of sandbars in a broader scale was of
crucial importance. Moreover, the horizontal andvertical velocity vectors indicated that the local
flow structure around the termination was ofthree-dimensional nature.

-10
-20

2.0m/sVortex-30-40707100707200707300707400707500

North (m)Fig.10 Vertical velocity vectors at section P in 2009

― 485 ―

4

Spurr dykes

4.1 Desccription of Betil/Enayetpu
ur spur dykees
The Betil and Enayyetpur spur dykes
dare loccatedin the easternepart of Belkuchhi and Chow
whali
Upazilla,, about 25km
m south of Sirrajganj town. The
Betil andd Enayetpur Bazar areass are historiccallywell knoown for their handloom teextile production.
There arre lots of immportant public and priivate
establishm
ments such as schools, maadrasahs, collleges,
mosques, hospitals annd industries. This area is oneof the higgh priority arreas to be prootected designnatedby the goovernment.
Since 1914, the bannkline of thiss area has shifted
towards west by arouund 5km (Haalcrow, 1994). In
order to protect the bank, two spursdykes werew
constructted in 2002. The
T distance between the two
spur dykes is arouund 2.5km. The Betil andEnayetpuur spur dykees basically follow the samesdesign methods.
mEithher of themm consists off anearthen shank and a RCC (Reeinforced cem
ment
concrete)) head. Thee RCC heaad is not deepdpenetrateed into the riverbedr
but is supportedd byin-situ caasted concretee piles with a diameter of 0.5m.
0Therefore, the lower part will be permeable iff the
bed leveel is degradded enough. The connecction
between the RCC heaad and the earrthen shrank is in
the form of a bell-moouth and has been found tot be
one of thhe weakest pooints accordinng to past failure
experiencces. Althouggh RCC bloccks and geo-bags
are dumpped into thee water to prrovide additiional
protection to the spurr dykes, the eartheneparts havehsuffered from damaages frequenntly in monssoon
seasons since theiir completioon. The mainmparameteers related tot the Betill/Enayetpur spurdykes aree listed in Tabb.2.
Tablee 2 Basics of Betil/Enayetp
B
pur spur dykees
Parameteers

Betil

Enayetppur

Length of earth shankk

801m

1050m

Length of RCC part

150m

150m

Crest levvel over PWD
D
Width off crest

15.50m
6m

15.30mm
6m

Side sloppes

1:2 (up)
1:3 (downn)

1:2 (up)
1:3 (down)

Design sccour depth
Completiion of spur

18.5m
2002

17.0m
2002

Embaankment

Piles

Photto3 The Betil spur dyke

R
RCC
head
Shank

Photo4 The Enayetppur spur dyke
4.2 Bed deform
mation arouund Betil/Enayetpur
spur dykes during flooddThe
T Betil/Enayyetpur spur ddykes were co
onstructedalon
ng the seconddary channel oof the river ass depicted
in Fig.2.
F
In the dry season, aalmost no flo
ow passed
the channel. Buut during thhe flood period, thechannnel might becomebactivee and made threatsttothe embankmentt. The tempooral variation
ns of thebed bathymetriess around the ttwo spur dyk
kes during
the 2008 flood werew
plotted in Fig.11 an
nd Fig.13.
Thee information on the flow discharge off the riverat th
he upstream Bahadurabadd gauging staation wasplottted in Fig.12 for referencee.
Due
D to severee local scour at the toe of the Betil
spurr dyke, the lower portioon of the RCC
R
part
becaame permeabble. Thereforre, the hydraaulic andmorrphological fuunction of thee RCC part off the Betil
spurr dyke wouldd be similar to those of a Bandal.
Thee downstreamm Enayetpur spur dyke, however,wass still imperm
meable. It was evident fro
om Fig.11
thatt the local scoour area at thhe toe of the Betil
B
spur
dyk
ke exhibited innsignificant cchanges although there
wass slight increease of the mmaximum sco
our depth
afteer the flood peak.
pTaking into account previous
scou
ur records (e..g. Zhang et al., 2008), it might beconcluded that the flood ddischarge wouldw
notalways enlarge the local sccour. The coontinuous
supp
ply of sedim
ment from tthe upstream
m channelmig
ght prevent the bed from coontinuous deg
gradation.

― 486 ―

470500

4707004706004708004709006N2-2682000Discharge (m3/s)

2
-6
-10
681900-666681800-22681700June 21, 2008

65000
60000
55000
50000
45000
40000
35000
30000
25000
20000
15000
10000
5000
0

August 20
July 19
September 10
June 214/275/17 6/6 6/267/16 8/5 8/259/14
Date (Month/Date)
Fig.12 Discharge in 2008 monsoon at Bahadurabad

6N
2679500

-6

2

6
2

-6
-2

6

-2

-10
6

471400471200-22679500-2-6679400470600470800-2
2
6471000June 21, 2008
July 19, 2008
2

-2

2

6

2
-2
-6

-6

-2

-2

-6
6

-10

2

July 19, 2008
6

6

2

-6
-2

6
2

2
-2

-6

August 20, 2008

2
6

6
2

-2

August 20, 2008

-6
-10
-14
6

2

2
-2

-6

-2

-2

-10
6

-2
2

-6

6

2

6

September 10, 2008
Fig.11 Bathymetry at the Betil spur

September 10, 2008
Fig.13 Bathymetry at the Enayetpuril spur

― 487 ―

Vertical (m)

-10
-15
-20

2.0m/s-25
470500470600470700470800

East (m)Fig.15 Velocity vectors at Section A

Vertical (m)

0
-5
-10
-15
-20

2.0m/s-25-30
470800470850470900470950471000

East (m)Fig.16 Velocity vectors at Section B
Vertical (m)4.3 Flow structure around Betil/Enayetpur spurdykes during flood
The velocity field was measured on July15, 2008
and the resulted velocity vectors in a horizontal
plane near the water surface around the two spurdykes were plotted in Fig.14. Major sandbars werealso sketched in the figure according to field
investigations and satellite images. Due to theexistence of various sandbars, the approach flow
did not attack the head of either spur dyke, which
made the scour phenomena around the two spurdykes somewhat unique.

0-5

North (m)Fig.17 Velocity vectors at Section C
0

Vertical (m)A large area of deposition was found behind the
Betil spur dyke, and the Enayetpur spur dyke wassituated immediately downstream of the area. The
bed level around the Enayetpur spur dyke did notchange so much in the flood season (Fig.13),
indicating the flow and the bed morphology were
well controlled by the upstream Betil spur dyke.
Moreover, the lowest bed located along the earthen
shank of the Enayetpur spur dyke instead of the toe,
which was completely different from that around atypical impermeable spur dyke. Similar to that of
the Betil spur dyke, land was created behind the
Enayetpur spur dyke due to sediment deposition
there.

-5

-10
-15
-20
6793801.0m/s679390679400679410679420679430

North (m)Fig.18 Velocity vectors at Section D
Vertical (m)North (m)

0
-5
-10
-15

1.0m/s-20-25679350679400679450679500

North (m)Fig.19 Velocity vectors at Section E

East (m)Fig.14 Velocity vectors around the Betil/Enayetpur
spur dykes 0.86m beneath the water surface

When the flow approached the spur dykes, it was
blocked, changed its direction paralleling to thespur dyke and flowed towards the spur dyke heads.
The parallel flow had a potential to remove
sediment on its way to the head which gave the― 488 ―

answer whyw
severe scour took place along theearthen shanksof the Enayetpur sppur dyke. Att thehead of either spur dyke, flow circulation wasrecognizaable althoughh the data wass not fine enoough
to resolvee the details.
The velocity vectoors in severaal vertical plaanes
were alsso presented from Fig.155 to Fig.19. The
locationss of the vertical planes werew
depictedd in
Fig.14. The
T velocity vectorsvin Secction A described
the typical parallel floow along the earthen shannk of
the Betil spur dyke. And
A similar fllow structure was
expectedd around thee Enayetpur spur dyke.. In
Section B,
B strong doownward flow
w detached fromf
the Betil spur dyke was
w observed.. This downw
ward
flow wass the main cauuse for the latteral enlargem
mentof the local scour hole. The downward flowfattacked the bed, turneed upward annd mixed withh the
downwarrd flow fromm the sandbarr in front off thespur dykke. Section C showed the longituddinalvariationn of the flow
w velocity at the head off the
Betil sppur dyke. Two
Tcirculatting cells werew
recognizeed: one was situated in thet upstream andthe otherr one was inn the downstrream of the spurdyke. They
T
were thetmajor enginesefor thedevelopm
ment of the scourshole in the longituddinal
directionn. The circulaating cells reesulted from theblockagee of the approoach flow at thet earthen shhank
and the RCC
R
head of the Enayetpuur spur dyke werew
shown inn Section D and Section E, respectivvely.
Althoughh the bed proofiles were quuite different,, thecirculatinng flows folloowed the samme direction. BothBof them area capable of removing seediment fromm the
bed and resultrin scouur at the foot ofo the spur dyykes.
5

the water coursee with a lenggth of about 10m andmadde an angle of 50o-60o to the bankkline. The
Banndal was maade of groups of verticall bamboo
pilees with a spaccing of 46cm
m. The diametters of thebammboo piles raanged from 66cm to 9cm. The piles
werre connected with each otther by cross bamboos
withh a vertical spacing of aroound 75cm. Moreover,M
incllined bambooos were set at an angle of 45
4 o to thebammboo piles to enhance the stability of th
he Bandal
stru
ucture. The uppper part of tthe Bandal was
w closedwithh bamboo thaatch while thhe lower partt is open.
Thee Bandals weere arranged in a group along theseco
ondary channnel of the rivver as show in Fig.2.
Thee distance between two cconsecutive Bandals
B
is
arou
und 32m. Thee details of oone of the Baandals areshow
wn in Photo11 which was taken during
g the field
inveestigation in July, 2008. On the othher hand,Pho
oto2 demonsstrates the sediment depositiondbehind the Banddal structure aalong the ban
nk and thephoto was takenn in July 20009, one year after the
firstt field trip. It is found thatt a large area of land iscreaated behind thetBandal w
within only one year.
Thee two photoos give visibble evidencee on thedeposition-promooting perform
mance of the Bandals.
BBand
dal and Banddal-like strucctures

5.1 Introoduction
The handloom ennriched Randdhunibari loccatesalong thee west bank and immediaately downstrreamof the Jaamuna Bridgee as shown ini Fig.2. Obllique
flow tow
wards the baankline durinng the monssoon
season put
p the Randdhunibari marrket nearby in avery danngerous situaation. In ordeer to protectt thebank from erosion annd to test thee performancce oftraditionaal river trainning structurres, a groupp ofBandals was constrructed by RRI
R(the RiverR
Researchh Institute) during the period of year
2007-20008. Each Banndal structurre protruded into

― 489 ―

Photo 5 Ranndhunibari Bandal (July, 2008)2

Photo 6 Ranndhunibari Bandal (July, 2009)
2

100

37.5

365

80

A

A

297.5

y
x

Inlet tankReservoir(a) Plan view

Instrument carriage
Fixed bed

Bandal-like structure
z

Flow
Pump

Sediment trapx200(b) Section A-A

14.4

0.6

2

3

Flume side

Movable bed

1.2(c) Structure details: side view, top view & piles details
Fig.20 Experiment setup (Unit: cm)
Table 3 Hydraulic conditions of the experimentu* : friction velocity,y (cm)

5.2 Experiments
A structure, having the similar functions as a
Bandal, is named a Bandal-like structure herein
despite its construction materials, shape and layout.
Hence, the Betil spur dyke may be considered as a
Bandal-like structure and the flow velocity
information of which may be of reference for the
Randhunibari Bandal site. In order to understandthe flow structure more precisely, laboratory
experiments are conducted in the Ujigawa Open
Laboratory, Kyoto University. In the experiment,
powdered anthracite is used as model sediment,
which has a mean size of 0.835mm and a specificgravity of 1.41. A pair of Bandal-like structures is
set perpendicular to the left side of a flume. The
flume is 10m-long and 80cm-wide and has a slopeof 1/800. Each Bandal-like structure is made of 12
brass cylindrical piles, having a vertical steel plate
mounted at the upper part. The details of the
Bandal-like structure and the experiment setup areshown in Fig.20.
The experiment is carried out under live-bed
scour condition, i.e. the approach flow velocity islarger than the critical flow velocity for the
sediment entrainment. As a result, bed forms
develop in the whole movable bed area. It is thecase in the Jamuna River. The hydraulic parametersin the experiment are shown in Tab.3.
The experiment starts from a flat bed andcontinuous sediment supply is ensured from the
upstream of the flume. The total amount of
sediment supplied is the same as that collected fromthe downstream, which is determined by some trial
experiments. After 6 hours, a dynamic equilibrium
condition is reached. The pump is then stopped.
When the bed is drained out, a laser displacement
meter is then used to measure the bed deformation.
The measured bed configuration is shown in Fig.21.
In the deformed bed, bed forms and large areas ofsandbars occur due to sediment movement in thewhole movable bed domain. Local scour takes placeat the toes and along the bodies of the Bandal-likestructures, particularly the upstream one. Moreover,sediment deposition is observed in the wake zones
behind both structures. The local bed morphology
around the Bandal-like structure exhibits typicalfeatures of both an impermeable spur dyke and apermeable spur dyke (Zhang and Nakagawa, 2009).

u*c : critical friction velocity

Flow discharge

7.76 l/s

Mean velocity
Approach flow depth

24.25 cm/s
4.0 cmu*/u*c1.91 (live-bed scour)Reynolds number

7,460

Froude number

0.387

120
100
-6 -5 -4 -3 -2 -1 0 1 2 z (cm)
80
60
40
20
0
-200 -160 -120 -80 -40
040
80 120 160

x (cm)
Fig.21 Final bed configuration in the experimentInstant cement is sprayed to the bed surface and thedeformed bed becomes fixed. Water is then pumped
to the flume again. The velocity on the water
surface is obtained with PIV (Particle Image
Velocimetry) techniques and two electromagnetic
velocimetries are used to measure the 3D velocity― 490 ―

200

in the waater column. Water
W
level is recorded withw a
point gauuge.

Flow
Messh number: 588842
Nodde number: 578870

5.4 Resu
ultsFlow velocitiesv
ressulted from bothb
experimments
and simuulations are plottedpin Figg.23-27. The flowfvelocity on the wateer surface, nearnthe bed andtypical laateral and longitudinal secttions is shownn.

Bandal-like sttructures

Fig.22 Compputational meesh around Baandal-likestructuures.
20
0cm/s

80

y (cm)

60

40

20

0
-60

-40
0

-20

0

20

40

60

x ((cm)Fig.23a Flow velocity (u, v) oon water surfaace (PIV)
20cm/s80

60

y (cm)

5.3 Nummerical modeel
Since the flow arouund a Bandall-like structurre ishighly 3D,3the detaails of the flow cannott be
resolved without a 3D model. Moreoverr, aBandal-liike structure is generallyy sophisticateed inshape annd the bounndary of an actual riveer iscommonlly irregular. Hence, the numerical
n
m
model
should bee capable of resolving com
mplex geomeetries
as well. The authors have develooped a numerricalmodel saatisfying the abovearequireements (Zhanng etal., 20066). In the nuumerical moddel, the com
mplex
flow fieeld is obtaiined based on the RA
ANS(Reynoldds-averaged Navier-StokesNs) equations withthe k-ε model for thhe turbulencee closure. Inn thenear-walll domain, thhe wall-functtion approach isapplied anda the resistance of the wall
w can be eaasily
accounted for. The widely useed FVM (Finite
Volume Method)M
is adoptedain thhe formulation of
the numeerical model and the govverning equattions
are discreetized based ono an unstrucctured mesh.
The flow field ono the deforrmed bed inn thelaboratorry experimennt is investtigated with theproposedd numerical model. The measured waterw
stage andd bed level arre used in the mesh generation.
In order to reproduce the details ofo the Bandal-likestructures, a hybrid mesh consissting of diffeerent
types of polyhedra iss employed. The
T mesh sysstem
includes 58842 cells and 578870 nodes. The
computattional mesh in the prroximity of the
Bandal-liike structuress is depicted in Fig.22. Att the
inlet boundary, all flow variables area known andd areprescribeed according to the meann velocity. Att theoutlet booundary, zero diffusion fluux is assumedd for
all flow variables.
v
The free surfacee is considereed asa rigid lid and iss unchangeaable during thecomputattion. Near the wall bounddaries such ass thechannel bed,b
the Banddal-like struccture and the sideof the fllume, the floow velocity isi assumed too beparallel tot the walls.

40

20

0
-60

-4
40

-20

0

20

40

60

x (ccm)Fig..23b Flow vellocity (u, v) oon water surfaace (Sim.)
AccordingA
to Fig.23 and F
Fig.24, it is foundfthat
the flow velocitty on the waater surface isi affectedsignnificantly by thet blockage of the upper part of theBanndal-like strucctures and 3D
D vortices in the scour.
On the other haand, the flow
w velocity at the lower
layeer is closely relatedrto the bed geometriies and theexissting of the piles of the Baandal-like stru
uctures. In
general, the num
merical modell reasonably reproducesrthe experimentall results.

― 491 ―

20cm/s

80

60

y (cm)60

y (cm)

20cm/s

80

40

40

20

20

0

0
-40

-20

0

20

40

60

80

-40

100

-20

0

20

Fig.24a Flow velocity (u, v) at 2cm from the initialbed (Exp.)z (cm)z (cm)

Vortex5 10 15 20 25 30 35 40 45 50 55 60 65 70 75

y (cm)y (cm)

Vortex20cm/sFig.25b Flow velocity (v, w) at x=-22cm (Sim.)Up flowz (cm)z (cm)

100

5 10 15 20 25 30 35 40 45 50 55 60 65 70 75

6
3
0
-3
-6

5 10 15 20 25 30 35 40 45 50 55 60 65 70 75

20cm/s

6
3
0
-3
-6

5 10 15 20 25 30 35 40 45 50 55 60 65 70 75

y (cm)y (cm)

Fig.26a Flow velocity (v, w) at x=-16cm (Exp.)

Vortex

Vortex
VortexDown flow
-40

-30

-20

-10

0

10

Fig.26b Flow velocity (v, w) at x=-16cm (Sim.)20cm/sz (cm)z (cm)

80

20cm/s

6
3
0
-3
-6

Fig.25a Flow velocity (v, w) at x=-22cm (Exp.)

6
3
0
-3
-6

60

Fig.24b Flow velocity (u, v) at 2cm from the initialbed (Sim.)

Down flow

20cm/s

6
3
0
-3
-6

40

x (cm)x (cm)

Down flow
20

30

40

50

9
6
3
0
-3
-6

20cm/s60-40 -30 -20 -10

x (cm)

0

10

20

30

40

50

60

x (cm)

Fig.27a Flow velocity (u, w) at y=72cm (Exp.)

Fig.27b Flow velocity (u, w) at y=72cm (Sim.)If one takes a closer look at the free surface
velocity field, one may note that bed morphology
influences a lot on the flow structure. The flow
separation angle at the head of the structure is muchsmaller compared with that of the PIV measurement.
As has been argued in previous research (Zhang etal., 2009), the over-estimation of the current PIV
method due to an over-accumulation of tracers is
considered to be a probable reason. The inherent
deficiency of the eddy viscosity based turbulencemodel is also another probable cause, but further
evidences are needed.

3D vortices are obviously confirmed in Fig.25-27,
concentrated in the extent of local scours and being
similar to those observed around spur dykes (Zhang
et al., 2009). These vortices are engines for the scour
development. However, there are many properties
unique to the Bandal-like structures. Fig.26 andFig.27 indicate that the wake vortex behind the
upstream structure is not well developed. The vortex
is very weak and is confined in the shade of theimpermeable part of the structure other than inside
the scour hole. It is attributed to strong flows whichpass the piles of the structure and make great― 492 ―

disturbances to the wake zone. It is also found in
Fig.27 that there are vortices located in front of andbehind the downstream structure. Due to the vortex
in front of the structure, flows passing the piles lose
their energy and the vortex behind the structuredevelops more freely and even extends to the insideof the scour hole. The vortex in front of thedownstream structure is resulted from the down flow
and the relatively small scour depth (hence, a small
opening ratio). Similar vortex pair appeared in thefield measurements of the Betil spur dyke as shownin Fig.17.
6

Discussions

The information on the flow field and bed
deformation of the study sites provides a key tounderstand the fundamental working principles andperformances of typical structural measures againstbank erosion along the Jamuna River. Comparedwith conventional revetments and spur dykes, theflow structure and the bed morphology around the
Sirajganj Hardpoint (revetment type) and the
Betil/Enayetpur spur dykes (spur dyke type) aremuch more complex due to the complex flow andbed conditions.
Owing to its unique location, the upstream
termination of the Sirajganj Hardpoint works likean impermeable spur dyke to some extent. Several
vortex systems of obviously 3D nature form in theproximity of the termination and they are strongly
dependent on the local bed morphology as well as
the migration of the sandbars nearby. The
termination is a weak point of the structure due tolocal scour at the toe and possible sliding down of
the sand materials on which the apron is set. The
area a little downstream of the termination is also
very weak due to the direct attack of the returncurrents. Both parts experienced failures in the past
several years. The comparison of the flow
structures in 2008 and 2009 suggests that theperformance of the Sirajganj Hardpoint strongly
depends on the sandbars in its neighborhood andthat the evolution of the sandbars necessitates
special attention.
By introducing long earthen shanks, the area
protected by the Betil/Enayetpur spur dykes isenlarged and the construction cost is reduced. On

the other hand, the earthen shanks become the mostfrequently failed parts of the two spur dykes due tothe direct hit by the approach flow and the parallel
flow along the shanks. Countermeasures against theparallel flow are hence important for the stability ofthe two spur dykes. The RCC part of the Betil spurdyke works like a Bandal structure which blocks theflow in the upper layer and allows the water to flowat the lower elevation. This kind of structure takes
the advantages of both the impermeable spur dyke
and the pile dyke. As a result, excessive local scouris avoided but either the mainstream degradation or
the wake deposition is promoted. Moreover, thereturn currents behind the spur dyke are also
weakened due to the joining of the flow passing
through the piles of the RCC part. The downstream
Enayetpur spur dyke furthermore prevents thepossible return currents caused by the upstream
Betil spur dyke from direct attacking theembankment. Due to the influence of the Betil spurdyke, the impermeable Enayetpur spur dyke does
not suffer too much from local scour at the RCC toe.
The maximum scour depth around the Enayetpur
spur dyke is about 9.5m smaller than that of the
Betil spur dyke during the field measurement.
However, the parallel flow is more dangerous along
the Enayetpur spur dyke compared with that along
the Betil spur dyke since the former owns a longer
earthen shank and an almost impermeable RCC part.
Making the lower part of the RCC part permeable toreduce the effective length of the spur dyke might
be a solution to reduce the erosion along the foot of
the shank.
Although detailed measurements are notconducted for the Bandal structures in site, theeffectiveness of the structures are confirmed fromboth the Randhunibari test Bandal site and theBandal-like part of the Betil spur dyke. In addition,
the laboratory experiment and numerical simulation
preliminarily provides the working mechanisms forthis kind of structure. Recently, Teraguchi et al.
(2011) have made more systematic comparisons on
the hydraulic and morphological characteristics
among the conventional impermeable spur dykes,
pile dykes and the newly highlighted Bandals. It has
to be mentioned that the original Bandals are
generally made from locally available bamboos andare constructed without reliable scientific design or― 493 ―

effective quality management. Therefore they arequite cheap (Table4) and easily implemented but
generally suffer from stability problems especiallyduring the flood.
Table 4 Cost of bank protection measures
Measures

Site

AgencyCost(US$/m)

Guide
Bank

JamunaBridgeForeign

33,000Hardpoint

Sirajganj

Foreign

21,000Solid spur
Revetment(Geobags)

Kalitola
JamunaRiver

Foreign

12,500Foreign

2000-3000

Revetment

JamunaRiver

BWDB

3800-4000

RCC spurJamuna
River

BWDB

950

Bandalstructures

Sirajganj

RRI

9*compiled based on Rahman et al., 2007
Comparing the Bandals with the conventional
impermeable spur dykes, it is noted that the Bandals
will generally exert less impact on the flow andchannel dynamism. Hence, it has a potential toprovide a more nature friendly alternative for the
management of large alluvial rivers like the Jamuna
River. The Bandals are physically smaller andweaker than conventional impermeable spur dykes.
Furthermore, the Bandals may be quickly buried
due to huge deposition around it and their working
life will be much shorter compared with theconventional impermeable spur dykes. Therefore,
the Bandal structures should be better basicallyused as temporary structures in a recurrent way tocope with changing conditions adaptively. The total
cost will include initial investments and timely
maintenances.
7

Conclusions

This paper presented a study on hydraulic andmorphological consequences of typical bankprotection measures along the Jamuna River ofBangladesh. The results indicate that conventionalbank protection measures such as revetments andspur dykes play important roles in protecting the
BRE and that historied river training measures suchas Bandals based on indigenous knowledge exhibit
high potential for wider applications.
The flow field and bed morphology around thebank protection structures, either intrusive or
nonintrusive, vary spatially and temporally but
maintain certain common features depending on thetype of the structures. The strong vortex system infront of and the return currents behind the upstream
termination of the Sirajganj Hardpoint are the major
engines for the morphological variations and arealso the main causes of the structure failure.
Moreover, the intensity of the flow around thetermination is closely related to the evolution of thesandbars nearby. The vortex system and the parallel
flow resulted from the flow blockage at the longshanks of the Betil/Enayetpur spur dykes are great
threats to the stability of the two spur dykes. The
RCC part of the Betil spur dyke works as a
Bandal-like structure, providing a well control of
the incoming flow and sediment transport for thedownstream Enayetpur spur dyke. Compared withthe local scour at the toe of the RCC part of the
Enayetpur spur dyke, the erosion along the foot of
the long earthen shank is a more serious concern. It
might be a challenging problem unique to relatively
huge structures constructed in large rivers.
Field investigation suggests that the Bandals
developed from indigenous knowledge are capableof promoting sediment deposition efficiently.
Considering the migration nature of the Jamuna
River, the cost-effectiveness and the environmental
harmony, the wider application of Bandals or
Bandal-like structures is recommended. However,more research is suggested to clarify the associatedmechanisms and to formulate guidelines on theirdesigns, constructions and maintenances.
Acknowledgements
This research is supported by Grant-in-Aid for
Scientific Research B, MEXT, Japan (PI: Dr. H.
Nakagawa, Grant No. 18404010) and the JST-JICA
Program on Science and Technology ResearchPartnership for Sustainable Development (PI: Dr. H.
Nakagawa). The authors would also like to express
their sincere gratitude to a lot of participating― 494 ―

members from BUET, CEGIS, BWDB, RRI andKyoto University.
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― 495 ―

バングラデシュ国ジャムナ川における河岸侵食防止対策が流れ及び地形に及ぼす影響

張浩・中川一・馬場康之・川池健司・ラマン ムンスン*・ウヂン ナジム***

バングラデシュ工科大学水資源・洪水管理研究所**

ダッカ工科大学土木工学研究科

要 旨
バングラデシュ国の河川では，河岸侵食が大きな問題となっている。河岸侵食は科学と工学的な問題だけでなく，この南アジアの国にとっては，大きな社会や経済問題でもある。本稿は，ブラマプトラ川の下流（バングラデシュ国内ではジャムナ川と呼ばれる）における河岸侵食防止対策及びそれらが流れ及び地形に及ぼす影響について報告する。数回の現地調査及び計測により，ジャムナ川における河岸侵食のメカニズムの解明を図った。また，既存の河岸侵食防止対策の機能を評価し，可能な改善策を提案した。特に，現地の歴史的な方法で土着工法の代表であるバンダル型水制について着目し，ジャムナ川の河道安定及び河岸侵食防止方策としては，バンダル型水制の設置の繰り返しが現地に適応した工法となり得る可能性を示唆した。

キーワード: ブラマプトラ川/ジャムナ川，河岸侵食防止，河道安定化，現地計測，バンダル，水制― 496 ―