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J. Civil Eng. Architect. Res. Vol. 1, No. 6, 2014, pp. 414-427 Received: July 28, 2014; Published: December 25, 2014
Journal of Civil Engineering
and Architecture Research
Sustainability of a Dredged Channel for River Training:
A Case Study Using Satellite Image and HEC-RAS 1D
Model
Md. Mosiur Rahman, Pk. Shubhra, Md. Abu Hena Mostofa Kamal, Md. Munsur Rahman Institute of Water Modelling (IWM), Institute of Water and Flood Management (IWFM), BUET, Dhaka, Bangladesh
Corresponding author: Md. Mosiur Rahman (mosiur26ce@yahoo.com or mosiur26ce@gmail.com)
Abstract: To mitigate the erosion and shifting of river courses, Bangladesh Water Development Board (BWDB) has initiated a 20 km capital dredging on the River Jamuna as pilot basis in order to guide the flow to reduce the risk of failure of Sirajganj Hard Point and right guide bundh of the Bangabandhu Bridge. The sustainability of the dredged channel was assessed by analyzing the time-series satellite images and HEC-RAS 1D model. Satellite image indicates that, the channel is developing very fast along the western bank through deviation of flow towards the river bank and there is no significant positive impact of dredging has observed near Sirajganj Hardpoint area as well as the downstream of Banghabandhu Bridge. It is also observed that the rate of siltation is higher, where the dredging alignment passes through the existing char. It was happening, because the hydraulic condition around the dredged area as well as upstream river morphology remain favorable for the siltation on the existing char. Analysis of observed data indicates that, more than 1.75 m/s flow velocity persists during the peaks which covering the entire Sirajganj Hardpoint area and it persists from mid of May to September. It is also observed that, if the dredging alignment passes through the existing channel not over the char area, the dredged channel would be more sustainable. The major finding of this study is that the dredging of a braided river would not be a sustainable solution without changing upstream river morphology as well as hydraulic conditions.
Key words: River morphology, sustainability, dredged channel, satellite image, sediment concentration.
1. Introduction
The erosion and shifting of river courses, loss of
land, especially along the Jamuna river have long been
recognized as a national problem that affects a sizable
population [1, 2] in Bangladesh. The secondary
current in a bend of a third order channel of braided
river is similar to that of the single thread meandering
channel [3]. The influence of secondary currents on
flow and sediment dynamics causes meander shifting
through bank erosion and bar formation in typical
meandering river [4-9]. The bank material
characteristic along the Jamuna River is fine sand and
almost uniform with respect to flow resistance [10].
The overall width of the river exhibits an increasing
trend and there is tendency of shifting westwards,
especially at the upstream part of the Jamuna river
[10-12]. To mitigate the above problem, historically
Bangladesh Water Development Board (BWDB) is
being implemented a number of river bank protection
structures such as embankments, groins, revetments,
spurs and hard points etc. [13]. Since last sixties [10]
BWDB have mixed experiences of failures and
successes and each year a big amount of money is
required for the maintenance these river training
structures. As an alternative BWDB has initiated
capital dredging on the River Jamuna as a pilot basis
in order to guide the flow away from the west channel
into a mid channel to reduce the risk of failure of
Sirajganj Hard Point and to guide the flow along the
Sustainability of a Dredged Channel for River Training: A Case Study Using Satellite Image and HEC-RAS 1D Model
415
middle of the existing char through the Bangabandhu
Bridge to near Dhaleswari Offtake [14]. Total length
of the pilot dredging is about 20 km from upstream of
Sirajgonj hard point to downstream of Banghabandhu
Bridge is shown in Fig. 1. Before adopting these
measures a thorough understanding of flow
characteristics and their interaction with channel
geometry and planform is essential [15]. This study is
intended to assess the sustainability of the above 20
km dredged channel along the braided Jamuna River.
2. Methodology
The sustainability of the dredged channel was
assessed by analyzing the data so far available both on
primary and secondary data. The data collection map
around the study area is shown in Fig. 1. Bathymetric
data, water level, discharge and sediment concentration
data were collected from pilot capital dredging project
and used as a primary data in this study which is
formally permitted from BWDB and IWM. Primary
data collection included the following items: A)
Discharge data B) Suspended sediment and C) Water
Level. Secondary data were collected from IWM,
BWDB and CEGIS. Historical hydrometric data such
as water level, discharge, sediment concentration, bed
Fig. 1 Data collection location map around the study area.
Sustainability of a Dredged Channel for River Training: A Case Study Using Satellite Image and HEC-RAS 1D Model
416
material, etc. were also collected from IWM and
BWDB. Satellite images were used in this study,
which are collected from Center for Environmental
and Geographic Information Services (CEGIS). All
image acquisition dates are in dry season.
2.1 Image Analysis
GIS techniques were used to analyze the satellite
images. Time series satellites images were used in
studying the flow and erosion processes of a
developing bend. The time-series geo-referenced
satellite images were superimposed to assess the
historical trend of the shifting of river courses.
Geo-referenced images were used to delineate the
large-scale bed-forms such as sandbars. The
delineated sand bars were superimposed in GIS
environment to assess the shifting pattern and to
assess the translation process of large scale bed forms.
2.2 Model Development
One dimensional HEC-RAS (Hydrologic
Engineering Centers River Analysis System) model
has been developed for this study. The model
geometry is developed firstly. The initial and
boundary condition is applied for the year 2012
bathymetry data. The model is also calibrated and
validated using observed data and finally analyzed the
sedimentation, bed gradation and sediment load to
assess the sustainability of the dredged channel.
3. Results and Discussions
River bank erosion is a severe problem in
Bangladesh especially along the braided Jamuna River
considering both the scale and intensity of erosion.
Thousand hectares of floodplain are eroded each year.
Due to the dynamic nature of river morphology, it
causes the sufferings to the people along with
damages to public and natural resources. Braided
rivers are strongly influenced by high sediment
delivery from nearby sources (e.g. glacial outwash,
torrential tributaries) coupled with lower sediment
throughput due to hydraulic conditions (primarily
gentle slopes). They are sensitive to changes in their
flood regime or sediment influx, and can completely
modify their geometry over a few decades (Ferguson,
1993). To prevent the erosion, different types of
protection structures have already been constructed in
different locations in both banks of the Jamuna River.
The structural measures are sometimes ineffective due
to morphological changes of the river.
3.1 Satellite Image Analysis
ASTER (2007) and IRS LISS (2008, 2009, 2010,
2011 and 2012) images were used to identify the
sandbars and their translation process. These sandbars
are different size and shape and bars are translating
both laterally and longitudinally at different rate. The
sand bars at the upstream and adjacent to the study
area are referred as sandbar-1 and sandbar-2 are
shown in Figs. 2-4. In this analysis, lateral translation
in each year was measured from the centreline of the
2007 bar towards the western direction. Eventually,
longitudinal translation was measured from the head
end of the sand bar of 2007 towards the downward
direction.
3.1.1 Lateral Translation
The maximum lateral translation of sandbar-1 was
occurred during 2007 and 2008 which was just
upstream of the upstream termination of Sirajganj
Hardpoint. In 2009 and 2012 bar was translated
downstream from the upstream termination which
caused damages of the hardpoint downstream from the
termination. The west ward translation of the
sandbar-1 diverted the flow towards the Hardpoint and
caused undermining and damages during different
years. Due to the position of sandbar-2 in 2007 a very
narrow channel (100 m to 150 m) was flowing through
between Sirajganj Hardpoint to Jamuna bridge guide
bundh bend. Due to lateral translation, flow was
diverted by the sandbar-2 extreme west point towards
the bank and by eroding the bank, the bend
consequently developed are shown in Figs. 2 and 3.
Sustainability of a Dredged Channel for River Training: A Case Study Using Satellite Image and HEC-RAS 1D Model
417
The lateral translation of sandbar-1 and sandbar-2
during 2007-2012 is summarized in Table 1 and
represent in Figs. 4 and 5.
3.1.2 Longitudinal Translation
Sandbars are also moving towards longitudinal
direction. Longitudinal movement is measured as a
distance travelled by the upstream end of the bar
towards the downstream direction in the consecutive
Fig. 2 The sandbar movements of Jamuna River, year 2007 to 2010.
Sustainability of a Dredged Channel for River Training: A Case Study Using Satellite Image and HEC-RAS 1D Model
418
Fig. 3 The sandbar movements of Jamuna River, year 2011 to 2012.
Fig. 4 Schematic diagrams of lateral and longitudinal translation of sandbars. (Source: Farzana Mahmud, M.Sc. thesis, 2011).
Lateral Translation of Sandbars Longitudinal Translation of Sandbars
2007
2008
2010
Flow
Centerline of 2007 bar
Bankline N
2009
2011
2012
2010 Bar
2010
2007
2008
2009
Centerline of 2007 bar
Flow
Bankline
N
2011
2012
Sustainability of a Dredged Channel for River Training: A Case Study Using Satellite Image and HEC-RAS 1D Model
419
Table 1 Lateral and longitudinal translation of the sandbar-1 and sandbar-2.
Year Lateral translation Longitudinal translation
Sandbar-1in (m) Sandbar-2 in (m) Sandbar-1in (m) Sandbar-2 in (m)
2007 1,540 730 0 0
2008 1,690 910 650 1,400
2009 1,870 1,060 800 1,800
2010 1,360 560 1,300 2,200
2011 1,100 720 1,950 2,500
2012 800 680 2,100 3,500
years. The longitudinal translation of sandbar-1 and
sandbar-2 during 2007-2012 is summarized in Table 1
and shown in Figs. 4-6. During 2007 to 2012,
sandbar-1 translated 150 to 600 meter per year and
sandbar-2 moved 300 to 1400 meter per year. The rate
of movement of sandbar-2 is higher than that of
sandbar-1 because sandbar-1 was obstructed by the
revetment structure as well sandbar-2 could move
downstream without facing any obstacle. In 2010 and
2012, sandbar-2 diverted flow towards downstream
bend and caused huge erosion at the downstream part
of the bend. As a whole, due to the translation of
sandbars along downstream, flows were diverted
towards the bank and caused bank erosion.
Fig. 5 Lateral translation of the sandbar-1 and sandbar-2.
Fig. 6 Longitudinal translation of the sandbar-1 and sandbar-2.
Sustainability of a Dredged Channel for River Training: A Case Study Using Satellite Image and HEC-RAS 1D Model
420
3.2 Analysis from Morphological Model
3.2.1 Model Geometry
The one dimensional (HEC-RAS) model boundary
was developed with a 54 km long Jamuna River from
Tarakandi-Meghai in the upstream and ending at Betil,
downstream of Bangabandhu Bridge. Only 2012
bathymetric survey data has been incorporated over
this 54 km reaches of Jamuna River around 2 km
interval. Hec-GeoRAS is used to link the HEC-RAS
model with ArcGIS 10. The model geometry mode is
shown in Fig. 7.
3.2.2 Initial & Boundary Condition
For unsteady flow simulation HEC-RAS 1D
requires boundary conditions at all open boundaries.
Here the model has two boundaries. Upstream
boundary at Bahadurabad is given as discharge data of
the year 2012; downstream boundary condition is
water level at Sirajganj Hardpoint of the year 2012.
All the observed data for this model is collected from
BWDB and IWM. Discharge Hydrograph at
Bahadurabad and Water Level Hydrograph at
Sirajganj Hardpoint are shown in Figs. 8 and 9
accordingly.
3.2.3 Calibration and Validation of Model
The model was calibrated with (March-June), 2013
and validated (July-September), 2013 water level data.
The model simulation results were carried out with
and without dredging conditions for one monsoon
ahead. The discharge time series applied at the
upstream boundary is based on the rating curve
derived at Bahadurabad on the basis of the data
measured by BWDB corresponding to the observed
water level and discharge. The water level time series
used at the downstream boundary of the model is
based on interpolation using the average slope
between Bahadurabad and Sirajganj. The calibration
and validation hydrograph are shown in Figs. 10 and
11.
jamuna
5400052000500004800046000440004200040000380003600034000
3200030000
2800026000
24000220002000018000160001400012000
100008000
600040002000
Jam
un
a
Fig. 7 Study area & model setup with boundaries.
Sustainability of a Dredged Channel for River Training: A Case Study Using Satellite Image and HEC-RAS 1D Model
421
0 50 100 150 200 2500
10000
20000
30000
40000
50000
60000
70000
80000
River: Jamuna Reach: jamuna RS: 54000
Sim ulation Time (days)
Flo
w (
m3/
s)
Legend
Flow
Fig. 8 Discharge hydrographs at Bahadurabad for the year-2012.
0 50 100 150 200 2504
5
6
7
8
9
10
11
12
River: Jamuna Reach: jamuna RS: 1000
Sim ulation Time (days)
Sta
ge
(m
)
Legend
Stage
Fig. 9 Water level hydrographs at Sirajganj Hard point for the year-2012.
Sustainability of a Dredged Channel for River Training: A Case Study Using Satellite Image and HEC-RAS 1D Model
422
Fig. 10 Calibration of HEC-RAS 1D Model for March-June, 2013.
Fig. 11 Validation of HEC-RAS 1D Model for July-September, 2013.
3.2.4 Analysis from Morphological Model
After introducing geometric and hydraulic
specifications of flow to the model and running the
model, one can observe the modeling results in
different forms. In this study, the effects of parameters
like velocity profile, bed profile and sediment etc.
were studied. The results of cross-section comparison
at different points and longitudinal changes of bed
profile for the study period are shown in Figs. 12-17.
The model area was selected after the following
considerations:
The boundaries should be far enough away from the
proposed dredging location to exclude any significant
influence from those at the location of interest;
The model domain should not extend into areas
without reliable bathymetric data;
The model should not be time consuming to
simulate;
3.2.5 Observation Form Morphological Model
The morphological change of the sand-bed braided
river like the Jamuna River is a great challenge for the
structural stability of river training works. This study
investigates the unsteady pattern of river flow as well
as variations of river bed elevation and sediment
transport due to 2013. Model results are generated for
velocity filed and specific discharge with assessment
of morphological changes such as
erosion/sedimentation of the river. Through the
application of this model, the change in river behavior
Sustainability of a Dredged Channel for River Training: A Case Study Using Satellite Image and HEC-RAS 1D Model
423
Fig. 12 General velocities profile plot of Jamuna River (Year-2012) along the model boundaries.
0 2000 4000 6000 8000 10000 12000 14000 16000
0
5
10
15
54000
Station (m)
Ele
vatio
n (m
)
Legend
04Apr2013 0600
02May2013 2100
11May2013 0600
21Jul2013 0900
Fig. 13 Cross-section comparison at upstream of Sirajganj Hardpoint with and without dredging condition at different dates of 2013.
Sustainability of a Dredged Channel for River Training: A Case Study Using Satellite Image and HEC-RAS 1D Model
424
0 2000 4000 6000 8000 10000 12000 14000-30
-20
-10
0
10
20
32000
Station (m)
Ele
vatio
n (m
)
Legend
04Apr2013 0600
03Jun2013 1200
29Jun2013 0300
18Aug2013 0900
27Oct2013 0600
Fig. 14 Cross-section comparison near Sirajganj Hardpoint with and without dredging condition at different dates of 2013.
0 2000 4000 6000 8000-10
-5
0
5
10
23000
Station (m)
Ele
vatio
n (m
)
Legend
04Apr2013 0600
03Jun2013 1200
29Jun2013 0300
18Aug2013 0900
27Oct2013 0600
Fig. 15 Cross-section comparison upstream of Banghabandhu Bridge with and without dredging condition at different dates of 2013.
Sustainability of a Dredged Channel for River Training: A Case Study Using Satellite Image and HEC-RAS 1D Model
425
0 10000 20000 30000 40000 50000 60000-30
-25
-20
-15
-10
-5
0
5
C:\Documents and Settings\NKB\Desktop\mmn on home (10.0.3.13)\JAMunaDefault\HECRAS\jamuna.sed03
Main Channel Dis tance (m)
Ch
Inve
rt E
l (m
)
Legend
04APR2013 06:00:00-Ch Invert El (m )
13MAY2013 15:00:00-Ch Invert El (m)
25JUN2013 13:00:00-Ch Invert El (m )
21JUL2013 14:00:00-Ch Invert El (m)
12SEP2013 12:00:00-Ch Invert El (m )
27OCT2013 02:00:00-Ch Invert El (m )
Jam una-jamuna
Fig. 16 Longitudinal bed profile of Jamuna River with and without dredging condition around the model boundaries (Year-2013).
Fig. 17 Longitudinal bed profile at the study area (with and without dredging condition) at different dates of 2013 from chainage 51 km to 54 km.
Sustainability of a Dredged Channel for River Training: A Case Study Using Satellite Image and HEC-RAS 1D Model
426
under different scenarios can also be assessed for
proper planning of any development project on the
braided Jamuna River. The results from data analysis
and morphological model outputs are summarized as
follows:
Model simulations reveal that for both flood events,
more than 1.75 m/sec flow velocity persists during the
peaks which covering the entire Hardpoint area (SHP)
and it persists from the period of mid of May to
September only.
There is no significant positive impact of dredging
has observed near Hardpoint and downstream of
Banghabandhu Bridge. But at upstream of Hardpoint
the siltation rate is higher and the dredge channel
almost silted up just after 1 year of monsoon flood.
4. Conclusions
River channels tend to a dynamic equilibrium
driven by the dynamics of discharge of water and
sediment transport. Different river training works,
upstream interventions increasingly change the natural
drivers of the channel morphology. The major
findings of this study are:
Due to translation of sandbars along downstream,
flow were diverted towards the bank and caused bank
erosion. The channel is developing very fast along the
western bank of the Jamuna River and erodes the river
bank.
Satellite image analysis indicates that the flow
processes and the location of active bank erosion
changes significantly due to changes in large scale
sand bar movement (both laterally and longitudinally).
Analysis of observed data indicates that, during
May-September, 2012 the depth average velocity near
the Sirajganj Hardpoint area exceeds 1.75 m/s and
consequently during the above period sedimentation
did not occurred.
There is no significant impact of dredging has
observed near Hardpoint as well as the downstream of
Banghabandhu Bridge. It is also observed that the rate
of siltation is higher, where the dredging alignment
passing through the existing char. It was happening,
because we cannot change the hydraulic condition
around the dredged area as well as upstream river
morphology. It seems that the dredge channel may silt
up within a very short period.
The major finding of this study is that the dredging
of a braided river would not be a sustainable solution
without changing upstream river morphology as well
as hydraulic conditions. It is also observed that, if the
dredging alignment passes through the existing
channel not over the char area, the dredged channel
would be more sustainable.
In this study, one dimensional morphological
equation has been used to predict erosion/deposition
process. In natural rivers there are always some
uncertainties in estimating the relevant parameters due
to their variation in space/time using one dimensional
equation. Three dimensional analyses may provide
better understanding on the above issues.
Acknowledgments
I am heartily thankful to my respective supervisor
Dr. Md. Munsur Rahman, Professor, IWFM, BUET. I
gratefully acknowledge to “The Project Director,
Capital (Pilot) Dredging of River System of
Bangladesh” and Institute of Water Modelling (IWM)
to allow me using capital dredging project data and
necessary support. I also acknowledge the support and
cooperation needed from Abu Salah Khan, Deputy
Executive Director, IWM and Md. Amirul Islam,
Director, Survey and Data Division, IWM.
References
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