5 Sustainability of a Dredged Channel for River Training A ... capital dredging on the River Jamuna...

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 ([email protected] or [email protected])

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.


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.


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.


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


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.


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.


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.


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


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.


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.


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.


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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[3] M.N. Uddin, Md.M. Rahman, Flow and erosion processes at bends and around river training works in a sand bed


427

braided river, Ph.D. thesis, IWFM, BUET, 2010. [4] G.T. Dury, Theoretical implications of under fit streams,

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[12] China-Bangladesh Joint Expert Team, Study report on flood control and river training project on the Brahmaputra river in Bangladesh, 1991.

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[14] Contractual agreement between BWDB and IWM, Quality control, monitoring and impact assessment pilot dredging of Jamuna River at 2 locations from Sirajganj Hardpoint to Dhaleswari Offtake (20 Km) and Near the Nalin Bazar (2 Km), January 9, 2011.

[15] J.C. Bathurst, Environmental river flow hydraulics, in: C.R. Thorne, R.D. Hey, M.D. Newson (Eds.), Applied Fluvial Geomorphology for River Engineering and Management, Wiley, Chichester, 1997, pp. 69-93.

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