SEDIMENTATION AND ITS COUNTERMEASURE AT
THE OFF-TAKE AREA OF NEW DHALESWARIRIVERTanjir Saif AHMED*MEE15634

Supervisors: Prof. EGASHIRA Shinji**
Assoc. Prof. YOROZUYA Atsuhiro***

ABSTRACT
Present study proposes a method to mitigate sediment deposition at the off-take region based on an idea
for the relation of the stream line curvature and associated secondary currents. We investigatednumerically effects of spur dyke on the mitigation of sediment deposition, focusing on flow pattern, bedvariation and off-take flow discharge as well as on location and size of spur dyke. A suitable size of spurdyke and its construction site are proposed based on the numerical predictions.
Keywords: Sediment deposition, curvature of flow, secondary current, bed variation, suitable spur dyke.
INTRODUCTION
New Dhaleswari River is the most prominent distributary channel of the mighty river Jamuna in termsof discharging water to the four major peripheral rivers of the Capital city of Bangladesh, Dhaka. Thisriver route is expected to carry necessary discharge through the whole hydrological cycle every year.
Jamuna, being a suspended sediment dominated river, injects huge volume of sediment in suspension
with the discharge through this channel. Due to massive sediment deposition at the New Dhaleswari offtake area, the channel remains almost closed for five to six months in the dry season. Adjacent rivers of
the capital city, Dhaka as well as the river network of central part of Bangladesh are badly affected withthe reduced flow coming from Jamuna through this river route. The present study is to find a technicalsolution to the sedimentation problem at the New Dhaleswari Off-take area. The study includes the study
area, problem to be solved, over-all goal, hydrologic and hydraulic characteristics of the study area,
formulation of problem, explanation about the idea of the countermeasure, technique applied to test thevalidity of the idea, the governing equations employed in numerical computation, calculation conditions,scenario of flow pattern in case of flow and bed variation cases, sediment transportation characteristics
and analysis of the effect of the idea over flow and sediment control and conclusion.
THEORY AND METHODOLOGY
The objective is to find a technical solution to closing of channel due to massive deposition of sedimentat the New Dhaleswari Off-take area. To solve the problem, the idea of employing a spur dyke along themain channel at the off-take of Branch channel is introduced. The expectation is the spur dyke would
control the curvature of flow and generate secondary current. That secondary current is expected totransport sediment from the off-take area towards the center part of the river and it diverts more clear
water (less turbid water) into the branch channel. The spur dyke controls the flow pattern at the off-takearea and resist closing of channel. Figure 1 summarizes the idea of present study. The numerical model
proposed by Takebayashi et.al. (2009) is employed. This model is based on the depth integrated 2-D
governing equations for water flow as well as on the governing equations on sediment transportation
and associated bed evolution. These equations are as follows.
𝜕ℎ𝜕𝑢ℎ𝜕𝑣ℎ+ 𝜕𝑥 + 𝜕𝑦 = 0(1)𝜕𝑡In which, h is the flow depth, u and v are the x-component and y-component of velocity, respectively.
The momentum conservation equations for flow along x and y-directions are described as𝜕𝑢ℎ𝜕𝑢𝑢ℎ𝜕𝑢𝑣ℎ𝜕𝜏𝜕𝜕++= −𝑔ℎ (ℎ + 𝑧𝑏 ) − 𝑥 +(ℎб𝑥𝑥 ) +(2)(ℎ𝜏𝑦𝑥 )
𝜕𝑡𝜕𝑥
𝜕𝑦𝜕𝑥
𝜌
𝜕𝑥
𝜕𝑦𝜏𝑦𝜕𝑣ℎ𝜕𝑢𝑣ℎ𝜕𝑣𝑣ℎ𝜕𝜕𝜕++= −𝑔ℎ (ℎ + 𝑧𝑏 ) − +(ℎб𝑦𝑦 ) +(ℎ𝜏𝑦𝑥 )
𝜕𝑡𝜕𝑥
𝜕𝑦𝜕𝑦
𝜌
𝜕𝑦𝜕𝑥

* Assistant Engineer, Directorate of Planning-1, Bangladesh Water Development Board (BWDB).
**Professor, GRIPS.
***Associate Professor, GRIPS.

(3)

Figure 1 Idea and theoretical background of study.

In which,𝜏𝑥 = 𝜏𝑏𝑢𝑏
2 +𝑣 2√𝑢𝑏𝑏, 𝜏𝑦 = 𝜏𝑏𝜕𝑢𝑣𝑏

𝜏

2 +𝑣 2√𝑢𝑏𝑏, 𝜌𝑏 = 𝑢∗2𝜕𝑣

(4)
𝜕𝑣

𝜕𝑢

б𝑥𝑥 = 2𝜀 𝜕𝑥 , б𝑦𝑦 = 2𝜀 𝜕𝑦 , 𝜏𝑥𝑦 = 𝜏𝑦𝑥 = 𝜀 (𝜕𝑥 + 𝜕𝑦)(5)

Where, t = Time; x and y = the coordinates along the longitudinal and the transverse directions; u, v =depth-averaged flow velocity on bed along the longitudinal and the transverse directions; z = Water
surface level; 𝑧𝑏 = Bed elevation; h = Surface flow depth; 𝜏𝑥 , 𝜏𝑦 = Shear Stress along the longitudinal
and the transverse directions; 𝜏𝑏 = Bed Shear Stress; ε = Coefficient of eddy viscosity; 𝑢𝑏 𝑎𝑛𝑑 𝑣𝑏 =Velocity near the bed surface along the longitudinal and the transverse directions;
Mass Conservation Equation of Bed Sediment case, including suspended sediment part is as follows,𝜕𝑧𝑏
𝜕𝑡1

𝜕𝑞+ 1− ∑𝑖 ( 𝜕𝑥𝑏𝑖𝑥 +𝜕𝑞𝑏𝑖𝑦
𝜕𝑦+ 𝐸𝑖 − 𝐷𝑖 ) = 0(6)

Where, 𝑧𝑏 = Bed elevation. 𝑞𝑏𝑖 = bed load transport rate for grain size 𝑑𝑖 .
𝑢𝑏𝑥
𝑞𝑏𝑥 = x-component of the bed load rate, 𝑞𝑏𝑥 = 𝑢̅̅̅̅∗ 𝑞𝑏 .
𝑏𝑢𝑏𝑦

𝑞𝑏𝑦 = y-component of the bed load rate, 𝑞𝑏𝑦 = 𝑢̅̅̅̅ ∗ 𝑞𝑏 .
𝑏̅̅̅𝑏 = velocity at bed surface, h = bed elevation and flow depth, 𝐸𝑖 , 𝐷𝑖 = Erosion and Deposition Rate𝑢of Suspended Sediment for grain size 𝑑𝑖 ; 𝑞𝑏 is directed to the velocity. Velocity is a vector. Afterevaluating 𝑞𝑏 , 𝑞𝑏𝑥 and 𝑞𝑏𝑦 are determined automatically. Bed load discharge in the direction of bedload 𝑞𝑏𝑘 is calculated by Ashida and Michue’s equation as follows:3⁄𝜏𝑢𝑞𝑏∗ = 17 𝜏∗𝑒2 (1 − 𝜏∗𝑐 ) (1 − 𝑢∗𝑐 )∗∗(7)

Where, 𝜏∗𝑒 = Non dimensional effective bed shear stress; 𝜏∗𝑐 = Non dimensional critical bed shear
stress for initiating sediment movement which is evaluated by Shields diagram. 𝑢∗𝑐 = Critical shearvelocity; The effective bed shear stress is defined as𝜏∗𝑒 =2𝑢∗𝑒(8)б( −1)𝑔𝑑𝜌Therein 𝑢∗𝑒 was evaluated using𝑣ℎ= 6.0 + 2.5 ln 𝑑(1+2𝜏 )𝑢∗𝑒∗Where, v = spatial average velocity;
Mass conservation equation of suspended sediment within the flow body is given by(9)𝛿𝑐̅ℎ𝛿𝑡+̅̅̅̅ℎ𝜕𝑟1 𝑐𝑢𝜕𝑥+̅̅̅ℎ𝜕𝑟1 𝑐𝑣𝜕= 𝜕𝑥𝜕𝑦𝜕𝑐̅𝜕𝜕𝑐̅(ℎ𝜀𝑥 𝜕𝑥 ) + 𝜕𝑦 (𝜀𝑦 𝜕𝑦 ) + E – D(10)Here, 𝑐̅, 𝑢̅, 𝑣̅ = depth avg. values for sediment concentration, x and y component of velocity.
𝜀𝑥 , 𝜀𝑦 = x and y components of dispersion co-efficient (similar to turbulent diffusion co-efficient).
E & D = Erosion and Deposition rate of sediment, 𝑟1 = correction factor.
Deposition rate can be evaluated using the formula written as
𝐷 = 𝑟𝑤0 𝑐̅
(11)Where, 𝑤0 = fall velocity of sediment particle, 𝑐𝑏 = sediment concentration at reference level in thevicinity of bed surface.
𝑐𝑐𝑏 = 𝑟 𝑐̅ , (𝑟 = 𝑐̅𝑏 ≥ 1, co-efficient)(12)
Non-dimensional form of deposition rate formula is𝐷𝑤= 𝑟 𝑢0 𝑐̅
(13)𝑢
∗

∗

Generally, deposition rate is evaluated using Eq. (11) or Eq. (13) in which r is estimated using sedimentconcentration profile in equilibrium condition. If it is supposed that the erosion rate is determined byflow characteristics (turbulence characteristics) near the bed, the erosion rate can be evaluated using
equilibrium sediment concentration 𝑐𝑏𝑒 at reference level,
𝐸 = 𝐷 (= 𝑤0 𝑐𝑏𝑒 )(14)
The equilibrium concentration of the suspended load at the reference height 𝑐𝑏𝑒 of Lane & Kalinske’s
equation is as follows,
1 𝑢𝑤

1.61

𝑐𝑏𝑒 = 5.55 (2 𝑤∗ exp (− 𝑢0 ))
0∗𝑓𝑏𝑘

(15)

Where, 𝑓𝑏𝑘 = Concentration of bed load layer for of k sediment size class. 𝑟𝑏 = Function of theexchange layer thickness.
Sediment transport model under Morpho2D, iRIC software is used in this study. Morpho 2D is theunsteady horizontal two dimensional bed deformation analysis solver which is developed by Hiroshi
Takebayashi, Kyoto University. The governing equations are written in boundary fitted generalcoordinate system (Takebayashi, 2009). Users can choose the calculation of water flow only or bed
deformation analysis. Users can choose the calculation of bed load only or bed load + suspended load.
Both uniform sediment and non-uniform sediment can be treated. When users choose non-uniform
sediment, size distribution of sediment can be calculated (Takebayashi, 2009).
RESULTS AND DISCUSSION
To check the validity of the idea and establish the concept with a view to solving the sedimentation
problem at the off-take area, a rectangular shaped simple experimental channel is made.
10 km

500 m

100 m

10 km
AB

CD

EF

1.5 km100 m

G

Figure 2: Computational domain
with 10 m grid size to observe flow
pattern over rigid and erodible bed
conditions.

Figure 4: 07 types of spur dykes are
Figure 3: Computational
considered for computation. A=100 m spurdomain with 20 m grid size to 1, B=100 m spur 2, C=100 m inclined spur,
observe sediment transport
D=150 m spur 1, E=150 m spur 2,characteristics over erodible bed F=Inclined Spur 2, G= Inclined spur withconditions.
wall.

(A) Analysis of annual and temporal changes along the Study reach

(a)

(b)

(c)

(d)

(e)

(f)

Figure 5: Annual and temporal changes along Jamuna River before and after construction of Jamuna
Bridge. (a) 1973, (b) 1980, (c) 2000, (d) 2010, (e) Changes in bank line from 1976 to 1999, (f)Changes in bank line and sand bars from 1999 to 2011.
There used to be 02 off-takes of New Dhaleswari River (Green and Red channels) before constructionof Jamuna Bridge (Fig. 5 (a) & (b)), whereas after construction of Jamuna Bridge, green channel isclosed ((fig. 5 (c) & (d)).
There is massive change isriver shape before and after
construction of Jamuna
Bridge (fig. 5 (e)). Fig. 6
shows that the off-takepoint used to shift its
locationand
riveralignment used to vary in
different years. Analysis isdone from 1999 to 2011,
this time period is after
construction of Jamuna
Bridge.
Figure 6: Annual changes of New Dhaleswari River off-take area.

(B) Comparison on flow pattern
After observing performances of 07 types of spurs dykes, 100 m spur 2 (Type B) is found most suitableform of spur dyke. For erodible bed case, from comparison of flow pattern, it is seen that without spurdyke, there is drop in velocity at the mouth and inside the branch channel at the off-take area, whereas
after employing 100 m spur 2, velocity is increased considerably inside the branch channel at the offtake area.

Figure 7: Flow pattern over erodible bed, without spur and 100 m spur 2 cases.
(C) Comparison of bed deformation
After 16 days of computation withFlow direction
discharge equivalent to 20000 m3/sec,
without spur dyke, there is clear
tendency of closing of channel due tomassive sediment deposition at the offtake area (blue marked). After
employment of 100 m spur 2, it is seenthat bed level is deepened and waterWithout spurcan easily enter into the branchchannel. For average flood dischargecase i.e. discharge equivalent to 60000
m3/sec, similar performance isobserved in terms of sediment transport
Flow direction
characteristics.
(D) Comparison of Discharge along
branch channel
Figure 9 shows the comparison ofdischarge at erodible bed case and rigid
bed case at different time steps from 0to 16 days. Here, Qo is discharge over
100 m spur 2
Rigid Bed case and Qt is Dischargeover erodible bed at different time
steps. Discharge is analysed at 1 km
downstream of the off-take, along theFigure 8: Comparison of bed deformation.
1Qt/Qo0.95
0.90.85
0.8

Without spur

0.75100 m spur 20.7

2468No. of Days

10121416Figure 9: Comparison of Discharge for erodible bed case over rigid bed case for without spur dyke, 100m spur 1 and 100 m spur 2 scenario.

branch channel. The figure says that flow discharge through the branch channel is increased
considerably as effect of spur dyke.
(E) Comparison of non-dimensional bed shear stressWithout spur dyke case, from the off-take towards the branch channel, deposition character is found.
But with 100 m spur 2 case, deposition character is reversed to erosion character which means deposition
tendency at the off-take is reversed to erosion tendency. So, there is little possibility of closing of channel
after employment of spur dyke.
0.34

τ*0.29Without spur

0.24

Deposition

100 m spur 2

0.19
0.14

Erosion

0.09
0100

200

300

400

500

600

700

800

Length of Branch Channel in Meters (from Off-take towards the Branch)

Figure 12: Comparison of non-dimensional bed shear stress for without spur and 100 m spur 2 cases.
CONCLUSIONSThe present study evaluates a technical solution to sedimentation problem at the off-take area of branchchannel with introduction of spur dyke to control stream line curvature of flow and associated generationof secondary currents. That secondary current transports sediment from the off-take area towards thecenter part of the river and it diverts more clear water (less turbid water) into the branch channel.
Sedimentation problem at the off-take of New Dhaleswari River, the most prominent distributary ofJamuna River is addressed under this study. Annual and temporal changes of Jamuna River at thevicinity of the off-take area are analyzed from 1972 to 2011. Numerical computations are done alongexperimental channels for different shapes of spur dykes at varying locations and flow pattern, bedvariation characteristics, sediment transport behavior and discharge scenario are observed. 100 m spur
2 (100 m spur constructed at 50 m downstream of the off-take point) is found the most suitable form of
spur dyke that establishes the idea of study. After employment of spur dyke, average velocity as well as
discharge are increased at the Off-take area, possibility of closing of channel due to sediment deposition
is diminished and sediment is being flushed from the off-take. The established idea can be implemented
as solution to sedimentation problem at the New Dhaleswari River off-take area.
ACKNOWLEDGEMENT
I express my heart-felt gratitude to my supervisor for this study Shinji EGASHIRA, Dr. Engr. Professor,
GRIPS for his necessary guidance and valuable suggestions whole of my study period. I thankfullyrecall his contribution in study period that influenced me to choose and study sediment dynamics in myindividual study. It is him having the biggest influence on me about creating endeavor to study sedimentphenomena along the river reaches and find a distinct target about working on the relevant field. Inaddition, I express my heart-felt thanks and gratitude to Dr. Atsuhiro YOROZUYA, Associate Professor
for his active cooperation, suggestion and inspiration at each and every step of study period. Experiencegathered while working with such learned personalities will be milestone for my future life-span.

REFERENCES
EGASHIRA, P. S. (2015). Lecture Note. Mechanics of Sediment Transportation and Channel

Changes.

EGASHIRA, P. S. (2015). Mechanics of Sediment Transportation and Channel Changes.
Takebayashi, H. (2009). iRIC Software.