A Thesis of NUMERICAL SIMULATION OF FLOW THROUGH OPEN CHANNEL WITH SERIES OF GROINS

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NUMERICAL SIMULATION OF FLOW THROUGH
OPEN CHANNEL WITH SERIES OF GROINS
SUMAN JYOTI
LAL BABU RAY
DEPARTMENT OF CIVIL ENGINEERING
DHAKA UNIVERSITY OF ENGINEERING AND TECHNOLOGY, GAZIPUR
APRIL, 2021

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NUMERICAL SIMULATION OF FLOW THROUGH
OPEN CHANNEL WITH SERIES OF GROINS
A Thesis By
Suman Jyoti
Student No: 191125
Reg No: 8199 Session: 2019-2020
In Co-operation with
MD. ZINNAT ALI
Student No: 141096
Supervised by
Dr. Mohammed Alauddin
Professor
Dept. of Civil Engineering
Submitted to the
DEPARTMENT OF CIVIL ENGINEERING
DHAKA UNIVERSITY OF ENGINEERIMG AND TECHNOLOGY, GAZIPUR
In partial fulfillment of the requirements for the award of the degree
of
BACHELOR OF SCIENCE IN CIVIL ENGINEERING
APRIL, 20Project Report Series 2021

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This series is published by
Bachelor of Science in Engineering Project Report
Department of Civil Engineering
Dhaka University of Engineering and Technology, Gazipur
The conclusion and viewpoints presented in this study are those of the
authors and do not necessarily coincide with those of the department.

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ABSTRACT
Rivers in Bangladesh are highly unstable due to their loose boundaries,
mild slope at bed and water surface, irregular siltation of huge
sediment load coming from upstream, and so on. Groins are installed in
river bank in order to deflect the flowing water away from vulnerable
zone. In many cases, conventional groins are not functioning
successfully. The effect of groins with different configurations needs
to be intensively studied to defend the river bank from erosion,
improve navigation, enrich the biodiversity of aquatic species, and so
on. The main objective of this study is to simulate flow through open
channel with series of groin models by using 2D numerical model,
iRIC Nays2DH. In this numerical simulation, K-ε model for
turbulence and Cubic Interpolation Pseudo-particle (CIP) method
for advective terms are utilized. Simulation of flow fields due to
interaction of series of impermeable, permeable and combined groins
placed in a straight channel has been made. A given flow condition is
applied for all the groins. The simulation results depict that combined
groins in series influence favorable flow fields compared to impermeable
and fully permeable groins. This causes still water zone near bank at
the downstream of impermeable part, then slow flow through the
permeable part. No strong circulation of flow at groin field and strong
current after the groin head is present.

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ACKNOWLEDGEMENT
All praises go to almighty Allah, the most magnificent merciful. The
author gratefully acknowledge their profound gratitude and
indebtedness to project supervisor, professor Dr. Mohammed
Alauddin, Department of Civil Engineering, Dhaka University of
Engineering and Technology (DUET), Gazipur for his keen interest
in this project, continuous supervision, guidance, encouragement,
inspiration and thoughtful suggestion in completing the work.
The authors also grateful to honorable Head of the Department of
Civil Engineering, Professor Dr. Md. Kamal Hossain.
Finally authors would like to express gratitude to all the faculty
members of the Department for their kind Co-operation.
Authors

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LIST OF FIGURES
Fig. 2. 1 Finite difference grid..............................................................................................................14
Fig. 2. 2 Conceptual relationship between consistency, stability and convergence.
..........................................................................................................................................................................................................15
Fig. 2. 3 An outline of the iRIC Software, its functions and features.21
Fig: 3. 1 Channel with series of impermeable groin....................................................................33
Fig: 3. 2 Channel with series of impermeable groin (with grid) on iRIC Nays2DH
.............................................................................................................................................................................................................33
Fig: 3. 3 Channel with series of permeable groin...........................................................................34
Fig: 3. 4 Channel with series of permeable groin (with grid) on iRIC Nays2DH
............................................................................................................................................................................................................34
Fig: 3. 5 Channel with series of combined groin..............................................................................35
Fig: 3. 6 Channel with series of combined groin on iRIC Nays2DH...................36
Fig: 4. 1 Channel and grids with groin (90° orientation)................................................40
Fig: 4. 2 Comparison of resultant velocity profile with the available previous study
for 90° groin............................................................................................................................................42
Fig: 4. 3 Simulated velocity vectors around the groins for (a) series of impermeable,
(b) series of permeable groin, and (c) series of combined groins................44
Fig. 4.4 Simulated streamlines around the groins for (a) series of impermeable, (b)
series of permeable, and (c) series of combined groin..........................................46

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Fig: 4. 5 Velocity magnitude around the groins for (a) series of impermeable (b) series
of permeable and (c) series of combined groin.............................................................48
Fig: 4. 6 Simulated velocity fields in the 1st groin field.................................................49
Fig: 4. 7 Comparison of simulated velocity profiles along the channel at D/S of 1st
groin......................................................................................................................................................................51
Fig: 4. 8 Simulated velocity fields at the mid portion of 3rd and 4th groin.51
Fig: 4. 9 Comparison of simulated velocity profiles at the mid portion between 3rd and
4th groin...........................................................................................................................................................53

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LIST OF TABLES
Table: 3. 1 Model Constants............................................................................................................26
Table: 3. 2 Channel dimension...........................................................................................................31
Table: 3. 3 Groin Parameter (Impermeable)................................................................32
Table: 3. 4 Groin parameter (Permeable)..............................................................................33
Table: 3. 5 Groin parameter (Combined)...............................................................................34
Table: 3. 6 Flow conditions.......................................................................................................................36

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Contents
ABSTRACT...............................................................................................................................................................i
ACKNOWLEDAGEMENT..................................................................................................................vi
NOTATIONS.......................................................................Error! Bookmark not defined.
LIST OF FIGURES.............................................................................................................................vii
LIST OF TABLES...................................................................................................................................ix
CHAPTER I INTRODUCTION.....................................................................1
1.1 General................................................................................................1
1.2 Background........................................................................................1
CHAPTER II LITERATURE REVIEW.................................................6
2.1 General...................................................................................................6
2.2 Literature review............................................................................6
2.3 Generalized simulation procedure ...................................11
2.3.1 Simulation and modeling ..........................................................12
2.3.2 Components of a numerical solution ..................................12
2.3.3 Discretization approaches .....................................................13
2.3.4 Numerical grid..............................................................................13
2.3.5 Properties of numerical solution methods .....................14
2.4 iRIC software ...........................................................................15
2.4.1 Features of the flow field calculation model .............21
CHAPTER III METHODOLOGY.....................................................................24
3.1 Introduction................................................................................24
3.2 Software used .............................................................................24
3.3 Basic equation ...............................................................................25
3.3.1 K-ε model.......................................................................................25

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3.3.2 Equation of continuity .............................................................26
3.3.3 Equation of motion......................................................................26
3.3.4 Bottom friction calculation method..................................29
3.4 Operational procedure of software ..................................30
3.5 Calculation conditions..................................................................36
3.6 Working procedure......................................................................37
CHAPTER IV SIMULATION OF FLOW................................................39
4.1 General...............................................................................................39
4.2 Verification of numerical model............................................39
4.3 Simulated flow fields ................................................................42
4.4 Simulated streamlines ................................................................44
4.5 Comparison of simulated velocity profiles ....................... 46
CHAPTER V CONCLUSION AND RECOMMENDATION55
5.1 Introduction.................................................................................55
5.2 Conclusion..........................................................................................55
5.3 Recommendations for further study............................... 56
REFERENCES..............................................................................................................................................58

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CHAPTER I
INTRODUCTION
1.1 General
Bangladesh is a great delta formed by the three mighty river
systems: the Ganges, the Brahmaputra and the
Meghna. There are around 400 rivers in the country. Most
of the basin area of major rivers of the country is located
outside the country, serving as the predominant sources for
rivers that go with flow through the nations China, Bhutan,
Nepal and India, and eventually passing into the Bay of Bengal
to the south of Bangladesh. Every year hundreds of hectares
of land are eroded by means of the rivers (NWMP, 2001).
No other disasters are as disastrous as riverbank erosion
in phrases of long-term effect on people and society (Elahi,
1991). To shield river erosion, use of groins is
very popular and effective. The following sections describe
background, objectives and scope of the present study.
1.2 Background

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Recurrent flood phenomena occurred after heavy rainfall,
riverbed and bank of the rivers tend to erode due to high
velocity. Also, there are some other reasons for bank erosion like
very mild slope in riverbed and water surface, loose sedimentary
formation in channel boundaries, small depth of flow due to
siltation of huge sediment load coming from upstream, and so on.
The simulation of water flow in rivers has been the subject of
many researches in the field of hydraulics and river engineering
(Azevedo and Gates, 2000). In river and coastal
engineering, groins are very important structures for river
navigation, coastal protection, and beach reclamation (Sarveram
and Shamsai, 2012). These hydraulic structures have been
constructed in river bank in order to deflect the flowing water
away from vulnerable zone. The main purpose of building up of
such obstacles on natural river bank is to divert the direction of
water flow so that bank erosion can be eliminated. Construction
of such obstacles against water flow causes significant change in
flow patterns, sediment transport and bed topography. Many

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experimental and numerical researches have been done in order
to examine flow pattern and scouring around groins (Francis et
al., 1968), (Rajaratnam and Nwachukwu, 1983),
(Choudhury et al, 1995), (Tang and Ding, 2007),
(Hossain et al., 2012), (Pandy et al., 2015), (M.
Shahjahan et al, 2018). In different conditions of groin
length, groin installation angle towards the approaching flow,
permeable or impermeable states, submerged and non-submerged
states and number of groins, and so on (Yeo et al., 2005).
Due to the variety in the configuration of groins, flow
separation and recirculating length would be greatly different,
which is a challenge to the applications of numerical models
(Quanhong and Pengzhi, 2007).
In stabilizing river channels, groins are extensively used.
Impermeable groin imposes a huge obstruction to flow, and
change the flow profile abruptly. Fully permeable groin cannot
modify the flow environment rightly, and flow near bank occurs,
which could affect the bank when associated with oblique flow.

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Combined groins, which have impermeable parts near bank, could
improve the flow field to have favorable environment for groin
and bank stability.
In this study, open channel flow with three different
arrangements of groins in series – impermeable, permeable and
combined groins, is simulated with a two-dimensional (2D)
numerical model, iRIC Nays2DH.
1.3 Objectives
The main objective of the present study is to investigate flow
profiles around different types of groins. So that, the effects
of groins in an open channel can be evaluated. The specific
objectives are as follows:
 To simulate the flow through open channel with groins placed in a series.
 To determine the flow fields and compare the channel responses due to three different groin structures.
1.4 Scope of the study
In nature, there are various flow conditions, complex channel
geometry and boundary materials. The channel boundaries are
composed of loose sedimentary materials, very irregularity in the

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geometry of channel boundary, flow varies highly there.
However, a simplified open channel has been considered in the
present study. A straight channel with immoveable boundary is
considered and constant flow is maintained in the study to
investigate the effects of groin structures.

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CHAPTER II
LITERATURE REVIEW
2.1 General
Groin is an elongated obstruction having one end on the bank of
the stream and the other end projecting into the current. It
reduces the flow velocity in the critical area and encourage
deposition of sediment in the downstream area of the structures
near the bank line. The simulation of flow in rivers has been the
subject of many researchers in the field of hydraulics and river
engineering. In this chapter, the available previous studies made
on channels with groins are reported. Besides, some basic
information on groin functioning, concepts of numerical modeling
and description of iRIC Nays2DH software are illustrated.
2.2 Literature review
Groins have been used extensively all over the world as river training and bank
protection structure to reduce the current along the stream bank, thus
reducing the erosive capacity of the stream and in some cases including

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sedimentation between groins. In the groin-flow study, the dead-water zone
between groins is called the groin field. Secondary flow produces and develops
in the groin field, and one or more vortices dominate the flow structures in
the zone. A mixing layer is prevailed at the interface between the groin field
and the main channel. Eddies are produced near the groins and then move
downstream owing to the disturbance of the flow by the groins. The mean
velocity in the main channel exceeds that of the groin field. The properties
of groin flow significantly influence the velocity profile, transport of sediment
and pollutants in rivers. Groin flow properties, including the velocity
distribution, turbulent kinetic energy and flow structures, are related to
groin parameters, such as the groin height, groin length, angle to the bank,
and aspect ratio (the ratio of the groin length to the distance between two
adjacent groins).
In broad aspect, the functions of groins not only to keep banks from erosion
(Przedwojski 1995), they improve the navigation capability of the main
channel, restores river ecosystems (Hood, 2004), and increases biological
diversity. Groins can protect neighboring banks from scouring by guiding flow
to a certain direction. The flow is concentrated in the center of the river,
and the velocity simultaneously increases in the main channel and the vicinity of
groin heads (Wu et al., 2005). In contrast, the groins can induce a rising
of water level during extreme conditions and hence, increase the risk of levee
overtopping and collapsing (Pinter et al. 2001).

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The first research carried out in this field by Francis et al.
(1968). They considered separation zone for various types
of groin in a rectangular flume, but they did not measure the
flow velocities. Researches done by Rajaratnam and Nwachukwu
(1983) did velocity measurements in the flow induced by
groins. Also, Choudhury et al. (1995) applied numerical
methods for the solution of flow field around groins. Molls et al.
(1995) also developed a general mathematic model to solve the
unsteady two-dimensional depth averaged equations by combining
it with a constant eddy viscosity turbulent model. Various models
in considering turbulence under different flow conditions of and
groin dimensions are utilized to investigate scour around groins by
Zhanfeng and Xiaofeng (2006), Quanhong and Pengzhi
(2007), Tang and Ding (2007), and so on.
Kim and Choi (2003) conducted an experiment on numerical
simulations of open-channel flows in a bend using the finite element
method. They used the 2D numerical algorithm for this

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simulation. The proposed model was applied to 180° bend
experimented by Rozovskii (1961). The simulated results
compare favorably to measured data. Then, the model was used
to simulate flows in a 7.7 km curved reach in the Han River,
Seoul, Korea. They also investigate the impact of planting
vegetation on each side of floodplain.
Masjedi and Foroushani (2012) studied the effect of
different shape of single spur dike in river bend on local scour.
Hossain et al. (2012) applied a numerical model on a
rectangular straight channel which is 13.3m long and 0.8m
wide. The barb structure is installed as a obstacle at 45°,
90° and 135° on the upstream portion of river channel. The
length of the barb is 24cm, one third of the cross-section. The
numerical simulations have been done considering sediment
transport. Barbs have been used by Natural Resources
Conservation Service (NRCS) of US department of
agriculture, in Origon for river and stream bank protection since
the late 1980.

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Permeable spur dikes have the advantages of being more stability
requiring easier maintenance than impermeable ones (Kang et al,
2011). Scour around spur dikes occurs under both clear
water and live bed conditions. Where there is no transport of
sediment by the approaching flow to the region of scour activities
around the spur dike or under live bed scour where sediment is
transported by the approaching flow as bed load or suspended
load to the scour hole at the spur dike (Pandy et al., 2015).
M.Shahjahan et al, (2018) predicted velocity profiles and
bed shear stress profiles for 45º, 90º and 135º angles
of groin in a straight channel.
Most of the studies through experiments or numerical simulations
were done about a single groin with different orientation, and
they found out several merits and demerits of impermeable and
permeable groins. For impermeable groins, flow is obstructed
largely, and because of flow separation and strong vortices
developed near the groin head, huge scour occurs near groin,

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which hampers stability of groins. In case of permeable groins,
the flow partly penetrates the structures which results in a
considerable reduction in velocity, vortex strength and shear
force at the nose of the spur dike (Li et al. 2005). Fully
permeable groins allow flow near bank, and do not divert the flow
far from the bank. However, from the experimental
investigation, this has been found that a combined groin (a
combination of impermeable and permeable part) can influence and
develop dead zone in the groin field (Alauddin et al., 2011).
In the present study, numerical simulations have been
conducted for investigating flow fields around groins of three
different designs. These groin models are – impermeable,
permeable and combined. The comparison of change in velocity
profiles due to different groin models can give important
information to explore a suitable design of a groin.
2.3 Generalized simulation procedure

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2.3.1 Simulation and modeling
Simulation of a system is used for understanding the physics of
the process of the system in time and space. Generally the
simulation process contains following steps:
 Conceptualization of process of physical system.
 Transforming the process of physical system by into partial
differential equation.
 Transforming the partial differential equation into
algebraic equations or numerical equations.
 Development of the solution algorithm of the numerical
equations.
 The computed codes are written.
 Finally, the model is validated.
2.3.2 Components of a numerical solution
A numerical solution method contains following components –
 Mathematical Model,

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 Discretization Approach,
 Coordinate and Basis Vector Systems,
 Numerical Grid,
 Finite Approximations,
 Solution Method,
 Convergence Criteria,
Discretization Approach and Numerical Grid are two most important
components. Here, these have explained.
2.3.3 Discretization approaches
There are three discretization approaches –
 Finite Difference Method
 Finite Volume Method
 Finite Element Method
2.3.4 Numerical grid
The discrete locations at which the variables are to be calculated
are defined by numerical grid which is essentially a discrete
representation of the geometric domain. It divides the solution

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domain into a finite number of sub domains (grids, elements,
control volumes, etc).
 Continuous function is replaced by discrete function as grid
or elements in a process, which is called discretization. Finite
difference grid is shown below (Fig. 2.1).
 Discrete time steps approximate continuous time.
Fig. 2. 1 Finite difference grid
2.3.5 Properties of numerical solution methods
The solution method should have certain properties. In most
cases it is not possible to analyze the solution method. One analyzes
the components of the method; if the components do not possess
the desired properties, neither will the complete method but
the reverse is not necessarily true. Some important properties
are given below –

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 Consistency
 Stability
 Convergence
 Conservation
 Roundedness
 Realize ability
 Accuracy
Fig. 2. 2 Conceptual relationship between consistency, stability and convergence.
2.4 iRIC software
The International River Interface Cooperative (iRIC) is an
informal organization made up of academic faculties and

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government scientists with the goal of developing, distributing,
and providing education for a public-domain software interface
for river modeling. Nays2D is an analytical solver for calculation
of unsteady 2D plane flow and riverbed deformation using
boundary-fitted coordinates within general curvilinear
coordinates. Professor Yasuyuki Shimizu of Hokkaido University
firstly developed the solver’s prototype in the 1990s. After
many improvements, first it was adopted in 2004 as a
calculation solver for incorporation in the iRIC-Nays riverbed
deformation calculation pre post software of the Foundation of
Hokkaido River Disaster Prevention Research Center (Version
1.0).
Later refined by the inclusion of dynamic memory allocation by
Ichiro Kimura of Hokkaido University and outfitted with a bank
erosion model by Yasuyuki Shimizu and a HotStart function by
Toshiki Iwasaki of Hokkaido University, Nays2DH (Version
2.0) was distributed as one of the solvers included with iRIC
in 2010 at the release of iRIC Version 1.0.

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Further functional additions included the incorporation of a
mixed-diameter multilayer model proposed by Toshiki Iwasaki. A
river confluence model proposed by Takuya Inoue and Michihiro
Hamaki of Kaihatsu Koei Co. Ltd. Nays2DH was registered as
a calculation solver for iRIC Version 2.0 in March 2011
under the planning and supervision of Kazutake Asahi from the
Foundation of Hokkaido River Disaster Prevention Research
Center (Version 3.0).
This model has an established reputation for calculation of
unsteady flows accompanied with turbulence and laminar flow and
it is capable of dynamically showing the realistic motions of
unsteady eddies. In addition, its riverbed deformation
calculations can reproduce the generation, development and
migration of sandbars with high precision. There have been many
examples of Nays2D being used on actual rivers for purposes
such as assessing the effects of trees and vegetation, making
flood calculations, studying the effects of inflowing rivers and

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simulating bank erosion disasters. Nays is a model developed at
Hokkaido University; the details of the model are described in
Shimizu (2002). The title of the model is not an acronym; it
is an Ainu word meaning “small river“.
There are four primary features of Nays:
 It is two-dimensional (vertically integrated).
 It is fully unsteady.
 It is cast in a general, non-orthogonal coordinate system
with variable cell size.
 It includes a much more sophisticated method for treating
turbulence that includes both a horizontal large eddy
simulation and a suite of turbulence closures.
The non-orthogonal coordinate system allows more precise fitting
of the coordinate system to suit arbitrary channel curvature
and variable width. More importantly, the more detailed
treatment of turbulence and large eddies allows predictions of
time-variable behavior even for steady discharges. Nays is

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currently the most sophisticated model within iRIC in terms of
handling advection of momentum and strong local unsteadiness.
Nays also includes full sediment-transport capabilities and
morphologic change prediction, so the impacts of unsteadiness on
bed change can assessed with this approach. Furthermore, Nays
provides a variety of particle-tracking information.
Formed in late 2007, the group released the first version
of this interface, iRIC, in 2009. The purposes of the
activities of this project are creation of opportunities for
interaction and provision and exchange of information on issues to
utilize knowledge and engineering about rivers. To support
creation of more beneficial and sustainable river environments
among administrative engineers, construction consultant
companies, river researchers and students focusing on
development of software for analysis of stream flows, river bed
fluctuations and floods. The iRIC software interface includes
models for two- and three-dimensional flow, sediment transport,
bed evolution, groundwater-surface-water interaction,

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topographic data processing, and habitat assessment, as well as
comprehensive data and model output visualization, mapping, and
editing tools. All of the tools within iRIC are specifically
designed for use in river reaches and utilize common river data
sets. The models are embedded within a single graphical user
interface so that many different models can be made available
to users without requiring them to learn new pre- and post-
processing tools. iRIC provides a comprehensive, unified
environment in which data that are necessary for river analysis
solvers (hereafter, solvers) can be compiled, rivers can be
simulated and analytical results can be visualized. The highly
flexible iRIC interface allows various solvers to be imported.
Upon selecting the solver, iRIC selects functions suitable for
the solver and prepares the optimal simulation environment.
Because the iRIC functions vary depending on the solver, the
method of using the iRIC application depends on the solver.
The iRIC software consists of three functions: preprocessor,
postprocessor, and solver (Figure 2.3)

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Fig. 2. 3 An outline of the iRIC Software, its functions and features.
2.4.1. Features of the flow field calculation model
As a coordinate system, the general curvilinear coordinate
system is adopted, allowing direct consideration of complex
boundaries and riverbed shapes.
 Calculations involving the confluence of a main channel and a
tributary can be performed.
 For the finite-difference method applied to the advection
terms in equations of motion, the user can choose the upwind
difference method (primary precision) or the CIP method.

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This is a high-order finite-difference method. By using a
cubic polynomial as an interpolation function, numeric
diffusion is reduced, thus enabling high-precision local
interpolation.
 For the turbulent field calculation method, the user can
select one from Constant eddy viscosity model, Zero-
equation model, and K-ε model.
 Various settings are possible for boundary conditions of the
upstream and downstream ends, including periodic
boundary conditions, downstream end water surface
elevation setting and upstream end velocity setting. This
makes it easy to set boundary conditions from limited
observation data.
 For setting the initial water surface profile, the user can
select from among [Constant slope], [Line], [Uniform flow
calculation] and [Non-uniform flow calculation].

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 The bottom friction evaluation method is set by using
Manning's roughness parameter. This parameter can be
set to each computational cell.
 Any obstacle within the calculation target area can be
taken into account on a calculation-cell basis. For each
calculation cell, a flag can be set to define an obstacle. By
this means, river structures such as bridge piers can be
easily incorporated in calculation.
 The effect of vegetation for the flow calculation can be
introduced as a drag force. The user can set the density
of vegetation in each computational domain.

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CHAPTER III
METHODOLOGY
3.1 Introduction
In the present study, three different types of series groin
models were considered for investigating the flow pattern due
to a constant flow. In this chapter, the details of flow domain,
flow parameters, and simulation of flow due to different groin
series are presented. The simulation works were conducted using
iRIC Nays2DH. A brief description of 2D numerical
model, iRIC Nays2DH and its operational procedure have
been given in the following sections.
3.2 Software used
In this study, iRIC Nays2DH is used to simulate the flow
fields in an open channel with groins. iRIC (International River
Interface Cooperative) is a pre- and post-processing software
application and framework for computation of flow and sediment
transports in rivers. The application through a Graphical User
Interface (GUI) allows the model user to build, run, and

25
visualize the results from the system. The GUI provides tools
for building both structured and unstructured grids, defining
topography and other boundary conditions on the grid, and
defining grid-dependent values such as grain size, vegetation,
and obstacles by mapping measured values to the grid or by
creating user-defined polygons with attributes of grid
dependent value. It combines the functionality of
MD_SWMS, developed by the USGS (U.S. Geological
Survey) and iRIC-Nays, developed by the Foundation of
Hokkaido River Disaster Prevention Research Center.
3.3 Basic equation
3.3.1 K-ε model
The eddy viscosity coefficient in the standard k-ε model can be
expressed by the following equation:
where, Cμ is a model constant. k and ϵ are obtained by the
following equations:

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Table: 3. 1 Model Constants
Where C1ε, C2ε, σk and σε are model constants whose
respective values are shown in Table 2.1 note that Pkv and
Pεv are calculated with the following equations:
3.3.2 Equation of continuity
The equation of continuity is given below:
3.3.3Equation of motion
The equation of motion is given as:
Cμ C1ε C2ε σk σε
0.09 1.44 1.92 1.0 1.3

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Where,
As for diffusion terms Dξ and Dη in the motion equation in
general coordinates, since developing those terms will make the

28
number of terms huge, they are simplified by assuming the
following conditions:
The second-order derivative for the metric coefficient is
assumed to be locally zero. Those terms are locally treated as
pseudo-orthogonal coordinates. As a result, the diffusion terms
can be approximated as follows:
Where, ξr and ηr are parameters each representing the ratio
of the local grid size in general coordinates to the full-scale
length of the grid. They are defined as follows:
Note that to derive the approximate equations of Dξ and Dη
above; the following relations are used, based on the assumption
of a relationship of local orthogonally.

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Where, θ represents the angle formed by the x axis and the
ξ axis (or the y axis and the η axis).
3.3.4Bottom friction calculation method
In Nays2DH, bottom friction is set using Manning's
roughness parameter. For Manning's roughness parameter, the
user can define this parameter in each computational cell.
Riverbed shearing forces τx and τy are expressed by using
coefficient of riverbed shearing force Cf. The coefficient of
riverbed shearing force Cf is estimated by Manning's roughness
parameter nm as follows:

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This Manning’s roughness parameter can be estimated from the
relative roughness height, ks, by using the Manning – Strickler
equation as follows:
3.4 Operational procedure of software
The following are the basic procedures for operating Nays2DH with iRIC:
1. Launching Nays2DH
Prepare to use Nays2DH with iRIC
2. Create new project
Create a new project select Nays2DH
iRIC.3x 1.0 64bit
3. Creating Calculation grids
Create a grid for calculation import
channel measured data. Which prepare in
excel at .riv format.
5. Making simulations
Use Nays2DH to run the simulation
6. Visualizing the calculation results
Visualize the simulation results, such as
flow velocity, water depth, by using
vector map to see whether the simulation
has successfully run and save the file.
4. Setting calculation conditions
Set simulation discharge, boundary
conditions, roughness and other items

31
Three different types of groins in series were considered in the
present study. These models are – Impermeable, permeable
and combined (combination of impermeable and permeable parts)
groins. All groins are placed perpendicular to the channel bank. A
nonlinear 𝑘-𝜀 model is used to predict the turbulent flow field
by capturing the anisotropic turbulence. Cubic interpolation
pseudo-particle (CIP) method was used for advection term.
The basic equations are discretized as fully explicit forms and
solved successively with the time increment in step by step. It is
solved using iterative procedure at each time step.
The concrete channel under hydraulic engineering laboratory of
DUET, Gazipur, which has the dimension of length 20.50
m, width 1.52 m and depth 1.12 m is considered in the
numerical simulation. The information of channel and groin models
have been presented in Tables 3.2 - 3.5.
Table: 3. 2 Channel dimension
Length of the channel 20.50 m

32
Table: 3. 3 Groin Parameter (Impermeable)
Fig. 3.1 shows model setup and Fig. 3.2 shows calculation grid
for impermeable groin.
Width of the channel 1.52 m
Depth of the channel 1.12 m
Length of the groin 0.45m
Thickness of the groin 5cm
Number of groin 5 Nos
Position of first groin 7.5 m from U/S
section
c/c distance between groin 0.90m

33
Fig: 3. 1 Channel with series of impermeable groin
Fig: 3. 2 Channel with series of impermeable groin (with grid)
on iRIC Nays2DH
Table: 3. 4 Groin parameter (Permeable)
Length of the each groin 0.45m
Size of block(each) 3cm
Size of opening(each) 6cm
Total impermeable portion 15cm
Total permeable portion 30cm

34
Fig: 3. 3 Channel with series of permeable groin
Fig: 3. 4 Channel with series of permeable groin (with grid)
on iRIC Nays2DH
section
Table: 3. 5 Groin parameter (Combined)

35
Fig: 3. 5 Channel with series of combined groin
Length of the each groin 0.75m
Total impermeable portion 0.45m
First impermeable portion 0.30m
Size of block (each) 3cm
Size of opening (each) 6cm
section

36
Fig: 3. 6 Channel with series of combined groin on iRIC Nays2DH
3.5 Calculation conditions
A certain flow condition was considered in this study, and it was
maintained same for all cases. This condition has been explained
below:
A constant discharge of 0.05 m3/sec is used in the simulation. The
depth of flow in the channel was kept constant, and it was around
22.5 cm in the channel. The flow discharge remains same for
all the cases in this study. Related parameters are given in Table
3.6 below:
Table: 3. 6 Flow conditions
Discharge (m3/sec) 0.05
Depth of flow (m) 0.225
Time period (minutes) 30

37
Bed slope (𝑆𝑜) 0.001
3.6 Working procedure
The numerical simulation works were done as mentioned in the
following procedure:
i. First, the channel geographic data have been collected and
prepared for making calculation grid.
ii. Then, the geographic data is imported, and grid is
created.
iii. In the grid made in the previous step, obstacle cells are
defined in some certain locations to function these as groins
as per groin parameters mentioned earlier.
iv. After that, calculation condition is given for computation.
Constant discharge at upstream and constant depth with
zero velocity gradients was given as downstream boundary
conditions.

38
v. The simulations are done and the calculation results are
extracted for further analyses.

39
CHAPTER IV
SIMULATION OF FLOW
4.1 General
2D numerical model Nays2DH is used to simulate the flow
field in a straight open channel with 90º groin angled with the
flow direction with three different series of groin models. The
simulation has been performed utilizing channel and flow conditions
and analyzed for comparing velocity profiles induced by
different groins. These have been explained in the following
sections.
4.2 Verification of numerical model
The model is verified with experimental data of Rajaratnam and
Nwachukwu (1983), where they investigated the flow-fields
in a laboratory flume near a groin-like structure. The flow-fields
of the physical model are well reproduced by the numerical model
outside of the groin field, but some discrepancies are present
downstream of the groin where the flow is highly skewed.

40
The sketch of the flow domain for flows in an open channel with
groin of 90° angled to the direction of flow which was
performed under the same conditions of the experiments
conducted by Rajaratnam and Nwachukwu (1983). The
length of the channel 6m, width of the channel 0.9m, discharge
0.0430 𝑚3
/𝑠, water depth at downstream 0.001m,
manning’s roughness co-efficient 0.01 were taken .
Fig: 4. 1 Channel and grids with groin (90° orientation).
For 90° groins, the resultant velocity profile is compared with
available experimental result measured at y/l =2 and 3, where

41
l = 141mm is the length of the groin. Fig. 4.2 shows the
comparison of velocity profile for lateral distance (y/l =3 ). Here
the initial flow velocity profiles for different lateral distance
(y/l). Here the initial flow velocity and water depth were 𝑈0 =
0.50 m/s and H = 0.189m, respectively. In the figure,
velocity is normalized by 𝑈0. In the figure, x/l = 0 indicated
the groin position along flume direction.
Good agreements are found between experimental and
calculated results. For the velocity profile, the result obtained
by iRIC Nays2Dh is very near to the experimental result.
The calculated results under predict the experimental data in
the downstream zone of the groin. The numerical result provide
by (Quanhong and pengzhi, 2007) and (Sarveram et al.,
2012) also under predicted the experimental data largely
in this region. This may be due to the very high velocity gradient
arising in this region which makes the depth-averaged model
inapplicable. Otherwise, the possible reason may come from the
experimental measurement errors in this region. In the figure

42
only present data compared with the experimental data by
Rajaratnam and Nwachukwu (1983).
(a)Velocity along y/l= 2
(b)Velocity along y/l= 3
4.3 Simulated flow fields
Fig. 4.3 shows the simulated velocity vectors of the flow field
around the groin for three different series of groin with a
0
0.5
1
1.5
2
-6 -4 -2 0 2 4 6
w/u
x/l
Calculated (present study)
Measured (Rajaratnam & Nwachukwu, 1983)
0
0.5
1
1.5
2
-6 -4 -2 0 2 4 6
w/u
x/l
Calculated (present study)
Measured (Rajaratnam & Nwachukwu, 1983)
Fig: 4. 2 Comparison of resultant velocity profile with the available previous study for 90°
groin

43
constant flow. The upstream flow of channel started from right
side. For all the cases, the model is found to reproduce the
general flow features of the flow field around the groin
successfully. From the simulated results, it is seen that at the
upstream boundary the flow is uniform and hence the flow
vectors are straight and parallel to bank. However, when the
flow approaches the groin, the flow is deviated towards the
right bank compared to the left due to obstruction of flow by
the groin. Recirculation of flow zone is observed just downstream
region of the first groin for impermeable series of groins. In
permeable condition, the velocity vectors are not disturbed as
found in the impermeable case, though these have same length.
For the impermeable series portion, the velocity vectors are too
closed at the main channel area after the first groin. In
combined series of groin, the velocity vectors are modified
compared to both impermeable and permeable cases. Small
recirculation of flow occurs at the downstream of each groin along
the length.

44
(a)
(b)
(c)
Fig: 4. 3 Simulated velocity vectors around the groins for (a) series of impermeable, (b) series of permeable groin, and (c)
series of combined groins.
4.4 Simulated streamlines
Fig. 4.4 shows the streamline around series of groin for all cases such as impermeable, permeable and combined series
condition, which is obtained by the present study. From the streamline contour it is found that at the position of groin
head the flow is deviated towards the opposite bank and at the downstream of the groin. In permeable series of groins
the flow pass through the permeable portion. The flow stream line is less disturb as impermeable groins. Due to the
U/
S
U/
S
U/
S

45
permeable portion on the combined series of groin the flow can occur along the permeable portion so that the velocity
intensity reduced on the upper portion of the groins. The Figures 4.2 depict that series of impermeable groin
deviations of streamline are higher compared to series of combined groin. In the combined series of groin shows the least
deviation compared to series of impermeable groin.
(a)
(b)
(c)
U/
S
U/
S
U/
S

46
Fig. 4.4 Simulated streamlines around the groins for (a) series of impermeable, (b) series of permeable, and (c) series of
combined groin.
4.5 Comparison of simulated velocity profiles
Fig. 4.5 shows the predicted velocity magnitude of the flow
field around the groin for three different series of groin with
a constant flow. The results are found to be similar in nature.
For all cases, the velocity contour along the left bank is higher
than right bank at the downstream region of the groin; it
indicates the deflection of flow towards right bank and
sheltering of flow by groin at left bank. For the impermeable
series of groin the velocity of flow is very high at just upper
position of groins head. The velocity of flow is pick at the first,
fourth and fifth groin head. Though the same length between
impermeable and permeable groin the velocity is very low for
permeable groins. It is due to in the permeable series of groin
the total impermeable length is 15cm, where the impermeable
groins total impermeable length is 45cm. At the same time for
the combined series of groin the velocity is pick after the groins
head but the intensity of flow velocity is comparatively low than

47
impermeable series of groin. For the combined series of groin the
velocity intensity at the impermeable zone is very low and in the
permeable zone flow intensity increase very slowly. Though large
dead zone occurs at the downstream of series impermeable groin
but here also produced one or more recirculation with high
velocity. Scouring have done in this area. So the stability of groins
structure is affected by scouring. In the series combined groin
the velocity intensity very large at the 5th groin head. In the
interior of the series combined groin, the dead zone occurs that
is comparatively large and the recirculating area is small with low
velocity with respect to impermeable series of groin.
(a)

48
(b)
(c)
Fig: 4. 5 Velocity magnitude around the groins for (a) series of impermeable (b) series of permeable and (c) series of combined
groin.
The figure given below (Fig. 4.6) shows the predicted velocity
profiles for series of impermeable, permeable and series of
combined groin at different points along the channel. For each
circumstance, the longitudinal velocity profiles are compared at
a lateral position of the channel bed. In the graph, horizontal
axis presents the velocity magnitudes of the flow and the vertical
axis represent the lateral distance of the channel bed.

49
Fig: 4. 6 Simulated velocity fields in the 1st groin field.
Figure 4.6 represent the position of 1st groin D/S and 2nd
groin U/S portion in different series groin case. Figure 4.7
represents the flow velocity at the downstream of the 1st groin
along the channel width. Both figure shows the comparison the
velocity intensity in these area produced by the series of
impermeable, permeable and series of combined groin. The
velocity intensity produced by the impermeable groin at the D/S
along the groin length is very low intensity. The velocity intensity
reached at pick position just upper the position of the groin head.
For permeable series of groin condition the velocity along the
groin length is high with respect to impermeable series of groins.

50
The velocity intensity through the width of channel is medium
due to the resistant portion is very small comparatively
impermeable groins. At the same time for the combined groin,
the velocity intensity in the impermeable zone is lower than the
impermeable groin. After the impermeable zone the velocity
intensity rise a small portion and varies depending upon the
permeable and impermeable portion along the length of the groin.
At the groin head the velocity increase slightly but the intensity
is low rather than impermeable groin head. In this point a dead
zone produced by impermeable and combined groin but for
permeable groin no dead zone produced. Single but large
recirculation produced by impermeable groin, for permeable
groin no recirculation produced and for combined groin single
recirculation produced a low flow intensity.

51
Fig: 4. 7 Comparison of simulated velocity profiles along the channel at D/S of 1st groin.
Fig: 4. 8 Simulated velocity fields at the mid portion of 3rd and 4th groin.
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8
Velocity
(m/s)
Transverse Distance (m)
Impermeable Permeable Combined

52
Fig. 4.8 represents the mid groin field, i.e area bounded by
3rd and 4th groin for all groin cases. Fig. 4.9 represents the
flow velocity at the middle of the 3rd and 4th groin along the
channel width. The graph shows the evaluation of velocity
intensity in middle position along the width produced by the series
of impermeable, permeable and series of combined groins. Here
the flow velocity intensity for the impermeable groin is very low
along the length of groin and after the groin head the velocity
is instantly higher. For the permeable groins the velocity have a
small change with respect to impermeable groins. At the same time
the velocity intensity for the combined groin is comparatively low
in the impermeable zone. In the permeable zone the velocity
intensity is increase but lower than velocity intensity produced
by impermeable groin. The pick flow occur after a certain
distance from the head of combined groins. This makes the
combined groin more stable in sustainable life. In impermeable
groin condition approximate two large recirculation produced and
for combined groin two small recirculation produced. Due to the
high velocity no deposition occurs in that area and maintain the

53
flow depth. Though the total impermeable portion between
impermeable and combined groins are same but the velocity
reduction pattern is very good for combined groins and the dead
zone produced by combined groins is large. The size of
recirculation area also small.
Fig: 4. 9 Comparison of simulated velocity profiles at the mid portion between 3rd and 4th groin.
From the simulation data, this can be found that installation of
permeable groin produces slow flow near side or bank. Although,
a dead zone is created in the groin field due to impermeable
groin, strong recirculation of flow might be present which can
0
0.1
0.2
0.3
0.4
0.5
0.6
0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8
Velocity
(m/s)
Transverse Distance (m)
Impermeable Permeable Combined

54
attack the bank line. The flow velocity near the head of the
impermeable groin is found very high. So that the possibility of
scouring near groin is very high, and thus stability of the groin
might be hampered. As the length of permeable groin was kept
same of impermeable one, velocity at main channel area is not
increased enough, which could assist improving navigation facility.
In permeable groin condition though the velocity reduced but
no dead zone produced at the groin bank side. In the combined
series of groins, the total impermeable portion is same as
impermeable groin. Even though, this has same obstruction,
because of the presence of permeable part with impermeable
part, velocity of flow is not increased much near the head of
groin; so that, less scour might be expected. In addition, due
to the impermeable portion near bank, a dead zone of less
circulation of flow is created which might influence deposition of
sediment near bank.

55
CHAPTER V
CONCLUSION AND RECOMMENDATION
5.1 Introduction
Two-dimensional numerical model Nays2DH was employed in the
present study to investigate the characteristics of flow field
developed around groin structures. From the numerical simulation
performed in the study, the velocity profiles are compared for
different types of series groin for evaluating their performance.
The following sections describe conclusion and recommendations.
5.2 Conclusion
This study has given the detailed information regarding the flow
fields due to groins in an open channel. From the numerical
simulation, the general flow features around a groin is
reproduced successfully. The flow fields found in the study can
be described as:
i. The flow at upstream side is uniform.

56
ii. Circulation of flow is found at the downstream of the
groin that is created due to sheltering the flow by
impermeable groin.
iii. High velocity zone is created after the groin due to the
deflected flow.
From the comparison of flow profiles due to series of
impermeable, permeable and combined groins, this is observed
that the velocity is maximum near the head of impermeable groin,
and this is much less for permeable and combined groins. Strong
circulation of flow is present for impermeable groin; however, a
dead zone of less circulation is created near side of channel due
to combined groin.
5.3 Recommendations for further study
Following recommendation can be made for further research:
i. The study can be extended to simulate the sediment
transport, nutrient transport and bed deformation due to
groins.

57
ii. As the flow near a groin is distinctly three-dimensional, it is
recommended to use, a 3D numerical model needs to study
flow vortex and local scour.
iii. The model can be applied for different spacing between two
groins to study the effect of spacing.
iv. The study can be extended for different groin length to
channel width ratios to study the effect of groin length to
the flow field and sediment transport.

58
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A Thesis of NUMERICAL SIMULATION OF FLOW THROUGH OPEN CHANNEL WITH SERIES OF GROINS

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