Mitigation of Suspended Sediment From Hydropower Projects On Himalayan Rivers

Mitigation of Suspended Sediment from
Hydropower Projects on Himalayan Rivers

A Dissertation
Submitted for the partial fulfilment of

the requirements for the award of the degree of
MASTER OF TECHNOLOGY
In
WATER RESOURCES ENGINEERING

(CIVIL ENGINEERING)
By
ADITYA THAKARE
Roll No.: 32112502
Supervisors
Dr. Arun Goel M. K. Verma

(Professor & H.O.D) (Scientist ‘D’)
Department of Civil Engineering Central Water & Power Research Station
National Institute of Technology, (CW&PRS) Pune
Kurukshetra
DEPARTMENT OF CIVIL ENGINEERING
NATIONAL INSTITUTE OF TECHNOLOGY, KURUKSHETRA
JUNE 2023
CANDIDATE’S DECLARATION
I hereby declare that the work being presented in this dissertation entitled “Mitigation of
Suspended Sediment from Hydropower Projects on Himalayan Rivers” submitted towards the
partial fulfillment of the requirements for the award of degree of Master of Technology in Civil
Engineering (Water Resources Engineering), National Institute of Technology, Kurukshetra, is an
authentic record of my work carried out from July 2022 to June 2023,under the guidance of Dr.
Arun Goel, Department of Civil Engineering, National Institute of Technology, Kurukshetra and
Shri M. K. Verma, Scientist ‘D’, Central Water Power & Research Station, Pune.
I have not submitted the matter embodied in the dissertation for the award of any other degree.
Dated:
Place: Kurukshetra
Aditya Thakare
(32112502)
Water Resources Engineering
Department of Civil Engineering
N.I.T Kurukshetra
CERTIFICATION
This is to certify that the above statement made by the candidate is true to the best of my
knowledge and belief.
Dr. Arun Goel Shri M. K. Verma

Professor Scientist ‘D’
Department of Civil Engineering Central Water & Power Research Station
N.I.T Kurukshetra Pune
ACKNOWLEDGEMENTS
I would like to express my sincere thanks with profound sense of respect and gratitude for my
respected guides Dr. Arun Goel, Professor, Civil Engineering Department, National Institute of
Technology Kurukshetra and Shri M. K. Verma, Scientist ‘D’, CW&PRS, Pune. The words prove
to be insufficient to express my deep feelings for their benevolence and elaborative guidance
throughout the study period.
I express my sincere gratitude to Head of Civil Engineering Department, NIT Kurukshetra for giving
me permission to work at CW&PRS and Director of CW&PRS for allowing me to perform physical
model studies for completing dissertation work and all the professors from Water Resources
Engineering who educated me and offered me useful advice. I am also thankful to Lab assistant Hem
sir for helping me in due course. I express my heartfelt thanks and gratitude to Shri M.Z. Qamar,
Scientist ‘C’ CW&PRS, Pune for his contribution during the course to fill the capacity in me in
meeting the inputs required by this dissertation work.
I am also thankful to Shri A.P Meshram, Scientist ‘B’ and the staff members Shri Sahu Sir, Mr.
Dhananjay of Sediment Management Division CW&PRS for their help and gentleness during the
dissertation work.
I also extend my heartful thank to Ajai S, Scientist ‘C’ HMC division, CW&PRS for his valuable
guidance in simulating the CFD model using Flow 3D.
Finally, I'd want to express my gratitude to my parents for believing in me and supporting me
throughout my academic career from whom I received the untiring help and encouragement to
complete this dissertation work.
Aditya Thakare
(32112502)
Water Resources Engineering
Department of Civil Engineering
N.I.T Kurukshetra
i
ABSTRACT
Himalayan region of India has great hydropower potential capacity. Since majority of perennial
rivers originates from the Himalayas and having steep topography, rivers carry huge suspended
sediment load especially Quartz. Due to suspended sediments which enters into the water conduction
system major loss in terms of economy has to be faced by hydropower projects, since these
suspended sediments damages the turbines. In order to control those economic losses, there is need
to provide the desilting chamber. Desilting chamber proves to be an effective measure to prevent
the entry of coarse particle into the Head race tunnel thus increasing the life of turbines. Generally,
three types of desilting chambers are available such as Pressure type desilting basin, Open channel
type and Vortex desilting basin. Depending upon the site conditions, topography and availability of
space specific type of desilting chamber is used.
Out of which for the dissertation work, Pressure type desilting basin is taken into consideration.
Physical model studies on Pressure type desilting basin have been carried out in CW&PRS, Pune in
sediment management division.
Total 13 experimental runs have been performed on model of Pressure type desilting basin for
Kholongchhu H.E.P. Experiments have been carried out taking into the consideration design
discharge and 10 % overload discharge. It has been found out that the length of desilting chamber
that was originally proposed was on greater side so efforts have been made to reduce the length of
basin in order to achieve economy. For the model studies conducted on 350 m length of basin
equivalent to prototype, overall average settling efficiency for design discharge came out to be 85.2
% and with 10% overload discharge average settling efficiency was observed to be 84.55 %. For the
model studies conducted on 300 m length of basin equivalent to prototype, overall average settling
efficiency for design discharge came out to be 83.99 % and with 10% overload discharge average
settling efficiency was observed to be 83.27 %. Desilting chamber is generally designed to remove
the sediments of size 0.2 mm and above because they cause wear and tear of turbines.
In addition to the physical model studies, numerical simulation was also carried out on 3D geometry
of desilting basin. Flow 3D software was used to perform the numerical simulation for the desilting
chamber have length of basin 350 m and 300 m and check the performance of desilting chamber. It
has been found out that the new dimensions of desilting basin to be adopted are 300 m (L)× 13 m
(W) × 18 m (H) which is adequate for 90 % settlement of sediment size of 0.2 mm and above.
ii
TABLE OF CONTENTS
ACKNOWLEDGEMENTS i
ABSTRACT ii
TABLE OF CONTENTS iii
LIST OF TABLES vi
LIST OF FIGURES viii
LIST OF SYMBOLS xii
LIST OF ABBREVIATIONS xiv
CHAPTER 1 1
INTRODUCTION 1
1.1 GENERAL 1
1.2 DESILTING BASINS 4
1.3 RESEARCH GAPS 7
1.4 OBJECTIVES 7
1.5 LAYOUT OF DISSERTATION WORK 7
CHAPTER 2 9
LITERATURE REVIEW 9
2.1 GENERAL 9
2.2 OPEN CHANNEL TYPE DESILTING BASIN 9
2.3 PRESSURE TYPE DESILTING BASIN 11
CHAPTER3 14
METHODOLOGY 14
3.1 GENERAL 14
3.2 EMPIRICAL AND THEORETICAL APPROACHES 14
3.3 DESIGN SPECIFICATION OF DESILTING CHAMBER 18
CHAPTER 4 24
EXPERIMENTAL WORK ON PHYSICAL MODEL 24
4.1 INTRODUCTION 24
iii
4.2 DESILTING CHAMBERS AND SEDIMENT DATA 25
4.3 EXPERIMENTAL SETUP 25
4.4 EXPERIMENTAL PROCEDURE 27
4.4.1 INITIAL FILLING OF DESILTING CHAMBER 27
4.4.2 SEDIMENT INJECTION STAGE 29
4.4.3 DEWATERING STAGE 32
CHAPTER 5 34
NUMERICAL MODELLING 34
5.1 GENERAL 34
5.2 USE OF CFD SOFTWARE 34
5.3 PROCEDURE FOR DEVELOPING 3D GEOMETRY & IMPORTING IN 35
FLOW3D
5.4 INITIAL AND BOUNDARY CONDITIONS 38
5.4.1 INITIAL CONDITIONS 39
5.4.2 BOUNDARY CONDITIONS 39
5.5 CONDITIONS FOR PARTICLES INCLUSION 40
CHAPTER 6 42
RESULTS & DISCUSSION 42
6.1 GENERAL 42
6.2 RESULTS FOR EXPERIMENTAL STUDIES 42
6.2.1 BASIN LENGTH EQUIVALENT OF 350 M AS THAT OF 42
PROTOTYPE
6.2.1.2 EXPERIMENTAL RUN WITH 10% OVERLOAD DISCHARGE 43
0.0119 m3/s
6.2.1.3 COMPARISON OF RESULTS FOR DESIGN DISCHARGE & 10% 47
OVERLOAD DISCHARGE
6.2.1.4 ANALYTICAL STUDIES FOR 350M LENGTH DESILTING 48
BASIN
6.2.2 BASIN LENGTH EQUIVALENT OF 300 M AS THAT OF PROTOTYPE 57
6.2.2.1 EXPERIMENTAL RUN WITH DESIGN DISCHARGE 0.0108 m3/s 58
6.2.2.2 EXPERIMENTAL RUN WITH 10% OVERLOAD DISCHARGE 60
0.0119 m3/s
iv
6.2.2.3 COMPARISON OF RESULTS FOR DESIGN DISCHARGE & 10% 62
OVERLOAD DISCHARGE
6.2.2.4 ANALYTICAL STUDIES FOR 300M LENGTH DESILTING 63
BASIN
6.3 RESULTS FOR NUMERICAL SIMULATION 70
6.3.1 RESULTS FOR THE 350 M LENGTH OF BASIN 70
6.3.2 RESULTS FOR THE 300 M LENGTH OF BASIN 76
CHAPTER 7 85
CONCLUSIONS 85
REFERENCES 87
LIST OF PUBLICATIONS 90
v
LIST OF TABLES
TABLE 1 CLASSIFICATION OF SETTLING BASINS AS PER CWPRS 5
(1992)
TABLE 4.1 DIMENSION AND DETAILS OF DESILTING CHAMBER FOR 26
DESIGN DISCHARGE (MODEL SCALE 1:30 GS)
TABLE 5.1 PARTICLE SOURCE DATA 40
TABLE 6.1 SPECIFICATION OF PHYSICAL MODEL FOR BASIN LENGTH 43
OF 350 M EQUIVALENT OF PROTOTYPE
TABLE 6.2 RESULTS FOR DESIGN DISCHARGE OF 0.0108 m3/s (350 M) 43
TABLE 6.3 RESULTS FOR DESIGN DISCHARGE OF 0.0108 m3/s AFTER 44
ADJUSTMENTS (350 M)
TABLE 6.4 RESULTS FOR 10% OVERLOAD DISCHARGE OF 0.01191 m3/s 45
(350 M)
TABLE 6.5 RESULTS FOR 10% OVERLOAD DISCHARGE OF 0.01191 M3/S 46
AFTER ADJUSTMENT (350 M)
TABLE 6.6 SETTLING EFFICIENCY ESTIMATION FOR SEDIMENT DATA 49
FOR PROTOTYPE (350 M)
FOR MODEL (SAMPLE 1)
FOR MODEL WITH 10% OVERLOAD DISCHARGE (SAMPLE 1)
TABLE 6.13 SPECIFICATION OF PHYSICAL MODEL FOR BASIN LENGTH 58
OF 300 M EQUIVALENT OF PROTOTYPE
TABLE 6.14 RESULTS FOR DESIGN DISCHARGE OF 0.0108 m3/s (300 M) 59
TABLE 6.15 RESULTS FOR DESIGN DISCHARGE OF 0.0108 m3/s AFTER 59
ADJUSTMENTS (300 M)
vi
(300 M)
AFTER ADJUSTMENT (300 M)
FOR PROTOTYPE (300 M)
TABLE 6.25 SETTLING EFFICIENCY OF DESILTING CHAMBER OF 74
LENGTH 350 M AT DIFFERENT TIME STEPS
TABLE 6.26 SETTLING EFFICIENCY OF DESILTING CHAMBER OF 81
LENGTH 300 M AT DIFFERENT TIME STEPS
vii
LIST OF FIGURES
FIGURE 1.1 DAMAGED TURBINE DUE TO SUSPENDED SEDIMENTS 1
FIGURE 1.2 METHODS TO AGGRAVATING EFFECT OF SEDIMENTATION 4
FIGURE 1.3 MODEL OF PRESSURE TYPE DESILTING BASIN (CWPRS) 5
FIGURE 1.4 MODEL OF URI-II H.E.P OPEN CHANNEL TYPE DESILTING 6
BASIN (CWPRS)
FIGURE 1.5 VORTEX TYPE DESILTING BASIN 6
FIGURE 3.1 CAMP AND DOBBINS GRAPH REPRESENTING FUNCTION 15
OF SHEAR VELOCITY
FIGURE 3.2 CAMP AND DOBBINS GRAPH REPRESENTING FUNCTION 15
OF MIXING COEFFICIENT
FIGURE 3.3 RELATION BETWEEN λ and β 16
FIGURE 3.4 GRAPH OF VELIKANOV (λ vs EFFICIENCY) 17
FIGURE 3.5 LAYOUT OF WATER CONDUCTOR SYSTEM FROM CWPRS 19
(2005)
FIGURE 3.6 IMAGE OF INLET TRANSITION OF DESILTING CHAMBER 19
FIGURE 3.7 CONTINUES HOPPER ADOPTED FOR MODEL STUDIES 20
FIGURE 3.8 FIRST OPENING INTO THE FLUSHING TUNNEL 21
FIGURE 3.9 DUNE FORMATION IN SETTLING TRENCH 22
FIGURE 3.10 FLUSHING GATE ON DOWNSTREAM OF MODEL 22
FIGURE 3.11 IMAGES OF OUTLET TRANSITION WITH (A) GRADUAL 23
CONTRACTION, (B) VERTICAL WALL
FIGURE 4.1 LOCATION MAP OF KHOLONGCHHU H.E.P 24
FIGURE 4.2 PROJECT LAYOUT PLAN 25
FIGURE 4.3 MODEL OF KHOLONGCHHU DESILTING BASIN 350 M 26
EQUIVALENT OF PROTOTYPE
FIGURE 4.4 WALNUT SHELL POWDER 27
FIGURE 4.5 AIR VENTS ON THE ROOF OF DESILTING CHAMBER 27
FIGURE 4.6 (A) REHBOCK WEIR AND (B) WATER LEVEL BETWEEN 28
MDDL AND FRL
FIGURE 4.7 (A) GAUGE IN REHBOCK WEIR, (B) GAUGE IN HRT TANK, 29
(C) GAUGE IN SFT TANK
FIGURE 4.8 (A) IMAGE OF MANUAL WAY TO INJECT SEDIMENTS, (B) 30
SEDIMENT STORAGE TANK
viii
FIGURE 4.9 INLET TRANSITION WITH HIGH SEDIMENT 30
CONCENTRATION
FIGURE 4.10 SEDIMENT SETTLEMENT ON HOPPER SLOPE 31
FIGURE 4.11 CONCENTRATION OF SEDIMENTS AT THE OUTLET OF 31
MAIN BASIN
FIGURE 4.12 OUTLET TRANSITION WITH OUTLET AND SILT FLUSHING 33
TUNNEL
FIGURE 4.13 (A), (B) SEDIMENTS COLLECTION IN SFT AND HRT TANKS 33
FIGURE 5.1 SKELETON OF KHOLONGCHHU DESILTING CHAMBER 36
FIGURE 5.2 3D WATER-TIGHT GEOMETRY OF BASIN 36
FIGURE 5.3 .STL FORMAT OF DESILTING CHAMBER 37
FIGURE 5.4 THREE MESH APPLIED TO BASIN 37
FIGURE 5.5 RECTANGULAR BOX EMBEDDED WITH DESILTING 38
CHAMBER
FIGURE 5.6 BAFFLE WALLS PROVIDED AT OUTLET 38
FIGURE 6.1 MODEL OF DESILTING BASIN 350 M EQUIVALENT TO 42
PROTOTYPE
FIGURE 6.2 REPRESENTATION OF RESULTS FOR 350 M BASIN LENGTH 44
(DESIGN DISCHARGE)
(10% OVERLOAD DISCHARGE)
FIGURE 6.4 REPRESENTATION OF OBSERVED VALUES OF SETTLING 47
EFFICIENCY FOR 350 M BASIN LENGTH
FIGURE 6.5 REPRESENTATION OF MODIFIED VALUES OF SETTLING 48
FIGURE 6.6 GRADATION CURVE OF SUSPENDED SEDIMENT FOR 48
PROTOTYPE
FIGURE 6.7 PARTICLE SIZE DISTRIBUTION CURVE FOR SAMPLE 1 50
FIGURE 6.10 REPRESENTATION OF ANALYTICAL SEDIMENT REMOVAL 55
EFFICIENCY FOR MODEL AND PROTOTYPE FOR 350 M
LENGTH BASIN FOR DESIGN DISCHARGE
ix
LENGTH BASIN FOR 10 % OVERLOAD DISCHARGE
FIGURE 6.12 REPRESENTATION OF LENGTH OF DESILTING BASIN VS 56
SETTLING EFFICIENCY (DESIGN DISCHARGE)
FIGURE 6.13 REPRESENTATION OF LENGTH OF DESILTING BASIN VS 57
SETTLING EFFICIENCY (10% OVERLOAD DISCHARGE)
FIGURE 6.14 MODEL FOR 300 M DESILTING CHAMBER EQUIVALENT TO 58
PROTOTYPE
(DESIGN DISCHARGE)
(10% OVERLOAD DISCHARGE)
FIGURE 6.17 REPRESENTATION OF OBSERVED VALUES OF SETTLING 62
FIGURE 6.18 REPRESENTATION OF MODIFIED VALUES OF SETTLING 63
LENGTH BASIN FOR DESIGN DISCHARGE
LENGTH BASIN FOR 10% OVERLOAD DISCHARGE
FIGURE 6.21 PRESSURE PROFILE INSIDE DESILTING CHAMBER FOR 350 70
M LENGTH
FIGURE 6.22 VELOCITY PROFILE AT T=0 SEC FOR 350 M LENGTH 71
FIGURE 6.26 SEDIMENT TRANSLATION PROFILE AT T=1000 SEC FOR 350 72
M LENGTH
M LENGTH
x
M LENGTH
M LENGTH
FIGURE 6.30 PRESSURE PROFILE INSIDE DESILTING CHAMBER FOR 300 77
M LENGTH
M LENGTH
M LENGTH
M LENGTH
M LENGTH
FIGURE 6.40 REPRESENTATION OF SETTLING EFFICIENCY VS TIME FOR 83
NUMERICAL SIMULATION
xi
LIST OF SYMBOLS
Symbols Description
ach Approach conduit cross-sectional area (m2)
A Plan Area (m2)
Ax Cross-sectional area (m2)
B* Non dimensional length
b Width of approach channel (m)
β Rouse number
D Depth of desilting basin (m)
D* Non dimensional depth
ℇ Mixing coefficient (m2/s)
g Acceleration due to gravity (m/s2)
h Depth of flow in approach channel (m)
ĸ Von Karman constant
L Desilting basin length (m)
L* Non dimensional length
Lr Scale Ratio
λ Velikanov’s function
n Manning’s roughness coefficient
ƞ Desilting efficiency
ƞd Flushing efficiency
Q Discharge (m3/s)
R Hydraulic radius (m)
U Flow through velocity (m/s)
xii
Symbols Description
µ* Shear Velocity (m/s)
w settling velocity (m/s)
W Width of basin (m)
we Effective fall velocity (m/s)
xiii
LIST OF ABBREVIATIONS
Abbreviation Meaning
CFD Computational Fluid Dynamics
d/s Downstream
EL Elevation
FRL Full Reservoir Level
H.E.P Hydro Electric Project
HRT Head Race Tunnel
MDDL Minimum Drawdown Level
R.O.R Run-of-River
SFT Silt Flushing Tunnel
u/s Upstream
xiv
CHAPTER 1
INTRODUCTION
1.1 General:
India's hydroelectric potential is largely influenced by the Himalayan region. The excessive
suspended sediments (silt) carried by the Himalayan rivers harm the mechanical components
of hydropower plants as shown in Fig 1.1. The high silt load and concentration make the total
system non-operational. This effect of sediments is aggravating, and it is skill of an engineer
to decrease the damage that sediments cause to hydraulic machinery. To ease this task different
types of desilting basins also called as silting tank, sediment trap, etc. are provided in close
vicinity of headwork of hydroelectric projects to exclude the sediments.
Figure 1.1: Damaged turbine due to suspended sediments

To benign the effect harmful suspended sediment which passes through the trash rack of power
intake into the water conductor system of hydroelectric project, Run-of-the river reservoir
schemes are generally provided with desilting chambers. Generally, ROR are provided for
hydro power generation which have live storage limited from few hours to few days, are built
1
specifically. This reservoir has insignificant storage capacity compared to the average annual
inflow, thus the trapping efficiency of the sediment in such reservoir is very limited.
Management of silt in the catchment area of reservoir is at most important therefore
management of silt is done during planning, designing and operational stage of hydropower
project and they are as follows as mentioned in standard operation procedure (2018):
1]Planning and design stage:

 For ROR & canal hydel project proper sediment exclusion devices should be installed
to prevent the inclusion of particle >0.2mm into turbine such one device is desilting
basin also called as silting tank, settling basin, sediment trap.
 Chemical characteristics of water and petrographic analysis should be done while

designing turbine, main inlet valve etc.
 Hydraulic hoist should be used for operation of various gates as compared to drum
hoists.
 Hydro suction system which is sediment removal device should be installed.
 Use of low-level sluice is an effective solution for flushing of sediments.
2]Operational stage consideration:
 Underwater part of hydroelectric station due to high silt load can be protected using
protective hard coating (Tungsten Carbide) by HVOF spray method.
 Sediment rating curves, discharge vs suspended sediment load should be prepared for
monsoon & non-monsoon season.
 For ROR scheme to reduce siltation of live storage reservoir is operated at MDDL
during high flood period.
 For project lying in close vicinity proper co-ordination of flushing of sediment should
be made i.e sediment which gets flushed out from u/s reservoir does not settle in d/s
reservoir.
 Dredging operation should be carried out as and when required.
 Blank panel at intake crest level should be made and hydrographic survey in reservoir
should be carried out every year at fix location.
2
Construction of a reservoir on a river causes drastic changes in natural flow and sediment
conditions; following are multiple effects of sedimentation as per Verma et al. (2012):
 Reduction in useful and economic life of reservoir, water supply and power generation.
 Chocking of sluices and intakes and rapid degradation on the downstream of dams.
 Abrasion of turbines and other equipments of power house.
 Additional lateral forces on the dam wall exerted.
However, Bharat Heavy Electricals Ltd. who are hydro power equipment manufacturer in India
have fixed following guidelines as mentioned by Acharekar et.al (1978):
 Particles of > 0.25 mm size and hardness > 5 on Mohr’s scale are harmful.
 If the concentration of particle is > 200 ppm, desilting measures are required.
 Concentration of harmful particles should be reduced by 85% to 95%.
 Tong (1981) has concluded from experimental results that damages take place due to
cavitation and this process is accelerated by sediments. As per his findings, if the
turbine is likely to be eroded within 5-10 years of installation, then the sediment
exclusion measures are necessary and if the life of the turbine works out to be more
than 20 years, the sediment exclusion measures can be eliminated.
Khurana et al. (2014) after performing experimental work derived the relation for erosion due
to silt in Turgo impulse turbine. Using data, relation he developed is shown in equation 1.1 as
follows:
W=1.976×10-10 S0.118 C0.967 V1.368 t1.117…………………(eq 1.1)
Where W= Wear rate, S=silt size, C=concentration of silt, V=jet discharge velocity and
t=working hours of turbine affects the erosion in hydro turbines. They have observed that silt
size, concentration of silt, jet discharge velocity, working hours of turbine affects the the
erosion in hydro turbines. They concluded that these parameters are mainly responsible for the
efficiency loss in the turbine. Efficiency suggested by Khurana et al. (2014) is shown in
equation 1.2 as:
Ƞ %=2.93×10-8 S0.212 C1.113 V1.368 t1.117………………….(eq 1.2)
Padhy & Saini (2009) gave the correlation for erosion due to silt for Pelton wheel turbine
shown in equation 1.3 as:
W=2.43×10-10 S0.099 C0.93 V3.40 t0.75……...………….……(eq 1.3)
3
Thus, to reduce the aggravating effect of sedimentation methods shown in Fig 1.2 are followed
generally in catchment area or at dam site as mentioned in standard operation procedure (2018):
Measure Against Reservoir Sedimentation
In Catchment In Reservoir On Dam

Area
1]Soil Conservation 1]Dredging 1]Dredging

2]Settling Basin 2]Dead Storage 2]Sluicing
3]Slope and Bank 3]Flushing 3]Turbining
Protection 4]Hydro Suction suspended
4]Bypassing 5]Avoiding setting of sediments
Structure fine sediment(sluicing) 4]Dam heightening
5]Off Stream 6]Swaying the 5]Heightening of
Storage Reservoir turbidity currents intake & bottom
Figure 1.2: Methods to aggravating effect of sedimentation

Sedimentation is a major obstacle to the effective use of water resources in the Himalayan
region, where the steep slopes promote erosion and the monsoon rains deliver enormous loads
of material downstream. The idea of desilting chambers has developed as a workable and
successful approach to deal with this problem. A desilting chamber, also known as a
sedimentation tank or settling basin, is a structure designed to capture and remove sediment
from flowing water. It acts as a sediment trap, allowing the water to slow down and the
sediment to settle at the bottom. By removing sediment from the water before it reaches
downstream infrastructure, desilting chambers help maintain the efficiency and longevity of
irrigation systems, hydropower plants, and other water-related infrastructure in the Himalayan
region. This thesis mainly focuses on the various aspects for the design of desilting chamber
and their enhancement.
1.2 Desilting Basins

Desilting basins/ chambers are one of the many crucial parts of hydropower project plans,
which guards hydraulic equipment against wear and tear brought on by the silt carried by the
conveyance system. The main purpose of desilting basins is to induce the settlement of
sediments by reducing velocity and turbulence & to skim sediment free layer of the water from
4
the surface at the outlet. Desilting chambers are normally designed for 90-95% efficiency to
remove particle of size coarser than 0.2 mm, since sediments coarser than 0.2mm causes
abrasion of turbine components. It is proposed that before entering the headrace tunnel all the
coarser sediments greater than 0.2 mm size should be discarded from the water. To prevent
entry of coarser particles, trash-rack of size 80×80 mm is provided at entrance of the intake of
power canal. In Himalayan region mainly Quartz particle causes major damage to turbine due
its high level of hardness i.e 7 on Mohr’s scale and in order to induce the settlement of these
particles before entering into head race tunnel desilting chambers are provided. CWPRS (1992)
classified the settling basin as shown in Table 1:
Table 1.1: Classification of settling basins as per CWPRS (1992)

Basis of Classification Type of basin
Mode of construction Natural or artificial
Method of cleaning Mechanical/Manual
Mode of operation Intermittent/Continuous
Type of flow Pressure flow/OCF
Configuration Single or multiple layout
Depending upon the type of flow generally, three types of desilting basins are available for the
removal of sediments i.e Open channel type, Pressure type and Vortex type desilting basin. In
Himalayan region due to limited available space and steep topography normally Pressure type
desilting basins are adopted. Pressure type desilting basin provides an effective measure to
control the inclusion of suspended sediments into the Head race tunnel. Model of Pressure type
desilting chamber is shown in Fig 1.3.
Figure 1.3: Model of Pressure Type Desilting basin (CWPRS)
5
Another effective means to prevent the entry of sediment into the Head race tunnel is Open
channel type desilting chamber. This type of desilting chambers are generally adopted in region
where adequate space is available and topography is not steep. One such type of model is
available in CWPRS adopted for the Uri-II hydroelectric power project as shown in Fig 1.4.
Figure 1.4: Model of Uri-II H.E.P Open Channel Type Desilting basin (CWPRS)
Third type of desilting chamber is of Vortex type desilting basin as shown in Fig 1.5. In this
type a high velocity in tangential direction is imparted in cylindrical basin which contains
orifice at center which is located on bottom side and due to tangential entry of flow, free vortex
is formed near the orifice along with combination of vortex conditions formed at outer
periphery. Many research works have been carried out on Vortex type basin including the
contribution made by Garde et al. (1990), Sumer (1991), Athar et.al (2002) and Ansari (2013).
Advantage of using Vortex type desilting basin is that it has relatively simple design as
compared to other two types of basins and also it is cost effective. Though many researches
have been carried out to improve the design of vortex desilting basin as well as having
advantages in terms of economy this desilting basin is not adopted for hydropower projects in
Himalayan regions. The main reason behind it is they are operated for lower value of discharge
and efficiency of vortex desilting basin is maximum for sediment size 0.055<d<0.22 mm.
Figure 1.5: Vortex Type Desilting basin
6
1.3 Research Gaps
 Inadequate design for flushing arrangement.
 Improper transition design
 No specific criteria are provided for spacing of flushing holes
 Need for numerical as well as experimental modelling
1.4 Objectives
 Perform experimental run with physical model.
 Developing numerical models of Pressure type desilting basin and performing
simulation in CFD software i.e FLOW 3D.
 Designing effective transition section for maximizing the sediment settling
efficiency.
1.5 Layout of Dissertation Work
This thesis is organized into the following chapters:
CHAPTER I: INTRODUCTION
This chapter explains the need and types of desilting chambers that are adopted for
H.E.P. Also, the research gap and objectives of the study is included in this chapter.
CHAPTER II: LITERATURE REVIEW
Brief summary of research work that has been carried out in field of desilting basins
adopted for H.E.P are mentioned in this chapter.
CHAPTER III: METHODOLOGY
This chapter provided Empirical and Theoretical Approaches associated to find out
settling efficiencies for Open and Pressure type desilting basins. Design specifications
of desilting basins are also included in this chapter.
CHAPTER IV: EXPERIMENTAL WORK ON PHYSICAL MODEL

This chapter includes the information of desilting chamber provided in Kholongchhu
H.E.P, experimental setup and procedure to conduct physical modelling.
CHAPTER V: NUMERICAL MODELLING

This chapter consists of need for numerical simulation, procedure to construct the 3D
geometry of desilting basin and different conditions to be used while performing
7
simulation in Flow 3D.
CHAPTER VI: RESULTS & DISCUSSION

This chapter provides the results carried out on physical modelling, analytical studies
and numerical simulations.
CHAPTER VII: CONCLUSION
This emphasizes the findings of the current study and invites for more research into the
subject.
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8
CHAPTER 2
LITERATURE REVIEW
2.1 General
In this chapter various researches carried out on Open channel type and Pressure type desilting
basins are discussed briefly. In case of Open channel type desilting basin Garde et al. (1990),
CWPRS (1992), Nils (1992), Ranga Raju et al. (1999), Mazumder & Kumar (2001), Vasquez
(2007), Weerakoon & Rathnayake (2007), Shah et al. (2008), etc. have remarkable contribution
in terms of their findings and experiments. For Pressure type desilting basin Olsen &
Chandrashekhar (1995), CWPRS (2005), Verma et al. (2015), Sinha et al. (2018), Qamar et al.
(2022), etc. have contributed in terms of experimental as well as numerical modelling.
2.2 Open channel Type Desilting Basin
Following are the researchers who have carried out their research on Open channel type
desilting basin:
Garde et al. (1990)

Total 162 experiments were performed on Open channel type rectangular flume by inducing
modifications in slope, sediment size and concentration, discharge and basin length. Author
derived new relation of efficiency as ƞ = ƞ0(1-𝑒 −𝐾𝐿/𝐷 ), this relation was developed from the
results obtained from experiments and were cross checked by preparing graphs with equations
proposed by Camp-Dobbin (1944), Cecen et al. (1969), Vanoni (1975) & Sumer (1977).
CWPRS (1992)
Based upon the model studies conducted on various hydropower project in CWPRS, further
discussed are some modifications made while performing experiments. If angle of 12o to 14o is
introduced on either side of the centre line of the jet which covers major portion of region, this
results in expansion ratio approximately 1:4 to 1:5. This provision is modified little bit for the
wide desilting basin in open area, the inlet divergence provided should be flatter than 1:4 to
1:5.
Ranga Raju et al. (1999)

Total 99 experimental runs were conducted on Open channel type rectangular flume out of
which 23 were carried out without considering flushing effect and remaining with flushing
effect. For this experiments variations in slopes, discharge, sediment size and flushing
9
discharge were considered. Efficiency obtained from the readings were compared with
efficiency obtained by using empirical and analytical methods proposed by Camp, Dobbins,
Summer, Vanoni and Garde et al. and it was found that for only coarse sediments i.e d > 0.4mm
efficiency came out to be accurate while for finer sediments results were inaccurate.
Mazumder & Kumar (2001)
Author suggested that when the maximum area expansion ratio is kept at 1:5 the flow in basin
gets stabilized. By providing lower Coriolis coefficient value and higher hydraulic efficiency
the author indicated that the flow at expansion becomes steady.
Yeb and Lin (2002)

For a limited range of discharge (0.1–10 cms), the author used multiple regression analysis on
parameters created by Vittal et al. (1997) to identify non-dimensional parameters like L*, B*,
and D*. Author created 25 geometrical models of settling basins by varying length and width.
With the help of 2-D finite analytical movable bed model, 450 experimental runs were
performed totaling 3 hours each were completed. The results showed that for low discharge &
finer material, efficiency can be maximized by providing an ideal horizontal basin area.
Vasquez (2007)
Author discovered that when a channel expands symmetrically from narrow to wide, flow
concentration occurs on either side of the basin, creating eddies and a significant recirculation
zone. To tackle these issues, he introducing dividing walls and central vanes in the inlet
transition to prevent asymmetric flow.
Weerakoon & Rathnayake (2007)

Based upon the experimental studies carried out by the authors, they concluded that by
changing the inlet transition angle from 30o to 10o trapping efficiency of basin varied from 50%
to 85%.
Shah et al. (2008)

Based upon the model studies conducted on Open channel desilting basin of Uri-II H.E.P,
authors have made modifications in order to enhance the settling efficiency of desilting
chamber. Following are some modifications made in model, low height weir was introduced in
inlet channel, bed slope has been changed from 1V:10.9H to 1V:7.09H, slope of flushing tunnel
has been changed from 1V:400H to 1V:200H in 36 meters of initial reach of tunnel, provision
10
of central divide wall through-out basin, provision of cross-wall in the vicinity of inlet
transition and Hump was introduced at the end of inlet transition.
Janssen (2009)
As per the findings of author, the efficiency of the settling basin is reduced by 35% both with
and without the employment of contraction and expansion of the flow at the outlet and inlet. In
order to obtain a uniform distribution of intake across the whole width of the desilting chamber,
baffle walls are used.
Verma et al. (2012)

This paper focuses on the various types of desilting basin (Open channel & Pressure type) and
other desilting devices like Weir type sediment excluder & diversion tunnel, on which model
studies have been performed & their design aspect. Author has mentioned maximum forward
velocity for effective removal of different sediment size which is to be maintained in main
basin and are as follows:
For 0.1 mm particle – 0.15 m/s

Paschmann et al. (2017)

Author has made assessment of the flux and flow field of suspended sediment in three desilting
basins that are of open channel type and concluded that introducing tranquillizing racks in the
inlet transition phase can minimize the turbulent kinetic energy.
2.3 Pressure Type Desilting Basin

Following are the researchers who have contributed their research on Pressure type desilting
basin:
Olsen & Chandrashekhar (1995)
The author used the SSIIM (CFD model) for the Nathpa Jhakari Project's desilting chamber.
For the computation of water motion in turbulent flow, it solves the continuity equation and
the Reynold averaged Navier Stokes equation in three dimensions. The transient convection
diffusion equation can be used to compute the transportation of suspended sediment, and Van
Rijn's formula can be used to determine the transportation of bed load. The pattern of sediment
deposition and the effectiveness of the trapping are predicted using this model.
11
CWPRS (2005)
From the various model studies carried out at CWPRS, hereby are some important guidelines
to be considered while designing the pressure type desilting basin: bed slope of inlet transition
should be kept in between 1:2 to 1:2.3, to prevent the deposition of sediment on hopper slope
the hopper angle of 400 is provided. To prevent obstructions in the entrance section of the main
basin, a flushing discharge of around 15 to 20 % of Head Race Tunnel discharge is maintained.
Verma et al. (2015)

Author focused on need of optimization of design of desilting chamber using model studies
and reasons to performed them and are as follows:
 To determine design parameters for effective operation.

 Reviewing empirical design methods for preliminary design.
 Evaluation of model study outcomes based on settling and flushing efficiency.
CFD software like FLOW-3D is utilized for mathematical modelling throughout the
optimization phase.
Sinha and Singh (2018)

Authors have made modifications in existing empirical formulae suggested by Camp’s-
Dobbins (1946), USBR approach (1949), Mosonyi’s approach (1991), Summer’s approach
(1977), Garde et al. approach (1990) & Ranga Raju et al. approach (1999). Modifications were
made taking into consideration parameters such as friction factor, plan area/cross-sectional area
and hydraulic radius. The results obtained from physical model investigations were then
compared with modified formulas, and were found to be satisfactory.
Verma et al. (2021)

Authors carried out the model study on Etalin Hydropower Project which is of Pressure type
desilting basin. Three dufour type underground chamber were proposed for the hydropower
project to obtain particle removal efficiency of 90%. Overall experimental settling efficiency
was 62.26%, with the analytical settling efficiency of the prototype coming out to be 55.63%
and that of 0.2mm size particles to be 95%.
Iyer et al. (2022)

This paper reviews model studies for Pressure-type and Open-channel desilting chambers. The
review paper mentions a number of studies on inlet transition and changes to the basin designs
12
of the aforementioned two desilting basins. Following conclusions have been drawn from the
review work:
 The insufficient design methodologies for constructing inlet transitions depending on

sediment water.
 Inlet transitions must be designed effectively.
 Physical model studies and numerical model studies can be combined to create a
economic desilting basin.
Qamar et al. (2022)

The main objective of the authors is to perform the numerical simulation on the 3-D geometry
for model of Pressure type desilting basin using Flow-3D. The efficiency tabulated from the
numerical modelling is then compared with the sediment removal efficiency of actual model.
Simulation has been performed for 0.2mm particle size and settling efficiency for the model
study came out as 90% and that for numerical modelling came out to be 91.3%.
****************************************************************
13
CHAPTER 3
METHODOLOGY
3.1 General
This chapter provides complete overview on various empirical as well as theoretical
approaches generally adopted for the design of desilting basin.
3.2 Empirical and Theoretical Approaches

Different theories are associated to find out settling efficiencies for Open and Pressure type
desilting basins are Camp-Dobbin’s approach (1946), USBR (1949), Summer’s approach
(1977), Mosonyi’s approach (1991), Garde et al. (1990) & Ranga Raju et al. approach (1999).
1] Camp-Dobbin’s approach (1946)

As per Camp-dobbin’s approach settling efficiency is function of shear velocity & mixing
coefficient and assumed that they are same through-out the fluid following relationship is
derived:
 Camp-dobbin’s approach (function of shear velocity) (1946)
Ƞ=f [𝑤𝑤 , µ𝑤 ]
0 ∗
𝑤 𝑤𝐿 𝑤 𝑤𝐷1/6
= 𝑈𝐷 , =
𝑤0 µ∗ 𝑈𝑛√𝑔
 Camp-dobbin’s approach (function of mixing coefficient) (1946)
Ƞ=f [𝑤𝑤 , 𝑤𝐷
0 2ℇ
]
𝑤 𝑤𝐿 𝑤𝐷 𝑤
= 𝑈𝐷, =122
𝑤0 2ℇ 𝑈
where,
w= settling velocity (m/s)
w0= Overflow rate (m/s)
U= Flow through velocity (m/s)
L,D= Length & Depth of basin
µ∗ = Shear Velocity (m/s)
14
ƞ = desilting efficiency
(f) = friction factor = 0.03
ℇ = mixing coefficient
and desilting efficiency (ƞ) is tabulated from Fig 3.1and Fig 3.2 as shown below.
Figure 3.1: Camp and Dobbins graph representing function of shear velocity
122 W/ V
Figure 3.2: Camp and Dobbins graph representing function of Mixing coefficient
15
2] USBR approach (1949)
As per USBR approach:
Ƞ=f [𝑤𝑤 ]0
Where,
w= settling velocity (m/s)
w0= Overflow rate (m/s)
Desilting efficiency as per USBR is given by equation 3.1 as:
−𝑤𝐿
Ƞ = 1-𝑒 𝑈𝐷 …………………………………(eq 3.1)
3] Summer’s Approach (1977)
Ƞ=f [µ𝑤 , 𝑤µ ]
∗
∗
0
𝑤
…….(This method is valid only if β < 4; µ < 1.6)
∗
Desilting efficiency is given by equation 3.2 as:

ĸ𝜆 𝐿µ
−( 6 )( 𝑈𝐷∗)
Ƞ =1-𝑒 ……………………………(eq 3.2)
where, ĸ = Von Karman constant= 0.4
λ = constant
β = Rouse number = (ĸµ𝑤 )

∗
where relation between λ and β are obtained from Fig 3.3 as shown below.
Figure 3.3: Relation between λ and β
4] Mosonyi’s Approach (1991)
16
Ƞ=f [µ𝑤 , 𝐷𝐿 ]
∗
Mosonyi’s approach considered the retarding effect of turbulence taken into

consideration by lowering settling velocity (we):
0.132𝑈
we = w-
√𝐷
The settling length (L) incorporating reduction in settling velocity is given as:
𝑈𝐷
L=
𝑤𝑒
Desilting efficiency is calculated from Velikanov graph as shown in Fig 3.4.

Where,
7.51𝑤 2 𝐿
λ =√ 1
𝑈 2 (𝐷2 −0.2)2
Figure 3.4: Graph of Velikanov (λ vs efficiency)
5] Garde et al. Approach (1990)

As as per Garde et al., approach desilting efficiency is influenced significantly by L/D
ratio.
Ƞ=f [µ𝑤 , 𝐷𝐿 ]
∗
Settling efficiency(Ƞ) is given by equation 3.3 as:

−𝐶𝐿
Ƞ = ƞ0(1-𝑒 𝐷 )……………………………..(eq 3.3)
17
Where,
C= constant
𝑤
Depending upon value of , C and ƞ0 are calculated from values shown below:
µ∗
𝑤 0.7 0.9 1.2 1.6 2.0 >2.2
µ∗
C 0.02 0.03 0.06 0.14 0.215 0.24
ƞ0 34 40 50 70 97 100
6] Ranga Raju et al. approach (1999)

1/6
Ƞ=f [𝑤𝑈 , 𝐿𝐵 ,
𝐷
ℎ𝑏 𝑛√𝑔
]
From basin studies carried out in laboratory following conclusions are made:
Ƞ =100% ……………... w/u* > 2.5
𝑤 𝐿𝐵 0.23 𝐷1/6 0.98
Ƞ=11.7( )0.81 ( ) ( ) …………. w/u* < 2.5
𝑈 𝑏ℎ 𝑛 √𝑔
Ranga Raju considered the effect of flushing on efficiency (ƞ𝑑 )and suggested equation
3.4 as:
ƞ𝑑 𝑤
=1-0.12𝑄𝑓 −0.105 (µ )0.312 …………………..( eq 3.4)
ƞ ∗
Where,
𝑄𝑓 = Flushing discharge
A = 𝐿 × 𝐵= plan area of desilting basin
ach =𝑏 × ℎ= cross sectional area of approach channel
3.3 Design Specifications of desilting chamber
Following are the design aspects required to be taken into consideration while designing
desilting chamber as per CWPRS (2005):
1. Location and Orientation:

The most important aspect taken into consideration is the location and the orientation
of the desilting chamber, Fig 3.5 shows the arrangement of water conductor system as
mentioned in CWPRS (2005). In order to maintain the desired control to lower the
amount of sedimentation in the approach channel, the desilting basin should generally
be situated in close proximity to the head works/intake as is practical. However, the
turbulence on downstream of the intake/head regulator would be an issue if the basin
were positioned too close to the intake/head works. Proper alignment should be
maintained between the feeder tunnel and inlet transition so that the flow is uniformly
18
distributed throughout the basin and no turbulence is produced in inlet transition. For
this purpose, generally it is advised to provide the straight length equal to ten times the
diameter of feeder tunnel or width of basin on upstream side of basin.
Figure 3.5: Layout of Water conductor system from CWPRS (2005)
2. Inlet Transition:
Design of inlet transition is utmost important because in this part high velocity flow
from the power intake is reduced to much lower velocity by increasing the area by
providing suitable horizontal and/or vertical divergence. Special attention is given
while designing the transition part so that eddies are not generated which would disturb
the flow and will eventually affect the efficiency of basin. To achieve the desirable inlet
transition CWPRS (1992) suggested that angle of 12o to 14o is introduced on either side
of the centre line of the jet which covers major portion of region, this results in
expansion ratio approximately 1:4 to 1:5. This provision is modified little bit for the
wide desilting basin in open area, the inlet divergence provided should be flatter than
1:4 to 1:5. Fig 3.6 shows the inlet transition of desilting chamber model.
Feeder
Tunnel
Inlet Transition
Figure 3.6: Image of Inlet Transition of desilting chamber
19
3. Size and Slope of the Hoppers:
Hopper is a part of main basin and its arrangement depends upon the size and length of
desilting basin. The bed of the desilting basin will need to be divided into several
hoppers in the case of a continuous flushing system. One row of hoppers may be
adequate for long and narrow basins. However, more rows of hoppers would be
required for wide basins. For the narrow desilting basin continuous hopper with
sediment accumulation trench below it is adopted instead of individual rectangular
hopper. Fig 3.7 shows the image of continues hopper adopted for the model of desilting
chamber.
Hopper
Figure 3.7: Continues hopper adopted for model studies

Generally, slope adopted for the hopper is greater than 400 so as to ensure that no
sediments accumulate on it, instead they slide over the slope and eventually fall inside
the sediment collection trench.
4. Size and Spacing of Openings at bottom of Sediment Trench into the Flushing Tunnel:
The first opening into the flushing tunnel should be of larger dimension as compared to
rest of the openings as shown in Fig 3.8. The reason behind to provide larger dimension
is to flush out the larger size sediments which generally settles just on downstream of
inlet transition and to also carry discharge of 20 to 30% into the flushing tunnel. As the
basin progresses in downstream direction the size of opening goes on reducing because
the settlement of sediments goes on reducing in downstream direction. It is to be noted
that the last opening provided in the settling trench is of relatively larger dimension
than the previous opening because at the end of basin sediments form the reverse ramp.
20
Opening
Figure 3.8: First opening into the flushing tunnel
5. Sediment Settling Trench:

Since sediment settling trench is provided on the bottom of hopper as discussed below,
designing its height is an important task. As the sediments settles into trench after
getting sliding from hopper slope, the heigh of dunes rises as time passes and in no
condition the height of dunes should obstruct the flow in settling zone. The width of
settling trench is provided in such a way that the base width of the dunes in the direction
of flow is about 3 times the height of the dunes. Fig 3.9 shows the settlement of
sediments and dunes formation around the openings.
Dunes
Figure 3.9: Dune formation in settling trench

6. Size of Flushing Conduit:
Flushing conduits are provided at the bottom of desilting chamber in order to flush out
the settled suspended sediments in the basin. For continuous flushing operation from
the past studies, it has been found that minimum velocity of 2.50 m/s is required for the
21
efficient functioning of the tunnel. The major discharge through the flushing tunnels is
obtained from the first opening provided in sediment settling tunnel as discussed
previously and the rest of discharge is obtained from the remaining openings. The
flushing discharge is controlled by a gate at the downstream end as shown in Fig 3.10
for the model of desilting basin.
Flushing Gate
Figure 3.10: Flushing Gate on downstream of model

7. Outlet Transition:
Outlet transition should be placed in straight alignment with inlet transition. Unlike
inlet transition, the design of outlet transition also affects the settling efficiency. Outlet
transitions are designed to skim relatively less sediment water and from past studies it
has been concluded that outlet should be placed at relatively greater height and it should
be as wide as possible. Generally, there are two ways in which outlet transition can be
designed one is by providing gradual contraction and other way is to provide straight
vertical wall and from its top outlet is drawn. These both arrangement is shown in Fig
3.11 (a) and (b) respectively.
22
(a) (b)
Figure 3.11: Images of outlet transition with (a) gradual contraction, (b) Vertical wall
***************************************************************************
23
CHAPTER 4
EXPERIMENTAL WORK ON PHYSICAL MODEL
4.1 Introduction
The Kholongchhu Hydro Electric Project is located at the lower course of Kholongchhu River
just before its confluence with Dramengchu (Gongrichu) in Trashiyangtse District of Bhutan.
The area lies partly in the Khamdang Gewog (block) on the left bank and partly in the Tomi
Jangsa Gewog on the right bank. Location map of the project is shown in Fig 4.1.
Kholongchhu HEP
Figure 4.1: Location map of Kholongchhu H.E.P
The proposed project is a run- of- the river scheme with diurnal storage for peaking purposes
and envisages utilization of a net head of about 761.53 m between Full reservoir level (FRL,
EL 1572 m) at Dam site and maximum Tail water Level (EL.768 m) at power house Site. Two
number straight type intakes with inclined trash rack are provided for smooth entry of water
from the reservoir to power intake / headrace tunnel. The invert level of intakes is kept at
1546.65 m. Two no. of feeder tunnel of 4.70 m diameter, horse shoe shaped with intake
discharge of 106.8 m3/s (including 20% flushing discharge) are proposed from power intake to
desilting chambers. Two D-Shaped desilting chambers of 13.00 m (W) X 18 m (H) X 350 m
24
(L) have been provided for exclusion of sediment size of 0.2 mm and above. The project layout
plan is shown in Fig 4.2.
Figure 4.2: Project layout plan
4.2 Desilting chambers and sediment data
Impact of silt-laden water on operation of hydropower station is one of the most significant
problems wherein silt abrasion causes rapid wear and tear of turbine and its degradation. The
high silt load and silt concentration makes the total system non-operational. The proposed
desilting chamber arrangement comprises two D-Shaped type desilting chambers of 13.0 m
(W) X 18 m (H) x 350 m (L) for exclusion of 90% suspended particles coarser than 0.2 mm
size and maximum sediment concentration of 4000 ppm. The intake design discharge for each
desilting chamber is 53.4 m3/s including flushing discharge of 8.9 m3/s. The average flow
through velocity in the desilting chamber works out to be 0.282 m/s. Average concentration of
the suspended sediment as per data from the site recorded over five years is in the percentage
of coarse, medium and fine sediment as 36%, 24% and 40% respectively.
4.3 Experimental Setup
Experiments have been performed at Central Water and Power Research Station, (CWPRS)
Pune in sediment management department. Model of desilting chamber for Kholongchhu
25
H.E.P was used for the experimentation purpose. Table 4.1 consist of details and dimension of
desilting chamber of model as well as prototype.
Table 4.1: Dimension and details of Desilting chamber for design discharge (Model
Scale 1:30 GS)
Parameters Prototype Model

Inlet discharge (m3/s) 53.4 0.0108
Outlet discharge (m /s)3 With design 44.5 0.009
discharge
Flushing discharge (m3/s) 8.9 0.00181
Depth of flow (m) 18 0.6

Length of chamber (m) 350 11.67
Width of chamber (m) 13 0.43
Flow area (m2) 173.75 0.1931
Average flow though velocity (m/s) 0.282 0.0514
Size of settling trench at the beginning (m) 2.0 (W) x 0.6 (D) 0.067 (W) x 0.02 (D)
Size of settling trench at the end (m) 2.0 (W) x 2.0 (D) 0.067 (W) x 0.067 (D)
Size of flushing tunnel at beginning (m) 1.0 (W) x 0.7 (D) 0.033 (W) x 0.023 (D)
Size of flushing tunnel at the middle
1.4 (W) x 1.35 (D) 0.047 (W) x 0.045 (D)
Size of flushing tunnel at the end
1.4 (W) x 2.0 (D) 0.047 (W) x 0.067 (D)
Model of desilting chamber is casted with fibre glass with transparent Perspex window to
obtain visuals of sediment distribution throughout the model as shown in Fig 4.3. For the
experimental work Walnut shell powder is used as sediment as shown in Fig 4.4 whose specific
gravity is 1.4 and d50= 0.11mm, this specific gravity was accounted on basis of fall velocity
criteria.
Figure 4.3: Model of Kholongchhu desilting basin 350 m equivalent of prototype
26
Figure 4.4: Walnut shell powder
4.4 Experimental Procedure
Normally desilting chamber is run for one day equivalent of prototype and whole run is divided
into the three stages. First stage is the initial filling of desilting chamber, second stage is to
inject the sediments and the third stage is dewatering the basin as mentioned in CWPRS (2005).
4.4.1 Initial Filling of Desilting Chamber

 In light of sediment deposition in the desilting basin and air expulsion from the desilting
basin, the first filling operation is crucial. It is also necessary to adhere to the other
water conductor system filling standards.
 Initial filling rate should be kept very low so as to avoid air-water column separation
along the water conductor system. Therefore, the intake gates should be operated
partially. The air vent provided at the roof of the desilting basin as shown in Fig 4.5
should be checked to see that it is free from any choking.
Air Vents
Figure 4.5: Air vents on the roof of Desilting chamber
27
 The flushing tunnel gate should be kept open such that the discharge passing through
the tunnel will be at least 10% more than the design flushing discharge. This will help
in generating higher flushing velocities required to transport coarse sediment.
 Once the filling of the water conductor system is completed as shown Fig 4.6 (a) and
the reservoir water level is held at MDDL or above as shown in Fig 4.6 (b), the intake
gates should be fully opened for normal operation. This will help to have lower
velocities at intake for drawing the design discharge, which is controlled at powerhouse.
Rehbock
Weir
Water Level
(a) (b)
Figure 4.6: (a) Rehbock weir and (b) water level between MDDL and FRL
 Once the basin is completely filled with water and gauge level in rehbock weir, HRT
as well as SFT tanks are set as shown in Fig 4.7 (a), Fig 4.7 (b) and Fig 4.7 (c)
respectively the outlet valve is partially closed and required discharge is allowed
through desilting chamber.
28
(a) (b) (c)
Figure 4.7: (a) Gauge in rehbock weir, (b) gauge in HRT tank, (c) gauge in SFT
tank
 Gauge level maintained at inlet weir, HRT as well as SFT are 56.81 cm, 40.80 cm and
45 cm respectively throughout the experiment.
4.4.2 Sediment Injection Stage

 Once the gauges are set, sediments can be induced in desilting chamber, but before that
the sediment concentration required to be maintained in desilting chamber is computed
as follows:
Sediment concentration from site: 4000 ppm,
% Gradation of Particles: Coarse=36%, Medium=24%, Fines=40%,
% Gradation of Particles considered for the experiment: Coarse=36%, Medium=24%,
Since the volume of vessel is 10 Liters so the time required to inject 10 Lit of
sediments is 9 mins.
 Now with the help of provision made in the intake tank as shown in Fig 4.8 sediments
are injected in desilting chamber manually.
29
(a) (b)
Figure 4.8: (a) Image of manual way to inject sediments, (b) sediment storage tank
 For the experiment purpose out of single batch of sediments three samples were taken
out for the analytical studies and different quantities of sediments were taken into
considered for the experiments.
 Following were the observations made during the injection of particles:
I. When sediments are released in the basin their concentration in inlet transition
is very high as shown in Fig 4.9 because cross-sectional area of inlet transition
increases gradually due to which velocity is reduced drastically.
Figure 4.9: Inlet transition with high sediment concentration
II. Beyond inlet transition since velocity is reduced sediments settles on hopper
slope as shown in Fig 4.10 and slides into the settling trench which is extended
below the hopper slope.
30
Hopper Slope
Settled Sediments
Figure 4.10: Sediment settlement on hopper slope
III. As we move towards downstream of the basin, concentration of suspended

sediment goes on reducing and quantity of settled sediments are also less near
the end of main basin as shown in Fig 4.11 as compared to one which is
observed in Fig 4.9.
Figure 4.11: Concentration of sediments at the outlet of main basin
 Continuous monitoring of the suspended sediment content in the upstream of intake is

necessary. The intake gates should be immediately closed if the suspended sediment
concentration in ppm exceeds the design limit.
 The designed flushing discharge should be passed through the flushing tunnel
irrespective of the available discharge, for model studies of Kholongchhu H.E.P
flushing discharge of 0.00181 m3/s equivalent to that of prototype is allowed through
flushing tunnel.
31
 If the flushing tunnel is operated at a discharge less than the designed flushing discharge
then openings connecting the main basin to the flushing tunnel at the downstream end
of the basin will only be effective resulting in deposition of sediment in the upstream
region of the flushing tunnel, as well as in the basin. This will lead to choking of the
flushing tunnel.
 When the discharge available is more than the design discharge, the powerhouse is run
with overload using more water. This increases forward velocity in the basin and
reduces the detention time resulting into lesser settling efficiency. Under these
circumstances, the concentration of sediment at inlet must be observed and the system
should be run only when sediment concentration is less than the design value. For this
reason, model is tested for 10% overload discharge to check whether the desilting
chamber could serve its purpose satisfactorily by withstanding the heavy load on it.
4.4.3 Dewatering Stage

 The dewatering of the desilting basin is necessary for inspection and maintenance.
 Once the sediments are completely injected, at the end of run outlet gate on HRT tunnel
is closed gradually followed by closer of intake gate.
 The water in the basin should be drained out through flushing tunnel by operating the
flushing tunnel gate as shown in Fig 4.12, such that the discharge in the flushing tunnel
is about 10% more than the flushing designed discharge, which would generate higher
velocities for flushing of sediment.
 Once the deposited sediments in the basin are completely flushed out, HRT and SFT
tanks are then set ideal for almost 48 hrs so that sediments received by them gets settled
in due coarse.
 After the sediments are settled both the tanks are decanted to collect the sediments as
shown in Fig 4.13 (a) and (b).
 Once the sediments are collected settling efficiency can be computed as given in
equation 4.1 :
𝑞𝑠𝑖 −𝑞𝑠𝑒
Settling efficiency (Ƞ) = …………………...(eq 4.1)
𝑞𝑠𝑖
Where,
qsi, qse= amount of sediment entering and leaving the basin per unit time
 Efficiency obtained for different sediment quantities have been discussed in Result and
discussion chapter.
32
Outlet
gate
Flushing Tunnel
gate
Figure 4.12: Outlet transition with outlet and silt flushing tunnel gate
(a) (b)
Figure 4.13: (a), (b) Sediments collection in SFT and HRT tanks
****************************************************************
33
CHAPTER 5
NUMERICAL MODELLING
5.1 General
This chapter consist of need and applicability of numerical modelling to compute the settling
efficiency of desilting basin. Numerical modelling has been proven to be important tool in
solving the complex problems. It is crucial because it allows us to comprehend systems and
processes that are challenging or impossible to investigate directly and to predict and simulate
complicated events. Here are some key reasons why numerical modelling is important:
1. Assessing intricate structures: Researchers can analyse complex systems by simulating

them with mathematical equations using numerical models. These models may
replicate behaviour of physical, biological, or social systems, assisting researchers in
understanding how those systems work together.
2. Cost-effective experimentation: Conducting physical experiments can frequently be

time-consuming, expensive, or unethical. Researchers can replicate experiments using
numerical modelling, which eliminates the need for expensive and resource-intensive
physical testing. This is particularly beneficial in fields such as hydraulic engineering,
where extensive testing is required before building prototypes.
3. Design optimization: One of the most important advantages of numerical modelling is

that it enables engineers to optimize designs before physically adopting them. By
running simulations along with modifying various design parameters, they can assess
the performance of different design options and identify the most efficient and effective
solutions.
Numerical modelling can be performed with the help of different Computational Fluid
Dynamics (CFD) software.
5.2 Use of CFD Software
Computational Fluid Dynamics (CFD) software is used to simulate and analyse fluid flow,
sediment load transport, turbulence, heat transfer, and other related phenomena using
numerical methods. There are several CFD software packages available, each with its own
34
features and capabilities. Here are some commonly used CFD software: ANSYS Fluent, Open
FOAM, FLUENT, Flow 3D, etc. For the current study purpose FLOW 3D software is used.
Flow 3D is a computational fluid dynamics (CFD) software that offers advanced capabilities
for simulating fluid flow, sediment load translation, and other related phenomena. In order to
evaluate and optimize the design of desilting basins, numerical simulation utilising CFD
software like Flow 3D can be beneficial. For Desilting chamber Flow 3D could be applied as
follows:
1. Geometric modelling: Flow 3D make use of actual 3D model of the desilting basin. The
software offers capabilities for creating precise representations of the inlet and outflow
structures, sedimentation zones, and baffles that make up the geometry of the basin.
2. Flow simulation: To simulate the flow of water and sediment inside the basin, Flow 3D
uses numerical techniques. Multiphase flows (a water-sediment mixture) and turbulent
flows are a couple of the complex flow conditions that the software can manage. It
considers elements including silt concentration, flow velocity, and turbulence.
3. Sediment transport and settling: With the help of Flow 3D actual profile for the
sediment transport and settling processes within the desilting basin can be obtained. It
considers factors such as sediment size distribution, settling velocities, and interaction
with the basin walls and structures. The simulation can shed light on the patterns and
effectiveness of sedimentation in the basin.
4. For analysis: Flow 3D provides visualization and post-processing tools to analyse and
interpret the simulation results. Users can visualize flow patterns, sediment distribution
in the desilting chamber, and other relevant variables. This enables a detailed
understanding of the hydraulic behaviour of the desilting.
5.3 Procedure for Developing 3D Geometry & Importing in Flow 3D
Before performing actual simulation, there are some pre-requirements of software that are to
be accomplished first. Water tight geometry in .stl format is basic requirement for the
simulation in Flow 3D. So here are the steps adopted for developing 3D geometry of Prototype
of Desilting chamber as follows:
1. Model of prototype for Kholongchhu basin was formed by first creating 10 cross-
section and placing them in straight alignment. Once the sections are in proper
35
alignment by using “lofting” command all sections are joined together forming the
continuous geometry as shown in Fig 5.1.
Figure 5.1: Skeleton of Kholongchhu Desilting chamber
2. After using loft command, the next step is to make the geometry as solid and water tight
and for this purpose, “surface sculpt” command is used. Every face, openings, surface
of geometry is selected and by right click geometry is converted into 3D solid-water
tight basin as shown in Fig 5.2.
Figure 5.2: 3D water-tight geometry of basin
3. Once water tight section is formed then it is converted into .stl format before importing
it into FLOW 3D software. Fig 5.3 shows image of desilting basin in .stl format.
36
Figure 5.3: .stl format of desilting chamber
Since 3D model is now created, the next step is to import the geometry into Flow 3D
software for which next steps are to be followed:
4. In FLOW 3D new work space is created for simulation and .stl file is imported.
5. Three meshes are created covering flushing trench, middle and top portion of basin
throughout its length as shown in Fig 5.4.
Figure 5.4: Three mesh applied to basin
6. Once the meshes are applied, next step is to create the confined region for the geometry
for which rectangular box is created in which geometry is embedded as shown in Fig
5.5. The reason to provided such confined region is to create solid surface surrounding
the basin so that flow condition could be established in it.
37
Figure 5.5: Rectangular box embedded with desilting chamber
7. Baffle walls are then provided at outlet section at end of basin and at bottom of flushing
trench through which sediments are flushed out as shown in Fig 5.6.
Figure 5.6: Baffle walls provided at outlet
Once the baffle walls are provided numerical simulation can be performed by giving initial and
boundary conditions which are discussed in section 5.4.
5.4 Initial and Boundary Conditions
Accurate initial and boundary conditions are to be provided so that the simulation could
proceed in required direction. While performing the numerical simulation special care is taken
while giving boundary conditions because if in case proper conditions are not given then
pressure will not get converge inside the basin thus resulting into poor flow distribution
throughout the basin. Initial and boundary conditions remain same for all the simulations.
38
5.4.1 Initial Conditions
Initial conditions are required so as to attain the actual flow conditions in the desilting chamber
and to record the results for each and every time step. Below mentioned are the initial
conditions adopted for the simulation:
 Flow velocity: 0.282 m/s (X direction);

 Pressure condition: Hydrostatic Pressure;
 Fluid Initialization: Use Fluid Elevation;
 Volume of fluid advection: One fluid, no free surface (confined flow only);
 Momentum advection: First order;
 Fluid flow solver option: Solve momentum and continuity equation;
 Initial time step: 1𝑒 −15 ;
 Pressure solver option: Implicit and GMRES;
 Restart data interval: 50 sec (To obtain results after every 50 sec);
 History data interval: 20 sec (To store the results after every 20 sec);
 Fractional interval: Fluid velocities and Particle information.
 Fluid 1: Water at 200C;
 Gravity and non-inertial frame of reference: X=0, Y=0, Z= -9.81;
 Viscosity and turbulence: Viscosity options= viscous flow;
 Turbulence model = Two-equation (k-e);
 Run time adopted = 845 sec.
5.4.2 Boundary conditions
Boundary conditions are basically needed so that inlet and outlet of the section could be defined
w.r.t flow. With help of it we can control the discharge in the desilting chamber. Following are
the boundary conditions adopted for the three different meshes for simulation:
For Top mesh:
 X min: Pressure of 2.5𝑒 5 bars with fluid fraction as 1;

 X max: Volume flow rate of 44.5 m3/s with fluid elevation of 46 m;
 Y min: Symmetric condition;
 Y max: Symmetric condition;
 Z min: Symmetric condition;
39
 Z max: Wall.
For Middle mesh:
 For this particular mesh all conditions are set to Symmetry because this portion lies in
the middle of chamber due to which no new conditions are to be adopted within this
particular portion.
For Bottom mesh:
 X min: Wall;
 X max: Wall;
 Y min: Wall;
 Y max: Symmetric condition;
 Z min: Volume flow rate of 8.9 m3/s with Z- flow direction vector as -1;
 Z max: Wall.
5.5 Conditions for Particles Inclusion
Particles are injected in the basin once the steady flow condition is achieved in desilting basin.
One can know the achievement of steady state condition once it is reflected by the solver.
Following are the conditions inculcated for inclusion of particles:
 Mass particles: Variable diameter;

 Particle density: 2650 kg/m3
 Particle/void: Particles cannot escape to voids (particles initialized in voids will be
deleted);
 Particle/fluid interaction: Partial fluid particle interaction (fluid motion not affected by
particles);
 Maximum allowed number of particles: 5000000
 Run time adopted: from 850 sec to 10000 sec or by choosing restart simulation from
100 sec to 10000 sec;
 Particle Initialization: Particle source as shown in Table 5.1.
Table 5.1: Particle source data
Name X X Y Y Z Z # of Min Max Rate
low high low high low high species size size
P1 25.3 27.3 5.8 7.8 18.7 20.7 1 0.19mm 0.21mm 500
40
Once the particles are injected the simulation is run till 10000 sec so that whole particle
translation profile is achieved. The results for the sediment settling efficiency and sediment
translation profile are mentioned in Results and discussion chapter.
****************************************************************
41
CHAPTER 6
RESULTS & DISCUSSION
6.1 General
This chapter consist of results obtained from the experimental studies as well as from numerical
simulation. Total 13 experimental run were performed on physical model with equivalent basin
length of 350 m and 300 m of that of prototype. Apart from physical modelling, numerical
simulations were performed on 3D geometry of prototype for the basin length 350 m & 300 m
and the results were then compared to check the applicability of numerical modelling.
6.2 Results for Experimental Studies
Physical model studies were carried out for two different basin length i.e for prototype
equivalent of 350 m and 300 m length sediment size greater than 0.2 mm. Below mentioned
are the results of settling efficiency for these two models.
6.2.1 Basin length equivalent of 350 m as that of prototype
For the model studies 1:30 geometrical similar scale is chosen. Table 6.1 shows the
specifications of the physical model of Kholongchhu desilting basin and Fig 6.1 shows the
image for the model of desilting chamber 350 m equivalent of prototype.
Figure 6.1: Model of desilting basin 350 m equivalent to prototype
42
Table 6.1: Specification of physical model for basin length of 350 m equivalent of
prototype
Specification Model
Dimension (m) 11.67(L)×0.6(H)×0.433(W)
Discharge(m3/s) 0.0108
10% Overload discharge 0.01191
Particle size(mm) 0.11
Flushing discharge(m3/s) 0.00181
Concentration(ppm) 4000
% Gradation of Particles Coarse=36%, Medium=24%,
Bed Slope (Inlet transition) 1:2.255
Flow through velocity(m/s) 0.0514
Flow area (m2) 0.2067
Fall Velocity(m/s) 0.0026
6.2.1.1 Experimental Run with Design Discharge of 0.0108 m3/s
For 350 m length, total 7 experimental runs were performed considering design discharge and
10% overload discharge for different sediment quantities. For the runs performed using the
design discharge of 0.0108 m3/s, four different quantities were taken into account i.e 320 lit,
300 lit, 260 lit and 240 lit and average overall settling efficiency for 350m basin came out to
be 84.33%, whose results are been discussed in Table 6.2 as shown below.
Table 6.2: Results for design discharge of 0.0108 m3/s (350 m)

Material Material collected Observed
injected (liters) settling
(liters) HRT SFT efficiency
(Ƞ)
(%)
320 35 270 84.38
260 30 220 84.62
240 30 200 83.33
300 30 255 85
Average overall settling efficiency = 84.33 %
43
Since while conducting the experimental run fine particles / wash load were not been able to
trap in HRT and SFT tanks. In order to account those lost sediments, adjustments of lost
materials were made based on the sediment quantities that are trapped in respective tanks. After
performing adjustments, the average overall modified settling efficiency of basin came out to
be 85.2%. Table 6.3 refers to the adjustment made in the collected material.
Table 6.3: Results for design discharge of 0.0108 m3/s after adjustments (350 m)
Material Material collected Modified
injected settling
(liters)
(liters) efficiency
HRT SFT (Ƞ)
(%)
320 47 273 85.31
260 38 222 85.38
240 38 202 84.17
300 42 258 86
Average overall modified settling efficiency = 85.2%
Comparison of the results for overall settling efficiency before and after the adjustment made
for design discharge of 0.0108 m3/s from above two tables have been shown in Fig 6.2 as shown
below.
Figure 6.2: Representation of results for 350 m basin length (design discharge)
44
Since four different quantities of material was considered for study out of which for 240 lit of
sediment overall observed settling efficiency came out to be 83.33 % and overall modified
efficiency came out to be 84.17%. For the sediment quantity of 260 lit the overall observed
settling efficiency and overall modified efficiency came out to be 84.62% and 85.38%
respectively. For the sediment quantity of 300 lit the overall observed settling efficiency and
overall modified efficiency came out to be 85 % and 86 %, while for 320 lit sediment overall
observed and modified efficiency were 84.38% and 85.31 %.
6.2.1.2 Experimental Run with 10% overload discharge of 0.01191 m3/s
Apart from the design discharge, the turbines in hydropower plants are sometimes supplied
with 10 % overload discharge to meet the excessive peak load. Due to this desilting chamber
are operated for 10% overload discharge greater than the discharge for which they are designed
for. So, to analyse the effect of 10% overload discharge three experimental runs were
performed considering discharge as 0.01191 m3/s. The average overall settling efficiency was
observed to be 83.6% and results for the study are represented in Table 6.4.
Table 6.4: Results for 10% overload discharge of 0.01191 m3/s (350 m)
(Ƞ)
(%)
250 27 213 85.20
340 41 279 82.06
340 41 284 83.53
Since while performing experimental run fine particles / wash load were not been able to trap
in collection tanks so adjustments of lost materials were made based on the sediment quantities
that are trapped in respective tanks. After performing adjustments, the average overall modified
settling efficiency of basin came out to be 84.55 %. Table 6.5 refers to the adjustment made in
the collected material.
45
Table 6.5: Results for 10% overload discharge of 0.01191 m3/s after adjustment (350 m)
injected settling
(liters)
(liters) efficiency
HRT SFT
(Ƞ)
(%)
250 35 215 86
340 57 283 83.24
340 53 287 84.41
Average overall modified settling efficiency = 84.55 %
Results from the Table 6.4 and Table 6.5 have been compared in Fig 6.3 as shown below.
Figure 6.3: Representation of results for 350 m basin length (10% Overload discharge)
From Fig 6.3 it can be concluded that for sediment quantity of 250 lit the average overall
settling efficiency and modified settling efficiency came out to be 85.20 % and 86 %
respectively. For 340 lit quantity variation in results were observed due to human error while
collecting sediments in order to calculate efficiency, so overall observed efficiency came out
to be 82.06% and 83.53% while for the overall modified efficiency results came out to be
83.24% and 84.41%.
46
6.2.1.3 Comparison of results for design discharge & 10% overload
discharge
As mentioned earlier necessity for testing the model for 10% overload discharge as it becomes
crucial to check the performance of desilting basin so as to ensure that its settling efficiency is
affected within permissible range. So, the results of design discharge and 10% overload
discharge are compared for the observed values and modified values of efficiency. Fig 6.4
shows the comparison made between observed values of settling efficiency.
320
Figure 6.4: Representation of observed values of settling efficiency for 350 m basin
length
From Fig 6.4 it can be concluded that the difference in average results for the 10% overload
discharge (84.55%) and the average results for the design discharge (84.33%) is 0.73 %. Since
the results are within permissible limits for high discharge than the designed one so desilting
basin satisfies its purpose.
Same comparison has been made for the modified values which are represented in Fig 6.5 as
shown below.
47
Figure 6.5: Representation of modified values of settling efficiency for 350 m basin
length
From Fig 6.5 it can be concluded that the difference in modified average results for the 10%
overload discharge (83.6%) and the average results for the design discharge (85.2%) is 1.6%.
Since the results are within permissible limits so desilting basin satisfies its purpose.
6.2.1.4 Analytical Studies for 350 m length desilting basin
For the analytical studies estimation of settling efficiency is done using camp’s criteria
(function of mixing coefficient) applied on the sediments that enters through water conductor
system whose d50=0.11 mm and as mentioned earlier for prototype suspended sediments
consists of 36% coarse, 24% medium and 40% fine particles. The graph for the gradation of
sediments is represented in Fig 6.6 as shown below.
100
90
80
70
60
% FINER
50
40
30
20
10
0
0.01 0.1 1 10
PARTICLE DIA (mm)
Figure 6.6: Gradation curve of suspended sediment for prototype
48
From Fig 6.6 settling efficiency for suspended sediments have been tabulated using camps
criteria as shown in Table 6.6.
Table 6.6: Settling efficiency estimation for sediment data for prototype (350 m)
V=0.282m/s Length = 350 m V0=0.01449 m/s
Sr. No % Finer Particle Fall 𝑊 𝑊 Removal
122
𝑉 𝑉0
Dia (mm) Velocity(m/s) Ratio
W
1 5 0.023 0.00046 0.2 0.032 0.032
2 15 0.032 0.00089 0.386 0.061 0.061
3 25 0.045 0.00175 0.759 0.121 0.121
4 35 0.062 0.00329 1.426 0.227 0.224
5 45 0.090 0.00671 2.907 0.463 0.450
6 55 0.130 0.01288 5.577 0.889 0.790
7 65 0.210 0.02612 11.312 1.803 0.990
8 75 0.580 0.06730 29.146 4.645 1
9 85 1.800 0.13188 57.111 9.102 1
10 95 5.300 0.23069 99.897 15.922 1
5.668
Overall Settling efficiency (%) = 56.68
Where,
0.132𝜔
W = Fall velocity of a particle after taking into effect for reduction due to turbulence [ ]
√𝐷𝑒𝑝𝑡ℎ
𝜔 = Terminal fall velocity of individual particle in still water;

V = Average forward velocity (m/s);
V0 = Required fall velocity (m/s);
𝜔 = Fall velocity (m/s) given by Ruby’s equation:
[1636(𝜌𝑠 −𝜌)𝑑3 +9𝜇2 ]0.5 −3𝜇
𝜔=
500𝑑
where,
𝜌𝑠 , 𝜌= density of sediment and water resp. in kg/m3 =2650, 1000 kg/m3 resp.
𝜇= dynamic viscosity (Ns/m2) = 1.002×10-3 Ns/m2
d= particle diameter to be excluded (m)
49
Based on the results as observed in Table 6.6 for 0.2 mm particle size removal efficiency would
be 98% for prototype.
For the model studies from the one batch of sediments three samples were collected and
analysed by preparing particle size distribution curves for three different samples as shown in
Fig 6.7, 6.8 and 6.9.
SUSPENDED SEDIMENT USED IN MODEL (SAMPLE 1)

110
100
90
80
PERCENTAGE FINER
70
60
50
40
30
20
10
0
0.01 0.1 1 10
PARTICLE DIA (mm)
Figure 6.7: Particle size distribution curve for sample 1

110
100
90
80
PERCENTAGE FINER
70
60
50
40
30
20
10
0
0.01 0.1 1 10
PARTICLE DIA (mm)
50
110
100
90
PERCENTAGE FINER 80
70
60
50
40
30
20
10
0
0.01 0.1 1 10
PARTICLE DIA (mm)

Above gradation curves are analysed with the help of Camp’s criteria (function of mixing
coefficient) and the removal ratio are tabulated for model parameters as shown in Table 6.7,
6.8 and 6.9 for design discharge and Table 6.10, 6.11 and 6.12 for the 10 % overload discharge.
Table 6.7: Settling efficiency estimation for sediment data for model (Sample 1)
V=0.0514 m/s Length = 11.67 m V0=0.0026 m/s
122
𝑉 𝑉0
W
1 5 0.045 0.00029 0.694 0.111 0.11
2 15 0.150 0.0031 7.380 1.176 0.92
3 25 0.170 0.00392 9.301 1.482 0.97
4 35 0.190 0.00479 11.358 1.810 0.99
5 45 0.2 0.00524 12.427 1.981 1
6 55 0.230 0.00664 15.749 2.510 1
7 65 0.250 0.00760 18.022 2.872 1
8 75 0.270 0.00856 20.314 3.238 1
9 85 0.290 0.00953 22.603 3.602 1
10 95 0.350 0.01236 29.320 4.673 1
8.99
Overall Average Settling efficiency (%) = 89.91
51
V=0.0514 m/s Length = 11.67 m V0=0.0026 m/s
122
𝑉 𝑉0
W
1 5 0.035 0.00018 0.420 0.067 0.07
2 15 0.170 0.00392 9.301 1.482 0.97
3 25 0.180 0.00435 10.314 1.644 0.98
4 35 0.190 0.00479 11.358 1.810 0.99
5 45 0.205 0.00547 12.970 2.067 1
6 55 0.220 0.00617 14.626 2.331 1
7 65 0.240 0.00712 16.882 2.691 1
8 75 0.270 0.00856 20.314 3.238 1
9 85 0.290 0.00953 22.603 3.602 1
10 95 0.350 0.01236 29.320 4.673 1
9.007
V=0.0514 m/s Length = 11.67 m V0=0.0026 m/s
122
𝑉 𝑉0
W
1 5 0.030 0.00013 0.309 0.049 0.05
2 15 0.160 0.00351 8.321 1.326 0.95
3 25 0.170 0.00392 9.301 1.482 0.97
4 35 0.185 0.00457 10.833 1.727 0.98
5 45 0.200 0.00524 12.427 1.981 1
6 55 0.220 0.00617 14.626 2.331 1
7 65 0.240 0.00712 16.882 2.691 1
8 75 0.270 0.00856 20.314 3.238 1
9 85 0.300 0.01001 23.742 3.784 1
10 95 0.350 0.01236 29.320 4.673 1
52
8.949
Table 6.10: Settling efficiency estimation for sediment data for model with 10%
overload discharge (Sample 1)
V=0.057 m/s Length = 11.67 m V0=0.0029 m/s
122
𝑉 𝑉0
W
1 5 0.045 0.00029 0.631 0.101 0.10
2 15 0.150 0.0031 6.709 1.069 0.87
3 25 0.170 0.00392 8.455 1.348 0.95
4 35 0.190 0.00479 10.325 1.646 0.99
5 45 0.2 0.00524 11.297 1.801 1
6 55 0.230 0.00664 14.317 2.282 1
7 65 0.250 0.00760 16.384 2.611 1
8 75 0.270 0.00856 18.467 2.943 1
9 85 0.290 0.00953 20.548 3.275 1
10 95 0.350 0.01236 26.655 4.248 1
8.91
V=0.057 m/s Length = 11.67 m V0=0.0029 m/s
122
𝑉 𝑉0
W
1 5 0.035 0.00018 0.379 0.061 0.06
2 15 0.170 0.00392 8.393 1.352 0.94
3 25 0.180 0.00435 9.308 1.500 0.98
4 35 0.190 0.00479 10.249 1.651 0.99
53
5 45 0.205 0.00547 11.704 1.886 1
6 55 0.220 0.00617 13.198 2.126 1
7 65 0.240 0.00712 15.234 2.454 1
8 75 0.270 0.00856 18.331 2.953 1
9 85 0.290 0.00953 20.397 3.286 1
10 95 0.350 0.01236 26.458 4.263 1
8.971
V=0.057 m/s Length = 11.67 m V0=0.0029 m/s
122
𝑉 𝑉0
W
1 5 0.030 0.00013 0.279 0.045 0.05
2 15 0.160 0.00351 7.509 1.210 0.92
3 25 0.170 0.00392 8.393 1.352 0.95
4 35 0.185 0.00457 9.775 1.575 0.99
5 45 0.200 0.00524 11.214 1.807 1
6 55 0.220 0.00617 13.198 2.126 1
7 65 0.240 0.00712 15.234 2.454 1
8 75 0.270 0.00856 18.331 2.953 1
9 85 0.300 0.01001 21.424 3.452 1
10 95 0.350 0.01236 26.458 4.263 1
8.905
The average overall settling efficiency for design discharge with physical modelling came out
to be 85.2 % and that from analytical studies from Table 6.7, 6.8 and 6.9 was observed to be
89.82 % i.e difference in efficiency came out to be 4.62%. While average overall settling
efficiency for 10% overload discharge with physical modelling came out to be 84.55 % and
54
that from analytical studies from Table 6.10, 6.11 and 6.12 was observed to be 89.29 % i.e
difference in efficiency came out to be 4.74%
Based on the results obtained from Table 6.7, 6.8, 6.9, 6.10, 6.11 and 6.12 the curve for the
settling efficiency vs particle diameter is prepared for model and its equivalent size in prototype
for design discharge as well as for 10 % overload discharge which is represented in Fig 6.10
and 6.11 respectively.
100
90
80
SETTLING EFFICIENCY (%)
70
Model Curve Proto Curve
60
50
40
30
20
10
0
0.00 0.05 0.10 0.15 0.20 0.25
PARTICLE DIA (mm)
Figure 6.10: Representation of analytical sediment removal efficiency for model and
prototype for 350 m length basin for design discharge
SETTLING EFFICIENCY CURVE (10% OVERLOAD)

100
90
80
70
60
50
40
30
20
10
0
0.00 0.05 0.10 0.15 0.20 0.25
PARTICLE DIA (mm)
prototype for 350 m length basin for 10 % overload discharge
55
From the curve of prototype as shown in Fig 6.10 and Fig 6.11 it can be observed that for 0.2
mm sediment size settling efficiency comes out to be 98%. If the difference of 4.62 % is
considered, the settling efficiency for 0.2 mm sediment size is observed to be 93.38 % which
is much greater for which desilting chamber is designed i.e for ƞ= 90%. Thus, it is concluded
that the length of desilting chamber is greater than what it is actually required. This gave the
scope to reduce the length of desilting basin.
From the prototype settling efficiency curve, various length of desilting chamber for 0.2 mm
sediment size was prepared based on analytical studies taken into consideration for design
discharge and 10 % overload discharge as shown in Fig 6.12 and Fig 6.13.
SETTLING EFFICIENCY Vs LENGTH

100
90
80
200 250 300 350 400 450
LENGTH OF DESILTING CHAMBER (m)
Figure 6.12: Representation of length of desilting basin vs settling efficiency (design

discharge)
From Fig 6.12 it can be observed that with curve generated from analytical results considered
for the design discharge, efficiency for 300 m came out to be 96.5 % but as we found out that
difference in analytical and experimental results is of 4.62 % so for the prototype expected
efficiency is 91.88 % which is in close range for which desilting chamber is needed to be
designed.
56
SETTLING EFFICIENCY Vs LENGTH
100

95
90
85
80
75
200 250 300 350 400 450
LENGTH OF DESILTING CHAMBER (m)
Figure 6.13: Representation of length of desilting basin vs settling efficiency (10%

overload discharge)
From Fig 6.13 it can be observed that with the curve generated from analytical results
considered for 10 % overload discharge, the efficiency for 300 m came out to be 95 % but as
we found out that difference in analytical and experimental results is of 4.74 % so for physical
modelling expected efficiency is 90.26 % which is in close range for which desilting chamber
is needed to be designed. Therefore 300 m basin length is adopted whose details have been
discussed in section 6.2.2.
6.2.2 Basin length equivalent of 300 m as that of prototype
As discussed in section 6.2.1.4 the settling efficiency of desilting chamber for sediment size
0.2 mm and above is more than 90% for design as well as 10% overload so the model for 300
m length equivalent to prototype is constructed with scale of 1:30 geometrical similar as shown
in Fig 6.14. Table 6.13 shows the specifications of the physical model of Kholongchhu desilting
basin for 300 m length equivalent of that of prototype.
57
Figure 6.14: Model for 300 m desilting chamber equivalent to prototype
Table 6.13: Specification of physical model for basin length of 300 m equivalent of
prototype
Specification Model
Dimension (m) 10(L)×0.6(H)×0.433(W)
Discharge(m3/s) 0.0108
10% Overload discharge 0.01191
Particle size D50(mm) 0.11
Flushing discharge(m3/s) 0.00181
Concentration(ppm) 4000
% Gradation of Particles Coarse=36%, Medium=24%,
Bed Slope (Inlet transition) 1:2.255
Flow through velocity(m/s) 0.0514
Flow area (m2) 0.2067
Fall Velocity(m/s) 0.0031
6.2.2.1 Experimental Run with Design Discharge of 0.0108 m3/s
For 300 m length, total 6 experimental runs were performed considering design discharge and
10% overload discharge for different sediment quantities. For the runs performed using the
design discharge of 0.0108 m3/s, three runs were performed using two different quantities i.e
240 lit & 300 lit and average overall settling efficiency for 300m basin came out to be 83.22%,
whose results have been discussed in Table 6.14 as shown below.
58
Table 6.14: Results for design discharge of 0.0108 m3/s (300 m)
(Ƞ)
(%)
240 32 202 84.2
240 30 198 82.5
300 39 249 83
Since while conducting the experimental run fine particles / wash load were not been able to
trap in HRT and SFT tanks. In order to account those lost sediments, adjustments of lost
materials were made based on the sediment quantities that are trapped in respective tanks. After
performing adjustments, the average overall modified settling efficiency of basin came out to
be 83.99%. Table 6.15 refers to the adjustment made in the collected material.
Table 6.15: Results for design discharge of 0.0108 m3/s after adjustments (300 m)
(Ƞ)
(%)
240 36.8 203.2 84.7
240 39.6 200.4 83.5
300 48.6 251.4 83.8
Average overall modified settling efficiency = 83.99%
Comparison of the results for overall settling efficiency before and after the adjustment made
for design discharge of 0.0108 m3/s from above two tables have been shown in Fig 6.15 as
shown below.
59
Figure 6.15: Representation of results for 300 m basin length (design discharge)
Since two different quantities of material was considered for study out of which two quantities
of 240 lit of sediments were taken into consideration whose overall observed settling efficiency
came out to be 82.5 % and 84.2% and overall modified efficiency came out to be 83.5% and
84.7% respectively. For the sediment quantity of 300 lit the overall observed settling efficiency
and overall modified efficiency came out to be 83% and 83.8 %.
6.2.2.2 Experimental Run with 10% overload discharge of 0.01191 m3/s
Experimental runs were carried out using 10% overload discharge as mentioned earlier to meet
the excessive peak load. So, to analyse the effect of 10% overload discharge three experimental
runs were performed considering discharge as 0.01191 m3/s. The average overall settling
efficiency was observed to be 82.33% and results for the study are represented in Table 6.16.
Table 6.16: Results for 10% overload discharge of 0.01191 m3/s (300 m)
(Ƞ)
(%)
300 42 243 81
300 39 246 82
300 36 252 84
60
Since while performing experimental run fine particles / wash load were not been able to trap
in collection tanks so adjustments of lost materials were made based on the sediment quantities
that are trapped in respective tanks. After performing adjustments, the average overall modified
settling efficiency of basin came out to be 83.27%. Table 6.17 refers to the adjustment made in
the collected material.
Table 6.17: Results for 10% overload discharge of 0.01191 m3/s after adjustment (300
m)
injected settling
(liters) efficiency
(liters)
HRT SFT (Ƞ)
(%)
300 54 246 82
300 51 249 83
300 45.6 254.4 84.8

Average overall modified settling efficiency = 83.27 %
Results obtained from the Table 6.16 and Table 6.17 have been compared in Fig 6.16 as
shown below.
Figure 6.16: Representation of results for 300 m basin length (10% Overload discharge)
61
From Fig 6.16 it can be observed that for the sediment quantity of 300 lit the average overall
settling efficiency considering design discharge for three sets of experiments came out to be
81%, 82% and 84% respectively and that for modified settling efficiency came out to be 82%,
83% and 84.8% respectively.
6.2.2.3 Comparison of results for design discharge and 10% overload

discharge
Since the length of basin has been reduced by 50 m so it becomes crucial to check the
performance of desilting basin so as to ensure that its settling efficiency is affected within
permissible range. The results of design discharge and 10% overload discharge are compared
for the observed values and modified values of efficiency. Fig 6.17 shows the comparison made
between observed values of settling efficiency.
Figure 6.17: Representation of observed values of settling efficiency for 300 m basin
length
From Fig 6.17 it can be observed that the difference in average results for the 10% overload
discharge (82.33%) and the average results for the design discharge (83.22%) is 0.87 %. Since
the results are within permissible limits so desilting basin satisfise its purpose.
Also, the comparison has been made for the modified values which are represented in Fig 6.18
as shown below.
62
Figure 6.18: Representation of modified values of settling efficiency for 300 m basin
length
From Fig 6.18 it can be concluded that the difference in modified average results for the 10%
overload discharge (83.27%) and the average results for the design discharge (83.99%) is
0.72%. Since the results are within permissible limits so desilting basin satisfise its purpose.
6.2.2.4 Analytical Studies for 300 m length desilting basin
Since the suspended sediment data is same for the prototype so particle distribution curve and
for the suspended sediments which are most likely to entered in the water conductance system
is same as Fig 6.6, but there is variation in the estimation for settling efficiency due to change
in length of model which is represented in Table 6.18 as shown below.
Table 6.18: Settling efficiency estimation for sediment data for prototype (300 m)
V=0.282m/s Length = 300 m V0=0.0169 m/s
122
𝑉 𝑉0
W
1 5 0.023 0.00046 0.2 0.027 0.027
2 15 0.032 0.00089 0.386 0.053 0.053
3 25 0.045 0.00175 0.759 0.104 0.104
4 35 0.062 0.00329 1.426 0.195 0.195
5 45 0.090 0.00671 2.907 0.397 0.390
6 55 0.130 0.01288 5.577 0.762 0.710
63
7 65 0.210 0.02612 11.312 1.545 0.980
8 75 0.580 0.06730 29.146 3.982 1
9 85 1.800 0.13188 57.111 7.802 1
10 95 5.300 0.23069 99.897 13.647 1
5.459
From Table 6.18 it can be concluded that the overall settling efficiency of desilting chamber
came out to be 54.59 % and from Table 6.6 for 350 m length basin overall settling efficiency
of basin worked out to be 56.68 % i.e for reduction in 50 m length of basin analytical overall
settling efficiency reduced by 2.09 %.
Where,
0.132𝜔
W = Fall velocity of a particle after taking into effect for reduction due to turbulence [ ]
√𝐷𝑒𝑝𝑡ℎ
𝜔 = Terminal fall velocity of individual particle in still water;

V = Average forward velocity (m/s);
V0 = Required fall velocity (m/s);
𝜔 = Fall velocity (m/s) given by Ruby’s equation:
[1636(𝜌𝑠 −𝜌)𝑑3 +9𝜇2 ]0.5 −3𝜇
𝜔=
500𝑑
where,
𝜌𝑠 , 𝜌= density of sediment and water resp. in kg/m3 =2650, 1000 kg/m3 resp.
𝜇= dynamic viscosity (Ns/m2) = 1.002×10-3 Ns/m2
d= particle diameter to be excluded (m)
Based on the results as observed in Table 6.18 for 0.2 mm particle size removal efficiency is
observed out to be 97% for prototype.
For the model studies from the same batch of sediments three samples were collected as that
for the experimental study of 350 m length desilting basin and were analysed by preparing
particle size distribution curves for three different samples as shown in Fig 6.7, 6.8 and 6.9 in
section 6.2.1.4. Due to change in length of basin (300 m) removal ratio changes for the three
samples which is shown in Table 6.19, 6.20 and 6.21 for design discharge and Table 6.22, 6.23
and 6.24 for 10 % overload discharge.
64
V=0.0514 m/s Length = 10 m V0=0.0031 m/s
122
𝑉 𝑉0
W
1 5 0.045 0.00029 0.694 0.095 0.10
2 15 0.150 0.0031 7.380 1.008 0.86
3 25 0.170 0.00392 9.301 1.271 0.94
4 35 0.190 0.00479 11.358 1.552 0.98
5 45 0.2 0.00524 12.427 1.698 0.99
6 55 0.230 0.00664 15.749 2.151 1
7 65 0.250 0.00760 18.022 2.462 1
8 75 0.270 0.00856 20.314 2.775 1
9 85 0.290 0.00953 22.603 3.088 1
10 95 0.350 0.01236 29.320 4.006 1
8.865
V=0.0514 m/s Length = 10 m V0=0.0031 m/s
122
𝑉 𝑉0
W
1 5 0.035 0.00018 0.420 0.057 0.06
2 15 0.170 0.00392 9.301 1.271 0.94
3 25 0.180 0.00435 10.314 1.409 0.96
4 35 0.190 0.00479 11.358 1.552 0.98
5 45 0.205 0.00547 12.970 1.772 0.99
6 55 0.220 0.00617 14.626 1.998 1
7 65 0.240 0.00712 16.882 2.306 1
8 75 0.270 0.00856 20.314 2.775 1
9 85 0.290 0.00953 22.603 3.088 1
65
10 95 0.350 0.01236 29.320 4.006 1
8.93
V=0.0514 m/s Length = 10 m V0=0.0031 m/s
122
𝑉 𝑉0
W
1 5 0.030 0.00013 0.309 0.042 0.04
2 15 0.160 0.00351 8.321 1.137 0.91
3 25 0.170 0.00392 9.301 1.271 0.94
4 35 0.185 0.00457 10.833 1.480 0.97
5 45 0.200 0.00524 12.427 1.698 0.99
6 55 0.220 0.00617 14.626 1.998 1
7 65 0.240 0.00712 16.882 2.306 1
8 75 0.270 0.00856 20.314 2.775 1
9 85 0.300 0.01001 23.742 3.243 1
10 95 0.350 0.01236 29.320 4.006 1
8.852
V=0.057 m/s Length = 10 m V0=0.0034 m/s
122
𝑉 𝑉0
W
1 5 0.045 0.00029 0.631 0.086 0.09
2 15 0.150 0.0031 6.709 0.917 0.82
3 25 0.170 0.00392 8.455 1.155 0.92
4 35 0.190 0.00479 10.325 1.411 0.97
66
5 45 0.2 0.00524 11.297 1.543 0.98
6 55 0.230 0.00664 14.317 1.956 1
7 65 0.250 0.00760 16.384 2.238 1
8 75 0.270 0.00856 18.467 2.523 1
9 85 0.290 0.00953 20.548 2.807 1
10 95 0.350 0.01236 26.655 3.641 1
8.77
V=0.057 m/s Length = 10 m V0=0.0034 m/s
Sr. No % Finer Particle Fall 122
𝑊 𝑊 Removal
𝑉
Dia (mm) Velocity(m/s) 𝑉0 Ratio
W
1 5 0.035 0.00018 0.379 0.052 0.05
2 15 0.170 0.00392 8.393 1.153 0.91
3 25 0.180 0.00435 9.308 1.279 0.95
4 35 0.190 0.00479 10.249 1.408 0.97
5 45 0.205 0.00547 11.704 1.608 0.99
6 55 0.220 0.00617 13.198 1.814 1
7 65 0.240 0.00712 15.234 2.093 1
8 75 0.270 0.00856 18.331 2.519 1
9 85 0.290 0.00953 20.397 2.803 1
10 95 0.350 0.01236 26.458 3.636 1
8.872
67
V=0.057 m/s Length = 10 m V0=0.0034 m/s
Sr. No % Finer Particle Fall 122
𝑊 𝑊 Removal
𝑉
Dia (mm) Velocity(m/s) 𝑉0 Ratio
W
1 5 0.030 0.00013 0.279 0.038 0.04
2 15 0.160 0.00351 7.509 1.032 0.87
3 25 0.170 0.00392 8.393 1.153 0.91
4 35 0.185 0.00457 9.775 1.343 0.96
5 45 0.200 0.00524 11.214 1.541 0.98
6 55 0.220 0.00617 13.198 1.814 1
7 65 0.240 0.00712 15.234 2.093 1
8 75 0.270 0.00856 18.331 2.519 1
9 85 0.300 0.01001 21.424 2.944 1
10 95 0.350 0.01236 26.458 3.636 1
8.76
The average overall settling efficiency for design discharge with physical modelling came out
to be 83.99 % and that from analytical studies from Table 6.19, 6.20 and 6.21 was observed to
be 88.81 % i.e difference in efficiency came out to be 4.82%. While average overall settling
efficiency for 10% overload discharge with physical modelling came out to be 83.27 % and
that from analytical studies from Table 6.22, 6.23 and 6.24 was observed to be 88.02 % i.e
difference in efficiency came out to be 4.75%
Based on the results obtained from Table 6.19, 6.20, 6.21, 6.22, 6.23 and 6.24 the curve for the
settling efficiency vs particle diameter is prepared for model and its equivalent size in prototype
for design discharge as well as for 10 % overload discharge which is represented in Fig 6.19
and 6.20 respectively.
68
SETTLING EFFICIENCY CURVE (DESIGN DISCHARGE)
100
90
80
70
Proto Curve
60
50
Model Curve
40
30
20
10
0
0.00 0.05 0.10 0.15 0.20 0.25
PARTICLE DIA (mm)
prototype for 300 m length basin for design discharge
SETTLING EFFICIENCY CURVE (10% OVERLOAD)

100
90
80
70 Model Curve Proto Curve

60
50
40
30
20
10
0
0.00 0.05 0.10 0.15 0.20 0.25
PARTICLE DIA (mm)
prototype for 300 m length basin for 10% overload discharge
From the curve of prototype as shown in Fig 6.19 and 6.20 it can be concluded that the
analytical settling efficiency of desilting chamber for design discharge and 10 % overload
discharge would be 96 % and 95.5 % respectively. If the difference of 4.82 % and 4.75 % is
considered between analytical and experimental values for design discharge and 10 % overload
discharge then the settling efficiency for particle size of 0.2mm obtained is 91.18 % for design
discharge and 90.75 % for 10 % overload discharge. Therefore, so desilting chamber of size
300 m (L)× 13 m (W) × 18 m (H) is adequate for 90 % settlement of sediment size 0.2 mm and
69
above which is also proven by performing numerical simulation on prototype mentioned in
section 6.3.
6.3 Results for Numerical Simulations
Numerical simulations have been carried out on 3D model of prototype using Flow 3D software
whose details related to construction and preprocessing conditions have been discussed in
chapter 5 for the basin length of 350 m and 300 m. Results of settling efficiency obtained from
the simulations are discussed further.
6.3.1 Results for the 350 m length of basin

The simulation has been carried out in two different parts i.e from 0 to 850 sec and from 850
to 10000 sec. As in CFD software like Flow 3D, solver tries to converge the pressure as per the
given initial conditions as mentioned in section 5.4.1, due to which the simulation has to be
performed in two parts. In the first part pressure gets converge and uniform pressure is
maintained throughout the run as shown in Fig 6.21.
Figure 6.21: Pressure profile inside desilting chamber for 350 m length
Along with the pressure conversion, velocity profile of fluid inside the desilting chamber is
formed. It was observed that within first 850 sec steady velocity profile was formed throughout
the length of basin which is confirmed by the solver by reflecting the message as ‘steady state
condition has been achieved’. Velocity profiles for different time steps have been shown in Fig
6.22 to 6.25.
70
Figure 6.22: Velocity profile at t=0 sec for 350 m length
71
The next part of simulation is injection of sediments. Once the flow become steady after 850
sec the simulation is restarted from t = 100 sec to t = 10000 sec in order to obtain complete
profile for the settling efficiency of desilting chamber. Before injection of sediments, proper
conditions as well as proper particle source data are to be given as input which is discussed in
section 5.5. For different time steps sediment translation profiles have been obtained as shown
in Fig 6.26 to 6.29.
Figure 6.26: Sediment translation profile at t=1000 sec for 350 m length
72
73
Since baffle walls were introduced at outlet as well as in the bottom of basin to capture the
sediments passing through SFT and HRT due to which results are obtained which are
represented in Table 6.25.
Table 6.25 shows the results obtained for hundred different time steps for the 350 m length
desilting basin as mentioned below.
Table 6.25: Settling efficiency of Desilting chamber of length 350 m at different time
steps
Time Sediment count for particle size 0.2 mm Efficiency of

(x100 for 350 m length desilting basin
sec) Particles leaving Particles leaving (Ƞ%)
SFT HRT
1 0 0 100
2 -1 0 100
3 -332 0 100
4 -600 0 100
5 -715 0 100
6 -829 0 100
7 -1427 0 100
8 -3653 0 100
9 -5847 0 100
10 -7714 0 100
11 -9120 0 100
12 -10045 0 100
13 -11105 0 100
14 -13012 0 100
15 -14346 0 100
16 -14614 0 100
17 -15035 0 100
18 -16941 2 99.99
19 -19478 81 99.59
20 -21130 422 98.04
21 -22300 755 96.73
22 -22959 944 96.05
23 -23657 1114 95.50
24 -23914 1178 95.31
25 -23661 1269 94.91
26 -24417 1325 94.85
27 -24568 1370 94.72
28 -24750 1424 94.56
29 -25001 1448 94.53
30 -24964 1479 94.41
31 -25285 1580 94.12
74
32 -25254 1616 93.99
33 -25094 1658 93.80
34 -25360 1627 93.97
35 -25187 1632 93.91
36 -25318 1699 93.71
37 -25520 1722 93.68
38 -25481 1775 93.49
39 -25437 1799 93.39
40 -25712 1709 93.77
41 -25514 1816 93.36
42 -25561 1751 93.59
43 -26165 1830 93.46
44 -25828 1846 93.33
45 -25588 1940 92.95
46 -25868 1819 93.43
47 -25976 1869 93.29
48 -26127 1841 93.42
49 -25976 1865 93.30
50 -25932 1917 93.12
51 -25821 1844 93.33
52 -25929 1805 93.49
53 -26149 1832 93.45
54 -25869 1855 93.31
55 -26282 1857 93.40
56 -26551 1836 93.53
57 -26430 1882 93.35
58 -26090 1879 93.28
59 -26386 1865 93.40
60 -26195 1948 93.08
61 -26264 1840 93.45
62 -26386 1839 93.48
63 -26316 1906 93.25
64 -26213 1971 93.01
65 -26373 1873 93.37
66 -26586 1915 93.28
67 -26237 1881 93.31
68 -26416 1936 93.17
69 -26643 1876 93.42
70 -26445 1913 93.25
71 -26545 1982 93.05
72 -26430 1800 93.62
73 -26783 1913 93.33
74 -26627 1914 93.29
75 -26694 1897 93.37
76 -26725 2017 92.98
77 -26765 1864 93.49
75
78 -26669 1887 93.39
79 -26775 1997 93.06
80 -26726 1848 93.53
81 -26608 1948 93.18
82 -26750 1961 93.17
83 -26802 1966 93.17
84 -26973 1978 93.17
85 -26505 1971 93.08
86 -26811 1940 93.25
87 -27040 1893 93.46
88 -27026 1967 93.22
89 -26779 1965 93.16
90 -26860 1948 93.24
91 -27015 1902 93.42
92 -27005 1911 93.39
93 -27290 1940 93.36
94 -26643 1876 93.42
95 -26445 1913 93.25
96 -26545 1982 93.05
97 -26430 1800 93.62
98 -26783 1913 93.33
99 -26627 1914 93.29
100 -26694 1897 93.37
From the Table 6.25 it can be concluded that the sediment settling efficiency of 350 m desilting
basin come out to be 93.327 %. The negative values of SFT represents that the flow occurred
in downward direct due to initial condition given in section 5.4.1 before simulation i.e Gravity
and non-inertial frame of reference: X=0, Y=0, Z= -9.81. Since from Fig 6.10 the expected
settling efficiency of prototype for 350 m length basin after considering the difference of 4.62
% is 93.38 %, there is a difference of 0.05 % observed for the result of the numerical simulation
which is considerable. But since the efficiency for the particle size of 0.2 mm is greater than
90 %, therefore the length of basin is needed to be reduced and from the curve as shown in Fig
6.12, length of about 50 m can be curtailed as per analytical studies. Thus, there is need to
perform numerical simulation on 300 m length of desilting basin to check its effectiveness.
6.3.2 Results for the 300 m length of basin
From Fig 6.19 analytical results were plotted and curve was formed from which the settling
efficiency for the prototype came out to be 96 % for 300 m basin length and by applying the
difference of 4.82 %, expected efficiency was 91.18 % for prototype. In order to check the
expected settling efficiency for decreased length of prototype desilting chamber the simulation
76
was performed on 300 m length desilting chamber. The simulation has been carried out in two
different parts i.e from 0 to 850 sec and from 850 to 10000 sec. As in CFD software like Flow
3D, solver tries to converge the pressure as per the given initial conditions as mentioned in
section 5.4.1, due to which the simulation has to be performed in two parts. In the first part
pressure gets converge and uniform pressure is maintained throughout the run as shown in Fig
6.30.
Figure 6.30: Pressure profile inside desilting chamber for 300 m length
With every time step as the flow advances in downstream direction along the length of basin
velocity profiles are created. Normally for it took about 850 sec to achieve the steady velocity
profile across the length of basin. Fig 6.31 to 6.35 shows the velocity profile at different time
steps i.e at 0 sec, 200 sec, 300 sec, 500 sec and 850 sec.
77
78
As discussed earlier in chapter 5, with the attainment of steady velocity profile, we can proceed
for the next part of simulation i.e sediment inclusion. For injection of sediments proper
conditions as well as proper particle source data are to be given as input which is discussed in
section 5.5. Once the flow become steady after 850 sec, constant velocity field is selected in
Fluid flow solver option and the simulation is restarted from t = 100 sec to t = 10000 sec in
order to obtain complete profile for the settling efficiency of desilting chamber. Fig 6.36 to
6.39 shows sediment translation profiles that have been obtained for different time steps.
79
80
Since baffle walls were introduced at outlet as well as in the bottom of basin to capture the
sediments passing through SFT and HRT the obtained results are shown in Table 6.26.
Table 6.26 shows the results obtained for hundred different time steps for the 300 m length
desilting basin as mentioned below.
Table 6.26: Settling efficiency of Desilting chamber of length 300 m at different time
steps
Sediment count for particle size 0.2 Efficiency of

Time mm for 300 m length desilting basin
(x100 sec) Particles leaving Particles leaving (Ƞ%)
SFT HRT
1 0 0 100
2 -1 0 100
3 -412 0 100
4 -632 0 100
5 -1099 0 100
6 -6490 0 100
7 -10041 0 100
8 -13387 0 100
9 -16123 0 100
10 -23816 0 100
11 -29079 0 100
12 -32514 0 100
13 -35714 0 100
14 -38435 0 100
15 -39930 0 100
16 -40609 0 100
17 -42449 0 100
18 -43855 26 99.94
19 -44962 584 98.72
20 -44600 2065 95.57
21 -45286 3539 92.75
22 -45399 3996 91.91
23 -45246 4385 91.16
24 -44843 4518 90.85
25 -45615 4598 90.84
26 -44976 4689 90.56
27 -44922 4586 90.74
28 -45249 4626 90.72
29 -45372 4662 90.68
30 -45171 4567 90.82
31 -45200 4647 90.68
32 -45516 4685 90.67
33 -45512 4634 90.76
81
34 -45472 4714 90.61
35 -45656 4619 90.81
36 -44952 4771 90.40
37 -45524 4656 90.72
38 -45442 4715 90.60
39 -45063 4647 90.65
40 -45698 4523 90.99
41 -45433 4584 90.84
42 -45258 4668 90.65
43 -45156 4632 90.70
44 -45288 4672 90.65
45 -45135 4651 90.66
46 -45245 4583 90.80
47 -44856 4641 90.62
48 -45677 4744 90.59
49 -45376 4714 90.59
50 -45396 4620 90.76
51 -45404 4562 90.87
52 -45368 4764 90.50
53 -45166 4691 90.59
54 -45410 4654 90.70
55 -44982 4619 90.69
56 -45368 4626 90.75
57 -45043 4627 90.68
58 -44878 4654 90.60
59 -45675 4751 90.58
60 -45419 4588 90.83
61 -45590 4677 90.70
62 -45290 4744 90.52
63 -44953 4695 90.54
64 -45446 4611 90.79
65 -45050 4582 90.77
66 -45252 4656 90.67
67 -45623 4496 91.03
68 -45444 4764 90.51
69 -45389 4607 90.79
70 -45398 4671 90.67
71 -45469 4602 90.81
72 -45159 4531 90.88
73 -45124 4667 90.63
74 -45324 4615 90.76
75 -45337 4750 90.52
76 -45097 4526 90.88
77 -45058 4725 90.51
78 -45677 4522 90.99
79 -45319 4641 90.71
82
80 -45405 4501 90.98
81 -45211 4698 90.59
82 -45377 4602 90.79
83 -45644 4634 90.78
84 -45204 4616 90.73
85 -44928 4634 90.65
86 -45523 4682 90.67
87 -45545 4684 90.67
88 -45260 4590 90.79
89 -45020 4715 90.52
90 -45225 4623 90.73
91 -44899 4667 90.58
92 -45262 4594 90.79
93 -45668 4722 90.63
94 -45214 4717 90.55
95 -45303 4696 90.61
96 -45437 4585 90.83
97 -45412 4656 90.70
98 -45506 4733 90.58
99 -45172 4592 90.77
100 -22416 2376 90.42
From the Table 6.26 it can be concluded that the sediment settling efficiency of 300 m desilting
basin come out to be 90.69%. The negative values of SFT represents that the flow occurred in
downward direct due to initial condition given in section 5.4.1 before simulation i.e Gravity
and non-inertial frame of reference: X=0, Y=0, Z= -9.81.
From the results obtained from Table 6.25 and Table 6.26 the graph for the settling efficiency
has been plotted for 350 m and 300 m basin length for the analysis as shown in Fig 6.40 below.
Settling Efficiency vs Time 300 m

110
350 m
105
Time steps choosen for computing
100 average efficiency
ƞ%
95
90
85
80
0 10 20 30 40 50 60 70 80 90 100
Time (x 100 sec)
Figure 6.40: Representation of settling efficiency vs time for numerical simulation
83
From Fig 6.40 it can be observed that since the efficiency for the particle size of 0.2 mm is
greater than 90 %, therefore the reduced length of 50 m has proven to be effective. Thus, the
new dimensions of desilting basin to be adopted are 300 m (L)× 13 m (W) × 18 m (H) which
is adequate for 90 % settlement of sediment size of 0.2 mm and above and it is proven through
numerical simulation. Therefore, it can be concluded that numerical simulation can be adopted
for the studies of desilting chamber.
****************************************************************
84
CHAPTER 7
CONCLUSIONS
In steep topography of Himalayan region and due to limited availability of space, normally
pressure type desilting basins are provided. Desilting basins are proposed in hydroelectric
power plants situated in Himalayan regions to remove the harmful suspended sediments from
water conductor system. Two desilting chambers of pressure type which were proposed for the
Kholongchhu H.E.P were designed to remove the sediment size of 0.2 mm and greater which
90% efficiency, for which the dimensions of basin were 350 m (L)× 13 m (W)× 18 m (H).
Physical model studies were conducted in CWPRS on the model of 350 m length equivalent to
prototype considering design discharge and 10 % overload discharge. For the model studies
considering design discharge average overall settling efficiency came out to be 85.2 % while
from the analytical studies carried out average overall settling efficiency was observed to be
89.82 % after making adjustments for the lost material, thus there was a difference of 4.62%
between these two efficiencies. Considering 10 % overload discharge, from model and
analytical studies average overall settling efficiency came out to be 84.55 % and 89.29 %
respectively with difference in efficiency as 4.74 % between them. From the curve plotted in
Fig 6.10 and 6.11 for design discharge and 10 % overload discharge for prototype, the settling
efficiency for 0.2 mm particle came out to be 98 % and by applying the difference of 4.62 %
efficiency obtained is 93.38% which is greater than 90 % for which prototype is designed.
Thus, there is need to reduce the length of basin and from Fig 6.12 and 6.13 it was observed
that 300 m length might serve the purpose.
Therefore, model for 300 m equivalent to prototype was fabricated and total six experiments
were performed on it considering each three runs for design discharge and 10 % overload
discharge. From the physical model studies and analytical studies considering design
discharge, average overall settling efficiency came out to be 83.99 % and 88.81 % respectively,
thus there was a difference of 4.82% observed between these two efficiencies. Considering 10
% overload discharge, from model and analytical studies average overall settling efficiency
came out to be 83.27 % and 88.02 % respectively with difference in efficiency as 4.75 %
between them. From the curve plotted in Fig 6.19 and 6.20 for design discharge and 10 %
overload discharge for prototype, the settling efficiency for 0.2 mm particle came out to be 96
85
% and 95.5 % respectively and by applying the difference of 4.82 % and 4.75 % efficiency
obtained is 91.18% and 90.75 % respectively.
Before opting for the model studies for model of 300 m length equivalent to prototype,
numerical simulation is performed on the 3D geometry of prototype considering two different
length i.e 350 m and 300 m. From the Table 6.25 for 350 m length desilting basin overall
efficiency for design discharge was observed to be 93.327 % which is in close range of
expected efficiency for prototype as observed from the analytical studies. From Table 6.26 for
300 m length desilting basin overall efficiency for design discharge was observed to be 90.69
% which is observed to be in range of expected settling efficiency from analytical studies.
Hence it can be concluded that new dimensions of desilting basin for Kholongchhu H.E.P to
be adopted is 300 m (L)× 13 m (W)× 18 m (H) which is proven experimentally as well as
through numerical simulation.
Hydraulic model studies are essential to finalize the dimension of desilting basin in absence of
any definite design criteria and to avoid any costlier mistake for faulty design. Finally, as the
cost of desilting basin is very high (sometimes even higher than the cost of main dam) so
comparison should be done on basis of cost of desilting basin and life of turbine before taking
decision to provide desilting basin. Every study is site specific i.e discharge, topography,
available head, etc. are different according to site so some data base can be prepared so that it
proves to be helpful for further studies.
****************************************************************
86
REFERENCES
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extractor. ISH Journal of Hydraulic Engineering, 8(2), 1-16.
Ansari, M. A., & Athar, M. (2013). Artificial neural networks approach for estimation of
sediment removal efficiency of vortex settling basins. ISH Journal of Hydraulic Engineering,
19(1), 38-48
Camp, T. R. (1946). “Sedimentation and the design of settling tanks.” Transactions of the
American Society of Civil Engineers, 111(1), 895-936.
CECEN, K., BAYAZIT, M., & SUMER, M. (1969). “Distribution of suspended matter and
similarity criteria in settling basins.”
CWPRS. (2005). Guidelines for design of desilting basins (Pressure Flow). Central Water &
Power Research Station (CWPRS) (Government of India), Pune.
CWPRS. (2022). Technical report on hydraulic model studies of desilting chamber of

Kholongchhu H.E. Project, Bhutan.
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235–262.
Garde, R. J., Ranga Raju, K. G., & Sujudi, A. W. R. (1990). “Design of settling basins.” Journal
of Hydraulic Research, 28(1), 81-91.
Iyer, J. C., & James, E. J. (2022). “Model studies for the design of inlet transition of settling
basins of hydropower projects in high sediment yield areas: a review.” ISH Journal of
Hydraulic Engineering, 1-9.
Khurana, S., Varun, & Kumar, A. (2014). Effect of silt particles on erosion of Turgo impulse
turbine blades. International Journal of Ambient Energy, 35(3), 155-162.
Mosonyi, E. (1991). “High-head power plants.” Akadémiai Kiadó.
Padhy, M. K., & Saini, R. P. (2009). Effect of size and concentration of silt particles on erosion
of Pelton turbine buckets. energy, 34(10), 1477-1483.
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Qamar, M. Z., Verma, M. K., & Meshram, A. P. (2014). “Importance of desilting basins in run-
of-river hydro projects in Himalayan region.” International Journal of Emerging Technology
and Advanced Engineering, 4(3), 407-412.
Qamar, M. Z., Verma, M. K., & Meshram, A. P. (2018). “Reservoir Silt Management in
Himalayan Region.” International Dam Safety Conference, Thiruvananthapuram.
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Desilting Chamber Using Flow 3D.” In River and Coastal Engineering (pp. 177-186). Springer,
Cham.
Raju, K. R., Kothyari, U. C., Srivastav, S., & Saxena, M. (1999). “Sediment removal efficiency
of settling basins.” Journal of Irrigation and Drainage Engineering, 125(5), 308-314.
Singh, G., & Kumar, A. (2013). “A Review of Desilting Basins Used in Small Hydropower
Plants.” vol, 3, 440-444.
Singh, G., & Kumar, A. (2016). “Performance evaluation of desilting basins of small
hydropower projects.” ISH Journal of Hydraulic Engineering, 22(2), 135-141.
Sinha, S., & Singh, A. P. (2019). “Sediment removal efficiency estimation criteria for modern
day desilting basins.” ISH Journal of Hydraulic Engineering, 25(1), 104-117.
“Standard operating procedure for management of silt in hydroelectric project”, 2018
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Hydraulics Division, 103(11), 1323-1337.
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open channel flow.” ISH Journal of Hydraulic Engineering, 14(1), 117-125.
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projects.” Seminar on sedimentation in reservoir- BIS New Delhi.
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88
Verma M.K., Qamar, M. Z., Isaac, N., & Bhosekar V.V. (2015). “Design optimization of
desilting chamber by model studies.” 20th International Conference on Hydraulics, Water
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89
LIST OF PUBLICATIONS
1. “Review of Sediment Removal Measures for Hydropower Projects” in 27th

International Conference on Hydraulics, Water Resources, Environmental and
Coastal Engineering (HYDRO 2022 INTERNATIONAL) at Punjab Engineering
College Chandigarh, India during December 22 -24, 2022.
90
2. “Performance Assessment of Desilting Chamber using FLOW-3D” in National
Conference on Sustainable Development of Smart Cities Infrastructure (SDSCI –
2023) organized by National Institute of Technology Kurukshetra.
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91

Mitigation of Suspended Sediment From Hydropower Projects On Himalayan Rivers

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Mitigation of Suspended Sediment From Hydropower Projects On Himalayan Rivers

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Mitigation of Suspended Sediment from

Hydropower Projects on Himalayan Rivers

Submitted for the partial fulfilment of

WATER RESOURCES ENGINEERING

Dr. Arun Goel M. K. Verma

DEPARTMENT OF CIVIL ENGINEERING

NATIONAL INSTITUTE OF TECHNOLOGY, KURUKSHETRA

Dr. Arun Goel Shri M. K. Verma

ach Approach conduit cross-sectional area (m2)

A Plan Area (m2)

Ax Cross-sectional area (m2)

B* Non dimensional length

b Width of approach channel (m)

D Depth of desilting basin (m)

D* Non dimensional depth

ℇ Mixing coefficient (m2/s)

g Acceleration due to gravity (m/s2)

h Depth of flow in approach channel (m)

ĸ Von Karman constant

L Desilting basin length (m)

L* Non dimensional length

n Manning’s roughness coefficient

R Hydraulic radius (m)

U Flow through velocity (m/s)

µ* Shear Velocity (m/s)

w settling velocity (m/s)

W Width of basin (m)

we Effective fall velocity (m/s)

Figure 1.1: Damaged turbine due to suspended sediments

1]Planning and design stage:

 Chemical characteristics of water and petrographic analysis should be done while

 Hydro suction system which is sediment removal device should be installed.

 Use of low-level sluice is an effective solution for flushing of sediments.

2]Operational stage consideration:

 Dredging operation should be carried out as and when required.

 Abrasion of turbines and other equipments of power house.

 Additional lateral forces on the dam wall exerted.

W=2.43×10-10 S0.099 C0.93 V3.40 t0.75……...………….……(eq 1.3)

Measure Against Reservoir Sedimentation

In Catchment In Reservoir On Dam

1]Soil Conservation 1]Dredging 1]Dredging

Figure 1.2: Methods to aggravating effect of sedimentation

1.2 Desilting Basins

Table 1.1: Classification of settling basins as per CWPRS (1992)

Figure 1.3: Model of Pressure Type Desilting basin (CWPRS)

Figure 1.5: Vortex Type Desilting basin

1.5 Layout of Dissertation Work

This thesis is organized into the following chapters:

CHAPTER II: LITERATURE REVIEW

CHAPTER III: METHODOLOGY

CHAPTER IV: EXPERIMENTAL WORK ON PHYSICAL MODEL

CHAPTER V: NUMERICAL MODELLING

CHAPTER VI: RESULTS & DISCUSSION

CHAPTER VII: CONCLUSION

2.2 Open channel Type Desilting Basin

Garde et al. (1990)

Ranga Raju et al. (1999)

Mazumder & Kumar (2001)

Yeb and Lin (2002)