sediment management in run-of the-river schemes - Sedinet

SEDIMENT MANAGEMENT IN
RUN-OF THE-RIVER SCHEMES
(Reservoir Flushing & Desilting Basins)
Dr. I. D. Gupta, Director
&
Mrs. V. V. Bhosekar, Joint Director
Central Water and Power Research, Pune-411024

ABOUT CWPRS
Established in 1916 by the then Bombay Presidency,
today a premier research institute under MOWR in
hydraulics and allied areas.
The research activities are mainly based on conducting
physical and mathematical model studies
supplemented with field and laboratory experiments.
CWPRS receive references mainly from Central/ State
Govt. organizations, Municipal Corporations, Ports,
Public/ Private Sector undertakings, etc.
UN Recognized Regional Laboratory for ESCAP since
1971. Also takes problems from neighbouring
countries.

Main Functions
Applied research supported by necessary basic research
leading to safe and cost effective designs of projects
dealing with water resources, river engineering, power
and coastal developments.
Offering advisory services to various ministries and
departments within the sphere of its activities by
participation in Experts’ Committees.
Disseminating research findings amongst hydraulic
research fraternity by way of publications and training
programmes.
Evolving and updating standards (ISO & BIS) and
advising the apex regulatory agencies to ascertain
compliance to required stipulations.

Major Disciplines and Technical Divisions
1. RIVER ENGINEERING
River Hydraulics
Hydraulic Analysis and Prototype Testing
Bridge Engineering
2. RIVER AND RESERVOIR SYSTEMS MODELING
Hydrometeorology
Surface Water Hydraulics
Water Quality Analysis and Modeling
3. RESERVOIR AND APPURTENANT STRUCTURES
Spillways and Energy Dissipators
Control Structures and Water Conductor Systems
Sediment Management
4. COASTAL AND OFFSHORE ENGINEERING
Ports and Harbours
Coastal Hydraulic Structures
Mathematical Modeling for Coastal Engineering
5. FOUNDATION AND STRUCTURES
Geotechnical Engineering
Modeling and Analysis of Structures
Concrete Technology
6. APPLIED EARTH SCIENCES
Engineering Seismology
Vibration Technology
Geophysics
Isotope Hydrology
7. INSTRUMENTATION, CALIBRATION AND
TESTNG SERVICES
Hydraulic Instrumentation
Hydraulic Machinery and Cavitation
Current Meter Calibration

SEDIMENTATION IN HIMALAYAN RESERVOIRS
High erosion rates due to steep
river slopes and fragile geology
Himalayan rivers carry huge
quantity of sediment.
Many reservoirs are losing
capacity at the rate of about 2 %
per annum
Orifice spillways & sediment
exclusion devices

METHODS FOR MITIGATING RESERVOIR SEDIMENTATION
To increase the passing of sediment through reservoir
during high floods with heavy sediment concentrations.
To bypass high flow with heavy sediment concentration
from entering the reservoir.
To flush sediment accumulation through the reservoir.
To remove reservoir sediment by mechanical means
such as dredging and siphoning

SILTATION PROFILE USING MATHEMATICAL MODEL
Rate and pattern of deposition in reservoir can be
assessed using following numerical models:
HEC – RAS
MIKE – 21
CHARIMA
At CWPRS, we use HEC - RAS which takes into
account both suspended sediments and bedload

DATA REQUIRED FOR HEC-RAS
(Topographic Data)
L-section and plan of the river
Cross sections of river
Hydraulic details of bridges and weirs/ barrages
Elevation-area-capacity curve of the reservoir

DATA REQUIRED FOR HEC-RAS
(Hydraulic Data)
Daily water level and discharge data
Records of High Flood Level
Rule curve for operation of reservoir

DATA REQUIRED FOR HEC-RAS
(Sediment Data)
Suspended sediment concentration along with
gradation curve
Bed load data with gradation curve

Elevation.(m)
185
180
175
170
165
160
155
150
MDDL
FRL
SPILLWAY CREST 157 m
1.25 m.cum
SILTATION PROFILE
L I V E S T O R A G E
12.27 m.cum
S I L T A T I O N
145
0 500 1000 1500 2000 2500 3000 3500
Chainage (m)
ORIGINAL BED
0.253 m.cum

PHYSICAL MODEL STUDIES - FLUSHING OF RESERVOIR
Geometrical similar scale model based on Froudian
criteria for simulation
Model scale depends upon
– fetch of reservoir
– bed material/suspended load gradation curves
Entire fetch of reservoir, intake, spillway, river bed with
suitable graded material is modeled
Selection of bed material depends upon
Shield criteria
Rouse criteria
Young’s incipient motion

RESERVOIR FLUSHING MODEL

SILTATION IN MODEL

FLUSHNG OPERATION

RIVER BED AFTER FLUSHING OPERATION (12 Hrs)
-1.0
m
-8.0
m
-3.0
m
-3.0
m

Elevation.(m)
185
180
175
170
165
160
155
150
C.S.180
C.S.30
MDDL
LONGITUDINAL PROFILE AFTER FLUSHING
FOR 12, 24 & 36 Hrs
FRL
C.S.300
C.S.900
C.S.1500
C.S.3000
145
0 500 1000 1500 2000 2500 3000 3500
Chainage (m)

CROSS SECTION AFTER FLUSHING FOR 12, 24 & 36 HRS
Elevation(m)
Elevation(m)
170
168
166
164
162
160
158
156
154
152
150
0 100 200 300 400
180
175
170
165
160
155
150
C.S. 30 m C.S. 180 m
Chainage (m)
0 100 200 300 400
Chainage (m)
Elevation(m)
Elevation(m)
180
175
170
165
160
155
150
0 100 200 300 400
180
175
170
165
160
155
Chainage (m)
C.S. 300 m C.S. 900 m
150
0 50 100 150 200 250
Chainage (m)

2.4
2
1.6
1.2
0.8
0.4
0
DIFFERENT DISCHARGES
Quantity of sediment flushed ( m.cum ) TIME Vs QUANTITY OF SEDIMENT FLUSHED FOR
0 12 24 36
Flushing duration (Hrs)

FLUSHING IN FRONT OF POWER INTAKE

IMPORTANT PROJECTS STUDIED
1. Teesta Low Dam Project,Stage III and IV, West Bengal
2. Kotlibhel IA, IB, II, Uttarakhand
3. Chuzachen H.E. Project, Sikkim
4. Sewa-II H.E. Project, Jammu and Kashmir
5. Parbati-II H.E. Project, Himachal Pradesh
6. Subansiri lower H.E. Project,Assam
7. Baira Siul Project, Himachal Pradesh
8. Teesta H.E. Project, Stage-V, Sikkim
9. Chamera H.E. Project, Stage-II, Himachal Pradesh
10.Tala H.E. Project, Bhutan.

SIGNIFICANCE OF RESERVOIR FLUSHING STUDIES
Volume of sediments flushed can be quantified for
different spillway crest levels with varying discharges
and flushing time
Studies are useful to assess the optimum discharge and
duration for most efficient flushing
Optimizing the crest level of spillway for efficient
flushing
Optimizing the layout of power intake with respect to
spillway to have silt free area in front of the intake

HAZARDS OF SEDIMENTATION TO POWER HOUSE
Damage of the turbine runner blades / buckets
due to abrasion and subsequent cavitation.
Shut down of units for considerable duration.
To induce settlement of suspended sediment by
reducing the velocity / turbulence, desilting basins
are provided

TYPICAL DAMAGE TO TURBINES
Highly Abrasive Silt Causes Erosion/ Cavitation damages
Projects affected: Salal, Baira Siul, Maneri Bhali, Chilla, Nathpa Jakhri, etc.

RESERVOIR
TYPICAL LAYOUT OF POWER INTAKE AND
DESILTING BASINS
INLET TRANSITION
DAM AXIS
RIVER
INTAKE
DESILTING BASIN
FLUSHING TUNNEL
OUTLET TRANSITION
RIVER
HRT

COMPONENTS OF DESILTING BASIN
1. Inlet transition
2. Main basin
3. Flushing Tunnel
4. Outlet transition
5. Air Vent and Outlet Gate

FLOW
MODEL OF DESILTING BASIN
INLET TRANSITION
FIRST OPENING
HOPPER SLOPE
SETTLING TRENCH
MAIN BASIN

SHAPES OF DESILTING CHAMBER

DESIGN PARAMETERS AND CONSIDERATIONS
PARAMETERS
a) Length - depends on the width of desilting basin
b) Width - depends on the rock condition
c) Depth - as per cross sectional area required for
settlement of the desired size of sediment
d) Flare angle - optimized by model studies
CONSIDERATIONS
a) Sufficiently high velocity in the transition to avoid
settlement downstream
b) No separation of flow
c) Uniform velocity distribution
d) Drop in forward velocity

INLET TRANSITION
Due to gradual increase in cross-sectional
area, the velocity of flow entering the
desilting basin is reduced to desired extent.

INLET TRANSITION – DESIGN CONSIDERATIONS
1) Steeper Bed Slope - Return flow
2) Flatter Bed Slope - Deposition
3) Larger Flare Angle - Return flow
CWPRS experience indicates:
• Bed slope - 1(v) : 2.00 to 2.3(h)
• Flare angle - 6º to 9º

MAIN BASIN – DESIGN CONSIDERATIONS
• Fall velocity -
depends on size,
shape, specific
gravity of sediment
and viscosity of
water
• Forward velocity –
depends on shape
and size of the
basin
Fall velocity of quartz spheres

MAIN BASIN – HOPPER BOTTOM OPENINGS
Openings are provided to connect the settling
trench with the flushing tunnel.
First opening is required to be larger to allow
for higher rate of deposition of larger size
particles at the beginning.
The last opening is kept a little larger than the
opening just to its upstream to accommodate
higher rate of deposition of finer particles

FORMATION OF DUNES IN FLUSHING TUNNEL
DUNES
FLUSHING TUNNEL
HOPPER SLOPE
OPENINGS

FLUSHING TUNNEL
Generally rectangular in shape, and size increases
gradually from upstream to downstream.
The minimum velocity of flow in the beginning of
flushing tunnel is 3.0 m/s, which increases to 3.5 to
4.0 m/s at the end of desilting basin.
Flushing discharge is controlled by a gate at the end,
which also isolates the desilting basin.

Efficiency of the
desilting basin also
depends upon proper
arrangement at the
outlet for skimming off
the relatively less
sediment laden top
layers of flow.
Settling efficiency
improves with
provision of outlets
having higher sill level.
OUTLET TRANSITION
OUTLET TRANSITION
END OF THE BASIN
FLUSHING TUNNEL
CONTROL GATE

During initial filling of the basin, the air within the
basin is required to be expelled out.
During dewatering of the basin, the air is required to
be supplied to avoid generation of negative
pressures.
Therefore air vent is provided after the outlet
transition.
AIR VENT AND OUTLET GATE
Outlet gate is provided at the end of outlet transition
of the desilting basin for initial filling and
dewatering, which also isolates the desilting basin.

OPERATION OF MODEL OF DESILTING BASIN

PROTOTYPE DATA COLLECTION
The basin gives the designed settling efficiency only if
the design discharge is drawn and the sediment
concentration is less than the design value.
In case the discharge is less, reduction in forward
velocity will result in more deposition of sediment.
In case the sediment concentration is more, more
sediment will enter into the turbines.
Continuous data collection regarding discharge and
sediment concentration is thus useful to ensure proper
performance of the desilting basin.

IMPORTANT MODEL STUDIES CONDUCTED
Chukha, Tala Project- -Bhutan
Nathpa Jhakri
Dhauli Ganga
Chamera II
Teesta V
Parbati II & III
Sewa
Dul-hasti

OBSERVATIONS FROM MODEL STUDIES
• Validity of Camp’s criteria is borne out
• Turbulence prevails-15% to 20% of length
• Hopper angle - 40º
• Settling trench is useful to enhance performance
• Pressure flow - flushing tunnel
• Flushing discharge - 15% to 20%
• Maximum forward velocity in basin
Particle dia. Φ(mm) Max Forward Velocity (m/s)
0.1 0.15
0.2 0.3
0.3 0.35
• Sizes & spacing of opening are optimized

SIGNIFICANCE OF MODEL STUDIES
These basins are substantially large in size, underground
structure and are cost prohibitive.
Once put into operation, it is very difficult to maintain
and repair the desilting basin
Various theoretical approaches involve assumptions
which leaves the designer in doubt while choosing any
particular method
Hence hydraulic model studies are essential, to optimize
the size and shape for desired settlement of suspended
sediment and smooth functioning under all operating
conditions
Considering experience of CWPRS and non-availability
of standard procedure, two Technical Memoranda have
been published

GOVERNMENT OF INDIA
MINISTRY OF WATER RESOURCES
GUIDELINES
FOR
DESIGN OF DESILTING BASINS
(Pressure Flow)
CENTRAL WATER & POWER RESEARCH STATION
Khadakwasla, Pune-411 024