This slideshare is brief introduction about the sedimentation encountered in storage projects (reservoir) with the mitigation measures that could be applied.

1. A
Paper On
Sedimentation Challenges in
Reservoir
And it’s Mitigative Measures
Er. Tejaswi Sharma

2. Outlines of the Presentation
 Introduction
 Necessity of Storage Project
 Effects of Sedimentation in storage project
 Techniques of estimation of sediment
 Mitigation methods for storage preservation
 Conclusion

3. Introduction
 When a dam is built across a stream, the flow cross
section progressively increases and the flow velocity
decreases toward the dam
 This leads to a decrease in sediment transport
capacity, causing deposition of sediments, first in the
backwaters created by the reservoir and then in the
reservoir
 Sediment accumulation in in-channel reservoirs
reduces their storage capacity and yield and limits
their useful life if it is not controlled in some manner

4. Dams built across the world
for different purposes
 provide a supply of water for towns, cities and mining
sites; eg Warragamba Dam in Australia
 contain and store waste (tailings) from mines; eg Omai
Tailings Dam, Guyana, South America
 provide a supply of water for the irrigation of crops; eg
Burrinjuck Dam, Australia
 generate electricity ; eg Itaipu Dam, Brazil is the largest
hydro-electric power station in the world.
 To help control or mitigate floods; eg the Tennessee
Valley Authority dams in the U.S.A

5. Challenges of Sedimentation
 Depletion of reservoir capacity
 Aggradation in the backwater area
 Degradation in the downstream reach
 Abrasion of structural components of
dams,metallic part(gates,valves,liners etc)
 Clogging of narrow water passages like
tunnels,conduits and small openings
 Environmental,technical and economic
devastating effect

6. Case study
Graph: Storage of sediment in Kulekheani Dam (CIDA, WECS)
1993 storm
indicate that
about 4.8
million m3 of
sediment was
deposited
below the High
Water level
(Galay, 1995)
Meant that the project life of power production would be
18 years instead of 100 years
0
DAM
LWL: EL.1476
Dead storage
1000 2000 3000 4000 5000 6000 7000
1971
Sep 1994
Live storage
1425
HFL : EL.1530
1450
1475
1500
1525

7. Techniques of estimation of sediment
 Stream sediment load, S, generally increases as a power
function of stream flow or discharge Q:
S = a*Q b ……………(1)
where a is a constant and b is an exponent. The
parameters a and b are evaluated by statistical regression
of the observed S and Q values.
log S = log a + b log Q …………(2)
 S is determined from the fitted equation 1 or 2

8. A holistic management framework for
evaluating the effects of sediment
accumulation in rivers (from Wood & Armitage,
1997)

9. Sediment Mgmt approach
Types of job Location Details of sediment control measures
Sediment
Management
Reducing sediment
inflow
Sediment routing
Removal of
deposited sediment
Watershed area of the
reservoir
Lowest part of the
reservoir
(End of reservoir)
Lowest part of the
reservoir
(End of reservoir)
Within the reservoir
Partially or fully drawdown flushing
Dregging (mechanical or hydraulical)
Dry excavation (mechanical removal), disposal sites are
necessary.
Within the reservoir
Reduce erosion by watershed management by land treatment
measures (vegetative cover, critical area planting, contour
farming), stream channel improvement and stabilization, debris
and sabo dams to temporarily store sediments.
Stop sediment entering to the reservoir by constructing check
dam
(evacuation or recycling is needed of upstream of check dam)
Sediment bypass, off-channel storage
Sediment sluicing (seasonal drawdown, flood drawdown),
bottom outlets are necessary.
Density current venting (release turbid underflow from low level
outlets)
Types of job Location Details of sediment control
measures
Sediment
Management
Reducing
sediment
inflow
Sediment
routing
Removal of
deposited
sediment
Watershed area
of the reservoir
Lowest part of the
reservoir
(End of reservoir)
Lowest part of the
reservoir
(End of reservoir)
Within the
reservoir
Partially or fully drawdown flushing
Dregging (mechanical or hydraulical)
Dry excavation (mechanical removal), disposal
sites are necessary.
Within the
reservoir
Reduce erosion by watershed management by
land treatment measures (vegetative cover,
critical area planting, contour farming), stream
channel improvement and stabilization, debris
and sabo dams to temporarily store sediments.
Stop sediment entering to the reservoir by
constructing check dam (evacuation or
recycling is needed of upstream of check dam)
Sediment bypass, off-channel storage
Sediment sluicing (seasonal drawdown, flood
drawdown), bottom outlets are necessary.
Density current venting (release turbid
underflow from low level outlets)

10. Watershed Rehabilitation(Structural
and non-structural measures)
 reducing the soil erosion by practising contour farming
and terracing;strip cropping;crop rotation;gully erosion
control;stabilization of critical areas by their return to
grasslands or forests etc
 reducing the debris flow by the construction of debris
basin(debris dam),which are low dams built across the
sediment contributing tributaries of reservoir in order to
trap the sediment
 construction of check dams,which are effective for
reservoirs where bed load or relatively coarse grain size
accounts for larger sediment inflow

11. Sediment Routing
 it is the technique to route the sediment
inflow not allowing them to deposit within
the reservoir.The techniques used are-
 Sediment Bypass,to divert the sediment
laden flows around a reservoir
 off channel storage,built adjacent to main
river channel in which water is diverted
from main river into this during low
sediment concentration time

12. cont
 Sediment sluicing,in
which sediment laden
inflows are released
through a reservoir
before the particles can
settle thereby reducing
the trap efficiency
 Density current
venting,by passing the
density current through
low level gates sediment
that would have
deposited downstream
thus reducuing chance
of storage loss Fig:Sediment management
strategy
(Sumi,2011)

13. Sediment Flushing
 uses drawdown or
emptying of reservoir
during flood to scour
 flow velocities in a
reservoir are increased
such that sediments are
re-mobilized and
transported through low
level outlets
 restores tractive force
beyond it’s critical force
by drawdown and flushes
the deposits
Reservoir level is drawdown
for flushing
Maximum water level
Fig:Welbedacht dam during
flushing

14. Dredging Removal System
 it is a method of
sediment removal
below the underlying
water
 Mechanical dredge,lifts
sediments mechanically
to surface via barge
 Hydraulic dredge,
pumps sediments from
the bottom of river in
enclosed pipelines to
on-shore location
 hybrid dredge
Fig: Dredge euipments

15. Sustainability of Reservoirs
 RESCON(Reservoir Conservation)and
Computer Program(Palmieri et.al,2003) for
existing dams and deteriorating reservoirs
 Life-cycle management approach
 Technical feasibility,according to location i.e.u/s
and d/s of the reservoir
 Economic feasibility,based on B/C ratio using
NPV over “design life” of project
 Environmental and social safeguards,a rating
system to estimate EIA and relative social
impacts

16. CONCLUSION
 Reservoirs are a vital source of water supply, provide
recreational opportunities, support diverse aquatic
habitat, and provide flood protection
 Reservoir sediment studies are important because of
the effect that sediment accumulation has on the
quality of water and useful life of the reservoir
 The increase in sediment deposit reduces the life of
the project, eventually the project may not exist
 Proper management of sediment control and its
mitigative measures must be strictly applied

17. Thank You