Reservoir Planning

RESERVOIRS
(WATER RESOURCES ENGINEERING – II)
UNIT – I
Rambabu Palaka, Assistant ProfessorBVRIT

Learning Objectives
1. Reservoir Classification
2. Investigations
3. Selection of Site for Reservoir
4. Zones of Storage
5. Storage Capacity and Yield

Learning Objectives
6. Mass Inflow Curve & Demand Curve
7. Calculation of Reservoir Capacity
8. Reservoir Sedimentations
9. Life of Reservoir
10. Selection of Dam

3. Distribution Reservoir
4. Multi-purpose Reservoir
Reservoir Classification
1. Storage Reservoir
2. Flood Protection Reservoir
Flood Reservoirs are those which store
water during flood and release it
gradually at a safe rate when flood
reduces
Distributed Reservoir is a small storage
reservoir used for water supply in a city
A multi-purpose reservoir is that which is
designed for
Flood Protection,
Irrigation,
Hydroelectric development,
domestic and industrial supplies
Storage Reservoirs are primarily used for
water supplies for
Irrigation,
Hydroelectric development,
domestic and industrial supplies

Investigations
1. Engineering Surveys
2. Geological Investigations
3. Hydrological Investigations
1. Survey to prepare contour map
2. Prepare physical characteristics
Area-Elevation curve
Storage-Elevation curve
Map showing land property
Suitable site selection
From the contour map
1. Calculate Area using Planimeter
2. Calculate Volume using Prismoidal
Formula
1. Water tightness of reservoir
2. Suitability of foundation for dam
3. Geological structural features such as folds,
faults, fissures etc.
4. Location of permeable and soluble rocks
5. Groundwater conditions
6. Location of quarry for materials required for
dam construction
1. Study of runoff pattern at the
proposed dam to determine the
storage capacity to a given demand.
2. Determine hydrograph for the worst
flood to determine the spillway
capacity and design

Selection of site for Reservoir
1. Low percolation losses in catchment area
2. Quantity of Leakage should be minimum
3. Percolation below dam should be minimum
4. Reservoir basin should have narrow opening
5. Cost of Real Estate should be less

Selection of site for Reservoir
6. Less submerging of land & other properties
7. Deep reservoirs are preferred to avoid
evaporation loss and weed growth
8. Silt from tributaries should be minimum
9. Rocks and soils at reservoir must not contain
any objectionable minerals and salts

Zones of Storage
1. Useful Storage
2. Surcharge Storage
3. Dead Storage
4. Bank Storage
5. Valley Storage

Storage Capacity and Yield
Yield is the amount of water that can be
supplied from reservoir in a specified interval of
time.
1. Safe Yield
2. Secondary Yield
3. Average Yield
Max. quantity of water that can be
guaranteed during a critical dry period
Quantity of water available in excess of
safe yield during periods of high flood
Arithmetic average of safe yield and
secondary yield over a long period of
time

Mass Inflow Curve
Mass Inflow Curve is a plot between the
cumulative inflow in the reservoir with time.
Used to find reservoir capacity corresponding
to a specific yield with the help of Demand
Curve

Demand Curve
Demand Curve is a plot between the
cumulative demand with time.
 Uniform rate of demand curve
 Variable rate of demand curve

Calculation of Reservoir Capacity
Procedure:
1. Prepare Mass Inflow Curve & Demand Curve
2. Draw tangents parallel to demand curve
3. Measure the max. vertical intercepts
4. Biggest the vertical intercept represents the
required storage capacity

Calculation of Safe Yield from Reservoir
Procedure:
1. Prepare the Mass Inflow Curve
2. Draw diagram for various demand rates
3. Draw tangents to specified reservoir capacity
4. Measure the slopes of each tangent. The
Slope of flattest demand line is safe yield

Reservoir Sedimentation
River catchments carry silt during heavy rains.
Factors affecting erosion and silt load are
 Nature of soil of the catchment area
 Topography of the catchment area
 Vegetation cover
 Intensity of rainfall
 Soft soil leads to Sheet Erosion
 Hard soils carries less silt Steep slopes give rise to high velocity
 High velocity erodes surfaces easily
Higher intensity of rainfall causes greater
runoff and more erosion
If a catchment area has sufficient
vegetation cover the higher velocities are
checked and erosion is very much
reduced.

Reservoir Sedimentation
Sediment transported by the river can be
divided into two heads:
1. Bed Load
2. Suspended Load
The bed load is dragged along the bed of
the stream and is about 10 to 15% of
suspended load
The suspended load is kept in
suspension because of the vertical
component of the eddies formed due
friction of flowing water against bed
Density Current is gravity flow of a fluid under another fluid.
 Clear water is in upper layer
 Muddy or turbid water flows along the channel bottom
towards dam.

Life of Reservoir
The useful life of reservoir is terminated when
its capacity is reduced to 20% of the designed
capacity.
This will occur over a time period when the
dead storage is reduced by siltation.
Probable rate of siltation should be considered
in reservoir planning

Life of Reservoir
The reservoir sedimentation is measured in
terms of its Trap Efficiency (η)
It is a percent of inflowing sediment which is
retained in reservoir
Measurement of Sediment:
 Collect sample from various depths
 Filter sample and measure dried silt in
the units of ‘ppm’

Example
No. of Years = Reduced Capacity / Annual Sediment Trapped
*Trap Efficiency decreases
with decrease in Capacity-Inflow Ratio

 If the dam is constructed lower in first
instance and is being raised in stages
increases Life of Reservoir
 Sufficient outlets should be provided in
different elevations to discharge flood
water

Dams
Dam is a hydraulic structure constructed across
river to store water on its upper stream.
Most common types of Storage Dams:
Rigid Dam: Masonry or Concrete Gravity Dam
Non-Rigid Dam: Earth, Rockfill, Combined
Earth and Rockfill Dam

Selection of site for Dam
1. Topography
2. Geology and foundation conditions
3. Availability of construction materials
4. Spillway size and location
5. Reservoir and Catchment Area

Selection of Dam
The selection of type of dam is depends on
1. Topography
2. Geology and Foundation Conditions
3. Materials of construction
4. Spillway size and location
 Earth dam with a separate spillways
are suitable for low rolling plains.
 An Arch dam is suitable for a low
narrow V-shaped valley
 If foundation consists of sound rock with no
fault and fissures, any type of can be
constructed
 Rocks like Granite, Gneiss and Schist are
suitable for Gravity Dams
 Poor rock or gravel foundations are suitable
for Earth dams and Rock fill dams
 If sand, gravel and crushed stone is
available, a concrete gravity dam is preferred
 If coarse and fine grained soils are
available, an earth dam is suitable
 If large spillway capacity is required, overflow
concrete gravity dam is preferable
 If small spillway capacity is required, earth dams
are preferable
 If large discharge is required, concrete gravity dam
is preferable
If no other site is available for spillway, earth dam
with central overflow section of concrete

Selection of Dam
5. Roadway
6. Length and Height of Dam
7. Life of Dam
 If a roadway is to be passed over top
of the dam, an earth or gravity dam is
preferred
 If length of a dam is very long and its
height is low, an earth dam is preferred
 If the length of dam is small but height
is more, gravity dam is preferred
 Concrete or masonry gravity dams
have very long life
 Earth and Rockfill dams have
intermediate life

Previous Questions
1. Explain the types of reservoirs? What do you understand by a multipurpose
reservoir?
2. Describe in brief, various investigations for reservoir planning?
3. Define the following:
a) surcharge storage b) valley storage c) safe yield d) secondary yield
4. What do you understand by a demand curve? Explain the method of calculating
reservoir capacity for a specified yield, from the mass inflow curve.
5. What do you mean by mass-inflow curve and how it is prepared?
6. Discuss various methods of reservoir sedimentation control?
7. What are the various factors on which, the selection of a site of a reservoir
depend upon?

Reference
Chapter 6 & 7
Irrigation and Water Power Engineering
By Dr. B. C. Punmia,
Dr. Pande Brij Basi Lal, Ashok Kr. Jain,
Arun Kr. Jain, Punmia
Laxmi Publications

Reservoir Planning

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