Reservoir Planning

15CE5164
Advanced Water Resources Engineering
© 2016 KL University – The contents of this presentation are an intellectual and copyrighted property of KL University. ALL RIGHTS RESERVED 1

Reservoir Planning
•Water storage reservoirs may be created by
constructing a dam across a river, along with
suitable appurtenant structures.
•Reservoirs are also meant to absorb a part
of flood water and the excess is discharged
through a spillway.

Basic Reservoir Types:
• Impounding reservoirs, into which a river flows
naturally. Impounding or storage reservoirs are
intended to accumulate a part of the flood flow of the
river for use during the non-flood months.
• Service or balancing reservoirs, which receiving
supplies that are pumped or channeled into them
artificially. These are relatively small in volume
because the storage required by them is to balance
flows for a few hours or a few days at the most.

Investigations:
Engineering surveys
Geological investigations
Hydrological investigations
© 2016 KL University – The contents of this presentation are an intellectual and copyrighted property of KL University. ALL RIGHTS RESERVED. The
materials used in the ppt has been compiled from different sources
4

Engineering surveys-
Conducted for dam, reservoir and other associated work.
Topographic survey of the area is carried out and the contour
plan is prepared.
The horizontal control is usually provided by triangulation
survey, and the vertical control by precise levelling.
5

Geological investigation-
Geological investigations of the dam and reservoir site are done
for the following purposes.
(i) Suitability of foundation for the dam.
(ii) Watertightness of the reservoir basin
(iii)Location of the quarry sites for the construction materials.
6

Hydrological investigations-
The hydrological investigations are conducted for the
following purposes :
(i) To study the runoff pattern and storage capacity.
(ii) To determine the maximum discharge at the site.
7

Site selection:
Large storage capacity
River valley should be narrow, length of dam to constructed
is less.
Watertightness of reservoir.
Good hydrological conditions
Deep reservoir
8

Small submerged area
Low silt inflow
No objectionable minerals
Low cost of real estate
Site easily accessible
9
Site selection:

Reservoirs levels:
•Full Reservoir Level (FRL): It is the level
corresponding to the storage which includes
both inactive and active storages and also
the flood storage, if provided for.
•In fact, this is the highest reservoir level that
can be maintained without spillway
discharge or without passing water
downstream through sluice ways.

Reservoirs levels:
•Minimum Drawdown Level (MDDL): It is the
level below which the reservoir will not be
drawn down so as to maintain a minimum head
required in power projects.
•Dead Storage Level (DSL): Below the level, there
are no outlets to drain the water in the reservoir
by gravity.

Reservoirs levels:
•Maximum Water Level (MWL): This id the water
level that is ever likely to be attained during the
passage of the design flood.
•It depends upon the specified initial reservoir
level and the spillway gate operation rule.
•This level is also called sometimes as the Highest
Reservoir Level or the Highest Flood Level.

Reservoirs storage Zones:
• Live storage: This is the storage available for the intended
purpose between Full Supply Level and the Invert Level of
the lowest discharge outlet.
• The Full Supply Level is normally that level above which
over spill to waste would take place.
• This may also be termed as the volume of water actually
available at any time between the Dead Storage Level and
the lower of the actual water level and Full Reservoir Level.

• Dead storage: It is the total storage below the invert level
of the lowest discharge outlet from the reservoir.
• It may be available to contain sedimentation, provided the
sediment does not adversely affect the lowest discharge.
• Outlet Surcharge or Flood storage: This is required as a
reserve between Full Reservoir Level and the Maximum
Water level to contain the peaks of floods that might occur
when there is insufficient storage capacity for them below
Full Reservoir Level.

• Buffer Storage: This is the space located just above the
Dead Storage Level up to Minimum Drawdown Level.
• As the name implies, this zone is a buffer between the
active and dead storage zones and releases from this zone
are made in dry situations to cater for essential
requirements only.
• Dead Storage and Buffer Storage together is called
Interactive Storage.

Storage capacity and yield:
Yield – It is the amount of water that can be
supplied from the reservoir in a specified
interval of time which is chosen for the design
varies from a day for small distribution reservoirs
to a year for large conservation reservoirs.
Safe yield – It is the maximum quantity of water
that can be guaranteed during a critical dry
period.

Storage capacity and yield:
Secondary Yield – It is the quantity of
water available in excess of safe yield
during periods of high flood.
Average yield – It is the arithmetic
average of the firm and the secondary
yield over a long period of time.

Mass Inflow Curve - It is a plot of accumulated flow in
a stream against time.

Mass Demand Curve - It is a plot of accumulated
demand with time.

Reservoir capacity:
Depends upon the inflow available and demand
Inflow in the river is always greater than the
demand, there is no storage required
If the inflow in the river is small but the demand
is high, a large reservoir capacity is required

The required capacity for a reservoir
can be determined by the following
methods:
1. Graphical method, using mass curves.
2. Analytical method

Graphical method-
1. Prepare a mass inflow curve from the flow hydrograph
of the site for a number of consecutive years including
the most critical years (or the driest years) when the
discharge is low.
2. Prepare the mass demand curve corresponding to the
given rate of demand.
• If the rate of demand is constant, the mass demand
curve is a straight line.
•The scale of the mass demand curve should be the same
as that of the mass inflow curve.

3. Draw the lines AB, FG, etc. Such that
(I) they are parallel to the mass demand curve, and
(Ii) they are tangential to the crests A, F, etc. Of the mass
curve.
4. Determine the vertical intercepts CD, HJ, etc. between the
tangential lines and the mass inflow curve.
• These intercepts indicate the volumes by which the inflow
volumes fall short of demand.

•Assuming that the reservoir is full at point A, the inflow
volume during the period AE is equal to ordinate DE and
the demand is equal to ordinate CE.
• Thus the storage required is equal to the volume
indicated by the intercept CD.
5. Determine the largest of the vertical intercepts found
in step (4).
• The largest vertical intercept represents the storage
capacity required.

Determination of Safe yield from a reservoir of a
given capacity:
•Prepare the mass inflow curve. Draw st. lines
from a common origin, representing demands at
various rates, say ranging from 0 to 5000 ha-m
per year.
•From the apices A1, A2, A3, etc. of the mass curve
draw tangents in such a way that their maximum
departure from the mass curve does not exceed
the specified reservoir capacity.

Determination of Safe yield from a reservoir of a
given capacity:
•Thus in fig. the ordinates E1 D1, E2 D2, E3 D3 etc.
are all equal to the reservoir capacity (say 1500
ha-m).
•Measure the slopes of each of these tangents.
•The slopes indicate the yield which can be
attained in each year from the reservoir of given
capacity.

Procedure for calculation reservoir life:
•Knowing the inflow rate calculate the (capacity/inflow) ratio
and obtain the trap efficiency from the curve.
•Divide the total capacity into any suitable interval, say 10%.
Assuming the 10% capacity has been reduced due to
sediment deposit, find the trap efficiency for reduced
capacity (i.e. 90% of the original) and the inflow ratio.
•For this interval of 10% capacity, find the average trap
efficiency by taking average if η found in step 2 and 3.

•Determine the sediment inflow rate by taking
water samples and drying the sediment.
•Multiply the total annual sediment transported by
the trap efficiency found in step 3.
•Divide the volume interval by the sediment
deposited to get the number of years to fill this
volume interval of 10% capacity.

•Repeat the procedure for further intervals of the
capacity.
•The total life of the reservoir will be equal to the
total number of years required to fill each of the
volume intervals.

Reservoir Sedimentation:
It is a difficult problem for which an economical
solution has not yet been discovered, except by
providing a “dead storage” to accommodate the
deposits during the life of the dam.
Disintegration, erosion, transportation, and
sedimentation, are the different stages leading to
silting of reservoir.

Causes of sedimentation:
Nature of soil in catchment area
Topography of the catchment area
Cultivation in catchment area
Vegetation cover in catchment area
Intensity of rainfall in catchment area

Sediment Management:
• Maximum efforts should water should be released so that less sediments
should retain in reservoir.
• Following options are:
• Catchment vegetation
• Construction of coffer dams(a watertight enclosure pumped dry to
permit construction work below the waterline)/low height barriers
• Flushing and desilting of sediments
• Low level outlets / sediment sluicing (sliding gate)

Stepped watershed for sediment
control

Flushing of sediments from
reservoir

Mechanical desilting from reservoir

Flow Routing:
• Procedure to determine the
flow hydrograph at a point on
a watershed from a known
hydrograph upstream
• As the hydrograph travels, it
• Attenuates
• Gets delayed
Q
t
Q
t
Q
t
Q
t

Why to route flows?
 Account for changes in flow hydrograph as a flood wave passes
downstream.
 This helps to:
 Calculate storages
 Studying the attenuation of flood peaks
Q
t

Method of flood routing:
• It is the process of calculating water levels in reservoir, the storage
quantities and outflow rates corresponding to a particular inflow
hydrograph at various instants.
• Flood routing is carried out in a reservoir to determine what will be
maximum rise in its water surface and what will be the discharge in
the downstream channel when particular flood passes through it.
Some of the important methods which have a more practical bearing
are:
• Calculus method
• step by step method: (1) Graphical method (2) Trial and error
method

Graphical method / Inflow Storage Discharge method
•The inflow storage discharge method was first
developed by L.G puls of U.S army corps of
engineers. According to this method
•In the above equation all the quantities on left hand
side are known and hence the quantity is
determined.

•From this quantity the value of Q2 can be
found out from the storage-discharge
relationship as explained below.
•From the available storage discharge curve,
the curves of (S-Q/2 ∆T) verses Q and
(S+Q/2 ∆T) verses Q are developed.
•Such curves are called routing curves.

•From the known value of Q1 , the value of (S1-
Q1/2∆T) is read from Q verses (S-Q/2 ∆T) curve.
•This value is added to (I1+I2/2)∆t to give
(S2+Q2/2∆T).
• Entering the graph with this value of (S2+Q2/2 ∆T)
, the value of Q2 is read out from Q verses
(S+Q/2∆t) curve.
• The value of Q2 thus determined becomes Q1 for
the next time interval.

•If the inflow and outflow hydrographs are now plotted on
the same time scale, it will be observed that the peak flow
of outflow hydrograph is less than the peak flow of the
inflow hydrograph.
•Similarly, the time to peak in the outflow hydrograph is
more than the time to peak in the inflow hydrograph.
•These are the effects of reservoir storage on the movement
of flood wave through the reservoir.
•The reduction of peak is known as the attenuation and the
difference in times to peak is known as the reservoir lag.

material used in the ppt has been compiled from different sources.
Spillways: Passages constructed either within a dam or in the
periphery of the reservoir to safely pass the excess of the river during
flood flows are called Spillways
The capacity of a spillway is seen to depend upon the following major factors:
• The inflow flood
• The volume of storage provided by the reservoir
• Crest height of the spillway
• Gated or ungated

Free Overfall Spillway:
• In this type of spillway, the water
freely drops down from the crest,
as for an arch dam.
• It can also be provided for a
decked over flow dam with a
vertical or adverse inclined
downstream face.

Overflow Spillway: The overflow type spillway has a crest shaped in
the form of an ogee or S-shape. The upper curve of the ogee is made to
conform closely to the profile of the lower nappe of a ventilated sheet of
water falling from a sharp crested weir.

Chute Spillway:
It is one whose discharge
is conveyed from the
reservoir to
the downstream river
level through an open
channel, placed either
along a dam abutment or
through a saddle.

Side channel Spillway:
A side channel spillway is one in which the control weir is placed
approximately parallel to the upper portion of the discharge
channel.

Shaft Spillway:
A Shaft Spillway is one where water enters over a horizontally positioned
lip, drops through a vertical or sloping shaft, and then flows to the
downstream river channel through a horizontal or nearly horizontal conduit
or tunnel.

Tunnel Spillway:
• Where a closed channel is used
to convey the discharge around
a dam through the adjoining
hill sides, the spillway is often
called a tunnel or conduit
spillway.
• The closed channel may take
the form of a vertical or
inclined shaft, a horizontal
tunnel through earth or rock,
or a conduit constructed in
open cut and backfilled with
earth materials.© 2016 KL University – The contents of this presentation are an intellectual and copyrighted property of KL University. ALL RIGHTS RESERVED. The

Siphon Spillway:
 A siphon spillway is a closed
conduit system formed in the
shape of an inverted U, positioned
so that the inside of the bend of
the upper passageway is at normal
reservoir storage level.
• Siphon spillways comprise usually
of five components, which include
an inlet, an upper leg, a throat or
control section, a lower leg and an
outlet.

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