Reservoir Planning

❑ Definition
❑ Type of reservoirs
❑ Storage zones of a reservoir
❑ Investigations for reservoir site
❑ Mass inflow curve and demand curve
❑ Determination of reservoir capacity
❑ Safe yield
What is a
reservoir?
❑ A water supply scheme drawing
water directly from a river or a
Kanakanala Dam Reservoir on river Kanakanala/
stream may fail to satisfy the
Tungbhadra Raichur, Karnataka
consumers demands during
extremely low flows, while during
high flows it may become difficult to
carry out its operation due to
devastating floods.
❑ When a barrier is constructed across
a river in the form of a dam, water
gets stored on the upstream side of
the barrier, forming a pool of water
generally called a
dam reservoir/ impounding
reservoir/ river reservoir/ storage
reservoir.
❑ The water so stored
reservoir in a artificial
is an given reservoir
lake during rainy season can be easily used almost throughout the
year, tillin
created thea time
riverofvalley
arrival of the next
through damrainy season, to refill the empty reservoir again.
❑ Hence, the most
construction, and critical purpose
the quality of reservoirs is flood risk management. Reservoirs collect water
of water
during
stored istimes of highdifferent
not much rainfall, from
reducing
that flood risk, and then release the water slowly over the following
weeks and months.
of a natural lake.
Types of
reservoirs
❑ Storage/conservation reservoir ❑ Multipurpose reservoir
❑ Flood control/flood mitigation ❑ Distribution reservoir
reservoir

❑ Storage reservoir is also called

conservation reservoir because they are Ladybower Reservoir, in Derbyshire, England
used to conserve water.

❑ When the reservoir remains in water

supply during the rainy season and
releases it during dry period as per
requirement, it is called storage or
❑ A storage orreservoir.
impounded a conservation reservoir can
retain such excess supplies during periods
of peak flows, and can release them
gradually during low flows as and when
the need arises.
❑ Advantages of conserving water
Controlling flood
Reduce flood damage below the
reservoir
Types of
reservoirs
❑ Storage/conservation reservoir ❑ Multipurpose reservoir
❑ Flood control/flood mitigation ❑ Distribution reservoir
reservoir
Forms of Storage Reservoir

❑ Reservoirs formed by a dam across the course of a river, with subsequent inundation of the upstream
land surface are often called impoundments.

❑ Water bodies not constructed within the course of the river and formed by partially or completely
enclosed water-proof banks (and usually filled by diverted river flows or pipes) are often referred to
as off-river, or bonded, reservoirs.

❑ Reservoirs created by dams or weirs serially along a river course form a cascade.
❑ The various purposes for which the storage works
are required are
Irrigation Navigation
Hydro-electric power generation Low water regulation for generation
Domestic/municipal and industrial water Recreation
supply Preservation and breeding of useful aquatic
Control of destructive floods life
Types of
reservoirs
❑ Storage/conservation reservoir ❑ Multipurpose reservoir
❑ Flood control/flood mitigation ❑ Distribution reservoir
reservoir
❑ Flood Mitigation/Control reservoirs collect
water at times of high (flood) flow and then Flood Control Structure on Lake Okeechobee,
release it slowly over the course of the flood Florida
and during the subsequent days or weeks.
❑ The main difference between a flood control
reservoir and a conservation reservoir is the
need for a large sluiceway capacity to permit
rapid drawdown before or after a flood.
❑ Flood control reservoir are further sub
classified as
1) Detention basins/storage reservoirs
A reservoir with uncontrolled and ungated
outlets is known as a retarding basin or
retarding reservoirs.
2) Retarding basins/reservoirs
A reservoir having gates and valves
installation at its spillway and at its sluice
Types of
reservoirs
❑ Storage/conservation reservoir ❑ Multipurpose reservoir
❑ Flood control/flood mitigation ❑ Distribution reservoir
reservoir
❑ It is a reservoir constructed to store the
portion of the flood water in such a way as to
minimize the flood peaks to protect the area
on the downstream side from flood damage.
❑ To accomplish this, the entire flow entering
the reservoir is discharged till the outflow
reaches the safe capacity of the channel
downstream.
❑ The inflow in excess of this rate is stored in
the reservoir, which is then gradually
❑ released,
It may failso
toas to recover
satisfy the storage
the consumers capacity
demands during:
for1)the next flood.
Extremely low flows
❑ The2) flood peaks at the d/s of the reservoir are
High flow
❑ thus
It may reduced
becomeby an amount
difficult AB,
to carry outastheir
shown in
operations due to
Types of
reservoirs
❑ Storage/conservation reservoir ❑ Multipurpose reservoir
❑ Flood control/flood mitigation ❑ Distribution reservoir
reservoir
Retarding basins
❑ Usually provided with an uncontrolled spillway and an uncontrolled orifice type sluiceways.
❑ The automatic regulation of outflow, depending upon the availability of water, takes place from
such a reservoir.
❑ The max. discharging capacity of such a reservoir should be equal to the maximum safe carrying
capacity of the channel d/s.
❑ As floods occur, the reservoir gets filled, and discharges through-sluiceways.
❑ As the reservoir elevation increases, the outflow discharge increases.
❑ The water level goes on rising until the flood has subsided, and the inflow becomes equal to or less
than the outflow.
❑ After this, the water gets automatically withdrawn from the reservoir until the stored water is
completely discharged.
❑ The advantages of a retarding basin over a gate controlled detention basin are:
Types of
reservoirs
❑ Storage/conservation reservoir ❑ Multipurpose reservoir
❑ Flood control/flood mitigation ❑ Distribution reservoir
reservoir
Storage Reservoir
❑ A storage reservoir with gated spillway and gated sluiceways, provides more flexibility of operation,
and thus gives us better control and increased usefulness of the reservoir.
❑ Storage reservoirs are, therefore, preferred on large rivers, which require better control; while
retarding basins are preferred on small rivers.
❑ In storage reservoirs, the flood crest downstream, can be better controlled and regulated properly, so
as not to cause their coincidence.
❑ This is the biggest advantage of such a reservoir and out- weighs its disadvantages of being costly and
involving risk of human error in installation and operation of gates.
Types of
reservoirs
❑ Storage/conservation reservoir ❑ Multipurpose reservoir
❑ Flood control/flood mitigation ❑ Distribution reservoir
reservoir
❑ The term multipurpose reservoir includes all reservoirs actually designed and operated to serve more
than one function.
❑ Reservoir, designed for one purpose, incidentally serving other purposes, shall not be called a
multipurpose reservoir, but will be called so, only if designed to serve those purposes also in addition
to its main purpose.
❑ Hence, a reservoir designed to protect the downstream areas from floods and also to conserve water
for water supply, irrigation, and other purposes shall be called a multipurpose reservoir.
❑ Bhakra dam and Nagarjun Sagar dam are the important multipurpose projects of India.
❑ Multipurpose reservoirs may be managed to balance some or all of the following activities:
► Water supply
► Flood control
► Soil erosion
► Environmental management
► Hydroelectric power generation
Types of
reservoirs
❑ Storage/conservation reservoir ❑ Multipurpose reservoir
❑ Flood control/flood mitigation ❑ Distribution reservoir
reservoir
❑ Distribution reservoirs, also called service reservoirs, are the storage reservoirs, which store the
treated water for supplying water during emergencies (such as during fires, repairs, cleaning etc.)
and also to help in absorbing the hourly fluctuations in the normal water demand by the ultimate
consumers during various parts of the day.
❑ Such a reservoir can be filled by pumping water at a certain rate and can be used to supply water
even at rate higher than inflow rate during period of maximum demands called critical periods of
demand.
❑ Such reservoirs are, therefore, helpful in permitting the pumps or the water treatment plants to work
at a uniform rate, and they store water during the hours of no demand or less demand, and supply
water from their storage during the critical periods of maximum demand.

Functions of Distribution Reservoirs:

❑ To equalize and absorb the variation in hourly demand of water by the consumers to a uniform rate
of supply from the source either by gravity or pumping,
Types of
reservoirs
❑ Storage/conservation reservoir ❑ Multipurpose reservoir
❑ Flood control/flood mitigation ❑ Distribution reservoir
reservoir
❑ Distribution Reservoirs should be located as close as possible to the center of demand.
❑ Water level in the reservoir must be at a sufficient elevation to permit gravity flow at an adequate
pressure.
Types of distribution
Reservoirs Small ground
Overhead level reservoirs
tanks
Large ground level reservoirs

Underground
reservoirs
Storage Zones of
Reservoirs
Full Reservoir Level (FRL):
The FRL encompasses both active and inactive
storage, including flood storage, if
incorporated. It represents the highest
attainable reservoir level that can be
maintained without the need for spillway
discharge or sluice-way operation.

Minimum Draw-down Level (MDDL):

The MDDL signifies the minimum water level
below which draw-down is restricted to
maintain the requisite head for power projects.

Dead Storage Level (DSL):

Below the DSL, there are no outlets for gravity
drainage, leading to silt accumulation over the
design lifespan.

Maximum Water Level (MWL) or High Flood

Level (HFL)/High Reservoir Level (HRL):
The MWL is the level likely to be reached
during the design flood, serving as a crucial
Storage Zones of
Reservoirs
Live/Useful Storage:
This represents the volume of water available
between the dead storage level and the full supply
level at any given time. The minimum operating
level should prevent vortex formation and air
entrainment.

Dead Storage:
Total storage below the inverted level of the lowest
discharge outlet, remaining unusable under
ordinary operating conditions. Dead Storage and
Buffer Storage together is called Interactive Storage.

Outlet Surcharge of Flood Storage:

This required storage between FRL and the
maximum water level helps contain flood peaks
when storage capacity below FRL is insufficient.

Buffer Storage:
Storage above the dead storage level up to the
minimum draw-down level. Releases from this
Storage Zones of
Reservoirs
Bank Storage
It is the volume of water that is stored temporarily
in the permeable reservoir banks.

Valley storage
It is the volume of water that can be held by natural
river channel on the soil above it.

Storage Capacity and Yield

Yield: It is the amount or quantity 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 (firm): It is the maximum amount or
quantity of water that can be guaranteed during a
worst or critical dry period.
Secondary Yield: It is the quantity of water
available in excess of safe yield during periods of
high flood flows.
Average yield: It is the arithmetic average of the
Investigations for
Reservoirs
❑ Engineering ❑ Geological ❑ Hydrological
Surveys Investigations Investigations
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.
❑ Contour map gives the following information:
1) Water spread
2) Capacity of the reservoir
3) Suitable dam site
4) Site for waste weir and outlets
5) Storage Elevation Curve
6) Arrangement of lines of communication
7) Map of the area to indicate the land property to be surveyed
Hydrological investigations
❑ The hydrological investigations are conducted for the following purposes :
1) To study the runoff pattern of the river or stream and to compute the storage capacity
2) To determine the maximum discharge at the site
3) To determine the hydrograph of the worst flood
Investigations for
Reservoirs
❑ Engineering ❑ Geological ❑ Hydrological
Surveys Investigations Investigations
Geological Investigations

❑ Geological investigations is done in two stages:

❑ Reconnaissance Survey and surface mapping: In this survey inspection of region is done and
data collected physically without using precise survey instrument.
❑ Sub surface exploration: The surface and subsurface investigation or site exploration is carried
out to collect the information about physical properties and characteristics of the subsoil material
as well as the details of other geological features of the site area.

❑ Geological investigations of the dam and reservoir site are done for the following purposes:
i) Suitability of foundation for the dam
ii) Water tightness of the reservoir basin or permeable pockets
iii) Faulty structure in the basin
iv) Type and depth of overburden to be excavated
v) Ground water condition of region
vi) Location of the quarry sites for the construction materials and their quantities.
Site Selection for
Reservoirs
❑ Large storage capacity
❑ Availability of suitable dam site
❑ Water tightness of reservoir to ensure minimum leakage (Proper Geological formations)
❑ River valley should be narrow, length of dam to constructed is less
❑ Good hydrological conditions
❑ Geology of catchment area should be such as to entail minimum water losses through
evaporation and percolation
❑ Deep reservoir or water-spread area should be less
❑ Land under Submergence should be minimum
❑ Silt free water or Low silt inflow
❑ Quality of water reasonably good or No objectionable minerals
❑ Low cost of real estate and Site easily accessible
Reservoir Sedimentation
❑ Sediment is defined as the fragmental material
either transported by suspended in or
deposited by the water. The deposition of
sediment in the reservoir is known as
'Reservoir Silting' or 'Reservoir Sedimentation'.
❑ All rivers carry certain amount of sediment
load during heavy rains due to erosion from
❑ The sediment particles try to settle down to the river bottom due to the gravitational force, but may be
catchments.
❑ kept in suspension
Disintegration, due to the
erosion, upward currents
transportation and in the turbulent flow which may overcome the gravity
force. Due to these
sedimentation are reasons, the river
the different carries
stages fine sediment in suspension as suspended load, and larger
leading
solids
to along
silting the river bed as bed load. When the silt laden water reaches a reservoir in the vicinity of a
of reservoir.
dam, the velocity and the turbulence are considerably reduced. The bigger suspended particles and
most of the bed load, there- fore, gets deposited in the head reaches of the reservoir. Fine particles may
travel some more distance and may finally deposit farther down in the reservoir. Some very fine
particles may remain in suspension for much longer period, and may finally escape from the dam along
with the water discharged through the sluiceways, turbines, spillway, etc.
Reservoir Sedimentation
❑ Factors Affecting Sedimentation
The quality of sediment depends upon the extent
of erosion in the catchment area, which depends
upon nature and type of soil, topography,
cultivation or vegetation cover and intensity of
rainfall in catchment area

❑ Sediment Management
❑ Soil conservation of drainage basin
❑ Flushing and de-silting of sediments or
Mechanical stirring
❑ Catchment Vegetation cover will minimize
impact of rain drops
❑ Retarding overland flow by terraces, contour
bunding etc.
❑ Construction of coffer dams (a watertight
Reservoir Sedimentation: Methods to reduce
sediments
Stepped Watershed
Calculation of Useful Life of
Reservoirs
❑ The deposition of the sediment will automatically reduce the water storing capacity of the reservoir,
and if this process of deposition continues longer, a stage is likely to reach when the whole reservoir
may get silted up and become useless.
❑ With the passage of time, the reservoir capacity will go on reducing.
❑ Therefore, in order to see that the capacity does not fall short of requirement ever during the design
period, we must take this silting into account. The total volume of silt likely to be deposited during
the designed life period of the dam is, therefore, estimated; and approximately that much of volume
is left unused to allow for silting, and is known as dead storage. The remainder is known as the
effective storage or the live storage.
❑ The dead storage generally varies between 15 to 25% of the total capacity.
❑ For example in Bhakra dam, the gross capacity of the dam is 9,344 million cubic metres and the dead
storage provided is 2,054 million cubic metres.
❑ All the outlets fetching water from the reservoir are provided above the dead storage level.

❑ Example: Let the total capacity of a reservoir be 30 million cubic metres and the provision of dead
storage be 6 million cubic metres. Let the average volume of sediment deposition be 0.15 million
cubic metres per year. Then the dead storage will be filled up in 6/0.15 = 40 years, and the total
storage in about 30/0.16 = 200 years.

❑ Hence, the usefulness of this reservoir would start reducing after 40 years, and after 200 years it
would be nothing but a collection of sand and sediment with no water in it, provided the siltation
3
Reservoir Sedimentation
Density Currents

❑ In a reservoir, the coarser sediment settles down along the bottom of the reservoir, as the muddy flow
approaches the reservoir; while the finer sediment usually remains in suspension, and moves in a
separate layer than the clear reservoir water.
❑ This layer of water, containing the fine sediment, moves below the clearer reservoir water, as a density
current, since its density is slightly more than the density of the main body of the reservoir water.
❑ Because of their density difference, the water of the density current does not mix easily with the
reservoir water, and maintains its identity for a considerable time.
❑ The density current can thus be removed through the dam sluiceways, if they are located properly and
at the levels of the density current.
❑ A lot of sediment load can, thus, be passed out of the reservoir, if it is possible to locate the dam outlets
and sluiceways in such a fashion, as to vent out the density currents.
❑ Trap efficiency of reservoirs may thus be decreased by about 2 to 10%, if it is possible to vent such
density currents through the outlets and sluiceways of the dams.
Reservoir Sedimentation
Trap Efficiency
Trap efficiency is defined as the percentage of the sediment deposited in the reservoir even in spite of
taking precautions and measures to control its deposition.
Therefore,

Trap Efficiency (ɳ) = Total sediment deposited in the reservoir/Total sediment flowing in the
river

Most of the reservoirs trap 95 to 100% of the sediment load flowing into them. Even if various feasible silt
control measures are adopted, it has not been possible to reduce this trap efficiency below 90% or so.

Capacity Inflow Ratio

The ratio of the reservoir capacity to the total inflow of water in it, is known as the capacity-inflow ratio.
It is a very important factor, because the trap efficiency (n) has been found to be a function of capacity-
inflow ratio i.e.

ɳ = f * (Capacity/Inflow)

The graph obtained for the existing reservoirs between trap efficiency and log of (Capacity/Inflow) has
been shown here.
Reservoir Sedimentation

It is evident from the curve, that if capacity reduces

(with constant inflow), trap efficiency reduces, and
hence, lesser sediment is trapped. Therefore, the
silting rate in the reservoir shall be more in the
beginning, and as its capacity reduces due to silting,
the silting rate will reduce. Hence, the complete
reservoir-silting may take longer period.

It can also be concluded that for small reservoirs

(having small capacity) on large rivers (having
large inflow rates), the trap efficiency is extremely
low, because the capacity inflow ratio is very small.
Such reservoirs silt very little and most of their
sediment is passed downstream. On the other hand,
large reservoirs on smaller rivers shall silt
tremendously and almost complete deposition of
sediment may take place.
Silting Control in Reservoirs
Silting Control in Reservoirs
Silting Control in Reservoirs
Capacity of Reservoirs

Fixing the capacity of the Reservoirs

In general, storage capacities have to be designed based on certain specified considerations,
which are:

❑ Precipitation, Runoff and silt records

❑ Erosion of Catchment
❑ Losses in the reservoir
❑ Trap efficiency
❑ Water Demand
❑ Density and location of outlets
❑ Economic analysis
❑ Engineering and Geological Aspects
Mass Inflow Curve

It is a plot of accumulated flow in a stream against time.

Demand Curve
It is a plot of accumulated demand against time.
Determination of Capacity of Reservoirs: Demand
Known
Determination of Capacity of Reservoirs: Demand
Unknown
Determination of Dependable Rainfall
Determination of Dependable Rainfall
Determination of Dependable Rainfall
Determination of Dependable Rainfall
Determination of Dependable Rainfall
Determination of Dependable Rainfall