Classification of Reservoirs

1. Reservoirs
Classification of reservoirs - Site
selection - General principles of design
- Spillways -Elevation- Area-Capacity
curve - Storage estimation -
Sedimentation - Life of reservoirs – Rule
curve.

2. Reservoirs

3. Reservoirs
A reservoir is the large water body that
forms behind a wall constructed across
a river or a large valley. In some
instances, the reservoir accumulates
large amounts to form a human-made
lake.
A large lake where water is stored to be
used by a particular area, city, etc.

4. Reservoirs
• Dams and weirs are some of the barriers which when constructed
across the rivers and streams, cause accumulation of water behind
them.
• The water thus accumulated in form of an artificial lake is known
as reservoir.
• In broad sense, any collected water in form of a pool or lake, may
be termed as reservoir. Dams and reservoirs are the most
important elements of any multipurpose river basin development.

5. Reservoirs
• River water may be used for generation of hydroelectric power,
irrigation, and water supply source for some town or city.
• All these uses require a constant or almost constant supply of
water. But we know that discharge in rivers and streams remains
fluctuating.
• During rains, the water in the river may be more than the
requirements and during dry period the discharge in the river may
become less than the minimum requirements.

6. Reservoirs
• Thus to store the excess water, flowing during floods, dams are
constructed across the rivers.
• This stored water is used to augment the supplies of the river
during dry weather.
• By this arrangement it is possible to generate hydroelectric power,
provide irrigation facilities and make available drinking water for
whole of the year.

7. Reservoirs
• Actually dams and reservoirs are complementary to each other.
• Reservoir can be developed only by constructing dam, conversely
when dam is constructed reservoir is bound to develop.
• Both dams and reservoirs, being very important elements of any
multipurpose irrigation project, have to be very carefully planned,
designed and operated.
• This involves proper selection of site, for dam and reservoir,
fixation of capacity of reservoir, safe yield of the storage and other
connected works.

8. Classification of Reservoirs
On the basis of purpose of construction
of reservoir, it is classified into three
types:
• Storage reservoir
• Flood Control reservoir
• Distribution reservoir

9. 1. Storage Reservoirs
• One constructed this reservoir to store water which flows during the
high flow, during rainy season and one uses the same during the dry
summer seasons.
• It can serve the purpose of irrigation, hydel power generation,
domestic and industrial water supply.
• In some cases, reservoirs also help to control the floods.
• So on the basis of requirement, these reservoirs can be single
purposed or multi- purposed.

10. 1. Storage Reservoirs
• We know that discharge in the rivers remains changing day to day,
and season to season.
• During high floods the excess water in the river goes waste while in
dry months it may not be sufficient to meet the minimum needs.
• The storage reservoir is constructed to store the excess water of
floods and released gradually as and when required.

11. 2. Flood Control Reservoirs
• To reduce the danger and the damage due to the floods, One
constructs the flood control reservoirs.
• One calls it as flood protection reservoir.
• Generally these reservoirs are of single purposed type of reservoirs.
• One divided Flood control reservoirs into two types:
• Retarding Reservoirs
• Detention Basins
• Retarding Reservoirs: It has outlets and one does not control spillway
by valves or gates.
• Detention Basins: It has outlets and one controls spillways by gates or
valves.

12. 2. Flood Control Reservoirs
• This reservoir is also called flood mitigation reservoir.
• The main purpose of this reservoir is to temporarily store the flood
water and release slowly at a safe rate after the floods, so that it may
not cause any damage on the downstream side. So this reservoir may
be said a flood prevention reservoir also.
• This reservoir requires provision of large spillways and sluice-ways so
that excess stored flood water is rapidly released downstream, but
only at the safe rate.

13. Storage Reservoirs or Detention basins
• The reservoirs, whose spillways and sluice outlets are fitted with gates
and valves, are known as storage reservoirs or detention basins.
Gated spillways and gated sluiceways provide more flexibility in
operation. They help in exercising better control on the reservoir and
thus reservoir water can be used more wisely and usefully.
• This reservoir is costly as it involves cost of gates and valves.
Detention basins or storage reservoirs are preferred on large rivers, as
better controls on flood waters can be exercised and water can be
released from the reservoir at better controlled rate so as not to
cause any damage on the downstream side.

14. Retarding Basins or Retarding Reservoirs
• In these reservoirs there are no gates at spillways and sluice outlets.
In this case sluice and spillway’s joint maximum discharging capacity
is at the most equal to the maximum safe carrying capacity of the
channel downstream. As floods occur, the reservoir first of all gets
filled up, upto normal level.
• At this time sluice outlets are discharging out water from the
reservoir. As level in the reservoir increases further, discharge through
sluice ways also increases. At certain level of reservoir, water also
starts escaping through un-gated spillways. As the level of water
further rises the discharge over spill ways also increases.

15. Retarding Basins or Retarding Reservoirs
• At some particular level a balance will be struck between the inflow
and outflow of the reservoir. At this time level of water in the
reservoir will become stable. This condition happens only when flood
inflow in the reservoir is equal to the outflow from the reservoir.
• Now when floods occurring recede, inflow in the reservoir will also
decrease but the outflow is at the same maximum rate. Hence flood
water which had accumulated in the reservoir will now be slowly
flowing out of the reservoir.

16. Advantages of Retarding Basins over detention
basin
(a) Gates are not required to be provided at sluice ways and spillway
crests.
(b) Since there are no gates at the spillways, chances of human error in
opening the gates during floods cannot take place.
(c) Since water from the reservoir is driven out in few days after floods,
the land during maximum floods remains submerged only temporarily.
This submerged land can be used for growing very good crops. But no
habitation on this land should be allowed. Retarding basins are
preferred on small rivers.

17. Advantages of Retarding Basins over detention
basin
(a) Gates are not required to be provided at sluice ways and spillway
crests.
(b) Since there are no gates at the spillways, chances of human error in
opening the gates during floods cannot take place.
(c) Since water from the reservoir is driven out in few days after floods,
the land during maximum floods remains submerged only temporarily.
This submerged land can be used for growing very good crops. But no
habitation on this land should be allowed. Retarding basins are
preferred on small rivers.

18. Distribution Reservoirs
• It is a small storage reservoir which one uses to make domestic water
supply to the urban areas to meet the varying demands of the
consumer at different period of a day.
• The one pumps the water into the reservoir, at a constant rate and
one supplies it to the consumers when people reduces the water
demand.
• And when the demand is high, one supplies water through the
storage tank.

19. Distribution Reservoirs
• It is a small capacity reservoir which is mainly constructed to meet the
water supply requirements of a particular city.
• It is made of masonry or cement concrete and may be covered from
the top.
• This reservoir is filled by treated water at some constant rate. Since
demand of water remains fluctuating during the day, water may have
to be drawn from this reservoir at times at rate much more than the
inflow rate.

20. Distribution Reservoirs
• Hence these reservoirs allow pumping units and treatment units to
work at predetermined constant rates.
• During no demand or very little demand the water coming from
treatment units and pump units goes on storing in the reservoir.
• During peak demand this stored water from the reservoir is used to
make up the required supplies.

21. Purpose of reservoirs

22. Purpose of reservoirs
1. To act as source of water for any public water supply scheme.
2. To augment the irrigation supplies when discharge in the river
is smaller than the demand.
3. To maintain some minimum level of water for generation of
hydroelectric power during lean months of discharge.
4. To increase the depth of water to facilitate navigation.

23. Purpose of reservoirs
5. For reducing the flood havoc downstream. It is achieved as
flood waters are temporarily held up in the reservoir.
6. To render water comparatively slit and debris free so that it
may not cause any damage in hydroelectric generation
equipment or pumping equipment if water is to be pumped for
some public water supply scheme or any other purpose.
7. Growing useful aquatic life
8. Recreation.

24. Selection of Reservoir Site
Before finally selecting the reservoir site following factors
should be seriously considered:
1. Catchment area should have such geological conditions that
percolation and absorption losses are minimum.
2. Available run-off should be maximum.
3. The site should be free from fissured rocks. This will avoid
possibilities of leakage when reservoir is full to capacity.
4. The reservoir site must have adequate capacity.
5. The reservoir basin should have a deep narrow opening in the
valley so that the length of the dam may be kept minimum.

25. Selection of Reservoir Site
6. Heavily silt laden tributaries should not lead their discharge to
the reservoir.
7. Suitable site for dam should be available. It will be an ideal site
if dam is constructed at the narrow and shallow part of the river
which lies down stream of the deep river. It is very important
point as cost of dam is often a controlling factor in selection of
the reservoirs site.
8. Site should be such that deep reservoir is formed. Deep
reservoir would store more of water and expose minimum area
at the surface for evaporation.
9. If earthen dam is proposed to be constructed, then separate
suitable site for spillway works should be available.

26. Selection of Reservoir Site
10. Reservoir site should be well connected by rail and road.
11. Materials for the construction of dam should be available nearby.
12. The soil formation at reservoir site should be free from harmful
salts.
13. If reservoir water is to be used for irrigation, the dam site should
be near the area proposed to be irrigated. This would reduce the
length of the canal system and consequently the cost of the project.
14. Reservoir should not submerge habited area or areas of fertile
lands or gardens.
15. River banks should be hard, strong and high so that cost on river
training works is minimum.