1. RESERVOIRS
A reservoir – artificial lake or dam is used to store water.
Reservoirs may be created in river valleys by the construction of a
dam or may be built by excavation in the ground or by
conventional construction techniques such as brickwork or
cast concrete.
The term reservoir may also be used to describe underground
reservoirs such as an oil or water well.
2. Five thousand years ago, the craters of extinct volcanoes in Arabia were
used as reservoirs by farmers for their irrigation water.
Dry climate and water scarcity in India led to early development of
water management techniques, including the building of a
reservoir at Girnar in 3000 BC.
Artificial lakes dating to the 5th century BC have been found in ancient
Greece.
An artificial lake in present-day Madhya Pradesh province of India,
constructed in the 11th century, covered 650 square metres
(7,000 sq ft).
In Sri Lanka large reservoirs have been created by ancient Sinhalese
kings in order to save the water for irrigation.
The famous Sri Lankan king Parākramabāhu I of Sri Lanka stated " do not
let a drop of water seep into the ocean without benefiting
mankind ".
He created the reservoir named Parakrama Samudra (sea of King
Parakrama), which has astonished archaeologists.
3. Types of Reservoirs
Valley dammed reservoir –
A dam constructed in a valley relies on the natural
topography to provide most of the basin of the reservoir.
Dams are typically located at a narrow part of a valley
downstream of a natural basin. The valleysides act as
natural walls with the dam located at the narrowest
practical point to provide strength and the lowest practical
cost of construction. In many reservoir construction
projects people have to be moved and re- housed, historical
artifacts moved or rare environments relocated.
Where the topography is poorly suited to a single large
reservoir, a number of smaller reservoirs may be
constructed in a chain
5. Lake Vyrnwy Reservoir. The dam spans the Vyrnwy Valley and was the first large stone dam
built in the United Kingdom.
6. Bank-side reservoir
Where water is taken from a river of variable quality or quantity, bank-
side reservoirs may be constructed to store the water
pumped or siphoned from the river.
Such reservoirs are usually built partly by excavation and partly by the
construction of a complete encircling bund or embankment
which may exceed 6 km in circumference.
Both the floor of the reservoir and the bund must have an impermeable
lining or core
7. Service reservoir
Service reservoirs store fully treated potable water close to the point
of distribution.
Many service reservoirs are constructed as water towers, often as
elevated structures on concrete pillars where the landscape is
relatively flat.
Other service reservoirs are entirely underground, especially in more
hilly or mountainous country.
Service reservoirs perform several functions including ensuring
sufficient head of water in the water distribution system and
providing hydraulic capacitance in the system to even out peak
demand from consumers enabling the treatment plant to run at
optimum efficiency.
Large service reservoirs can also be managed to so that energy costs in
pumping are reduced by concentrating refilling activity at times
of day when power costs are low.
8. Storage works are constructed to serve many purposes, such as for
Irrigation
Domestic uses
Industrial uses
Hydro electric power
Navigation
Flood control
Land reclamation in low lying areas
Debris control
Aquaculture
Preservation of aquatic life
Recreation
Depending on the purposes, the reservoir may be classified as
Storage or conservation reservoirs
Flood control reservoirs
Distribution reservoirs
Multipurpose reservoirs
9. Storage or conservation reservoirs – are constructed to store water
received through excess rainfall and is released gradually when
it is needed for the following purposes -
irrigation, hydroelectricity, domestic, industrial etc.
Flood control reservoirs – storage of water received through excess
rains causing floods.
Then after gradually released when the flood rate decreases.
This kind of storage avoids or decreases the intensity of the
damage and loss due to floods.
Distribution reservoir – is a small storage reservoir used for water
supply in a city.
Accounts for varying rate of water supply during the day.
Such distribution permits pumping plants and water treatment
works etc., to operate at constant rate.
Multipurpose reservoir – serves more than one purpose.
10. Selection of a site for a Reservoir –
Depends on the factors like
Geological condition of the catchment area – less infiltration and
maximum runoff preferred.
Leakage should be minimum – i.e through rocks – preferred rocks area
shales, slates, schists, gneisses and crystalline rocks such
granites etc.
Suitable dam site – water tight rock base, percolation below the dam
should be minimum. Cost of dam is often a controlling factor.
Narrow valley opening – the reservoir basin should have a narrow
opening in the valley to have a shortest length of the dam
Cost – cost of real estate for the reservoir, road, rail road, re location of
dwellings etc., should as low as possible.
11. HYDROLOGICAL INVESTIGATIONS
It is an important aspect of the reservoir planning.
The capacity of the irrigation canals and the installed capacity of the power
houses depends on the available supplies from the reservoir.
Investigations are divided into two heads.
1.Study of run off pattern at the proposed dam site and to determine the
storage capacity to meet the given demand
2.Determination of the hydrograph of the worst flood, to determine the spillway
capacity and design.
12. Topography – should have adequate capacity without submerging
excessive land and villages and other properties
Capacity of the reservoir – depth of the reservoir must be as deep as
possible to hold more water within a short area.
Shallow reservoirs occupy more land and involves high costs of
acquiring land.
Less evaporation loss when compared to wide reservoirs
Less likelihood of weed growth.
Desilting can be done at longer intervals which is recurring
expenditure and maintenace compared to shallow reservoirs
Suitability of the water in reservoir – should be free from natural
pollutants such as mineral deposts especially from the radioactive
mineral deposits. Otherwise the water enriched in such elements as U,
Th and other objectionable elements as Na, F, K and Nitrates,
Carbonates will cause health hazards and cannot be used even for
irrigation purposes.
13. Zones of storage in Reservoir
Normal pool level – Maximum level of water in the reservoir during
operation conditions.
It corresponds to the level of spillway crest, or to the top of the
spillway crest gates.
Maximum pool level – corresponds to the water level during the design
flood.
Minimum pool level – the lowest level of water in the reservoir where
water is to be drawn under ordinary operating conditions
14. The different zones are
Useful storage – the water stored between the normal pool level and the minimum pool level.
Surcharge storage – the water stored between the normal pool level and the maximum level
corresponding to a flood
Dead storage – the water below the minimum pool level not useful under normal operating
conditions.
Bank storage and Valley storage – referred to the water stored in the pervious formation of the
river banks and the soil above it. It depends on the geological conditions of the river
banks.
The bank storage effectively increases the capacity of the reservoir above that
indicated by the elevation-storage curve
15. Reservoir storage is divided into four zones, or pools.
These include, from top to bottom, the flood-control zone, conservation zone,
buffer zone and inactive zone.
The conservation and buffer pools, together, constitute the reservoir's active
storage.
Ensure that the flood-control zone is always kept vacant, i.e., the volume of
water in the reservoir cannot exceed the top of the conservation pool.
16. Reservoir Yield
It is also storage capacity and yield.
Yield – it is the amount of water that can be supplied from the reservoir
in a specified interval time.
The interval of time is dependent on the size of the reservoir
from a day for small reservoir to a year for large conservation
reservoirs.
Ex – if 35,000 cubic meters of water is supplied from a reservoir
in one year, its yield is 35,000 cu.m/yr or 3.5 hectare meters/yr.
Safe Yield or Firm Yield – Maximum quantity of water that can be
guaranteed during a critical dry period
Secondary Yield – the amount of water available in excess of safe yield
during high flood periods.
Average Yield – the arithmatic average of the first and the secondary
yield over a long period of time is called average yield.
17. Flood Hydrograph of Inflow
shows inflow of several years.
A hydrograph is a graph showing the rate of flow (discharge) versus time past a
specific point in a river, or other channel or conduit carrying flow. The rate of
flow is typically expressed in cubic meters or cubic feet per second (cms or cfs).
18. Mass Inflow Curve
A mass inflow curve is a plot between the cumulative inflow in the reservoir with time.
The reservoir capacity corresponding to a specified yield is determined with the help
of mass inflow curve and the demand curve.
A mass inflow curve continuously rises as it shows accumulated inflow.
If there is no inflow during
certain period, the mass inflow
curve will be horizontal
during that period.
The mass inflow curve will
rise very sharply during the
period of high flood.
The curve will rise sharply
during the period of high
flood.
The steepness of the curve
indicates the rate of inflow.
19. Demand Curve
It is a plot between accumulated demand with time. Uniform demand
leads to a straight line. Demand curve indicates variable demand rate
over a time.
20. Estimation Of Reservoir Capacity
Reservoir capacity may be estimated by using mass inflow curve.
A mass inflow curve is prepared over the years with cumulative inflow of water. A mass
demand curve is also prepared on the same scale.
The tangent drawn at the apices of mass inflow curve A1, A2, A3, …is the Mass Demand
Curve
E1D1, E2D2, E3D3 etc., are between the
tangent and the mass inflow curve.
The vertical intercepts indicate the
volume by which inflow falls short of
demand.
C1D1 – net flow
C1E1 – demand
E1D1 – to be provided from the
reservoir
E1D1, E2D2, E3D3 etc. – required
reservoir capacity
It should be noted that the vertical
distances between successive tangents
represents water wasted ove the
spillway.
21. Reservoir Sedimentation
Silting –
Sedimentation in the reservoir is known as silting.
silting is the accumulation of the transported material/debris/rock
debris/silt or mud in the reservoir.
Many of the reservoirs have become extinct due to bad maintenance with
regard to desilting from time to time.
Silting in the reservoir depends on the amount of load carried by the river
or stream, weathering and erosional capacity, transportation over
a period of time.
The other factors are –
Nature of the soil in the catchment area
Topography of the catchment area
vegetation cover and
intensity of rainfall.
22. Important factors – nature of soil at the catchment area
The sediment transported by the river - divided into two heads.
1.Bed load – the bed load is dragged along the bed of the stream.
2.Suspended load – the load is kept in suspension because of the vertical component of
the eddies formed due to friction of flowing water against the bed. The bed load is
generally smaller. It is only 10-15% of the suspended load. The coarser particles settle
down near the dam in the dead storage due to reduced velocity.
23. Density currents – defined as a gravity flow of fluid under another fluid of approximately
equal density. In the reservoirs the water stored is usually clear and the inflow
is generally muddy. The two fluids have different densities and the heavy turbid
water flows along the channel bottom towards the dam under the influence of
the gravity. This is known as density current. The rate of silting in case of
reservoirs reduces if the density currents are vented by proper location and
operation of outlet and sluice gates.
24. Measurement of sediment load – sediment sample to be collected at regular intervals of
ime and at different depths of the reservoir. The sample are to be dried and the
amount of sediment material is calculated. It is calculated as ppm or weight
percentage.
Resevoir life – It is dependent on the maintenance of the reservoir and the amount of
siltattion taking place at every flood season. A dead storage in the reservoir is
provided for silting the fine materials. The water in the dead storage is
unutilized. If proper care is not taken in the maintenance of the reservoir i.e .,
desilting then the effective storage or live storage will be affected leading to failure
of the reservoir or reducing the effective or live storage of the reservoir.
Trap efficiency (η) – This is a measure of reservoir sedimentation.
The investigations show that the trap efficiency is a function of the ratio of
reservoir capacity to the total inflow
The rate of silting is much higher in the initial stages of the inflow into the
reservoir, then the rate of silting will reduces as silting goes on. Therefore the
complete filling of reservoir may take quite a long period.
with regards to the small reservoir on a large stream
25. A small reservoir on a large dam having a small capacity has a large inflow
rates.
It has a small capacity/inflow ratio.
The trap efficiency of such reservoirs is extremely small and the stream passes
most of its inflow quickly and the finer sediments are discharged
without getting time to settle.
On the contrary A large reservoir on a small stream with less inflow rates has
higher capacity/inflow ratio.
Such reservoirs has a greater trap efficiency.
Such reservoirs retain water for several years and permit almost complete
deposition of the sediment.
26. Life of a reservoir
1. Capacity of reservoir – is calculated from the study of inflow rates, leakage
factors, type of rocks and structure, topography, drainage basin analysis and
trap efficiency etc.
2. Assume 10 % trap efficiency for reduced capacity due to silting.
3. For the interval of 10 % capacity, average trap efficiency is calculated by taking the
average of trap efficiency (η).
4. Sediment inflow rate is determined by drying the sediments from the water
samples.
5. The total annual sediment transported is multiplied by the trap efficiency and is
converted into hectare-metre (volumetric) units for one year.
6. Volume interval (10 % of the capacity) is divided by the sediment deposited in
order to get number of years to fill this volume interval of 10 % capacity.
7. This procedure is repeated further at 80 %, 70 %, 60 %,….. 20 % of the
capcity. The total life of the reservoir = total number of years required to fill each
of the volume intervals.
27. Reservoir Sediment Control
1.Proper selection of reservoir site
2.Control of sediment Inflow
3.Proper designing and reservoir planning
4.Control of sediment deposit in the reservoir
5.Removal of sediment deposit
6.Erosion control in the catchment area
