Reservoir capacity

Instructional objectives
On completion of this report, we shall learn:
 1. The usual classification of the zones of a reservoir
 2. The primary types of reservoirs and their functions
 3. The steps for planning reservoirs
 4. Effect of sedimentation in reservoirs
 5. What are the geological explorations required to be carried out
for reservoirs
 6. How to reduce the loss of water from reservoirs
 7. How to control sedimentation of reservoirs

These specific levels and parts are generally defined as follows:
 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.
 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.
 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.
RESERVOIR STORAGE ZONE AND USES OF RESERVOIR

 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. The minimum operating level must be sufficiently above the lowest
discharge outlet to avoid vortex formation and air entrainment. 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.

Some other terms related to reservoirs are defined as follows:
 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.
 Within-the-Year Storage: This term is used to denote the storage of a reservoir
meant for meeting the demands of a specific hydrologic year used for planning
the project.
 Carry-Over Storage: When the entire water stored in a reservoir is not used up in
a year, the unused water is stored as carry-over storage for use in subsequent
years.
 Silt / Sedimentation zones: The space occupied by the sediment in the reservoir
can be divided into separate zones. A schematic diagram showing these zones is
illustrated in Figure 2 .

Freeboard : It is the margin kept for safety between the level at which
the dam would be overtopped and the maximum still water level. This is
required to allow for settlement of the dam, for wave run up above still
water level and for unforeseen rises in water level, because of surges
resulting from landslides into the reservoir from the peripheral hills,
earthquakes or unforeseen floods or operational deficiencies.

2. PRIMARY TYPES OF
RESERVOIRS AND
THEIR FUNCTIONS

TYPES OF RESERVOIRS
The Bureau of Indian Standards code defines the following types of
reservoirs:
 Auxiliary or Compensatory Reservoir : A reservoir which supplements and
absorbed the spill of a main reservoir.
 Balancing Reservoirs : A reservoir downstream of the main reservoir for
holding water let down from the main reservoir in excess of that
required for irrigation, power generation or other purposes.
 Conservation Reservoir : A reservoir impounding water for useful
purposes, such as irrigation, power generation, recreation, domestic,
industrial and municipal supply etc.
PRIMARY TYPES OF RESERVOIRS AND THEIR FUNCTIONS

 Detention Reservoir : A reservoir where in water is stored for a relatively brief
period of time, past of it being retained until the stream can safely carry the
ordinary flow plus the released water . Such reservoirs usually have outlets without
control gates and are used for flood regulation. These reservoirs are also called as
the Flood Control Reservoir or Retarding Reservoir.
 Distribution Reservoir : A reservoir connected with distribution system a water
supply project, used primarily to care for fluctuations in demand which occur over
short periods and as local storage in case of emergency such as a break in a main
supply line failure of a pumping plant.
 Impounding or Storage Reservoir : A reservoir with gate-controlled outlets wherein
surface water may be retained for a considerable period of time and released for
use at a time when the normal flow of the stream is in sufficient to satisfy
requirements.
 Multipurpose Reservoir : A reservoir constructed and equipped to provide storage
and release of water for two or more purposes such as irrigation, flood control,
power generation, navigation, pollution abatement, domestic and industrial water
supply, fish culture, recreation, etc.

The functions of reservoirs are to provide water for one or more of the
following purposes. Reservoirs that provide water for a combination of
these purpose, are termed as ‘Multi Purpose’ reservoirs.
 Human consumption and/or industrial use:
 Irrigation : usually to supplement insufficient rainfall.
 Hydropower : to generate power and energy whenever water is
available or to provide reliable supplies of power and energy at all
times when needed to meet demand.
FUNCTIONS

 Pumped storage hydropower schemes : In which the water flows from an
upper to a lower reservoir, generating power and energy at times of high
demand through turbines, which may be reversible, and the water is pumped
back to the upper reservoir when surplus energy is available. The cycle is
usually daily or twice daily to meet peak demands. Inflow to such a reservoir
is not essential, provided it is required to replace water losses through
leakage and evaporation or to generate additional electricity. In such
facilities , the power stations, conduits and either or both of the reservoirs
could be constructed underground if it was found to do so.
 Flood control: storage capacity is required to be maintained to absorb
foreseeable flood inflows to the reservoirs, so far as they would cause excess
of acceptable discharge spillway opening. Storage allows future use of the
flood water retained.
 Amenity use: this may include provision for boating, water sports, fishing,
sight seeing.

The first step in planning the construction of a reservoir with the help
of a dam is for the decision makers to be sure of the needs and
purposes for which the reservoir is going to be built together with the
known constraints (including financial), desired benefits. There may be
social constraints, for examples people’s activism may not allow a
reservoir to be built up to the desired level or even the submergence
of good agricultural level may be a constraint. Some times, the
construction of a dam may be done that is labor intensive and using
local materials, which helps the community for whom the dam is being
built. This sort of work is quite common in the minor irrigation
departments of various steps, especially in the drought prone areas.
The Food-for-Work schemes can be utilized in creating small reservoirs
that helps to serve the community.

The second step is the assembly of all relevant existing information, which includes the
following:
 Reports of any previous investigations and studies, if any.
 Reports on projects similar to that proposed which have already been constructed in
the region.
 A geographical information system (GIS) for the area of interest may be created using
a base survey map of the region.
 Topographical data in the form of maps and satellite pictures, which may be
integrated within the GIS.
 Geological data in the form of maps and borehole logs, along with the values of
relevant parameters.
 Meteorological and hydrological data - of available parameters like rainfall,
atmospheric and water temperatures, evaporation, humidity, wind speed, hours of
sunshine, river flows, river levels, sediment concentration in rivers, etc.

 For water supply projects, data on population and future population growth based
on some acceptable forecast method, industrial water requirement and probable
future industrial development.
 For irrigation projects, data on soils in the project area and on the crops already
grown, including water requirement for the crops.
 For hydropower projects, data on past demand and forecasts of future public and
industrial demand for power and energy; data on existing transmission systems,
including transmission voltage and capacity.
 Data on flora and fauna in the project and on the fish in the rivers and lakes,
including data on their migratory and breeding habits.
 Data on tourism and recreational use of rivers and lakes and how this may be
encouraged on completion of the proposed reservoir.
 • Seismic activity data of the region that includes recorded peak accelerations or
ground motion record.

4. EFFECT OF
SEDIMENTATION
IN RESERVOIRS

It is important to note that storage reservoirs built across rivers and
streams loose their capacity on account of deposition of sediment.
This deposition which takes place progressively in time reduces the
active capacity of the reservoir to provide the outputs of water
through passage of time. Accumulation of sediment at or near the dam
may interfere with the future functioning of water intakes and hence
affects decisions regarding location and height of various outlets. It
may also result in greater inflow into canals / water conveyance
systems drawing water from the reservoir. Problems of rise in flood
levels in the head reaches and unsightly deposition of sediment from
recreation point of may also crop up in course of time.

In special cases, where the effects of sedimentation on backwater
levels are likely to be significant, backwater studies would be useful
to understand the size of river water levels. Similarly, special studies
to bring out delta formation region changes may be of interest. The
steps to be followed for performance assessment studies with
varying rates of sedimentation are as follows:
a. Estimation of annual sediment yields into the reservoir or the
average annual sediment yield and of trap efficiency expected.
b. Distribution of sediment within reservoir to obtain a sediment
elevation and capacity curve at any appropriate time.
c. Simulation studies with varying rates of sedimentation.
d. Assessment of effect of sedimentation.

In general, the performance assessment of reservoir projects has to be
done for varying hydrologic inputs to meet varying demands. Although
analytical probability based methods are available to some extent,
simulation of the reservoir system is the standard method. The method
is also known as the working tables or sequential outing. In this
method, the water balance of the reservoir s and of other specific
locations of water use and constraints in the systems are considered.
All inflows to and outflows from the reservoirs are worked out to
decide the changed storage during the period. In simulation studies,
the inflows to be used may be either historical inflow series, adjusted
for future up stream water use changes or an adjusted synthetically
generated series.

5. GEOLOGICAL
EXPLORATIONS FOR
RESERVOIR SITES

Though a dam is constructed to build a reservoir, a reservoir has a large
area of spread and contained in a big chunk of the river valley upstream
of the dam. Hence, while identifying a suitable site for a proposed dam,
it is of paramount importance that the proposed reservoir site is also
thoroughly investigated and explored. The basis of planning for such
explorations is to have a rapid economical and dependable pre-
investment evaluation of subsurface conditions. It is also necessary that
a degree of uniformity be followed while carrying out subsurface
explorations so that the frame of reference of the investigation covers
all requisite aspects.

Since reservoir projects in river valleys are meant to hold water;
therefore, the following aspects of the reservoirs have to be
properly investigated.
(a) Water tightness of the basins
(b) Stability of the reservoir rim
(c) Availability of construction material in the reservoir area
(d) Silting
(e) Direct and indirect submergence of economic mineral wealth
(f) Seismo-techtonics

6. RESERVOIR
LOSSES AND THEIR
MINIMIZATION

Loss of reservoir water would mainly take place due to evaporation and a
number of methods have been suggest for controlling such loss. As such,
percolation or seepage loss is small for most of the reservoirs and
progressively gets lowered with the passage of time since the sediment
getting deposited at the reservoir bottom helps to reduce percolation
losses. Of course, in some hills and valleys forming the reservoir, there may
be continuous seams of porous rock strata or limestone caverns which
cause huge amount of water to get drained out of the reservoirs.

A number of factors affect the evaporation from open water surface, of
which, the major factors are water spread area and frequent change of
speed and direction of wind over the water body. Other meteorological
factors like.
a) Vapour pressure difference_ between water surface and the layer of
air above;
b) Temperature of water and air;
c) Atmospheric pressure;
d) Radiation;
e) Heat storage in water body; and
f) Quality of water,
have direct influence on the rate of evaporation.

Since the meteorological factors affecting evaporation cannot be
controlled under normal conditions, efforts are made for inhibition of
evaporation by control of flow of wind over water surface or by protection
of the water surface area by physical or chemical methods. The methods
generally used are as follows:
a) Wind breakers,
b) Covering the water surface,
c) Reduction of exposed water surface,
d) Integrated operation of reservoirs, and
e) Treatment with chemical water evaporetardants (WERs).

Wind Breakers
Wind is one of the most important factors which affect rate of
evaporation loss from water surface. The greater the movement of air
over the water surface, greater is the evaporation loss. Planting of trees
normal to windward direction is found to be an effective measure for
checking of evaporation loss. Plants (trees, shrubs or grass) should be
grown around the rim of tanks in a row or rows to act as wind breaker.
These wind breakers are found to influence the temperature,
atmospheric humidity, soil moisture, evaporation and transpiration of
the area protected.
Plants to act as wind breakers are usually arranged in rows, with tallest
plants in the middle and the smallest along the end rows, so that more
or less conical formation is formed.

Covering the Water Surface
Covering the surface of water bodies with fixed or floating covers
considerably retards evaporation loss. These covers reflect energy inputs
from atmosphere, as a result of which evaporation loss is reduced. The
covers literally trap the air and prevent transfer of water vapour to outer
atmosphere.
Fixed covers are suitable only for relatively small storages. For large
storages, floating covers or mat or spheres may be useful and effective.
However, for large water surfaces the cost of covering the surface with
floats is prohibitive, Further in case of reservoirs with flood outlets, there
is also the danger of floats being lost over spillway or through outlets.
The floating covers are thus of limited utility to larger water bodies.

Reduction of Exposed Water Surface
In this method shallow portions of the reservoirs are isolated or
curtailed by construction of dykes or bunds at suitable locations.
Water accumulated during the monsoon season in such shallow
portions is diverted or pumped to appropriate deeper pockets in
summer months, so that the shallow water surface area exposed to
evaporation is effectively reduced.

7. CONTROL OF
SEDIMENTATION
IN RESERVOIRS

Sedimentation of a reservoir is a natural phenomenon and is a matter of
vital concern for storage projects in meeting various demands, like
irrigation, hydroelectric power, flood control, etc. Since it affects the
useful capacity of the reservoir based on which projects are expected to be
productive for a design period. Further, the deposited sediment adds to the
forces on structures in dams, spillways, etc.
The rate of sedimentation will depend largely on the annual sediment load
carried by the stream and the extent to which the same will be retained in
the reservoir. This, in turn, depends upon a number of factors such as the
area and nature of the catchment, level use pattern (cultivation practices,
grazing, logging, construction activities and conservation practices), rainfall
pattern, storage capacity, period of storage in elation to the sediment load
of the stream, particle size distribution in the suspended sediment, channel
hydraulics, location and size of sluices, outlet works, configuration of the
reservoir, and the method and purpose of releases through the dam.

There are different techniques of controlling sedimentation in reservoirs
which may broadly be classified as follows:
• Adequate design of reservoir
• Control of sediment inflow
• Control of sediment deposition
• Removal of deposited sediment.

Design of reservoirs
From the point of view of sediment deposition , the following points may
be given due consideration:
a) The sediment yield which depends on the topographical, geological set
up , land use/land cover, intercepting tanks, etc. ;
b) Sediment delivery characteristics of the channel system;
c) The efficiency of the reservoir as sediment trap;
d) The ratio of capacity of reservoir to the inflow;
e) Configuration of reservoir;
f) Method of operation of reservoir;
g) Provisions for silt exclusion.

Control of sediment inflow
There are many methods for controlling sediment inflows and they
can be divided as under:
1) Watershed management/soil conservation measures to check
production and transport of sediment in the catchment area.
2) Preventive measures to check inflow of sediment into the reservoir.
The soil conservation measures are further sub-divided as:
a) Engineering,
b) Agronomy, and
c) Forestry.

The engineering methods include:
a) Use of check dams formed by building small barriers or dykes across stream
channels.
b) Contour bounding and trenching;
c) Gully plugging;
d) Bank protection.
The agronomic measures include establishment of vegetative screen, contour farming,
strip cropping and crop rotation.
Forestry measures include forest conservancy, control on grazing, lumbering,
operations and forest fires along with management and protection of forest
plantations.
Preventive measures to check inflow of sediment into the reservoir include
construction of by-pass channels or conduits.

Check Dams
Check dams are helpful for the following reasons:
a) They help arrest degradation of stream bed thereby arresting the
slope failure;
b) They reduce the velocity of stream flow, thereby causing the
deposition of the sediment load.
Check dams become necessary, where the channel gradients are steep
and there is a heavy inflow of sediment from the watershed. They are
constructed of local material like earth, rock, timber, etc. These are
suitable for small catchment varying in size from 40 to 400 hectares. It
is necessary to provide small check dams on the subsidiary streams
flowing into the main streams besides the check dams in the main
stream.

Proper consideration should be given to the number and location of
check dams required. It is preferable to minimize the height of the
check dams. If the stream ha, a very-steep slope, it is desirable to start
with a smaller height for the check dams than may ultimately be
necessary.
Check dams may generally cost more per unit of storage than the
reservoirs they protect. Therefore, it may not always be possible to
adopt them as a primary method of sediment control in new reservoirs.
However , feasibility of providing check dams at a later date should not
be overlooked while planning the protection of-a new reservoir.

Contour Bunding and Trenching
These are important methods of controlling soil erosion on the hills
and sloping lands, where gradients of cultivated fields or terraces are
flatter, say up to 10 percent. By these methods the hill side is split up
into small compartments on which the rain is retained and surface
run-off is modified with prevention of soil erosion. In addition to
contour bunding , side trenching is also provided sometimes.

Gully Plugging
This is done by small rock fill dams. These dams will be effective in
filling up the gullies with sediment coming from the upstream of
the catchment and also prevent further widening of the gully.

Removal of deposited sediment
The most practical means of maintaining the storage capacity are
those designed to prevent accumulation of permanent deposits as the
removal operations are extremely expensive, unless the material
removed is usable. Therefore, the redemption of lost storage by
removal should be adopted as a last resort. The removal of sediment
deposit implies in general, that the deposits are sufficiently
compacted or consolidated to act as a solid and, therefore, are unable
to flow along with the water. The removal of sediment deposits may be
accomplished by a variety of mechanical and hydraulic or methods,
such as excavation, dredging, siphoning, draining, flushing, flood
sluicing, and sluicing aided by such measures as hydraulic or
mechanical agitation or blasting of the sediment. The excavated
sediments may be suitably disposed off so that, these do not find the
way again in the reservoir.

Excavation
This involves the removal of deposits from the bottom of a reservoir
and their conveyance to some other point by mechanical or hydraulic
means, while water storage is being maintained.
Dredging practices are grouped as:
a) Mechanical dredging by bucket, ladder, etc;
b) Suction dredging with floating pipeline and a pump usually mounted
on a barrage; and
c) Siphon dredging with a floating pipe extending over the dam or
connected to an opening in the dam and usually with a pump on a
barrage.

Draining and Flushing or Sluicing
The method involves relatively slow release of all stored water in a
reservoir through gates or valves located near bottom of the dam
and the maintenance thereafter of open outlets for a shorter or
longer period during which normal stream flow cuts into or directed
against the sediment deposits. Therefore, this method may be
adopted in flood control reservoirs.

Reservoir capacity

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