ROHINI COLLEGE OF ENGINEERING AND TECHNOLOGY

4.1 RESERVOIR TYPE AND SITE SELECTION

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

Reservoirs are of two main categories: (a) Impounding reservoirs into which a river flows
naturally, and (b) Service or balancing reservoirs receiving supplies that are pumped or
channelled into them artificially. In general, service or balancing reservoirs are required to
balance supply with demand. Reservoirs of the second type 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. Impounding or storage reservoirs are intended to accumulate a part of the flood flow of
the river for use during the non-flood months

4.1.1 FUNCTIONS OF THE RESERVOIRS

1. Human consumption and/or industrial use:

2. Irrigation: usually to supplement insufficient rainfall.

AI3404 HYDROLOGY AND WATER RESOURCES ENGINEERING

 ROHINI COLLEGE OF ENGINEERING AND TECHNOLOGY

3. 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.

4.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.
4. 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.

5. Amenity use: this may include provision for boating, water sports, fishing, sight seeing.
Formally, the Bureau of Indian Standards code IS: 4410 (part 6)1983 “

4.1.2 types of reservoirs:

1. Auxiliary or Compensatory Reservoir: A reservoir which supplements and absorbed the spill
of a main reservoir.

2. 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.

3. Conservation Reservoir: A reservoir impounding water for useful purposes, such as

irrigation, power generation, recreation, domestic, industrial and municipal supply etc.

4. 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.

5. 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

AI3404 HYDROLOGY AND WATER RESOURCES ENGINEERING

 ROHINI COLLEGE OF ENGINEERING AND TECHNOLOGY

as local storage in case of emergency such as a break in a main supply line failure of a pumping
plant.

6. 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.

7. 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.

4.1.3 FACTORS GOVERNING THE SELECTION OF SITE FOR THE RESERVOIR

IS: 13216 - 1991 “Code of practice for geological exploration for reservoir sites”, that discusses
the relevant aspects

(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 These aspects are determined through investigations carried out by
surface and sub- surface exploration of proposed basin during the reconnaissance, preliminary
investigation, detailed investigation, construction and post-construction stages of the project.
The two basic stages of investigation: reconnaissance and preliminary investigations are
explained below: Reconnaissance In the reconnaissance stage, the objective of investigation is
to bring out the overall geological features of the reservoir and the adjacent area to enable the
designers, construction engineers and geologists to pinpoint the geotechnical and ecological
problems which have to be tackled. The scale of geological mapping for this stage of work
need not be very large and the available geological maps on 1:50,000 or 1: 250,000 scales may
be made use of. It is advantageous to carry out photo geological interpretation of aerial
photographs of the area, if available. If a geological map of the area is not available, a traverse
geological map should be prepared at this stage preferably using the aerial photos as base maps
on which the engineering evaluation of the various geotechnical features exposed in the area

AI3404 HYDROLOGY AND WATER RESOURCES ENGINEERING

 ROHINI COLLEGE OF ENGINEERING AND TECHNOLOGY

should be depicted. A topographical index map on 1: 50 000 scales should be used at this stage
to delineate the areas which would require detailed study, subsequently. To prevent an
undesirable amount of leakage from the reservoir, the likely zones of such leakage, such as
major dislocations and pervious or cavernous formations running across the divide of the
reservoir should be identified at this stage of investigation for further detailed
investigations.Major unstable zones, particularly in the vicinity of the dam in tight gorges,
should be identified at this stage for carrying out detailed investigations for the stability of the
reservoir rim. The locations for suitable construction material available in the reservoir area
should be pin pointed at this stage so that after detailed surveys such materials can be exploited
for proper utilisation during the construction stage prior to impounding of reservoir.The rate of
silting of the reservoir is vital for planning the height of the dam and working out the economic
life of the project. Since the rate of silting, in addition to other factors, is dependent on the type
of terrain in the catchment area of the reservoir, the major geological formations and the
ecological set up should be recognized at this stage to enable a more accurate estimation of the
rate of silting of the reservoir. For example, it should be possible to estimate at this stage that
forty percent of the catchment of a storage dam project is covered by Quaternary sediment and
that this is a condition which is likely to a yield a high silt rate or that ninety percent of the
catchment of another storage dam project is composed of igneous and metamorphic rocks and
is likely to yield a relatively low sediment rate. This information will also be useful in
examining whether or not tributaries flowing for long distances through soft or unconsolidated
formations, prior to forming the proposed reservoir, can be avoided and if not, what remedial
measures can be taken to control the silt load brought by these tributaries. The impounding of
a reservoir may submerge economic/strategic mineral deposits occurring within the reservoir
area or the resultant rise in the water table around the reservoir may cause flooding, increased
seepage in quarries and mines located in the area and water logging in other areas. It is,
therefore, necessary that the economic mineral deposits, which are likely to be adversely
affected by the reservoir area, are identified at this stage of the investigation. For example, if
an underground working is located close to a proposed storage reservoir area, it should be
identified for regular systematic geo-hydrological studies subsequently. These studies would
establish whether the impoundment of the water in the reservoir had adversely affected the
underground working or not. References should also be made to various agencies dealing with
the economic minerals likely to be affected by the impoundment in the reservoir for proper
evaluation of the problem and suitable necessary action. A dam and its reservoir are affected
by the environment in which they are located and in turn they also change the environment.

AI3404 HYDROLOGY AND WATER RESOURCES ENGINEERING

 ROHINI COLLEGE OF ENGINEERING AND TECHNOLOGY

Impoundment of a reservoir sometimes results in an increase of seismic activity at, or near the
reservoir. The seismic activity may lead to microtremors and in some cases lead to earthquakes
of high magnitude. It is, therefore, necessary to undertake the regional seismotechtonic study
of the project area. The faults having active seismic status should be delineated at this stage.
Simultaneous action to plan and install a network of seismological observatories encompassing
the reservoir area should also be taken. Preliminary Investigation The object of preliminary
investigation of the reservoir area is to collect further details of the surface and subsurface
geological conditions, with reference to the likely problems identified during the
reconnaissance stage of investigation by means of surface mapping supplemented by photo
geological interpretation of aerial photographs, hydro geological investigations, geophysical
investigations, preliminary subsurface exploration and by conducting geo-seismological
studies of the area. On the basis of studies carried out during the reconnaissance stage it should
be possible to estimate the extent of exploration that may be required during the preliminary
stage of investigation including the total number of holes required to be drilled and the total
number and depth of pits, trenches and drifts as also the extent of geophysical surveys which
may be necessary. For exploration by pits, trenches, drifts and shafts guidelines laid down in
IS 4453: 1980 Name of IS code should be followed. The potential zones of leakage from the
reservoir and the lateral extent of various features, such as extent of aeolian sand deposits,
glacial till, land slides, major dislocations or pervious and cavernous formations running across
the divide, should be delineated on a scale of 1: 50000. The geo-hydrological conditions of the
reservoir rim should be established by surface and sub-surface investigation as well as
inventory, as a free ground water divide rising above the proposed level of the reservoir is a
favourable condition against leakage from the reservoir. The level of water in a bore hole should
be determined as given in IS 6935: 1973. The extension of various features at depth, wherever
necessary, is investigated by geophysical exploration and by means of pits, trenches, drifts and
drill holes. For example, the resistivity survey should be able to identify water saturated zones

****************

AI3404 HYDROLOGY AND WATER RESOURCES ENGINEERING