Introduction, Term related to reservoir planning (Yield, Reservoir planning and operation curves, Reservoir storage, Reservoir clearance), Investigation for reservoir planning, Significance of mass curve and demand curves, Applications of mass-curve and demand curves, Fixation of reservoir capacity from annual inflow and outflow, Fixation of reservoir capacity.
4. The water required for drinking purposes, Industrial purposes, Irrigation,
Navigation and generation of hydropower (Electricity) is mostly drawn
from the surface source (i.e. river or lake).
During low flow in the river (ex. Summer season, Drought), it may not
possible to meet the above demands of the consumers.
Reservoir is, an enlarged natural or artificial lake, pond or impoundment
created using a dam to store water.
Water storage reservoirs may be created by constructing a dam across a
river, along with suitable appurtenant structures.
4
INTRODUCTION
5. Reservoirs are also meant to absorb a part of flood water and the excess
is discharged through a spillway.
Impounding reservoirs, into which a river flows naturally. Impounding
or storage reservoirs are intended to accumulate a part of the flood flow
of the river for use during the non-flood months.
Service or balancing reservoirs, which receiving supplies that are
pumped or channeled into them artificially. These 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. 5
INTRODUCTION
6. TYPES OF RESERVOIRS
Storage Reservoirs
Flood Mitigation or control reservoirs
Distribution reservoirs
Multipurpose Reservoirs
6
7. Storage Reservoirs or Conservation
Storage reservoir is also called conservation reservoir because they are used
to conserve water.
The pool of water created by the construction of a dam across a river is
called reservoir.
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
impounded reservoir.
7
9. Flood Mitigation or Control reservoirs
Flood control reservoirs collect water at times of high flow and then release it
slowly over the course of the flood and during the subsequent days or weeks.
It is a reservoir constructed to store the portion of the water so as to protect the
area on the downstream side from flood damage.
Flood control reservoir are further sub classified as
i) Detention basins reservoirs
ii) Retarding reservoirs
9
11. Distribution reservoirs or service
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.
It is small storage reservoir constructed for the supply of water in a city
during there emergencies need and satisfies the varying demand by the
ultimate consumers during various parts of the day.
11
13. Multipurpose Reservoirs
It is a reservoir that serves two or more purposes.
The term multipurpose reservoir includes all reservoirs actually
designed and operated to serve more than one function.
The most common purposes of these reservoirs are to generate
hydroelectric power, provide flood control, store water, irrigation,
drinking purpose, industrial purpose and navigation.
13
16. 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 leveling.
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.
16
17. Geological investigation
Geological investigations is done in two stages.
i) Reconnaissance Survey and surface mapping: In this survey
inspection of region is done and data collected physically without using
precise survey instrument.
ii) 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.
17
18. Geological investigation
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. 18
19. Hydrological investigations
The hydrological investigations are conducted for the following
purposes :
i) To study the runoff pattern of the river or stream and to compute the
storage capacity
ii) To determine the maximum discharge at the site
iii) To determine the hydrograph of the worst flood
19
20. Site Selection for a Reservoir
Large storage capacity
Water tightness of reservoir
River valley should be narrow, length of dam to constructed is less
Good hydrological conditions
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 20
21. Zones in the Reservoir Storage
Full Reservoir Level (FRL): It is the maximum normal level that can be
conveniently stored in the reservoir under normal operating condition. It is the
level corresponding to the storage which includes both inactive and active
storages and also the flood storage, this is the highest reservoir level that can be
maintained without spillway discharge without passing water downstream to
sluice ways.
Minimum water Level (Min. WL): It is the lowest level in the reservoir to
which water from the reservoir can be withdrawn. (Minimum pool level)
21
23. Maximum Water Level (Max. WL): This is the maximum level in the
reservoir to which the water will rise. This is 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
Maximum Pool Level.
23
Zones in the Reservoir Storage
24. 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.
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.
24
Zones in the Reservoir Storage
25. Zones in the Reservoir Storage
Live Storage or Useful Storage: This is the storage available for the
intended purpose between normal pool level and minimum pool level of
the reservoir. This may also be termed as the volume of water actually
available at any time between the Dead Storage Level and Full Reservoir
Level.
Dead Storage Level (DSL): Below this level, there are no outlets to
drain the water in the reservoir by gravity. The volume of water below
the minimum pool level that cannot be utilized. 25
27. Zones in the Reservoir Storage
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.
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.
27
28. Zones in the Reservoir Storage
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.
28
29. 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 firm and secondary yield
over a long period of time.
29
30. Fixing the capacity of the Reservoirs
In general, storage capacities have to be designed based on certain specified
considerations, which have been discussed separately in the flowing Bureau of Indian
Standard codes:
IS:5477 Fixing the capacities of reservoirs
(part 1): 1999 General requirements
(part 2): 1994 Dead Storage
(part 3): 1969 Live Storage
(part 4): 1911 Flood Storage
30
31. Fixing the capacity of the Reservoirs
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
31
34. Computation of Reservoir capacity
Depends upon the inflow available and demand.
Inflow inthe river is always greater than the demand, there is storage
required.
If the inflow in the river is small but the demand is high, a large reservoir
capacity is required.
The required capacity for a reservoir can be determined by the following
methods:
1) Graphical method, using mass curves
2) Analytical method
34
35. Graphical Method (MASS CURVE)
Prepare a mass inflow curve from the flow hydrograph of the site for a
number of consecutive years including the most critical years (or the
driest years) when the discharge is low.
Prepare the mass demand curve corresponding to the given rate of
demand.
1) If the rate of demand is constant, the mass demand curve is a straight
line.
2) The scale of the mass demand curve should be the same as that of the
mass inflow curve.
3) Draw the lines AB, FG, etc. Such that (i) they are parallel to the mass
demand curve and (ii) they are tangential to the crests A, F, etc. of the
mass curve. 35
37. 4) Determine the vertical intercepts CD, HJ etc. between the tangential lines
and the mass inflow curve.
5) These intercepts indicate the volumes by which the inflow volumes fall
short of demand.
6) Assuming that the reservoir is full at point A, the inflow volume during
the period AE is equal to ordinate DE and the demand is equal to ordinate
CE.
7) Thus the storage required is equal to the volume indicated by the intercept
CD.
8) Determine the largest of the vertical intercepts found in step (4).
9) The largest vertical intercept represents the storage capacity required.
37
38. Prepare the mass inflow curve. Draw lines from a common origin,
representing demands at various rates, say ranging from 0 to 5000 ha-m
per year.
From the apices A1, A2, A3 etc. of the mass curve draw tangents in such a
way that their maximum departure from the mass curve does not exceed
the specified reservoir capacity.
38
Determination of Safe Yield from a
Reservoir of given Capacity
39. Thus in given figure the ordinates E1 D1, E2 D2, E3 D3 etc. are all equal
to the reservoir capacity (say 1500 ha-m).
Measure the slopes of each of these tangents this slopes indicate the yield
which can be attained in each year from the reservoir of given capacity.
The demand line if extended in forward direction should intersect the mass
curve.
39
Determination of Safe Yield from a
Reservoir of given Capacity
41. RESERVOIR SEDIMENTATION
Sediment is defined as the fragmental material either transported by
suspended in or deposited by the water.
All rivers carry silt during heavy rains due to erosion from catchments.
Disintegration, erosion, transportation and sedimentation are the different
stages leading to silting of reservoir.
It is a difficult problem for which an economical solution has not yet been
discovered, except by providing a “dead storage” to accommodate the
deposits during the life of the dam.
41
43. Factors Affecting Sedimentation
The quality of sediment depends upon the extent of erosion in the in
the catchment area, which depends upon following factors:
Nature and type of soil in catchment area
Topography of the catchment area
Cultivation or vegetation cover in catchment area
Intensity of rainfall in catchment area
43
45. Sediment Management
Maximum efforts should water should be released so that less sediments
should retain in reservoir are as follows:
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 enclosure pumped dry to
permit construction work below the waterline)
low height barriers / Low level outlets / sediment sluicing(sliding gate)
45
50. Procedure for calculation reservoir life
The useful life of reservoir is terminated when its capacity is reduced to
20% of the designed capacity.
This will occur over a time period when the dead storage is reduced by
siltation.
Probable rate of siltation should be considered in reservoir planning.
Knowing the inflow rate calculate the (capacity/inflow) ratio and obtain
the trap efficiency from the curve.
Divide the total capacity into any suitable interval, say 10%. Assuming the
10% capacity has been reduced due to sediment deposit, find the trap
efficiency for reduced capacity (i.e. 90% of the original) and the inflow
ratio. 50
51. For this interval of 10% capacity, find the average trap efficiency by
taking average if η found in step 2 and 3.
Determine the sediment inflow rate water samples and drying the
sediment.
Multiply the total annual sediment transported by the trap efficiency
found in step 3.
Divide the volume interval by the sediment deposited to get the number
of years to fill this volume interval of 10% capacity.
Repeat the procedure for further intervals of the capacity.
The total life of the reservoir will be equal to the total number of years
required to fill each of the volume intervals. 51
Procedure for calculation reservoir life
