PLANNING
OF RESERVOIR & DAM
DEFINITION AND TYPES:
When a barrier is constructed across some river in the form of a dam, water gets stored on the upstream side of the barrier, forming a pool of water, generally called a dam reservoir or an impounding reservoir or a river reservoir, or a storage reservoir.Depending upon the purpose served by a given
reservoir, the reservoir may be broadly divided into the following three types:
a. Storage or Conservation reservoir
b. Flood Control reservoir
c. Multipurpose reservoir
(a) Storage or
Conservation Reservoir: A city water supply, irrigation water supply,
or a hydroelectric project drawing water directly from a river or a stream may
fail to satisfy the consumer's demands during extremely low flows; while during
high flows, it may become difficult to carry out their operations due to
devastating floods. A storage or a
conservation reservoir can retain such excess supplies during periods of
peak flows and can release them gradually during low flows as and when the
need arises.
Incidentally,
in addition to conserving water for later use, the storage of flood waters may
also reduce flood damage below the reservoir. Hence, a reservoir can be used
for controlling floods either solely or in addition to other purposes. In the
former case, it is known as a ‘Flood
Control Reservoir’ or a ‘Single Purpose Flood Control Reservoir’; and in
the latter case, it is called a ‘Multipurpose
Reservoir’.
(b) Flood Control
Reservoir: A flood control reservoir, generally called a flood
mitigation reservoir. Stores a portion of the flood flows in such a way as to
minimize the flood peaks at the areas to be protected downstream. To accomplish
this, the entire inflow entering the reservoir is discharged till the outflow
reaches the safe capacity of the channel downstream. The inflow in excess of
this rate is stored in the reservoir, which is then gradually released, so as
to recover the storage capacity for the next flood.
Types of flood control reservoirs: There are two basic
types of flood-mitigation reservoirs; i.e.
i.
Storage reservoirs or Detention basins; and
ii.
Retarding basins or Retarding reservoirs
A reservoir having gates and valves installed at
its spillway and at its sluice outlets is known as a storage reservoir; while on the other hand, a reservoir with uncontrolled
and un-gated outlets is known as a retarding
basin.
Functioning
and Advantages of a Retarding Basin:
A retarding
basin is usually provided with an uncontrolled spillway and uncontrolled orifice-type sluiceways. The automatic regulation of outflow, depending upon
the availability of water, takes place from such a reservoir. The maximum discharging
capacity of such a reservoir should be equal to the maximum safe carrying
capacity of the channel downstream. As floods occur, the reservoir gets filled and discharges through sluiceways. As the reservoir elevation increases, the outflow
discharge increases. The water level goes on rising until the flood has subsided,
and the inflow becomes equal to or less than the outflow. After this, the water
gets automatically withdrawn from the reservoir until the stored water is completely
discharged. The advantages of a retarding basin over a gate-controlled detention
basin are:
(i) Cost of the
gate installation is saved.
(ii) There are no
gates and hence, the possibility of human error and negligence in their
operation is eliminated.
(iii) Since such a
reservoir is not always filled, much of the land below the maximum reservoir
level will be submerged only temporarily and occasionally and can be
successfully used for agriculture, although no permanent habitation can be
allowed on this land.
Functioning
and Advantages of a Storage Reservoir:
A storage reservoir
with gated spillway flexibility and gated sluiceways provides more operation, and flexibility of operation, and thus gives us better control and
increased usefulness of the reservoir. Storage reservoirs are, therefore,
preferred on large rivers, which require better control; while retarding basins
are preferred on small rivers. In storage reservoirs, the flood crest downstream can be better controlled and regulated properly, so as not to cause their
coincidence. This is the biggest advantage of such a reservoir and outweighs
its disadvantages of being costly and involving the risk of human error in the installation
and operation of gates.
(c) Multipurpose
Reservoirs: A reservoir planned
and constructed to serve not only one purpose but various purposes together cares a multipurpose reservoir. Reservoir, designed for one purpose, incidentally
serving other purposes, shall not be called a multipurpose reservoir, but will
be called so, only if designed to serve those purposes also in addition to its
main purpose. Hence, a reservoir designed to protect the downstream areas from
floods and also to conserve water for water supply, irrigation, industrial
needs, hydroelectric purposes, etc. shall be called a multipurpose. Bhakra Dam
and Nagarjuna Sagar Dam are important multipurpose projects of India.
(d) Distribution Reservoirs: A distribution reservoir is a small storage reservoir
constructed within a city water supply system. Such a reservoir can be filled
by pumping water at a certain rate and can be used to supply water even at
rates higher than the inflow rate during periods of maximum demand (called critical
periods of demand). Such reservoirs are, therefore, helpful in permitting the
pumps or the water treatment plants to work at a uniform rate, and they store
water during hours of no demand or less demand and supply water from their
‘storage’ during critical periods of maximum demand.
In this chapter, we shall, however, confine ourselves to the river reservoirs only.
SELECTION OF A SUITABLE SITE FOR A RESERVOIR:
It is almost
impossible to select a perfect ideal reservoir site. But its selection is
guided by the following factors:
- i. A suitable dam site is available. The cost of the dam is generally a controlling factor in the selection of a reservoir site.
ii. The
geological formations for the reservoir banks, walls, etc. should be such as to
entail minimum leakage.
iii. The
geology of the catchment area should be such as to entail minimum water losses
through absorption and percolation.
iv. The
site should be such that a deep reservoir is formed. A deep reservoir is
preferred to a shallow one, because of lower land cost per unit of capacity,
less evaporation loss, and less possibility of weed growth.
v. The
reservoir site must have adequate capacity.
vi. Too
many silt-laden tributaries should be avoided as far as possible.
vii. The reservoir basin should have a deep narrow
opening in the valley so that the length of the dam is minimum.
STORAGE ZONES OF A RESERVOIR:
These zones
are defined w. r. to Fig. 18.5
Normal Pool Level or Maximum Conservation
Level. It is the maximum
elevation to which the reservoir water surface will rise during normal operati.ng
conditions. (See Fig. 18.5), It is equivalent to the elevation of the spillway crest
or the top of the spillway gates, for most of the cases.
Minimum Pool Level. The lowest water surface elevation, which has to be
kept under normal operating conditions in a reservoir, is called the minimum pool
level (See Fig. 18.5). This level may be fixed by the elevation of the lowest outlet
in the dam or may be guided by the minimum head required for efficient functioning
of turbines.
Useful and Dead Storage. The volume of water stored in a reservoir between the
minimum pool and normal pool levels is called the useful storage. Water stored in the reservoir below the minimum
pool level is known as the Dead Storage,
and it is not of much use in the operation of the reservoirs. The useful
storage may be subdivided into conservation storage and flood mitigation
storage, in a multipurpose reservoir.
Maximum Pool Level or Full Reservoir Level. During high floods, water is discharged over the
spillway, but will cause the water level to rise in the reservoir: above the
normal pool level. The maximum level to which the water rises during the worst
design flood is known as the maximum pool level.
Surcharge Storage. The volume of water stored between the normal pool
level and the maximum pool level is called surcharge
storage. Surcharge storage is an uncontrolled storage, in the sense that it
exists only till the flood is in progress and cannot be retained for later use.
Bank storage. When the reservoir is filled up, certain amount of water
seeps into the permeable reservoir banks. This water comes out as soon as the
reservoir gets depleted. This volume of water is known as the bank storage, and
may amount to several percent of the reservoir volume depending upon the geological
formations. The bank storage effectively increases the capacity of the reservoir
above that indicated by the elevation capacity curve of the reservoir.
Valley Storage. Even before a darn is constructed, certain variable amount
of water is stored in the stream channel, called, valley storage. After the reservoir
is formed, the storage increases, and the actual net increase in the storage is
equal to the storage capacity of the reservoir minus the natural valley
storage. The valley storage thus reduces the effective storage capacity of a
reservoir. It is not of much importance in conservation reservoirs, but the
available storage for flood mitigation is reduced, as given by the following
relation:
Effective storage
for flood mitigation
CATCHMENT YIELD AND RESERVOIR YIELD:
Long-range
runoff from a catchment is known as the yield of the catchment. Generally, a
period of one year is considered for determining the yield value. The total
yearly runoff, expressed as the volume of water entering/passing the outlet point
of the catchment, is thus known as the catchment
yield and is expressed in M.m3 or M.ha.m.
The annual
yield of the catchment up to the site of a reservoir, located at the given point
along a river, will thus indicate the quantum of water that will annually enter
the reservoir, and will thus help in designing the capacity of the reservoir. This
will also help to fix the outflows from the reservoir since the outflows are dependent
upon the inflows and the reservoir losses.
The amount of
water that can be drawn from a reservoir, in any specified time interval,
called the reservoir yield, naturally depends upon the inflow into the reservoir
and the reservoir losses, consisting of reservoir leakage and reservoir evaporation.
The annual
inflow to the reservoir, i.e. tine catchment yield, is represented by the mass
curve of inflow; whereas, the outflow from the reservoir, called the reservoir
yield, is represented by the mass demand line or the mass curve of outflow. Both
these curves decide the reservoir capacity, provided the reservoir losses are ignored
or separately accounted
The inflows
to the reservoir are, however, quite susceptible to variation in different
years, and may therefore vary throughout the prospective life of the reservoir. The past available data of
rainfall or runoff in the catchment is therefore used to work out the optimum value
of the catchment yield. Say for example, in the past available records of say 35
years, the minimum yield from the catchment in the worst rainfall year may be
as low as say 100 M.ha.m; whereas, the maximum yield in the best rainfall year
may be as high as say 200 M.ha.m. The question which then arises would be whether the reservoir capacity should correspond to 100 M.ha.m yield or 200 M.ha.m
yield. If the reservoir capacity is provided corresponding to 100 M. ha.m yield,
then eventually the reservoir will be fitted up every year with dependability
of 100%; but if the capacity is provided corresponding to 200 M.ha.m yield,
then eventually the reservoir, will be filled up only in the best rainfall year
(i.e. once in 35 years) with a dependability of about
In order to
obtain a sweet agreement, a via media is generally adopted and an intermediate
dependability percentage value (p), such as 50% to 75% may be used to compute
the dependable yield or the design yield.
The yield which corresponds to the worst or the most critical year on record is
however, called the firm yield or the
safe yield. Water available in excess of the firm yield during years of
higher inflows, is designated as the secondary
yield. Hydropower may be developed from such secondary water, and sold to
the industries ‘on and when available basis’. The power commitments to domestic
consumers must, however, be based on the firm basis, and should not exceed the
power which can be produced with the firm yield, unless thermal power is also
available to support the hydroelectric power.
The
arithmetic average of the firm yield and the secondary yield is called the average yield.
FIXATION OF RESERVOIR CAPACITY WITH THE HELP OF
MASS CURVES OF INFLOW AND OUTFLOW:
After the
flow hydrographs for the stream at the dam site have been plotted for a large
number of years (say 25 to 30 years), the required storage capacity for a
reservoir with a given outflow pattern can be approximately calculated with the
help of mass curves. A hydrograph is a plot of discharge vs. time, while a mass
curve is a plot of accumulated flow vs. time. The area under the hydrograph between
times t = 0 and t = t will represent nothing but the accumulated flow up to the
time t, and hence, the ordinate of the mass curve at time t.
A typical
annual inflow hydrograph is shown in Fig.18.8.(a), and the mass curve for this
inflow hydrograph is plotted in Fig. 18.8.(b). The area under the first curve
upto a time (t) is equal to the ordinate of the second curve at the same time (t).
Adjustments for the scales and units of the two curves must be made while
plotting.
It is evident
that a mass curve will continuously rise, as it is the plot of the accumulated
inflow would be represented by the horizontal lines on the mass inflow curve.
To differentiate such a mass curve of runoff from the mass curve of rainfall,
this mass curve is usually called as the flow
mass curve and is an integral of the flow hydrograph.
The mass curve
may also be called the ripple diagram. The slope of the mass curve at any time
is a measure of the inflow rate at that time.
After the
inflow mass curve has been plotted, the mass curve of demand may also be
plotted by accumulating the required outflow. If a constant rate of withdrawal
is required from the reservoir, the mass curve of demand will be a straight
line having a slope equal to the demand rate. Demand curves or demand lines are
generally straight lines (representing uniform withdrawal) although, in
practice, they may be curved also [See Fig. 18.9 (a) and (b)].
Determining Reservoir Capacity for a Given Demand:
The mass curve of
inflow and the demand line can be used to determine the required storage
capacity. In Fig. 18.10, it is evident that the demand lines drawn tangent to
the high points A1, A2, A3... of the mass curve,
represent the rate of withdrawal from the reservoir. Assuming the reservoir to
be full whenever the demand line intersects the mass curve (points F1,
F2...), the maximum departure (B1C1, B2C2...)
between the two curves represents the reservoir capacity just required to
satisfy the demand.
Fig. 18.10
has been drawn for a demand of 2,000 hectare- metres/year. The biggest departure
ordinate (i.e. the maximum of B1C1, B2C2...)
works out to be 1,950 hectare-metres/year, which represents the required
storage capacity for the reservoir.
The vertical
distance between the successive tangents A1B1, and A2B2,
etc., represent the water wasted over the spillway. The spillway must have
sufficient capacity to discharge this flood volume.
For Fig.
18.10, the spillway capacity works out to be 600 hectare-metres, and the
reservoir capacity as 1,950 hectare-metres. It can also be observed that:
i.
Assuming
the reservoir to be full at A1, it is depleted to 1950 - 1500 = 450
ha-m at C1 and is again full at F1.
ii.
The
reservoir is full between F1 and A2, and the quantity of
water spilled over the spillway is equal to 600 ha-m
iii.
From
A2, the water starts reducing in the reservoir till it becomes fully
empty at C2.
iv.
The
water again starts collecting in the reservoir and it is again full at F2.
Note 1. It may
also be noted that a demand intersect line, when extended, must the mass curve.
If it does not, the reservoir will not refill.
Note 2. When the
demand curve is not a straight line, then the two mass curves are superimposed
over each other in such a way that their origins and axis coincide. The larger ordinate
between the two, gives the required storage capacity, as shown in Fig 18.11.
Fixing the Demand for a Reservoir of a Given Capacity:
In the previous
article, we have explained how the reservoir capacity can be determined
for a given demand. The reverse, i.e. fixing the demand for a given reservoir
capacity, may also be done with the help of a mass curve of inflow. In this case,
the tangents are drawn to the high points (A1, A2,..) of
the mass inflow curve in such a way that the maximum departure from the mass
curve is equal to the reservoir capacity. The slopes of the lines so drawn
represent the demand rate that can be obtained with this capacity during different
periods. The minimum value of these slopes will represent the withdrawal rate,
which can certainly be obtained from the given reservoir, and will, thus,
represent its firm yield.
Problem
(1) The amounts of water flowing from a certain
catchment area at the proposed dam site are tabulated as follows. Determine:
(7 mark) May
2015
i.
The minimum
capacity of the reservoir if the water is to be used to feed the turbines of the hydropower plant at a uniform rate and no water is to be spilled over.
ii.
The initial
storage required to maintain the uniform demand as above.
|
Month |
Jan |
Feb |
Mar |
Apr |
May |
Jun |
Jul |
Aug |
Sep |
Oct |
Nov |
Dec |
|
Inflow x105m3 |
2.83 |
4.25 |
5.66 |
18.4 |
22.64 |
22.64 |
19.81 |
8.49 |
7.10 |
7.10 |
5.65 |
5.66 |
Answer: The computation are done in a tabular form below
Average demand =
This value has
been entered in column (3) of the below table.
|
Month |
Inflow (x105m3) |
Av.
Demand (x105m3) |
Deficit (x105m3) |
Surplus (x105m3) |
|
1 |
2 |
3 |
4
= 3 - 2 |
5
= 2 - 3 |
|
January |
2.83 |
10.8533 |
8.0233 |
|
|
February |
4.25 |
10.8533 |
6.6033 |
|
|
March |
5.66 |
10.8533 |
5.1933 |
|
|
April |
18.40 |
10.8533 |
|
7.5467 |
|
May |
22.64 |
10.8533 |
|
11.7867 |
|
June |
22.64 |
10.8533 |
|
11.7867 |
|
July |
19.81 |
10.8533 |
|
8.9567 |
|
August |
8.49 |
10.8533 |
2.3633 |
|
|
September |
7.10 |
10.8533 |
3.7533 |
|
|
October |
7.10 |
10.8533 |
3.7533 |
|
|
November |
5.66 |
10.8533 |
5.1933 |
|
|
December |
5.66 |
10.8533 |
5.1933 |
|
|
Sum |
130.24x105 |
130.24x105 |
|
40.0766x105 |
Since no
water is to be spilled, the minimum capacity will be equal to the sum of the
surplus water.
1.6 RESERVOIR SEDIMENTATION:
Every river
carries a certain amount of sediment load. The sediment particles try to settle
down to the river bottom due to the gravitational force, but may be kept in
suspension due to the upward currents in the turbulent flow which may overcome
the gravity force. Due to these reasons, the river carries fine sediment in
suspension as suspended load, and larger solids along the river bed as bed
load. When the silt loaded water reaches a reservoir in the vicinity of a dam,
the velocity and the turbulence are considerably reduced. The bigger suspended
particles and most of the bed load, therefore, gets deposited in the head
reaches of the reservoir. Fine particles may travel some more distance and may
finally deposit farther down in the reservoir, as shown in Fig. 18.24. Some
very fine particles may remain in suspension for much longer period, and may
finally escape from the dam along with the water discharged through the
sluiceways, turbines, spillway etc.
The
deposition of sediment in the reservoir is known as 'Reservoir Silting’ or ‘Reservoir
Sedimentation’.
The
deposition of the sediment will automatically reduce the water storing capacity
of the reservoir, and if this process of deposition continues longer, a stage
is likely to reach when the whole reservoir may get silted up and become useless.
Moreover,
with the passage of time, the reservoir capacity will go on reducing. Thus, if
today at the time of construction, a reservoir can store 10,000 cubic metres of
water, tomorrow say after five years, it may be able to store only say 8,000
cubic metres of water. Therefore, in order to see that the capacity does not
requirement fall short of ever during the design period, we must take this
silting into account.
The total volume
of silt likely to be deposited during the designed life period of the dam is, therefore,
estimated; and approximately that much of volume is left unused to allow for
silting, and is known as dead storage.
The remainder is known as the effective storage or the live storage. The dead storage generally varies between l5 to 25%
of the total capacity. For example in Bhakra dam, the gross capacity of the dam
is 9,344 million cubic metres and the dead storage provided is 2,054 million
cubic metres. All the outlets fetching water from the reservoir are provided above
the dead storage level.
The
importance of this silting can be understood by considering the following
example:
Let the total
capacity of a reservoir be 30 million cubic metres and the provision of dead
storage be 6 million cubic metres. Let the average volume of sediment deposition
be 0.15 million cubic metres per year. Then it is evident, that the dead storage
will be filled up in
Hence, the
usefulness of this reservoir would start reducing after 40 years, and after 200
years it would be nothing but a collection of sand and sediment with no water
in it, provided the siltation rate remains constant at 0.15 M.m3/yr.
1.6.1. Density Currents.
In a reservoir,
the coarser sediment settles down along the bottom of the reservoir, as the muddy
flow approaches the reservoir; while the finer sediment usually remains in
suspension, and moves in a separate layer than the clear reservoir water, as
shown in Fig.18.24. This layer of water
containing the fine sediment moves below the upper clearer reservoir water as a
density current is slightly more than the density of main body of the reservoir
water. Because of their density difference, the water of the density current
does not mix easily with the reservoir water, and maintains its identity for a
considerable time. The density current can thus be removed through the dam
sluiceways, if they are located properly and at the levels of the density
current. A lot of sediment load can, thus, be passed out of the reservoir, if
it is possible to locate the dam outlets and sluiceways in such a fashion, as
to vent out the density currents. Trap
efficiency of reservoirs may thus be decreased by about 2 to 10%, if it is
possible to vent such density currents through the outlets and sluiceways of
the dams.
1.6.2. Trap Efficiency. Now, we introduce another very important term called
Trap efficiency. Trap efficiency is defined as the percentage of the sediment deposited
in the reservoir even inspite of taking precautions and measures to control its
deposition.
Therefore,
Trap Efficiency (η)
Most of the
reservoirs trap 95 to 100% of the sediment load flowing into them. Even if various
feasible silt control measures are adopted, it has not been possible to reduce
this trap efficiency below 90% or so.
Problem (1) As impounding reservoir had an original storage
capacity for 738 ha-m. The drainage area of the reservoir is 80 sq.km. from
which, annual sediment discharges into the reservoir at the rate 0.1153 ha-m
per sq.km. of the drainage area. Assuming the trap efficiency as 80 percent,
find the annual capacity loss of the reservoir in percent per year. (6 mark) Dec
2015 (Old)
Answer: Given
Reservoir capacity = 738
ha-m
Drainage area = 80 sq.km
Annual sediment discharge =
0.1153 ha-m per sq.km
Trap efficiency = 80% = 0.80
We know
1.6.3. Capacity Inflow Ratio. The ratio of the reservoir capacity to, the total
inflow of water in it, is known as the capacity-inflow ratio. It is a very important
factor, because the trap efficiency (η) has been found to be a function of capacity-inflow
ratio i.e.
The graph
obtained for the existing reservoirs between trap efficiency and log of
It is evident
from the above curve that if capacity reduces (with constant inflow), trap efficiency
reduces, and hence, lesser sediment is trapped. Therefore, the silting rate in the
reservoir shall be more in the beginning, and as its capacity reduces due to
silting, the silting rate will reduce. Hence, the complete reservoir silting may
take longer period.
It can also
be concluded that for small reservoir (having small capacity) on large rivers (having
large inflow rates), the trap efficiency is extremely low, because the capacity
inflow ratio is very small. Such reservoirs silt very little and most of their sediment
is passed downstream. On the other hand, large reservoirs on smaller rivers
shall silt tremendously and almost complete deposition of sediment may take
place.
1.6.4. Silting Control in Reservoirs: In order to increase the life of a reservoir it is
necessary to control the deposition of sediment. Various measures are
undertaken in order to achieve this aim. The various methods which are adopted
can be divided into two parts:
(a) Pre-constructing
measures; and (b) Post-constructing measures.
These measures are
discussed below:
a) Pre-constructing measure. They are those measures before which are adopted and
during the execution of the project. They are innumerated below :
(i) Selection of Dam
site. The silting depends upon the amount of erosion from the catchment If the
catchment is less erodible, the silting will be less. Hence, the silting can be
reduced by choosing the reservoir site in such a way as to exclude the runoff
from the easily erodible catchment.
(ii) Construction
of the Dam in Stages. The design capacity plays an important role in the
sitting of a reservoir. When the storage capacity is much less than the average
annual runoff entering the reservoir, a large amount of water will get out of the
reservoir, thereby, reducing the silting rate compared to what it would have
been if the entire water would have been stored. Therefore, the life of a reservoir
can be prolonged by constructing the dam in stages. In other words, first of
all, the dam should be built lower, and raised subsequently when some of its
capacity gets silted up.
(iii) Construction
of Check Dams. The sediment inflow can be controlled by building check dams
across the river streams contributing major sediment load. These are smaller
dams and trap large amounts of coarser sediments. They are quite expensive.
(iv) Vegetation Screens.
This is based on the principle that vegetations trap large amounts of sediment.
The vegetation growth is, therefore, promoted at the entrance of the reservoir
as at the entrance of the reservoir as well as in the catchment. These
vegetative covers, through which flood waters have to pass before entering the
reservoirs, are known as vegetation screens, and provide a cheap and a good
method of silt control.
(v) Construction
of Under-sluices in the Dam. The dam is provided with openings in its base, so
as to remove tire more silted water on the downstream side.
The sediment
concentration will be more at some levers than at others. There-fore, sluices
are located at the levels of higher sediment concentration. The method in
itself, is not sufficient because the water digs out a channel behind the sluice
for movement and leaves most of the sediment undisturbed. Therefore, this is simultaneously
supplemented with mechanical loosening and scouring of the neighbouring
sediment in order to increase its effectiveness. But to provide large sluices
near the bottom of the dam, is again a structural problem. The use of this
method is, therefore, limited.
(b) Post constructing Measures. These measures are undertaken during the operation of
the project. They are given below:
(i) Removal of
Post Flood Water. The sediment content increases just after the floods;
therefore, attempts are generally made not to collect this water. Hence, the
efforts should be made to remove the water entering the reservoir at this time.
(ii) Mechanical
stirring of the Sediment. The deposited sediment is scoured and disturbed by
mechanical means, so as to keep it in a moving state, and thus, help in pushing
it towards the sluices.
(iii) Erosion control
and soil conservation. This includes all those general methods which are
adopted to reduce erosion of soil and to make it more and more stable. This
method is the most effective method for controlling siltation, because when the
soil erosion is reduced, the sedimentation problem is reduced automatically.
But the methods of treating the catchment in order to minimise erosion are very
costly. It has been estimated that the investment required for treating 16% of
the Indian catchment area is more then Rs. 2,000 crores. In India, only 1.5% of
the catchment area has been treated to minimise silting.
1.7. RESERVOIR SEDIMENTATION STUDIES ON EXISTING
RESERVOIRS:
Sedimentation
of storage reservoirs is a
natural process, since large part of the silt eroded from the catchment and
transported by the river, gets deposited on the bed of the reservoir. This
causes reduction in the live as well as dead storage capacities of the
reservoir. Progressive loss of capacity due to sediment accumulation results in
reduced benefits and may even cause operational problems. It therefore, becomes
necessary to monitor the sedimentation rates in the existing reservoirs at regular
intervals, to help in planning and executing suitable remedial measures for controlling
sedimentation in order to prolong the life of the reservoir and. its benefits
Conventional
hydro-graphic surveys are conducted at, regular intervals at the existing
reservoirs to determine the available capacities at different elevations, and
to help compute the sedimentation volume at such regular intervals. Such conventional
surveys will require computation of water spread areas at different water
levels, and is quite a tedious and a costly process. Remote sensing techniques do offer a modern answer to the costly
conventional surveys, as it offers a great potential for application in capacity
evaluation of medium to large reservoirs. From the data provided by the remote
sensing satellites, it has now been possible to compute loss of reservoir
capacity due to sedimentation, and its distribution. The results obtained from
this technique have been found to be quite comparable with those obtained from
the costly and cumbersome conventional methods. One of the greatest advantage of
this technique is that the capacity evaluation could be easily computed on
yearly basis.
The methodology
involved in this technique requires the use and analysis of the satellite
imageries provided by the Remote Sensing satellites, which collect the data of
the Earth surface features in different bands at regular intervals. In the case
of Indian Remote Sensing Satellites IRS-1A and IRS-1B (both identical satellites),
this periodicity is 22 days for Indian sub-continent. These two satellites together
are thus capable to provide us data of all our reservoirs at 11 days interval.
The third satellite of this series IRS-1C had also been launched on 28.01.1996
and helps in providing better pictures even of cloud bound areas. IRS-1D was
further launched on 29.09.1997, through our first Indian rocket launcher.
Due to the
water withdrawals from an existing reservoir, its water spread area goes on
changing throughout the year. The reservoirs are generally full just after the
mansoon period in October, and get depleted to almost, dead storage/minimum drawdown
level just before start of monsoon season in May or June, every year. The
satellite data of various dates during the period from October to May, provide us
an array of water spread areas between maximum water level (i.e. around the FRL)
and the minimum reservoir level (i.e. around the dead storage level minimum drawdown
level). From the whole set of the satellite data, a few of them which are cloud
free and of good quality and representative of the whole range of reservoir levels
at close intervals, are selected for analysis.
The method of
analysis depends upon the data products. The selected CCT's/FCC's of various
dates are analysed for determining the water spread areas. The corresponding
water levels are obtained from the daily gauge record of the reservoir. From these,
the water volumes between two consecutive water levels are computed using
Prismoidal or any appropriate formula. Volume of water below the minimum water
level (as recorded by the satellite) and the “new zero” elevation, are
estimated based upon the previous hydro-graphic surveys. In case, these information’s
are not available in the hydro-graphic survey data, then the elevation-area relationship
obtained from the hydro-graphic surveys as well as the one obtained from the
satellite data interpretation, should be extended to get the new zero elevation,
and then the volume between the minimum mapped water level and this new zero
level is estimated. After this, the cumulative water volume at each reservoir
level is computed, and then the revised elevation-area-capacity curve is drawn.
By comparing the original area-capacity curve or any other such curve, the total
sediment volume and its distribution can be computed.
1.8. RESERVOIR LOSSES:
Huge quantity of
water is generally lost from an impounding reservoir due to evaporation,
absorption and percolation. Depending upon which, the following losses may
occur from such a reservoir:
a.
Evaporation losses
b. Absorption
losses
c.
Percolation losses or Reservoir leakage
These losses are
discussed below
a. Evaporation losses: The evaporation losses from a reservoir depend upon several
factors, such as: water surface area, water depth, humidity, wind velocity,
temperature, atmospheric pressure and quality of water. The evaporation loss
from a reservoir under the given atmospheric conditions can be easily estimated
by measuring the standard pan evaporation and multiplying the same by the pan
coefficient. The evaporation losses become very significant in a hot and humid
country like India; and realistic estimation of these losses is quite
important. These losses in fact vary from place to place and from season to
season, and hence monthly values of these losses are usually determined.
On the basis of a review conducted on 130 sample
reservoirs, the Central Water Commission, in 1990, has, however, estimated the average
annual evaporation loss to be 225 cm; and the total water lost from all the
existing reservoirs to be 27000 Mm3 per annum. What a tremendous
waste of precious water!
In order to
control such large scale wastage of water, several methods have been devised by
engineers and scientists. All these methods are based upon the efforts made to
reduce the evaporation rate from the surface of the water bodies by physical or
chemical means, since the basic meteorological factors affecting evaporation cannot
be controlled under normal conditions. The following methods are generally used
far evaporation control:
1. Wind
breakers
2. Covering
of the water surface
3. Reduction of
the exposed water surface
4. Use of underground
storage rather than the use of surface storage
5. Integrated
operation of reservoirs
6. Use of chemicals
for retarding the evaporation rate from the reservoir surface.
Out of all
these methods, the last method has evoked the maximum response from all over
the world, and has been considered to be the only practical solution for
conservation of fresh water, inspite of its various limitations and
disadvantages in high cost of application in normal conditions. The use of chemicals,
called Water Evapo-Retardants (WERs), for controlling the evaporation rate from
the surface of reservoirs is therefore, discussed here in details.
A non toxic chemical,
capable of forming a thin monomolecular film over the water surface, is
generally spread river the reservoir water surface in powder, liquid or
emulsion form. The resulting film prevents energy inputs from the atmosphere,
thus reducing evaporation. Such a film, however, allows the passage of enough
air through it, to avoid any harmful effects on the aquatic life due to
shortage of oxygen.
Fatty
alcohols of different grades like: Cetyl
alcohol (C16.H33.OH) popularly called hexa decanol, Stearyl alcohol (C18.H37.OH)
popularly called Octadecanol, and Behenyl
alcohol (C22.H45.OH) called docosanol, or a mixture
of these chemicals, have been generally used and found to be quite suitable.
These chemicals should, however be 99% pure for getting the desired properties
of monolayer. National Chemical Laboratory, Pune, has developed one more
compound by synthesising alkoxy ethanols.
In general,
all such chemical compounds should possess the following properties:
(i) the chemical
compound (WER) should be tasteless, odourless, non-toxic, non-inflammable, and
should not produce any effect on the quality of water.
(ii) the
chemical should easily spread and form an even compact cohesive and efficient
monomolecular film on the water surface.
(iii) the
thin film formed by the chemical should be pervious to oxygen and carbon dioxide,
but tight enough to prevent escape of water molecules.
(iv) the thin
film formed by the chemical should be durable, and should be able to re-seal
itself, when broken due to external disturbances such as wind, waves, etc.
(v) the chemical
and the film formed by it should not be adversely affected by the water borne
bacteria, proteins and other impurities present in the water body.
The use of
chemical WERs has, however not been found to be cost effective for mass scale
use, and has further not been found to be suitable under the following conditions
:
(a) when the
wind velocities exceed 10 km/hr or so.
(b) when the
temperature rises above 400C or so.
(c) when the
size of the water body is relatively large.
Development
of cheaper WERs capable of withstanding higher wind speeds upto about 20 km/hr
and having strong cohesive forces and properties of self spreading and
re-uniting to maintain the monolayer in resilient state even at high wind
velocities, is therefore of vital importance. Moreover, the life of the film formed,
must be longer, so as to reduce the frequency of application to about 3 to 7
days from its present frequency of 24 hours. Development of such chemical WERs is
the subject matter of present research.
Other long
term evaporation control measures like plantation of trees to act as wind breakers,
reduction of exposed, water surface by covers, underground storage of water,
integrated operation of reservoirs etc. have been employed in some parts of the
country. The effectiveness and economics of these methods are, however, yet to
be established.
In India, the
water conservation methods are presently being adopted only in draught prone and
scarcity areas, since large scale use of such methods on all the reservoirs of
the country is not found to be economical or practically unfeasible due to
their large size and adverse meteorological factors.
b. Absorption Losses: These losses do
not play any significant role in planning, since their amount, though sometimes
large in the beginning, falls considerably as the pores get saturated. They certainly
depend upon the type of soil forming the reservoir.
c. Percolation Losses or Reservoir Leakage. For most of the reservoirs, the banks are permeable
but the permeability is so low that the leakage is of no importance. But in
certain particular cases, when the walls of the reservoir are made of badly fractured
rocks or having continuous seams of porous strata, serious leakage may occur.
Sometimes, pressure grouting may have to be used to seal the fractured rocks.
The cost of grouting has to be accounted in the economic studies of the
project, if the leakage is large.
1.9. ECONOMIC HEIGHT OF DAM:
The economic height of a dam is that height
of the dam, corresponding to which, the cost
of the dam per unit of storage is minimum. For this purpose, the estimates
are prepared for construction costs, for several heights of the dam, somewhat
above and below the level at which the elevation-storage curve shows a fairly
high rate of increase of storage per unit rise of elevation, keeping the length
of the dam moderate. The construction cost is found to increase with the dam height,
as shown in Fig. 18.26.
For each dam
height, the reservoir storage is known from the reservoir capacity curve. The
construction cost per unit of storage for all the possible dam heights can then
be worked out and plotted, as shown in Fig. 18.27.
The lowest
point A on this curve, gives the dam height for which the cost per unit of
storage is minimum, and hence, most economical.
DAMS
DEFINITION AND USES/NECESSITY OF DAMS:
A dam may be
defined as an obstruction or a barrier built across a stream or a river. At the
back of this barrier, water gets collected, forming a pool of water. The side
on which water gets collected is called the upstream side, and the other side
of the barrier is called the downstream side. The lake of water which is formed
upstream is often called a reservoir, or a dam reservoir, or a river reservoir,
or a storage reservoir.
The water
collected in this reservoir can be supplied for irrigating farm lands through a
system of canal network, or may be supplied for drinking purposes. The lake so
formed can be used for recreation uses. The energy of this collected water can be
used to turn a mill to grind wheat or to turn the blades of a turbine to
generate electrical power. And in times of floods, the dams can serve as
protections for the towns and cities farther down the river.
Apart from
these numerous advantages and uses (such as navigation, irrigation, electricity,
flood control, etc.) of a dam, it sometimes helps us in planning war strategy
and help us in controlling the advancement of enemies and their forces. Dams
have been frequently opened in times of war. The Dutch breached their dikes
during Second world war, to bedevil the invading Germans. Chinese used to destroy
their dikes to flood out the enemy. Russian army retreating from the Nazi
marauders, partly destroyed the famous Dneprostroi Dam in the Ukraine to keep its
power plant from falling into the hands of Hitler’s men.
VARIOUS TYPES OF DAMS:
Before we
describe some of the famous dams of the world, it is worth while to classify
the various types of dams.
Most
engineers recognise seven general types of dams. Three of them are ancient in origin,
and four have come into general use only in the last about 100 years or so.
The three older
types of dams are:
1) Earth Dams
2) Rock-fill
dams
3) Solid masonry
gravity dams.
These three
types of dams were all found in ancient days. In recent times, four other types
of dams have come into practice. They are:
4) Hollow
masonry gravity dams
5) Timber
dams
6) Steel dams
7) Arch dams
These types of dams
are discussed below:
1.11.1 Earth Dams. Earth dams are made of soil that is pounded down solidly.
They are built in areas where the foundation is not strong enough to bear the weight
of a concrete dam, and where earth is more easily available as a building material
compared to concrete or stone or rock.
Some
important earth dams of the world are:
(i) Green
mountain dam on Colorado river in U.S.A.
(ii) Swift
dam in Washington in U.S.A.
(iii) Side
flanks of Nagarjun Sagar dam in India.
(iv) Trinity
Dam in California in U.S.A.
(v) Maithan
Dam in India (which is partly Earthen and partly Rockfill).
1.11.2 Rock-fill Dams. Rock-fill are formed of loose rocks and boulders
piled in the river bed. A slab of reinforced concrete is often laid across the
upstream face of a rock-fill dam to make it water-tight.
Some
important rock-fill dams of the world are;
(i) The Salt
Springs Dam in California (345’ high) in U.S.A.
(ii) The San
Gabriel No. 1 Dam (321’ high) in U.S.A.
(iii) Cougar
Dam on Mc-Knezie River in Oregon (445’ high) in U.S.A.
1.11.3 Solid masonry Gravity dams. These are familiar to us by now, after we have talked
about Aswan, Roosevelt, Hoover, and above all Bhakra dam.
These big
dams are expensive to be built but are more durable and solid than earth and
rock dams. They can be constructed on any dam site, where there is a natural
foundation strong enough to bear the great weight of the dam.
1.11.4 The hollow masonry gravity dams. These are essentially designed on the same lines on
which the solid masonry gravity dams are designed. But they contain less concrete
or masonry; about 35 to 40% or so. Generally, the weight of water is carried by
a deck or R.C.C. or by arches that share the weight burden. They are difficult
to build and are adopted only if very skilled labour is easily available;
otherwise the labour cost is too high to build its complex structure.
1.11.5 Steel dams. These are not used for major works. Today, steel dams
are used as temporary coffer dams needed for the construction of permanent dams.
Steel coffer dams are usually reinforced with timber or earthfill.
1.11.6 Timber dams. These are short lived, since in a few years time,
rotting sets in. Their life is not more than 30 to 40 years and must have
regular maintenance during that time. However they are valuable in agricultural
areas, where a cattle raiser may need a pool for his live stock to drink from,
and for meeting other such low-level needs.
1.11.7 Arch dams. Arch dams are very complex and complicated. They make
use of the horizontal arch action in place of weight to hold back the water.
They are best suited at sites where the dam must be extremely high and narrow.
Some examples are :
(i) Sautet dam
on the Drac River in France, 414’ high, but only 230’ long at top and 85’, long
at bottom of the gorge, 56’, thick at bottom and 8’ thick at top.
(ii) The
Tignes dam in France (592’ high).
(iii) Mauvoisin
dam on the Drause River in switzerland, (780’ high).
(iv) Idduki
dam in Kerala State, across the Periyar river, which is the only arch dam in
India. It is 366 m (1200’) long double curvature arch dam, made in concrete, and
has a height of about 170 m (560’).
1.12 SELECTION OF THE TYPE OF DAM AND THEIR
CLASSIFICATIONS:
Dams can be
classified in various ways depending upon the purpose of the classification.
(1) Classification According to the Material used for
Dam Construction:
The dams
classified according to the material used for construction are: Solid masonry
gravity dams, Earthen dams, Rockfill darns, Hollow masonry gravity dams, Timber
dams, Steel dams, and R.C.C. Arch dams. They have already been explained in a
previous article.
(2) Classification According to Use
(i) Storage Dams.
They are constructed in order to store water during the periods of surplus
water supply, to be used later during the periods of deficient supply. The
stored water may be used in different seasons and for different uses. They may
be further classified depending upon the specific use of this water, such as
navigation, recreation, water supply, fish, electricity, etc.
(ii) Diversion
Dams. These small dams are used to raise the river water level, in order to
feed an off-taking canal and or some other conveyance systems. They are very
useful as irrigation development works. A diversion dam is generally called a
weir or a barrage.
(iii) The
Detention Dams. They detain food-waters temporarily so as to retard flood
runoff and thus minimise the bad effects of sudden flood. Detention dams are
sometimes constructed to trap sediment. They are often called debris dams.
(3) Classification According to Hydraulic Designs
(i) Overflow Dams.
They are designed to pass the surplus water over their crest. They are often
called Spillways. They should be made of materials which will not be eroded by
such discharges.
(ii) Non-overflow
Dams. They are those which are not designed to be overtopped. This type of
design gives us wider choice of materials including earthfill and rockfill
dams.
Many a times,
the overflow dam and the non-overflow dam are combined together to form a
composite single structure.
(iii) Rigid Dams
and Non-rigid Dams. Rigid dams are those which are constructed of rigid
materials like masonry, concrete, steel, timber, etc.; while non-rigid dams are
constructed of earth and rock-fill. They have already been explained.
1.13 FACTORS GOVERNING THE SELECTION OF A PARTICULAR
TYPE OF DAM
Whenever we
decide to construct a dam at a particular place, the first baffling problem
which faces us, is to choose the kind of the dam. Which type will be the most
suitable and most economical? Two, three kinds of dams may be technically feasible,
but only one of them will be the most economical. Various designs and their
estimates have to be prepared before signalling one particular type. The various
factors which must be thoroughly considered before selecting one particular type
are described below:
(1) Topography. Topography dictates the first choice of the type of
dam. For example:
(i) A narrow
U-shaped valley, i.e. a narrow stream flowing between high rocky walls, would
suggest a concrete overflow dam.
(ii) A low,
rolling plain country, would naturally suggest an earth fill dam with a
separate spillway.
(iii) The
availability of a ‘Spillway Site’ is very important while selecting a particular
kind of a dam.
(iv) A narrow V-shaped
valley indicates the choice of an arch dam. It is preferable to have the top
width of the valley less than one-fourth of its height. But a separate site for
the spillway must also be available.
(2) Geology and Foundation Conditions. The foundations have to carry weight of the dam. The
dam site must be thoroughly surveyed by geologist, so as to detect the
thickness of the foundation strata, presence of faults, fissured materials, and
their permeability, slope, and slip, etc.
The various kinds
of foundations generally encountered are discussed below:
(i) Solid Rock
Foundations. Solid rock foundations such as granite, gneiss, etc. have a strong
bearing power. They offer high resistance to erosion and percolation. Almost
every kind of dam can be buiit on such foundations. Sometimes, seams and fractures
are present in these rocks. They must be grouted and sealed properly.
(ii) Gravel Foundations.
Coarse sands and gravels are unable to bear the weight of high concrete gravity
dams and are suitable for earthen and rock-fill dams. Low concrete gravity dams
up to a height of 1b m may also be suggested on such foundations.
These
foundations have high permeability and, therefore, subjected to water percolation
at high rates. Suitable cut-offs must be provided to avoid danger of undermining.
(iii) Silt and
Fine Sand Foundations. They suggest the adoption of earth dams or very low
gravity dams (upto height of 8 m). A rockfill dam on such a foundation is not
suitable. Seepage through such a foundation may be excessive. Settlement may
also be a problem. They must be properly designed to avoid such dangers. The protection
of foundations at the downstream toe from erosion must also be ensured.
(iv) Clay Foundations.
Unconsolidated and high moisture clays are likely to cause enormous settlement
of the dam. They are not fit for concrete gravity dams or for rock-fill dams.
They may be accepted for earthen dams, but that too, after special treatment.
(v) Non-uniform
Foundations. At certain places, a uniform foundation of the types described
above may not be available. In such a case, a non-uniform foundation of rock
and soft material may have to be used if the dam is to be built. Such unsatisfactory
conditions have to be dealt with by special designs. However, every problem is
an individual problem and a solution has to be found by experienced engineers. For
example- The Jawahar Sagar Dam in Rajasthan offered such a problem. A bed of
clay was encountered, between the base of the dam and solid rock foundation. It
was not economically feasible to remove this clay bed. The solution adopted was
to anchor the base of the dam to the foundations below, by means of pre-stressed
cables.
(3) Availability of Materials. In order to achieve economy in the dam, the materials
required for its construction must be available locally or at short distances from
the construction site.
Sometimes,
good soil is easily available, which naturally calls for an earthen dam. If
sand, cement and stone, etc., are easily available, one should naturally think
of a concrete gravity dam. If the material has to be transported from far off distances,
then a hollow concrete dam (Buttress) is a better choice.
(4) Spillway size and Location. Spillway, as defined earlier, disposes of the surplus
river discharge. The capacity of the spillway will depend on the magnitudes of
the floods to be by- passed. The spillway will, therefore, become much more important
on streams with large flood potential on such rivers, the spillway may become
dominant structure, and the type of dam may become the secondary consideration.
The cost of constructing
a separate spillway may be enormous or sometimes a suitable separate site for a
spillway may not be available. In such cases, combining the spillway and the
dam into one structure may be desirable, indicating the adoption of a concrete
overflow dam.
At certain
places, where excavated material from a separate spillway channel may be
utilised in dam embankment, an earthfill dam may prove to be advantageous. Small
spillway requirement often favours the selections of earthfill or rockfill dams
even in narrow dam sites.
The practice
of building a concrete spillway on earth and rock embankments is being
discouraged these days, because of their conservative design assumptions and
the vigil and watch that has to be kept during their operations.
(5) Earthquake Zone. If the dam is to be situated in an earthquake zone,
its design must include the earthquake forces. Its safety should be ensured
against the increased stress induced by an earthquake of worst intensity. The type
of structures best suited to resist earthquake shocks without danger are
earthen darns and concrete gravity dams.
(6) Height of the Dam. Earthen dams are usually not provided for heights more
than 30 m or so. Hence, for greater heights, gravity dams are generally preferred.
(7) Other Considerations. Various other factors such as, the life of the dam, the
width of the roadway to be provided over the dam, problem of skilled labour, legal
and aesthetic point must also be considered before a final decision is taken. Overall
cost of construction and maintenance and the funds available will finally decide
the choice of a particular kind of a dam at a particular place.
1.14 SELECTION OF DAM SITE
The selection
of a site for constructing a dam should be governed by the following factors:
(1) Suitable
foundations (as determined in the previous article) must be available.
(2) For economy,
the length of the dam should be as small as possible, and for a given height,
it should store the maximum volume of water. It, therefore, follows, that the
river valley at the dam site should be narrow but should open out upstream to
provide a large basin for a reservoir. A general configuration of contours for
a suitable site is shown in Fig. 17.1.
(3) The general
bed level at dam site should preferably be higher than that of the river basin.
This will reduce the height of the dam and will facilitate the drainage
problem.
(4) A suitable
site for the spillway should be available in the near vicinity. If the spillway
is to be combined with the dam, the width of the gorge should be such as to
accommodate both.
The best dam
site is one, in which a narrow deep gorge is separated from the flank by a hillock
with its surface above the dam, as shown in Fig. 17.2.
If such a
site is available, the spillway can be located separately in the flank, and the
main valley spanned by an earthen or similar dam. Sometimes, the spillway and
concrete masonry dam may be compositely spanned in the main gorge, while the
flanks are in earth at low cost.
(5) Materials
required for the construction should be easily available, locally or in either
the near vicinity, so that the cost of transporting them is as low as possible.
(6) The reservoir
basin should be reasonably water-tight. The stored water should not escape out
through its side walls and ted.
(7) The value of land
and property submerged by the proposed dam should be as low as possible.
(8) The dam site
should be easily accessible, so that it can be economically connected to
important towns and cities by rails, roads, etc.
(9) Site for
establishing labour colonies and a healthy environment should be available in
the near vicinity.
1.15 Introduction
to Dam Instrumentations:
Normally,
instruments are installed in a concrete gravity dam to measure the various
parameters that indicate the structural health of the dam and the state of the
foundation. These instruments have been classified into two types: obligatory
and optional, by the Bureau of Indian Standards code IS 7436(part2)-1997 “Guide
for types of measurements for structure in river valley projects and criteria
for choice and location of measuring instruments”. These two types of
instruments are explained in the following paragraphs.
Obligatory Measurements
The following
measurements are obligatory for all dams:
a) Uplift pressure at the base of
the dam at a sufficient number of transverse sections
b) Seepage into the dam
and appearing downstream there-from;
c) Temperature of the
interior of the dam; and
d) Displacement measurements - Except for very
small structures (of height 20 m and below not involving any foundation
complications). Displacement measurements should include one or more of the
following types of -measurements:
1) Those determined by suspended plumb lines;
2) Those determined by geodetic measurements where
warranted by the importance of the structure;
3) Those determined by embedded resistance joint
meters at contraction joints where grouting is required to be done.
Optional Measurements
The
following measurements are optional and may be undertaken where warranted by
special circumstances of project. These would be beneficial for high dams, for
structures of unusual design, for structures where unusual or doubtful
foundations exist, for the verification of design criteria and for effecting
improvement in future designs:
a) Stress
b) Strain
c) Pore pressure (as distinct from uplift pressure),
and
d) Seismicity of the area and dynamic characteristic
of the structure.
Due to ever increasing demand of power,
emphasis has been laid to construct large size of hydro electric project with
very high dam. With the present trend, dam sites are neither geologically
seismically suitable as most of the best sites have already been considered for
the purpose. Due to this reason those measurements that were considered
optional at the time of framing this code may become obligatory now for high
dams specially in the Himalayan region.
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