Design
of
Hydroelectric
Power
Stations
Reservoirs
Reservoirs
The
natural
stream
of
water
if
used
for
hydro
electric
project,
may
be
unable
to
satisfy
the
demands
of
its
consumers
during
extremely
low
flows.
The
quantity
of
water
flow
is
extremely
large
during
flood
period
whereas
the
quantity
of
water
flow
is
very
little
during
dry
period
of
the
year
as
mentioned
earlier.
Therefore,
there
is
necessity
to
store
the
water
during
high
flow
of
the
river
and
supply
the
same
to
the
power
plant
during
low
flow
period.
A
storage
used
to
retain
such
excess
water
from
periods
of
high
flow
and
supplies
the
retained
water
during
low
flow
is
commonly
known
as
reservoir.
The
storage
of
water
during
high
flow
period
(flood)
may
also
reduce
Flood
damage
to
the
area
below
the
reservoir
in
addition
to
conserving
water
for
later
use.
The
main
function
of
the
reservoir
is
to
store
and
supply
the
water
according
to
demand
by
regulating
the
quantity
of
water
supplied.
Once
the
dam
site
is
selected,
the
other
important
problem
is
to
find
out
the
most
economical
cost
to
provide
the
necessary
storage
volume.
The
increase
in
dam
height
produces
increase
in
head
and
mean
output.
This
gain
in
mean
output
requires
an
economic
analysis
because
gain
in
output
must
counter
balance
the
increase
in
cost
due
to
increased
height.
If
the
head
of
the
power
plant
is
solely
created
by
the
dam
then
it
may
permit
the
increase
in
height
to
increase
the
installed
capacity.
However,
when
fairly
high
natural
head
is
available,
it
does
not
usually
pay
to
increase
it
beyond
the
point
necessary
to
provide
adequate
storage.
The
economic
analysis
of
storage
possibilities
is
not
rigid
and,
therefore
the
designer
must
take
his
own
analysis
of
storage
cost
balancing,
the
cost
of
each
increase
in
dam
height
against
the
advantage
gained
from
extra
firm
output
shape
of
the
valley
and
type
of
construction.
The
high
value
of
x
is
expected
with
an
increase
in
height
compared
with
increase
in
length
of
the
dam.
Reservoir
capacity
The
capacity
of
the
reservoir
on
natural
sites
is
determined
with
the
help
of
topographic
surveys
as
its
shape
is
always
irregular.
The
available
storage
capacity
of
the
site
is
generally
measured
in,
surveying
a
series
of
water
level
counters
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