The
storage of reservoir includes the Active Storage (or Conservation
Storage) and the Buffer Storage. Active or conservation storage
assures the supply of water from the reservoir to meet the actual
demand of the project whether it is for power, irrigation, or any
other demand water supply. The active or conservation storage in a
project should be sufficient to ensure success in demand
satisfaction, say 75 percent of the simulation period for irrigation
projects, whereas for power and water supply projects success rates
should be 90 percent and 100 percent respectively. These percentages
may be relaxed in case of projects in drought prone areas. The
simulation period is the feasible service period, but in no case be
less than 40 years. Storage is also provided to satisfy demands for
maintaining draft for navigation and also maintaining water quality
for recreation purpose as envisaged in design.
Live
storage capacity of a reservoir is provided to impound excess waters
during periods of high flow, for use during periods of low flow. It
helps the usage of water at uniform or nearly uniform rate which is
greater than the minimum flow live storage has to guarantee a certain
quantity of water usually called safe (or firm) yield with a
predetermined reliability. Though sediment is distributed to some
extent in the space for live storage, the capacity of live storage is
generally taken as the useful storage between the full reservoir
level and the minimum draw-down level in the case of power projects
and dead storage in the case of irrigation projects.
The
design of the line storage include certain factors, of which the most
important in the availability of flow, since, without an adequate
flow, it is not possible to cope up with the demand at all periods
and seasons throughout the year. When adequate flow is available,
there may still be certain problems like the possible maximum
reservoir capacity from physical considerations may be limited and
then this becomes the governing criteria. Even if an adequate
reservoir capacity may be possible to be built, the governing factor
may have to be based on the demand.For fixing the live storage
capacity, the following data should be made use of:
a) Stream
flow data for a sufficiently long period at the site;
b)
Evaporation losses from the water-spread area of the reservoir and
seepage losses and also recharge into reservoir when the reservoir is
depleting;
c) The
contemplated irrigation, power or water supply demand;
d) The
storage capacity curve at the site.
Stream
flow records are required at proposed reservoir site. In the absence
of such records the records from a station located upstream or
downstream of the site on the stream or .on a nearby stream should be
adjusted to the reservoir site. The run off records are often too
short to include a critical drought period. In such a case the
records should be extended by comparison with longer stream flow
records in the vicinity or by the use of rainfall run off
relationship. The total evaporation losses during a period are
generally worked out roughly as the reduction in the depth of storage
multiplied by the mean water-spread area between the full reservoir
level and the minimum draw-down level. For accurate estimation,
monthly working tables should be prepared and the mean exposed area
during the month is
found out
and the losses should be then worked out on the basis of this mean
exposed area, and the evaporation data from pan evaporimeter at the
reservoir site. The details are expected to be covered in the draft
‘Indian Standard criteria for determination of seepage and
evaporation losses including the code for minimizing them. In the
absence of’ actual data these may be estimated from the records of
an existing reservoir with similar characteristics, like elevation,
size, etc, in the neighbourhood.
Source: CE,IIT Kharagpur
