Inactive
storage including dead storage pertains to storage at the lowest
level up to which the reservoir can be depleted. This part of the
storage is set apart at the design stage for anticipated filling,
partly or fully, by sediment accumulations during the economic life
of the reservoir and with sluices/outlets so located that it is not
susceptible to full depletion. In case power facility is provided, it
is also the storage below the minimum draw down level (MDDL).
Sill
level of lowest outlets for any reservoir is fixed from command
considerations in case of irrigation purposes and minimum draw down
level on considerations of efficient turbine operation in the case of
power generation purpose. The lowest sill level should be kept above
the new zero elevation expected after the feasible service period
according to IS 12182 which is generally taken as 100 years for
irrigation projects and 70 years for power projects supplying power
to a grid. By providing extra storage volume in the reservoir for
sediment accumulation, in addition to live storage, it is ensured
that the live storage although it contains sediment, will function at
full efficiency for an assigned number of years. The distribution
pattern of sediments in the entire depth of a reservoir depends upon
many factors, such as slope of the valley, length of reservoir,
constriction in the reservoir, particle size of the
suspended
sediment and capacity inflow ratio, but the reservoir operation has
an important control over the factors. However, the knowledge of the
pattern is essential, especially, in developing areas, in order to
have an idea about the formation of delta and recreational spots.
The dead
storage of a reservoir depends upon the sediment yield of the
catchment. The measurement of sediment yield is done as follows:
(a)
Measurement of sediment yields
The
sediment yield in a reservoir may be estimated by any one of the
following two
methods:
i)
Sedimentation surveys of reservoirs with similar catchment
characteristics, or
ii)
Sediment load measurements of the stream.
(b)
Reservoir Sedimentation Survey
The
sediment yield from the catchment is determined by measuring the
accumulated sediment in a reservoir for a known period, by means of
echo sounders and other electronic devices since the normal sounding
operations give erroneous results in large depths. The volume of
sediment accumulated in a reservoir is computed as the difference
between the present reservoir capacity and the original capacity
after the completion of the dam. The unit weight of deposit is
determined in the laboratory front the representative undisturbed
samples or by field determination using a calibrated density probe
developed for this purpose. The total sediment volume is then
converted to dry- weight of sediment on the basis of average unit
weight of deposits. The total sediment yield for the period of record
covered by the survey will then be equal to the total weight of the
sediment deposited in the reservoir plus that which has passed out of
the reservoir based on the trap efficiency. In this way, reliable
records may be readily and economically obtained on long-term basis.
The density of deposited sediment varies with the composition of the
deposits, location of the deposit within the reservoir, the
flocculation characteristics of clay content and water, the age of
deposit, etc. For coarse material (0.0625 mm and above) variation of
density with location and age may be unimportant. Normally a time and
space average density of deposited materials applicable for the
period under study is required for finding the overall volume of
deposits. For this purpose the trapped sediment for the period under
study would have to be classified in different fractions. Most of the
sediment escape front getting deposited into the reservoir should be
front the silt and clay fractions. In some special cases local
estimates of densities at points in the reservoir may be required
instead of average density over the whole reservoir. The trap
efficiency mainly depends upon the capacity-in-flow ratio but may
vary with location of outlets and reservoir operating procedure.
Computation of reservoir trap efficiency may be made using trap
efficiency curves, such as those developed by Brune and by Churchill
(IS: 12182-1987).
(c)
Sediment Load Measurements
Periodic
samples front the stream should be taken at various discharges along
with the stream gauging observations and the suspended sediment
concentration should be measured as detailed in IS 4890: 1968. A
sediment rating curve which is a plot of sediment concentration
against the discharge is then prepared and is used in conjunction
with stage duration curve (or flow duration) based on uniformly
spaced daily or shorter time units data in case of smaller river
basins to assess sediment load. For convenience, the correlation
between sediment concentration against discharge, may be altered to
the relation of sediment load against run-off for calculating
sediment yield. Where observed stage/flow data is available for only
shorter periods, these have to be suitably extended with the help of
longer data on rainfall. The sediment discharge rating curves may
also be prepared from hydraulic considerations using sediment load
formula, that is, modified Einstein’s procedure.
The bed
load measurement is preferable. How- ever, where it is not possible,
it may be estimated using analytical methods based on sampled data or
as a percentage of suspended load (generally ranging from 10 to 20
percent). This should be added to the suspended load to get the total
sediment load.
Source: CE, IIT Kharagpur
