The standard procedure that needs to be carried out for
planned storages requires an assessment of the importance of the
problem to classify the reservoir sedimentation problem as
insignificant, significant, or serious. Assessment of reservoir
sedimentation problem, in a particular case may be made by comparing
the expected average annual volume of sediment deposition with the
gross capacity of the reservoir planned. If the ratio is more than
0.5 percent per year, the problem is usually said to be serious and
special care is required in estimating the sediment yields from the
catchment. If it is less than 0.1 percent per year, the problem of
siltation may be insignificant and changes in reservoir performance.
For cases falling between these two limits, the sedimentation problem
is considered significant and requires further studies. The following
studies are required if the problem is insignificant:
1. No simulation studies with sediment correlation is
necessary.
2. The feasible service time for the project may be
decided. Sediment
distribution studies to ensure that the new
zero-elevation does not exceed
the dead storage level may be made.
In the above, the following terms have been used, which
are explained below:
• Feasible Service Time: For
a special purpose, the period or notional period for which a
reservoir is expected to provide a part of the planned benefit in
respect of storage in the reservoirs being impaired by sedimentation.
Customarily, it is estimated as the time after which the new zero
elevation of
the reservoir would equal the sill of the outlet
relevant for the purpose.
• New Zero Elevation: The
level up to which all the available capacity of the reservoir is
expected to be lost due to progressive sedimentation of the
reservoir up to the specified time. The specified time should be any
length of time such as Full Service Time, Feasible Service time, etc.
• Full Service Time: For a
specified purpose, the period or notional period for which the
reservoir provided is expected to provide, a part of the full planned
benefit inspite of sedimentation.
The
following studies are required if the problem of sedimentation in the
reservoir is assessed to be significant,
but not serious.
1.
Both the full service time and feasible service time for the
reservoir may be decided.
2.
Simulation studies for conditions expected at the end of full service
time may be made to ensure that firm outputs with required depend
ability are obtained. The studies used also assess non-dependable
secondary outputs, if relevant, available at the end of this period.
Studies without sedimentation, with the same firm outputs should
bring out the additional potential secondary outputs which may be
used, if required in economic analysis, using a linear decrease of
these additional benefits over the full service time.
3.
No simulation studies beyond full service time, is required.
4.
Sediment distribution studies required for feasible service time are
essential.
The
following studies are required if the problem of sedimentation is
serious.
1.
All studies described for the ‘Significant’ case have to be made.
2.
The secondary benefits available in the initial years should be more
in such cases. If they are being utilized, for a proper assessment of
the change of these, a simulation at half of full service time should
be required.
3.
In these cases, the drop of benefits after the full service time may
be sharper. To bring out these effects, a simulation of the project
at the end of the feasible service time is required to be done.
Source: CE, IIT Kharagpur
Source: CE, IIT Kharagpur
