Procedure for Planning a New Reservoir

Procedure for Planning a New Reservoir

The standard procedure that needs to be carried out for planned storage 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: 

  • No simulation studies with sediment correlation is necessary. 

  • 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:  

The following studies are required if the problem of sedimentation in the reservoir is assessed to be significant, but not serious. 

  1. 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. 

  2. 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. 

  3. 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. 

    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.