Under engineering and geological aspects the following items of work shall invariably be carried out:
Preliminary surveys to assess the catchment and reservoir,
Control surveys like topographical surveys,
Location of nearest Railway lines/Roads and possible access, and 4) Detailed survey for making area capacity curves for use in reservoir flood routing.
General formations and foundation suitability;
Factors relating to reservoir particularly with reference to water tightness;
Deleterious mineral and salt deposits;
Location of quarry sites, etc.
Important aspects of the methods are briefly described in the subsequent sections.
Fixing of Inactive Storage including Dead Storage
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:
Measurement of Sediment Yields
The sediment yield in a reservoir may be estimated by any one of the following two methods:
Sedimentation surveys of reservoirs with similar catchment characteristics
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 (see IS: 12182-1987).