What is Sedimentation of Reservoir ?
When the silt laiden water reaches a reservoir in vicinity of the dam, the velocity and turbulence are considerably reduced. The biggest particle which moves along the bed load, gets deposited in the head or upper reaches of reservoir. While the fine particles which are kept in suspension as suspended load may travel some more distance and gets deposited further down in the reservoir. If this process of deposition continues longer, a stage is likely to reach when the whole reservoir may get silted up and become useless. The total volume of silt is likely to be deposited during the design life period of dam, is therefore, estimated and approximately that much volume is left unused to allow for silting. This volume is known as dead storage volume. It normally ranges from 15-25 %. This overall phenomena may be termed as sedimentation in reservoir.
Factors Governing Sedimentation of Reservoir
- The rate of sedimentation will depend largely on the annual sediment load carried by the steam and the extent to which the same will be retained in reservoir.
- Level use pattern like grazing, logging, conservation practices etc.
- Particle size distribution in the suspended sediment.
- Location and size of sluices, outlet works through the dam.
- The area and nature of the catchment.
- Rainfall pattern, storage capacity, sediment load of the stream.
- Channel hydraulics.
Control Measures for Sedimentation in Reservoir
1. Adequate Design of Reservoir :
- The capacity of the reservoir and the size and characteristics of the reservoir and its drainage system are the main factors governing the annual rate of accumulation of sediments.
- The percentage of sedimentation trapped by a reservoir with given drainage area increases with capacity.
- The practice for design of reservoir includes the observed suspended sediment data from hydrological network and hydrographic survey.
2. Control of Sediment Inflow :
- Engineering measures includes : Use of check dams formed by building small barriers, contour bunding and trenching, gully plugging, bank protection.
- Agronomic measures include : Establishment of vegetative screen, contour farming, strip cropping and crop rotation.
- Forestry measures include : Forest conservancy, control on grazing, lumbering and forest sites along with management and protection of forest plantation.
3. Control of Sediment Deposition :
- Density current :
Water at various level contains different concentrations of suspended particles during and after flood flows. If all waste water could be withdrawn at those levels where the concentration is highest, a significant amount of sediment might be removed from the reservoir.
- Waste water release :
This method is applicable only when a reservoir is of such size that a small part of large flood flow will fill it. In the design of the dam, sediments may be passed through or over it as an effective method of silt control by placing a series of outlets at various elevations.
4. Removal of Deposited Sediment :
The removal of sediment deposits may be accomplished by a variety of mechanical and hydraulic methods such as excavation, dredging, siphoning, flushing, flood sluicing or by mechanical agitation or blasting of the sediments.
Surveys for Reservoir Sedimentation
1. Sedimentation survey of reservoir with similar catchment characteristics:
- The sediment yield from the catchment is determined by measuring the accumulated sediment in a reservoir for a known period by means of echo-sound.
- 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 deposit is determined by using calibrated density probe or in laboratory front.
- 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 to 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.
2. Sediment load measurement of the stream :
- Periodic sample front should be taken at various discharge along with the stream gauging and suspended sediment concentration should be measured.
- In case of smaller river basins to access sediment load, a sediment rating curve which is a plot of sediment concentration discharge is used in conjunction with stage duration curve of flow duration curve.
- Where observed stage flow data is available for only short period, these have to be suitably extended with the help of longer data on rainfall.
- The sediment discharge rating curves may be prepared from hydraulic consideration using sediment load formula.
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