Sedimentation of reservoirs
ü It is important to note that storage reservoirs built across rivers and streams loose their capacity on account of deposition of sediment.
ü This deposition which takes place progressively in time reduces the active capacity of the reservoir to provide the outputs of water through passage of time.
ü In this regard, the Bureau of Indian Standard code IS: 12182 - 1987 “Guideline determination of effects of sedimentatiois an important document which discusses some of the aspects of sedimentation that have to be considered while planning reservoirs.
ü Some of the important points from the code are as follows:
ü While planning a reservoir, the degree of seriousness and the effect of sedimentation at the proposed location has to be judged from studies, which normally combination consists of:
1. Performance Assessment (Simulation) Studies with varying rate of sedimentation.
2. Likely effects of sedimentation at dam face.
ü In special cases, where the effects of sedimentation on backwater levels are likely to be significant, backwater studies would be useful to understand the size of river water levels.
ü Similarly, special studies to bring out delta formation region changes may be of interest. The steps to be followed for performance assessment studies with varying rates of sedimentation are as follows:
a. Estimation of annual sediment yields into the reservoir or the average annual sediment yield and of trap efficiency expected.
b. Distribution of sediment within reservoir to obtain a sediment elevation and capacity curve at any appropriate time.
c. Simulation studies with varying rates of sedimentation.
d. Assessment of effect of sedimentation.
ü In general, the performance assessment of reservoir projects has to be done for varying hydrologic inputs to meet varying demands.
ü Although analytical probability based methods are available to some extent, simulation
of the reservoir system is the standard method.
ü The method is also known as the working tables or sequential routing. In this method, the water balance of the reservoir s and of other specific locations of water use and constraints in the systems are considered.
ü All inflows to and outflows from the reservoirs are worked out to decide the changed storage during the period.
ü In simulation studies, the inflows to be used may be either historical inflow series, adjusted for future up stream water use changes or an adjusted synthetically generated series.
Control of sedimentation in reservoirs
ü Sedimentation of a reservoir is a natural phenomenon and is a matter of vital concern for storage projects in meeting various demands, like irrigation, hydroelectric power, flood control, etc.
ü Since it affects the useful capacity of the reservoir based on which projects are expected to be productive for a design period.
ü Further, the deposited sediment adds to the forces on structures in dams, spillways, etc.
ü The rate of sedimentation will depend largely on the annual sediment load carried by the stream and the extent to which the same will be retained in the reservoir.
ü This, in turn, depends upon a number of factors such as the area and nature of the catchment, level use pattern (cultivation practices, grazing, logging, construction activities and conservation practices), rainfall pattern, storage capacity, period of storage in relation to the sediment load of the stream, particle size distribution in the suspended sediment, channel hydraulics, location and size of sluices, outlet works, configuration of the reservoir, and the method and purpose of releases through the dam.
ü Therefore, attention is required to each one of these factors for the efficient control of sedimentation of reservoirs with a view to enhancing their useful life and some of these methods are discussed in the practice for Bureau of Indian Standard code IS: 6518-1992 “Code of control of sediment in reserve”
ü In this section, these factors are briefly discussed.
There are different techniques of controlling sedimentation in reservoirs which may broadly be classified as follows:
• Adequate design of reservoir
• Control of sediment inflow
• Control of sediment deposition
Removal of deposited sediment.