Reservoir Sedimentation

1. 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.
2. Since it affects the useful capacity of the reservoir based on which projects are expected to be productive for a design period. Further, the deposited sediment adds to the forces on structures in dams, spillways, etc.
3. 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. 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.
4. 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

• Adequate design of reservoir
• Control of sediment inflow
• Control of sediment deposition
• Removal of deposited sediment.

1. Adequate design of reservoir:

• The rate of sediment delivery increases with the volume of discharge. The percentage of sediment trapped by a reservoir with a given drainage area increases with the capacity. In some cases, an increased capacity will however, result in greater loss of water due to evaporation. However, with the * The capacity of the reservoir and the size and characteristics of the reservoir and its drainage area are the most important factors governing the annual rate of accumulation of sediment.
• Periodical reservoir sedimentation surveys provide guidance on the rate of sedimentation. In the absence of observed data for the reservoir concerned, data from other reservoirs of similar capacity and catchment characteristics may be adopted. Silting takes place not only in the dead storage but also in the live storage space in the reservoir.

2. Control of sediment inflow:

There are many methods for controlling sediment inflows and they can be divided as under:

a) Watershed management/soil conservation measures to check production and transport of sediment in the catchment area.

b) Preventive measures to check inflow of sediment into the reservoir. The soil conservation measures are further sub-divided as:

(i) Engineering,

(ii) Agronomy, and

(iii) Forestry.

The engineering methods include:

(i) Use of check dams formed by building small barriers or dykes across stream channels.

(ii) Contour bounding and trenching;

(iii) Gully plugging;

(iv) Bank protection.

The agronomic measures include establishment of vegetative screen, contour farming, strip cropping and crop rotation. Forestry measures include forest conservancy, control on grazing, lumbering, operations and forest fires along with management and protection of forest plantations. Preventive measures to check inflow of sediment into the reservoir include construction of by-pass channels or conduits.

Check Dams: Check dams are helpful for the following reasons:

a) They help arrest degradation of stream bed thereby arresting the slope failure;

b) They reduce the veIocity of stream flow, thereby causing the deposition of the sediment load Contour Bunding and Trenching: These are important methods of controlling soil erosion on the hills and sloping lands, where gradients of cultivated fields or terraces are flatter, say up to 10 percent. By these methods the hill side is split up into small compartments on which the rain is retained and surface run-off is modified with prevention of soil erosion. In addition to contour bunding, side trenching is also provided sometimes

Gully Plugging: This is done by small rock fill dams. These dams will be effective in filling up the gullies with sediment coming from the upstream of the catchment and also prevent further widening of the gully.

3. Control of sediment deposition:

1. The deposition of sediment in a reservoir may be controlled to a certain extent by designing and operating gates or other outlets in the dam in such a manner as to permit selective withdrawals of water having a higher than average sediment content.

2. The suspended sediment content of the water in reservoirs is higher during and just after flood flow. Thus, more the water wasted at such times, the smaller will be the percentage of the total sediment load to settle into permanent deposits.

3. There are generally two methods: (a) density currents, and (b) waste-water release, for controlling the deposition and both will necessarily result in loss of water.

4. Removal of deposited sediment:

1. The most practical means of maintaining the storage capacity are those designed to prevent accumulation of permanent deposits as the removal operations are extremely expensive, unless the material removed is usable.
2. Therefore, the redemption of lost storage by removal should be adopted as a last resort. The removal of sediment deposit implies in general, that the deposits are sufficiently compacted or consolidated to act as a solid and, therefore, are unable to flow along with the water.
3. The removal of sediment deposits may be accomplished by a variety of mechanical and hydraulic or methods, such as excavation, dredging, siphoning, draining, flushing, flood sluicing, and sluicing aided by such measures as hydraulic or mechanical agitation or blasting of the sediment. The excavated sediments may be suitably disposed of so that, these do not find the way again in the reservoir