INTRODUCTION

1.Fundamentally, a reservoir serves to store water and the size of the reservoir is governed by the volume of the water that must be stored, which in turn is affected by the variability of the inflow available for the reservoir. Reservoirs are of two main categories:

(a) Impounding reservoirs into which a river flows naturally, and

(b) Service or balancing reservoirs receiving supplies that are pumped or channeled into them artificially. In general, service or balancing reservoirs are required to balance supply with demand. Reservoirs of the second type are relatively small in volume because the storage required by them is to balance flows for a few hours or a few days at the most. Impounding or storage reservoirs are intended to accumulate a part of the flood flow of the river for use during the non-flood months.

2.The functions of reservoirs are to provide water for one or more of the following purposes. Reservoirs that provide water for a combination of these purpose, are termed as ‘Multi-Purpose’ reservoirs.

• Human consumption and/or industrial use
• Irrigation: usually to supplement insufficient rainfall.
• Hydropower: to generate power and energy whenever water is available or to provide reliable supplies of power and energy at all times when needed to meet demand.
• Pumped storage hydropower schemes: in which the water flows from an upper to a lower reservoir, generating power and energy at times of high demand through turbines, which may be reversible, and the water is pumped back to the upper reservoir when surplus energy is available. The cycle is usually daily or twice daily to meet peak demands. Inflow to such a reservoir is not essential, provided it is required to replace water losses through leakage and evaporation or to generate additional electricity. In such facilities, the power stations, conduits and either or both of the reservoirs could be constructed underground if it was found to do so.
• Flood control: storage capacity is required to be maintained to absorb foreseeable flood inflows to the reservoirs, so far as they would cause excess of acceptable discharge spillway opening. Storage allows future use of the flood water retained.
• Amenity use: this may include provision for boating, water sports, fishing, sight-seeing.

3.Formally, the Bureau of Indian Standards code IS: 4410 (part 6)1983 “Glossary of terms relating to river valley projects -Reservoirs" defines the following types of reservoirs:

• Auxiliary or Compensatory Reservoir: A reservoir which supplements and absorbed the spill of a main reservoir.
• Balancing Reservoirs: A reservoir downstream of the main reservoir for holding water let down from the main reservoir in excess of that required for irrigation, power generation or other purposes.
• Conservation Reservoir: A reservoir impounding water for useful purposes, such as irrigation, power generation, recreation, domestic, industrial and municipal supply etc.
• Detention Reservoir: A reservoir where in water is stored for a relatively brief period of time, past of it being retained until the stream can safely carry the ordinary flow plus the released water. Such reservoirs usually have outlets without control gates and are used for flood regulation. These reservoirs are also called as the Flood Control Reservoir or Retarding Reservoir.
• Distribution Reservoir: A reservoir connected with distribution system a water supply project, used primarily to care for fluctuations in demand which occur over short periods and as local storage in case of emergency such as a break in a main supply line failure of a pumping plant.
• Impounding or Storage Reservoir: A reservoir with gate-controlled outlets wherein surface water may be retained for a considerable period of time and released for use at a time when the normal flow of the stream is in sufficient to satisfy requirements.
• Multipurpose Reservoir: A reservoir constructed and equipped to provide storage and release of water for two or more purposes such as irrigation, flood control, power generation, navigation, pollution abatement, domestic and industrial water supply, fish culture, recreation, etc.

RESERVOIR SEDIMENTATION

All the rivers carry certain amount of silt eroded from the catchment area during heavy rains. The extent of erosion, and hence the silt load in the stream depends on the following factors:

(a) Nature of the soil of the catchment area

(b) Topography of the catchment area.

(c) Vegetation cover

(d) Intensity of Rainfall.

1. The nature of the soil of the catchment area is an important factor. If the soil is soft, there is always a possibility of sheet erosion. The tributaries collecting the water of the catchment area containing hard sol carry lesser silt. Steep slope give rise to high velocities and erode the surface soil easily.
2. Similarly, higher intensity of rainfall causes greater runoff and more erosion. If the catchment area has sufficient vegetation cover, the higher velocities are checked and erosion is very much reduced. Areas having poor or practically no vegetal cover are productive of more silt. The river or tributaries passing through such areas carry more silt load with it, causing quick silting of the reservoir.
3. The sediment transported by the river can be divided into heads: (i) bed load, (ii) suspended load. The bed load is dragged along the bed of the stream. The suspended load is kept in suspension because of the vertical components of the eddies formed due to the friction of flowing water against the bed.
4. The bed load is generally much smaller i.e., 10 to 15% of the suspended load. When the stream approaches the reservoir, the velocity is very much reduced. Due to this reduction, the coarser particles settle in the head reaches of the reservoir, while the finer particles are kept in suspension for sufficient time till they settle just to the upstream side of the dam. Some fine particles may pass through sluice ways, turbines or spillways.

Sediment Control in reservoir

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
5. Adequate design of reservoir
6. Control of sediment inflow
7. Control of sediment deposition
8. Removal of deposited sediment.

• Adequate design of reservoir:

1. 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 progress of sedimentation, there is decrease of storage capacity which in turn lowers the trap efficiency of the reservoir.

2. 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.

3. 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.

• 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.

• 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.

• 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