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In this article we will discuss about:- 1. Introduction to Reservoirs 2. Geological Investigations for Reservoirs 3. Silting.
Introduction to Reservoirs:
Reservoirs may broadly be defined as artificially created water storage basins with storage capacity that may range from a few thousand cubic meters to thousands of million cubic meters.
Depending on the purpose of storage, reservoirs are classified into three main categories:
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(i) Storage and conservation reservoirs, where river water is stored by creating barriers or dams in its path and is then released from gated or ungated outlets. These feed the canal systems for irrigation and power generation.
(ii) Flood control reservoirs which have as their main function accommodating large volumes of surplus water during peak flow times of a river. The surplus water is released after the flood abates. Such reservoirs are provided with large sluice ways to discharge the inflow received by the reservoir during a flood up to a volume which could be safely accommodated in the channel downstream. Excess or surplus inflow is retained back till a desirable time.
(iii) Distribution reservoirs are actually small storage reservoirs which hold water supplies in a water- supply system for short spells of time. Water is constantly pumped into these reservoirs, from where it is distributed for drinking and other purposes in a regulated manner.
Many reservoirs serve more than one of the above objects and are called multipurpose reservoirs.
Most important terms related to the design of a reservoir are pool level, storage capacity and reservoir yield.
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Pool Level, indicates the designed level up to which the reservoir shall be ‘full of water’ at a particular point of time. There is, thus, the maximum pool level, the minimum pool level and the normal pool level. No water can be stored beyond maximum poll level and no water shall be available when the storage is below the minimum pool level.
Storage Capacity of the reservoir is the single most important quality of a dam.
It is expressed by such terms as:
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i. Useful Storage:
Volume of water in cubic meters available between minimum pool level and normal pool level;
ii. Dead Storage:
Volume of water which is not available for use and is represented by the water stored in the reservoir below the minimum pool level;
iii. Surcharge Storage:
Volume of water which is available between the maximum reservoir level and normal pool level.
Reservoir Yield is a measure for the volume of water that can be drawn from a given reservoir in a certain interval of time and is also expressed in terms of safe or guaranteed yield, secondary yield and design yield etc.
Geological Investigations for Reservoirs:
Since reservoirs are essentially water storage basins, the area should possess such geological characteristics that favour holding of water in the basin so created. Such characters are – Topographic suitability, ground water conditions, permeability and structural stability.
(a) Topographically, the area should be a broad natural valley preferably ending in a narrow gorge where a barrier could be placed. The valley could be a U-shaped glacial valley, or V-shaped river valley or a broad synclinal valley. Flatlands and plains cannot make convenient places for reservoirs. In alluvial plains, the reservoir area has to be an extensive low-lying stretch bordered by high-lands on flanks.
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(b) Groundwater conditions in the proposed reservoir area must be thoroughly established for an approximately accurate evaluation of storage capacity of the reservoir.
Any one of the three possibilities could be there:
(i) The general water table is much above the top reservoir level (TRL) in the proposed reservoir. This is a very favourable situation, as there is no risk of water loss from the reservoir to the surrounding area.
(ii) The general water table is much below the minimum water level in the reservoir. Obviously, depending upon the permeability of rocks along the base and the flanks of the reservoir, water would be lost from the reservoir to the surroundings thereby effecting the total storage inventory considerably.
(iii) The general water table of the area and the top water level of the proposed reservoir are almost the same. This is an ideal situation ensuring a no loss no gain condition but generally not available on a large scale.
In view of such a close relationship between water table and storage capacity of the proposed reservoir, it is absolutely essential that water table position all along the reservoir areas, especially along the flanks, must be thoroughly established. This may necessitate drilling test holes upto required depths and numbering many hundreds or even thousands.
(c) Permeability is a critical property in reservoir area studies, especially in those areas where groundwater table is below the minimum water level of the reservoir. Permeability may be of primary nature, due to inherent porous texture of the rocks, or, of secondary character, where it is caused due to structural deformations of the rocks subsequent to their formation. Primary permeability may be extensive and all prevailing along the rock body whereas secondary permeability is often localized and amenable to treatment.
In critical area where water loss from the proposed reservoir could be expected due to seepage and under hydraulic head difference, permeability-values, whether primary or secondary, must be thoroughly established. This may also require drilling of test-wells at numerous places. Such studies then can be analysed to calculate possible loss due to expected seepage and suggest methods of treatment of rocks in critical zones to minimize the expected loss.
(d) Structural constitution of the area of the reservoir is important to identify those zones along the flanks which are liable to failure by sliding, creep or subsidence. The worst known dam disaster of Vaoint, Italy that happened on October 9, 1963 was actually caused by sudden slippage of millions of cubic meter of mass from the flanks into the reservoir creating waves as high as 270 m above reservoir which took the form of a flash flood and killed 3000 people in a matter of minutes.
It has been established that the “disaster was entirely by dangerous geological conditions, accentuated by groundwater changes due to the filling of the reservoir”. Such geological setting is not unique to Vaoint, Italy; it could be found repeated at many places in the world. Hence great precaution is necessary in selection of reservoir locations.
(e) Trend and rate of weathering in catchment area is of considerable importance in determining the age of the reservoir. If the catchment happens to be made up predominantly of weathered, barren slopes which are broken, falling, shattered and jointed profusely, the run off and slope wash would contribute heavy load of sediments to the reservoir after every rain.
This would obviously decrease the effective storage capacity of the reservoir on the one hand and its total life on the other hand. However, stable rock slopes covered with thick forests and vegetation would make ideal catchment areas for reservoirs.
Examples:
Some of the major reservoirs are created by dams. Many of them are multipurpose reservoirs.
In Table 23.1, the main features of these reservoirs are summarized for a quick review:
Silting of Reservoirs:
The Problem:
Sedimentation of the reservoirs with the passage of time is the most important problem to solve in reservoir design. Uncontrolled sedimentation will lead to the indirect failure of the project by gradually reducing the storage capacity. And removal of silt or methods of sedimentation control may be so much time and energy and money consuming that the economy of the project is sometimes threatened. These facts prove the complicated nature of the problem.
Streams:
Along with their tributaries are the major agents that contribute to the silting of reservoirs. Their contribution is much effected by factors like their velocities, their catchment zones, and the climate of the region, its structural constitution, topographic and lithological characters and also by the vegetation of the area.
Control:
The problem has been treated in different ways in different countries. More important of these methods of treatment include better reservoir designs, installation of check dams at the inlet points and water-shed improvement by forestation on an extensive and intensive scale. Removal of the silt after fixed intervals by dredging and similar methods are also practised in many cases.
All these are, however, very costly methods and any one or more of them are adopted in combination after careful analysis of cost benefit ratio. If the rate of sedimentation is such that the safe reservoir yield would be available up to the designed life period of the reservoir, then spending too-heavily on desilting measures might not be advised. However, when the rate of silting threatens the storage capacity and safe yield, effective silt control methods would have to be adopted in a proper planned manner.