Reservoirs: Types, Planning and Selection of Site

After reading this article you will learn about:- 1. Types of Reservoirs 2. Planning of Reservoirs 3. Selection of Site.

Types of Reservoirs:

A reservoir can be either:

(i) A storage or conservation reservoir, or

(ii) A flood control reservoir.

A storage reservoir stores excess water that flows in a river during the flood season for meeting different kinds of demand (water supply, power generation etc.) at a later date. Any storage reservoir also helps in moderating the floods and, therefore, provides protection against floods to some extent.

The purpose of constructing a flood control reservoir is to protect the area downstream of the reservoir from the damages due to floods. A flood control reservoir holds up flood waters when the river stage is rising and thus reduces the peak rate of flow and, therefore, the stage, i.e., water level in the river downstream. The stored flood water is released when the flood is receding so that enough space is available in the reservoir for storing the excess water of the subsequent flood.

Planning of Reservoirs:

The main factors to be considered in the planning of a reservoir are:

(i) Rim stability

(ii) Water-holding capability

(iii) Evaporation

(iv) Storage zones

(v) Seismicity

(vi) Sedimentation

Rim stability and water-holding capability are interrelated. Rim failure can be caused due to either sliding or erosion of a segment of the reservoir rim. Seepage of water is mainly responsible for such failures. Major slides into a reservoir would, obviously, reduce the reservoir capacity considerably.

Similarly, snow avalanches and masses of ice falling from hanging glaciers can cause serious problems. Besides reducing the capacity of the reservoir, a rapidly moving slide may also generate waves. A dam may be overtopped due to the resulting wave action or rise of the water surface on account of a major slide into the reservoir.

If the reservoir site is likely to be affected by the slides and it cannot be abandoned, some restraining steps in reservoir opera­tion should be taken to avoid serious failure. These steps could be in the form of limiting the filling and drawdown rates or imposing the maximum allow­able water surface at a level lower than the maximum normal water surface.

Alternatively, installation of drains to relieve water pressure along likely slip surfaces, some form of impervious lining and pinning the unstable mass to its parent formation by rock bolting can be resorted to for preventing slides.

Stabilisation of the unstable mass can also be achieved by strengthening or replacing weak material. Grouting is the most common remedy for strengthening such weak masses. It may be desirable to plan the steps to be taken to mitigate the effects of a potential slide after it has occurred in spite of all preventive steps.

Reservoir water loss either to the air or to the ground can be a controlling factor in the selection of site for a conservation reservoir. For a flood control reservoir, water loss is of concern only if it relates to safety of the project.

Lining of the surface through which seepage is expected is one of the preven­tive measures to reduce the reservoir water loss to the ground. At times, a blanket of impervious material extending from the heel of the dam is required. This too serves to control the seepage from the reservoir.

Loss of reservoir water to the air occurs due to direct evaporation from the reservoir surface. The evaporation losses are affected by the climate of the region, shape of the reservoir, wind conditions, humidity and temperature.

From the considerations of evaporation, a reservoir site having a small surface area to volume ratio will be better than a saucer-shaped reservoir of equal capacity. Evaporation-retardant chemicals increase the surface tension of water by forming mono-molecular film and, thus, reduce evaporation.

The maximum elevation to which water level in a reservoir generally reaches during normal operating conditions is known as full (or maximum) reservoir level. This level usually coincides with the level of the spillway crest.

The minimum elevation to which water level in a reservoir can be drawn under normal operating conditions is called minimum reservoir level (or dead storage level). This level is fixed by the level of the lowest outlet in the dam. The volume of water stored below this level is not available for use under normal conditions and, accordingly, this storage is termed as dead storage.

The storage volume between the minimum and maximum reservoir levels is called useful storage (or live storage). During floods the reservoir level exceeds the full reservoir level and water starts flowing over the spillway. The storage volume between the maximum reservoir level and the reservoir level when the design flood passes over the spillway is called the surcharge storage.

Bank storage is the water which spreads out from a body of water, filling interstices of the surrounding earth and rock mass. This water is assumed to remain in the surrounding mass and does not continue to move to ultimately join the ground water or surface water as seepage water does.

The storage zone is theoretically recoverable but, in a sense, is considered a loss. Storage zone is not mitigable. It must, however, be estimated for feasibility investiga­tions and measured during reservoir operation for providing guidelines for reservoir regulation.

It appears that there is some effect of reservoir impoundment on the increased seismic activity of an area in which a large reservoir (having a storage capacity of more than 12 x 10⁸ m³ behind a dam higher than 90 m) has been constructed. However, there have been large reservoirs without increasing the seismic activity of the region.

The increased seismic activity is attributed to the changes in the normal effective stresses in the underlying rock because of the increased pore pressure. The transmission of hydrostatic pressure through discontinuities in the underlying rock can have a triggering effect where a critical state of stress already exists. The relationship between the reservoir impoundment and the earthquake relationship is not fully understood.

Hence, it is necessary that every large reservoir site is subjected to detailed geologic, geodetic and seismic studies for feasibility decision. These observations must be continued during the reservoir operation too in order to better understand the relationship between reservoir impoundment and seismic activity.

The streams bringing water to the reservoir bring sediments too. The sediment gets deposited in the reservoir due to the reduced stream velocity. The capacity of the reservoir is reduced on account of sediment deposition in the reservoir. Usually, a portion of the reservoir storage is reserved for the storage of the sediment.

The life of a reservoir is predicted on the basis of the amount of sediment delivered to it, the reservoir size and its ability to retain the sediment. Sediment deposition at the initial stage may be beneficial in the sense that it may have the effect of a natural blanket resulting in reduced seepage loss.

Measures to minimise sediment deposition in reservoirs include watershed protection through a vegetative management programme to prevent soil erosion, silt detention basins at inlets of smaller reservoirs and low level outlets in dams to provide flushing action for removal of sediment from the reservoir.

Of the various measures, the watershed protection is the most effective as well as the costliest method. Erosion in the catchment area can be reduced by soil conservation methods, such as afforestation, control of deforestation, control of grazing, checking gully formation by providing small embankments, etc.

Selection of Site for a Reservoir:

Based on planning and other considerations, the guidelines for selection of site for a reservoir are as follows:

(i) Availability of a suitable site for construction of dam.

(ii) The hills surrounding the reservoir and the bed of the reservoir should be impervious.

(iii) Availability of good storage capacity with minimum submergence of the adjacent land.

(iv) A reservoir should not be sited downstream of such tributaries which bring-in excess sediment into the river.

(v) Availability of deep gorge which results in larger capacity with lesser water surface area and, therefore, lesser evaporation loss.

(vi) The cost of other associated works is less.

(vii) The site with the possibility of land slides into the reservoir must be avoided.

(viii) The site should not be, as far as possible, on valuable land being used for some other purposes, such as agriculture, forestry, communica­tion and habitation by people,

(ix) Sites with mineral deposits in and around the reservoir area should also be avoided.