Dams

DAMS

1 Introduction

2 Purpose of a dam

3 Factors governing selection of site for dam

4 Classification of dams

 INTRODUCTION

A dam is an impervious barrier construction across a river to store water. The side on which water gets collected is called the upstream side, and the other side of the barrier is called the downstream side. The lake of water which is collected in the upstream side is called as reservoir. This water is then utilized as and when it is needed.

PURPOSE OF A DAM

1. To store and control the water for irrigation

2. To store and divert the water for domestic uses

3. To supply water for Industrial uses

4. To develop hydroelectric power plant to produce electricity

5. To increase water depths for navigation

6. To create storage space for flood control

7. To preserve and cultivate the useful aquatic life

8. For recreational purposes.

Multipurpose Reservoirs

A reservoir planned and constructed to serve not only one purpose but various purposes together is called a multipurpose reservoir.

Reservoir, designed for one purpose, incidentally serving other purposes, shall not be called a multipurpose reservoir.

Hence a reservoir designed to protect the down stream areas from floods and also to conserve water for water supply, irrigation, industrial needs, hydroelectric purposes etc. shall be called as MULTIPURPOSE RESERVOIR.

FACTORS GOVERNING SELECTION OF SITE FOR DAM

1.     Suitable foundations should be available at the site selected for a particular type a dam. For gravity dams, sound rock is essential. For earth dams, any type of foundations is suitable with proper treatment.

2.     The river cross-section at the dam site should preferably have a narrow gorge to reduce the length of the dam. However, the gorge should open out u/s to provide large basin for a reservoir.

3.     The general bed level at dam site should preferably be higher than that of the river basin. This will reduce the height of the dam and will facilitate the drainage.

4.     A suitable site for the spillway should be available in the near vicinity. If the spillway is to be combined with the dam, the width of the George should be such as to accommodate both.

5.     Materials required for the construction should be easily available, either locally or in the near vicinity, so that the cost of transporting them is as low as possible.

6.     The reservoir basin should be reasonably water tight. The stored water should not escape out through its side walls and bed.

7.     The value of land and property submerged by the proposed site should be as low as possible.

8.     The dam site should be easily accessible so that it can be economically connected to important towns and cities by rails, roads, etc.

9.     To establish site for labour colonies, a healthy environment should be available in the near vicinity.

CLASSIFICATION OF DAMS

Dams may be classified into different categories, depending upon the basis of the classification.

(a) Classification according to use : Based on use, dams are classified as follows

:

i. Storage dam ii. Diversion dam      iii. Detention dam

i. Storage Dam : Storage dam is constructed to store water to its upstream side during the periods of excess supply in the river (i.e. during rainy season) and is used in periods of deficient supply.

ii. Diversion Dam : Diversion dam supply raises the water level slightly in the river and thus provides head for carrying or diverting water into ditches, canals or other conveyance systems to the place of use.

iii. Detension Dam : A detention dam is constructed to store water during floods and release it gradually at a safe rate when the flood reduces.

(a)  Classification According to Hydraulic Design : According to hydraulic design, dams may be classified81 as follows :

i. Non – Overflow dam           ii. Overflow dam

i. Non-Overflow Dam :

A non-overflow dam is the one in which the top of the dam is ke pt at a higher elevation than the maximum expected high flood level.

ii. Over Flow Dam :

An overflow dam is the one which is designed to carry surplus discharge (including floods) over its cr est.

Usually, in a river valley project, the two types of dams are combined. The main dam is kept as a non – over flow dam and some portion of dam is kipt as o verflow dam (spill way)at some suitable lo cation along the main dam.

c) Classification according to material : According to this classification, dam may be classification, dam may be classified as follows : i. Rigid dam ii. Non-rigid d am

i. Rigid Dams : Rigid dams are those which are constructed of rigid m aterials such as masonry, concrete, steel o r timber. Rigid dams may be further classified as follows:

1. Solid Masonry gravity damm

2. Solid concrete gravity dam

3. Arched masonry dam

4. Arched concrete dam

5. Concrete buttress dam

6. Steel dam

7. Timber dam

ii. Non-Rigid Dams : Non -rigid dams are those which are constructed of non-rigid materials such as earth and or rockfill. The most common types of non-rigid dams are :

1. Earth dam        2. Rockfill dam

Gravity dams : A gravity dam is the one in which the external forces (s uch as water pressure, wave pressure, silt pressure, uplift pressure etc.) are resisted by the weight of the dam itself. A gravity dam may be constructed either of masonry or of concrete. Masonry gravity dams are now-a-days constructed of only small heights. A ll major and important gravity dams are now constructed of concrete only. A gravity dam may be either straight or curved in plan.

Arch dams : An arch dam is a dam curved in plan and carries a maj or part of its waterload horizontally to the abutments by arch action. The thrust developed by the water load carried by arch action e ssentially require strong side walls of the canyon to resist the arch forces. The weight of arch dams is not counted on to assist mat erially in the resistance of external loads.

Butter Dams :

A buttress dam consists of a number of buttresses of piers. These pi ers divide the space (i.e. the space to be d ammed) into number of spans. Between these piers, panels are constructed of horizontal arches or flat slabs. When the panels consist o r arches, it is known as Multiple arches type buttress dam. If the panels consist of flat slab, it is known as Deck type buttress dam.

Steel Dams : Steel dams are constructed with a frame work of steel with a thin skin plate as deck slab, on the upstream side. Steel dams are generally of two types.

i.                   Direct Strutted ty pe

ii.                 Cantilever type

In the direct strutted type, the load on the deck plate is carried directly to the foundation through inclined struts. In the cantilever type, the deck is formed by a cantilever truss i.e. the deck i s anchored to the foundation at the u/s toe.

Timber Dams : A timber dam is constructed of framework of timb er struts and beams, with timber plan faci ng to resist water pressure. They are suitable in places where timber can be available in plenty.

Earth Dams : Earth dams are made of locally available soils and gravel s. Therefore, these type of dams are used upto moderate heights only. Their construction involves utilization of materials in the natural state requiring a minimum of processin g.

Following are the three types of earth dams

i.                   Homogeneous em bankment type

ii.                 Zoned embankme nt type

iii.              Diaphragm emba nkment type

i.  Homogeneous embankm ent type : In this type, dam is composed of a s ingle kind of material. But this dam is structurally weak. To check the seepage throug h the dam a horizontal filter drain or rock toe is provided.

ii. Zoned embankment type : In this type, the dam is made up of more thn one material. Usually this dam consists of central impervious core and outer previous shell as shown in figure. A suitable drainage s ystem, in the form of horizontal drain or a ro ck toe is also provided.

iii. Diaphragam type emba nkment : In this type, a thin diaphragam of impermeable materials is pro vided at the centre of the section to check the seepage. The diaphgram may be made of cement masonry, cement concrete or impervious soils.

Rock fill dams : In this type, variable sizes of rocks are used to form the embankment. The rock fill d am usually consists of the following four parts.

i.                   Main rock fill at the down – stream side

ii.                 Up -  stream rock – cushion of laid – up stone.

iii.              Up – stream impervious membrane resting on the upstream rock cushion.

iv.              Up stream cut-off  to check the sub soil seepage

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