In this article we will discuss about the classification of dams.
Classification of Dams:
Dams may be classified in several ways as follows:
1. Classification Based on Materials of Construction:
According to this classification the following may be the types of dams:
(i) Earth Fill Dams:
Earth fill dams are the oldest types of dams. This dam structure consists of local soil mainly. This dam is not 100% impervious. Water percolates through the body of the dam but rate of percolation is kept under check and percolation line of hydraulic gradient line of percolating water is not allowed to get exposed anywhere.
This dam does not require much of skill. They also prove economical as mostly local soil is used in their construction. The biggest drawback of these dams is that they cannot be constructed in large heights.
(ii) Rock Fill Dam:
These dams mostly comprise rock boulders. Only boulders cannot prevent the flow of water and as such some impervious layer is laid on the upstream face of the Dam, which may be in form of cement concrete slab or earth fill covered by rip rap or any other arrangement. Rock fill provides the stability whereas impervious layer provides imperviousness to the dam to prevent flow of water through them. These dams are not much in use.
(iii) Concrete Dams:
They are made either from plain cement concrete or reinforced concrete. They may be gravity dams or may be in form of arch, buttress or any other form of dams.
(iv) Masonry Dams:
They are made from stone masonry in cement mortar. They are mostly gravity dams.
(v) Steel Dams:
They are not used much. They are used for secondary purpose small dams.
(vi) Timber Dams:
They are also of secondary nature and used for storing small amounts of water temporarily.
2. Classification Based on the Flow over Its Top:
Under this classification the dams may be:
(i) Overflow Dams:
In the case of over flow dams, the water may flow over the top. Spillways and weirs, are the examples of over flow dams. The dam after storing its designed capacity allows surplus water to flow downstream of the dam by passing over the top of the dam. This dam is mostly made of plain concrete or masonry and not of earth fill or rock fill.
(ii) Non Overflow Dams:
In this case top of the dam is kept higher than the maximum expected high flood level. Water is never allowed to over top the dam. As water is not allowed to over top the dam, they may be constructed from any material.
In practically all the river valley projects overflow and non-overflow dam are combined. The main dam is constructed as non-overflow dam but a part of the dam is kept as overflow dam to act as spillway.
3. Classification Based on the Use of the Dam:
Depending upon the use of the reservoir developed by the dam, the dams may be classified as follows:
(i) Storage Dam:
This dam is constructed mainly to impound the excess river water on its upstream side during floods. The stored water is used when river runs with deficient supply. Stored water may be used for irrigation, water power generation, water supply schemes, or for multi-purpose projects. The dam may be made of concrete, earth, masonry etc.
(ii) Diversion Dam:
The purpose of this dam is not to store excess water behind it, but to raise the level of water so that water may be diverted into a system of canals. The height of this dam is, therefore, relatively small as no reservoir is to be formed to store water. During floods water passes over or through the dam to downstream side, while during normal flow the river water is wholly or partly diverted to the irrigation canals. The common examples of diversion dam are weirs and barrages.
(iii) Detention Dam:
This dam is constructed to control the flood water. The flood water is stored behind the dam during floods. The stored water is released gradually at a safe rate when floods recede. In such dams water is temporarily stored and released through suitable outlet structures. The temporarily held water seeps into the banks and foundation and thus causes rise in ground water table in adjoining area, which may be exploited in from of lift irrigation. The submerged area during floods holds usually sufficient amount of water which may be enough for growing a crop without any irrigation.
Such a detention dam is known as water spreading dam or dike. Detention dams are also sometimes constructed across tributaries carrying large silt and sediment. Such a detention dam is then called as debris dam whose main function is to trap the sediment before water of that tributary meets the main reservoir.
Today in all the river valley projects, the dam may serve the purposes of storage, flood protection, irrigation, power generation and other purposes.
4. Classification Based on the Mod or Resistance Offered by the Dam against External Forces:
Under this classification, dams may be of following types:
(i) Gravity dams
(ii) Buttress dams
(iii) Arch dams.
Gravity dams resist all the external forces acting on the dam by virtue of its weight. Every effort is made to make dam more heavy so as to increase its stability.
Arch dams resist effect of external forces by arch action. The dams are curved in plan and are subjected to compressive stresses only.
Buttress dam is also subjected to same forces as gravity and arch dams. The total uplift pressure is reduced to a very small magnitude because of the gaps between the buttresses.
5. Classification Based on Rigidity of the Dam:
According to this classification dams may be:
(i) Rigid dams.
(ii) Non-rigid dams.
Rigid dams are those which are constructed of rigid materials like concrete, masonry, steel or timber.
The dams made from materials like earth, rock fill etc. are known as non-rigid dams.
Factors that Govern Selection of Type of Dam:
It is very difficult to make decision as to what type of dam will suit best to the particular site condition. Topographical, geological and foundation conditions, availability of materials, suitable site for location of spillways, etc. influence the selection of type of dam. Availability of labour, equipment, accessibility of site etc. also affect selection of the dam to some extent.
The most common physical factors have been discussed as follows:
1. Materials of Construction:
Dams require very large quantities of construction materials. Only that dam may prove economical whose materials are available locally or in the vicinity of the site. If gravel, sand and crushed stone are available locally, concrete gravity dam may be most suitable. If only fine and coarse grained soils are available earth dam may prove suitable.
2. Geological Conditions:
If available foundation is solid sound rock, having no faults or fissures, any type of dam can be constructed on it. If rocks are available but they have faults or fissures, they will have to be grouted first before founding gravity dam over them. Poor rock or gravel foundations are suitable for earth or rock fill dams but not for gravity dams.
Deep cut-offs may be constructed under such conditions of foundation to check the seepage. Fine sand or silt, foundations are suitable only for either earth dam or for low gravity dams but not for rock fill dams. Gravity dams and rock fill dams are not suitable on clay foundations. Clay foundations are suitable for earth dams, but only after giving proper treatment.
3. Topography:
This is one of the most important considerations. A narrow deep valley suggests adoption of gravity dam. If however the width of the valley at top is less than about one fourth the height and separate site for spillway is available, arch dam may suit best. In case of plain country, having very flat slopes of banks, earth dam is the best choice.
4. Spillways:
For the safety of the dam, safe and efficient disposal of flood discharge is very essential. This requires a suitable site for the spillways. If they large spillway discharge is required, an overflow concrete gravity dam may be adopted. If separate site for spillway is available and spillway capacity required is also small, earth dam should be preferred.
If no separate site for spillway is available and it has to be accommodated in the dam proper, then concrete gravity dam may be most preferred choice. Sometimes even earth dam may be constructed but some part of the dam which is to be used as spillway may be constructed of concrete.
5. Roadway over the Dam:
If a roadway is to be passed over the top of the dam, it can be none else than earth or gravity dam.
6. Life and Height of the Dam:
If length of the dam is quite large, but height not much, an earth dam would be the best choice. On the other hand if length is small but height is more gravity dam should be the only choice.
7. Life of Dam:
If life of the reservoir has to be very large, concrete or gravity dam should be adopted. Earth dams and rock fill dams require constant supervision and in long life they prove uneconomical.
8. Generation of Hydroelectric Power:
In this case dams are exclusively made of concrete or masonry gravity dams. Earth dams, being not very high, do not develop sufficient head for running the turbines.
Selection of Site for Dam:
Following points should be considered while selecting the site for the dam:
1. Suitable foundation for the dam should be available.
2. The width of the river at dam site should be minimum so as to keep the length of the dam minimum and thus economical. In order to provide large basin for the reservoir, the site should open out upstream.
3. If river is deep, but there is raised bed level existing, the dam should preferably be located at the raised bed. This will help in keeping height of the dam less.
4. Proper separate site for location of spillways should be available. If separate spillway site is not available, the width of the river should be sufficient to locate spillway in the dam proper.
5. The materials of construction for the dam should be locally available.
6. Very costly land should not submerge in water because of the construction of the dam.
7. The site should be well connected either with railway or highway or both. This will facilitate transport of cement, machinery and other materials required in the construction.
8. The site should be such that leakage through its sides and bed is minimum. Highly permeable rocks should be absent from the site.
9. Site should such that the reservoir developed is deep and having minimum spread of water.
10. Site should be such that the tributaries carrying very high content of silt do not lead to the reservoir developed by the dam.