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:
- Earth fill dams
- Rock fill dams
- Concrete dams.
- Masonry dams.
- Steel dams.
- Timber dams.
- Earth Fill 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. Masonry dams and concrete dams have been discussed in details in Chapter 13 under common heading ‘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) Over flow dams
(ii) Non overflow dams.
(iii) Overflow Dams.
(i) Over flow 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 down stream of the dam by passing over the top of the dam.
- This dam is mostly made of plain concrete or masonry and not of earthfill or rockfill.
(ii) Non Overflow Dams.
- In this case top of the dam is kept higher than the maximum expected high flood level.
- Water is never allow to over top the dam.
- As water is not allow 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
(ii) Diversion dam
(iii) Detention dam.
(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 down stream 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.
- Figure 11.1 (a), (b), (c), (d) shows flood hydrograph before the construction of detention reservoir.
- This flood hydrograph gets modified to AB’CD after the construction of the dam.
- 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.
(i) Gravity 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, rockfill etc. are known as non-rigid dams.