Classification of Reservoirs.pptx

Classification of Reservoirs

1. Reservoirs Classification of reservoirs - Site selection - General principles of design - Spillways -Elevation- Area-Capacity curve - Storage estimation - Sedimentation - Life of reservoirs – Rule curve.

2. Reservoirs

3. Reservoirs A reservoir is the large water body that forms behind a wall constructed across a river or a large valley. In some instances, the reservoir accumulates large amounts to form a human-made lake. A large lake where water is stored to be used by a particular area, city, etc.

4. Reservoirs Dams and weirs are some of the barriers which when constructed across the rivers and streams, cause accumulation of water behind them. The water thus accumulated in form of an artificial lake is known as reservoir. In broad sense, any collected water in form of a pool or lake, may be termed as reservoir. Dams and reservoirs are the most important elements of any multipurpose river basin development.

5. Reservoirs River water may be used for generation of hydroelectric power, irrigation, and water supply source for some town or city. All these uses require a constant or almost constant supply of water. But we know that discharge in rivers and streams remains fluctuating. During rains, the water in the river may be more than the requirements and during dry period the discharge in the river may become less than the minimum requirements.

6. Reservoirs Thus to store the excess water, flowing during floods, dams are constructed across the rivers. This stored water is used to augment the supplies of the river during dry weather. By this arrangement it is possible to generate hydroelectric power, provide irrigation facilities and make available drinking water for whole of the year.

7. Reservoirs Actually dams and reservoirs are complementary to each other. Reservoir can be developed only by constructing dam, conversely when dam is constructed reservoir is bound to develop. Both dams and reservoirs, being very important elements of any multipurpose irrigation project, have to be very carefully planned, designed and operated. This involves proper selection of site, for dam and reservoir, fixation of capacity of reservoir, safe yield of the storage and other connected works.

8. Classification of Reservoirs On the basis of purpose of construction of reservoir, it is classified into three types: Storage reservoir Flood Control reservoir Distribution reservoir

9. 1. Storage Reservoirs One constructed this reservoir to store water which flows during the high flow, during rainy season and one uses the same during the dry summer seasons. It can serve the purpose of irrigation, hydel power generation, domestic and industrial water supply. In some cases, reservoirs also help to control the floods. So on the basis of requirement, these reservoirs can be single purposed or multi- purposed.

10. 1. Storage Reservoirs We know that discharge in the rivers remains changing day to day, and season to season. During high floods the excess water in the river goes waste while in dry months it may not be sufficient to meet the minimum needs. The storage reservoir is constructed to store the excess water of floods and released gradually as and when required.

11. 2. Flood Control Reservoirs To reduce the danger and the damage due to the floods, One constructs the flood control reservoirs. One calls it as flood protection reservoir. Generally these reservoirs are of single purposed type of reservoirs. One divided Flood control reservoirs into two types: Retarding Reservoirs Detention Basins Retarding Reservoirs: It has outlets and one does not control spillway by valves or gates. Detention Basins: It has outlets and one controls spillways by gates or valves.

12. 2. Flood Control Reservoirs This reservoir is also called flood mitigation reservoir. The main purpose of this reservoir is to temporarily store the flood water and release slowly at a safe rate after the floods, so that it may not cause any damage on the downstream side. So this reservoir may be said a flood prevention reservoir also. This reservoir requires provision of large spillways and sluice-ways so that excess stored flood water is rapidly released downstream, but only at the safe rate.

13. Storage Reservoirs or Detention basins The reservoirs, whose spillways and sluice outlets are fitted with gates and valves, are known as storage reservoirs or detention basins. Gated spillways and gated sluiceways provide more flexibility in operation. They help in exercising better control on the reservoir and thus reservoir water can be used more wisely and usefully. This reservoir is costly as it involves cost of gates and valves. Detention basins or storage reservoirs are preferred on large rivers, as better controls on flood waters can be exercised and water can be released from the reservoir at better controlled rate so as not to cause any damage on the downstream side.

14. Retarding Basins or Retarding Reservoirs In these reservoirs there are no gates at spillways and sluice outlets. In this case sluice and spillway’s joint maximum discharging capacity is at the most equal to the maximum safe carrying capacity of the channel downstream. As floods occur, the reservoir first of all gets filled up, upto normal level. At this time sluice outlets are discharging out water from the reservoir. As level in the reservoir increases further, discharge through sluice ways also increases. At certain level of reservoir, water also starts escaping through un-gated spillways. As the level of water further rises the discharge over spill ways also increases.

15. Retarding Basins or Retarding Reservoirs At some particular level a balance will be struck between the inflow and outflow of the reservoir. At this time level of water in the reservoir will become stable. This condition happens only when flood inflow in the reservoir is equal to the outflow from the reservoir. Now when floods occurring recede, inflow in the reservoir will also decrease but the outflow is at the same maximum rate. Hence flood water which had accumulated in the reservoir will now be slowly flowing out of the reservoir.

16. Advantages of Retarding Basins over detention basin (a) Gates are not required to be provided at sluice ways and spillway crests. (b) Since there are no gates at the spillways, chances of human error in opening the gates during floods cannot take place. (c) Since water from the reservoir is driven out in few days after floods, the land during maximum floods remains submerged only temporarily. This submerged land can be used for growing very good crops. But no habitation on this land should be allowed. Retarding basins are preferred on small rivers.

17. Advantages of Retarding Basins over detention basin (a) Gates are not required to be provided at sluice ways and spillway crests. (b) Since there are no gates at the spillways, chances of human error in opening the gates during floods cannot take place. (c) Since water from the reservoir is driven out in few days after floods, the land during maximum floods remains submerged only temporarily. This submerged land can be used for growing very good crops. But no habitation on this land should be allowed. Retarding basins are preferred on small rivers.

18. Distribution Reservoirs It is a small storage reservoir which one uses to make domestic water supply to the urban areas to meet the varying demands of the consumer at different period of a day. The one pumps the water into the reservoir, at a constant rate and one supplies it to the consumers when people reduces the water demand. And when the demand is high, one supplies water through the storage tank.

19. Distribution Reservoirs It is a small capacity reservoir which is mainly constructed to meet the water supply requirements of a particular city. It is made of masonry or cement concrete and may be covered from the top. This reservoir is filled by treated water at some constant rate. Since demand of water remains fluctuating during the day, water may have to be drawn from this reservoir at times at rate much more than the inflow rate.

20. Distribution Reservoirs Hence these reservoirs allow pumping units and treatment units to work at predetermined constant rates. During no demand or very little demand the water coming from treatment units and pump units goes on storing in the reservoir. During peak demand this stored water from the reservoir is used to make up the required supplies.

21. Purpose of reservoirs

22. Purpose of reservoirs 1. To act as source of water for any public water supply scheme. 2. To augment the irrigation supplies when discharge in the river is smaller than the demand. 3. To maintain some minimum level of water for generation of hydroelectric power during lean months of discharge. 4. To increase the depth of water to facilitate navigation.

23. Purpose of reservoirs 5. For reducing the flood havoc downstream. It is achieved as flood waters are temporarily held up in the reservoir. 6. To render water comparatively slit and debris free so that it may not cause any damage in hydroelectric generation equipment or pumping equipment if water is to be pumped for some public water supply scheme or any other purpose. 7. Growing useful aquatic life 8. Recreation.

24. Selection of Reservoir Site Before finally selecting the reservoir site following factors should be seriously considered: 1. Catchment area should have such geological conditions that percolation and absorption losses are minimum. 2. Available run-off should be maximum. 3. The site should be free from fissured rocks. This will avoid possibilities of leakage when reservoir is full to capacity. 4. The reservoir site must have adequate capacity. 5. The reservoir basin should have a deep narrow opening in the valley so that the length of the dam may be kept minimum.

25. Selection of Reservoir Site 6. Heavily silt laden tributaries should not lead their discharge to the reservoir. 7. Suitable site for dam should be available. It will be an ideal site if dam is constructed at the narrow and shallow part of the river which lies down stream of the deep river. It is very important point as cost of dam is often a controlling factor in selection of the reservoirs site. 8. Site should be such that deep reservoir is formed. Deep reservoir would store more of water and expose minimum area at the surface for evaporation. 9. If earthen dam is proposed to be constructed, then separate suitable site for spillway works should be available.

26. Selection of Reservoir Site 10. Reservoir site should be well connected by rail and road. 11. Materials for the construction of dam should be available nearby. 12. The soil formation at reservoir site should be free from harmful salts. 13. If reservoir water is to be used for irrigation, the dam site should be near the area proposed to be irrigated. This would reduce the length of the canal system and consequently the cost of the project. 14. Reservoir should not submerge habited area or areas of fertile lands or gardens. 15. River banks should be hard, strong and high so that cost on river training works is minimum.