In this article we will discuss about:- 1. Introduction to Reservoir 2. Classification of Reservoirs 3. Purposes of Water Storage 4. Selection 5. Storage Zones 6. Various Yields 7. Life 8. Silting Power 9. Silt Control 10. Reservoir Losses.
Contents:
- Introduction to Reservoir
- Classification of Reservoirs
- Purposes of Water Storage in Reservoir
- Selection of Reservoir Site
- Storage Zones of a Reservoir
- Various Yields of Reservoir
- Life of Reservoir
- Silting Power of Reservoirs
- Silt Control of Reservoirs
- Reservoir Losses
1. Introduction to Reservoir:
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.
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.
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.
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.
2. Classification of Reservoirs:
Depending upon the purposes served, the reservoirs may be classified into following categories:
1. Storage or Conservation Reservoirs:
These reservoirs are primarily used to maintain minimum supplies of water for irrigation, hydroelectric generation, domestic and industrial water supply schemes, etc. during lean months of discharge in the rivers. 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.
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. Figure 9.1 shows hydrographs at certain point on the river. ABC is the natural hydrograph at the dam site having a maximum flood discharge Q1.
After construction of the dam the hydrograph gets modified because of the development of a reservoir at the back of the dam. AB’DC is the modified hydrograph showing maximum flood discharge as Q2. Thus, because of reservoir the flood discharge has reduced from Q1 to Q2. Shaded are ABDB’A represents the storage to be provided in the reservoir. The area DEC represents the excess volume being released from the reservoir after the floods have receded.
Flood control reservoirs may be further classified into two categories:
(i) Storage reservoir of Detention basins.
(ii) Retarding basins or retarding reservoirs.
(i) 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.
(ii) 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.
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.
This reservoir has the following advantages 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.
3. Distribution Reservoir:
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.
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.
4. Multipurpose Reservoirs:
The reservoir planned and designed keeping only one purpose in view is known as single-purpose reservoir. Reservoir planned and designed keeping more than one purpose in view is called Multi-purpose reservoir. For example – a reservoir designed to protect the downstream areas from floods and also to conserve water for irrigation, water-supply, hydroelectric purpose etc. shall be called a Multi-purpose reservoir. Bhakra Dam, Nagarjun Sagar Dam are the examples of important multi-purpose projects of India.
3. Purposes of Water Storage in Reservoir:
The storage of water may be done for following purposes:
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.
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.
4. 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.
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.
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.
5. Storage Zones of a Reservoir:
1. Dead Storage:
The volume of water stored below the minimum pool level of the reservoir is known as dead storage. This storage is not of such use in the operation of the reservoirs.
2. Useful Storage:
The volume of water stored in a reservoir between minimum pool level and normal pool level is known as useful storage. The useful storage may be further classified into conservation storage and flood mitigation storage in a multi-purpose reservoir.
3. Surcharge Storage:
The volume of water stored between normal pool level and the maximum pool level is known as surcharge storage. This storage is an uncontrolled storage. It exists only till floods are in progress and cannot be retained for later use.
4. Bank Storage:
When reservoir is full of water some amount of water seeps in the permeable banks of the reservoir. This seeped water comes out as soon as the reservoir level gets depleted. This amount of seeped water which becomes available after the reservoir is depleted is known as Bank storage.
Amount of bank storage may amount to several per cent of the reservoir storage depending upon the geological formations of the banks. This storage increases the capacity of the reservoir above that indicated by the elevation-volume curve as shown in Fig. 9.2.
5. Valley Storage:
Some amount of water is stored by the stream channel even before a dam is constructed. This storage is known as valley storage. Amount of valley storage is variable as it depends upon the rate of flow in the reservoir. After the construction of dam the valley storage increases. The net increase in the storage is equal to the storage capacity of the reservoir and less than natural valley storage. The valley storage thus reduces the effective storage capacity of the reservoir.
This storage is not of much consequence in conservation reservoirs, but the available storage for flood mitigation is reduced as follows:
Effective storage for flood mitigation
= Useful storage + Surcharge storage – Valley storage.
6. Various Yields of Reservoir:
1. Yield of the Reservoir:
It is the amount of water that can be supplied by the reservoir in a specified interval of time. The specified time interval may vary from a day for a small distribution reservoir to a month or year for large conservation reservoirs. If we say that three million cubic metres of water can be supplied from a reservoir in a year then its yield is 3000000 m3/year. The yield of the reservoir is dependent upon the inflow and thus varies from time to time.
2. Safe Yield:
It is also known as firm yield. It is the maximum quantity of water that can be supplied from the reservoir with full guarantee during the worst dry period.
3. Design Yield:
The critical period for a reservoir is generally considered, when natural flow in the reservoir is minimum. There is possibility that sometimes the minimum natural flow in the reservoir may even fall short of guaranteed yield. Hence a lower value than the guaranteed yield or safe yield may be taken for design purpose.
This yield whose value is smaller than the safe or firm yield is known as design yield. The value of design yield for a reservoir to be used for water supply is taken less than the safe yield. In the case of reservoirs used for irrigation purpose the design yield may be taken slightly more than the safe yield as crops can tolerate some deficiency of water during exceptionally dry season.
4. A Secondary Yield:
The quantity of water available in excess of safe yield is known as secondary yield. This yield is available during period of high inflows. This secondary yield of the reservoir can be used either to generate extra hydroelectric power or for irrigation of extra lands.
5. Average Yield:
The arithmetic average of the safe yield and the secondary yield considered for a number of years is known as average yield.
The storage capacity of the reservoir and its yield are very much interdependent. The water is stored in the reservoir to fulfill the safe yield requirements. If capacity of the reservoir is more it can certainly provide more water and hence yield is more. The reservoirs are designed to meet a specific demand of water.
The capacity of the reservoir and the yield are governed by the following storage equation:
Inflow – outflow = Increase in storage.
7. Life of Reservoir:
Any reservoir cannot last forever. Ultimately all the reservoirs get silted up. Silting of the reservoir starts from the day it is created. When reservoirs are created some of its capacity is left unused. This is the capacity of the reservoir lying below the crest level of the bottom most under sluices.
This storage capacity which remains unused is known as dead storage. This dead storage capacity is used to accommodate deposited silt so that effective storage of the reservoir is not affected. So long as dead storage capacity of the reservoir is not silted completely, effective storage or useful storage capacity is not affected.
The process of silting continues even after complete silting of dead storage. The further silting affects the effective storage of the reservoir and the reservoir does not have enough water to fully fulfill its obligations. Generally useful life of the reservoir is considered terminated when its effective storage is reduced by 20% of the designed capacity of the reservoir.
8. Silting Power of Reservoirs:
For reservoirs constructed only for power generation, silting is not as important as for the reservoirs constructed for irrigation, and water supply. This is due to the fact that for running turbines a minimum head of water is required.
So long as this head is available the working of power reservoir is not affected. Obviously, if the silting has affected the minimum capacity of the reservoir required for generation of power, the generation of power will not be maintained, as it should be as revervoir will deplete very fast and there will be no water to run the turbines.
Hence while silting of the reservoir is not affecting the head of water over the turbines, it is certainly affecting the reservoir capacity.
9. Silt Control of Reservoirs:
Entrance of silt into the reservoir may be controlled by adopting following preventive measures:
1. Reservoir site should be properly selected. If nature of catchment soil contributing water to the reservoir is very soft, the rate of silting will he more, as soft sand will be easily carried by run-off. If slope of the catchment is steep, it will also cause more of silting, as steepness would cause increased velocity of flow of run-off and thus more erosion of the catchment soil.
2. The tributaries carrying more of silt load to the reservoir should be provided with check bunds somewhere upstream of the tributaries. Check bunds would cause sedimentation of silt load upstream and comparatively clear water will reach the main reservoir. The small reservoirs created at the back of the check bunds also supply stored water to the main reservoir when supplies in the tributaries have receded considerably.
3. By increasing the vegetation growth on the catchment, silt entry into the reservoir can be reduced.
4. Silt deposition in the reservoir can be considerably reduced and deposited silt can be scoured by operating sluice gates properly. During floods the inflow of silt is more and scour sluices should be kept open.
5. The deposited silt should be dredged from time to time.
6. If dam is constructed in stages, the rate of silting in the reservoir is kept low. In this measure the dam should be constructed of smaller than the required height and operated for five years. After this, dam is further increased in height and again operated for few years. By this method (capacity / inflow) rate is kept small and trap efficiency is kept under check.
10. Reservoir Losses:
Reservoir losses may be classified under following three heads:
1. Evaporation Losses:
These are the major losses from a reservoir. This loss is affected by exposed surface area of the reservoir, wind velocity, temperature, relative humidity etc. This loss is expressed in cm of water depth and varies from place to place depending upon the local conditions such as temperatures, relative humidity, wind etc.
Average values of losses in cm for North and South India for various methods of the year are given as follows:
2. Absorption Losses:
This loss of water is considerable in the beginning, but falls to very small values after some time when pores get saturated. These losses depend upon the soil forming the reservoir. These losses are not considered while planning a reservoir.
3. Reservoir Leakage or Percolation Losses:
Reservoirs being very large, their banks are permeable. But permeability of the soil is generally very low and hence these losses do not carry any importance. But in certain cases the banks of the reservoir may be made of badly fractured rocks or having continuous seams of porous strata.
Such conditions may cause serious leakage. To stop such leaks grouting with cement may have to be resorted. Hence while selecting the site of the reservoir, this aspect of the site is investigated in great length and if any such fault is noticed, it is rectified before reservoir is constructed.