Here is a term paper on ‘Reservoirs’ for class 7, 8, 9, 10, 11 and 12. Find paragraphs, long and short term papers on ‘Reservoirs’ especially written for school and college students.
Term Paper on Reservoirs
Term Paper Contents:
- Term paper on the Introduction to Reservoirs
- Term paper on the Types of Reservoirs
- Term paper on the Selection of Site for Reservoirs
- Term paper on Investigations for Reservoir and Dam Site
- Term paper on the Silting or Sedimentation of Reservoirs
- Term paper on Water Losses from Reservoirs
- Term paper on Reservoir Capacity from Mass Curves
Term Paper # 1. Introduction to Reservoirs:
A dam is constructed to create reservoir by storing the river flow in the valley. Thus storage work comprises a reservoir created by constructing a dam across the river. In the body of the dam a spillway is provided to pass down the flood flow safely. Also, sluices and outlets are provided to release the water for irrigation purposes.
Lakes, ponds, streams, pervious soil mass and the water bearing rock formations are the best examples of natural water storages. Absence of, natural storages of adequate capacities necessitate construction of some artificial storage works.
In rainy season there is excess flow down the valley in a river. An impounding reservoir can be constructed in the valley to store this excess water. This stored water meets the demand in dry periods. Reservoirs may be constructed to serve a single purpose or many (multi) purposes.
The various purposes for which reservoirs are required are mentioned below:
i. Irrigation.
ii. Hydro-electric power generation.
iii. Control of destructive floods.
iv. Low water regulation for navigation.
v. Domestic and industrial water supply.
vi. Recreation.
vii. Preservation and cultivation of useful aquatic life.
Term Paper # 2. Types of Reservoirs:
Reservoirs are constructed to provide flexibility to the supply for satisfying demand of water of adjacent region.
Depending upon the functions served following categories of reservoirs can be recognised:
a. Conservation Reservoirs:
They are constructed to store water during the period of excess flow. Stored water is used subsequently when demand is more because of less natural water storage. The demand of water may be for irrigation or hydro-electric power generation or domestic and industrial water supply.
b. Food Control Reservoirs:
In this case the reservoirs absorb flood water during the period of excess flow, to reduce the severity of flood. The water which is absorbed during the period of excess flow is released through the reservoirs gradually.
c. Multipurpose Reservoirs:
They are constructed to serve more than one purpose. For example, common combinations of functions may be of irrigation, water power generation, domestic and industrial water supply or irrigation, flood control and water power generation.
Term Paper # 3. Selection of Site for Reservoirs:
While selecting a site for a reservoir following points should be taken into account:
(a) Geological Factors:
(i) Reservoir area should be away from earth-quake epicentre.
(ii) Reservoir catchment must not have loose soil cover. It produces more silt and fills up the reservoir.
(iii) Reservoir bed should not allow quick percolation of stored water.
(iv) Soil in the reservoir area should not contain harmful salts and minerals.
(v) Reservoir site should not lie over excessively faulted and folded rock formations.
(b) Topographical Factors:
(i) The reservoir basin should have narrow outlet. It reduces the length of dam to be constructed.
(i) The reservoir basin should be wide above the dam site to afford more storage of water.
(iii) The reservoir should have steep and high side slopes to give maximum storage.
(c) Other Factors:
(i) The reservoir should not submerge valuable land and property.
(ii) The reservoir site should not be too far away from the irrigable area.
(iii) Catchment area of the reservoir should produce good run-off.
Term Paper # 4. Investigations for Reservoir and Dam Site:
Following methodical investigations are generally carried out for selecting site of storage works:
A. Reconnaissance:
When it is decided to start a storage project in any region, there may be various localities where storage of water can be done. By very rough type of survey, which is also known as reconnaissance, we can actually find out number of sites where storage of water can be done.
In reconnaissance inspection of the region is done physically without using precise surveying instruments. Thus reconnaissance provides number of sites where construction of storage project is feasible. Next step is to study merits and demerits of each site.
B. Primary Surveys:
To study the sites in all respects various types of investigations are necessary. Following three types of investigations may be said to fall in primary surveys. They furnish required data.
(a) Engineering Surveys:
This category includes various types of engineering surveys, for example, plane-table survey, traverse survey, aerial and photographic survey, etc. The aim of this type of investigation is to prepare a contoured map or a topographic map of the site.
If the area is fairly plain, the contour interval may be about 2 metres. But if the area is very hilly, contour interval may be kept 5 to 10 metres. For a dam site careful triangulation survey is required. Here contour interval required is obviously less; say upto 1 m. Horizontal scale may be from 10 to 5 metres per cm.
The contoured map once prepared, will furnish necessary information about:
(i) Water spread.
(ii) Arrangement of lines of communication.
(iii) Capacity of the reservoir.
When the height upto which water is to be stored, is decided from the contour map the extent of area over which water will spread can be calculated accurately. It is given by the area enclosed in the contour at that height. The area enclosed in the contour can be measured by planimeter.
In the same way the map furnishes extent of water spread at any height. After knowing the area going to be submerged land can be acquired accordingly. The compensation to be paid to the land owners can now be estimated correctly.
Knowing topographical features of the area, roads, railways and other lines of communication can be best aligned and constructed.
The capacity of a reservoir basin can also be calculated mathematically from contour maps.
The formulae used are:
(i) Trapezoidal Formula:
Where V is the volume of space enclosed between two contours.
A1 and A2 are the areas enclosed in two contours.
H is the contour interval. This method is useful in finding out volume of the space between two successive contours only.
(ii) Simpson’s Formula:
The reservoir capacity is generally given in million cubic metres or hectare- metres.
(b) Geological Exploration:
Geological investigations cost little in comparison with the total cost of the scheme. This relatively small amount represents a valuable insurance against many unforeseen difficulties which might arise during construction programme.
Geological exploration is generally done in two stages:
(i) Surface inspection and surface mapping.
(ii) Sub-surface exploration.
Sub-surface exploration is generally done by digging open trenches, core drilling or tunneling into the sides of the valley.
Geological Exploration Gives Valuable Information Regarding:
(a) Water-tightness of the basin.
(b) Depth of over-burden.
(c) Nature of rock formation.
(d) Geological structural features tor example faults, folds, joints fissures etc.
(e) Properties of subsurface at various depths.
(f) Availability of construction materials locally.
(g) Ground water conditions in the region.
(c) Hydrologic Survey:
Whenever any storage work is to be constructed, it is very essential to ascertain that required amount of water will be available for storage.
In this category, the runoff pattern at the site of a dam is studied for number of years. On the basis of past records, future predictions are made. The highest water levels attained during worst floods in a season are noted and studied to design the spillway capacity.
Thus primary survey gives complete data of various sites in view. At this stage some sites can be discarded, being uneconomical. Here engineers are left with few sites for final and detail considerations.
Detailed and Precise Investigations:
After primary surveys only few sites are selected for further considerations. In this type of investigation office studies and estimates for each of the selected sites is prepared. Detailed design of various storage structures are prepared in the drawing office and the cost is worked out. Thus relative merits and demerits of two or more sites are taken into consideration so as to adopt one site finally.
Term Paper # 5. Silting or Sedimentation of Reservoirs:
During the floods, runoff water carries large amount of clay, silt and sand. This load may be in suspension or rolling over the bed. This runoff water comes in the reservoirs along with its silt and sand load.
The reservoir arrests the flood flows and a large part of its silt load is deposited in the reservoir. Thus the accumulation of rather coarse material goes on in the reservoir. In course of time the reservoir gets silted up. It reduces the storage capacity meant for various purposes. This in turn reduces useful life of the reservoir.
Since the entry of silt and sand load cannot be stopped completely dead storage capacity is kept reserved for containing the silt accumulation. At the same time it is necessary to adopt remedial measures to prolong the time of complete silting up of the dead storage capacity.
Following methods can be successfully used to control silting of reservoirs:
(i) Construction of Check Dams:
Small bunds may be constructed across the tributaries of the main drainage. The low check dams allow clear water to pass over them but arrest sand and silt load of the water. Thus it controls sediment inflow into the reservoir.
(ii) Dredging:
The method involves mechanical removal of the sediment from the reservoir by excavation under water. Other methods of removal of sediment deposit are excavation and sluicing aided by mechanical agitation of the sediment.
(iii) Scouring Sluices in Dams:
They are outlets in the dam at lower levels. Lower layers of water contain high concentration of silt load. The outlets drain-off this water during the time of heavy inflow in the reservoir. The outlets are controlled by means of shutters as shown in Fig. 13.15. This method aims at controlling the sediment deposition in the reservoir.
It is very essential to adopt some preventive method. The root cause of this silting problem is soil erosion in the catchment of a reservoir. To prevent silting soil erosion has to be checked. Steps taken to check soil erosion are called soil conservation methods. Soil erosion takes place through the agencies like wind and flow of water and ice. Soil conservation methods can be divided into two groups, viz., engineering practices and agricultural practices.
In engineering practices following methods may be included:
(a) Excavation of Ditches:
They are the channels excavated at suitable locations to divert excess runoff water. The runoff water is thus prevented from flowing over the fields which causes erosion.
(b) Construction of Terraces:
It is a broad flat field constructed on the slope of a hill. It is given slight longitudinal slope. The shooting rain water is obstructed by the terrace. On the terrace this water flows in a lateral direction along the longitudinal slope.
(c) Construction of Check Dams:
In agricultural methods following are worth mentioning:
(d) Afforestation:
It is not sufficient to plant more trees. Deforestation should be legally restricted. Forests should be protected against fires, tree cutting etc.
(e) Crop Rotation:
Field erosion can be checked by adopting suitable sequence of crops for rotation. The sequence commonly followed is a cultivated crop, a small grain and then grass. The cycle is repeated over and over again.
(f) Strip Cropping:
In this method cultivated crops and cover crops are sown in alternate strips. The strips are generally kept parallel to the contours.
Term Paper # 6. Water Losses from Reservoirs:
Water spreads over a large area to form a storage reservoir. It is impossible to come across a fully impervious and impermeable basin. Water surface of a reservoir is exposed to the atmosphere. Hence loss of water always occurs from the reservoir storage. For successful reservoir operation it is very essential to have full idea about water losses from the reservoir. There are three kinds of significant losses.
They are:
(i) Evaporation loss
(ii) Absorption loss, and
(iii) Percolation loss.
(i) Evaporation Loss:
Due to the heat energy of sun water is lost in evaporation from the free water surface of the reservoir. It is directly proportional to the exposed water surface area. Loss of water by evaporation is a very serious problem in arid regions.
The loss is increased when temperature is more or wind is more and humidity is less. The evaporation loss can be expressed in cm depth of water like rainfall. The loss can be measured very conveniently by keeping water in a pan at the reservoir site. Loss can be measured daily or weekly.
(ii) Absorption Loss:
In the initial stages loss of water due to absorption may be quite significant. In course of time the soil pores became completely saturated. Then the loss due to absorption is negligible.
(iii) Percolation Loss:
The reservoir water is always lost in percolation since it is impossible to come across water-tight basin. Geological investigations should be carried out before finally selecting a reservoir site. When it is ascertained that there is no excessive leakage the site can be adopted. Sometimes, treatment of some parts of reservoir bed and sides may become necessary to make them water-tight.
Term Paper # 7. Reservoir Capacity from Mass Curves:
Availability of water is ascertained before any project is contemplated. The next step in reservoir planning is to fix the reservoir capacity.
The reservoir has to provide sufficient storage for various purposes, namely:
(i) Dead storage to contain silt deposition;
(ii) Storage to account for evaporation loss;
(iii) Live storage to meet the downstream demand for irrigation; domestic or industrial supply, power generation etc., and
(iv) Storage to act as flood cushion.
The basis of fixing storage capacity for dead storage and evaporation loss depends upon the amount of incoming sediment and the annual evaporation loss respectively. Requirement of flood cushion depends on the intensity and volume of flood flow. The live storage capacity of a reservoir depends on the demand for various purposes. It can be arrived at by plotting the mass curves of demand and inflow on the same graph.
That mass curve is a graph of cumulative runoff plotted against time. The capacity of the reservoir is fixed in such way as to take care of the demands during the minimum flow period in the driest year on record. In some cases it is necessary to cover a period of successive dry years to consider storing of sufficient water to meet the demands during the periods of prolonged drought.
The mass curves of inflow and demand can be plotted as shown in Fig. 13.16. The discharges are added month by month and then plotted for all the years for which data is available. Similarly a mass curve of demand is also plotted on the same graph.
Sometimes the demand of water is constant throughout the year. It is generally so in case of a hydro-electric project. In such cases the mass curve of demand represents a straight rising line as shown in Fig. 13.16.
Mass curve may be a straight line or a curve; it is always a rising curve. It is obvious from the fact that it is plotted for cumulative flow. If there is no flow for a particular period, addition to the accumulated flow for that period is nil. Then the curve is parallel to horizontal axis for that particular period but in any case it does not fall below horizontal line.
Naturally if the slope of mass curve is steep then it is clear that the instantaneous addition of flow is more. That is the rate of flow is more. But if the slope is flat it indicates a dry period. The method of finding the storage capacity is based on this fact.
Just by visual inspection we can see when the inflow rates are low, that is when the inflow mass curve is flat. At this time if the slope of mass curve of demand is more, naturally it shows that present rate of demand is more than the rate of supply or inflow. Obviously to fulfill the demand some storage is required. The question now is how much storage should be done. It can be best understood from Fig. 13.17 which is an enlarged part of Fig. 13.16.
To find out the required storage capacity a line is drawn parallel to the mass curve of demand from the beginning of a dry period. In Fig. 13.17 beginning of the dry period is shown by a point P. The line cuts the mass curve of inflow or supply in B. That is at this stage again the instantaneous rate of supply is more than the demand. So water storage is required only for the period P-B.
As the curves are of accumulated values the maximum ordinate in deficiency area gives the storage required. At section XZ the demand is XZ whereas total supply upto that time from the beginning of dry period is only YZ. Obviously the storage capacity of amount XY is required to take care of the dry period.
Sometimes a mass curve of demand may not be a straight line because of varying demands. In this case also storage capacity is calculated on the same basis.