Here is a compilation of notes on water engineering.
Note # 1. Construction of Open Wells:
From the point of view of construction the open wells may be classified in the following three types:
Type I: Wells with Impervious Lining:
These are the most stable and useful type of wells. For constructing such a well first of all a pit is excavated using hand tools such as pick and shovel upto the soft moist soil. A curb is then lowered into the pit. A curb is a circular ring of R.C.C., timber or steel having a cutting edge at the bottom and a flat top wide enough to support the well lining (or steining) of the desired thickness. Masonry or concrete lining is then built up on the curb upto a few metres above the ground surface.
ADVERTISEMENTS:
The soil in the pit is then excavated and as the excavation proceeds below the curb then the curb along with the lining sinks down. If the lining does not sink under its own weight then it is loaded on the top by sand bags etc. As the lining sinks down it is further extended at the top. To ensure vertical sinking of the well lining plumb bobs are used and if it starts getting tilted it is corrected by adjusting the loads or by removing the soil from below the curb which may be causing the tilt.
After the well has gone upto the water table, further excavation may be done either by continuously removing the water through pumps or the excavation may be carried out from the top by jhams. A jham is a self-closing bucket which is tied to a rope and worked up and down over a pulley. When a jham is thrown into the well, its jaws strike the bottom of the well, dislodging some of the soil materials. As the jham is pulled, the soil cuttings get retained but the water oozes out.
In the case of a deep well the sinking is continued till the impervious or mota layer is reached. A bore hole of small diameter is then made through the mota layer in the centre of the well, which is generally protected by timber lining.
In the case of a shallow well since mota layer is not available, the well is sunk as described above upto the required depth and it is partly filled with gravel or stone ballast or brick ballast. This will function as a filter through which sand particles will be prevented from rising up.
ADVERTISEMENTS:
In a well lined with an impervious lining on its sides, the flow is not radial. The water enters the well only from the bottom and the flow becomes spherical when once the cavity has been formed at the bottom.
Type II: Wells with Pervious Lining:
In this type of wells, brick or stone lining without mortar or binding material is used on the sides of the wells. The water thus enters the well from the sides through the pores in the lining. These wells are generally plugged at the bottom by means of concrete. Therefore the flow in these wells is radial.
However, if the bottom is not plugged, then the flow will be a combination of radial and spherical flows. Such wells are generally suitable in gravel or coarse sand strata. When such a well is constructed in finer soil, the pervious lining is surrounded by gravel or stone ballast or brick ballast to prevent the entry of sand into the well along with the percolating water.
ADVERTISEMENTS:
Type III: Kachha Wells:
These wells are not provided with lining. As such these are temporary wells of very shallow depths. Such wells can be constructed in hard soils where the well walls can stand vertically without any support. Further these wells can be constructed only where water table is very near the ground surface. Though these wells are quite cheap and useful, they may collapse after some time and may sometimes prove to be dangerous.
Note # 2. Zones of Storage in a Reservoir:
The entire storage capacity of a reservoir may be divided into a number of zones by certain water surface levels or pool levels in the reservoir as indicated below:
Normal Pool Level (N.P.L):
ADVERTISEMENTS:
It is the maximum elevation to which the water surface will rise in the reservoir during ordinary operating conditions in the case of an ungated spillway the normal pool level is determined by the elevation of the spillway crest. However, if the spillway is gated then the normal pool level is determined by the top of the spillway gates. The normal pool level is also known as Full Reservoir Level (F.R.L) or Full Tank Level (F.T.L).
Minimum Pool Level:
It is the lowest elevation to which the water is drawn from the reservoir under normal conditions. This level may be fixed by the elevation of the lowest outlet in the dam, or in the case of hydroelectric reservoirs, by the minimum head required for efficient functioning of turbines.
Maximum Pool Level:
ADVERTISEMENTS:
It is the maximum elevation to which the water surface will rise in the reservoir during the design flood (or worst flood). It is also known as Maximum Water Level (M.W.L.) or Pool Level during Design Flood, or Highest Flood Level (H.F.L.)
The various zones of storage in a reservoir are as follows:
(a) Useful Storage:
The volume of water stored between the normal pool level and the minimum pool level of a reservoir is called the useful storage. The useful storage is also known as live storage as it can be used for various purposes required to be served by the reservoir.
(b) Dead Storage:
The volume of water held below the minimum pool level of a reservoir is known as dead storage. It cannot be used for any purpose under ordinary operating conditions.
(c) Surcharge Storage:
The volume of water stored between the normal pool level and the maximum pool level of a reservoir is called surcharge storage. The surcharge storage is an uncontrolled storage as it exists only while a flood is occurring and cannot be retained for later use.
(d) Bank Storage:
The bank storage is the volume of water that is temporarily stored in the permeable banks of a reservoir when the reservoir fills and drains out as the water level in the reservoir is lowered. The bank storage effectively increases the capacity of the reservoir above that indicated by elevation-capacity curve. The amount of bank storage depends on geologic conditions and may amount to several percent of the reservoir volume.
(e) Valley Storage:
The volume of water held by a natural stream channel or river is known as valley storage. Even before a dam is constructed and reservoir is created certain amount of water is stored in the natural stream channel as a valley storage which may however vary. After the creation of a reservoir the storage capacity increases but the net increase in the storage capacity is equal to the total storage capacity of the reservoir minus the natural valley storage.
This distinction of the total and net storage capacities is of no importance for storage reservoirs, but from the point of view of flood control the effective storage capacity in the reservoir is equal to the useful storage plus the surcharge storage minus the natural valley storage corresponding to the rate of inflow to the reservoir.
Note # 3. Divisions of Subsurface Water:
The subsurface water may be divided into the following two zones:
The zone of aeration overlies the zone of saturation and extends upward to the ground surface. In the zone of aeration the interstices are partially occupied by water and partially by air. The water that occurs in the zone of aeration is usually termed as vadose water.
The zone of aeration is further subdivided into the following three zones:
(i) Soil-Water Zone:
The soil-water zone extends from the ground surface down upto the major root zone. The soil in this zone becomes saturated either during irrigation or when rainfall occurs. However, the soil in this zone remains saturated only for a short duration after irrigation or rainfall, because the excess water drains through the soil under the influence of gravity.
This excess water which cannot be retained by the soil is known as gravitational water. After the gravitational water drains out the remaining water is held by surface tension forces and is known as capillary water.
The water of the soil-water zone is gradually depleted by evaporation from with in the soil and by transpiration from vegetal growth on the ground surface and if it is not replenished the water content may be reduced to such an extent that only thin film of moisture known as hygroscopic water remains absorbed on the surface of the soil particles.
(ii) Intermediate Zone:
The intermediate zone extends from the lower edge of soil-water zone to the upper limit of the capillary zone. This zone usually contains non-moving vadose water (or pellicular water) which is held by molecular
and surface tension forces in the form of hygroscopic and capillary water.
Temporarily, this zone may also contain some excess water which, however, moves downward as gravitational water. The thickness of this zone may vary from zero when the water table is high and is approaching the ground surface, to more than 100 m under deep water table conditions.
(iii) Capillary Zone:
The capillary zone extends from the water table upto the limit of capillary rise of water. The pore space may be considered to represent a capillary tube and hence just above the water table almost all pores contain capillary water which constitute this zone.
In the zone of saturation all the interstices are filled with water under hydrostatic pressure. The zone of saturation is bounded at the top by either a limiting surface of saturation called water table or an overlying impermeable strata, and extends down to underlying impermeable strata (or bed rock).
At the water table there is atmospheric pressure. Thus if a well penetrates the zone of saturation with water table forming its upper surface then the static water level in the well stands at the same elevation as the water table.
The saturation actually extends slightly above the water table due to capillary action, which constitutes the zone of aeration. On the other hand when the zone of saturation is bounded at the top by an impermeable strata, the water is in contact with the bottom of the impermeable strata and is under pressure and consequently there is no water table. When a well penetrates a zone of saturation of this type, the water will rise above the bottom of the confining strata.
The water occurring in the zone of saturation is known as groundwater. The thickness of the zone of saturation may vary from a few metres to hundreds of metres. The formation within the zone of saturation from which sufficient quantity of groundwater can be obtained may be termed as aquifer.
Note # 4. Loss of Water from a Reservoir:
Some quantity of water stored in a reservoir is always lost and hence it is necessary to account for the same in the planning and designing of a storage reservoir. The loss of water from a reservoir may be due to evaporation, absorption and percolation.
The evaporation loss mainly depends on the water surface area of the reservoir. The other factors influencing evaporation loss are temperature, wind velocity and relative humidity. The loss of water due to evaporation is usually expressed in terms of the depth of water measured in millimetres or centimetres, which thus represents the volume of water lost per unit area of the water surface of a reservoir.
The evaporation loss can be determined by measuring the depth of evaporation in a pan and multiplying the same by a predetermined pan coefficient. Various types of pans have been developed and standardized for the measurement of evaporation.
Though the evaporation loss may vary from place to place, the observed values of the same on existing reservoirs can be adopted for the design of a new reservoir after making due allowance for difference in the conditions of the two. The average values of the evaporation loss for North, and South and Central Indian reservoirs on mean exposed water surface area during each month of the year are as indicated below.
In order to reduce the loss of water due to evaporation in reservoirs the following measures may be taken:
1. By constructing reservoirs of less surface area and more depth, so that total evaporation is less due to less surface area being exposed to atmosphere.
2. By growing tall trees on the windward side of the reservoir which act as wind breakers and hence reduce evaporation.
3. By spraying certain chemicals or fatty acids and thus forming thin film of these substances on the water surface in the reservoir. For this purpose cetyl alcohol (Hexadecanol) is commonly used, which when sprayed on the water surface spreads as a monomolecular layer forming a thin film on the water surface which is only about 0.015 micron in thickness.
This film allows the precipitation from the top to enter the reservoir but does not allow water molecules to escape through it and hence reduces evaporation. However, this method is effective only when wind velocities are less. Further it is best suited only for small and medium size reservoirs.
4. By providing the outlets in such a way that the warmer water is released and hence the temperature of water is reduced and consequently the evaporation is also reduced.
5. By removing the weeds and plants from the periphery of the reservoir.
6. By providing coverings of thin polythene sheets for the water surface. This is possible only for small ponds and lakes.
7. By developing underground reservoirs, since the evaporation from a ground water table is very much less than the evaporation from a water surface.
8. By growing huge trees and forest around the reservoir so that due to cooler environment the evaporation will be less.
The loss of water from a reservoir due to absorption will depend on the type of soil forming the reservoir basin. This loss may be quite large in the beginning but will be gradually reduced as the pores get saturated. As such the absorption loss is not significant in reservoir planning and design.
The percolation loss is also small in most of the cases. However, it may be significant in some cases where severe leakage may occur under or through the surrounding hills or under the base of the dam through continuous seams of porous strata or cavernous or fissured rock. The reservoir basin and the surrounding hills forming its rim should therefore be carefully checked for watertightness during geological investigations and necessary remedial measures such as grouting etc., should be taken to prevent such leakage.
The total loss of water in a reservoir during a given period can be determined by measuring the fall in the reservoir level and knowing the inflow and outflow rates during this period. From the inflow and outflow rates the total inflow and outflow during a given period can be computed and the fall in the reservoir level multiplied by the average water surface area gives the change in storage during this period. The total loss of water in the reservoir during a given period is then given by-
Total loss = Inflow – Outflow + Change in Storage
The values of the total loss of water in existing reservoirs determined as indicated above usually provide valuable guidance for planning of future reservoirs.
Note # 5. Reservoir Yield:
Reservoir yield (or yield of a reservoir) is the amount of water which can be supplied from a reservoir in a specified interval of time. The time interval may vary from a day for a small distribution reservoir to a year for a large storage reservoir. Reservoir yield is dependent upon inflow and will vary from year to year.
For most of the storage reservoirs in addition to yield it is also necessary to know safe or firm yield and secondary yield which are defined below-
Safe or firm yield is the maximum quantity of water which can be supplied from a reservoir during a critical (or worst) dry period. In practice the period of lowest natural flow on record for the stream is usually taken as the critical period. However, there is a possibility that a drier period may occur in future with a yield even less than the safe yield determined on the basis of the past record of the stream flow.
Secondary yield is the quantity of water available in excess of safe yield during period of high flows.
Average yield is the arithmetic average of the yield (firm and secondary) over a long period of time.
Note # 6. Bacterial Efficiency of Chlorine in Chlorination:
The bactericidal efficiency of chlorine is affected by the following factors:
1. Turbidity:
The effect of turbidity in water is to make it difficult to obtain free residual chlorine. Further the penetration of chlorine and consequent destruction of bacteria in the particles of suspended matter of turbid water may be uncertain. Hence for effective chlorination the water should be free from turbidity.
2. Presence of Metallic Compounds:
The compounds of iron and manganese if present in water utilize large amount of chlorine to convert them into their higher stages of oxidation which are insoluble in water. This would result in reducing the quantity of chlorine to be utilized for disinfection. Hence it is essential to remove such metallic compounds to make chlorination more effective.
3. Ammonia Compounds:
The presence of ammonia with or without organic matter in water may form combined available chlorine which is not as effective as bactericide as free available chlorine. It has been reported that about 25 times as much combined available chlorine must be used to achieve the same degree of kill as free available chlorine. Further if similar doses of free and combined chlorines are used, then the combined chlorine will take 100 times as long as the free chlorine to achieve the same degree of kill.
Thus if ammonia is naturally present in water, it would be necessary to add sufficient chlorine to react with the natural ammonia present in water and also to create an excess of free chlorine for speedy disinfection.
4. pH Value of Water:
Increasing pH value of water reduces the effectiveness of chlorine as bactericide. The effective sterilising compound hypochlorous acid (HOCl) is formed in greater quantity at low pH values than at high pH values.
5. Temperature of Water:
Reduction in the temperature of water results in substantial decrease in the killing power of both free and combined chlorine. It has been reported that in order to achieve a higher bactericidal efficiency the requirement of residual chlorine increases with decrease in temperature and increase in pH value of water.
6. Time of Contact:
The bactericidal activity of chlorination is not instantaneous. The killing of bacteria and viruses depends upon the time of contact between the chlorine and the micro-organisms, and it increases with contact time available for disinfection.
The contact time required for satisfactory disinfection depends on various factors such as temperature, pH value, type and concentration of micro-organisms and form of chlorination. Generally for disinfection by free chlorine acting in clear water, a contact time of 30 minutes is required, while it is 60 minutes for combined chlorine such as chloramines etc.
In the case of large water treatment plants adequate time of contact is available because the chlorinated water has a considerable detention in the clear water reservoir before it is supplied to the consumers.
However, in the case of small water treatment plants where such storage is not provided, the contact time is determined by the time taken for the water to flow from the point of application of chlorine to the point of withdrawal of water by the first consumer. If the minimum contact time is not available the dose of disinfectant should be suitably increased.
7. Type Condition and Concentration of Micro-Organisms:
Bacteria and viruses are the two main types of micro-organism present in water. It has been observed that enteric pathogenic bacteria are less resistant to chlorine than E- coli bacteria, and hence if E-coli bacteria have been killed, satisfactory disinfection of water is ensured. Further viruses are more resistant than bacteria and hence require longer time of contact and higher dose of chlorine.
The condition in which the micro-organism occurs may also affect the efficiency of chlorination. Thus when bacteria are clumped together, the cell inside the clump may be protected against the action of chlorine. A higher concentration of microorganisms would require longer time of contact and higher dose of chlorine for satisfactory disinfection of water.