Water Engineering Exam Questions and Answers

Here is a compilation of water engineering exam questions and answers.

1. What are the Factors Considered While Selecting a Source of Water Supply?

The following factors are generally considered while selecting a source of water supply for a particular town or city:

i. Quantity of Water:

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The source should be able to supply enough quantity of water to meet the various demands of the town or city such as domestic, industrial, firefighting, etc., during the entire design period, which is usually 20 to 30 years.

ii. Quality of Water:

The source should have water which is wholesome, safe and free from pollution of any kind and other undesirable impurities. The water should not be toxic poisonous or in any other way injurious to health. Moreover, the impurities present in the water should be as less as possible, and these should be removed easily and cheaply by normal methods of treatment of water.

iii. Location of Source:

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The source of water supply should be situated as near to the town or city as possible and also it should be so located that as far as possible the water from the source would flow by gravity. Besides convenience this will result in reducing the overall cost of the project.

iv. Cost of Water Supply Project:

The source should be such that the overall cost of the water supply project is as far as possible minimum. The cost of a water supply project would very much depend on the location of the source of water supply. If the source of water supply is located at a sufficient higher elevation than the town or city to be supplied with water then the water from the source would flow by gravity and the operational costs would be less.

On the other hand if the source of water supply is located at a lower elevation than the town or city to be supplied with water then the water has to be pumped first before supplying which will involve large operational costs.

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Similarly the cost will also depend on the distance between the source of water supply and the town or city to which the water is to be supplied. Longer distance would require longer conduits and more number of other appurtenances which would result in greater overall cost.

2. How to Determine Average Depth of Precipitation Over an Area? Explain with Equation.

For the analysis of most of the hydrologic problems, it is necessary to determine the average (or mean) depth of rainfall over an area. For a small area the rainfall recorded at a single rain gage station located in that area may be taken as the average depth of rainfall over that area. For large areas there will be a network of rain gage stations as recommended by the Indian Standard IS: 4987-1991.

As the rainfall over a large area is not uniform, the average depth of rainfall over the area is determined by one of the following three methods:

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(i) Arithmetic Average Method:

In this method the average depth of rainfall over an area is obtained by dividing the sum of the depths of rainfall recorded at all the rain gage stations located in the area by the number of the rain gage stations. Thus it P₁ P₂ P₃….. P_n are the depths of rainfall recorded at the n rain gage stations distributed over an area then the average depth of rainfall P for the entire area is given as,

The results obtained by this method will be correct only if the rain gage stations are uniformly distributed over the area and the rainfall varies in a regular manner. This is so because in this method every rain gage station has equal weightage regardless of its location.

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(ii) Thiessen Polygon Method:

In this method the adjacent rain gage stations are joined by straight lines thus dividing the entire area into a series of triangles. On each of these lines perpendicular bisectors are erected, thereby forming a series of polygons, each containing only one rain gage station. The entire area within any polygon is nearer to the rain gage station contained therein than to any other station, and hence it is assumed that the rainfall recorded at that rain gage station is the representative rainfall for that area.

If P₁,P₂, P₃,……P_n represent the depths of rainfall recorded at the rain gage stations enclosed by polygons, the areas of which are respectively A₁, A₂, A₃,………. A_n then the average depth of rainfall P for the entire area A is given as-

In the Thiessen polygon method each rain gage station is given’ weightage according to its position with respect to the boundary of the area under consideration and hence this method is better than the arithmetic average method.

(iii) Isohyetal Method:

Isohyets are the contours of equal rainfall. In the isohyetal method isohyets are drawn on the map of the area under consideration. For drawing the isohyets the depth of rainfall recorded at each of the rain gage stations are noted on the map of the area at the respective rain gage stations. Then assuming a linear variation of rainfall between the two rain gage stations the possible positions of rainfall values at some interval are interpolated between the rain gage stations.

The points with equal values of rainfall are connected by smooth curves which forms the isohyetal pattern for the area. The area between the two adjacent isohyets is measured with the help of a planimeter and for this area the rainfall is presumed to be equal to the mean of the two isohyet values.

The average depth of rainfall P for the entire area A may be found by the following equation:

The isohyetal method is the most accurate method for computing the average depth of rainfall. This is so because the isohyetal method takes into account the actual spatial relationship of the rain gage stations.

3. Describe Hydrologic Cycle with Diagram.

The hydrologic cycle is the descriptive term applied to the general circulation of water from the oceans to the atmosphere, to the ground and back to the oceans again. Figure 3.1 shows the various phases of the hydrologic cycle. The cycle may be considered to begin with the water of the oceans. Water from the ocean surface is evaporated into atmosphere.

The vapour is condensed by various processes and falls to the earth as precipitation. Some of this precipitation falls directly on the ocean, and some falls on the land surfaces. A portion of that falling on the land is retained temporarily in the soil, in surface depressions (such as ponds, lakes etc.) and on vegetation and on other objects until it is returned to the atmosphere by evaporation and transpiration.

Another portion runs off from the ground surface into the streams or rivers and is returned to the ocean. Still another portion percolates into the ground and joins the ground water which also slowly finds its way to the streams or rivers as ground water flow.

However, some portion of the groundwater which percolates the great depths appears after long intervals as springs, artesian wells and geysers. Further the entire quantity of water that reaches the stream does not flow directly to the oceans, because throughout its travel a portion of it returns to the atmosphere by evaporation and transpiration and some portion seeps into the ground.

4. What are the Factors Causing Water Borne Diseases?

Water borne diseases are those diseases which spread mainly through contaminated water.

The various water borne diseases may be caused by the following factors:

I. Presence of Micro-Organisms:

Due to the presence of various micro-organisms in water such as bacteria, viruses and protozoa several diseases may be caused as indicated below:

(i) Diseases Caused by Bacteria:

(a) Typhoid, caused by Salmonella typhi.

(b) Paratyphoid, caused by Salmonella paratyphi.

(d) Bacillary dysentery, caused by Shigella dysenterial.

(e) Diarrhoea of travellers, caused by pathogenic Escherichia coli.

(ii) Diseases Caused by Viruses:

(a) Infectious hepatitis (or Jaundice), caused by Hepatitis A viruses.

(b) Infectious nonbacterial gastroenteritis, caused by Norwalk- type viruses.

(iii) Diseases Caused by Protozoa:

(a) Amebiasis or Amebic dysentery, caused by Entamoeba histolytica.

(b) Giardiasis (a type of diarrhoea or gastroenteritis), caused by Giardia lamblia.

In spreading the various diseases noted above water plays the part of a mechanical agent. The sequence of events being the faeces or urine of a person suffering from these diseases not disposed of properly and conveyed by storm water or through other agencies to a stream, well or other sources of water supply. The water gets contaminated and the persons taking this water may fall sick.

Sometimes the sewer pipe line may leak into a water supply pipe through a faulty joint, thereby contaminating the water in the supply pipe and the persons drinking this water may fall sick.

II. Presence of Parasitic Ova:

The eggs or the developed embryos of the eggs of round worms and tape worms are generally carried by water and cause entozoal diseases like bilharziasis, nematodes, flukes, guinea worm and hook worm infections. Besides this water also forms the medium to carry mosquito eggs which lead to malaria and yellow fever.

III. Presence of Inorganic Matter:

Some diseases may be caused from the presence or absence of certain minerals in water. Some minerals may be toxic even if they are present in very small quantity, while others become toxic when their quantity exceeds a certain limit. For example lead and arsenic are the two of the most toxic materials but these are generally not found in water.

Further fluorides when present in water in quantities less than 1 mg/l have a beneficial effect on young teeth and hence fluorine is required to be added to fluoride-free waters to bring it to a level of about 1 mg/l.

However, fluorides in excess of 1.5 mg/I cause fluorosis of erupting teeth. Presence of excessive amounts of sulphates of magnesium generally causes disorders of the alimentary tract leading to diarrhoea. In the case of infants fed with milk containing high concentration of nitrates Methemoglobinemia (or blue baby disease) is wide spread. In this disease because of the production of methemoglobin in blood, nitrates are not converted to reduced forms of nitrogen and the infant turns blue.

As a result blood is not able to carry oxygen, the infant becomes sick, vomits and in extreme cases dies. Due to deficiency of iodides in water Goitre is caused. This is manifested in human beings by the symptomatic enlargements or swelling in the anterior part of the neck.

IV. Presence of Organic Matter:

An excess amount of vegetable matter present in water, or the entrance of sewage effluents into the water bodies may lead to diarrhoea and other gastric disturbances in the human body.

In order to prevent the spreading of water borne diseases especially in the form of epidemics, the following preventive or precautionary measures may be taken:

(i) The water supplied through public water supply system should be thoroughly checked and disinfected before supplying to the public. Further the water directly obtained from hand pumps or wells should also be checked and if necessary remedial measures should be taken to make the water safe and wholesome.

(ii) The pipelines carrying water should be frequently tested, checked and inspected so as to detect any leakage and possible source of contamination from nearby surroundings. The leaking joints must be properly sealed.

(iii) While designing the water supply system and laying the pipes attempt should be made to keep the sewer lines and water lines as far away as possible.

(iv) In case of slightest doubts about the sources of water supply being contaminated (especially during rains or floods; or during dry weather flows) people should be forewarned and advised to use boiled water. Moreover, in such circumstances an extra dose of chlorine must be added to the water supplied to the public. The additional dose of chlorine would impart bad taste and odour to the water supplied to the public but it would ensure safety against the possible danger of any water borne disease being spread in the form of epidemic.

(v) It would be possible to control water borne diseases by instituting an environmental health programme which incorporates personal and household hygiene, control of fly species and other insects, monitoring of food processing, immunization of populace where possible, and proper, scientific waste disposal and water treatment to remove harmful constituents.

5. How Water is Supplied to the Consumers?

Water may be supplied to the consumers by the following two systems:

1. Continuous System:

In this system of supply, water is supplied to the consumers for all the 24 hours of day. This is the most ideal system of supply of water and it should be adopted as far as possible. The only disadvantage of this system is that considerable wastage of water occurs if there are some leakages and also if the consumers do not realize the importance of treated water. However, one way to avoid this defect of the system is to supply water through metres.

2. Intermittent System:

In this system of supply, water is supplied to the customers during certain fixed hours of the day only. The usual period of supply of water is about one to four hours in the morning and about the same period in the afternoon. For example water may be supplied from 6 A.M. to 10. A.M. and from 4 P.M. to 8 P.M. The timings of supply of water may be changed to suit the seasons of the year and it is more or less a matter of convenience only.

The intermittent system of supply of water is useful for the following two conditions:

(i) The quantity of water available is not sufficient to meet the various demands of water, and

(ii) The available pressure is poor.

In the intermittent system of supply of water the distribution area is divided into several zones and the timings for the supply of water to each zone are so adjusted that good working pressure is maintained in each zone.

The intermittent system of supply of water has several drawbacks as indicated below:

(i) Fire Demand:

If a fire breaks out during non-supply period, there may be great inconvenience in bringing it under control, because the supply of water may not be turned on quickly and also quick diversion of water from other zones may not be possible. This may cause considerable damage due to fire.

(ii) Number of Valves:

This system requires a large number of valves for its working because the supply of water in different zones needs to be properly regulated.

(iii) Staff Requirements:

In this system since a number of valves of different types are fitted on pipe lines for water supply, many of which may be non- automatic, extra staff will be required to operate and maintain these valves.

(iv) Wastage of Water:

The consumers usually store water for use during non-supply hours, but the unused stored water may be thrown away to replace it by fresh water when supply of water is resumed. This will result in considerable wastage of water. Further during non-supply period water taps may be left open unknowingly or due to negligence, which will lead to large wastage of water during supply period.

(v) Pollution in Water Supply:

When supply of water is stopped, all the water from the pipe is drawn off, which may create a partial vacuum in the pipe. This may induce suction through leaking joints in the pipe. Thus if the pipe is laid near sewers, etc., the effluent from the same may be sucked into the pipe. Now when supply of water is resumed the consumers will be supplied with polluted water instead of pure water.

(vi) Sizes of Pipes:

In this system since the supply of water for whole day is to be made during 6 to 8 hours only, water mains of greater size will be required.

(vii) Pollution of Stored Water:

The water stored by the consumers may be polluted due to any source of contamination. For example the domestic storage tank built for the purpose may suffer for want of proper maintenance and attention for a long time, resulting in a possible contamination of water.

However, inspite of various drawbacks, the intermittent system of supply of water is most commonly adopted in our country. This is mainly because of less quantity of water available from the source of water. Further it helps in supplying water with adequate residual pressures by dividing the distribution area into different zones. It also assists in carrying out repairs during non-supply period.

6. What are the Troubles Encountered in the Operation of Rapid Sand Filter?

Some of the potential filter troubles which may be encountered in the operation of rapid sand filters (gravity type) are as follows:

(i) Formation of Mud Balls:

Mud balls are the conglomerations of the sand grains, floe and other binding material, varying from the size of a pea to 25 to 50 mm or more in diameter. Because of their lighter specific gravity, the mud balls are found most densely collected at or near the surface of the sand bed.

However, when the mud balls are of specific gravity equal to or heavier than that of the sand they may be distributed throughout the sand bed, and in some cases attached firmly to the gravel. The cause of mud-ball formation is insufficient washing of the sand bed which permits gelatinous material to adhere to the surface of the sand grains resulting in the formation of mud balls.

The mud balls interfere with the normal working of the filter and also with the back washing of the filter. As such it is necessary to control the formation of the mud balls and also to remove them if formed. It has been found that a 50% expansion of sand during back washing is effective in minimising the formation of mud balls.

Methods of removing mud ball after they have formed include:

(a) Dipping them off the surface with strainers while wash water is running through the filter at a slow rate;

(b) Breaking them up with rakes and sharp hoes and subsequently washing off the particles;

(d) Washing the filter or allowing it to stand full for some time, up to 48 hr, with a solution of some chemicals such as caustic soda, sulphuric acid, hydrochloric acid, soda ash, sulphur dioxide and chlorine;

(e) Digging out with shovels the hard spots developed in the filter bed due to the accumulation of the mud balls;

(f) Removing, cleaning and replacing the sand.

(ii) Cracking and Clogging of Filter Bed:

During the operation of the filter as the head loss is increased, the soft gelatinous coating on the sand grains is compressed which allows the sand grains to push together, resulting in the shrinkage of the filter bed, thereby developing cracks in the filter bed and pulling away the sand from the side walls. These cracks are more prominent near the wall junctions. The effect of such cracks in the filter bed is to permit dirty matter to penetrate deeply into the bed, even into the gravel.

Due to this the filter bed gets clogged and the efficiency of filtration as well as the washing of the filter is impaired. The remedial measures to overcome this trouble are same as those adopted to overcome the formation of mud balls.

(iii) Air Binding:

The condition of air binding is caused by the release of dissolved air and gases from water and their accumulation in the voids of the filter media. The effect of air binding is that water does not pass through the portion of the filter where the bubbles of air and gas have accumulated. This may result in loss of capacity of the filter or an overloading of other portions of the filter. The trouble of air binding is more likely to occur if water is saturated or super-saturated with air in solution.

The release of the dissolved air and other gases from water may be caused due to the following reasons:

(a) Negative head developed due to excessive loss of head;

(b) Increase in temperature of water as it passes through the filter media; resulting in the reduction in its capacity to retain gases in solution;

The air binding can be minimised by:

(a) Avoiding excessive negative head by providing a water depth of at least 1.5 m above the surface of the sand bed;

(b) Avoiding the warming up of water as it passes through the treatment plant;

(d) Avoiding super-saturation of water with air.

(iv) Sand Incrustation:

Sand incrustation may be caused either due to deposition of sticky gelatinous materials such as floe from the influent water or due to an after-crystallization of calcium carbonate in case where heavy lime treatment of water is practiced. Due to sand incrustation the sand grains enlarge, and the effective size of sand is changed.

This can be minimized by carbonating the influent, thereby dissolving both calcium carbonate and any manganese that might have deposited on sand grains. Sodium-hexa-meta-phosphate may also be used in small treatment plants to keep calcium carbonate in dissolved state.

(v) Jetting and Sand Boils:

Jetting and sand boils may result during back washing of the filter when wash water follows the path of least resistance and break through to the scattered points due to small differences in porosity and permeability of sand and gravel. When jetting becomes severe, the sand boils up like quick sand, and gravel as well as sand is lifted to the surface. The tendency of jetting can be minimized by surface wash.

(vi) Sand Leakage:

Sand leakage or downward migration and escape of fines may result when the layers of smallest gravel are displaced during back washing. It can be minimized by properly proportioning the sand and gravel layers.