Water Engineering Questions and Answers for Engineering School

Here is a compilation of water engineering questions and answers for engineering school.

1. Define Storativity, Hydraulic Conductivity and Transmissivity of Aquifer. Explain with Diagram.

Storage Coefficient or Storativity:

Storage coefficient or Storativity is defined as the volume of water that an aquifer releases from or takes into storage per unit surface area of aquifer per unit change in the component of head normal to that surface. The storage coefficient is a dimensionless quantity because it represents the volume of water per unit volume of aquifer.

In a confined aquifer if a vertical column of unit cross-sectional area (say 1 m × 1 m) is considered which is extending upto the piezometric surface then the storage coefficient S is the volume of water released from the aquifer when the piezometric surface declines by unit distance (say 1 m). For most of the confined aquifers the value of the storage coefficient lies in the range 0.00005 to 0.005. For confined aquifers the value of S can be determined from pumping tests.

In an unconfined aquifer if a vertical column of unit cross-sectional area is considered which is extending up to the water table as shown in Fig. 4.6(b) then the storage coefficient is the volume of water released from the aquifer when the water table is lowered by a unit distance.

The water so released represents, for practical purposes, the gravity drainage from this portion of the column of the aquifer. As such for an unconfined aquifer the storage coefficient is equal to the specific yield. For unconfined aquifers the value of the storage coefficient lies in the range of 0.05 to 0.30.

Coefficient of Permeability or Hydraulic Conductivity:

The coefficient of permeability or hydraulic conductivity is defined as the rate of flow of water per unit cross-sectional area of aquifer under a unit hydraulic gradient. Thus coefficient of permeability has the dimensions of velocity and it is usually expressed in mm/s or cm/s and denoted by k. The representative values of the coefficient of permeability for some of the soils are given in Table 4.1.

Coefficient of Transmissibility or Transmissivity:

The coefficient of transmissibility or transmissivity is defined as the rate of flow of water through a vertical strip of aquifer of unit width and extending for the full saturated height under unit hydraulic gradient. Thus the coefficient of transmissibility T is equal to the coefficient of permeability k multiplied by the aquifer thickness b, i.e.-

T – bk ….(4.7)

The value of T for a confined aquifer can be determined by pumping test method or recovery test method.

2. How to Design Partially Penetrating Wells? Explain with Diagram.

A partially penetrating well is the one which extends only partially through the aquifer. Thus for a partially penetrating well the length of water entry is less than the thickness of the aquifer which it penetrates. In practice such wells are often encountered. Figure 4.36 shows partially penetrating well in confined and unconfined aquifers.

The flow pattern in aquifer in the vicinity of a partially penetrating well differs from that for a fully penetrating well. In the case of partially penetrating well there is a vertical convergence of streamlines near the well resulting in increased entrance velocity and hence a greater resistance to flow is encountered. This results in the following relationships between two similar wells, one partially and one fully penetrating the same aquifer.

If Q_P = Q’ then s_p > s; and if s_p = s, then Q_P < Q.

Here Q is well discharge, s is drawdown at the well, and the subscript p refers to the partially penetrating well.

However, beyond a radial distance equal to 2 times the saturated thickness of the aquifer the effect of partial penetration of well on the flow pattern and the drawdown is negligible.

For a well penetrating only the upper portion of a confined aquifer the total drawdown at the well is given as-

From Eq. 4.50 the effect of partial penetration on the yield of a well can be determined. Thus for example if a 300 mm well penetrates only 6 m in a 15 m thick confined aquifer having R = 150 m then from Eq. 4.50, (Q_p/Q) = 0.57 i.e., in this case a partially penetrating well yields 57 per cent of what a similar fully penetrating well would yield for the same drawdown.

For partially penetrating wells in unconfined aquifers if the drawdown is small in relation to the saturated thickness H, then as a good approximation the value of the ratio (Q_p/Q) is given by the following expression-

The corresponding expression for discharge Q_P of well Partially penetrating unconfined aquifer is as follows:

3. Describe Losses, Specific Gravity and Efficiency of Wells.

Well Losses:

When water is pumped from a well the total drawdown caused at the well face is made up of the following:

(i) The drawdown resulting from the head loss due to the resistance to the flow of water in the aquifer which is known as aquifer loss or formation loss.

(ii) The drawdown due to the well losses resulting from the flow through the well screen and the axial flow within the well to the pump intake.

Due to aquifer loss the logarithmic drawdown curve is developed and the resulting drawdown at the well is therefore equal to steady-state drawdown given by Eqs. 4.14 and 4.21 for the wells in confined and unconfined aquifers respectively.

The drawdown due to well loss will however be equal to the total well loss expressed in terms of the head of water. Since near the well face generally turbulent flow occurs, the well loss may be taken to be proportional to the nth power of discharge i.e., Qⁿ, where n is a constant greater than one. The well loss and the consequent drawdown is therefore equal to CQⁿ, where C is a constant which is known as coefficient of well loss. The value of C depends on the radius, construction and condition of the well.

Thus taking account of the well loss, the total drawdown s at the well in a confined or an unconfined aquifer may be obtained by adding the drawdown due to well loss to the steady-state drawdown given by Eq. 4.14 or 4.21, and hence-

(s/Q) is usually known as specific drawdown and it is defined as the drawdown at a well per unit discharge of the well. The specific drawdown is reciprocal of specific capacity (Q/s) defined later. By plotting (Q/s) versus Q and fitting a straight a line through the points, the slope of the line gives the coefficient of well loss C and the intercept on the (Q/s) axis at Q = 0 gives the aquifer loss coefficient B.

Figure 4.33 (b) shows the variation of total drawdown and well loss with the well discharge. It is apparent from Fig. 4.33 (b) that for relatively low pumping rates the well loss may be neglected, but for high pumping rates the well loss can be a substantial fraction of total drawdown. However, the well loss can be reduced by increasing the radius of the well, and also by using such well screen which have slots or openings of size compatible with the surrounding porous media and which do not deteriorate or get clogged.

Specific Capacity:

The specific capacity of a well is defined as the discharge per unit drawdown in a pumping well. In other words the specific capacity of a well is obtained by dividing the discharge by drawdown in a pumping well. Thus if s is drawdown and Q is well discharge then

Equation 4.36 indicates that specific capacity of a well is independent of the well discharge, but it gives only theoretical specific capacity because in actual practice well loss is not equal to zero (except at very low pumping rates when the well loss may be negligible).

The specific capacity of a well is a measure of its productivity, and hence larger is the specific capacity, better is the well. Further as indicated by equation 4.35 (a) for a given discharge a well may be assumed to have constant specific capacity, but in actual practice it is not correct because the specific capacity of a well is found to slightly vary with time.

A significant reduction in the specific capacity of a well may be due to an increase in the well loss resulting from clogging or deterioration of the well screen. Moreover, in the case of an unconfined aquifer due to lowering of the groundwater level the transmissibility may be reduced which in turn may also result in reducing the specific capacity of the well.

Well Efficiency:

The efficiency of a well E_w is defined as the ratio of the actual specific capacity (Q/s) of a well measured in the field and the theoretical specific capacity (Q/BQ) and is expressed as percentage.

Equation 4.38 indicates that when well loss is large the well is less efficient.

4. What are the Causes of Failure of Tube Wells?

In general a properly constructed and developed tube well can provide sufficient quantity of water continuously for a number of years and hence it is a reliable source of water supply for a town or city. Moreover the quality of tube well water is generally very good and in many cases it can be used without any major treatment.

It is, however, necessary to properly operate and maintain the tube wells so that they are able to provide a satisfactory service throughout their life which may be more than 20 to 25 years. Further various measures should be taken to avoid the failure of a tube well.

A tube well may generally fail due to the following two causes:

1. Corrosion:

If the groundwater is highly acidic and it contains lot of chlorides and sulphates then due to continuous action of such water the well pipe may get corroded in due course of time. The damaged strainer would allow sand particles to enter the well along with water. The water pumped from the well will then be contaminated with lot of sand which is not desirable. As such efforts should be made to-avoid the corrosion of the well pipe. Some of the precautions which may be taken to avoid corrosion of the well pipe and thereby increase the life of the tube well are as indicated below.

(i) The diameter of the tube well may be kept more so that the flow velocity of water entering the well is reduced.

(ii) The stainless steel pipes may be used for avoiding failure of tube well due to corrosion. However, the stainless steel pipes are very costly.

(iii)The pipes may be galvanised (i.e., zinc coated) or provided with a coating of other corrosion resistant material.

(iv) Thick pipes may be used.

2. Incrustation:

The deposition of alkali salts on the inside walls of the tube well is known as incrustation. The most important alkali salt causing incrustation is calcium carbonate. Sometimes the other salts such as sulphates and silicates of calcium and magnesium may also lead to incrustation. The incrustation of the well pipe reduces the diameter of the pipe as well as the effective area of the openings of the strainer and hence reduces the discharge of the tube well.

Some of the measures which may be taken to reduce the incrustation are as indicated below:

(i) The strainers made of resistant materials may be used so that the incrustation may be removed at a later stage by acids without damaging the strainers.

(ii) The strainers with slightly larger areas of the openings may be used so as to keep some allowance for the future incrustation.

(iii)The incrustating deposits may be periodically removed during maintenance of tube well.

(iv) The incrustation may be reduced by reducing the pumping rates or by using larger capacity tube wells.

5. How to Achieve Better Results in Lima-Soda Process of Water Treatment?

In order to achieve greater economy and better results in lime-soda process certain measures are taken as indicated below:

i. Addition of Aluminum or Ferrous Compounds:

In addition to lime and soda ash small quantities of aluminium or ferrous compounds may be added to water. The aluminium compounds commonly used are alum and sodium aluminate. The aluminium compounds coagulate finely divided particles and thus shorten the reaction and settling periods.

They also convert soluble magnesium salts into insoluble magnesium aluminates which are then deposited and removed in the settling tanks or filters. The use of alum, however, slightly increases the sulphate hardness, thus necessitating the use of additional soda ash. The ferrous compound viz., ferrous sulphate may also be used in place of alum.

ii. Use of Excess Lime:

A slightly more lime than necessary may be added to water which helps in complete precipitation of magnesium. However, the excess lime if present in water may increase alkalinity of water. The excess lime may be neutralised by adding sufficient quantity of soda ash to react with non- carbonate hardness and with excess lime.

iii. Split Treatment:

In this method a portion of water is given excessive treatment and its hardness is reduced to the maximum possible extent. This excessively softened water is then added to water to be treated for removal of hardness. This method is known as ‘split treatment’. It has been found that with the help of split treatment it is possible to save a considerable quantity of chemicals.

iv. Recirculation of Sludge:

In this method the sludge formed during the process is recirculated and it is allowed to get mixed with water. It has been found that recirculation of sludge may result in an effluent containing 50 to 70 p.p.m. of hardness, about 30 p.p.m less than that obtained without recirculation.

Moreover, recirculation of sludge also helps in considerable reduction of detention time. Various devices making use of this process have been developed, out of which the following two devices are commonly used.

They are described below:

(i) Water Softening Accelerator:

A water softening accelerator is a compact unit which combines mixing, flocculation, settling and sludge removal. As shown in Fig. 9.40 a water softening accelerator consists of (a) primary mixing and reaction zone, (b) secondary mixing and reaction zone, (c) return flow zone and concentrator, and (d) clarifying chamber.

A rotor impeller separates the primary and secondary mixing zones. Raw water (i.e., water to be treated for removal of hardness) enters the primary mixing zone through a horizontal pipe and the required dose of lime and soda ash mixture enters this zone through a vertical pipe A.

The water in the primary mixing and reaction zone also receives a certain quantity of settling particles from the return flow zone, which accelerates the precipitation. Water from the primary mixing and reaction zone is drawn up in the secondary mixing and reaction zone due to the rotation of the impeller. In the secondary mixing and reaction zone a dose of alum is given through pipe B to achieve quick coagulation.

From the secondary mixing and reaction zone, water enters the clarifying chamber, from which clear water flows to the collecting channel and the sludge is deposited in the conical bottom, known as concentrator.

The sludge deposited in the concentrator can be discharged through a pipe either continuously or at a pre-decided interval which may operate automatically. In the clarifying chamber an upward velocity is kept between 70 and 90 mm per minute. Detention periods are short varying from 1 to 2 hours.

(ii) Spiroacter:

A spiroacter consists of an inverted conical tank. Half of the tank is filled with marble chips of size 0.10 mm to 0.20 mm. Raw water (i.e. water to be treated for removal of hardness) and chemicals enter the tank through pipes placed at the bottom of the tank.

Water as well as chemicals is forced into the tank under pressure through nozzles in a tangential direction. Thus water which enters the tank is forced upwards spirally. The softening of water is achieved by this spiral motion of water.

The velocity of flow of water is so adjusted that the marble chips are kept in suspension only and not carried away by water. The calcium carbonate and magnesium hydroxide precipitate and accumulate around the marble chips. This increases the size of marble chips and consequently the volume occupied by them also increases.

The level of marble chips is maintained by periodical removal of marble chips from the bottom and by adding new chips from the top. The marble chips which are removed can be washed and used again. In this case wet sludge is not formed but instead of this crystalline and hard granules are formed which are easy to handle. Further in this case the detention period is only about 6 to 12 minutes.

6. How to Select Site for Location of an Intake for Collecting Surface Water?

The site for locating the intake should be selected carefully, keeping in view the following points:

(1) The site for locating the intake should be as far as possible near the treatment plant so that the cost of conveying water would be less.

(2) The intake must be located in the pure water zone of the source so that water of best possible quality is withdrawn from the source, which will reduce the load on the treatment plant.

(3) The intake must never be located at the downstream or in the vicinity of the point of disposal of the sewage or waste water of the city. This may be the case when water is directly withdrawn from a river or stream into which there might be a provision of disposal of sewage of the city.

(4) As far as possible the intake should never be located near or in the navigation channel, because there are chances of water of intake being polluted due to discharge of refuse and wastes from ships and boats. However, if the intake is required to be located in a navigation channel then its sides should be protected by a cluster of piles all around from the blows of moving ships.

(5) The intake should be located so as to ensure the supply of water even under the worst conditions, i.e., when the level of water in the source is minimum even then sufficient quantity of water should be available in the intake. Thus in the case of rivers the intake must be located in deep waters, sufficiently away from the shore line (i.e., khadir banks or permanent banks).

As otherwise during dry periods when the water level goes down and the water recedes from the permanent banks, the intake may be left dry without any water to be drawn around it. If the river has a wide basin then a cross approach channel may be constructed upto the intake to fetch water from the deeper portion of the river to the intake.

Further if the water level in the river is lowered considerably during summer season, then a weir or barrage may be constructed across the river to raise the water level in the river. This will thus ensure the availability of water at the intake in sufficient quantity at all times of the year.

(6) The site for locating the intake structure should be such that there is sufficient scope for future additions and expansions, i.e., it would permit greater withdrawal of water if required in future.

(7) The intake site should remain easily accessible during floods and it should not get flooded. Moreover, the flood waters should not be concentrated in the vicinity of the intake.

(8) In the case of a meandering river the intake should not be located on the curves. However, if the intake is to be located on the curve, it should be located on the concave bank (i.e., the outer bank) and not on the convex bank (i.e., the inner bank).

This is so because although the scouring tendencies will be more on the concave bank of the river, water will always remain available on this side. On the other hand near the convex bank of the river water may not always remain available due to silting and consequent blockage.

(9) The site for the location of the intake should be so selected that it is least affected by scouring, silting and storms. Further the site should be free from the attack of heavy water currents.

(10) The site of intake should be well connected by good approach roads.

(11) In the selection of the intake site the natural causes such as wind currents, seasonal variations in quantity and quality of water, climatic conditions, etc., should also be studied to ensure maximum stability and safety to the intake works.