chapter 1 & 2 ss (1)

Arba Minch university, IOT

Water supply & Environmental Eng’g

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1. INTRODUCTION TO SANITATION & SANITARY ENGINEERING

Objective of the chapter

At the end of successful completion, one can;

I. Identify the difference between conservancy and water carriage system of sanitation

II. Classify potential sources of wastewater

III. Know the systems of sewerage

IV. Identify the relative advantages and disadvantages of different sewerage systems

V. Identify component parts of wastewater collection system and

VI. Plan economically feasible sewerage system

1.1. Systems of Sanitation

The waste products of a society including the human excreta had been collected, carried and

disposed of manually to a safe point of disposal, by the sweepers, since time immemorial. This

primitive method of collecting and disposing of the society's wastes has now been modernized

and replaced by a system, in which these wastes are mixed with sufficient quantity of water and

carried through closed conduits under the conditions of gravity flow. This mixture of water and

waste products, popularly called sewage, thus automatically flows up to a place, from where it is

disposed of, after giving it suitable treatments; thus avoiding the carriage of wastes on heads or

carts.

The treated sewage effluents may be disposed of either in a running body of water, such as a

stream, or may be used for irrigating crops. This modern water-carried sewerage system has

completely replaced the old conservancy system of sanitation in the developed countries like

U.S.A. However, India being a developing country, still uses the old conservancy system at

various places, particularly in her villages and smaller towns. The metropolitan cities and a few

bigger towns of different countries, no doubt, have generally been equipped with the facilities of

this modern water carriage sewerage system.

The modern water-carried sewerage system is preferred to the old. Conservancy system, because

of its following advantages:

(i) The water carriage system is more hygienic, because in this system, the society's

wastes have not to be collected and carried in buckets or carts, as is required to be

done in the conservancy system. The free carriage of night soil in carts or as head load,

which is required in the conservancy system, may pose health hazards to The term

sewerage is applied to the art of collecting, treating and finally disposing of the

sewage. sweepers and other residents, because often possibilities of flies and insects

transmitting disease germs from these accessible carts to the resident's foods and



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eatables ; whereas, in modern sewerage system, no such danger exists, because the

polluted sewage is carried in closed conduits, as soon as it is produced.

(ii) In the conservancy system, the waste products are generally buried underground,

which may sometimes pollute the city's water supplies, if the water supply pipes

happen to pass through such areas or the wells happen to draw water through such

areas.

(iii) In the conservancy system of sanitation, the entire day's human feces are collected and

then disposed of in the morning, once a day, thus, from this type of latrines, pungent

smells may continue to pollute the surroundings for the entire day. But since in the

water, carried system, the human excreta is washed away as soon as it is produced, no

such bad smells are produced. Moreover, in the conservancy system of sanitation, the

waste waters from bath rooms, wash basins, kitchen sinks, etc.; is carried through open

road side drains, as this is supposed to be not so foul, since it does not contain human

excreta. But these road side drains are generally abused by children or adults for

passing their stools, particularly at night hours, thus creating foul and more unhygienic

conditions. No such problems exist in the water carriage system.

(iv) In water carriage system, the sewage is carried through underground pipes (popularly

called sewers) which owing to their being underground, do not occupy floor area on

road sides or impair the beauty of the surroundings. The road side drains carrying foul

liquid in the conservancy system, will no doubt pose such problems.

(v) The water-carried system may allow the construction of latrines and bath-rooms

together [popularly called water-closets (W.C)], thus occupying lesser space with their

compact designs. This system is also very helpful for multistoried buildings, where the

toilets, one above the other, can be easily constructed, and connected to a single

vertical pipe.

Inspire of these advantages of the modern water-carried system, it has not been possible to

completely replace the old conservancy system, mainly because huge capital funds are required

for the construction of such a system. Besides the huge initial investments, the MO expenses are

also high, which make it difficult to replace the simpler and cheaper conservancy system.

Moreover, for the functioning of sewerage system, ample amount of water must be made

available to the people, and hence, reliable and assured water supply must, first, be installed,

before installing the sewerage system.

1.2. Types and Sources of Sewage and Sewerage Systems

This modern water carriage sewerage system not only helps in removing the domestic and

industrial wastewaters, but also helps in removing storm water drainage. The run off resulting

from the storms is also sometimes carried through the sewers of the sewerage system, or more

generally is carried through separate set of drains (open or closed) directly discharging their

drainage waters into a body of water, such as a lake or a river. Since the rain run-off is not as



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foul as the sewage is, no treatment 'is generally required to be given to the drainage discharge.

When the drainage is taken along with sewage, it is called a combined system; and when the

drainage and sewage are taken independently of each other through two different sets of

conduits, it is called a separate system. Sometimes, a part of drainage water, especially that

originating from the roofs or paved courtyards of buildings, is allowed to be admitted into the

sewers ; and similarly sometimes, the domestic sewage coming out from the residences or

institutions, etc., is allowed to be admitted into the drains, the resulting system is called a

partially separate system.

Strictly speaking, it is generally advantageous and economical to construct a 'separate system' at

least in the bigger cities and towns. But in practice, it is generally not possible to attain a 'truly

separate system' because some rain water may always find its way into the sewers either through

wrong house sewer connections or through open manhole covers. Similarly, wherever the

authorities find insufficient sewer capacities, they divert part of the sewage into the storm water

drains, thus making most of our existing systems as 'partially separate' only.

Domestic sewage consists of liquid wastes originating from urinals, latrines, bathrooms, kitchen

sinks, wash basins, etc. of the residential, commercial or institutional buildings. This sewage is

generally extremely foul, because of the presence of human excreta in it.

Industrial sewage consists of liquid wastes originating from the industrial processes of various

industries, such as Dyeing, Paper making, brewing, etc. The quality of the industrial sewage

depends largely upon the type of industry and the chemicals used in their process waters.

Sometimes, they may be very foul and may require extensive treatment before being disposed of

in public sewers.

The sum total of domestic and industrial sewage may be termed as sanitary sewage or simply

sewage.

The run-off resulting from the rain storms was used to be called storm sewage, but the modern

approach is to call it storm drainage or simply drainage, so as to differentiate it from sewage,

which is much fouler as compared to drainage, and requires treatment before disposal

In the modern days, separate system is generally preferred to a 'combined system', although each

individual case should be decided separately on merits, keeping the following points into

consideration:

a) A separate system will require laying two sets of conduits, whereas, a combined system

required laying only one set of bigger sized conduits, thus making the former system

costlier. Moreover, the separate conduits cannot be laid in congested streets and

localities, making it physically unfeasible.

b) The sewer pipes in the combined system are liable to frequent silting during the non-

monsoon season (when the flows in them are quite less) unless they are laid at



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sufficiently steeper slopes, which, in turn, will make them deeper, requiring more

excavation and pumping, thereby making them costlier.

c) In a combined system, the less-foul drainage water gets mixed with the highly foul

sewage water, thus necessitating the treatment of the entire flow, needing more capacity

for the treatment plant, thereby making it costlier. Whereas, in the separate system only

sewage discharge is treated and the drainage discharge is disposed of without any

treatment

d) In case, flooding and backing up of sewers or drains occur due to excessive rains, more

foul and insanitary conditions will prevail in case of combined sewage than in the case of

storm drainage alone.

e) Since the sewer lines are generally laid deep and at steeper slopes, as compared to storm

water surface drains, pumping of sewage and often no pumping of drainage is required in

a separate system. Whereas, the entire discharge will have to be pumped if the sewage

and drainage discharges are mixed together; thereby making the combined system more

costly.

f) The economy of the two systems must be worked out for each individual project, and the

economical system should be adopted, if it is physically feasible.

1.3. Components of a Sewerage System

A sewerage system consists of a network of sewer pipes laid in order to carry the sewage from

individual homes to the sewage treatment plant. This network of sewers may consist of house

sewers (or individual house connections); lateral sewers; branch sewers (or sub mains); main

sewers (generally called trunk sewers); outfall *Since heavy rain storms concentrated for a

period of 3 months or so do occur, and there are poor water supplies here in India, the ratio of the

drainage to sewage works out to be as high as 20 to 30. Thus, during non-monsoon periods, only

1/20th or 1/30th of the designed discharge will be passing through the sewers, if the combined

system has been adopted. Sewer (the sewer which transports sewage to the point of treatment)

Manholes are provided in every sewer pipe at suitable intervals, so as to facilitate their cleaning

and inspection. In the sewers, which carry the drainage discharge either solely or in combination'

with sewage, inlets called catch basins are provided to permit entrance of storm water from street

gutters. In order to avoid the large scale pollution of the water sources and to keep them usable

for the downstream people, the original contaminated sewage is not allowed to be discharged

directly into the water sources. A complete treatment including screening sedimentation,

biological filtration (or activated sludge treatment), sludge digestion, etc. is therefore, given to

this extremely foul sewage, so as to bring down its BOD and concentrations of other constituents

to safer values, before discharging it into a national river resource. However, a recent use of

sewage is being made for irrigating crops. For this use also, the sewage has to be treated, so as to

bring down its constituents to permissible values, as per the requirements of LS. 3307-1965.

All these aspects are explained in details in subsequent chapters.



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1.4. Design and Planning of a Sewerage System

The sewerage system must be properly and skillfully planned and designed, so as to remove the

entire sewage effectively and efficiently from the houses, and up to the point of disposal. The

sewers must be of adequate size, so as to avoid their overflow and subsequent damages to

properties and health hazards. In order to provide economically adequate sized sewers, it is

necessary that the likely sewage discharge be estimated as correctly as possible. The sewer pipes

should then be designed to be laid on a slope that will permit reasonable velocity of flow. The

flow velocity should neither be so large, as to require heavy excavation and high lift pumping;

nor should it be so small, as to cause the deposition of solids in the sewer, bottoms.

The sewers are generally designed to carry the water from the basements, and should therefore,

be at least 2 to 3 m deep. As far as possible, they should be designed to flow under gravity with

1/2 or 3/4 full. Owing to the requirements of seeking gravity flow, the sewage treatment plant

should generally be located in a low lying area. The design of the treatment units also requires

good engineering skill. In order to provide adequate and economical treatment, it is necessary to

thoroughly study the constituents of the sewage produced in the particular project, and also the

quality and other characteristics of the body of water that will receive the sewage. The

permissible standards for effluents, and the possible uses of water downstream, should also be

studied. The legal bindings, if any, will also have to be taken into consideration, while deciding

upon the quantum of treatment required to be given. No fixed standards can be laid for fixing this

required treatment, as everything depends upon the exigencies of a particular project.

Since the treatment plant will have to be located at low level; the flood protection devices both

during construction and thereafter, should also have to be taken care of, by the design engineers.

SUMMARY

Systems of sanitation are Conservancy & water carriage the former is very old and non hygienic

where as water carriage system is hygienic and wastewater is conveyed to a central wastewater

treatment plant this can be separate, combined or partially separate. Each of the conveyance

systems has its own advantage and disadvantage attached to the way it functions. The potential

wastewater generation sources are residencies, industries commercial centers and institutions.

Sewerage system starts from this different sources and along the longitudinal direction to flow it

can have components like Manhole, drop manhole, clean-out street inlets etc. the more it

includes components and facilities based on the type of topography where sewerage is to be

constructed can make it the more expensive to implement.



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Activity Questions 1. Describe conservancy and water-carriage systems. What are the relative advantages and

disadvantages of the two systems?

2. Discuss briefly the necessity of replacing the conservancy system by the Water carriage

system of sanitation

3. Discuss the relative merits of the separate and the combined systems of sewage, and give

the conditions favorable for the adoption of each one of them

4. Differentiate between;

a) domestic industrial and sanitary sewage

b) combined and separate systems of sewage

c) sewage and drainage

5. Write short notes

(i) Financing the sewage projects

(ii) Types of sewages

(iii) Systems of sewerage



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2. DESIGN SEWAGE QUANTITY ESTIMATION

Objective of the chapter

At the end of successful completion, one can;

Understand how to quantify sanitary sewage

Identify the constraints that can affect design period of sewerage system

Understand how to estimate waste generation rate and its timely fluctuations

Precisely select the empirical relation to estimate peak drainage discharge

2.1. Estimating Dry-whether flow

The sewage discharge which has to pass through a sewer must be estimated as correctly as

possible; otherwise the sewers may either prove to be inadequate, resulting in their overflow, or

may prove to be of too much of size, resulting in unnecessary wasteful investments.

Theoretically speaking, the quantity of sewage (i.e., domestic sewage + industrial sewage) that is

likely to enter the municipal sewers under design should be equal to the quantity of water

supplied to the contributing area, from the water-works. But in actual practice, this is not the

precise quantity which appears as sewage, but certain additions and subtractions do take place

from it, as explained below:

1) Additions due to unaccounted private water Supplies

The accounted water supplied to the public through the public distribution system (the records of

which are easily available from the water-works office), is not necessarily the only water

consumed by the public. Some private wells and tube wells may sometimes be used by the public

for their domestic needs; and similarly, certain industries may utilize their own sources of water.

This extra quantity of water used by the town is generally small, unless there are large industrial

private water uses. This quantity can, however, be estimated by actual field observations.

2) Additions due to infiltration

Whenever, the sewer pipes are laid below the ground water-table, certain amount of ground

water generally seeps into them, through their faulty leaky joints or cracks formed in the pipes

due to bad materials or poor construction.

The quantity of the ground water entering these sewer pipes Depends mainly upon the height of

the water-table above the sewer invert level and the nature and extent of faults and fissures

present in the sewer pipes. However, if the ground water table is well below the sewer, the

infiltration can occur only after rain, when water is moving down through the soil. In that case,

the infiltration quantity will depend upon the permeability of the ground soil. Since these factors

cannot be precisely computed, the exact quantity of ground water infiltrating into the sewer pipes

cannot be estimated precisely. Only certain nominal allowance, based upon some experimental



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results, may be made on account of this factor. In U.S.A., an allowance varying from 11,000 to

2, 25,000 (average value = 1, 14,000) liters per day per kilometer length of sewer pipe, is

generally made in high water-table areas

No allowance for infiltration should, however, is made when sewers are provided with under-

drains which have free outlets.

Sometimes, the storm water drainage may also infiltrate into the sewers. This inflow cannot be

computed easily and generally left unaccounted without making any extra provision for it. This

additional water, if happens to enter the sewers, can be accommodated in the extra empty space

left at the top in the sewers, which are generally designed as running 3/4th full at maximum

designed discharge

3) Subtractions due to water losses.

The water lost, due to leakage in the distribution system and house connections of the water

supply scheme, does not reach the consumers, and hence, never appears as sewage.

4) Subtraction due to water not entering the sewerage 'system.

Certain amount of water may be used by the public and industries for such uses which may not

produce any sewage at all. For example, the water used in boilers for steam generation; the del'

sprinkled over the roads, streets, lawns and gardens; the water used for automobile washings; the

water consumed in industrial products, such as beverages, etc., the water used in air cooling etc.,

does not normally produce any sewage. Quantity of Sewage Produced. The net quantity of

sewage produced will be equal to the accounted quantity of water supplied by the water-works

plus the additions due to factors (1) and (2) minus the subtractions due to factors (3) and (4),

described above. The net value may vary between 70 to 130 per cent of the accounted water

supplied from the water-works.

However this value; generally taken as equal to 75 to 80% of the accounted water supplied from

the water works.

2.2. Design Periods for Different Components of Sewerage Scheme

A sewerage scheme involves the laying of underground sewer pipes and construction of costly

treatment units, which cannot be replaced or increased in their capacities easily or conveniently

at a later date. For example, addition of sewer pipes at a future date cannot be accomplished

without digging the roads and disrupting the traffic. In order to avoid such future complications,

and to take care of the future expansions of the city and consequent increase in the quantity of

sewage produced, it is necessary to design the various components of the scheme larger than

their present day requirements and of such sizes, as to serve the community, satisfactorily, for a

reasonable number of years to come. This future period for which the provision is made in

designing the capacities of the various components of the sewerage scheme is known as the



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design period. The design period should neither be too long nor it should be too short, and

moreover, it should not exceed the useful life of the component structures. The design period is

generally guided by the following considerations:

(i) Useful life of component structures, and the chances of their becoming old and obsolete.

Design period should not exceed those values.

(ii) Ease and difficulty, that is likely to be faced in expansion, if undertaken at future dates. For

example, more difficult expansions mean choosing a higher value of the design period.

(iii) Amount and availability of additional investment, likely to be incurred for additional

provisions. For example if funds are not easily available, then one has to keep a smaller design

period.

(iv) The rate of interest on the borrowings and the additional money invested. For example, if the

interest rate is small; a higher value of the design period may be economically justified, and

therefore, adopted.

(v) Anticipated rate of population growth, including possible shifts in communities, industries

and commercial investments. For example, if the rate of increase of population is less, a higher

figure for the design period may be chosen.

The following design periods are often used in designing the different components of a sewerage

scheme.

Table 2.1 Design Periods for Different Components of a Sewerage Scheme



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2.3. Future Forecasts and Estimating Design Sewage Discharge

The quantity of sewage that is likely to pass through a sewer (Q') at the end of the design period,

can be easily computed by multiplying the per capita production of sewage (q') by the expected

population at the end of the design period.

The per capita sewage which is produced (q') in a community can be easily estimated by

assuming it as 75 to 80 per cent of the per capital water supplied to the public (q). However, it

should also be kept in mind that the future increase in population may also increase the per capita

water demand, and consequently increasing the per capita production of sewage. The increase in

per capita water supply or sewage production with the increase in population obviously occurs

due to improved economical conditions in the city, implying higher standards of living and

greater consumption of water. In U.S.A., this increase in per capita water demand and sewage

production is generally assumed to be 5% of the percentage increase in population. However, for

normal Indian conditions, the following norms may be adopted:

Table 2.2 Variations in per capital water demand and sewage production with population case

study India.

The expected population at the end of the design period can be estimated by collecting the data

of the past populations of several decades from the Census Department, and then by

extrapolating the future population by using anyone of the different methods, such as:

o Arithmetical increase method;

o Geometrical increase method;

o Incremental increase method;

o Decreasing rate method;

o Simple graphical method;

o Comparative graphical method;



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o Master plan method;

o The apportionment method;

o The logistic curve method.

These methods for forecasting future populations have been described in details in water supply

modules determining the expected population as well as the per capita sewage contribution of the

town, by the end of the design period, the average quantity of sewage produced in liters/day

(then converted to cumecs) can be easily determined by multiplying both these figures.

2.4. Variations in Sewage Flow and their Effects on the

Components of a Sewerage Scheme

The per capita demand of waste production (q') so far discussed, are based upon annual defined

as annual average value is not sufficient, although design of various components of a sewerage

scheme; because there are wide variations in the actual flows that take place through the sewers

at a given time.

The f1ows in these sanitary sewers, though fluctuate seasonally, monthly, daily, as well as

hourly, with the water consumption*, yet they are sometimes delayed and less pronounced (Fig.

2.1) as they

Fig.2.1. Hourly variation of sewage flow compared to that of water supply

are damped because of the storage space in the sewers and because of the time required for the

sewage to reach the point of gauging. In other words, flattened, because it requires consider

point, and the high flows from various sections w various times of flow. Thus the time the flow

time in sewers and the type of district served. Hence, if the sewage is gauged near its origin, the

peak f1ow will be quite pronounced; whereas, if the sewage must travel a long distance before



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being gauged, the peak will be deferre Design of Various water (q) and the corresponding per

capital flow and are, therefore very useful, for the considerable sewage to fill the sewers to the

high flow will reach the gauging point as to when the peak flow occurs deferred. It, therefore,

follows, that the peak flows (expressed as number of times of their average values) will be much

greater for smaller lateral sewers, as compared to these for larger trunk sewers.

For areas of moderate sizes, such as involved for branch sewers, the maximum daily or hourly

sewage flows, can be expressed as:

Maximum daily flow = 2 times the average daily flow

Maximum hourly flow = 1.5 times the maximum daily

= 3 times the average daily

However, as pointed out earlier, the peak hourly flows will decrease, as the tributary area

increases. Therefore, the peak flow at the outfall of a city sewer system will be much less,

usually, about 1.5 times the average. The estimation of maximum hourly flows for different

types of sewers, within the city's sewerage system, are given below in table

Table 2.3 Hourly Variations in Sewage Flow

The sizes of the sewers can then be easily designed for carrying the computed maximum hourly

flows, with sewers running 3 /4th full

This peak sewage flow has been connected with the population by certain investigators by the

formula:

The minimum flow passing through a sewer is also an important factor in the design of the

particular sewer: because at low flow, the velocity will be reduced considerably, which may



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cause silting. Hence, the slope at which the sewer is to be laid has to be decided in accordance

with the requirement of minimum permissible velocity at the minimum flow.

The minimum flows occurring through the sewers during night hours will affect the laterals to a

maximum extent, and will affect the mains to a lesser extent. Thus, the minimum flows through

laterals, may be even lesser than 25 per cent of the average; while in the mains, they can be 50 to

70 per cent of the average. For 'moderate areas, such as involved for branch sewers, the

following minimum flows may be assumed:

2.5. Estimating the Peak Drainage Discharge

The sewers and the drains of a separate sewerage system should be designed to carry the

maximum sewage discharge and the maximum rain runoff, respectively. Were as, the sewers of a

combined sewerage system should be designed to carry the sewage discharge plus the rain

runoff. The sewers of a combined system should, therefore, in addition to passing this combined

maximum flow, should also be capable of passing the low sewage discharge during non-

monsoon periods, as dry weather flow, with minimum permissible velocities. The partially

separate sewers may be designed for carrying the sewage discharge plus part of the storm

drainage, particularly that coming from the roofs and courtyards.

In order to design the sewers and the drains properly, it is absolutely necessary to estimate the

sewage discharge and the urban storm drainage discharge that are likely to enter the sewers or

drains. The methods of estimating the maximum sewage discharge were discussed in the

previous chapter; and here we will discuss the methods of estimating the maximum rate of storm

run-off, popularly called peak drainage discharge.

2.6 The Run-off Process and Peak Run-off Rate

When a rain, falls n a certain area, a part of it is intercepted by the soil, a part of it is evaporated,

and the remaining water flows overland towards the valleys, as storm runoff.



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Since the storm runoff has to be removed through drains or through combined sewers, the

drainage engineer must evaluate the peak rate of run-off, which can be produced from a certain

catchment by the given rain, at any moment. Further, the more intense is the rain, the more will

be the peak run-off rate. Hence, a proper and economical value of rain frequency (or recurrence

interval**) must be chosen, which the drains must be designed. The frequency of rainfall to be

adopted in design should neither be so large, as to cause too heavy investments, nor should it be

so small, as to cause very frequent overflowing of the drains. For Delhi, the experts have

recommended a 2years rain frequency for designing smaller link drains, and 5 years Frequency

for designing all the major drains.

2.7 Estimating Peak Run-off

The peak rate of run-off that is produced from a particular catchment depends upon numerous

factors; such as, the type of precipitation, the intensity and duration of rainfall, the rainfall

distribution, the soil moisture deficiency, the direction of the prevailing storm, the climatic

conditions, the shape, size and type of catchment basin, etc. etc. Due to these 15 to 20 variables

involved in evaluating the run-off, it is not possible to precisely determine it, even with the help

of the most complicated mathematics, as all these variables are interdependent, and run off

cannot be easily expressed by an exact Equation. Hence, until about 40 years ago, the peak run-

off rate was used to be estimated by empirical formulas only, even in the developed countries

like U.S.A. Different empirical formulas were, therefore, developed¡¤ for different regions, by

the investigators, depending upon their actual experimental works. In recent years, however, a

rational method has been evolved to estimate the peak Drainage discharge.

This method, though called rational, is not rational in the sense that the results given by this

formula for larger areas (more than 500 hectares or so) are generally erroneous and misleading.

This method can be applied most precisely to smaller areas (preferably less than 50 hectares or

so). For large areas, empirical formulas are, however, continued to be used, although the most

modern method for computing urban storm drainage is by digital computer simulations. This is

an advance topic dealt under the subject of "Water Resources Systems Planning" and is beyond

the scope of the Undergraduate Courses. The rational formula and other empirical formulas for

determining peak drainage discharge are discussed here:

2.7.1 Computing the Peak Drainage Discharge by the Use of Rational Formula.

If a rainfall is applied to an impervious surface at a constant rate, the resultant runoff from the

surface would finally reach a rate equal to the rainfall. In the beginning, only a certain amount of

water will reach the outlet, but after some time, the water will start reaching the outlet from the,

entire area; and in this case, the run-off rate would become equal to the rate of rain The period

after which the entire area will start contributing to runoff is called the time of concentration.

The runoff resulting from a rain having a duration lesser than the time of concentration will not

be maximum, as the entire area will not contribute to run-off in this case Further, it has been



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established that the runoff is not maximum, even when the duration of the rain is more than time

of concentration, because in such a case, the intensity of rain reduces with the increase in its

duration. In other words, it has been established that the maximum runoff will be obtained from

rain having duration equal to the time of concentration, and this is called the critical rainfall

duration. Based upon these basic principles, the rational formula evolved, due to the efforts of

Fruhling of Germany, Kuichling America, and later Lloyd Davis of England. This formula states

that

Concentration in cm/hr

Coefficient of Runoff

The coefficient of runoff (K) is in fact, the impervious factor of runoff, representing, and the

ratio of precipitation to runoff. The value of K increases as the imperviousness of the area

increases**, thus tending to make K = 1 for perfectly impervious areas. It is generally taken as

equal to 0.9 for paved areas and 0.15 for lawns and gardens. The values of K can also be worked

out for different localities having different population densities. Various values of K which can

be of use in designing storm water drains are given in Tables 2.3 and 2.4.

Table 2.3 Values of Run-off coefficient (K) for various Surfaces



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Table: 2.4 values of runoff coefficient (K) for different types of localities



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Intensity of Rainfall



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The time of concentration

The time of concentration for a given storm water drain generally consists of two parts; viz.

The total time of concentration at a given point in the drain, for working out the discharge at that

point, can be easily obtained as

The intensity of rainfall during this much of time (for the given design frequency, of course) can

be easily obtained from the standard intensity duration curves or DAD curves.



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Figure Typical Intensity-Duration-Frequency (IDF) Curve

The value of intensity so obtained is still the rainfall intensity at the rain gauge station, and is

called the point rainfall intensity. In order to make it effective over the entire catchment area (in

which this rain gauge station lies), it is necessary to multiply it by a factor called dispersion

factor or areal distribution factor. The resultant value will be nothing but Pc, to be used in Eq.

(2.4). The areal distribution factor: the resultant value will be nothing but p; to be used in

equation (2.4) it is a well established fact that the intensity of rainfall recorded at a particular rain

gauge station in a catchment is not the same throughout the catchment. As the size of the

catchment increases, the average intensity of rainfall over the catchment as a whole goes on

decreasing compared to the point intensity recorded at a particular station. Therefore, the areal

distribution factor, also called, the dispersion factor, is always applied to the point rainfall for

working out the design rainfall intensity. In case of Delhi, it is seen that the intensity of rainfall

varies considerably from one part to another, and as such, the dispersion factor reduces

considerably with the increase in the catchment area, as shown in Table 3.3

Table 2.5 Values of Dispersion Factor for Delhi



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In the absence of standard intensity-duration curves, the value of c can also be determined in the

following two ways:

(i) The value of "one hour rainfall" of a given frequency at a given place can be found from the

charts,



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This value of "one hour rainfall" is multiplied by the areal distribution factor, so as to

The values of a and b have been found out by the Health Ministry of Britain as 75 and 10

respectively for T varying between 5 to 20 minutes ; and as 100 and 20 respectively for T

varying between 20 .to 100 minutes respectively. The formulas given by them, and generally

applicable in England, are, therefore, given as below:

Using Tc in minutes, in place of T in Eqn. (2.7) and (2.8), the values of p, i.e. Pc can be

evaluated.

Besides the above generalized equations, certain other empirical equations have been suggested

for determining rainfall intensity, as given below:



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(a) For localities where rainfall is frequent. .

The equations from (2.7) to (2.14) can, though be used for finding the value of Pc, yet they are

very empirical equations, and are not very reliable. They are, therefore, generally avoided in

designing storm water drains in modern days. They may, however, be used when absolutely no

rainfall records are available. .

2.7.2. Computing the Peak Drainage Discharge by the Use of Empirical Formulas

The Rational formula described above is also quite empirical in the sense that the value of K

considerably depends upon the judgment of the designer. Moreover, this method gives reliable

results only for smaller areas, and hence used only for the design of drains having catchments

less than 400 hectares or so. For the design of drains having larger catchments (say above 400

hectares or so), it is generally advisable to use the suggested empirical formula for the given

region.

Various empirical formulas for calculating storm water run-off have been suggested by various

investigators; some of these formulas are based on local conditions only, and can be adopted

only when certain specific requirements are specified. The other formulas are based on

experimental studies and results obtained over wide areas, and they can, therefore, be adopted for

many localities. Some of the leading empirical formulas are given below:



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(i) Burkli-Zieglerformula. This formula is the oldest empirical formula used for determining the

peak run off rate. It was devised by a Swiss engineer for local conditions, but was soon followed

in the entire U.S.A. In M.K.S. units, it states that

ii. Dickens¡¯ s formula. This formula is generally useful for Indian catchments and particularly

for northern India and states that,

The value of C must be ascertained for each catchment, and depends upon the nature of the

catchment and the intensity of rain fall. An average value of C equals to 11.5 is generally used

and it should be increased for hilly areas and vice versa. Secondly for the same type of

catchment, greater is the rain fall greater will be the value of C and vice versa.

iii. Ryve¡¯ s formula. This formula is almost similar to that of Dicken¡¯ s; the only difference is in

the value of the constants. It is generally applicable to south Indian catchments and states that



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Iv. Inglis Formula

This formula is applicable to the fan shaped catchments in old Bombay state of India.

It states that

Vi. Dredge or Burge¡¯ s formula. This formula is based upon Indian records

chapter 1 & 2 ss (1)

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