← Environmental Engineering & Pollution Control · TNPSC AE Civil

Chapter 1 of 16

Sources of water & water demand

In the TNPSC AE Civil syllabus under Environmental Engineering & Pollution Control · 2 parts

Sources of Water & Intake Structures · Water Demand & Population Forecasting

CIVILGYAN TEST SERIES · civilgyantests.com — free study notes for civil engineering exams

📑 Contents (13 sections)

Part 1 of 2

Sources of Water & Intake Structures

Last reviewed 16 Sept 2026 · 7 min read

Sources of water

Water sources are broadly surface or subsurface (groundwater) sources.

Surface sources

Source	Quantity	Quality
Rivers and streams	Large in perennial rivers; flows fluctuate — minimum (lean season) flow governs reliability	Turbid in floods; exposed to pollution from towns and industries upstream; needs full treatment
Natural lakes and ponds	Depends on catchment and storage; usually reliable for small towns	Less turbidity (settling), but algae, tastes and odours; eutrophication risk
Impounding reservoirs (dams)	Large, regulated supply — stores monsoon flows for dry seasons	Clearer water due to settling; stratification in deep reservoirs; algae; quality varies with depth (so multi-level intakes)
Canals	Depends on canal operation (closures)	Similar to river water, with contamination risk along the route

Subsurface sources

Source	Features
Springs	Natural outflow of groundwater; gravity springs (water table cuts the ground surface), surface springs, artesian springs; supply for hill towns
Infiltration galleries	Horizontal perforated conduits laid in sand/gravel along or under river beds; collect naturally filtered water
Infiltration wells	Shallow wells sunk in river beds, connected to a jack well
Open (dug) wells	Shallow unconfined aquifers; small supplies
Tube wells	Deep aquifers; large yields; common source for towns and rural schemes

Groundwater is generally clear, free of pathogens and cooler, but may be hard or contain iron, fluoride, arsenic, nitrate or salinity; quantity is limited by aquifer recharge.

Alternative sources

Rainwater harvesting — rooftop and surface runoff collection for storage or recharge.
Desalination of sea or brackish water (reverse osmosis, multi-stage flash distillation) — energy-intensive; used in some coastal cities.
Reuse of treated wastewater for non-potable uses (industry, gardening, flushing).

Selection of a source

Quantity — adequate for the design period, including in droughts.
Quality — minimum treatment required; free from pollution risk.
Distance and elevation — nearer and higher sources reduce conveyance and pumping cost (gravity supply preferred).
Cost — capital and operation and maintenance.
Reliability and legal rights — water rights, inter-state issues.

Intake structures

An intake is a structure placed in or near a source to withdraw water and discharge it into the conveyance system (intake conduit, jack well, pumping main). It is usually a masonry or concrete structure with openings fitted with screens.

Types

Type	Description
River intake	Circular masonry tower (intake well) in or near the river with ports at different levels; water flows through screens into the well and is pumped out
Reservoir intake	Tower near the dam (often part of it) with inlets at several levels so the best-quality water can be drawn as the level changes; access by a foot bridge
Lake intake	Pipe or conduit laid in the lake bed with a bell-mouth entry protected by a crib or screen; conveys water to a shore sump
Canal intake	Masonry chamber on the canal bank with a screened inlet pipe
Submerged intake	Entirely under water (a crib of timber/concrete with pipe entry); cheap, does not obstruct navigation, but hard to inspect and clean
Exposed intake	Tower above water level; accessible for maintenance and operation of gates
Wet intake tower	Water enters the tower well (water level inside ≈ source level) and then flows into the conduit through gates
Dry intake tower	Inlet pipes lead directly to the conduit; the tower is dry inside; water can be drawn from any level independently; tower must be designed for buoyancy/uplift

Site selection for intakes

Located upstream of towns and sewage or industrial outfalls, where water is purest.
Adequate depth of water at the lowest level (entry below the lowest water level) — preferably on a straight stable reach or the concave (deep) side of a bend.
Good foundations and safety against scour, floods, floating debris and ice.
Not in navigation channels; easy access.
Should not be where heavy currents or rapids cause damage; avoid sites with sand bars forming.
Close to the treatment plant and supply area where possible, and able to accommodate future expansion.

Design considerations

Screens: coarse screens (bars) at the entry to exclude large floating matter, and fine screens for smaller debris; cleaned manually or mechanically.
Low entrance velocity through the ports (commonly well below about 0.6 m/s) to avoid drawing in debris, sediment and fish.
Inlets at multiple levels (reservoir and river intakes) — withdraw clearer water as levels and quality vary.
Stability against water pressure, wave and ice forces, buoyancy (dry towers), scour.
Capacity for the maximum daily demand at the end of the design period (intake structures commonly designed for about 30 years).
Provision for isolation (gates/valves) and cleaning.

Conveyance of raw water

Gravity (free-flow) conduits — canals, aqueducts, flumes, tunnels, grade aqueducts; follow the hydraulic gradient; cheap where terrain permits.
Pressure conduits — pipelines (mild steel, ductile iron, prestressed concrete, GRP, HDPE) that can follow ground undulations below the hydraulic gradient line.
Pumping from a jack well/sump at the intake through a rising main to the treatment plant; pumps designed for maximum daily flow with standby units.

FormulaPumping power

P = \frac{γ _{w} Q H}{η}

$H$ = total head = static lift + friction and minor losses; $η$ = overall efficiency of the pump set.

Economical diameter of a rising main is found by balancing pipe capital cost (increases with diameter) and pumping energy cost (decreases with diameter); velocities of roughly 1–2 m/s are commonly economical.

Part 2 of 2

Water Demand & Population Forecasting

Last reviewed 16 Sept 2026 · 7 min read

Planning a water supply scheme

A public water supply scheme must supply enough water of acceptable quality at adequate pressure to the population at the end of the design period. The first steps are to estimate the per capita demand, the future population and the variations in demand.

Types of water demand

Demand	Includes
Domestic	Drinking, cooking, bathing, washing clothes and utensils, flushing toilets, gardening — usually the largest share in towns
Industrial	Factories, processing units (varies greatly with type of industry)
Institutional and commercial	Schools, hospitals, hotels, offices, restaurants, railway and bus stations
Public (civic) use	Street washing, sewer flushing, public parks, fountains
Fire demand	Fire fighting (small annual quantity but high rate)
Losses and wastage	Leakage from mains and fittings, unauthorised connections, meter errors — part of non-revenue water

Per capita demand norms (CPHEEO)

Classification of town/city	Recommended maximum water supply (litres per capita per day)
Towns with piped water supply but without sewerage	70
Cities with piped water supply where sewerage exists or is planned	135
Metropolitan and mega cities with piped supply and existing/planned sewerage	150
Towns supplied through public stand posts	40

These figures exclude unaccounted-for water; an allowance (commonly up to about 15%) is added for losses.
Rural household tap connections under the Jal Jeevan Mission are planned for 55 lpcd.

Factors affecting per capita demand

Size of the city (larger cities — higher demand); climate (hot, dry — higher); living standards and habits of people; industrial and commercial activity; pressure in the distribution system (higher pressure — more wastage); quality of water; metering and cost (metering reduces demand); sewerage system (flush toilets increase demand); policy on continuous or intermittent supply; efficiency of the water works administration (leak control).

Fire demand

FormulaFire demand formulas (

P

= population in thousands)

Formula	Fire demand
Kuichling	$Q = 3182 P$ (litres/min)
Freeman	$Q = 1136 (\frac{P}{10} + 10)$ (litres/min)
National Board of Fire Underwriters (central congested areas)	$Q = 4637 P (1 - 0.01 P)$ (litres/min)
Buston	$Q = 5663 P$ (litres/min)
Indian practice (CPHEEO)	$Q = 100 P$ (kilolitres/day)

Fire hydrants are provided at intervals along mains, and the distribution system (especially storage) is designed to meet fire flow at adequate pressure.

Variations in demand

Water demand varies:

Seasonally — higher in summer.
Daily — with days of the week and festivals.
Hourly — two peaks in most towns (morning and evening), minimum at night.

FormulaPeak demands (common rule-of-thumb values)

Maximum daily demand ≈ 1.8 × average daily demand
Maximum hourly demand ≈ 1.5 × maximum daily demand = 2.7 × average daily demand
Goodrich's formula: $p = 180 t^{- 0.10}$ — percentage of the annual average demand for a period of $t$ days (e.g. about 180% for a day, 148% for a week)

Peak factors for distribution systems (CPHEEO):

Population	Peak factor
Up to 50 000	3.0
50 000 to 2 lakh	2.5
Above 2 lakh	2.0
Rural water supply schemes	3.0

Design flows for components

Component	Designed for
Source, intake, raw water pumps and mains	Maximum daily demand (often average daily for sources with storage)
Treatment plant	Maximum daily demand
Service reservoirs	Hourly fluctuations (balancing), fire and emergency storage
Distribution system	Maximum hourly demand (or maximum daily + fire demand, whichever is greater)

Design period

The design period is the number of years for which a component is designed to be adequate. It depends on the useful life of the component, ease of future expansion, rate of population growth, cost and interest rates, and the performance of the component in early years (under-loading).

Design periods recommended by CPHEEO (typical):

Component	Design period (years)
Storage by dams	50
Intake structures, conveyance mains, distribution system, trunk mains	30
Pumping machinery	15
Water treatment units	15
Service (clear water) reservoirs	15
Land acquisition for future extensions	30

The design period is counted from the expected year of completion of the project, not from the start of design.

Population forecasting

The future population is estimated from past census data (India's census is decennial).

FormulaMathematical methods (

P_{0}

= latest population,

n

= number of decades)

1. Arithmetic increase method — constant increase per decade (old, large, saturated cities):

P_{n} = P_{0} + n \overset{x}{ˉ}

$\overset{x}{ˉ}$ = average increase per decade.

2. Geometric increase method — constant percentage growth (young, rapidly growing cities):

P_{n} = P_{0} (1 + r)^{n}

$r$ = geometric mean of the decadal growth rates $= (r_{1} r_{2} \dots r_{t})^{1/ t}$ . Gives the highest estimate.

3. Incremental increase method — combines arithmetic and geometric trends (average cities):

P_{n} = P_{0} + n \overset{x}{ˉ} + \frac{n ( n + 1 )}{2} \overset{y}{ˉ}

$\overset{y}{ˉ}$ = average of the increments (change in decadal increases).

4. Decreasing rate of growth method — the percentage growth rate decreases steadily; the average decrease in rate is subtracted from the latest rate each decade.

5. Logistic curve (S-curve) method:

P = \frac{P _{s}}{1 + m e ^{n t}}, P_{s} = \frac{2 P _{0} P _{1} P _{2} - P _{1}^{2} ( P _{0} + P _{2} )}{P _{0} P _{2} - P _{1}^{2}}

using three census populations $P_{0}$ , $P_{1}$ , $P_{2}$ at equal time intervals; $P_{s}$ = saturation population.

Other methods:

Simple graphical method — extend the population–time curve by eye.
Comparative graphical method — the city's growth is assumed to follow that of larger, similar cities when they were of the same population.
Master plan / zoning method — population based on planned densities of zones in the city's master plan.
Ratio and correlation method — city population as a ratio of the state or national population forecast.

Arithmetic increase generally gives low estimates, geometric increase high estimates, and incremental increase intermediate values.

Worked examples

Worked ExampleExample 1 — population forecasts

Census populations of a town are 40 000 (1981), 50 000 (1991), 62 000 (2001) and 76 000 (2011). Estimate the 2041 population by the arithmetic, geometric and incremental increase methods.

Solution. Decadal increases: 10 000, 12 000, 14 000 → $\overset{x}{ˉ}$ = 12 000; increments: 2000, 2000 → $\overset{y}{ˉ}$ = 2000; $n$ = 3.

Arithmetic: $P = 76000 + 3 \times 12000 = 112000$

Incremental: $P = 76000 + 36000 + \frac{3 \times 4}{2} \times 2000 = 124000$

Geometric: rates 25%, 24%, 22.58% → $r = (0.25 \times 0.24 \times 0.2258)^{1/3} = 0.2384$ $P = 76000 \times 1.238 4^{3} = 76000 \times 1.899 = 144300$ (approx.)

Worked ExampleExample 2 — fire demand

Find the fire demand for a city of 1 lakh population by Kuichling's and Freeman's formulas and the CPHEEO formula.

Solution. $P$ = 100 (thousands) Kuichling: $3182100 = 31820 L/min$ Freeman: $1136 (10 + 10) = 22720 L/min$ CPHEEO: $100100 = 1000 kL/day$

Worked ExampleExample 3 — design demands

A city with sewerage has a design population of 1 lakh. Find the average daily demand, maximum daily demand and maximum hourly demand (use 135 lpcd).

Solution. Average $= 100000 \times 135 = 13.5 \times 1 0^{6}$ L/day = 13.5 MLD Maximum daily $= 1.8 \times 13.5 = 24.3 MLD$ Maximum hourly rate $= 2.7 \times 13.5 = 36.45 MLD$ (as a rate)

Frequently tested points

CPHEEO: 70 lpcd (no sewerage), 135 lpcd (with sewerage), 150 lpcd (metro), 40 lpcd (stand posts); JJM rural 55 lpcd.
Metering and lower pressure reduce per capita demand; flush toilets increase it.
Kuichling $3182 P$ ; Freeman $1136 (P /10 + 10)$ ; CPHEEO $100 P$ kL/day.
Max daily = 1.8 × average; max hourly = 2.7 × average; Goodrich $p = 180 t^{- 0.1}$ .
Peak factor: 3.0 (≤ 50 000), 2.5 (50 000–2 lakh), 2.0 (> 2 lakh).
Design periods: dams 50 y; intake, mains, distribution 30 y; pumps, treatment plants, service reservoirs 15 y.
Arithmetic (old cities, lowest), geometric (young cities, highest), incremental (intermediate).
Treatment plant designed for maximum daily demand; distribution for maximum hourly demand.

Common MistakeCommon mistakes

Using the arithmetic mean of growth rates in the geometric method (use the geometric mean).
Taking $n$ in years instead of decades.
Designing distribution pipes for average daily demand.

Revision SummaryChapter summary

Water demand combines domestic, industrial, commercial, public and fire uses plus losses.
CPHEEO norms and local factors fix per capita demand.
Demand varies seasonally, daily and hourly; peak factors size different components.
Design periods depend on component life and expandability.
Population is forecast by arithmetic, geometric, incremental, logistic and graphical methods.

Downloaded from civilgyantests.com · Not for resale. Verify every code clause and date against the official source.

Finished reading? Test yourself.

A timed chapter test from the TNPSC AE Civil series, on exactly this chapter.

Practice this chapter →