water supply lecture 2

1. LECTURE 2
QUANTITY OF WATER
Instructor : Engr. Beenish Akbar Khan
B.Tech Civil
Iqra National University (INU)
Water Supply & Waste
Water Management

2. VARIATIONS IN DEMAND
 Variation in water demand:
 Season to season
 Within a day
 Within a week
 Flows measured at water source are very important in calculating
“variations in water demand”.
 In the absence of data, it is necessary to estimate the maximum
rates.
 R.O. Goodrich gave a formula for variation in demand as follow:
P = 180 * t – 0.1
where,
P = the percent of annual average consumption
T = time in days 2

3. 1. Average daily consumption (Q) or per capita demand (ADD):
𝑄 = 𝑞 ∗ 𝑃 (𝑙𝑝𝑐𝑑)
 q is total yearly water required for city in liters.
 P is design population.
2. Maximum daily consumption (MDC):
It is the max. daily water consumption during any one day in the year.
It is about 180 % of the average daily consumption.
Max daily demand = 1.8 * Average daily demand
MDD = 1.8 * ADD 3
VARIATIONS IN DEMAND

4. 3. Max. weekly consumption:
It is 148 % of average daily consumption.
Max. weekly consumption = 1.48 x Avg. daily consumption
4. Max. monthly consumption:
It is 128 % of avg. daily demand.
Max. monthly consumption = 1.28 x Avg. daily demand
5. Max. hourly consumption:
It is likely to be about 150 % of the average daily consumption for that
day or it is the max. water consumption during any one hour of any day
(24 hrs).
Max. hourly consumption = 1.5 x Avg. daily consumption 4
VARIATIONS IN DEMAND

5. 6. Peak Hourly Consumption (P.H.D):
It is the max. consumption occurring in a particular one hour
throughout the year (or)
It is the max. consumption on the max. hour of the max. day
throughout the year.
P.H.D = 1.8 x 1.5 x ADD
= 2.7 x ADD
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VARIATIONS IN DEMAND

6. 7. Minimum rate of Consumption:
Normally it is 25 – 50 % of the average daily consumption.
Min. rate of consumption is of less importance than max flow, but it is
required in design of pumping stations.
Hydraulic Capacity of Main distribution:
= PHD + FF
Capacity = Peak Hourly Demand + Fire Flow
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VARIATIONS IN DEMAND

7. PROBLEMS
1) The average daily consumption is 670 lpcd. Find the max.
daily and peak hourly?
Solution:
MDD = 1.8 * ADD
= 1.8 * 670
= 1206 lpcd
PHD = 1.8 * 1.5 * ADD
= 1809 lpcd
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8. PROBLEMS
2) What will be the average daily consumption, max. daily & peak
hourly demand. For a town with a population of 85,000 persons
and rate of water supply of 160 lpcd.
Solution:
ADD= Q * P
= 160 * 85000 = 13.6 * 10 6 lpcd
MDD = 1.8 * ADD
= 1.8 * 13.6 * 10 6
= 24.4 * 10 6 lpcd
PHD = 1.8 * 1.5 * ADD
= 36.7 * 10 6 lpcd
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9. FIRE DEMAND PROBLEMS
3) Determine the max. demand during a fire for a community with a
population of 22,000 persons and an average water consumption of
600 lpd. The fire flow is for a building of ordinary construction with a
floor area 1000 m2 and a height of 6 stories. The fire flow is
maintained for 10 hours per day.
Given Data:
P = 22,000 persons
q = 600 lpd
C = 1
Floor area = a= 1000 m2
No. of stories = 6
Duration of fire = 10 hours/day
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10. FIRE DEMAND PROBLEMS
Required: Q = ?
Solution:
Step 1: ADD = q * P = 600 * 22,000 = 13.2* 106 l/d
Step 2: MDD= 1.8 * ADD = 1.8 * 13.2* 106 = 23.76 * 106 l/d
Step 3: Fire Flow= F = 18 * C * 𝐴 gpm
= 18 * 1 * 1000 ∗ 6 ∗ 10.76
= 4573.55 gpm
= 4573.55 * 3.78 * 60* 24
= 24.89 * 106 l/day 10
dayhourLiters

11. FIRE DEMAND PROBLEMS
Step 4:
Max. demand during fire = q = MDD + FF
= 23.76 * 106 + 24.89 * 106
= 48.65 * 106 l/d
= 48.65 * 106 / 22,000 lpcd
= 2211 lpcd for 10 hrs a day
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12. DESIGN POPULATION
Population forecasting:
1. Arithmetic growth method
2. Geometric growth method
3. Curvilinear growth method
4. Logistic growth method
5. Decline growth method
6. Ratio growth method
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13. Arithmetic growth method:
It is based on the hypothesis that rate of growth of population is
constant. It means that the population increases by a constant
increment every year.
Pf = Po + Ka* Δt
where,
Pf = future population
Po = present population
Ka = Arithmetic growth rate constant
Δt = design period (tf – t0)
It is suitable for large & established towns; whose future
growth is practically controlled.
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DESIGN POPULATION

14. Geometric growth method:
It is based on the hypothesis that rate of change of
population is proportional to present population.
ln( Pf ) = ln ( Po ) + Kg (tf – t0 )
where,
tf = future time
t0 = present time
It is suitable for cities with unlimited scope of expansion.
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DESIGN POPULATION

15. Logistic growth method:
When the growth rate of population due to birth, death and migration are
under normal situation and not subjected to extraordinary changes due to
unusual situation like war, epidemics, earthquakes and refuges etc.
P = P sat
1 + (e) a+b*Δt
Psat = saturation population
a, b = constant
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DESIGN POPULATION

16. Decline growth method:
This method like , logistic method, assumes that city has some limiting
saturation population and that its rate of growth is a function of population
deficit (deficiency).
dp = K ( Psat – P)
dt
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DESIGN POPULATION

17. Curvilinear method:
In this method it is assumed that population of a city will grow in the same
manner as in other city in the past. This similarity between cities includes
geographical proximity (neighborhood), similarity of economic base, access to
similar transportation system etc.
In practice it is difficult to find similar cities.
Ratio Method:
It is based on the assumption that the population of a certain area/city will
increase in the same manner to a larger entity like a province, or a country.
It requires calculation of ratio of locals to required population in a series of
census year.
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DESIGN POPULATION

18. DESIGN PERIOD (N)
Type of Structures Years
Dams, Tunnels, Large canal 50
Wells 5
Pipeline ( Transmission line) 25
Water Treatment Plant 10 – 15
Pumping plant 10
Distribution main 50 – 100
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Standards

19. THE END
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