Page 109 - Water Engineering Hydraulics, Distribution and Treatment

Chapter4
Quantities of Water Demand
Knowledge of the required quantities of water flows is fun-
proposed system and its component structures and
damental to systems design and management. In the United 1. The number of years, or design period, for which the
States the volumes of water supplied to cities and towns or equipment are to be adequate
removed from them are expressed in US gallons per year 2. The number of people, or design population,tobe
(gal/yr), month (gal/month), day (gpd), or minute (gpm). served
3
The US gallon occupies a volume of 0.1337 cubic feet (ft )
3. The rates of water use and wastewater release, or
or 3.785 liters (L) and weighs 8.344 pounds (lb) or 3.785 kilo-
design flows, in terms of per capita water consump-
grams (kg). The fundamental metric (SI) unit in engineering tion and wastewater discharge as well as industrial
3
work is the cubic meter (m ), weighing l,000 kilograms (kg)
and commercial requirements
or 1 metric tonne (T) and equaling 1,000 liters (L). In the
United States, annual water or wastewater volumes are con- 4. The area to be served, or design area, and the
veniently recorded in million gallons (MG) or million liters allowances to be made for population density and
(ML). Daily volumes are generally expressed in million gal- areal water consumption as well as water supply and
lons per day (MGD) or million liters per day (MLD) if more wastewater release from residential, commercial, and
than 100,000 gpd (378,500 L/d). The gallons per capita daily industrial districts
volumes are stated in gpcd while the liters per capita daily 5. The rates of rainfall and runoff, or design hydrology,
volumes are stated in Lpcd. Connected or tributary popula- for storm and combined systems
tions and numbers of services or dwelling units may take the
place of total populations.
Per capita and related figures generalize the experi- 4.1 DESIGN PERIOD
ence. They permit comparison of the experience and prac-
New water and wastewater works are normally made large
tices of different communities and are helpful in estimat-
enough to meet the needs and wants of growing communities
ing future requirements of specific communities. Fluctu-
for an economically justifiable number of years in the future.
ations in flow are usefully expressed as ratios of maxi-
Choice of a relevant design period is generally based on
mum or minimum annual, seasonal, monthly, weekly, daily,
hourly, and peak rates of flow to corresponding average rates
1. The useful life of component structures and equip-
of flow.
ment, taking into account obsolescence as well as
Most water and wastewater systems include massive
wear and tear
structures (dams, reservoirs, and treatment works) that have
long construction timelines and are not readily expanded; 2. The ease or difficulty of enlarging contemplated
they also include pipes and other conduits sunk into city works, including consideration of their location
streets, which disrupt traffic while they are being laid. 3. The anticipated rate of population growth and water
Accordingly, the principal system components are pur- use by the community and its industries
posely made large enough to satisfy community needs
4. The going rate of interest on bonded indebtedness
for a reasonable number of years. For this reason, select-
5. The performance of contemplated works during their
ing the initial or design capacity is not simple. It calls
early years when they are expected to be under min-
for skill in interpreting social and economic trends and
imum load
sound judgment in analyzing past experience and predict-
ing future requirements. Among needed estimates are the
Design periods often employed in practice are shown in
following:
Table 4.1.
Water Engineering: Hydraulics, Distribution and Treatment, First Edition. Nazih K. Shammas and Lawrence K. Wang.
© 2016 John Wiley & Sons, Inc. Published 2016 by John Wiley & Sons, Inc.

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