Water Hydraulics - Mathematics and Statistics for the Environment - page 587

22 Water Hydraulics

Evidence conforms to conceptions just as often as conceptions conform to evidence.

— Ludwik Fleck (1896-1961)

22.1 INTRODUCTION
The word “hydraulics” is derived from the Greek words hydro (“water”) and aulis (“pipe”).
Originally, the term referred only to the study of water at rest and in motion (flowing through pipes
or channels). Today, it is taken to mean the flow of any liquid in a system. Hydraulics —the study
of fluids at rest and in motion—is essential for an understanding of how water/wastewater systems
work, especially water distribution and wastewater collection systems.

22.2 BASIC CONCEPTS

Air pressure (at sea level) = 14.7 pounds per square inch (psi)

This relationship is important because our study of hydraulics begins with air. A blanket of air many
miles thick surrounds the Earth. The weight of this blanket on a given square inch of the Earth's
surface will vary according to the thickness of the atmospheric blanket above that point. As shown
above, at sea level the pressure exerted is 14.7 pounds per square inch (psi). On a mountain top, air
pressure decreases because the blanket is not as thick.

1 ft 3 H 2 O = 62.4 lb

This relationship is also important; note that both cubic feet and pounds are used to describe a
volume of water. A defined relationship exists between these two methods of measurement. The
specific weight of water is defined relative to a cubic foot. One cubic foot of water weighs 62.4 lb
(see Figure 22.1). This relationship is true only at a temperature of 4°C and at a pressure of 1 atmo-
sphere, conditions which are referred to as standard temperature and pressure (STP). Note that
1 atmosphere = 14.7 lb/in. 2 at sea level and 1 ft 3 of water contains 7.48 gal.
The weight varies so little that, for practical purposes, this weight is used for temperatures rang-
ing from 0 to 100°C. One cubic inch of water weighs 0.0362 lb. Water 1 ft deep will exert a pressure
of 0.43 lb/in. 2 on the bottom area (12 in. × 0.0362 lb/in. 3 ). A column of water 2 ft high exerts 0.86 psi
(2 ft × 0.43 psi/ft); one 10 ft high exerts 4.3 psi (10 ft × 0.43 psi/ft); and one 55 ft high exerts 23.65
psi (55 ft × 0.43 psi/ft). A column of water 2.31 feet high will exert 1.0 psi (2.31 ft × 0.43 psi/ft). To
produce a pressure of 50 psi requires a 115.5-ft water column (50 psi × 2.31 ft/psi).

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