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Chapter 5

Water Hydraulics, Transmission,

and Appurtenances

5.1 FLUID MECHANICS, HYDRAULICS, AND WATER TRANSMISSION
5.1.1 Fluid Mechanics and Hydraulics
Fluids are substances that are capable of flowing, and that conform to the shape of the ves-
sel in which they are contained. Fluids may be divided into liquids and gases. Liquids are
practically incompressible, whereas gases are compressible. Liquids occupy definite vol-
umes and have free surfaces, whereas gases expand until they occupy all portions of any
containing vessels. Water, wastewater, oil, etc., are liquids.
Fluid mechanics is a branch of applied mechanics dealing with the behavior of fluids (liq-
uids and/or gases) at rest and in motion. The principles of thermodynamics must be included
when an appreciable compressibility does occur. Hydraulics is a branch of fluid mechanics
dealing with the behavior of particularly incompressible water, wastewater, liquid sludge, etc.
The characteristics of gases are determined by Boyle’s and Charles’s laws:

P a V s >T a R g (5.1)
3
2
where P a absolute pressure P gauge P atm , lb/ft ; V s 1>g specific volume, ft /lb;
T a absolute temperature T 460 F; R g gas constant, ft/ F; T temperature, F;
3
g specific weight, lb/ft ; and
g 1>V s P a >R g T a (5.2)
The specific weight, g, of a substance is the weight of a unit volume of the substance.
3
The specific (unit) weight of water is 62.4 lb/ft for ordinary temperature variations.
The specific gravity of a substance is that pure number, which denotes the ratio of the
weight of a substance to the weight of an equal volume of a substance taken as a standard.
Solids and liquids are referred to water (at 39.2 F 4 C) as standard, whereas gases are
referred to air free of hydrogen and carbon dioxide (at 32 F 0 C and at 1 atmosphere
2
14.7 lb/in. pressure) as standard. For example, if the specific gravity of a given oil is 0.755,
3
3
its specific weight is 0.755 (62.4 lb/ft ) 47.11 lb/ft .
The mass of a body, (rho), which is termed density, can be expressed by the follow-
ing equation:
g
r = (5.3)
g
3
2
In the U.S. customary system of units, the density of water is (62.4 lb/ft )>(32.2 ft/s )
3
3
3
1.94 slug/ft . In the metric system, the density of water is 1 g/cm 1 kg/L 1,000 kg/m
3
1 tonne/m at 4 C.
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