Water Hydraulics - Wastewater Operators - page 81

capacity of an existing line. Determining flows in pipes arranged in par-
allel is also accomplished by applying energy conservation principles;
specifically, energy losses through all pipes connecting common junc-
tion points must be equal. Each leg of the parallel network is treated as
a series piping system and converted to a single equivalent length pipe.
The friction losses through the equivalent length parallel pipes are then
considered equal and the respective flows determined by proportional
distribution.
Note: Computations used to determine friction losses in parallel com-
binations may be accomplished using a simultaneous solution approach
for a parallel system that has only two branches; however, if the paral-
lel system has three or more branches, a modified procedure using the
Hazen-Williams loss formula is easier (Lindeburg, 1986, p. 3-26).
2.12 oPen Channel floW
Water is transported over long distances through aqueducts to
locations where it is to be used or treated. Selection of an aqueduct type
rests on such factors as topography, head availability, climate, construc-
tion practices, economics, and water quality protection. Along with pipes
and tunnels, aqueducts may also include or be solely composed of open
channels (Viessman and Hammer, 1998, p. 119). In this section, we deal
with water passage in open channels, which allow part of the water to be
exposed to the atmosphere. This type of channel (an open flow channel)
includes natural waterways, canals, culverts, flumes, and pipes flowing
under the influence of gravity.
2.12.1 Characteristics of open Channel flow *
Basic hydraulic principles apply in open channel flow (with water
depth constant), although there is no pressure to act as the driving force.
Velocity head is the only natural energy this water possesses, and at
normal water velocities this is a small value ( v 2 /2 g ). Several parameters
can be (and often are) used to describe open channel flow; however, we
begin our discussion by addressing a few characteristics of open chan-
nel flow, including laminar or turbulent, uniform or varied, and subcriti-
cal, critical, or supercritical.
2.12.1.1 Laminar and Turbulent Flow
Laminar and turbulent flow in open channels is analogous to that
in closed pressurized conduits (e.g., pipes). It is important to point out,
however, that flow in open channels is usually turbulent. In addition,
laminar flow essentially never occurs in open channels in either water
or wastewater unit processes or structures.
* This section is adapted from McGhee, T.J., Water Supply and Sewerage , 2nd ed.,
McGraw-Hill, New York, 1991, p. 45.