Regulated Rivers - Hydro-Environmental Analysis: Freshwater Environments - page 60

existing natural channels to meet some design goal, such as for water supply, lood control or drain-
age, waterborne transportation, or to restrict lows to conined areas (Brookes 1981). Many of these
engineered channels, new or altered, were, to the extent possible, straight with regular cross sections
and with some near-constant design depth. Channels of this type are easier to design and build than
natural channels. For example, one of the most common equations used in channel design, and one
that all civil engineering students are exposed to, is Manning's equation:

δ 23

12

Q =

n RAS

(3.1)

where
δ is a unit conversion
n is Manning's roughness coeficient
R is the hydraulic radius of the channel
A is the wetted area of the channel
S is the channel slope

This equation assumes that the low is constant (steady, or does not change with time) and the
channel is prismatic (constant shape and slope) so that depths and velocities are constant throughout
the channel (uniform); therefore, this equation can only be used to design channels with regular
cross sections and a constant slope.
Examples of channels include those excavated for either a new or a reengineered river. Also,
commonly, some portion of the existing channel or waterbody could be excavated to produce a
navigation channel, as illustrated in Figure 3.17. In some cases, a conining structure is installed to
raise the water level and minimize the excavation necessary, or to separate the navigable from the
existing waterway as with a perched channel (Figure 3.18). As indicated in the preceding section, a
goal of the operation of control structures in a navigation channel is nearly constant water-surface
elevations. So, the control structures are designed to convert the natural river slope into a series of
pools with a relatively constant depth, creating a “stair-stepped” water surface, with the structures
connected by natural or excavated navigation channels.
One consequence of channelization is often the destabilization of the channel, as discussed in
Section 3.3.2. One common consequence is aggradation, or the accumulation of sediments that tend
to ill the channel. Many of the engineering practices involved in maintaining channelized rivers
and streams are designed to protect embankments and channels from erosion. For example, layers
of rock may be added to embankments (rock revetments) to prevent erosion. However, aggradation
from upstream or watershed sources may ill the channel.

Material removed

Depth Dug channel

Material
removed

FIGURE 3.17 Excavated channels: a dug or excavated channel in an existing waterway.