M. HANIF CHAUDHRY
ED. SPRINGER 2008
627.042/C4962 o 2 - topográfico biblioteca U. Nacional Manizales
280 9 CHANNEL DESIGN
The design of a channel involves the selection of channel alignment, shape,
size, and bottom slope and whether the channel should be lined to reduce
seepage and/or to prevent theerosion of channel sides and bottom. Since a
lined channel offers less resistance to flow than an unlined channel, the channel
size required to convey a specified flow rate at a selected slope is smaller for a
lined channel than that if no lining were provided. Therefore, in some cases,
a lined channel may be more economical than an unlined channel.
Procedures are not presently available forselecting optimum channel pa-
rameters directly. Each site has unique features that require special consider-
ations. Typically, the design of a channel is done by trial and error. Channel
parameters are selected and an analysis is done to verify that the operational
requirements are met with these parameters. A number of alternatives are
considered, and their costs are compared. Then, themost economical alter-
nativo that gives satisfactory performance is selected. In this process, it is
necessary to include the maintenance costs while comparing different alter-
natives. Similarly, the costs of energy required if pumping is involved and,
for power canals, the amount of revenues produced by hydropower generation
must be included in the overall economic analysis.
The channeldesign may be divided into two categories, depending upon
whether the channel boundary is erodible or non-erodible. For erodible chan-
nels, flow velocities are kept low so that, the channel bottom and sides are
not eroded. The minimum flow velocity in flows carrying a large amount of
sediment should be such that the material being transported is not deposited
in the channel.
In this chapter, wefirst consider the design of rigid-boundary channels and
then the design of erodible channels.
9-2 Rigid-Boundary Channels
In the design of a rigid-boundary channel, the channel cross section and size
are selected such that the required discharge is carried through the channel
for the available head with a suitable amount of freeboard. The freeboard is
defined as the vertical distancebetween the design water surface and the top
of the channel banks. Freeboard is provided to allow for unaccounted factors in
design, uncertainty in the selection of different parameters, and disturbances
on the water surface, etc.
The channel alignment is selected so that the channel length is as short as
possible and at the same time meets other site restrictions and requirements,
such asaccessibility, right of way, and balancing of cut and fill amounts. The
bottom slope is usually dictated by the site topography whereas the selection
of channel shape and dimensions take into consideration the amount of flow to
be carried, the ease and economy of construction and the hydraulic efficiency
9-2 Rigid-Boundary Channels 281
of the cross section. A triangular channel is used forsmall rates of discharge,
and a trapezoidal cross section is generally used for large flows. For structural
reasons, channels excavated through mountains or built underground usually
have a circular or horseshoe shape. Normally, the Froude number is kept low
(approximately up to 0.3) so that the flow surface does not become rough,
especially downstream of obstructions and bends. Similarly, theflow velocity
is selected such that the lining is not eroded and any sediment carried in the
flow is not deposited.
Normally, these channels are designed based on the assumption of uniform
flow, although in some situations gradually varied flow calculations may be
needed to assess the suitability of selected channel size for extreme events.
The maximum permissible velocity is not usually a...