Design Charts for Open - Channel Flow HDS 3 August 1961

Design of Highway Drainage Channels
The design of a highway drainage channel to carry a given discharge is accomplished in two
parts. The first part of the design involves the computation of a channel section which will carry
the design discharge on the available slope. This chapter briefly discusses the principles of flow
in open channels and the use of the Manning equation for computing the channel capacity.
The second part of the design is the determination of the degree of protection required to
prevent erosion in the drainage channel. This can be done by computing the velocity in the
channel at the design discharge, using the Manning equation, and comparing the calculated
velocity with that permissible for the type of channel lining used. (Permissible velocities are
shown in Table 2 and Table 3.) A change in the type of channel lining will require a change in
channel size unless both linings have the same roughness coefficient.
Types of Flow
Flow in open channels is classified as steady or unsteady. The flow is said to be steady when
the rate of discharge is not varying with time. In this chapter, the flow will be assumed to be
steady at the discharge rate for which the channel is to be designed. Steady flow is further
classified as uniform when the channel cross section, roughness, and slope are constant; and
as nonuniform or varied when the channel properties vary from section to section.
Depth of flow and the mean velocity will be constant for steady flow in a uniform channel.
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