Open Channel Flow - Sediment Transport in Irrigation Canals: A New Approach - page 10

applications the variation may be considered to be so slow that a steady (or
quasi steady) flow can be assumed. Considering the spatial distribution,
any flow is essentially three-dimensional, meaning that the magnitude and
direction of the flow vary from one point to another. Knowledge of this
three-dimensional flow behaviour is still limited; but in many engineer-
ing applications it is often sufficient to know particular mean or average
values.
The mean value can be presented for:

• a two-dimensional flow situation by averaging the value over the canal

depth at a certain point; an example is the depth-averaged flow velocity
in a vertical;

• a two-dimensional flow condition by averaging the value over the canal

width (in a lateral direction); an example is the average water depth in
a cross-section;

• a one-dimensional flow situation by averaging the value over the

whole cross-section. The resulting values depend on the longitudinal
coordinate (the x -values); an example is the average velocity in a
cross-section.

It is important to remember that most of the two- and one-dimensional
flow problems are simplified by averaging the flow characteristics; some
information relating to the 'un-averaged' three-dimensional situation
and the consequences of the process of averaging should be considered
when the mean quantities are interpreted.
Open channel flow can be classified in many ways. A very common
classification of open channel (gravity) flow is according to the change
of flow depth with respect to time and space and is shown in Figure 2.1.
Steady flow means that the main flow variables do not change and are
steady with time; the variables at every point remain constant with time.
An example is the flow in a channel where the water depth either does not
change with time or can be assumed to be constant during the time interval

Uniform flow

Steady flow

Gradualy

Varied flow
(Non-uniform)

Rapidly

Flow

Quasi uniform
flow

Unsteady flow

Gradualy

Varied flow
(Non-uniform)

Rapidly

Figure 2.1. Classification of

flow types.