Introduction
A critical topic in the area of open-channel hydraulics is the design of
channels capable of transporting water between two points in a safe, cost-
effective manner. Although economics, safety, and esthetics must always be
considered. In addition, this discussion will be limited to the design of
channels for uniform flow, and only three types of channels will be
considered : (1) lined or nonerodible ; (2) unlined, earthen, or erodible ; and
(3) grass lined. In examining the design procedures for three types of
channels, there are some basic concepts which are common to all three, and
these commonalities will be discussed first.

The proportions of the best hydraulic section of specified geometric shape
can be easily derived. It should be noted that from the point of view of
applications, the best hydraulic section is not necessarily the most
economic section. In practice the following factors must be considered :
1. The best hydraulic section minimize the area required to convey a
specified flow ; however, the area which must be excavated to achieve the
flow area required by the best hydraulics section may be significantly larger
if the over burden which must be removed is considered.
2. It may not be possible to contruct a stable best hydraulic section in the
available natural material. If the channel must be lined, the cost of the
linning may be comparable with the cost of excavation.
3. The cost of excavation depends not only on the amount of material which
must be removed, but also on the ease of access to the site and the cost of
disposing of the material removed.
4. The slope of the channel in may cases must also be considered a
variable since it is not necessarily completely defined by topographic
considerations..

In most design problems, the longitudinal slope of the channel is
determined by topography, the head required to carry the design flow, and
the purpose of the channel. For example, in a hydroelectric power canal, a
high head at the point of delivery is desirable, and a minimum longitudinal
channel slope should be used.
Table 1.2 Suitable side slope for channels built in various types of materials
(Chow, 1959)
Material Side Slope
Rock Nearly vertical
Muck and peat soils ¼ : 1
Stiff clay or earth with concrete lining ½ : 1 to 1:1
Earth with stone lining or earth for large channels 1 : 1
Firm clay or earth for small ditches 1,5 : 1
Loose, sand earth 2 : 1
Sandy loam or porous clay 3 : 1

In deep cuts, side slope are often steeper above the water surface than
they are below the surface. In small drainage ditches, the side slope are
steeper than they would be in irrigation canal excavated in the same
material. In many cases, side slopes are determined by the economics of
construction. With regard to this subject, the following general comments
are appropriate :
1. In many unlined earthen canals on federal irrigation projects, side slopes
are usually 1,5 : 1 however, side slopes as steep as 1 : 1 have been used
when the channel runs through cohesive materials.
2. In lined canals, the side slopes are generally steeper than in an unlined
canal. If concrete is the lining material, side slope greater the 1 : 1 usually
require the use of forms, and with side slopes greater than 0,75 : 1 the
lining must be designed to withstand earth pressures. Some types of
linning require side slopes as flat as those used for unlined channels.
3. Side slopes through cuts in rock can be vertical if this is desirable.

DESIGN OF LINED CHANNELS
Lined channels are built for five primary reasons :
1. To permit the transmission of water at high velocities through areas of
deep or difficult excavation in a cost-effective fashion
2. To permit the transmission of water at high velocity at a reduced
construction cost
3. To decrease canal seepage, thus conserving water and reducing the
waterlogging of lands adjacent to the canal
4. To reduce the annual costs of operation and maintenance
5. To ensure the stability of the channel section

The design of lined channels from the viewpoint of hydraulic engineering is
a rather elementary process which generally consists of proportioning an
assumed channel cross section. Some typical cross sections of lined
channels used on irrigation projects in United States are summarized in
Table 1.3. In This table, it is assumed that the design flow QD, the
longitudinal slope of the channel S, the type of channel cross section, e.g.,
trapezoidal, and the lining material have all been selected prior to the
initiation of the channel design process.

These channels are lined with materials that do not erode easily, e.g.
concrete, stone pitching, steel, wood, glass, plastic, etc.
The choice of material depends on availability and cost of respective
materials. The advantage of nonerodible channels is that lower roughness
values allow higher velocities to be maintained in a specific channel
resulting in the building of a smaller, cheaper structure. Costs must be
minimized when designing non-erodible channels. Two aspects need to be
taken into consideration, namely the quantity of lining material and
excavation required.
Trapezoidal channels are usually used where flows are > 8 m3/s, with side
slopes of 1:1,5 generally being used.
Rectangular channels should only be used where space is limited and
where small quantities of water are to be transported. In such cases
rectangular channels have the advantage of being more stable than
trapezoidal channels, therefore also requiring less maintenance. With large
channels the cost of a rectangular channel may be up to three times more
than the equivalent trapezoidal channel.

The dry board of a channel is chosen such, that the distance is sufficient to
prevent overtopping due to waves or variations in water level. There is no
generally accepted rule for determining dry board, as wave action and
variations in water level are caused by uncontrollable factors. A dry board
variation of 5% – 30% of the normal flow depth is generally accepted.

Picture 1.1 Types Of Open Channel,
Non-Erodible Channel
THE EXAMPLE OF LINED OR NON-ERODIBLE DESIGN CHANNEL

Picture 1.2 Geotextile Lined to Solve
California’s Drought Problems

Picture 1.3 Rehabs of Irrigation System

Example 1
The normal flow depth in a trapezoidal concrete channel is 2 m. The base
width is 5 m with side slopes 1:2. The channel slope is 0,001 and Manning's
n = 0,015. Determine the flow rate and average flow velocity.
Solution :
W = b + 2zy
= 5 + ( 2 x 2 x 2 )
= 13 m
A = ( b + zy ) y = ( 5 + 2 x 2 ) x 2 = 18 m2
P = 22
2122512  xzyb = 13,94 m2
2
1
3
2
1
AS
P
A
n
Q  = 2
1
3
2
)001,0(18
94,13
18
015,0
1
xxx = 45 m3
sm
A
Q
v /5,2
18
45


Exsample 2
Determine flow depth and average flow velocities for a concrete channel
with slope 1:2 500 changing to 1:3 000. Assume Manning's n-value = 0,017.
The channel is rectangular with a base width of 3 m and must be able to
handle a flow rate of 2 m3/s.

Thank you for your attention
And good afternoon

Lined or non erodible design channel study