![](https://cv01.studmed.ru/view/2e5aa509f99/bg6d.png)
sectional area of the canal and the smaller the volume
to be excavated. Canals for micro-hydropower schemes
are often unlined because of the cost savings this
implies. However, there is a maximum permissible
velocity above which the hanks and bottom of an unlined
canal will start eroding. The magnitude of this velocity
depends on the nature of the soil. For bare
c2nals,
approximate values of this maximum velocity are shown
in Table 5.1. APPRNDM C (p. 269) explains how to
ascertain the nature ol the soil at a specific site.
TABLE 5.1. Maxfmum pe*mbfWe vekxftfes to
awofd em
sion in an earth canal (38,40,83)
Typeofsofl
Fine sand
Sandy loam
Clayey loam
Clay
velocity
b/d
0.3-0.4
0.4-0.6
0.6-0.8
0.8-2.0
If the canal is lined, wear by abrasion sets the upper
limit on velocity. For clear water in concrete canals,
velocities above 10 m/s have been found to do no harm.
However, if the water contains sand, gravel,
or
stones,
damage may occur at much lower velocities. Unless the
abrasive material is particularly bad, velocities up to
4 m/s should not injure wood or quality concrete. Thin
metal flumes may be damaged by coarse sand or gravel
at 2-3 m/s, and the galvanizing that might cover the
sheet metal may be injured at even lower velocities
(401.
Roughne~ coefficient. The roughness coefficient ‘n”,
also called Manning’s coefficient, is an empirical neas-
ure of the roughness of a surface. Its value for canals
ranges from 0.010 for those with the smoothest finish to
about 0.050 and more for earth canals in very poor con-
dition and obstructed with weeds and debris. The value
of ‘In” for an average canal ranges from 0.012 to 0.023.
Value- of “n“ for various materials and finishes are
shown in Fig. 5.66. Although a specific value of “n” has
been assigned for each canal surface described, the
actual value of ‘ii” may vary by kO.005
or
even more.
For this reason, the figure has been prepared to give the
reader a feel for the sensitivity of “n” to the actual sur-
face roughness.
Cross-sectional profile. The material in which the canal
is excavaled or of which it is constructed generally dic-
tates its cross-sectional profile. The following para-
graphs describe the common profiles used for canals and
when each is used.
A semicircular cross-section is the
most efficient profile because, for
I-d-1
a given canal slope and cross-sec-
I
I
tional area, it passes the maximum
flow or discharge. However, this
form is impractical to excavate. It
is therefore used primarily with materials which lend
themselves to this shape. Examples are prefabricated
concrete, sheet-metal, and wood-stave sections. Illus-
trations of these can be found in Plumes (p. 112).
A trapexoldal cross-section is the most common profile
for both lined and unlined canals excavated in earth. If
the canal is unlined, the maximum side slope is set by
that slope at which the material
will permanently stand under
water. The nature of the soil in
which the canal is excavated is
the major determining factor.
Clay soils, for example, can have
steep slopes (see Fig. 5.71 whereas
sandy soils have flatter slopes.
Some soils can stand fairly steep
slope? when dry but disintegrate into a fluid mass
and assume a flatter slope when wet. Also, original
ground which has been cut into may safely be steeper
than slopes made of the same material but filled in
(Fig. 5.67). However, it is always safer to excavate the
portion of the canal carrying the water in original earth.
The level of the water table will also affect the stabil-
ity of the slope, because incoming ground water can
cause the walls
of
the canal to slough (38). Table 5.2
presents suggested slopes for the banks of unlined
canals. (See APPEND= C, p. 269, for guidelines for
determining the type of soil in which a canal is being
excavated.1
/l in 1.5~
FQ. 5.67. Com~xrrison
of
a
cam1
section witl’l slopes both
GUI
In
origina! ground
and made of the same
material but
filled
in.
The raagnitudz of the slope of a lined banks depends on
the nature
of
the material on whmhe lining will rest.
The banks of a canal madexalmost any free-draining
material can be lined if the slope is not steeper than 1
in 1. Clayey material, on the other hand, which is apt
to become saturated with water, requires a lining with a
flatter slope. Any lining on slopes steeper than 1 in 3/4
must be built to act as a retaining wall (40).
Une disadvantage of a trapezoidal cross-section is that,
if a canal is built across a hillside which has a signifi-
cant slope, excavation on the uphill side of the canal
may be significant (see Fig. 5.77).
For a trapezoidal canal with a given slope
for
its banks,
the most efficient trapezoidal cross-section is one in
which a semicircle can be inscribed in the wetted area
(Fig. 5.68). For this section, it can be shown that the
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