Sediment Transport Concepts - Sediment Transport in Irrigation Canals: A New Approach - page 107

Lateral discharge Central discharge Lateral discharge

dy

k si

h i

l

h

m

k sb

Figure 5.10. Composite
roughness in a trapezoidal canal.

B

coefficients for the walls and bottom follow from recommended values
for the type of material on the bottom and walls, respectively. Chow
(1983) gives an extensive list of roughness coefficients.

• Movable bed : When the canal bed is movable then the roughness may

change through the development of bed forms on the bottom, but not
on the sides. The sediment particles on the sides are subject to the
fluid force and to the gravity force. Therefore, the critical shear stress
for the initiation of motion of these particles is much smaller due to
the component of the gravity force along the slope. For slopes that are
steeper than the angle of repose the critical shear stress for the initiation
of motion is reduced to zero (Ikeda, 1982b). Recommended values of
m for unlined irrigation canals are between 1 and 3. For most cases the
slope angle is between 18.5 ◦ and 45 ◦ and exceeds the natural angle of
repose of wet sand, which is between 15 ◦ and 25 ◦ (Kinori, 1970). Due
to this fact, it is expected that sediment particles will be deposited on
the bottom and not on the slopes.
The two most common cases for composite roughness of canals with
a movable bed are:
• Bed forms on the bottom and flat sidewalls : the roughness of the bottom

follows from the bed form characteristics and the roughness of the
sidewall follows from the type of material.

• Bed forms on the bottom and vegetation on the sidewalls : vegetation on

the sidewalls is a type of roughness (Chow, 1983), which is often given
by a single value drawn from field measurements. However, the rough-
ness of canals with vegetation is more complex and is a function of
many variables related to the flow conditions and vegetation character-
istics, which cannot be expressed by a single value. The flow resistance
for a canal with vegetation is difficult to determine and requires ample
research before the phenomena involved are completely understood
(Kouwen, 1969 and 1992). So far, it is almost impossible to make a
proper estimate of the resistance based on only analytical or theoretical
considerations only (Querner, 1993).

Irrigation canals are normally not very wide ( B / y ratio < 8), hence
the influence of the roughness of the side slope will be significant in