Sediment Transport Concepts - Sediment Transport in Irrigation Canals: A New Approach - page 114

100

80

60

40

20

0

1.05 1.10 1.15 1.20

Error factor

Figure 5.12. Comparison of
well-predicted values for
different values of error factor
using Krüger's data.

Method 1
Method 2

Method 3

Method 5
Method 6

Method 4

central part without considering the differences in velocity or water
depth;

•

methods 3 and 4 are fairly similar; they differ in the description of the
mean velocity in the sub-sections.

5.2.7 Prediction of composite roughness in a rectangular canal

For canals with a rectangular cross section, the existing methods to find the
equivalent roughness cannot be directly used. Rectangular cross sections
do not have a clearly defined area that can be associated with each type
of roughness along the wetted perimeter. Therefore, the estimate of the
equivalent roughness follows the same principles as used for the sidewall
correction method, which is a calculation procedure initially proposed
by Einstein (1942) to determine the shear stress at the bottom as well as
the shear velocity, friction factor, etc. The method does not include any
correction for the effect of the sidewalls on the velocity distribution or
sediment transport (Ranga Rayu et al., 1977).
Méndez described a method for a non-wide rectangular canal with
bottom width B and water depth h (Figure 5.13). The rectangular canal is
replaced by another canal, namely a wide canal with a bottom width B ∗
( B ∗ = B + 2 h ) and a water depth R ( R = A/P ). The area A = R ∗ ( B + 2 h )

and the total discharge of the wide, rectangular canal is expressed by:

2 C L hR RS f + C b BR RS f Q = vA = (5.44)