Sediment Transport Concepts - Sediment Transport in Irrigation Canals: A New Approach - page 118

When using the same reasoning Manning's n results in:

n + n

=

n (5.60)

5.2.9 Determination of the friction factor

A summary of the most widely used methods to predict the friction factor
includes the methods:
- Van Rijn (1984c);
- Brownlie (1983);
- White, Paris and Bettes (1979);
- Engelund (1966).
The methods of Engelund, White et al. and Brownlie predict the friction
factor as a function of the flow condition and sediment size. No explicit
bed form characteristics are required. The van Rijn method is based on
flow conditions and sediment size as well as on bed form and grain-
related parameters such as bed form, length and height. More details
about the methods that can be used to predict the friction factor are given
in Appendix B.
The four methods to predict the flow resistance have been compared
to find the most appropriate method for situations similar to those
encountered in irrigation canals. The comparison has been based on field
and flume data, and includes:
- RIJ (van Rijn, 1984c);
- BRO (Brownlie, 1983);
- WBP (White, Paris and Bettess, 1979);
- ENG (Engelund, 1966).
The accuracy of the prediction methods is evaluated by:

C measured
f

C measured ∗ f ≤ C predicted ≤ (5.61)

Number of well predicted values
Total number of values

Accuracy = (5.62)

where:
C measured = measured de Chézy coefficients from the data set compiled

by Brownlie (1981a);

C predicted = predicted de Chézy coefficients as determined by one of the

four methods;

f = error factor

The overall accuracy of each prediction method is used to draw some
conclusions concerning the applicability of each method:
• the four methods use only the bottom friction for the prediction of the

friction factor. The B / y ratio of all the data used is larger than 10 and