Sediment Transport Concepts - Sediment Transport in Irrigation Canals: A New Approach - page 112

5.2.6 Comparison of the equivalent roughness predictors
in trapezoidal canals

All the 6 methods for the prediction of the roughness were compared
using the Krüger (1988) data set. Considering the general conditions of
irrigation canals, the following criteria have been used for selecting the
data:
- trapezoidal cross section;
- Froude number less than 0.5;
- B / y ratio less than 8;
- the ratio of equivalent roughness height on the bed and the sidewall
( k sb / k se ) in the range of 50 to 0.02.
A total of 19 records were selected from the compilation of Krüger.
Table 5.7 shows the summary of the selected data.

Table 5.7. Characteristics of selected data.

Test B / y ratio Side slope k sl (mm) k sb (mm) k sb / k sl No. of records

1 3.0-5.8 1 0.054 1.047 19.800 3

2 1.8-5.7 1 8.400 1.047 0.125 8

3 3.9-7.9 2 0.054 1.047 19.800 8

The calculation process used in the comparison of the selected data
comprises the following steps:
- since it is not possible to measure Manning's roughness coefficient
separately for the bed and side slopes, the representative size of the
particles ( d 50 or d 90 ) was used to determine the equivalent roughness
height ( k s ). Various authors have given different estimation methods.
Henderson (1966) suggests 2 3*

d 90 , while Krüger (1988) has suggested using d 90 for a plane bed.
For the calculations k s =

∼ 3* d 50 , van Rijn (1982) suggests 1 ∼

d 90 has been used;
- the local Manning's roughness coefficients ( n b , n i ) is determined from
(Henderson, 1966):
n 0 . 031(3 . 28 k s ) 1 / 6

the flow can be divided into hydraulically smooth, transition or rough
depending on the following conditions (van Rijn, 1993):
smooth flow:
u ∗ k s
ν

=

≤ 5 (5.39)

transition flow:
5 < u ∗ k s
ν

< 70 (5.40)