Environmental Engineering Reference
In-Depth Information
Characteristic values for the particle parameter
D
∗
in irrigation canals
are in the range of 1.5 to 7.3 (Table 5.2).
Table 5.2.
D
∗
parameter for sediment sizes encountered in irrigation canals.
d
50
(mm)
0.05
0.1
0.15
0.20
0.25
0.30
0.35
0.40
0.45
0.50
D
∗
1.2
2.5
3.7
5.0
6.2
7.5
8.7
10.0
11.2
12.5
- Excess bed shear stress parameter
T
which is defined by:
τ
−
τ
cr
T
=
(5.13)
τ
cr
with
18 log
12
h
4
.
5
d
90
ρg
v
C
2
τ
=
and
C
=
(5.14)
The ranges described by van Rijn (1993) are shown in Table 5.3.
Table 5.3.
Classification of bed forms according to van Rijn (1984c).
Particle parameter
Transport regime
1
≤
D
∗
≤
10
D
∗
>
10
Lower
0
≤
T
≤
3
Mini ripples
Dunes
3
≤
T
≤
10
Mega ripples
Dunes
10
≤
T
≤
15
Dunes
Dunes
Figure 5.6 shows the ranges established by van Rijn (1984c) to distin-
guish ripples, mega ripples and dunes. The curves of the maximum values
of the classification parameter
T
for small irrigation canals (hydraulic
radius
R
0.5 m) are given for Froude numbers of 0.15, 0.25, 0.35 and
0.45. For larger irrigation canals (
R
=
4 m), the curves are for Froude
numbers of 0.08, 0.12 and 0.16. Higher Froude numbers in large canals
show that the actual shear stress on the bottom is larger than the
approximate 4-5 N/m
2
≈
allowed.
5.2.3
Effects of grains and bed forms on the roughness of the bed
Methods based on bed form parameters separate the total roughness into
that due to grains and bed forms. The methods require the shape and
size of bed forms to determine explicitly the equivalent roughness height.
Méndez (1998) tested the accuracy of the friction factor predictors with
selected data and concludes that the method by van Rijn (1984b) based
on bed form parameters gives the best results.
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