Environmental Engineering Reference
In-Depth Information
APPENDIX B
Methods to Predict the Friction Factor
The selection of methods to predict the friction factor in irrigation canals
include:
•
van Rijn (1984c);
•
Brownlie (1983);
•
White, Bettess & Paris (1979);
•
Engelund (1966).
The methods to estimate the friction factor in irrigation canals will
make use of the de Chézy coefficient. The Darcy-Weisbach friction factor
f
as well as the coefficients of Manning (
n
) and Strickler (
k
s
) can be
applied when the following relationships are used:
8
g
f
R
1
/
6
n
C
=
or
C
=
(B.1)
where:
C
de Chézy coefficient (m
1
/
2
/s)
=
=
f
Darcy-Weisbach's friction factor
=
Manning's coefficient (m
1
/
3
/s)
n
k
s
=
Strickler coefficient (s/m
1
/
3
)
R
=
hydraulic radius (m)
g
=
gravity acceleration (m/s
2
)
B.1 VAN RIJN
The de Chézy coefficient depends on the type of flow regime. Based on
the bed-roughness condition the flow regime in open canals can be divided
in: smooth, rough and a transition regime. Roughness conditions on the
bottom are simulated by using an equivalent height of the sand roughness
k
s
, which is equal to the roughness of sand that gives a resistance similar to
the resistance of the bed form. The dimensionless value of
u
∗
k
s
/
v
is used
as the classification parameter to distinguish the type of flow regime. Van
Rijn (1993) described the type of flow regimes as presented in Table B.1.
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