Environmental Engineering Reference
In-Depth Information
The rational method includes, amongst others, the method of White,
Bettes and Paris (1982) and Chang (1985). The rational method is useful
for the design of stable canals with very specific flow conditions. For
canals with large variations in discharge and sediment load the method is
inadequate to describe accurately the sediment transport process and the
conveyance of the sediment load through the whole canal network.
Transport of suspended load
To describe the conveyance of suspended sediment through the whole
network it is practical to assume that the fine particles in suspension have
an almost constant distribution over the vertical. Normally the sediment
particles in suspension fall in the range 'very fine' and their size is less
than 50 to 70 * 10
−
3
mm. De Vos (1925) stated that the relative transport
capacity (
T/Q
) is proportional to the average energy dissipation per unit
of water volume.
T
Q
∝
ρ
w
gv
av
S
o
(4.14)
where:
T/Q
=
relative transport capacity
=
sediment transport load (m
3
/s)
T
Q
=
discharge (m
3
/s)
ρ
w
=
density of water (kg/m
3
)
g
=
acceleration due to gravity (m/s
2
)
v
av
=
average velocity (m/s)
S
o
=
bottom slope (m/m)
From energy considerations it follows that the sediment particles will
be transported in any concentration by the flowing water when the fall
velocity w is smaller than a certain threshold.
ρ
w
ρ
s
−
ρ
w
v
av
S
o
w
≤
(4.15)
where:
w
=
fall velocity (m/s)
ρ
s
=
density of the sediment particles (kg/m
3
)
ρ
w
=
density of water (kg/m
3
)
S
o
=
bottom slope (m/m)
To convey sediment in suspension the hydraulic characteristics of a
canal network should be such that
ρ
w
gv
av
S
o
or
v
av
S
o
either remains con-
stant or does not decrease in a downstream direction. From
v
av
=
C
(
yS
o
)
0
.
5
in which
C
is a general smoothness factor, it follows that
yS
o
or
y
1
/
3
S
o
should be constant or non-decreasing in order to convey the suspended
load in wide canals.
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