Environmental Engineering Reference
In-Depth Information
In order to have no erosion in the canal network the shear stress should
be constant or non-decreasing, meaning that
yS
o
should be constant or
non-decreasing.
It might be expected that when a flow only transports a small amount of
bed load, the criterion for a continuous conveyance of bed load should be
somewhere in between the two values. The criterion to convey any non-
suspended (bed) material depends strongly on the water and sediment
equations. Therefore, the criterion is that the relative transport capacity
for bed load (T/Q) should be non-decreasing, or in the case of potential
erosion, should remain constant; the numeric approximation for this bed
load transport is that
y
1
/
2
S
o
is constant or non-decreasing.
Summarizing the rational method criteria
The design criterion for the conveyance of sediment through a canal
system can be based on energy dissipation considerations; the relative
sediment transport capacity follows from these considerations and is given
by de Vos (1926) and Vlugter (1962).
Based on their works, Dahmen (1994) states that:
•
for the conveyance of sediment in suspension, the hydraulic character-
istics of the canal system should be such that:
ρgvS
o
=
constant or non-decreasing in downstream direction
(4.19)
•
for
the
conveyance
of
non-suspended
sediments,
the
hydraulic
characteristics of the canal system should be such that:
y
1
/
2
S
o
=
constant or non-decreasing in downstream direction
(4.20)
4.3.5
Final comments
The preceding sections clearly show that the design of stable irrigation
canals should be based on irrigation considerations, including engineer-
ing, agricultural, management and economic aspects. An optimal canal
design is difficult to achieve and the final canal design will have to bal-
ance all these criteria to find the best solution for the explicit conditions
of a specific irrigation system.
The existing design methods are based on the interrelation of equations
for specific water flow and sediment transport conditions in an effort to
design stable canals. However, the input variables will vary widely during
the irrigation season and, moreover, during the lifetime of the irrigation
network. Most of the time, non-equilibrium conditions prevail in irrigation
networks and therefore, the initial assumptions for a stable canal design
are no longer valid. Also, lined canals experience sedimentation problems
due to variations in either flow conditions or incoming sediment load
that produce non-equilibrium conditions for the transport of the sediment.
Therefore, sediment problems in irrigation canals should be analysed in a
more integrated context, including all the alternative operation scenarios
for water flow and sediment transport in time and space.
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