Environmental Engineering Reference
In-Depth Information
will be based upon the material transported by the river and not on the
local soils that form the bed. The local soils might be different from the
sediment transported by the river and only after a few years of operation,
the bed material will be adjusted to the sediment entering the canal. The
canal will then show a quasi-equilibrium condition. The maturing of the
canal includes the following negative aspects:
•
Development of aquatic weeds on the side-slopes;
•
Adjustments of the canal to the newly developed longitudinal slope;
•
Changes and improvements of off-takes, irrigation turnouts and bifur-
cation structures to adjust their levels to the changes in the canal after
maturing.
Stable alluvial canals are characterized by regular side slopes, which
are developed by the deposition of fine silt and clay particles. The side
slopes are generally much less permeable than the bed and may reduce
the seepage losses through the sides. They also have a higher erosion
resistance and limit any tendency to widen the canal.
D.3 SLOPE ADJUSTMENTS
The bottom slope of a canal after maturing will differ from the design and
the difference can be significant for long canals. Slope adjustments are
feasible by the construction of drop structures at regular intervals. During
the maturing process the crest levels of the drop structures have to be
raised or lowered to accommodate the slopes. The drops often form a part
of bifurcation structures
D.4 DIVERSION OF THE SEDIMENT
The distribution or division of the sediment over the canal branches is
an important aspect in the design and operation of alluvial canals. The
distribution of the flow velocity and of the sediment concentration over
the flow depth is different and therefore, it is realistic to assume that
the division of water and sediment at bifurcations is not proportional.
Some canals will receive a larger or smaller concentration of sediment
than the concentration conveyed by the parent canal. Remodelling of the
bifurcation structures might be necessary to adjust the distribution of the
sediment to the branches.
The aim of any irrigation network is to convey the water as well as the
sediment through the farm turnouts to the fields. The sediment transport
depends on the design and location of the turnout structures in relation
to the canal bed. The discharge capacity of a turnout depends on the
command area downstream of that turnout. When the command area
increases with the development of new irrigation canals, the discharge has
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