Environmental Engineering Reference
In-Depth Information
velocities. Moreover, the conveyance of the water and sediments should
take place without erosion or sedimentation in the network. Based on
the fact that the earthworks for an irrigation network form a considerable
part of the total project costs, an optimal canal design should result in
minimal earthwork, an acceptable balance between cut and fill and a min-
imum of borrow pits and spoil banks. If a substantial imbalance exists,
the canal design has to be changed and the bottom slope, water levels and
canal dimensions have to be adjusted within acceptable ranges to achieve
a well-balanced earthwork.
A correct canal design should be based on the design flows of each
canal reach and the final design includes:
•
The layout of the canal,
consisting of the horizontal and vertical
alignment.
•
The cross sections, including the bottom width, side slopes, water
depth, freeboard and width of the embankments and the right of way.
This appendix will discuss a canal design that is based on the uniform
flow equation, boundary shear stress and sediment transport relationship.
The sediment transport relationship will use the principle of minimum
stream power, which states that the sediment transport capacity of a
canal network should be constant or non-decreasing in the downstream
direction.
This approach is simple and straightforward, but there are a few
limitations, namely:
•
The approach does not take into account the effect of sediment size and
concentration; the stream power is related to velocity and bottom slope,
but basically the slope depends on the sediment size; in this method
the bottom slope is adjusted to get a low boundary shear stress and
the stream power can be increased without considering the sediment
characteristics;
•
It doe not include the effects of bed forms on the roughness; it does not
account for the effect of the side slope on the effective roughness.
C.2 ALIGNMENT OF AN IRRIGATION CANAL
The layout of a canal mainly depends on the topography but is also influ-
enced by specific geological, agricultural, engineering, and economic
considerations. The alignment is largely effected by the existing topo-
graphy and dominant soil conditions; areas with very steep cross-slopes
or permeable soils or areas subject to landslides or that have a natural
tendency for sliding should be avoided. The alignment is more economic
when the water levels are kept as low as possible in irrigation canals,
preferably below the ground surface. The actual water levels should never
be allowed to exceed the design levels and therefore surplus water must
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