The Sediment Transport Model and its Applications - Sediment Transport in Irrigation Canals: A New Approach - page 195

control measures is smaller than for the canal with one of the two con-
trol measures (scenarios 1 and 2). The amount of sediment deposition per
day for the deepening scenario after 60 days is almost in line with the
amount for the canal without measures, while the sediment deposition for
the widening scenario ( B 14 m) at the end of the simulation period is

still two times larger than for the deepening scenario.

=

7.6 CASE 4 FLOW CONTROL STRUCTURES

One of the main design criteria for irrigation canals is the command level,
which states that the canals should be able to deliver irrigation water at the
right elevation, at the correct time and at the right amount to the command
area. In view of this criterion, flow control structures are needed to keep
the water level at the command or target level for any discharge during
the irrigation season. Two cases with a different type of control structure
will be considered, namely:
- a structure with undershot flow (Case 4.1);
- an overflow structure (Case 4.2).

When an overflow structure has been correctly designed the flow veloc-
ity will not decrease for the design flow and no deposition will occur in
front of the weir. If the overflow structure has not been correctly designed
deposition will follow due to the backwater effect (reduction in veloc-
ity) that is created by the hydraulically faulty weir design. This aspect
will be discussed in the last example (Case 7) that will show the differ-
ence between a hydraulically well designed and a hydraulically poorly
designed flow control structure for fluctuating discharges. The example is
useful to analyse the correct design of a flow control structure for a given
water flow and sediment characteristics.
Case 4 will evaluate the ability of overflow and undershot structures to
convey sediments; operational as well as other aspects that might influence
the selection of a structure will not be discussed. Bed load and suspended
load are freely conveyed by all structures with undershot flow; overflow
structures will also convey the suspended load, but any bed load will be
trapped. For the comparison both control structures are placed 2 500 m
downstream from the headworks and they will maintain a uniform flow
for the design discharge (25.75 m 3 /s). Downstream of both structures is
a bottom drop of 0.40 m to guarantee modular flow over the structures,
which means that the downstream water level will not influence the
discharge and hence the water level upstream of the structure.
The behaviour of the two structures in respect of the sedimentation
will be compared on the basis of the changes in sediment concentration
and in bed level. The major changes in concentration for both structures
occur directly downstream of the headworks. The incoming concentration