Description of the Main Aspects of the Regime Theory - Sediment Transport in Irrigation Canals: A New Approach - page 268

to be increased. Turnouts on irrigation canals are relatively inexpensive
(adjustable pipe turn outs) and when the canals mature and the system
attains equilibrium, the pipe turnouts can be replaced by (semi-permanent)
modules. The crest level of the turnouts will then be based on sediment
transport considerations as well as on the discharge variation in the canals.
In smaller canals, the turnouts are important to the geometry of the canals.
It has been found from intensive observations that the average bed level of
distributaries adjusts to the mean crest level of these turnouts. In unstable
distributaries, which may be silted up, the lowering of the turnouts is one
of the recommended solutions.
D.5 MAINTENANCE ASPECTS
All irrigation canals require maintenance; earthen canal banks are con-
stantly eroded by wind and rainfall and have to be maintained. Their
stability is based upon the stability of the canal bed and the cross sections.
The canal width is generally maintained by providing bank protection by
permeable spurs normal to the banks. Normally, bed levels are excavated
during closures if the canals silt up.
The crest of drop structures might be raised if the bed shows scouring.
Maintenance of the bed elevation and canal width is expensive. Experi-
ences show that on larger canals, bank protection is rarely practised and
bed clearance is never carried out. In the smaller distributaries both the
maintenance of the width and the bed elevation are often done. The berms
along these canals may become a problem as they may become beyond
their design width and may have to be trimmed. The canal bed level may
also have to be re-excavated to near design levels. In general, the fre-
quency of these operations, where needed, is about once in the five years.
The medium size canals are more stable. In these canals like in the large
canals no bank protections are practised, but the bed level may be cleared
once in the ten years.
The variation in discharge in the canals is an important factor in the
stability of alluvial canals. Low discharges flowing in canals with high
capacities often adopt a winding thalweg that may later erode the banks by
concentrating flows against the banks. For this reason the minimum dis-
charge in mature canals is limited to 55% of the design capacity. Similarly,
the raising and lowering of the canal discharge is also controlled to prevent
the failure of banks due to seepage forces.
D.6 FLOW CAPACITY
In recent years, the design capacities in almost all canals have been
increased to meet the growing demand for irrigation water. In some areas,