The Sediment Transport Model and its Applications - Sediment Transport in Irrigation Canals: A New Approach - page 199

Two aspects of the effect of the bed and side slope roughness on the
sediment deposition as a result of maintenance activities will have to be
considered:
a. When the canal is ideally maintained the roughness remains equivalent
to the design condition; the roughness is the same as at the design state
with the water level at the required set point for the design flow. For
flows smaller than the design discharge a backwater condition will be
created to maintain the set point.
b. The case that the canal is poorly maintained and the roughness will
increase with time. After some time the water depth at the design
discharge will increase above the design water level, which means that
either less irrigation water flows through the canal for the same set
point or that the water flow can only be conveyed for water levels
higher than the set point.

In this example (Case 5) the set point will be adapted to the changing
roughness to maintain the normal depth for poor maintenance condi-
tions. This change in set point level is one of the solutions to cope with
the changes in roughness. Depending on the actual management policy
of an irrigation system other scenarios can also be analysed to evalu-
ate the effect of the changing roughness on the water management and
sediment transport. The model can be a unique tool to analyse the var-
ious scenarios to cope with specific maintenance activities in the canal
network.
The roughness depends on the construction of the canal (i.e. lining,
masonry, earthen canal) and on the maintenance during or after the irriga-
tion season. This section will describe a simple example for the impact of
the roughness on the sedimentation. In this case the canal from Case 1 will
be simulated for an ideally maintained canal as well as for a poorly main-
tained canal. The cross section, longitudinal profile and the length are the
same as in Case 1. For both cases the flow will be uniform and is con-
trolled by a gate at the downstream end of the canal. Upstream of this gate
the water depth is 2.45 m for the poorly maintained ( C at approximately
60) and 2.35 m for the ideally maintained canal ( C at approximately 64).
The canal receives a sediment load of 150 ppm at the headworks and
is not able to transport the sediment to the fields during the irrigation
season. The equilibrium concentration is 59 ppm for the canal with ideal
maintenance and 54 ppm for the canal with no maintenance,

both

according to the Ackers-White sediment predictor.
Figure 7.18 shows the changes in concentration for the canal with
ideal and with no maintenance. The equilibrium concentration in the canal
with ideal maintenance is higher than for the canal with no maintenance;
the progress of the changes in the canal with ideal maintenance is faster
than in the canal with no maintenance. After 90 days the ideally maintained
canal has accumulated 11 501 m 3 and the canal with no maintenance