Environmental Engineering Reference
In-Depth Information
variation in size and type of the cropping pattern occur frequently dur-
ing the life of an irrigation canal. Hence the canal must be designed
and constructed with a certain degree of flexibility to deliver different
amounts of water. Irrigation canals are designed for a specific design flow,
but most of the time they will convey a variety of discharges and therefore
it is often necessary to have a certain control to maintain the desired flow
rates and required water levels.
From a computational point of view, the significance of the unsteadi-
ness of the flow in irrigation canals must be considered from two
aspects:
•
Firstly, the computation of the flow at any point of the system requires
a good knowledge of the response time in order to deliver the right
amount of water to the right place and at the right time. The water
delivery requires proper planning and operation and the flow has to be
controlled. The delivery methods will result in unsteady flow conditions
due to the initiation and termination of the water supply, changes in flow
rate, stoppage of lateral flows, changes in gate settings, etc. Unsteady
flow conditions exist most of the time and will seriously affect the water
distribution. The response time of an irrigation system is a function of
the distance between the origin of the disturbance and the point of inter-
est, the celerity of the wave propagation and the operation time of the
structures (see Figure 5.1). It is essential that the response time is known
in view of the various changes in flow conditions (Schuurmans, 1991).
1
2
Q
n
Q
0
time
Discharge at 1
Response
ti
me Tr
Response
ti
me Tr
1
Storage prism
Q
n
2
Tw
Q
0
time
Canal-reach
Discharge at 2
Figure 5.1. Hydro-dynamic
performance of an irrigation
canal with the intended and
actual flow at cross section 1
and 2.
Hydro-dynamic performance of a canal reach
•
Secondly, the flow computations for the determination of the temporal
morphological changes in the canal can be based upon the assumption
that the flow can be easily schematised as quasi-steady. Observations
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