Environmental Engineering Reference
In-Depth Information
the water levels in a canal drop accordingly with a reduction in flow, but in
downstream control systems the water levels must increase with a decrease
in flow. The upstream controlled systems are usually manually operated,
while the downstream controlled systems are mainly automatically con-
trolled. In upstream controlled systems the set point (target water level)
is set at the upstream side of the flow and water level control structures,
while in downstream controlled systems these are fixed at the downstream
side of the regulating structures. Therefore the flow control algorithms
developed for upstream control cannot be directly applied to downstream
control.
Besides, downstream controlled systems are generally applied to
demand-based irrigation. The inflowing discharge at the canal headworks
remains variable in response to the operation of the secondary or tertiary
irrigation off takes. In steady state models the reduction of the flow at the
head causes a drop in the downstream water levels in the irrigation canals
and vice versa, which is in principle contradictory to the flow control
methods in downstream controlled systems. In downstream controlled
systems any decrease in canal flow causes an increase in the canal water
levels and vice versa. This increase provides water storage in the canal,
which is used to diminish the effect of canal filling or response time when
an offtake is opened again after closure. This water storage provides imme-
diate supply to the secondary or tertiary system without decreasing the
flows to other offtaking canals, which may otherwise be faced in the case
of manually upstream controlled systems.
In SETRIC this problem has been solved and there is no need to give
the incoming flow as a function of time at the canal headworks; only the
design discharge is needed at the canal headworks. Then according to the
operation of the secondary or tertiary offtakes the flow is automatically
adjusted. The water levels in the canal will automatically respond to flow
variations in the system and increase or decrease accordingly. As the inflow
is generated automatically, it eliminates the need for manually operated
flow control at the headworks for a steady state. Incorporation of these
options in SETRIC has resulted in a simple and easy tool to apply for flow
and sediment transport modelling in upstream and downstream controlled
irrigation canals.
The main objective of this section is to describe the functioning of auto-
matically downstream controlled irrigation systems and to focus on the
associated complications in hydraulic and sediment transport modelling
by a simple steady state model.
6.2.3
Aspects of the downstream control module
The mathematical model SETRIC simulates the water flow, sediment
transport and changes of the bottom level in downstream-controlled
irrigation systems in the same way as in open upstream-controlled
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