Design Criteria for Irrigation Canals - Sediment Transport in Irrigation Canals: A New Approach - page 59

- on-off or rotational flow . Different combinations of rotation are pos-
sible. Depending upon the water requirement and available water the
secondary canals (laterals) can be grouped in two or more groups and
the canals in one group are opened and closed simultaneously;
- adjustable flow . The adjustment in flow rate may be necessary in view
of a change in demand or change in the available water at the source.
Flow control systems manage the water flow at bifurcations to meet
the irrigation service criteria regarding flexibility, reliability, equity and
adequacy of delivery (van Lier et al., 1999). Flows can be regulated
through water level control, discharge control, and/or volume control.
A combination of water level and discharge control is most common in
irrigation systems. Flow rates at offtakes are often indirectly controlled
through water level control in the conveyance canals. Variation in dis-
charge at the offtake depends on the variations in upstream water level
and the sensitivity of the offtakes and water level regulators to those
variations.
The classification of the methods of flow control and water division
(Ankum, 2004) can be made according to the orientation of the control
(upstream, downstream and volume control), the degree of automation
(manual, hydraulic or automatic control) and the form of control (local or
central control). A short description of upstream and downstream control
systems will be given as these systems will be used in the applications
with the mathematical model (Chapter 7).
4.2.1 Upstream control
Upstream control is most commonly used around the world. Controlled
flows are released to a downstream reach from the upstream end according
to a pre-arranged schedule. The water level in the reach is maintained by
a water level regulator at the downstream end. The offtakes are located
upstream of the water level regulator. Since the flow rates are controlled
from the upstream end, increments in the offtake flow cannot be diverted
unless extra flow is released at the upstream end. This mode of operation
forms a negative dynamic storage within the canal reach (Figure 4.2).
From the operational point of view, there is no storage available to meet
the immediate demand of the offtakes. Moreover, when there is a sudden
decrease in the demand and the offtake gates are closed, this stored water
has to be released either to the drains or to the downstream reaches that may
or may not use the available water. Upstream control is a serial control,
is supply oriented, has limited flexibility and requires a well-equipped
centralized management to operate the system properly.
Upstream control can be made fully automatic and is an option for
semi-demand allocation to the tertiary offtakes. Automation is only suit-
able when there is sufficient water at the source and when the power supply
for the operation of the control structure is reliable.