Environmental Engineering Reference
In-Depth Information
CHAPTER 1
Introduction
The transport of sediment in irrigation canals influences to a great extent
the sustainability of an irrigation and drainage network. Unintentional
or unwanted erosion and/or deposition of sediment in canals will not
only increase the maintenance costs, but also lead to an unfair and inade-
quate distribution of irrigation water to the end users. Proper knowledge
of the behaviour and transport of sediment in these canals will help to
plan efficient and reliable water delivery schedules to supply water at the
required levels, to have a controlled deposition of sediments, to estimate
and arrange the required maintenance activities, and to determine the type
of desilting facilities and their efficiency, etc.
The study of sediment transport is mainly focused on the sediment and
erosion processes in irrigation canal networks. In view of maintenance
activities the head works should be designed in such a way that they
prevent or limit the entrance of sediment into canals. In addition, the
design of the canal network should be based upon the transport of all the
sediment to the fields or to specific places in the canal system, where
the deposited sediment can be removed at minimum cost. Sedimentation
should be prevented in canals and near structures, as it will hamper and
endanger a correct irrigation management, the main objectives of which
are to deliver irrigation water in an adequate, reliable, fair and efficient
way to all the farmers at the required water level, at the right time and at
the proper rate. Inadequate management will result in low efficiency and
unnecessary loss of the already scarce resource.
Irrigation canals are usually designed upon the assumption that the
water flow is uniform and steady and that the canals are able to carry the
water and sediments to the fields. The design supposes that an equilibrium
situation exists where the sediments and water entering into the irrigation
network will be transported to the fields without deposition or erosion.
However, a perfectly uniform and steady flow is seldom found. In the
operation of irrigation systems the flow is predominantly non-uniform,
with varying discharges and very often with a constant water level at the
regulation points where the water is supplied to the offtakes. The sediment
transport capacity of the canals greatly depends on the flow conditions,
which are variable. Although the water flow can be modelled with a high
degree of accuracy, sediment transport behaviour is only understood to
