Introduction - Sediment Transport in Irrigation Canals: A New Approach - page 3

Generally three methods for the design of irrigation canals are used:
namely Lacey's regime method and the tractive force method for large
irrigation schemes, and the permissible velocity method for irrigation
systems in hilly areas. Sediment characteristics and coefficients are either
simply borrowed from literature or chosen based on the experience of
the designer. Hence there exists a large difference in the design param-
eters from scheme to scheme and even from canal to canal within the
same scheme. Without incentives or obligations for any verification, most
designers do not investigate or evaluate the performance of their design
in the field.
At the moment, the improvement of the performance of existing
schemes is more pressing than the development of new irrigation sys-
tems, especially in view of the high investment costs required for the
construction and operation of new systems. Appropriate management of
sediment in canal networks is one of the major challenges of the improve-
ment works, as a major part of the available maintenance budgets is spent
annually on the removal of the sediment deposited. This type of scheme
imposes extra conditions on the designer as the canal slope, bed width,
structure control, and management practices already exist and in most
cases they cannot be changed due to economic resources and/or social
considerations. Hence, the selection of an appropriate design philosophy
and its applicability for these particular conditions is very important.
The design of irrigation canals is not as simple as normally perceived.
It is the final product of a merger of complex and undetermined parameters
such as water flows, sediment load, structure control and operation, and
management strategies. No design packages for irrigation canals are avail-
able that deal with all the parameters at the same time. To simplify the
design process some parameters are either disregarded or assumed to be
constant, which consequently will lead to a less adequate design. Many
failures and problems are caused by a design approach that pays insuf-
ficient attention to the operational aspects (FAO, 2003). Considering the
aforementioned parameters involved in the canal design and their impor-
tance in view of the sustainability of the system, numerical modelling may
be one option that can simultaneously simulate all the variables. However,
the selection of a model to represent the system and its validity in the
proposed environment will have a major influence on the results.
Developments in the knowledge of sediment transport in open canals
have mainly been derived from natural channels such as rivers. So far sedi-
ment transport theories, the development of bed forms, resistance factors,
etc. have been developed under assumptions applicable to the particular
conditions encountered in rivers. Even though certain similarities between
rivers and irrigation canals exist, the sediment concepts for rivers are not
entirely applicable to irrigation canals. Most of these irrigation canals are
man-made and the irrigation environment presents a number of typical