Introduction - Sediment Transport in Irrigation Canals: A New Approach - page 2

a limited extent. The predictability of sediment transport equations and
models in view of the quantity of sediment that needs to be removed is still
rather poor. Computations of the effects of the non-equilibrium flow con-
ditions on sediment transport are required to determine whether deposition
and/or entrainment will occur and to assess the amount and distribution of
the sediment deposition and/or entrainment along the canals. Mathemati-
cal modelling of sediment transport offers the possibility of estimating the
distribution of sediment deposition or entrainment rates for a particular
flow and a specific situation
The main criterion for a canal design is the need to convey different
amounts of water at a fixed level during the irrigation season in such a
way that the irrigation requirements are met. Furthermore, the design must
be compatible with the sediment load of a particular location in order to
avoid silting and/or scouring of the canal. The water supply should meet
the irrigation requirements and at the same time the supply should result in
the least possible deposition in and/or scouring of the canals. The design
process becomes more complicated when canals are unlined and pass
through alluvial soils.
The problems of design and maintenance of stable channels in alluvial
soils are fundamental to all irrigation schemes (Raudkivi, 1990). Stable
or regime canals present ideal conditions for non-scouring and non-silting
throughout the irrigation network after one or several seasons of intensive
operation. The search for the main characteristics of stable channels started
with the work of Kennedy between 1890 and 1894 (Kennedy, 1895).
Subsequently, different theories have been developed and are being used
around the world. All of them assume uniform and steady flow conditions
and try to find those canal dimensions that are stable for a given discharge
and sediment load.
In the past the irrigation canals used to be designed for protective irri-
gation, where the government or an irrigation authority ran and maintained
the system. The available water was spread over as large an area as possible.
The canals carried an almost constant discharge without significant con-
trol structures and to some extent the assumption of a uniform and steady
flow was realised. In these situations a prediction of the behaviour of the
sediment in the canal could be made with some reliability. The growing
need for reducing the continually increasing governmental investments in
irrigation demands a more economical system, so that the farmers can
pay for the operational and maintenance costs with the return that they
get from irrigated agriculture. In addition, farmers would like to have a
more reliable and flexible water delivery. Flexibility in the delivery sys-
tem demands more frequent regulation of the water flow, which will create
unsteadiness in the flow. This unsteadiness will make all the assumptions
made in the original design imprecise and reduces the accuracy of the
already poor sediment transport predictors.