Introduction - Sediment Transport in Irrigation Canals: A New Approach - page 4

problems that are rarely encountered in rivers. The need to control water
levels and discharges in the upstream and/or downstream direction, the
necessity to find an optimal cross section and the considerable influence
of the side banks on the velocity distribution normal to the flow direc-
tion create some of the main differences in both kinds of channels. Other
differences are the presence of a large number of flow control structures,
the occurrence of submerged gate flow and overflow structures, and the
distinct flow characteristics of inverted siphons and their multiple flow
paths. Table 1.1 shows some of the main differences between rivers and
irrigation canals.
Irrigation canals are different from natural rivers in terms of hydraulics
and sediment characteristics. The computational environment for the mod-
elling of water flows in irrigation canals with sediment transport is much
more demanding than for river flows due to the extreme variability and
unsteadiness of the flow, the presence of numerous hydraulic structures,
dynamic gate movements and pump operations, and the existing topo-
graphical complexity. The methods for designing stable canals are only
useful in very specific flow conditions. For large changes in discharge

Table 1.1. Characteristics of water flow and sediment transport in rivers and irrigation canals.

Water flow and sediment transport

Characteristics Rivers Irrigation canals

- Main function Conveyance of water and

sediment

Diversion, conveyance and distribution
of water for agriculture

- Discharge Not controlled; increasing in

downstream direction

Controlled by operation rules;
decreasing in downstream direction

- Alignment Rarely straight, bends, sinusoidal

meanders and braids

Straight, wide bends

- Topology Convergent Divergent

- Flow control (Almost) no control structures Several flow control structures for

water level and discharge

- Water profiles Generally without water level

control: nearly steady flow

Water level control: gradually varied
flow and unsteady flow

- Velocity distribution Nearly uniform velocity

distribution in lateral direction

Distribution greatly affected by side
walls and side slope

- Froude number Wide range Restricted by the operation of flow

control structures (Fr < 0.4)

- Width ( B )/depth ( h ) B / h > 15 (wide canals) B / h < 7-8

- Lining Alluvial river bed Man-made canals: lined or unlined

- Sediment concentration Wide range Controlled at head works

- Sediment size Wide range of sediment size Fine sediment

- Size distribution Graded sediment Nearly uniform distribution

- Sediment material River bed External sources

- Sediment transportation Suspended and bed load Mainly suspended load

- Bed forms Mostly dunes Mostly ripples and mega-ripples

- Roughness Skin and form friction Form friction