SETRIC, a Mathematical Model for Sediment Transport in Irrigation Canals - Sediment Transport in Irrigation Canals: A New Approach - page 160

and sediment size. Then the equivalent roughness is computed taking into
account the sidewall effect as proposed by Méndez (1998) and Paudel
(2010).
The sediment continuity equation is solved numerically by using the
Modified Lax scheme. A depth-integrated convection-diffusion model
(Galappatti, 1983) is applied to predict the actual sediment concentration
at any point under non-equilibrium conditions. For the prediction of the
total sediment transport under equilibrium condition an option is avail-
able to select one of the three sediment predictors, namely Ackers-White
(HR Wallingford, 1990), Brownlie (1981b) and Engelund-Hansen (1967).
The predicted sediment transport capacity is corrected for the B / y ratio
and side slope for non-wide irrigation canals.

6.1.2 Water flow equations

The schematization of the water flow in irrigation canals will have to
consider two types of aspect, namely the operational aspects and the sed-
iment transport aspect. The operational aspects include the situation that
the uniform water flow will become non-uniform and unsteady due to the
changing nature of the irrigation demand and the operation of the gates
to meet this demand and to maintain the water supply level for a proper
irrigation of the fields. The sediment transport aspect covers the changes
in the sediment morphology, which are much slower than the changes in
the water flow with time and space.
The water flow in irrigation canals during an irrigation season and
moreover during the lifetime of the canals is not constant. Seasonal
changes in crop water requirement, water supply and variation in size
and type of the planned cropping pattern are frequent events during the
lifetime of an irrigation canal. The operation of gates to adjust to the vari-
ation in supply is normally gradual to avoid the generation of waves. It is
common practice to allow sufficient time to move from one steady state
to another steady state by operating the gate in multiple stages. More-
over, the Froude number in irrigation canals is normally small ( < 0.4) to
maintain a stable water surface and small disturbances in bends and tran-
sitions (Ranga Raju, 1981). For small Froude numbers (Fr < 0.6-0.8),
the celerity of the water movement ( c w ) is much larger than the celerity of
the bed level change ( c s ) (de Vries, 1975): therefore the water flow and the
sediment transport calculations can be uncoupled. For the non-uniform
flow situations the water flow can be treated as a gradually varied flow.
Water flow in irrigation canals will be schematized as quasi-steady in
which the governing equations are represented as:

Continuity equation:

∂Q
∂x =

=

0 ∴ Q constant (6.1)