The Sediment Transport Model and its Applications - Sediment Transport in Irrigation Canals: A New Approach - page 182

availability or by a variation in cropping pattern and the need to deliver
water at the supply level (set point) to the command area.
For discharges smaller than the design discharge the flow in any canal
reach might be either uniform flow (normal state) or a backwater flow. In
this example (Case 1) the water level at the downstream end is kept constant
at 2.38 m for discharges smaller than the design value and the flow profile
will be a backwater curve. The sediment deposition that will occur in the
canal is due to the combined effect of a reduction in discharge and the
backwater effect. The reduction in discharge only will result in a smaller
velocity, water depth, hydraulic radius and an increase in roughness; the
backwater profile will result in a reduction in velocity and an increase in
water depth and hydraulic radius and in a decrease in roughness. At the
end of this example the two effects will be compared separately to show
the effect of a reduction in discharge and of a backwater profile.
First, the effect of a reduction in water supply on the sediment transport
and the deposition will be presented for a constant water level at the
downstream canal end (see Figure 7.1). The reduction of the discharge is
specified in terms of a relative discharge (Rd), which reads:

actual discharge
design discharge

25 . 75 m 3 / s) Rd = ( Q = (7.1)

14

13

12

11

10

9

0 5000

Distance (m)

10000

Bottom
Normal water level
Embankment
Gate

4

3

2

Figure 7.1. Longitudinal profile
and cross section of the
irrigation canal in Case 1.

1

0

0 10 20 30