The Sediment Transport Model and its Applications - Sediment Transport in Irrigation Canals: A New Approach - page 183

The canal design has been based on the criteria of the rational method
(see Chapter 4) and for uniform flow. From an irrigation and sediment
transport point of view the design is not the most optimal solution, but
otherwise these imperfections form an excellent basis to show the vari-
ous aspects of SETRIC and how the programme can help designers and
managers to improve their design or their operational activities. A bet-
ter design should result in a canal that has a steeper bottom slope, a
greater bottom width and a smaller water depth. The final design values
will greatly depend on the sediment predictor that is appropriate for the
specific design circumstances.
The design discharge (relative discharge Rd 1) transports the sed-

iment in equilibrium conditions according to Brownlie's predictor; the
equilibrium transport capacity (108 ppm) is slightly larger than the incom-
ing sediment load (100 ppm) and remains constant during the whole
simulation period. The normal water depth is 2.38 m, the roughness C
is 62 m 1 / 2 /s and the maintenance is ideal. The SETRIC results show that
there is no deposition or erosion in the whole canal. Figure 7.2 shows
the equilibrium and the actual concentration for the relative discharges
Rd

=

= 1 at the end of the simulation period of 90 days.

250

225

200

175

150

125

100

75

50

25

0

Figure 7.2. Equilibrium and
actual concentration according
to Brownlie's predictor at the
end of the simulation period for
Case 1 and Q = 25.75 m 3 /s.

0 1000 2000 3000 4000 5000 6000 7000 8000 9000 10000

Distance (m)

Equibrium concentration
Actual concentration

Once the discharge decreases a gradually varied flow will develop
due to the constant water level (set point) at the downstream canal end.
The sediment transport capacity (equilibrium concentration) will also
show a gradual decrease in the downstream direction due to the backwater
caused by the downstream structure, where the actual water depth is larger
than the normal water depth for flows smaller than the design discharge
(Rd < 1) (see Table 7.1). For these discharges the equilibrium concentra-
tion is smaller than the incoming sediment load and this will result in
a deposition that propagates from the upstream end in the downstream
direction.