Environmental Engineering Reference
In-Depth Information
•
the sediment size is kept constant for cases 1 to 4 and the incoming
sediment concentration (in ppm) is kept constant for cases 3 and 4
during the whole simulation period;
•
only sediment previously deposited on the canal bottom can be entrained
during a later period of the simulation;
•
the side slopes of the canal are stable;
•
the initial roughness will be characterized by one single roughness
along the whole wetted perimeter of the canal (Nikuradse, de Chézy or
Manning);
•
variations of the roughness might occur over space and time due to the
occurrence of bed forms and the obstruction by weed growth;
•
the sediment transport capacity at a certain place and time is given by
the equilibrium concentration expressed in ppm by weight (parts per
million);
•
the time step and length step are based on a Courant number smaller
than 1; for normal conditions a time step of 1-3 hours has been used
and the length step is smaller than
L
99%
=
4
.
61
L
A
(see Section 5.3.5).
The geometrical, hydraulic and sediment characteristics of the irriga-
tion canal are:
•
length (
L
)
=
10 000 m
•
bottom width (
B
)
=
10 m
•
side slope (
m
)
=
2
•
bottom slope (
S
o
)
=
0.00008 (0.08 m/km)
•
equivalent roughness (
k
s
)
=
0.018 m (Nikuradse)
•
bottom level (begin)
=+
10.00 m
•
bottom level (end)
=+
9.20 m
25.75 m
3
/s
•
design discharge (
Q
)
=
•
normal water depth
=
2.38 m
•
water level at the downstream end
=+
11.58 m
•
sediment size (
d
50
)
=
0.15 mm
•
=
incoming sediment concentration
100 ppm (only for Case 1)
•
=
simulation period
90 days
•
=
flow control structure
undershot type
•
flow condition
=
uniform flow for the design
discharge
7.3 CASE 1 CHANGES IN THE DISCHARGE
AT THE HEADWORKS
A main objective of the design and operation of irrigation canals is to find
the correct flow conditions for which the design water requirements are
met for minimum or no sediment deposition. The most important reason
for sediment deposition in the irrigation canals is a reduction in the water
supply, caused either by a drop in crop water requirement or in water
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