Environmental Engineering Reference
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the size of the sediment is uniform which can be represented by a single
fall velocity;
w s /u should be much smaller than unity (approximately 0.3 to 0.4).
Figure 5.32 shows the corresponding range of sediment size ( d 50 ) for
flow conditions normally found in irrigation canals. Considering the
maximum permissible tractive force on the canal bed under normal
condition ( < 5 N/m 2 ), the maximum sediment size for which this model
is applicable is 0.18 mm (Ghimire, 2003);
the maximum fall velocity w s should be always smaller than 0.028 m/s.
0.50
0.45
0.40
0.35
0.30
0.25
0.20
0.21 mm
0.15
0.18 mm
0.10
0.14 mm
0.05
0
0.05
0.10
0.15
0.20
0.25
d 50 (mm)
Fr
S
h
0.20
0.300
1.000
Figure 5.32. Validity range of
Galappatti's model under
different flow conditions
(Paudel, 2010).
0.26
0.500
0.850
0.33
0.800
0.730
0.41
1.200
0.640
0.48
1.600
0.580
5.4 MORPHOLOGICAL CHANGES OF THE BOTTOM LEVEL
5.4.1 The modified Lax method
The one-dimensional computation assumes fixed sidewalls and the occur-
rence of deposited or entrained sediments on the bottom of the irri-
gation canals. The interrelation between the water movement and the
morphological changes on the bottom can be given by the following
equations:
d h
d x =
S o
S f
(5.136)
Fr 2
1
 
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