Agriculture Reference
In-Depth Information
For design purposes, always the most economical section is designed.
We know, for most economical trapezoidal section,
Half of the top width
=
Slo
ping side
d
√
z
2
b
+
2
zd
2
That is,
=
+
1
2
d
(1.3)
2
Or,
b
+
2.6
d
=
+
1(
∵
z
=
1.3)
Or,
b
=
0.68d
(A)
We know, for most economical trapezoidal section another condition is
=
hydraulic radius
half of depth of flow
d
2
=
(B)
i.e.,
R
Now, area,
A
=
d
(
b
+
zd
)
=
d
(0.68
d
+
1.3
d
)(putting b
=
0.68
d
)
1.98
d
2
The discharge,
Q
or,
A
=
N
R
2
/
3
S
1
/
2
with usual notations.
1
=
AV
=
A
×
2
1
1,200
3
2
1.98
d
2
1
Putting the values, 2
=
×
0.02
×
×
or,
d
8
/
3
=
1.11
or,
d
=
1.04 m
Then,
b
0.7072 m
Assuming freeboard as 20% of flow depth, depth of channel would be
=
0.68
d
=
d
c
=1.04
×
1.20 = 1.248 m
≈
1.25 m
Top width
=
b
+
2
zd
c
+
(2
×
1.3
×
1.25)
=
3.957 m
Results
3.957 m
Freeboard, 0.24
m
1
1.25
m
1.04
m
1.3
0.707
m
Channel bed width,
d
=
1.04 m
Flow depth,
d
=
1.04 m
Depth of channel
=
1.25 m
Top width
=
3.957
Side slope
=
1.3:1(
H
:
V
)
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