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By
Assoc Prof Dr. Ir. Lariyah Mohd. Sidek Water Engineering Unit
Hydraulic Engineering CEWB222
HYDRAULICS -
CHAPTER 2 (OPEN CHANNEL
PROPERTIES)
Natural channel
• All watercourses that occurs naturally, for example streams
and rivers
• The Hydraulic properties are generally irregular
• Not regular nor prismatic and their materials of construction
can vary widely.
• The surface roughness will often change with time, distance
and elevation.
Types of open channel
Man-made channel Constructed in a regular cross-section shape throuhout and thus
are PRISMATIC CHANNELS.
Made from concrete, steel or earth with well defined surface roughness.
All watercourses that are constructed by human effort, for example :
Canal : a long and mild-sloped channel built in the ground that may be unlined or lined with stone masonry, concrete ,etc.
Flume : a channel of concrete, masonry, etc that is usually supported on or above the surface of the ground to convey water across a depression.
Chute : a channel with steep slopes.
Culvert : a covered channel of comparatively short length installed to drain water through highway and railroad embankments.
Types of open channel
Canal
Trunk Drain - Wet Period
Rigid Boundary Canal
FLUME
Existing Culvert
Existing Pipe Culvert
Existing Box Culvert
• Prismatic channel : a channel with
unvarying cross section and
constant bottom slope.
• Non-prismatic channel : a channel
with varying cross section and
constant bottom slope.
Channel Geometry
CHANNEL GEOMETRY
GEOMETRIC PROPERTIES • Depth (y) – the vertical distance from the lowest point of the
channel section to the free surface
• Stage (z) – the vertical distance from the free surface to an
arbitrary datum
• Area (A) – the cross-sectional area of flow, normal to the direction
of flow
• Wetted perimeter (P) – the length of the wetted surface measured
to the direction of flow
• Top Surface width (T) – width of the channel section at the free
surface
• Hydraulic Radius (R)- The ratio of area to wetted perimeter (A/P)
• Hydraulic Mean Depth (Dm) – The ratio of area to top surface
width (A/T)
12
CHANNEL GEOMETRY
13
Channel type Area
A
Wetted
Perimeter, P
Hydraulic radius
R
Top width
T
Hydraulic
depth, D
Section factor
Z
by
b+2y
y2b
by
b
y
5.1by
(b+zy)y
212 zyb
212
)(
zyb
yzyb
zyb 2
zyb
yzyb
2
)(
zyb
yzyb
2
5.1
2zy
212 zy
212 z
zy
zy2
y21
5.2
2
2zy
Ty3
2
T
yT
*2
3
8
22
2
83
2
yT
yT
Y
A
2
3
y32
5.169
2 Ty
2
0sin8
1 d
021 d
0
sin1
4
1d
ydy 02
21sin
sin
8
1
5.2
0
21
5.1
sin32
sin2d
y
b
RECTANGULAR CHANNEL
T Area (A) = by
Wetted perimeter (P) = b+2y
Hydraulic Radius (R) = A/P= y2b
by
Top width (T) = b
Hydraulic Depth (D) = y
b
y
TRAPEZOIDAL CHANNEL
Area = (b+zy)y
P = 212 zyb
212
)(
zyb
yzyb
R = A/P =
T = zyb 2
D = A/T = zyb
yzyb
2
)(
y
b
z
1
T
A concrete rectangular channel has a bottom
width of 4 meters. If the channel is on a 1%
slope and is flowing 1 meter deep throughout its
length, what is the cross sectional area, wetted
perimeter and hydraulic radius?
Solution:
If the shape of the channel is rectangular, the
discharge computed as follows:
A = by = (4) (1) = 4m2
P = b+2y = 4 + 2 (1) = 6m
R = 4m2 / 6m = 0.67m
Example 1
T
4
1
A concrete trapezoidal channel has a bottom width of 4
meters and 45 side slopes. If the channel is on a 1%
slope and is flowing 1 meter deep throughout its
length, what is the cross sectional area, wetted
perimeter and hydraulic radius?
Solution:
If the shape of the channel is trapezoidal, the discharge
computed as follows:
A = (b+zy)y = (4+(1)(1))1 = 5.00m2
P = (b+ 2y (1+ z2)0.5 )= 4 + 2 (1) (2)0.5 = 6.83m
R = A/P = 5.00m2/6.83m = 0.73m
Example 2
T
4
1 1
1
Water flows at a depth of 1.83 m in a trapezoidal,
concrete-lined section with a bottom width of 3 m and
side slopes of 2:1 (H:V). The slope of the channel is
0.0005 and the temperature is 20o C. Assuming uniform
flow conditions, estimate the cross sectional area (A),
wetted perimeter (P) and hydraulic radius (R)?
Solution:
Flow area, A = (b+zy)y = (3 + 2 (1.83)) x 1.83
= 12.2 m2
Wetted perimeter, P = 11.2 m
Hydraulic Radius, R = A/P = 12.2/ 11.2 = 1.09 m
Example 3
Example 4
Example 4 (cont’)
TEXTBOOKS REFERENCES
• Lariyah, M.S. & Norazli, O., Hydraulic Notes,
UNITEN, 2006
• Roberson, J. A., Cassidy, J. J. & Chaudry, H.,
Hydraulic Engineering, 1st S.I. Ed., John
Wiley, 1998
• Chin, D., Water Resources Engineering,
Prentice Hall, 2006