Fluid Mechanics
Chapter 8 – Open Channel Flow
8 OPEN CHANNEL FLOW
8.1 Classification & Definition
‹
Open channel flows are flows in rivers, streams, artificial channels,
irrigation ditches, partially filled pipe etc.
‹
Basically, it is a flow with free surface.
(Free surface is a surface with atmospheric pressure)
Open Channel Flow
Steady flow
Uniform
Unsteady flow
Varied
Gradually
varied
flow
Varied
Rapidly
varied
flow
Gradually
varied
flow
Uniform
Rapidly
varied
flow
Classifications of Open Channel Flow
8.1.1
1.
Open Channel Geometry
Depth of flow, y: vertical distance from the bottom to surface.
Free surface
B
flow
A
y
Bottom
(cross-section)
2
Top width, B:
– the width of the channel at the free surface
3
Flow area, A:
– cross-sectional area of the flow
4
Wetted perimeter, P:
– the length of the channel cross-section in contact with the fluid
P.8-1
Fluid Mechanics
5
Hydraulic radius (hydraulic mean depth), R:
R
6
Chapter 8 – Open Channel Flow
=
Flow area
A
=
Wetted perimeter P
Average depth (hydraulic average depth), yave:
yave
8.1.2
=
Flow area A
=
Top width B
Rectangular channel
– B
=b
y
– A
= b*y
– P
= b+2*y
– R
=
– yave
=y
8.1.3
b
b*y
b + 2* y
Trapezoidal channel
– B
= b + 2*m*y
y
– A
= y*(b+m*y)
– P
= b+2*y* 1 + m 2
– R
=
– yave
=
m
1
b
y * ( b + m * y)
b + 2 * y * 1 + m2
y * ( b + m * y)
b + 2*m* y
P.8-2
Fluid Mechanics
8.1.4
Triangular channel
– B
–
Chapter 8 – Open Channel Flow
= 2*m*y
2
A
1
= m*y
– P
= 2*y* 1+ m 2
– R
=
– yave
=
8.1.5
y
m
m*y
2 * 1 + m2
y
2
Circular channel
– B
= 2 * y * ( D − y)
– A
D 2 * (θ − sin θ )
=
8
– P
θ*D
=
2
– R
=
D ⎛ sin θ ⎞
⎜1 −
⎟
4⎝
θ ⎠
– yave
=
D * (θ − sin θ)
8 * sin θ
2
y
θ
D
(θ in radian)
P.8-3
Fluid Mechanics
Chapter 8 – Open Channel Flow
8.2 Steady Uniform Flow
‹
For a steady uniform flow
– depth is constant along the flow
– velocity is constant over the cross-section
– time independent
8.2.1
‹
Manning Equations
In 1890, Manning, an Irish engineer derived a better and more accurate
relationship, Manning equation, based on many field measurement.
V
•
‹
2
1
1
* R 3 *S 2
n
(8.1)
n - Manning’s coefficient, s/m1/3
(can be found in most of the hydraulic handbooks)
To incorporate the continuity equation, Manning equation becomes
Q
‹
=
2
1
A
3
2
*
R
*S
=
n
(8.2)
As the flow according to Manning equations is for normal steady
uniform flow,
– the flow is Normal Flow
– the depth is Normal Depth
P.8-4
Fluid Mechanics
Chapter 8 – Open Channel Flow
Worked examples:
1.
Water flows in a rectangular, concrete, open channel that is 12 m wide
at a depth of 2.5m. The channel slope is 0.0028. Find the water
velocity and the flow rate. (n = 0.013)
Answer
By Manning equation,
2
1
1
V = * R 3 *S 2
n
with n = 0.013
S = 0.0028
∴
A
P
R
hence
= 12* 2.5 m2 = 30 m2
= 12 + 2*2.5 m = 17 m
= A/P
= 30 / 17 m = 1.765 m
V
2
1
1
* (1.765) 3 * (0.0028) 2
0.013
= 5.945 m/s
=
Discharge, Q = A*V
= 30*5.945 m3/s
= 178.3 m3/s
P.8-5
Fluid Mechanics
2.
Chapter 8 – Open Channel Flow
Water flows in a rectangular, concrete, open channel that is 12 m wide.
The channel slope is 0.0028. If the velocity of the flow is 6 m/s, find
the depth of the flow. (n = 0.013)
h
12m
Answer
By Manning equation,
2
1
1
V = * R 3 *S 2
n
with V = 6 m/s
n = 0.013
S = 0.0028
A
P
∴
R
= 12* h m2
= 12 + 2*h m
A
12 * h
6*h
=
= =
P 12 + 2 * h 6 + h
6*h
= 1.790
6+h
h = 2.551 m
Depth of the flow = 2.551m
P.8-6
Fluid Mechanics
3.
Chapter 8 – Open Channel Flow
A trapezoidal channel with side slopes of 2/3, a depth of 2 m, a bottom
width of 8 m and a channel slope of 0.0009 has a discharge of 56 m3/s.
Find the Manning’s n.
14m
2m
1
1.5
3m
8m
3m
Answer
= (14+8)*2/2 m2
= 22 m2
P = 8 + 2* 2 2 + 32 m
= 15.211 m
A/P = 22 / 15.211 m
= 1.446 m
A
By Manning equation,
Q=
2
1
A
* R 3 *S 2
n
Q
= 56 m3/s,
S
= 0.0009
2
1
22
* (1.446) 3 * (0.0009) 2
n
= 0.01507
56 =
n
P.8-7
Fluid Mechanics
4.
Chapter 8 – Open Channel Flow
Determine the depth in a trapezoidal channel with side slopes of 1 to
1.5, a bottom width of 8 m and a channel slope of 0.0009. The
discharge is 56 m3/s and n = 0.017.
(8+3*y) m
y
1
1.5
8m
Answer
A
P
R
= (8+8+3*y)*y/2 m2
= (8+1.5*y)*y m2
= 8 + 2*y* 12 + 1.5 2 m
= 8+3.6056*y m
= A/P
= (8+1.5*y)*y / 8+3.6056*y
By Manning equation,
2
1
A
Q = * R 3 *S 2
n
Q = 56 m3/s, S = 0.0009
∴
or
(8 + 1.5 * y) * y ⎡ (8 + 1.5 * y) * y ⎤
56 =
*⎢
⎥
0.017
⎣ 8 + 3.6056 * y ⎦
2
3
* (0.0009)
1
2
[(8 + 1.5 * y) * y]5 3 = 31.7333
[8 + 3.6056 * y]2 3
[(1 + 0.1875 * y) * y]5 3 − 3.9667 = 0
[1 + 0.4507 * y]2 3
By trial & error, y = 2.137 m.
The depth of the trapezoidal channel is 2.137m.
P.8-8
Fluid Mechanics
5.
Chapter 8 – Open Channel Flow
Water flows in the triangular steel channel shown in the figure below.
Find the depth of flow if the channel slope is 0.0015 and the discharge
is 0.22 m3/s. (n=0.014)
y
o
60
Answer
A
P
R
= 2ytan30° * y/2 m2
= y2*tan30° m2
= 2y/cos30° m
= A/P = y2*tan30° / 2y/cos30° m
= ysin30°/2 m
By Manning equation,
2
1
A
Q = * R 3 *S 2
n
Q
= 0.22 m3/s, S = 0.0015
2
y 2 tan 30° ⎛ y sin 30° ⎞ 3
*⎜
0.22 =
⎟ * 0.0015
0.014
2
⎝
⎠
8/3
= y * 0.6338
or
⎛ 0.22 ⎞
y =⎜
⎟
⎝ 0.6338 ⎠
= 0.672 m
3
8
m
Depth of the channel is 0.672 m.
P.8-9
Fluid Mechanics
8.2.2
‹
Chapter 8 – Open Channel Flow
Optimum Hydraulic Cross-sections (REFERENCE ONLY)
From Manning equation,
5
1 A 3* S
Q = *
2
n
P 3
Hence, Q will be maximum when P is a minimum.
‹
For a given cross-sectional area, A of an open channel, the discharge, Q
is maximum when the wetted perimeter, P is minimum. Hence if the
wetted perimeter, P for a given flow area is minimised, the area, A will
give the least expensive channel to be construct.
‹
This corresponding cross-section is the optimum hydraulic section or
the best hydraulic section.
8.2.2.1
Rectangular section
width
=b
y
depth
=y
b
area, A = by
P
= b+2*y
A
= + 2y
y
Hence
dP
A
=− 2 +2=0
dy
y
i.e.
y=
A
or
2
b = 2y
Therefore, the optimum rectangular section is
y
2y
P.8-10
Fluid Mechanics
8.2.2.2
Chapter 8 – Open Channel Flow
Trapezoidal section
B
= b+2*m*y
y
A
= (b+m*y)*y
P
= b+2*y* 1+ m 2
m
1
b
By eliminating b from P,
A
P = + ( 2 * 1 + m 2 − m) * y
y
For a minimum value of P, δP = 0,
dP
dP
i.e.
=0
=0
and
dy
dm
From
dP
= 0,
dy
y2
=
A
3
dP
1
= 0,
m =
dm
3
It implies the side slope of the channel is 60° to horizontal.
A
y 2 3
=
y
b = − my = 3y −
y
3
3
2 3
4
y+
y = 2 3y
and
P =
3
3
i.e.
P = 3*b
From
The optimum section is given as follow:
o
b
60 b
b
P.8-11
Fluid Mechanics
8.2.2.3
Chapter 8 – Open Channel Flow
Other sections
N-side Channel
‹ from the conclusion of the previous two sections
– reflection of the rectangular optimum section about the water
surface will form a square of side b.
– reflection of the trapezoidal optimum section about the water
surface will form a regular hexagon of side b.
‹
For a N-side channel, the optimum hydraulic section should be in a
form of half a 2N-side regular polygon.
b
φ
⎛ N −1⎞
φ=⎜
⎟ *180°
N
⎝
⎠
b
b
b
b
b
Triangular Section
‹ N = 2, hence
φ = 90°
y
o
45
Circular Section
‹
From the result of N-side channel, it can be concluded that the
optimum section of a circular channel is a semi-circle.
‹
It is the most optimum section for all the possible open-channel crosssection.
D
P.8-12
Fluid Mechanics
Chapter 8 – Open Channel Flow
Worked examples
1. An open channel is to be designed to carry 1m3/s at a slope of 0.0065.
The channel material has an n value of 0.011. Find the optimum
hydraulic cross-section for a semi-circular section.
D
Answer
The optimum circular section is a semi-circular section with diameter D
which can discharge 1 m3/s.
For a semi-circular section,
A = π*D2/8
P = π*D/2
R = A/P
= D/4
As n = 0.011, S = 0.0065 and Q = 1 m3/s.
Q=
2
1
A
* R 3 *S 2
n
2
π * D2 ⎛ D ⎞ 3
* ⎜ ⎟ * 0.0065
=
8 * 0.011 ⎝ 4 ⎠
i.e. 1
2
8/3
D
D
1
−
8 * 0.011 3
=
* 4 * 0.0065 2
π
= 0.951 m
The diameter of this optimum section is 951mm.
P.8-13
Fluid Mechanics
Chapter 8 – Open Channel Flow
2. Find the optimum rectangular section from the last example.
y
2y
Answer
A
P
R
= 2*y2
= 4*y
= A/P = y/2
By Manning equation,
Q=
2
1
A
* R 3 *S 2
n
1
=
2
3
1* 0.011* 2
2 * 0.0065
= 0.434 m
y8/3 =
y
2
3
2* y ⎛ y ⎞
* ⎜ ⎟ * 0.0065
0.011 ⎝ 2 ⎠
2
The optimum rectangular section has dimension of width 0.868m and
depth 0.434m.
P.8-14
Fluid Mechanics
3.
Chapter 8 – Open Channel Flow
Find the optimum triangular section from the last example.
y
o
45
Answer
A
P
= y2
= 2 2*y
R
= A/P
= y
2 2
By Manning equation,
2
1
A
Q = * R 3 *S 2
n
2
3
y
⎛ y ⎞
*⎜
⎟ * 0.0065
0.011 ⎝ 2 2 ⎠
2
1
=
2
3
0.011* (2 2 )
0.0065
y = 0.614 m
The optimum triangular section is a right angle triangle with depth 0.614 m.
y8/3 =
P.8-15
Fluid Mechanics
Chapter 8 – Open Channel Flow
8.3 Non-Uniform flow - Specific Energy in Open Channel & Critical
Flow
energy line
2
v1 /2g
fluid surface
E1
hf
v22/2g
y1
y2
z1
channel bed
horizontal datum
‹
‹
z2
In open channel, the solution of many problems are greatly assisted by
the concept of specific energy, i.e.
v2
E =
+y
(8.3)
2g
In terms of flow rate, Q,
1 Q 2
E =
( ) +y
2g A
(8.4)
The minimum energy will be given as
dE
=0
dy
(8.5)
8.3.1
‹
E2
Let
∴
Rectangular Channel
Q
= v*y
(8.6)
b
q - the discharge per unit width of a rectangular channel
q
E
=
=
q2
2gy 2
+y
(8.7)
P.8-16
Fluid Mechanics
‹
Chapter 8 – Open Channel Flow
By assuming q is constant
or
yc
‹
‹
dE
q2
=1=0
(8.8)
3
dy
gy
y = yc
q 2 13
=( )
(8.9)
g
- critical depth at which the energy is minimum.
The corresponding energy, E is
From (8.6),
Emin =
3
yc
2
v
q
.
y
=
(8.10)
Substitute into (8.8),
v c2
=0
1gy c
v c2
=1
gy c
vc = gy c
or
‹
(8.11)
(8.12)
Since Froude number, Fr is defined as
Fr
=
v
gy ave
(8.13)
Hence, the minimum energy is occurred when
Fr2 = 1
‹
(8.14)
For a given discharge, Q, if the flow is such that E is a min., the flow
is critical flow.
–
critical flow
- flow with Emin
–
critical depth, yc - the depth of the critical flow
–
critical velocity - vc = gy c
P.8-17
Fluid Mechanics
Chapter 8 – Open Channel Flow
y
2
v1 /2g
y1
A
y
y1
subcritical
or slow
2
vc /2g
c
C
yc
supercritical
B or fast
o
45
y2
y2
2
v2 /2g
E
‹
If the flow with E > Emin, there are two possible depths (y1, y2).
(y1, y2) are called alternate depths.
‹
C divides the curve AB into AC and CB regions.
-
AC - subcritical flow region
CB - supercritical flow region
Depth of flow
Velocity of flow
Slope
Froude number
Other
Subcritical
y > yc
v < vc
Mild
S < Sc
Fr < 1.0
v2 yc
<
2g 2
Critical
y = yc
v = vc
Critical
S = Sc
Fr = 1.0
v2 yc
=
2g
2
Supercritical
y < yc
v > vc
Steep
S > Sc
Fr > 1.0
v2 yc
>
2g 2
P.8-18
Fluid Mechanics
8.3.2
Chapter 8 – Open Channel Flow
Non - Rectangular Channel
‹
If the channel width varies with y, the specific energy must be written
Q2
in the form E =
(8.15)
+y
2gA 2
‹
The minimum energy also occurs where
dE
= 0 at constant Q
dy
‹
Since A = A(y), therefore (8.15) becomes
2Q 2 A −3 dA
12g
dy
or
‹
Since
∴
or
dA gA 3
= 2
dy
Q
=0
(8.16)
dA
= B - the channel width at the free surface,
dy
gA 3
B = 2
Q
BQ 2 13
A =(
)
(8.17)
g
Q
vc =
A
gA 1
=( ) 2
(8.18)
B
‹
For a given channel shape, A(y) & B(y), and a given Q, (8.17) & (8.18)
have to be solved by trial and error to find the A and then vc.
♦
If a critical channel flow is also moving uniformly (at constant depth),
it must correspond to a critical slope, Sc, with yn = yc. This condition
can be analysed by Manning formula.
P.8-19
Fluid Mechanics
Chapter 8 – Open Channel Flow
Worked examples:
1. A triangular channel with an angel of 120° made by 2 equal slopes. For
a flow rate of 3 m3/s, determine the critical depth and hence the
maximum depth of the flow.
B
yc
o
30
Answer
∴
For critical flow,
v2 = g*yave
Q2 = g*yave*A2
gA 3
A
=
(yave = )
B
B
For critical flow,
B = 2*y*cot 30°
& A = y2*cot 30°
3g 5
Q2 =
y
2
Hence
y
2Q 2 15
=(
)
3g
2 * 32 15
=(
) m
3 * 9.81
= 0.906 m
The maximum depth is 0.906 m.
The critical depth, yc = yave =
A ( B * y ) / 2 y 0906
=
= =
= 0.453m .
B
B
2
2
P.8-20
Fluid Mechanics
Chapter 8 – Open Channel Flow
2. In the last example, the channel Manning roughness coefficient is
0.012 and the flow rate is 3 m3/s. What is the value of the channel
slope if the flow is critical, subcritical or supercritical?
B
y
o
30
Answer
B
A
P
= 2 3 *y
1
= *B*y
2
= 4*y
Using Manning equation,
2
1
A
Q = * R 3 *S 2
n
1
1 1
B 2
= * ( * B * y) * ( ) 3 * S 2
8
n 2
2 nQ B − 2 3
( )
Sc1/2 =
By 8
2 nQ
3 * y −23
=
(
)
2
4
2 3*y
For critical flow, y = y
2 * 0.012 * 3
3 * 0.906 − 2 3
∴
Sc1/2 =
(
)
4
2 3 * (0.906) 2
Sc = 0.0472
For flow is
critical,
S = 0.0472
subcritical, S < 0.0472
supercritical, S > 0.0472
- critical slope
P.8-21
Fluid Mechanics
Chapter 8 – Open Channel Flow
8.4 Frictionless Flow over a Bump
supercritical
approach flow
y1
v1
2
y2 v
subcritical
approach flow
∆h
‹
When fluid is flowing over a bump, the behaviour of the free surface
is sharply different according to whether the approach flow is
subcritical or supercritical.
‹
The height of the bump can change the character of the results.
‹
Applying Continuity and Bernoulli’s equations to sections 1 and 2,
&
‹
= v2*y2
v1*y1
2
v22
v1
+ y1 =
+ y 2 + ∆h
2g
2g
Eliminating v2 between these two gives a cubic polynomial equation
for the water depth y2 over the bump,
v12 * y12
y2 – E2*y2 +
2g
3
2
=0
(8.19)
v12
where E2 =
+ y1 - ∆h
(8.20)
2g
This equation has one negative and two positive solutions if ∆h is not
too large.
P.8-22
Fluid Mechanics
‹
Chapter 8 – Open Channel Flow
The free surface’s behaviour depends upon whether condition 1 is in
subcritical or supercritical flow.
y1
water
depth
y2
∆ hmax
2
1
subcritical
bump
∆h
yc
supercritical
bump
Ec
E2
E1 specific
energy
The specific energy E2 is exactly ∆h less than the approach energy, E1,
and point 2 will lie on the same leg of the curve as E1.
‹
A subcritical approach, Fr1 < 1, will cause the water level to decrease
at the bump.
‹
Supercritical approach flow, Fr1 > 1, causes a water level increase
over the bump.
‹
If the bump height reaches ∆hmax = E1 – Ec, the flow at the crest will
be exactly critical (Fr = 1).
‹
If the bump > ∆hmax, there are no physical correct solution. That is, a
bump to large will choke the channel and cause frictional effects,
typically a hydraulic jump.
P.8-23
Fluid Mechanics
Chapter 8 – Open Channel Flow
Worked example:
Water flow in a wide channel approaches a 10 cm high bump at 1.5 m/s
and a depth of 1 m. Estimate
(a) the water depth y2 over the bump, and
(b) the bump height which will cause the crest flow to be critical.
Answer
(a) For the approaching flow,
v1
15
.
=
Fr =
9.81 * 1
gy1
= 0.479 ⇒ subcritical
For subcritical approach flow, if ∆h is not too large, the water level over
the bump will depress and a higher subcritical Fr at the crest.
E1
Hence
E2
v12
=
+ y1
2g
= 1.115 m
= E1 - ∆h
= 1.015 m
15
. 2
. m
=
+ 10
2 * 9.81
= 1.115 – 0.1 m
Substitute E2 into (8.24),
y23 – 1.015*y22 + 0.115 = 0
By trial and error,
y2 = 0.859 m, 0.451 m and –0.296 m (inadmissible)
The second (smaller) solution is the supercritical condition for E2 and is
not possible for this subcritical bump.
Hence
Checking:
y2 = 0.859 m
v2 = 1.745 m/s (By continuity)
(OK)
Fr2 = 0.601 (> Fr1 and < 1)
P.8-24
Fluid Mechanics
(b)
Chapter 8 – Open Channel Flow
By considering per m width of the channel,
q = v*y = 1.5*1 m2/s
For critical flow,
E2 = Emin
yc
E2
∆hmax
=
q 2 13
=( )
g
15
. 2 13
=(
)
9.81
= 0.612 m
3
= *0.612
2
= 0.918 m
3
yc
2
m
= E1 - Emin
= 1.115 – 0.918 m
= 0.197 m
P.8-25
Fluid Mechanics
Chapter 8 – Open Channel Flow
8.5 Hydraulic Jump in Rectangular Channel
‹
A hydraulic jump is a sudden change from a supercritical flow to
subcritical flow.
‹
Assumptions:
– the bed is horizontal.
– the velocity over each cross-section is uniform.
– the depth is uniform across the width.
– frictionless boundaries.
– surface tension effects are neglect.
1
2
critical
depth level
y2
y1
v
1
v2
eddy currents
‹
Considering the control volume between 1 and 2, the forces are
b
y12
= ρgb*
(8.21a)
F31 = ρgy1* *y1
2
2
y22
Similarly
F32 = ρgb*
(8.21b)
2
‹
By continuity equation,
Q = b*y1*v1
‹
‹
= b*y2*v2
By the momentum equation,
F1 = F2 = 0
hence F31 – F32 = ρ*Q*(v2 - v1)
(8.22)
(8.23)
Sub. (8.21a, b) and (8.22) into (8.23), then
ρgb 2
Q
Q
( y1 − y 2 2 ) = ρQ(
−
)
2
y 2 b y 1b
ρQ 2 y1 − y 2
=
(
)
b
y1 y 2
(8.24)
P.8-26
Fluid Mechanics
‹
Chapter 8 – Open Channel Flow
In a hydraulic jump, y1 ≠ y2,
2Q 2
y1*y2*(y1+y2) =
gb 2
2Q 2
2
2
∴
y1 y2 + y1y2 =
gb 2
2Q 2
y2 2
y2
i.e.
( ) +( )− 2 3 =0
y1
y1
gb y1
Solving (8.25),
1
8Q 2
y2
= [ −1 + 1 + 2 3 ]
2
y1
gb y1
This is the hydraulic jump equation.
‹
(8.26)
Using Froude number,
Fr1
2
v12
Q2
=
=
gy1
gy13b2
1
= [ −1 + 1 + 8Fr12 ]
2
then,
y2
y1
or
1
y1
= [ −1 + 1 + 8Fr2 2 ]
2
y2
‹
(y1,y2) are called conjugate depths.
‹
The energy loss in a jump is given by
v22
v12
+ y1 =
+ y2 + h f
2g
2g
v − v2
=( 1
) + ( y1 − y 2 )
2g
2
i.e.
‹
(8.25)
hf
(8.27)
(8.28a)
(8.28b)
2
(8.29)
Sub. (8.22) into above,
hf
Q 2 y1 + y 2
= [−
(
) + 1 ](y1 – y2)
2gb 2 y12 y 2 2
(8.29)
P.8-27
Fluid Mechanics
‹
Using (8.25), (8.29) becomes
( y 2 − y1 ) 3
hf =
4 y1 y 2
Chapter 8 – Open Channel Flow
(8.30)
This is the energy loss equation for the hydraulic jump (y2>y1, hf>0).
‹
The power loss in a jump is
P = ρghf*Q
‹
This energy loss is useful for getting away with the unwanted energy
of a flow. The energy loss is due to the frictional forces amount the
eddy currents in the pump. It will increase the temperature of the fluid.
P.8-28
Fluid Mechanics
Chapter 8 – Open Channel Flow
Worked example:
Water flows in a wide channel at q = 10 m2/s and y1 = 1.25 m. If the flow
undergoes a hydraulic jump, calculate
(a) y2,
(b) v2,
(c) Fr2,
(d) hf,, and
(e) the percentage dissipation of the energy.
Answer
(a)
v1
Fr1
Since
or
(b)
y2
y1
y2
q
y1
10
=
m/s = 8 m/s
1.25
v1
=
gy1
8
= 2.285
=
9.81 * 125
.
1
= [ −1 + 1 + 8Fr12 ]
2
1
= [ −1 + 1 + 8 * (2.285) 2 ]
2
= 2.77
= 2.77*1.25 m
= 3.46 m
=
By Continuity equation,
y
v2 = v1*( 1 )
y2
125
.
= 8*
m/s
3.46
= 2.89 m/s
P.8-29
Fluid Mechanics
(c)
(d)
Chapter 8 – Open Channel Flow
v2
gy 2
2.89
=
9.81 * 3.46
= 0.496
Fr2 =
hf
( y 2 − y1 ) 3
=
4 y1 y 2
(3.46 − 125
. )3
=
4 * 3.46 * 125
.
= 0.625 m
(e)
E1
percentage loss
v12
+ y1
=
2g
82
+ 1.25 m
=
2 * 9.81
= 4.51 m
hf
* 100%
E1
0.625
* 100%
=
4.51
= 14 %
=
P.8-30
Fluid Mechanics
Chapter 8 – Open Channel Flow
8.6 Gradually Varied Flow
‹
It is not always possible to have uniform depth across the flow i.e.
normal flow with normal depth.
‹
The depth of flow can be changed by the conditions along the channel.
‹
Examples of Gradually Varied Flow are:
– backwater curve
water surface
dy >0
dx
yn
dam
– Downdrop curve
water surface
yn1
dy
dx <0
slope change
yn2
‹
In a uniform flow, the body weight effect in balanced out by the wall
friction.
‹
In gradually varied flow, the weight and the friction effects are
unable to make the flow uniform.
P.8-31
Fluid Mechanics
Chapter 8 – Open Channel Flow
1
2
δx
2
v1/2g
THL
s
f
dh
2
v2/2g
fluid surface
v1
y1
y2
z1
channel bed
θ
horizontal datum
v2
z2
‹
Basic assumptions are
slowly changing bottom slope
slowly changing water depth (no hydraulic jump)
slowly changing cross section
one dimensional velocity distribution
pressure distribution approximately hydrostatic
-
‹
Denoting
v1
z1
y1
p1
= v;
= z,
= y,
= p,
v2
z2
y2
p2
= v + dv
= z + dz
= y + dy
=p
Apply Bernoulli’s equation between section 1 and 2,
p ( v + dv) 2
p v2
+
+y+z = +
+ ( y + dy) + ( z + dz) + dh f
γ 2g
γ
2g
Neglecting higher order terms,
v
dhf + dy + dz + du
g
=0
When lim dx → 0,
dh f dy dz v dv
+
+
+
=0
dx dx dx g dx
(8.30)
P.8-32
Fluid Mechanics
Chapter 8 – Open Channel Flow
‹
In (8.30), the four terms are
dh f
- rate of head loss along the channel
dx
= S (head loss gradient in Manning equation)
dy
- rate of change of water depth
dx
- water surface profile’s gradient
dz
- rate of vertical change along channel
dx
= -sinθ
v dv
- rate of change of velocity head along the channel
g dx
‹
By Continuity equation
v*A = constant
dv
dA
+v
=0
i.e.
A
dx
dx
dv
dA dy
A
+v
=0
dx
dy dx
dy
dv
A
+ vB
=0
dx
dx
dv
vB dy
=or
A dx
dx
v dy
=y dx
Therefore
‹
v dv
g dx
v 2 dy
=gy dx
dy
= -Fr2
dx
Hence (8.30) becomes
dy
dy
- sinθ - Fr2
=0
s+
dx
dx
dy
sin θ − S
or
=[
]
dx
1 − Fr 2
- general equation of gradually varied flow.
(8.31)
(8.32)
P.8-33
Fluid Mechanics
‹
Chapter 8 – Open Channel Flow
(8.32) is a 1st order non-linear differential equation. Numerical
method is used to solve the equation.
The equation is rewritten as
dx
1 − Fr 2
=[
]
dy
sin θ − S
2
y 2 1 − Fr
x2
∫x1 dx = ∫y1 ( sin θ − S)dy
2
y 2 1 − Fr
x2 = x1 + ∫y (
)dy
1 sin θ − S
(8.33)
The simplest solution is the direct mid-point solution of the integral.
1 − Fr 2
i.e.
x2 = x1 + (
) y + y *(y2-y1)
(8.34)
sin θ − S ( 1 2 2 )
‹
(8.34) may be used to calculate the water profile in a step-by-step
sequence from a known (x1, y1) value.
P.8-34
Fluid Mechanics
Chapter 8 – Open Channel Flow
Worked example:
Determine the upstream profile of a backwater curve given:
Q = 10 m3/s, b = 3m, sinθ = 0.001, n = 0.022.
water surface
yn
5m
dam
θ
Answer
For normal flow, (S → sin θ)
2
1
A
Q = * R 3 *S 2
n
( 3y n )
3y n
*(
) * 0.001
i.e.
10 =
0.022 3 + 2y n
yn = 2.44 m
The water profile is from 2.44 m to 5 m along the channel.
From Manning equation,
2
1
1
v = * R 3 *S 2
n
n2v2
S = 4
R 3
Hence
x2
= x1 + (
1 − Fr 2
0.001 −
n 2 v2
) *(y2-y1)
R 4/ 3
P.8-35
Fluid Mechanics
section, I
1
yi (m)
5
2
4.75
3
4.25
5
4
7
8
9
10
11
dy (m)
yave (m)
v (m/s)
Fr
1-Fr*Fr
R (m)
So - Sf
dx (m)
0.25
4.875
0.684
0.099
0.990
1.147
0.000812
305
0.25
4.625
0.721
0.107
0.989
1.133
0.000787
314
0.25
4.375
0.762
0.116
0.986
1.117
0.000758
326
x (m)
0
305
4.5
4
6
Chapter 8 – Open Channel Flow
619
945
0.25
4.125
0.808
0.127
0.984
1.100
0.000722
341
0.25
3.875
0.860
0.140
0.981
1.081
0.000677
362
0.25
3.625
0.920
0.154
0.976
1.061
0.000622
392
0.25
3.375
0.988
0.172
0.971
1.038
0.000551
440
0.25
3.125
1.067
0.193
0.963
1.014
0.000459
524
0.25
2.875
1.159
0.218
0.952
0.986
0.000337
707
0.31
2.595
1.285
0.255
0.935
0.951
0.000146
1992
1285
3.75
1647
3.5
2040
3.25
2480
3
3004
2.75
3711
2.44
5703
From the table, the water level is not affected by the dam at 5.7 km
upstream.
5m
4m
3m
normaldepth
2m
dam
1m
8km 7km 6km 5km 4km 3km 2km 1km
0
From the graph, the water depth at any location can be obtained.
P.8-36
Fluid Mechanics
8.6.1
Chapter 8 – Open Channel Flow
Classifications of Surface Profile of Gradually Varied Flow
‹
It is customary to compare the actual channel slope, sinθ or So with
the critical slope Sc for the same Q.
‹
There are five classes of channel slope giving rise to twelve distinct
types of solution curves.
So > Sc - Steep
(S)
So = Sc - Critical
(C)
So < Sc - Mild
(M)
So = 0 - Horizontal (H)
So < 0 - Adverse
(A)
‹
There are three number designators for the type of profile relates to
the position of the actual water surface in relation to the position of
the water for normal and critical flow in a channel.
1 the surface of stream lies above both normal and critical
depth
2 the surface of stream lies between normal and critical
depth
3 the surface of stream lies below both normal and critical
depth
M1
yn
M2
M3
yc
Mild slope
S1
yc
S2
S3
yn
Steep slope
P.8-37
Fluid Mechanics
‹
Chapter 8 – Open Channel Flow
Combining the two designators, we have
Slope
class
sinθ > S
Slope notation
Steep (S)
Depth
class
yc > yn
sinθ = S
Critical (C)
yc = yn
sinθ < S
Mild (M)
yc < yn
sinθ = 0
Horizontal (H)
yn = ∞
sinθ < 0
Adverse (A)
yn = Im
‹
For type S
‹
For type C
Froude
number
Fr < 1
Fr > 1
Fr > 1
Fr < 1
Fr > 1
Fr < 1
Fr < 1
Fr > 1
Fr < 1
Fr > 1
Fr < 1
Fr > 1
Actual depth
Profile
y >yn; y>yc
yc > y > yn
y<yn; y<yc
y>yn = yc
y<yn = yc
y>yn; y>yc
yn > y > yc
y<yn; y<yc
y > yc
y<yn; y<yc
y > yc
y < yc
S1
S2
S3
C1
C3
M1
M2
M3
H2
H3
A2
A3
P.8-38
Fluid Mechanics
‹
For type M
‹
For type H
‹
For type A
Chapter 8 – Open Channel Flow
P.8-39
Fluid Mechanics
Chapter 8 – Open Channel Flow
Class Exercise 8.1:
A 500 mm-diameter concrete pipe on a 1:500 slope is to carry water at a
velocity of 0.18 m/s. Find the depth of the flow. (n=0.013)
500mm
θ
y
(y = 18 mm)
P.8-40
Fluid Mechanics
Chapter 8 – Open Channel Flow
Class Exercise 8.2:
What are the dimensions for an optimum rectangular brick channel (n =
0.015) designed to carry 5 m3/s of water in uniform flow with s = 0.001?
What will be the percentage increase in flow rate if the channel is a semicircle but retained the same sectional area?
(increase = 8.4%)
P.8-41
Fluid Mechanics
Chapter 8 – Open Channel Flow
Class Exercise 8.3:
A trapezoidal channel has a bottom width of 6.0 m and side slopes of 1:1.
The depth of flow is 1.5 m at a discharge of 15 m3/s. Determine the
specific energy and alternate depth.
(E = 1.59 m, y = 0.497 m)
P.8-42
Fluid Mechanics
Chapter 8 – Open Channel Flow
Class Exercise 8.4:
A triangular channel has an apex angle of 60° and carries a flow with a
velocity of 2.0 m/s and depth of 1.25 m.
(a)
(b)
(c)
(d)
Is the flow subcritical or supercritical?
What is the critical depth?
What is the specific energy?
What is the alternate depth possible for this specific energy?
(yc = 1.148 m, E = 1.454 m, y = 1.06 m)
P.8-43
Fluid Mechanics
Chapter 8 – Open Channel Flow
Class Exercise 8.5:
A rectangular channel is 4.0 m wide and carries a discharge of 20 m3/s at a
depth of 2.0 m. At a certain section it is proposed to build a hump.
Calculate the water surface elevations at upstream of the hump and over
the hump if the hump height is 0.33 m. (Assume no loss of energy at the
hump.)
water surface
y1
y2
Q
hump
P.8-44
Fluid Mechanics
Chapter 8 – Open Channel Flow
Class Exercise 8.6:
In a hydraulic jump occurring in a horizontal, rectangular channel it is
desired to have an energy head loss equal to 6 times the supercritical flow
depth. Calculate the Froude number of the flow necessary to have this
jump.
(Fr1 = 4.822 )
P.8-45
Fluid Mechanics
Chapter 8 – Open Channel Flow
Tutorial – Open Channel Flow
1.
Calculate the normal depth in a concrete trapezoidal channel with side
slope of 1 to 3, a bed slope of 0.00033, a bottom width of 4.0 m and a
water discharge of 39 m3/s. Manning coefficient is 0.013.
1
y
3
1
3
4m
2.
Determine the critical depth of the trapezoidal channel for a discharge
of 15 m3/s. The width of the channel bottom, b = 6 m, and the side
slope is 45°.
3.
Consider a flow in a wide channel over a bump with an approaching
velocity, v1 at the upstream is 1 m/s and the depth, y1 is 1 m. If the
maximum bump height is 15 cm, determine
(a) the Froude number over the top of the bump, and
(b) the depression in the water surface. (y2 > 0.5 m)
4.
Water flows in a trapezoidal channel at a rate of 8.5 m3/s. The
channel has a bottom width of 3 m and side slope of 1:1. If a
hydraulic jump is forced to occur where the upstream depth is 0.3 m,
what will be the downstream depth and velocity? What are the values
of Fr1 and Fr2?
5.
A wide canal has a bed slope of 1 in 1000 and conveys water at a
normal depth of 1.2 m. A weir is to be constructed at one point to
increase the depth of flow to 2.4 m. How far upstream of the weir will
the depth be 1.35 m? (Take n in the Manning equation as 0.013)
P.8-46