Hydraulic Structures – Canal Falls January 16, 2011 1 Canal Falls It is neither necessary nor economical to design every fall as meter flume. A vertical fall may also be used as a meter when working head is more than 0.33 H and the d.s F.S.L. is up to or lower than the crest level. (H is the depth of crest below T.E.L.). Hydraulic Design of Canal Falls 1. Vertical drop fall The energy is dissipated by means of impact and deflection of velocity, suddenly from the vertical to the horizontal direction. Cistern length 1/ 2 5 c L L HD Cistern depth 2/3 1 4 larger of 3 L c X H D X d X 2 3 c q d g Where: is the length of cistern, is the depression below d.s. bed, is the depth of crest below u.s. T.E.L., is drop in meters. c L L X D H 2. Glacis fall The energy is dissipated by the formation of hydraulic jump. From H L and q, Ef 2 can be found from Blench curves Fig. 3.5. To make sure that the hydraulic jump will form on the glacis, the depth of cistern is increased by 25% of Ef 2 .
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Hydraulic Structures – Canal Falls January 16, 2011
1
Canal Falls
It is neither necessary nor economical to design every fall as meter flume. A vertical fall may also be
used as a meter when working head is more than 0.33 H and the d.s F.S.L. is up to or lower than the
crest level. (H is the depth of crest below T.E.L.).
Hydraulic Design of Canal Falls
1. Vertical drop fall
The energy is dissipated by means of impact and deflection of velocity, suddenly from the
vertical to the horizontal direction.
Cistern length 1/2
5c LL H D
Cistern depth
2/31
4 larger of
3
L
c
X H D
Xd
X
2
3c
qd
g
Where: is the length of cistern,
is the depression below d.s. bed,
is the depth of crest below u.s. T.E.L.,
is drop in meters.
c
L
L
X
D
H
2. Glacis fall
The energy is dissipated by the formation of hydraulic jump. From HL and q, Ef2 can be
found from Blench curves Fig. 3.5.
To make sure that the hydraulic jump will form on the glacis, the depth of cistern is
increased by 25% of Ef2.
Hydraulic Structures – Canal Falls January 16, 2011
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In case the d.s. bed level is lower than cistern level, the cistern level should be provided at
bed level. Where the d.s. bed level is higher than cistern level, the cistern level is joined at a
slope 5H:1V.
Sufficient length of cistern is provided so that turbulence dies out and subcritical flows takes
place before the water leaves the floor. Length of cistern of 5Ef2 is provided for normal soils
and 6Ef2 for sandy soils.
2(5 6)cL Ef
Hydraulic Structures – Canal Falls January 16, 2011
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Design of Sharda Type Fall
1. Crest
i. Length of crest:
The length of crest is kept equal to bed width, it is also possible to extend the length
to bed width plus depth.
ii. Shape of crest
For Q < 15 cumec, the section is kept rectangular with the d.s. face absolutely
vertical. The top width 10.55tB D and minimum base width
1 2D where D1 is
the height of crest above d.s. bed level.
It may be capped with 25 cm 1:2:4 cement concrete with its both ends rounded.
For 15Q cumec , a trapezoidal section with top width 10.55tB D D with u.s.
side slope of 1H:3V and segmental top conforming to a quadrant of a circle of radius
0.3 m at d.s. edge of crest width and d.s. slope of 1H: 8V is adopted.
a. Rectangular crest fall
b. Trapezoidal fall
iii. Crest level
The following equation is used to determine the height of the crest:
1/6
3/2
t
t
DQ C L D
B
For submerged flow conditions (above 33% submergence) discharge passing over
crest is:
3/2 1/2
2
22 2
3d t L a a d t L aQ C L g H h h C L h g H h
Hydraulic Structures – Canal Falls January 16, 2011
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where,
Lt = Length of crest
Bt = Width of crest
HL = Drop in water surface
h2 = Depth of d.s. water level over top of crest
ha = Head due to velocity of approach 2 2aV g
C = 1.84 for rectangular crest
2.26 for trapezoidal crest
Crest level (free fall) d.s. F.S.L. ah D
Crest level (Submerged falls) 2d.s. F.S.L. a Lh H h
Degree of submergence 1 dh
D
2. U.S. Approaches
For discharges larger than 15 cumec, the wing walls are kept segmental with radius equal to
5 6 times D making an angle 60º at center, and carried tangentially into the berm. The
foundations of wing walls are laid on impervious floor itself.
For falls of discharges less than 15 cumec, the approach wings may be splayed straight at an
angle of 45º.
i. U.S. protection
Brick pitching in a length equal to u.s. water depth should be laid on u.s. bed towards
the crest at a slope 10H:1V.
ii. U.S. curtain wall (cutoff)
The thickness of curtain wall equal to 1½ brick and depth equal to 1/3 u.s. water
depth + 0.6 m, be provided with min. 0.8 m also considering Lacey’s criteria (Table
6.1).
HL
d
D
P
hd
T.E.L.
T.E.L.
Hydraulic Structures – Canal Falls January 16, 2011
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3. Impervious concrete floor
i. Total floor length and its disposition:
Khosla’s method is used for large works. Bligh’s theory is used for small works.
The minimum length of floor on the d.s. side for clear falls and submergence less
than 33% is:
10.53 4.877 1.5p c LL d H
For submerged falls with submergence greater than 33%,
6.77 5.182p c LL d H
The balance of total length maybe provided under and upstream of crest.
4. Cistern
i. Length of cistern
For clear falls and submergence less than 33%,
3.8 0.415c c LL d H
For submerged falls with submergence greater than 33%,
5.2 1.067c c LL d H
ii. Depth of cistern (X)
2/31
4 larger of
3
L
c
X H D
Xd
X
5. D.S. Protection
i. Bed protection
Brick pitching about 20 cm thick resting on 10 cm ballast in a length equal 3 times
d.s. water depth.
Toe wall 1½ brick thick and of depth = ½ (d.s. water depth) with minimum 0.6 m
provided at the end of pitching.
ii. Side protection
After wing walls, the side slopes of the channel are pitched with 1 brick on edge in a
length equal to 3 times d.s. water depth. The pitching should rest on a toe wall 1½
brick thick and of depth equal to ½ d.s. water depth.
iii. Curtain wall
The thickness of curtain wall may be 1½ brick thick and depth = ½ (d.s. water depth)
with minimum 1 m, also consider Lacey’s criteria.
iv. D.S. wings
For Q > 15 cumec, d.s. wings are kept vertical for a length 5to8 LD H and may
then be gradually warped. They should be taken up to the end of pucca floor.
Hydraulic Structures – Canal Falls January 16, 2011
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Design Example 1:
Design a Sharda type fall with the data given below:
i. Full supply flow rate . .
10. .
u scumec
d s
ii. Drop 1 m
iii. Full supply depth . . 1.5
. . 1.5
u s m
d s m
iv. Bed level . . 100
. . 99
u s m
d s m
v. Bed width . . 8
. . 8
u s m
d s m
vi. Side slopes 1 H: 1V
vii. Soil Good loam
viii. Bligh’s coefficient 7
Solution
1. Length of crest
Take crest length 8tL m
2. Crest level
Since Q < 15 cumec, use rectangular crest with both sides vertical.
1/63/21.84 t tQ L D D B
Assume 0.8tB m
3/2 1/6
1/6
110 1.840 8
0.8
0.776 say 0.78
D D
D m m
Velocity of approach (with 1:1 side slopes)
100.702
8 1.5 1aV m s
Velocity head 2 20.702
0.0252 2 9.81
aVm
g
U.S. T.E.L. u.s. F.S.L. 101.5 0.025 101.525
R.L. of crest u.s. T.E.L. 101.525 0.78 100.745 say 100.75
ah m
D m
Adopt crest level = 100.75 m
3. Shape of crest
i. Top width
1 10.55 , 100.75 99 1.75
0.55 100.75 99 0.73
tB D D m
m
Adopt Bt = 0.75 m.
Hydraulic Structures – Canal Falls January 16, 2011
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Check for Q,
1/6
1.67 110 1.84 8
0.75
0.741
D
D
ii. Width of base 10.5
0.5 1.75 0.875 say 1
D
m
Its top shall be capped with 20 cm thick cement concrete.
4. Side walls
The side walls may be splayed straight at an angle of 45º from the u.s. edge of the crest and
extending by 1 m in the earthen bank from the line of F.S.L.
5. D.S. expansion
Side walls shall be straight and parallel up to the end of floor and shall be kept vertical.
6. U.S. protection
Brick pitching in a length equal to u.s. water depth = 1.5 m should be laid on the u.s. with a
slope of 1:10 downwards and 3 pipes of 15 m diameter at the bed should be provided for
drainage during maintenance (cleaning).
7. Cistern elements
i. Depth of cistern
3
cdX , and
2 2
331.25
0.5429.81
c
qd m
g
0.5420.181
3X m
or
2/3
2/3
1
4
11 0.75 0.21
4
LX H D
m
Cistern depth X= 0.21 m say 0.25 m
ii. Length of cistern
3.8 0.415
3.8 0.542 0.415 1 3.47
c c LL d H
m
or
1/2 1/2
5 5 1 0.75 4.3c LL H D m
Provide 4.5 m long cistern at R.L. 98.75 m
8. Length of impervious floor
Bligh’s coefficient = 7
Maximum static head 100.75 99 1.75m
Hydraulic Structures – Canal Falls January 16, 2011
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Total floor length required 7 1.75 12.25m
Minimum d.s. floor length required
10.53 4.877 1.5
10.53 0.542 4.877 1.5 1.0
9.08 say 9.0
p c LL d H
m
Provide d.s. floor 9 m long and the balance 3.25 m under and u.s. of the crest.
9. Floor thickness
Minimum floor thickness of 0.3 m should be provided at the upstream region.
Maximum uplift head at the toe of crest
1.75
12.25 3.25 1.2912.25
m
Floor thickness required1.29
1.031.25
m
Provide 1.05 m thick concrete overlaid with 0.2 m thick brick pitching.
Max. uplift head at 2.25 m d.s. from the toe of crest
1.75
12.25 5.5 0.9612.25
Floor thickness required0.96
0.771.25
m
Provide 0.8 m thick concrete overlaid with 0.2 m brick pitching.
Floor thickness required at 4.5 m d.s. the toe of crest
1.75 12.25 7.75
0.5112.25 1.25
m
Provide 0.55 m thick concrete overlaid with 0.2 m brick pitching.
Floor thickness at 6.75 d.s. from the toe of crest
1.75 12.25 100.26
12.25 1.25m
Provide 0.3 m thick concrete overlaid with 0.2 m brick pitching.
10. Curtain walls
a. D.S. curtain wall
The curtain walls at d.s. end of floor should be 1½ brick thick and of depth 2 0.6d m
to a minimum of 1 m.
Depth of curtain wall at d.s. end floor1.5
0.6 1.352
m
Provide 0.4 1.4m m deep curtain wall.
b. U.S. curtain wall
Depth of curtain wall at u.s. end flooru.s. water depth
0.6 0.5 0.6 1.13
m
Provide 0.4 1.1m m deep curtain wall.
Hydraulic Structures – Canal Falls January 16, 2011
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11. D.S. Protection
a. Bed protection
Length of bed protection33 3 1.5 4.5D m
Provide 4.5 m long dry brick pitching resting on 10 cm ballast which should be protected
by a toe wall 0.4 m wide and 0.8 m deep (half d.s. water depth).
b. Side protection
For length similar to that of bed, provide dry brick pitching 0.2 m thick on sides resting
on a wall of 0.4 m and 0.8 m deep.
Hydraulic Structures – Canal Falls January 16, 2011
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Design Example 2:
Design an unflumed non-meter straight glacis fall with the following data:
ix. Full supply flow rate . .
30. .
u scumec
d s
x. Drop 1 m
xi. Full supply depth . . 1.4
. . 1.4
u s m
d s m
xii. Full supply level . . 102
. . 101
u s m
d s m
xiii. Bed level . . 100.6
. . 99.6
u s m
d s m
xiv. Bed width . . 23
. . 23
u s m
d s m
xv. Side slopes 1 H: 1V
xvi. Safe exit gradient 1/6
Solution
1. Length of crest
Take crest length equal to the width of channel 23tL m
2. Crest level
3/21.84 tQ L D
3/230 1.84 23
0.8
D
D m
30
0.8823 1.4 1.4
aV m s
Velocity head 2 20.88
0.0395 say 0.042 2 9.81
aVm m
g
U.S. T.E.L. u.s. F.S.L. 102 0.04 102.04
Crest level u.s. T.E.L. 102.04 0.8 101.24 say 101.2
ah m
D m
Provide crest at level = 101.2 m
3. Width of crest
2 20.8 0.53
3 3tB D m
Provide crest width Bt = 0.6 m.
4. D.S. glacis
Glacis slope of 2 in 1 joined tangentially to the cistern floor with a radius equal to D = 0.8 m
shall be provided.
Hydraulic Structures – Canal Falls January 16, 2011
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5. Approach
a. U.S. glacis: Glacis slope may be ½ H: 1 V joined tangentially to the crest with a radius
equal to D/2 = 0.4 m.
b. Sides: The side walls may be splayed at an angle of 45º from the end of u.s. glacis and
extended by 1 m into the earthen bank from the line of F.S.L.
6. U.S. cutoff
Depth of cutoff 1 1.40.6 0.6 1.067 say 1.1
3 3
dm m
But minimum depth in Table 6.1 is 1.2 m. Therefore, provide 0.5 1.2m m deep cutoff wall.
7. Cistern
a. Depth of cistern
2
2
301.305
23
1.0
1.22 Fig. 2.7
L
q m s
H m
Ef m
21.25 1.525Ef m
R.L. of cistern
2d.s. T.E.L. 1.25
101 0.04 1.525
99.515
Ef
m
Or R.L. of cistern 2d.s. bed level 0.25
99.6 0.25 1.22 99.295
Ef
m
Provide cistern level at R.L. 99.2 m
b. Length of cistern
5 1.22 6.1cL m
Provide cistern length = 6.4 m
The cistern shall be joined to the d.s. bed in a slope of 1:5 in a length of 2 m.
c. D.S. cutoff wall
Depth of cutoff wall below bed 3 1.40.6 0.6 1.3
2 2
dm
Provide 0.5 1.5m m deep cutoff wall and the cutoff wall shall project above bed level by
3 1.40.14 say 0.15
10 10
dm m to act as a deflector.
d. Bed protection: Nil
Hydraulic Structures – Canal Falls January 16, 2011
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e. Side protection
Length of d.s. side protection33 3 1.4 4.2d m
Provide 0.2 m thick dry brick pitching in 4.5 m length beyond impervious floor. The
pitching shall rest on 0.4 0.8m m toe wall including 0.3 m thick foundation concrete.
f. Friction blocks: No friction blocks are required.
8. Total floor length and exit gradient
Exit gradient 1 6EG
Max. static head 101.2 99.6 1.6H m
1/2
2
1
1 1.6 1
6 1.5
4.15
2 1 1
7.23
E
HG
d
Total floor length required
7.23 1.5 10.85
b d
m
Provide total floor length = 14.2 m
9. Pressure calculations
Assume u.s. floor thickness = 0.3 m and d.s. floor thickness near d.s. cutoff = 0.6
a. U.S. cutoff
1
14.2 , 1.2
1 1.20.08
14.2
100 100 18 82%D D
b m d m
d
b
1
26
100 26 74%
0.3 correction due to floor thickness 82 74 2%
1.2
E
C
C
b. D.S. cutoff
Max (H) Cutoff depth (m) Length (m)
1.6 1.3 13.10
1.6 1.4 11.90
1.6 1.5 10.85
2.0 1.5 17.90
Hydraulic Structures – Canal Falls January 16, 2011
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1.5
14.2
d m
b m
1 1
1 1.50.106
14.2
30%, 20%E D
d
b
1
1
0.6 correction for depth 30 20 4%
1.5
corrected 30 4 26%
E
E
c. At the toe of glacis
% Pressure 76 26
26 8.413.2
57.82%
Floor thickness0.5782 1.6 0.4
1.061.25
m
Provide 1.1 m thick concrete floor at the toe of glacis in a length of 2 m.
At 2m from the toe of glacis
% Pressure 76 26
26 6.413.2
50.24%
Floor thickness0.5024 1.6 0.4
0.9631.25
m
Provide 1.0 m thick concrete floor at the toe of glacis in a length of 2.5 m.
At 4.4 m from the toe of glacis,
% Pressure = 41.15%
Provide 0.9 m thickness.
At 6.9 m, % pressure = 31.68%
Provide 0.8 m thickness until the end of the floor.
Hydraulic Structures – Canal Falls January 16, 2011