Hydraulic Structures Structures – Canal Falls
January 16, 2011
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.
1/2
Cistern length Lc 5 H L D
1 2/3 X H D L 4 Cistern depth X larger of X d c 3
d c
3
q2 g
Where: Lc is the length of cistern, X is the depression below d.s. bed, D is the depth of crest below u.s. T.E.L., H L is drop in meters.
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 Ef 2. increased by 25% of Ef
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Hydraulic Structures – Canal Falls
January 16, 2011
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 5 H :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 5 Ef 2 is provided for normal soils and 6 Ef 2 for sandy soils. Lc (5
6) Ef 2
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Hydraulic Structures – Canal Falls
January 16, 2011
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 Bt 0.55 D1 and minimum base width D1 2 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 Q 15 cumec , a trapezoidal section with top width Bt 0.55 D D1 with u.s. side slope of 1 H :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 1 H : 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 D Q C Lt D Bt
For submerged flow conditions (above 33% submergence) discharge passing over crest is: Q
2 3
Cd Lt
3/2 1/ 2 2g H L ha ha Cd Lt h2 2g H L ha
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Hydraulic Structures – Canal Falls
January 16, 2011
where, Lt Bt H L h2
= = = = ha = C =
Length of crest Width of crest Drop in water surface Depth of d.s. water level over top of crest Head due to velocity of approach Va2 2 g
1.84 for rectangular crest 2.26 for trapezoidal crest Crest level (free fall) d.s. F.S.L. ha D Crest level (Submerged falls) d.s. F.S.L. ha H L h2 Degree of submergence 1
hd D
T.E.L. D
T.E.L.
hd
H L
d
P
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. U.S. protection i. Brick pitching in a length equal to u.s. water depth should be laid on u.s. bed towards the crest at a slope 10 H :1V . U.S. curtain wall (cutoff) ii. 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).
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Hydraulic Structures – Canal Falls
January 16, 2011
3. Impervious concrete floor Total floor length and its disposition: i. 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: L p 10.53d c 4.877 1.5H L
For submerged falls with submergence greater than 33%, L p 6.77dc 5.182 H L
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%, Lc 3.8dc 0.415 H L
For submerged falls with submergence greater than 33%, Lc 5.2dc 1.067 H L
ii.
Depth of cistern ( X ) 1 2/3 X H L D 4 X larger of X d c 3
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. D.S. wings iv.
For Q > 15 cumec, d.s. wings are kept vertical for a length 5to8 D H L and may then be gradually warped. They should be taken up to the end of pucca floor.
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Hydraulic Structures – Canal Falls
January 16, 2011
Design Example 1: Design a Sharda type fall with the data given below: u.s. 10 cumec i. Full supply flow rate d .s. ii. Drop 1m u.s. 1.5 m iii. Full supply depth d .s. 1.5 m iv.
Bed level
v.
Bed width
u.s.
d .s. u.s.
99 m
d .s.
Side slopes vi. vii. Soil viii. Bligh’s coefficient
100 m 8m 8m
1 H: 1V Good loam 7
Solution 1. Length of crest
Take crest length Lt 8 m 2. Crest level
Since Q < 15 cumec, use rectangular crest with both sides vertical. 1/6
Q 1.84 Lt D3/2 D Bt
Assume Bt 0.8 m 10 1.840 8 D3/ 2 D1/ 6
1 1/6
0.8
D 0.776 m say 0.78 m
Velocity of approach (with 1:1 side slopes) Va 2
Velocity head
V a
2g
0.702
10
8 1.5 1
0.702 m s
2
2 9.81
0.025 m
U.S. T.E.L. u.s. F.S.L. ha 101.5 0.025 101.525 m R.L. of crest u.s. T.E.L. D 101.525 0.78 100.745 say 100.75 m
Adopt crest level = 100.75 m 3. Shape of crest i.
Top width Bt 0.55 D1 , D1 100.75 99 1.75 m
0.55 100.75 99 0.73 m Adopt Bt = 0.75 m. 6
Hydraulic Structures – Canal Falls
January 16, 2011
Check for Q, 1/6
10 1.84 8 D
1.67
1 0.75
D 0.741
ii.
Width of base 0.5 D1
0.51.75 0.875 say 1 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 X
d c
X
0.542
X
1
3
, and dc
3
3
q
2
2
3
g
1.25
9.81
0.542 m
0.181 m
or
ii.
4 1
H L D
2/3
2/3 1 0.75 0.21 m
4 Cistern depth X= 0.21 m say 0.25 m Length of cistern Lc 3.8dc 0.415 H L
3.8 0.542 0.415 1 3.47 m or 1/2
Lc 5 H L D
1/2
5 1 0.75 4.3 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 1.75 m 7
Hydraulic Structures – Canal Falls
January 16, 2011
Total floor length required 7 1.75 12.25 m Minimum d.s. floor length required L p 10.53dc 4.877 1.5H L
10.53 0.542 4.877 1.5 1.0 9.08 say 9.0 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 1.29 m 12.25 1.29 Floor thickness required 1.03 m 1.25 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 0.96 12.25 0.96 0.77 m Floor thickness required 1.25 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 0.51m 12.25 1.25 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 10 0.26 m 12.25 1.25 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
d 2 0.6 m
to a minimum of 1 m. Depth of curtain wall at d.s. end floor
1.5 2
0.6 1.35 m
Provide 0.4 m 1.4 m deep curtain wall. b. U.S. curtain wall
Depth of curtain wall at u.s. end floor
u.s. water depth
Provide 0.4 m 1.1m deep curtain wall. 8
3
0.6 0.5 0.6 1.1 m
Hydraulic Structures – Canal Falls
January 16, 2011
11. D.S. Protection a. Bed protection
Length of bed protection 3 D3 31.5 4.5 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.
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Hydraulic Structures – Canal Falls
January 16, 2011
Design Example 2: Design an unflumed non-meter straight glacis fall with the following data: u.s. 30 cumec ix. Full supply flow rate d .s. x. Drop 1m u.s. 1.4 m xi. Full supply depth d .s. 1.4 m xii.
u.s.
Full supply level
d .s. u.s.
xiii. Bed level xiv.
Bed width
xv. xvi.
Side slopes Safe exit gradient
d .s. u.s. d .s.
102 m 101 m
100.6 m
23 m
99.6 m 23 m
1 H: 1V 1/6
Solution 1. Length of crest
Take crest length equal to the width of channel Lt 23 m 2. Crest level Q 1.84Lt D3/2 30 1.84 23 D
3/ 2
D 0.8 m Va
30
23 1.4 1.4
0.88 m s
2
Velocity head
V a
2
2g
0.88
2 9.81
0.0395 m say 0.04 m
U.S. T.E.L. u.s. F.S.L. ha 102 0.04 102.04 m Crest level u.s. T.E.L. D 102.04 0.8 101.24 say 101.2 m
Provide crest at level = 101.2 m 3. Width of crest Bt
2
D
2
0.8 0.53 m 3 3 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. 10
Hydraulic Structures – Canal Falls
January 16, 2011
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 d 1.4 0.6 1.067 m say 1.1 m Depth of cutoff 1 0.6 3 3 But minimum depth in Table 6.1 is 1.2 m. Therefore, provide 0.5 m 1.2 m deep cutoff wall. 7. Cistern a. Depth of cistern q
30
1.305 m2 s
23 H L 1.0 m
Ef2 1.22 m Fig. 2.7
1.25 Ef2 1.525 m
R.L. of cistern d.s. T.E.L. 1.25 Ef 2
101 0.04 1.525 99.515 m Or R.L. of cistern d.s. bed level 0.25 Ef 2
99.6 0.25 1.22 99.295 m Provide cistern level at R.L. 99.2 m b. Length of cistern Lc 5 1.22 6.1m
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
d 3
2
0.6
1.4 2
0.6 1.3 m
Provide 0.5 m 1.5 m deep cutoff wall and the cutoff wall shall project above bed level by d 3
10
1.4 10
0.14 m say 0.15 m to act as a deflector.
d. Bed protection: Nil
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Hydraulic Structures – Canal Falls
January 16, 2011
e. Side protection
Length of d.s. side protection 3d3 3 1.4 4.2 m Provide 0.2 m thick dry brick pitching in 4.5 m length beyond impervious floor. The pitching shall rest on 0.4 m 0.8 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 G E 1 6 Max. static head H 101.2 99.6 1.6 m G E 1 6
H
1
Max ( H ) 1.6 1.6 1.6 2.0
d
1.6 1.5
1
4.15 2 1 1 2
Cutoff depth ( m) 1.3 1.4 1.5 1.5
Length ( m) 13.10 11.90 10.85 17.90
1/2
7.23 Total floor length required b d
7.231.5 10.85 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 b 14.2 m, d 1.2 m
1
d
1.2
0.08 b 14.2 D1 100 D 100 18 82% E 26 C 1 100 26 74% C correction due to floor thickness b. D.S. cutoff
12
0.3 1.2
82 74 2%
Hydraulic Structures – Canal Falls
January 16, 2011
d 1.5 m b 14.2 m
1
d
1.5
0.106 b 14.2 E1 30%, D1 20% E 1 correction for depth
0.6
1.5 E 1 corrected 30 4 26%
30 20 4%
c. At the toe of glacis % Pressure 26
76 26
13.2 57.82%
Floor thickness
8.4
0.57821.6 0.4 1.25
1.06 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 26
76 26
13.2 50.24%
Floor thickness
6.4
0.50241.6 0.4 1.25
0.963 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.
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Hydraulic Structures – Canal Falls
January 16, 2011
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