Design of Hydraulic Structures and Cross Drainage Works

To develop capability to perform the design of minor irrigation structures such as; cross drainage works, canal falls, regulators and prepare drawings. Also to impart knowledge on causes of failure and design criteria of hydraulic structures like dams and canal structures.

1. DESIGN OF HYDRAULIC
STRUCTURES
CET 306: Dr Arun

2. AIM
To develop capability to perform the design of minor irrigation
structures such as; cross drainage works, canal falls, regulators
and prepare drawings. Also to impart knowledge on causes of
failure and design criteria of hydraulic structures like dams and
canal structures.

3. Module I
- Diversion headworks : components and functions
- Weirs : Types and causes of failure
- Impervious floor of hydraulic structures : Bligh’s theory

- Design of vertical drop weir
- Design of impervious floor of hydraulic structures by Khosla’s theory

4. Introduction on different types of
Irrigation Structures

5. Different types of Hydraulic Structures
1. Dams & Reservoirs
2. Barrages/Regulators & Weirs
3. Bridges and Culverts

4. Siphons and Aqueducts

6. Dams can vary in shape and size according
to the purpose

7. Farakka Barrage over River Ganga

8. Difference between Dams and Barrages
Dams
➢Used for storage of water.
➢Has spillways for overflow.
➢Multi purpose
➢Water Storage
➢ Power production
➢ Flood Control
➢ Constructed all over the world.
Barrages
➢Used for diversion of water.
➢Has gates to control water level.
➢Not for Storage Purpose, sends calculated amount
of water to canals (Control through Gates).
➢Mainly popular in India, Pakistan, Iran and Egypt.

9. Different types of Spillways

11. Spillway Tainter Gates

12. Typical Plan &
Cross Section of a Barrage
/Sluice Gates

13. Weirs

14. The Design and Construction Failures are very costly and
its better to learn from someone else’s mistakes !

15. Water Way +
Road Way
Water Way +
Water Way
Culverts/
Bridges
Siphons/
Aqueducts

16. Culverts

17. Bridges

18. Crossing El-Salam Canal with Suez Canal through a siphon

19. Pont du Gard Aqueduct (UNESCO World Heritage Site)

21. Diversion Head Works; Weirs & Barrages

24. Diversion Head Work: A birds eye view

25. Purposes of Diversion Head work
➢Raise water level to increase command area
➢Regulate water to canal
➢Controls silt entry to canal
➢Acts as a relief in shortages

26. Weir
➢In the case of weir, most of the raising of water level or ponding
is done by the solid weir wall and very little by the shutters.
➢It may be provided with small shutters (gates) on its top.
➢The difference between the pond level and the crest level is
less than 1·5 m

27. Types of Weir
➢Vertical Drop
➢Concrete Sloping
➢Dry stone Sloping
➢Parabolic Weir

28. Barrage
➢ A barrage has a low crest wall with high gates.
➢ As the height of the crest above the river bed is low most of the
ponding is done by gates.
➢ During the floods the gates are opened so afflux is very small.
➢ The difference between the pond level and the crest level is greater
than 1·5 m.

31. Why do failures of hydraulic
structures happen?
➢ Barrages and Weirs more prone to failures ?
➢ Dams face less challenges ?
➢ Difference in hydraulic & structural design ?

32. Failure of weir or barrage on permeable foundation
➢ Diversion Head works are usually provided in the reaches of a river
where permeable foundation exists.
➢ Barrages/weirs constructed on these foundations are subjected to
surface flow and seepage flow.
➢ The seepage flow may cause the failure of weirs/barrages by
➢ Piping or undermining
➢ Rupture of floor due to uplift
➢ Rupture of flow due to suction caused by standing wave (Hydraulic Jump)
➢ Scouring on the u/s and d/s side of weirs/barrages

33. Causes of failure of weir or barrage on
permeable foundation
If the hydraulic gradient at exit is more than the critical gradient for the subsoil,
the soil particles will move with the flow of water thus causing degradation,
resulting in cavities and ultimate failure. This is called undermining or piping or
sand boiling.

34. Hydraulic gradient between two points give the head loss per unit length

37. 1.Piping or undermining
➢Hydraulic gradient or exit gradient greater than the critical value for the foundation soil,
the soil starts boiling at the exit point.
➢Boiling of the soil indicates lifting of the soil against gravity and it happens only when
exist gradient of seeping water is greater than the safe limit for the foundation soil.
➢The soil gets wash out with boiling water.
➢With washing out of some soil from D/S side, the exit gradient increase and boiling of
soil start with even more vigor.
➢This process of erosion of soil from below the foundation, progressively, works backward
towards the U/S.
➢This process ultimately develops a channel or pipe, below the foundation of the weir
and causes failure of the weir.

38. ➢Provide sufficient length of impervious floor so that path of
percolation is increased and exit gradient is reduced.
➢Provide sheet piles at u/s and d/s to increase the flow path.

39. Prevention of Piping Failure

40. Sheet Piles

41. 2. By Uplift Pressure
➢Seeping water through the foundation, exerts uplift pressure, on the floor.
➢The uplift pressure is maximum at the point, just D/S of the weir wall or crest
wall, when water is full up to the top of the gates and there is no water on the
D/S side.
➢Hence critical section the floor is just at the D/S side of the weir’s chest wall.
➢If the thickness of the floor is insufficient, its weight would be inadequate to
resist the uplift pressure.
➢This may ultimately lead to bursting of the floor and thus failure of the weir may
occur.
➢Once the floor is burst due to uplift pressure the effective length of seepage is
very much reduced which causes a further increase in the exit gradient and
consequent failure by piping.

42. Failure due to Uplift can be avoided by
(i) Sufficient length of the impervious floor.
(ii) Provide impervious floor of sufficient thickness.
(iii) Provide a cut-off pile at the U/S end so as to reduce the effect of
uplift throughout the foundation.

43. 3. By Suction due to Standing Wave of Hydraulic Jump
➢Standing waves of hydraulic jump formed at the D/S of the weir cause suction.
➢This suction increases the effect of uplift.
➢If floor thickness is inadequate to sustain the combined effect of uplift and
suction, it will fail.
➢Such failures occurred at Maralaweir on the Chenab and Rasul weir.
Remedial measures may be the following:
(i) Provide increased thickness of floor to resist the additional effect of the
standing wave.
(ii) Floor should be laid in one layer of concrete, instead of several, thin layers of
masonry.

44. Standing Wave of Hydraulic Jump

45. 4. By scour on U/S and D/S of the weir.
➢The beds of alluvial rivers are scoured to considerable depths
especially during floods.
➢This scouring may take place on the U/S side also, but it is most
likely on the D/S side of pucca floors.
➢If this scour is allowed unchecked, it will form scour holes
underneath the pucca floor.
➢These holes may slowly progress towards the main weir and cause
its failure.
Failure can be prevented by adopting the following measures:
(i) Providing deep piles at U/S and D/S ends of rigid floor. The piles
should be taken much below the calculated scour depth.
(ii) Suitable lengths of launching aprons, both at U/S and D/S of
impervious floors should be provided.

46. Bligh’s Creep Theory
Assumptions
➢ Hydraulic gradient is constant throughout the impervious length of apron.
➢ The percolating water creeps along the contact surface of the base profile
of the structure with sub soil. Path traversed by the percolating water is
creep length (L).
➢ Head loss is proportional to the creep distance travelled.
➢ Mr Bligh made no distinction between the creep in the horizontal
direction along the floor of weir and in the vertical direction along theface
of sheet piles.
➢ Water percolation cannot be stopped along the creep length unless the
sheet piles reach an impervious floor.

47. Bligh’s Creep Theory
Creep length, L = l

48. Bligh’s Creep Theory
Creep length, L = 2d
1+ L +2d
2

49. Percolation Coefficient
If H is the total head loss, the loss of head per unit length of creep, c is
i) c = H/L
ii) c = H/(2d
1+l+2d
2)
c is known as percolation coefficient.
Coefficient of Creep = 1/c = C

51. Design Criteria for Subsurface Flow
1. Safety Against piping- Length of creep should be sufficient to provide
a safe hydraulic gradient as per type of soil.
L, creep length = CxH

53. Minimum thickness to prevent uplift , t = (4/3) *h/ (ρ-1)
ie , t= 1.33* (h/(ρ-1))

54. Limitations of Bligh’s Creep Theory
1.No distinction between horizontal & vertical creep.
2.Bligh’s method holds good as long as horizontal distance between
the piles is greater than twice their depth.
3.He couldn’t explain exit gradient.
4.Made no distinction between outer & inner faces of sheet piles.
5.Loss of head is not proportional to creep length, but he assumed it
that way.
6.Bligh’s does not specify the absolute necessity of d/s sheet pile.

56. A B C

58. Minimum thickness to prevent uplift , t = (4/3) *h/ (ρ-1)
ie , t= 1.33* (h/(ρ-1))
t = 1.33* (2.56/(2.24-1)) = 2.76 m @ A
Similarly find required thickness at B and C.

61. Assignment
1. Design a vertical drop weir on Bligh’s theory for the following site
conditions
(a) Maximum flood discharge - 3200 cumecs
(b) HFL before construction - 25.0 m
(c) Min water level (d/s bed level ) - 278.0 m
(d) FSL of canal - 24.0 m
(e) Allowable Afflux - 1 m
(f)Coefficient of Creep - 12
(g)Permissible exit gradient - 1 in 6
Please ref to IS: 6966 Part I also to get more clarity.

62. Criteria for Design of Weirs & Barrages
1.Hydraulic Design 2. Structural Design
These depends on
1. Pond Level
2. Afflux
3. Level of U/S floor crest of weir
4. Shape of Weir crest
5. Water Way
6. Effect of Retrogression
7. Discharge and Discharge Intensity
8. Energy dissipation

63. Pond level in the U/S of the canal head regulator may be obtained by adding
the working head and designed full supply level in the canal. (Ref IS 6966
Part I)
Afflux may be defined as the rise in water level on the U/S of a weir or
barrage over and above that on the D/S caused under free flow conditions
as a result of construction of weir or barrage on the river. (Ref IS 6966 Part I)
Crest level should be at pond level for weirs without shutters.
Cross section is Trapezoidal for Vertical Drop Weirs.
Length of Water way is equal to length of weir or barrage to pass safely the
max flood discharge.
Retrogression is the degradation of the downstream river bed.

67. Khosla’s Theory
➢ The seepage of water does not creep along the surface of hydraulic
structure as stated by Bligh but this water moves along a set of
stream lines. This steady seepage in a vertical plane for a
homogeneous soil can be expressed by Laplacian equation,

➢ Equation represents 2 sets of lines which intersect orthogonally,
called as stream lines and equi potential lines (Flow net).
ⅆ
2
??????
ⅆ �
2+
ⅆ
2
??????
ⅆ �
2 = 0
??????= flow potential

68. Stream lines represent path along which water flows through sub soil.
Equi potential lines join points of equal residual head.

69. Critical Exit Gradient of Soil, GEC
GEC = ( G - 1 ) ( 1 - n ) where S is the
specific gravity of the soil and n is its porosity.
G= 2.65 , for most river sand and avg value of porosity is 0.4
Critical Exit Gradient of Soil = (2.65-1) (1-0.4) = 0.99 or 1.0
From safety point of view, the Safe Exit Gradient
GES = 1.0/4= 0.25 or 1 in 4

94. 0.105
29
20

103. 0.181

106. 0.18