Hydraulic Structure-II, Lecture CHAPTER Three

1. Chapter-3
3. DIVERSION HEAD WORKS
1

2. INTRODUCTION
• Diversion head works are structures constructed across
a river to facilitate a regulated and continuous diversion
of water into the off-taking canal.
• In rivers, it is hardly possible to divert a regulated and
continuous flow into main canal without such
headwork.
• This is due to the fact that the flow in the river is never
uniform and varies from season to season.
2

3. INTRODUCTION
• Thus, there is a need to regulate the flow into
the canal system in order to ensure a
continuous diversion of water.
• There is practically no storage provided by a
diversion structure.
• The purpose is to raise and keep the water
level more or less constant (reduce the
fluctuation of water levels) at the head of the
canal.
3

4. TYPES OF DIVERSION STRUCTURES
• Diversion head works can be classified as
weirs and barrages based on the structures
provided at the crest.
• Weir: A weir is a barrier (structure) constructed
across a river to raise the water level in the river
behind it so as to enable regulated diversion of water.
• A weir has a raised crest behind which a small
ponding of water will take place.
• Can be constructed with or without shutters on the
crest of the weir.
4

5. Weir
• Types of weirs:
The following types based on the geometry of the
crest and materials used for construction
– Masonry weirs with vertical or slightly sloping u/s and d/s
face
– Rock fill weirs
– Concrete weirs with sloping glacis
– Ogee crest weirs
5

6. 1. Masonry vertical drop weirs
• Have a horizontal floor constructed of masonry and a crest wall
with vertical or slightly sloping downstream face.
• The height of the crest depends on the actual site conditions and
head required behind the weir.
• Sheet pile walls are driven at the upstream and downstream of the
floor.
• Are suitable whenever the drop in water level is small.
6

7. Vertical drop weirs
7

8. 2. Rock fill weirs
• Are constructed of rocks with extremely sloping downstream
face.
• Are suitable whenever there is excess stone available for
construction.
8

9. 3. Concrete sloping weir
• The crest of this weir has sloping glacis both on the upstream and
downstream.
• Cutoff sheets are provided at the upstream, intermediate and
downstream of the floor to the depth equal to the scour depth.
• Hydraulic jump is formed on the downstream slope for energy
dissipation.
• These weirs are suitable whenever the drop in water level is large.
9

10. Glacis weirs
10

11. Ogee crest weirs
• Is a weir whose crest wall is rounded to increase the
discharge coefficient.
• It consists of a concrete weir wall with vertical upstream
face and rounded top and downstream.
• It is designed as gravity section similar to vertical drop
weir.
11

12. Gravity and non-gravity weirs
• Seepage water causes uplift force on the base of the weir.
• Whenever the weight of the weir is sufficient to balance
the uplift pressure caused by seeping water, it is called
gravity weir.
• When the concrete slab (floor) is designed continuously
with the weir body to keep the structure safe against
uplift, it is called as non-gravity weir.
12

13. Barrage
• A barrage is also an obstruction constructed across a
river for raising the water level and regulate the
diversion of water to canals.
• However, the crest wall of a barrage is low and ponding
of water takes place by gates.
• The gates are fitted on the top of the crest wall and can
be closed and opened as required based on the flow in
the river.
13

14. Barrage
14
Piers
Gates

15. Advantages and disadvantages of weirs and
barrages
Weir:
Advantage
• Low initial cost
Disadvantage
• High afflux (increase in water level) during floods;
• Siltation or sedimentation problem due to relatively high
crest;
• Lack of effective control during floods.
15

16. Barrage
Advantage
– Effective control of flow is possible;
– Afflux and thus flooding is small during floods;
– Silt inflow into the off-taking canal can be
effectively controlled.
Disadvantage
– It has a disadvantage that its initial cost is high.
16

17. Some technical considerations for
diversion headwork's
When planning a new diversion headwork, investigations
to be made can be classified into:
– Reconnaissance study
– Preliminary investigation
– Detailed investigation
The technical considerations include:
• Location of headwork's
• Construction materials and resources
• Topographic survey
• Soil investigation
• Hydrological data
17

18. 1. Location of Headworks
• For best site of diversion headwork, one has to have
clear information of the site.
• Generally topographic maps are required for the
purpose.
• However, one can also have a walk along the river to
find out possible sites for the headwork during the
reconnaissance study.
18

19. Factors when selecting site for diversion
weirs.
• Location of the Irrigated Area
– If the area is too far away from the headwork, it necessitates
construction of long canals with high cost
– If close to the headwork, some area located on the upper
reach of the canal may not be commanded.
• Stability of the river bank
– Affects the cost and the performance.
– Ideal site: straight reach of a river with stable and narrower
section.
– River banks are unstable in shallow and wide cross sections;
thus larger and costlier structure is needed.
– Flow velocity at these sections is small and results in more
sedimentation and problem on the performance.
19

20. 2. Construction materials and resources
• Most important factor for the selection of construction
materials is the economic factor.
• Questions to be made during site visit of diversion
headwork's.
– What are the construction materials available in the area?
– Is there a shortage of required construction material in the
local market?
– Is it possible to hire construction machinery in the area?
– What is the availability of skilled labor in the area and the
rates?
20

21. 3. Topographic survey
• After site selection, the designer has to be able to have a
detailed in formation of the cross section and profile of the river
at this section.
• Thus a topographic survey of the site is needed. Moreover, the
topography of the command area is needed to determine
whether the highest spot points can be irrigated by gravity from
the selected site or not.
• Particularly, this is important in flat areas where head is not
available.
• Whenever, the site of the diversion is sufficiently higher than
the command area, loss of head is a not a problem.
21

22. 4. Soil investigation
• Preliminary soil investigation is needed during the first
visit of the site.
• The soil can be visually tested and its physical
characteristics described. Shallow pits can be dug to
describe the soil profile.
• The investigation is important to judge the suitability
of the soil for foundation, its seepage condition and
bearing capacity.
22

23. 5. Hydrological data
• Is needed in order to determine design discharges.
• The size of the structure depends on the maximum flood
discharge that has to pass over the structure.
• Moreover, the minimum flow in the river is also needed for the
design.
• The design engineer visiting the site for the first time has to find
out if there are river gauging and meteorological stations in the
area. If not, the local people can provide useful information on
the maximum and minimum flows.
23

24. Components and Layout of Diversion Headworks
Diversion headwork's generally consist of the following
components:
– Weir wall/Barrage
– Under sluices
– Divide wall
– Canal head regulator
– Silt excluder
– Guide banks
– Wing walls
24

25. Components and layout…
25

26. Under sluices
• Adjacent to the canal head regulators, under sluice section is
provided.
• When canal intake is only in one direction, the under sluice is
provided on that side only.
• There is a divide wall between the weir body and the under sluice
section to separate the two portions and to avoid cross flows.
• Its crest is at lower level than that of the crest of the weir (usually
at river bed.)
26

27. Functions of under sluice
• Maintains well defined river channel near the head regulator;
• To scour (remove) away the silt deposited in front of the head
regulator;
• To pass small floods of 10% to 20% Qd during rainy season;
• To quickly lower the u/s high flood level because the discharge
intensity over the sluice portion is greater than that in the weir
portion;
• To minimize the effect of main river water current on the head
regulator.
27

28. Divide wall
• This is a wall placed parallel to flow direction in river.
• Separates the weir section from the under sluice section
of the headwork to avoid cross currents.
• On the upstream, it extends to little upstream of the head
regulator and on the downstream it usually extends to the
end of loose protection.
28

29. Functions of divide wall
• Separates the floor of the under sluices and weir (floor of under
sluice at a lower level);
• Provides a clear pocket near the head regulator where silt can
accumulate;
• Isolates the silt accumulation pocket to ensure scouring;
• Helps to avoid cross currents which might cause deep scour of the
river bed;
• Helps to concentrate the scour action of the under sluices on only
the silt accumulation pocket;
• Minimizes the effect of river current on the head regulator.
29

30. Canal head regulator
• A structure provided at the head of the off-taking canal
to regulate and control the inflow into the canal.
• Usually provided at one or both banks of the river with
its axis making an angle 900 to 1200 to the weir axis.
• It will be sized in such a way that it can pass the
required design discharge of the canal when the water
level on the upstream is at the pond level.
30

31. Layout of head regulator
31

32. Functions of head regulator
• Regulates the supply of water into the off-taking canal;
• Controls silt entrance into the canal;
• Prevents flood water from entering the canal;
• Used to stop the water supply into the canal for:
– maintenance and
– when highly silt-laden water flows in the river.
32

33. Section through head regulator…
33

34. Head regulator
34

35. Silt excluder
• Provided in the under sluices portion to pass highly silt-
laden water through the under sluices.
• It enables only relatively clear water to enter the canal.
• Aligned at right angle to the axis of the canal.
• They are small lined tunnels through which the bottom
silt-laden water will be passed down to the scouring
sluices.
35

36. Alignment of silt excluder
36

37. Guide banks
• Are rigid structures provided on either side of the
headwork to:
– guide the river flow directly to the headwork and
– to avoid scouring and meandering of the river near the work.
• Particularly important when the headwork is located
near alluvial banks of the river where bank scouring and
meandering are evident.
• Wing walls (Marginal bunds): are used to protect valuable
areas and property from flooding.
37

38. PRINCIPLES FOR DESIGN OF DIVERSION
HEADWORKS
• The design of any hydraulic and irrigation structures have
to consider the hydraulics of surface and subsurface flow.
• Subsurface flow (Seepage) Theory
• seeping water under the structure causes upward uplift
pressure on the base of the structure.
• For safe design of hydraulic and irrigation structures on
permeable foundation, the hydraulic gradient should be
less than some allowable limit called critical hydraulic
gradient.
• There are some theories developed on seepage of water
under the foundation of hydraulic and irrigation structures.
• 38

39. 1. Bligh’s Creep Theory
• This theory assumes that the seeping water creeps from
the upstream to the downstream of the structure along
the contact base of the soil with the structure.
• The length of seepage path traversed by the seeping
water is called creep length (L).
• One of the shortcomings of the Bligh’s theory is that it
does not make differences between vertical and horizontal
creep.
• According to Bligh’s creep theory, the hydraulic gradient
(i) is constant throughout the seepage path.
39

40. Bligh’s Creep Theory
40

41. Bligh’s Creep Theory
41
• Considering the above figures, the hydraulic gradient of
seepage is given by:
• Where H is the seepage head (difference in water level
between upstream and downstream)
• L is the creep length:
• The uplift pressure u at any point along the
seepage path is given by:

42. Bligh’s Creep Theory
42
• Where ;
ɣ is unit weight of water and
h is the residual head at the point.
• The residual head at any point p is determined from:
h= H-Head loss from upstream end to p
• Or it is equal to the head loss between point p and the
downstream end,
• Where l’ is floor length from point p to the downstream
end.

43. Bligh’s Creep Theory
Design criteria based on Bligh’s creep theory
• The hydraulic gradient should be less than permissible
value
43

44. Bligh’s Creep Theory
• The floor thickness and weight should be sufficient to
withstand the uplift pressure.
44

45. 2. Khosla’s Theory
• A.N. Khosla and his associates investigated the actual pressure on
the base of structures and they found out that the actual up lift
pressures were different from those determined on the basis of
Bligh’s theory.
• According to him, the uplift pressure at any point, at distance X from
the entry point of the impervious floor is given by;
45

46. Khosla’s Theory
46
• Uplift pressure at key points
• Pile at d/s end

47. Khosla’s Theory
• Pile at u/s end
47

48. Khosla’s Theory
• Intermediate pile
48

49. Example: determine the uplift pressure heads at points E, D & C
in the following figure. Determine the exit gradient (GE).
49

50. Khosla’s Theory
50

51. Khosla’s Theory of Independent Variables
• The pressures obtained for each individual standard forms
are then super-imposed to determine the pressure at the
key points for the whole structure.
• Assumptions made: The following assumptions are made
in the solution of the elementary forms.
• The floor thickness is negligible
• Only one pile at a time
• Horizontal floor
• Because of the above assumptions, corrections to the
super-imposed pressures are applied
51

52. Correction for floor thickness
52
• Where øE øC’ øD are the pressure head or seepage head
expressed in percentage.
• For example; øE=
• The pressures at E’ and C’ can be obtained assuming
linear variation of pressure from top to bottom of pile.

53. con’t…
• E’ is on d/s of E along the flow path indicated ØE’< E
• C’ is on u/s of C along the flow path, ØC’> C
• Thickness correction at E
• Thickness correction at C =
• Generally: - correction positive – for C & Negative for E.
53

54. Correction for Mutual interference of pile
54
• when there are more than one pile line, there will be interference
between the pile
• correction for mutual interference between pile is given by ;
• Where: C = is percentage correction in pressure
b1 = is the distance between the two piles
b = is the total length of impervious floor .
d = is the depth of the pile on which effect is required
D = is the depth of the pile whose effect is required

55. Correction for slope of the floor
55

56. Con’t….
56

57. Causes for Failure of weirs on permeable
foundations
• Failure of weir can be due to sub – surface flow & surface
flow.
• Failure due to sub – surface flow: Failure can be by piping
or rupture due to uplift.
Piping failure- is a failure when the seeping water takes
place under high hydraulic gradient ic and thus more
seepage force
• Failure by uplift: the seeping water under the floor
exerts an upward take (pressure) on the floor
called uplift pressure
57

58. Con’t….
• Failure due to surface flow
• Such failure can be either due to suction pressure or scour.
• Suction pressure failure: when hydraulic jump forms on the
d/s glacis the water surface in the hydraulic jump trough is
lower than the subsoil HGL and thus additional thickness of
floor is required to balance this pressure.
• Failure by scour: during high flow, scouring of the river bed
occurs both on the u/s and d/s of the weir .Thus sheet piles
have to be provided to a depth of maximum scour depth
and appropriate protection works should be provided on
both sides. 58

59. Design of weir and under sluice bay sections
• The discharge which passes over the weir and
sluice bay sections has to be determined first.
• Discharge over the sluice bay should at least equal
to the larger of the following:
• Twice the Q design of the off taking canal
• 20% of the design discharge of the wire ( 80%
over weir bay)
59

60. Procedure for design
1. Fix the discharge over the sections of weir and under
sluice bays
2. Fix the crest levels of the weir and under sluice sections.
3. Fix the water ways of the weir and under slice sections
4. Determine the characteristics of the hydraulic jump with
and with out retrogression. When high flood passes over
the weir , the d/s bed of the river erodes and lowering
takes place called retrogression
5. Calculate the normal scour depth, R and determine the
upstream and downstream pile depths.
6. Fix the total length of the impervious floor from exit
gradient consideration. Fix the levels of the floors on
upstream & downstream 60

61. Procedure for design
7. calculate the uplift pressures at the key points of the piles
for
 No flow condition (Khosla's theory is generally used for sloping
glass weirs)
 High flood condition
8. Determine the uplift pressure from the subsoil HGL for no
flow condition and suction pressure in high flood condition.
9. Determine the floor thickness at various points for the
determined uplift pressure in step8
10. Provide protection works (block protection and launching
apron) both on the u/s and d/s side.
61

62. Vertical drop weir
• Such a weir consists of a crest wall with nearly vertical d/s
face. Generally hydraulic jump is not formed and energy is
dissipated by vertical impact of water.
• Design data
- Design discharge - High flood level before
construction
- Bed level of river - FSL of off taking canal
- Silt factor (f) - critical exit gradient(GE)
-Retrogression - stage –discharge r/ship
at the site
• The design of a vertical drop weir is generally made by
Bligh’s theory and the thickness of floor checked by
Khosla’s theory. 62

63. Procedure
1. Determine the water way from Lacey’s perimeter
• L= P = 4.75 * Q, Q = design discharge
2. Determine discharge intensity , q = Q/L
3. Determine the normal scour depth from, R = 1.35 ( q2/f )1/3
4 . Regime velocity of flow. V = q/R
Determine velocity head from ha = V2/2g.
5. TEL upstream of the weir and d/s of the weir from:
D/s TEL = HFL before construction + ha
U/s TEL = D/s TEL + Afflux
U/s HFL = U/s TEL – ha
63

64. 6. Head over the crest can be determined with a broad
crested weir formula : q = 1.70 *(He) 3/2
7. Determine the crest level: crest level = u/s TEL –He
8. Pond level can be determined from
pond level = crest level = u/s TEL –He = FSL of canal +
modular head .
9.Determine the depth of u/s and d/s piles from
– u/s pile depth = 1.5 * R
– d/s pile depth = 2.0* R
10. Determine the maximum seepage head for the worst
condition ( WL on u/s at pond level and no tail water )
from :
Hs = pond level - d/s bed level 64
Procedure

65. Procedure
65

66. Procedure
66

67. Procedure
67

68. Procedure
68

69. Procedure
69

70. Procedure
70

71. Procedure
71

72. 72