diversionheadworksajithamiss-140806045330-phpapp02.ppt

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1. DIVERSION HEADWORKS

2. CANAL HEADWORKS
Canal head works
–Structures/works constructed across river and at the
head of the off taking canal
Canal head works
Diversion head works
To raise water level in
river and divert the
required quantity
Storage head works
To store water on u/s
of river and divert the
required quantity

3. DIVERSION HEADWORKS
Purposes
–Raises water level in the river
–Regulates supply of water into the canal
–Controls the entry of silt into the canal
–Provides some storage for a short period
–Reduces the fluctuations in the level of supply in river

4. TYPES OF DIVERSION HEAD WORKS
1. Temporary diversion head works
–Consists of a bund constructed across river to raise the
water level in the river and will be damaged by floods
2. Permanent diversion head works
–Consists of a permanent structure such as a weir or
barrage constructed across river to raise water level in
the river

5. LOCATION OF CANAL HEAD WORKS
Depends on the stages of flow (reaches) of river
(i) Rocky stage
(ii) Boulder stage
(iii) Trough stage or alluvial stage
(iv) Delta stage
Both rocky and delta stages are not suitable for
location of diversion head works

6. SUITABLE SITE FOR DIVERSION
HEAD WORKS
Having selected the reach of the river, selection
suitable site in accordance with the following
considerations
1. As far as possible, a narrow, straight, well defined
channel confined between banks not submerged by the
highest flood
2. Should be possible to align the off taking canal in such a
way that command of its area is obtained without excessive
digging

7. 3. Materials of construction such as stone, sand etc. should
be available in the vicinity of the site
4. Site should be accessible by rail or road
SUITABLE SITE FOR DIVERSION
HEAD WORKS

8. COMPONENTS OF DIVERSION
HEADWORKS

9. 1. Weir or Barrage
2. Divide wall or divide groyne
3. Fish ladder
4. Pocket or approach channel
5. Under sluices or scouring sluices
6. Silt excluder
7. canal head regulator
8. River training works such as marginal bunds and guide
bunds
COMPONENTS OF DIVERSION
HEADWORKS

10. WEIR
Weir is a structure constructed across river to raise
the water level and divert the water into the canal
Weir aligned at right angle to the direction flow
Shutters are provided at the crest of the weir so
that part of raising up of water is carried out by
shutters

11. According to the material used for construction
and certain design features
1. Masonry weirs with vertical drop walls
2. Rock fill weirs with sloping aprons
3. Concrete weirs with a downstream glacis
CLASSIFICATION OF WEIRS

12. MASONRY WEIR WITH VERTICAL
DROP

13. Weir consists of
–Impervious horizontal floor or apron
–A masonry weir wall with either side vertical; or both
faces inclined; or u/s face vertical and d/s face inclined
–Curtain walls or cutoffs or piles are provided at the u/s
and d/s ends of the floor
–Block protection at the u/s end and graded inverted
filter at the d/s end
–Longing aprons or pervious aprons after block
protection graded filter
MASONRY WEIR WITH VERTICAL
DROP

14. ROCKFILL WEIRS WITH SLOPING
APRONS

15. Weir consists of
–A masonry weir wall
–Dry packed boulders laid in the form of glacis or
sloping aprons
–Some intervening core walls
ROCKFILL WEIRS WITH SLOPING
APRONS

16. CONCRETE WEIRS WITH
DOWNSTREAM GLACIS

17. Floor made of concrete
Sheet piles of sufficient depth provided at the u/s and d/s
ends
Sometimes intermediate piles are also provided
Hydraulic jump is developed at the d/s slope due to which
considerable amount of energy is dissipated
Suitable on pervious foundations
CONCRETE WEIRS WITH
DOWNSTREAM GLACIS

18. BARRAGE
Crest is kept at a low level
Raising up of water level is accomplished by means of
gates
During floods, these gates are raised and clear off the high
flood level

19. CAUSES OF FAILURES OF WEIRS ON
PERMIABLE FOUNDATIONS
Causes of failures
–Due to seepage or subsurface flow
–Due to surface flow

20. Due to subsurface flow
–Piping or undermining
–By uplift pressure
Due to surface flow
–By suction due to hydraulic jump
–By scour on the u/s and d/s of the weir
CAUSES OF FAILURES OF WEIRS ON
PERMIABLE FOUNDATIONS

21. DESIGN OF IMPERVIOUS FLOOR FOR
SUBSURFACE FLOW
Bligh’s creep theory
Khosla’s theory

22. BLIGH’S CREEP THEORY
Design of impervious floor or apron
–Directly depend on the possibilities of percolation in
the porous soil on which the apron is built
Bligh assumed that
–Hydraulic gradient is constant throughout the
impervious length of the apron
–The percolating water creeps along the contact of base
profile of the apron with the sub-soil, losing head
enroute, proportional to the length of its travel
–Stoppage of percolation by cut off (pile) possible only
if it extends up to impermeable soil strata

23. Bligh designated the length of travel as ‘creep
length’ and is equal to the sum of horizontal and
vertical length of creep
BLIGH’S CREEP THEORY

24. If ‘H’ is the total loss of head, loss of head per unit length
of creep (c),
c-percolation coefficient
Reciprocal of ‘c’ is called ‘coefficient of creep’(C)
BLIGH’S CREEP THEORY

25. Design criteria
(i)Safety against piping
Length of creep should be sufficient to provide a safe
hydraulic gradient according to the type of soil
Thus, safe creep length,
Where, C= creep coefficient=1/c
BLIGH’S CREEP THEORY

26. Design criteria
(ii) Safety against uplift pressure
Let ‘h
’
’ be the uplift pressure head at any point of
the apron
The uplift pressure = wh
’
This uplift pressure is balanced by the weight of
the floor at this point
BLIGH’S CREEP THEORY

27. If, t =thickness of floor at this point
G = specific gravity of floor material
Weight of floor per unit area
=
BLIGH’S CREEP THEORY

28. BLIGH’S CREEP THEORY

29. LIMITATIONS OF BLIGH’S THEORY
Bligh made no distinction between horizontal and vertical
creep
Did not explain the idea of exit gradient -safety against
undermining cannot simply be obtained by considering a
flat average gradient but by keeping this gradient will be
low critical
No distinction between outer and inner faces of sheet piles
or the intermediate sheet piles, whereas from investigation
it is clear, that the outer faces of the end sheet piles are
much more effective than inner ones

30. Losses of head does not take place in the same
proportions as the creep length. Also the uplift
pressure distribution is not linear but follow a sine
curve
Bligh did not specify the absolute necessity of
providing a cutoff at the d/s end
LIMITATIONS OF BLIGH’S THEORY

31. LANE’S WEIGHTED CREEP THEORY
An improvement over Bligh’s theory
Made distinction between horizontal and vertical
creep
Horizontal creep is less effective in reducing uplift
than vertical creep
Proposed a weightage factor of 1/3 for horizontal
creep as against the 1 for vertical creep

32. KHOSLA’S THEORY
Dr. A. N. Khosla and his associates done
investigations on structures designed based on
Bligh’s theory and following conclusions were
made
–The outer faces of sheet piles are much more effective
than inner ones and the horizontal length of floor
–The intermediate sheet piles, if smaller in length than
the outer ones were ineffective

33. –Undermining of floors started from the tail end. If
hydraulic gradient at exit is more than the critical
gradient, soil particles will move with water and leads
to failure
–It is absolutely essential to have reasonably deep
vertical cutoff at the d/s end to prevent undermining
KHOSLA’S THEORY

34. Khosla and his associates carried out further
research to find out a solution to the problem of
subsurface flow and provided a solution
–Khosla’s theory
–Considered the flow pattern below the impervious base
of hydraulic structures on pervious foundations to find
the distribution of uplift pressure on the base of the
structure and the exit gradient
KHOSLA’S THEORY

35. KHOSLA’S METHOD OF
INDEPENDENT VARIABLES
A composite weir section is split up into a number
of simple standard forms
The standard forms
(a) A straight horizontal floor of negligible thickness
with a sheet pile either at the u/s end or at the d/s end of
the floor

36. (b) A straight horizontal floor of negligible thickness with
a sheet pile at some intermediate point
(c) A straight horizontal floor depressed below the bed but
with no vertical cutoff
KHOSLA’S METHOD OF
INDEPENDENT VARIABLES

37. These standard cases were analyzed by Khosla
and his associates and expressions were derived
fordetermining
–The residual seepage head (uplift pressure) at key
points (key points are the junction points of pile and
floor, bottom point of pile and bottom corners of
depressed floor)
–Exit gradient
–These results are presented in the form of curves
KHOSLA’S METHOD OF
INDEPENDENT VARIABLES

38. The curves gives the values of Φ(the ratio of
residual seepage head and total seepage head) at
key points
The directions for reading the curves are given on
the curves itself

42. The curves are for specific cases only
In actual practice
–consider the assembled profile with piles at u/s end, d/s
end, intermediate point, floor has some thickness and
slope
–combination of simple profiles needs to be considered
–Corrections need to be applied
1. Correction for thickness of floor
2. Correction for mutual interference of piles
3. Correction for slope of the floor

43. (i) Straight floor of negligible thickness with pile at u/s end
(ii) Straight floor of negligible thickness with pile at some
intermediate point
(iii) Straight floor of negligible thickness with pile at d/s
end
The pressure obtained at the key points from curves are
then corrected for
(i) Thickness of floor
(ii) Interference of piles
(iii) Sloping floor

44. CORRECTION FOR THICKNESS OF FLOOR
Pressure at actual points C1 and E1 can be computed by
considering linear variation of pressure between point D
and points E and C
When pile is at u/s end,
Correction for
Pressure at

45. For the intermediate pile,
Correction for
Correction for
When pile at d/s end,
Correction for
CORRECTION FOR THICKNESS OF FLOOR

46. Percentage correction for mutual interference of piles (C)
d-depth of pile on which the effect of another pile of depth D is
required to be determined
D-depth of pile whose effect is to be determined on the neighbouring
pile of depth d
CORRECTION FOR MUTUAL
INTERFERENCE OF PILES

47. This correction is positive for points in the rear and
subtractive for points in the forward direction of flow
For example, if we want to find the interference of pile no.
2 on pile no.1, the correction will be positive as point C is
on rear side of pile 2
CORRECTION FOR MUTUAL
INTERFERENCE OF PILES

48. CORRECTION FOR SLOPE
The % pressure under a floor sloping down is greater than
that under a horizontal floor
The % pressure under a floor sloping up is less than that
under a horizontal floor
Correction is plus for down slopes and minus for up slopes
Slope (vertical/horizontal) Correction (%)
1 in 1 11.2
1 in 2 6.5
1 in 3 4.5
1 in 4 3.3
1 in 5 2.8
1 in 6 2.5
1 in 7 2.3
1 in 8 2.0

49. The corrections given table are to be further multiplied by
the proportion of horizontal length of slope to the distance
between the two pile lines in between which the sloping
floor is located
The slope correction is applicable only to that key points of
pile line which is fixed at the beginning or end of the slope
CORRECTION FOR SLOPE