Design of Hydraulic structures

2. Design of Stilling Basins and Roughness
Devices
Submitted To: Dr. Mashooque Ali
Talpur
Chairman & Professor
Department of Irrigation &
Drainage Engineering

3. Group Members
 Engr Zeeshan Soomro 2k19-ID-32
 Engr Zeeshan Memon 2k19-ID-33
 Engr Bilal Mirani 2k19-ID-25
 Engr Zafar-ul-haq 2k19-ID-30
 Engr Hammad Memon 2k19-ID-26
 Engr Asad Khatyan 2k19-ID-22
Subject
Design of Hydraulic Structures (ID-701)

4. Stilling Basin
 Stilling basins are external energy dissipators placed at
the outlet of a culvert, chute or rundown.
 In stilling basin the kinetic energy first causes turbulence
and is ultimately lost as heat energy.
 These basin are characterized by some combination of
chute block, bed block and end sill designed to trigger a
hydraulic jump in combination with a required tail water
condition.
 A hydraulic jump type stilling basin may either with
horizontal apron or vertical apron.

5. Stilling Basin

6. Hydraulic Jump Type Stilling Basin With
Horizontal Apron
 When tail water rating curve approximately follows the
hydraulic jump curve or is only slightly above or below
it, then hydraulic jump type stilling basin with horizontal
apron provides the best solution for energy dissipation.
 In the case of the require site depth may be obtained on a
proper apron near or the ground level so that is quite
economical.
 For the spillways on the week bed conditions, end weirs,
and the barrages on sand and loose gravel, hydraulic
jump type stilling basin are recommended.

7. Hydraulic Jump Type Stilling Basin With
Horizontal Apron
Conjugate depth:
 Water depths at the beginning and end of the horizontal
jump are related by related by the following formula for
horizontal aprons:
 Where F is the Froude number of the flow at the
beginning of the jump and general formula is:

8. Type Stilling Basin With Hydraulic Jump
Horizontal Apron
Classification:
 Hydraulic jump type stilling basin with horizontal apron
may be classified into the following two categories:
a. Basin type I
b. Basin type II

9. Basin Type I
 Stilling basin in
which Froude number
of the incoming flow
is less than 4.5 . This
case encountered on
weir and barrages.

10. Basin Type II
 Silting basin in which
Froude number of the
incoming flow is
greater than 4.5. This
case is general feature
for dams.

11. Design of Stilling Basin Type I & II
 The factor involves in the design of stilling basin are:
i. Determination of elevation of floor.
ii. Basin length.
iii. Basin Appurtenance (Latin word) means An accessory
or other item associated with a particular activity.

12. Design of Stilling Basin Type I & II
i. Elevation of basin floor:
Where as; HL= head loss in hydraulic jump, q = discharge
intensity, D1 and D2 = the elevation of basin floor. Dc
= conjugate depths.

13. Design of Stilling Basin Type I & II
ii. Basin length and
depth:
 The length of basin
will determined from
the curve.

14. Design of Stilling Basin Type I & II
iii. Basin Appurtenance:
 The requirement of
basin appurtenance such
as chute blocks, basin
blocks and end sill are
shown in figure.

15. Hydraulic Jump Type Stilling Basin With
Sloping Apron
 A hydraulic jump type stilling basin with sloping apron
should be preferred as it should an efficient jump to be
formed at suitable level on the sloping apron.
Conjugate depth:
 Water depths at the beginning and the end of the
hydraulic jump related by following formula for the fully
sloping apron.

16. Hydraulic Jump Type Stilling Basin With
Sloping Apron
D2 = conjugate depth to D1 for sloping apron.
F = Froude number
K = shape factor. It is define as dimensionless parameter
which varied with Froude number and slope of apron.

17. Hydraulic Jump Type Stilling Basin With
Sloping Apron
Classification :
 The hydraulic jump on a
sloping apron may occur
in four different forms
depending on the tail water
condition. Hydraulic jump
type stilling basin with
sloping apron may be
classified as;
i. Basin III
ii. Basin IV
Fig: Different forms of hydraulic
jumps on sloping apron

18. Basin III
 It is recommended for
the case where tail
water curve is higher
than the D2 curve at
all discharge.

19. Basin IV
 It is suitable for case
where the tail water depth
at the maximum
discharge exceeds D2
considerably but is equal
to or slight greater than
D2 at lower discharges.

20. Design Criteria
 It is not possible to standardize design criteria for sloping
aprons to the same extent as in a horizontal apron. In this
case, greater individual judgment is required.
 The slope and overall shape of the apron are determined
from economic consideration, the length being judged by
the type and soundness of the river bed downstream.

21. Design Criteria For Basin Type III
 Assume a certain level at which the front of a jump will
form for the maximum tail water depth and discharge.
 Determine D1 from the known upstream total energy line
by applying Bernoulli's theorem and calculate F. then find
out D2 from equation.

22. Design Criteria For Basin Type III
Assume a certain slope and determine the conjugate depth
D2 and the length of jump for above Froude number
respectively. The length of apron should be kept equal to 60
percent of the jump length.
Test weather the available tail-water depth at the end of the
apron matches the conjugate depth D2. if not, change the
slope or the level of upstream end of apron or both. Several
trails may be required before the slope and the location of
the apron are compatible with the hydraulic requirement.

23. Design Criteria For Basin Type III
 The apron designed for maximum discharge may be
tested at lower discharges, say ¼ , ½ and ¾ .
 If the tail-water depth is sufficient, or in excess of the
conjugate depth for the intermediate discharges, the
design is acceptable.
 If not, a flatter slope at lower apron level should be tried
or Basin IV may be adopted.
 The basin should be supplemented by a solid end sill of
height (h3) equal to 0.05 to 02 D2 with an upstream slope
of 2 : 1 to 3 : 1.

24. Design Criteria For Basin Type III
Fig: determination of D2 for basin III
Fig: length of jump in term of conjugate
depth D2 for basin III

25. Design Criteria For Basin Type IV
 Determine the discharge at which the tail-water depth is
more deficient.
 For the above discharge determine the level and length of
apron on the basis of criteria given for horizontal apron.
 Assume the certain level at which the front of jump will
form for the maximum tail-water depth and discharge.
 Determine the D1 from known upstream total energy line
by applying Bernoulli's theorem and calculate F. then find
out D2 from equation.
 Determine the suitable slope by trail and error so that the
available tail-water depth matches the required conjugate
depth D2.

26. Design Criteria For Basin Type IV
 Determine the length of jump from above slope. If the
sum of lengths of inclined portion and horizontal portion
is equal to above 60 percent of the jump , the design is
acceptable.
 If not, fresh trails may be done by changing the level of
the upstream end of the jump formation.
 The basin should be supplemented by a solid end sill of
height 0.05 to 0.2 D2 and upstream slope of 2 : 1 and 3 :
1.

27. Design Criteria For Basin Type IV
Fig: determination of D2 for basin IV Fig: Determination of shape factor k

28. Roughness Devices
 The design of various roughening devices depends upon
the experience, and no theoretical treatment is available.
Following are some roughness devices:
I. Baffle wall
II. Friction block or arrows
III. Dentated sill
IV. Deflector
V. Biff wall
VI. Cellular or ribbed pitching

29. I. Baffle Wall
 A baffle wall sort of low
weir constructed at the end
of the cistern two serve as
two purposes:
a. To head up water to
upstream to such a height
that hydraulic jump is
formed, and
b. To with stand the actual
end of high velocity jet to
dissipate the energy.

30. II. Staggered Friction Block or Arrows
 Staggered friction block are one of the most useful and
simple device to dissipate the energy. It consist of regular
blocks of concrete. There height may be ¼th water depth
and are 1.5 to 2 times the height of block. The distance
between successive lines is equal to twice the height.
 Arrows are also friction block which provide on
downstream floor of the channel.

31. II. Staggered Friction Block or Arrows

32. III. Dentated Sill
 A dentate sill is
provide at the end of
cistern if height of jet
persist to the end of
the cistern.
 the object of the sill
is to deflect up the
high velocity jet from
near the bed and to
break it.

33. IV. Deflector
 A deflector is uniform
height, unlike
dentated sill is to
deflect up the high
velocity of jet from
near the bed causing a
reverse roller.

34. V. Biff Wall
 It provide at the end
of cistern, causing a
deep pool of water
behind it in the
cistern.
 Its object is to deflect
back the water from
the cistern to create
super turbulence in it.

35. VI. Cellular or Ribbed Pitching
 Ribbed pitching is
constructed on the
sides by putting
bricks flat and edge
alternatively.
 This provides the
roughening of the
perimeter to destroy
surplus energy down-
stream of the fall.

36. Thanks