Stilling Basins

5/23/2018 Stilling Basins

1/10

Hydrauli c StructuresStilling Basins December 4, 2010

1

Stilling Basins

The basins are usually provided with special appurtenances including chute blocks, sills and

baffles piers.

Chute blocks:are used to form a serrated device at the entrance to the stilling basin. Their

function is to furrow the incoming jet and lift a portion of it from the floor producing a shorter

length of jump than would be possible without them.

The sill:is usually provided at the end of stilling basin. Its function is to reduce further the

length of the jump and to control scour. The sill has additional function of diffusing the residual

portion of high velocity jet that may reach the end of the basin.

Baffle piers:are blocks placed in the intermediate position across the basin floor. Their function

is to dissipate energy mostly by impact action. They are useful in small structures with low

incoming velocities. They are unsuitable where high velocities make cavitation possible.

Classification of Stilling Basins

Stilling basins can be classified into:

1. Stilling basins in which1

4.5F . This is generally encountered on weirs and barrages.

2. Stilling basins in which1

4.5F . This is a general feature for medium and high dams.

Stilling Basin Design for Low Froude Numbers F1


2/10


2

21 2 11.1 800 For 11 to 17wT F D F (3.10)

9. Height of side wall above tailwater2 3Z D .

10.Wing walls should be equal in height to stilling basin side walls. The top of the wing wallshould have a slope of 1:1.

11.The wing wall should be placed at an angle 45 to the outlet centerline.12.Cutoff wall of normal depth should be used at the end of stilling basin13.The effect of entrained air should be neglected in the design of stilling basin. See Fig.

3.20, Varshney.

II. Indian Standard Stilling Basin IDefinition sketches are given in Figs 3.16 and 3.17, Varshney.

III. U.S.B.R. Stilling Basin IVThis stilling basin is recommended forF1= 2.5-4.5 which usually occur on canal structures and

diversion dams. For this range of Froude number an oscillating jump will be produced in the

stilling basin generating a wave that is difficult to dampen. U.S.B.R. Stilling Basin IV is

designed to solve this problem. See Figure 22. For better performance, it is desirable to make the

blocks narrower thanD1preferable 0.75D1and to set the tailwater (Tw)= (1.05-1.1)D2. The

length of basin equals the length of the jump. Basin IV is applicable to rectangular cross-sectionsonly.


3/10


3

Stilling Basin Design for Froude Numbers F1>4.5

i. S.A.F. Stilling Basin.ii.

Indian Standard Stilling Basin II. The dimension sketches are given in Fig. 3.21,Varshney.

iii. U.S.B.R Stilling Basin II.This design is recommended for large and medium spillways and large canal

structures. The length of basin is 33% reduced with the use of appurtenances. The

basin contains chute locks and dentated sill. No baffle piers are used because high

velocities might cause cavitation on piers. See fig 15-16 (Chow) or Fig. 3.22,

Varshney.


4/10


4

Design Steps

1. Set apron elevation to use tailwater depth (Tw) plus an added factor of safety (use Fig. 15-16b, Chow) on the figure there is a minimum Twdepth line which indicates the point at

which the front of the jump moves away from the chute blocks. Any lowering would

cause the jump to leave the basin. Bureau recommends a minimum safety margin of 5%

ofD2be added to sequent depth.

2. Basin II maybe effective down to Froude No. 4.3. Length of the basin can be obtained from Fig. 15-16c, Chow.4. Height of chute blocks=D1, width and spacing=D1. Space of 0.5D1is preferable along

each wall to reduce spray and maintain desirable pressure.

5. Height of the dentated sill= 0.2D2. Maximum width and spacing= 0.15D2. The block isplaced adjacent to each side wall. The slope of the continuous portion of the end sill is

2H:1V.

6. The slope of the chute varies from 0.6H:1V to 2H:1V.7. This type of basin is suitable for spillways with fall up to 200 ftand flows 500 cfs/ft of

basin width.

8. Water surface and pressure profiles of a jump in the basin are shown in Fig. 15-16d,Chow.

Example

Design a U.S.B.R. Stilling Basin II for the overflow spillway of crest length 250ft(at El. 982.3

ft) and design flow rate 75 000ft3/s. The upstream water surface at design flow rate is at El. 1000

ft, the average channel floor is at elevation El. 880 and tailwater at El. 920.

Solution

Head above crest1000 982.3 17.7ft

Total fall 1000 880 120ft

From Fig. 14-15 (Chow) the velocity of flow at toe of spillway= 79ft/s.


5/10


5

1

1

1

750003.8

79 250

797.14

32.2 3.8

QD ft

L V

VF

gD

From Fig. 15-16b (Chow),F1=7.14

The solid line2

1w

T D gives 2 9.7wT D

1

22

9.7 9.7 3.8 36.9

1, 36.9

w

w

T D ft

DD ft

T

The elevation of the basin is placed at= 920-36.9=883.1ft,

The total fall=1000-883.1=116.9ft, but this will not change the flow velocity.

Length of basin (from Fig 15-16c)2

4.16L

D

4.16 36.9 154L ft

From min. curve1

9.2w

T D

19.2 9.2 3.8 35

with 8% margin of safety 37.8

w

w

T D ft

T ft

Height= width= spacing of chute blocks=D1= 3.8ft= 3ft10 inches

Width= spacing of dentated sill 20.15

0.15 36.9 5.5 or 5 6

D

ft ft inches

Top thickness of sill2

0.02 0.02 36.9 0.738D ft

Angle of water surface 9 from Fig. 15-16d, Chow.


6/10


6

Design Example:R.S. Varshney Stilling Basin

Water flows under a sluice gate discharges into a rectangular plain stilling basin. Determine the

stilling basin parameters for the following data.

334 / / , 1 , tailwater depth 8.3 , width of basin 20L

q m s m H m m m

Design

From Blench Curves (Fig. 3.5, Varshney),2

8.5Ef m

1 2 8.5 1 9.5LEf Ef H m

From Energy of flow curves, Fig. 2.7

1 2

3

2.9 , 7.3

8.3w

D m D m

D T m

1

1

11

1

2 2

33

3411.72 /

2.9

11.722.2 4.5 O.K.

9.81 2.9

344.9

9.81c

qV m s

D

VFr

gD

qD m

g

Length of Cistern

a. Length 2 16 6 7.3 2.9 26.4L D D m b. FromFrandL/D1 relation, Fig. 3.15:

1

12, 12 2.9 34.8L

L mD

.

26.4 34.830.6 31

2av

L m

Blocks

a. Chute blocksi. Height

From hcandFrrelation in Fig. 3.14,

hc=0.9 m

ii. Width and spacing 0.750.75 0.9 0.675 0.7

ch

m

b. Floor blocksi. Position


7/10


7

From3 3

and relation,BLL

D DFig. 3.14

0.9 8.3 7.5BL m

From and relation, 6.7

6.7 7.5Average 7.12

B B

B

L L L m

L m

ii. Height hBFrom 2

3 3

and BD h

D Drelation given in Fig. 3.14,

2

3

3

7.30.88

8.3

0.22, 0.22 8.3 1.83B B

D

D

hh m

D

From 2 and B

c c

D hD D

in Fig. 3.15

27.3

1.49, 0.314.9

0.31 4.9 1.52

B

c c

B

D h

D D

h m

Adopt mean value1.83 1.52

1.67say 1.72

m

iii. Width and spacing wB0.8

0.8 1.7 1.36 say 1.4

B

B

B

w

h

w m

Top width 0.2 0.34 say 0.4B

h m

c. End silli. Height,He

From Fig. 3.15, relation between3

and eH

FrD

,

3

0.18, 0.18 8.3 1.5e eH

H mD

From 3ande

c c

H D

D Din Fig. 3.15,

38.3

1.7, 0.3, 1.474.9

ee

c c

D HH

D D

Average1.5 1.47

1.485 say 1.52

eH m

ii. Top width2

0.02 0.02 7.3 0.146 say 0.15D m

iii. Width and spacing of dents 20.15 1.1D m iv. Width of basin=20 m


8/10


8

Design of S.A.F.

1 2

2

0.76

1

0.76

2.2 1.7 O.K., 7.3

4.5

4.5 7.3

18.04 say 18.52.2

B

F D m

DL

F

m

Height of chute blocks=D1=2.9 m

Width and spacing= 0.752.9= 2.18 mSay 2.2 m

Distance from chute blocks to floor blocks18.5

6.23 3

BL

m

Minimum distance between floor blocks and wall3 2.9

1.1

8

m

Distance between wall and chute blocks= 1m

Length of end sill= 0.077.3=0.5 m

2

12

2

1.1120

2.21.1 7.3 8.32

120

w

FT D

m

Height of side wall above 27.3

2.43 3

w

DT m


9/10


9

Design Example: S.A.F. Stilling Basin

Water flows under a sluice gate discharges into a rectangular plain stilling basin. Determine the

stilling basin parameters for the following data.

334 / / , 1 , tailwater depth 8.3 , width of basin 21.8L

q m s m H m m m

Design

From Blench curve (Fig. 2.5),2

8.5Ef m

1 2

1 2

3

1

1

11

1

8.5 1 9.5

2.9 , 7.3

8.3

3411.72 /

2.911.72

2.29.81 2.9

L

w

Ef Ef H m

D m D m

D T m

qV m s

DV

FrgD

11.7 17Fr , we can design SAF Stilling Basin.

Length of Basin (LB)

2

0.760.76

1

4.5 4.5 7.318.04 say 18

2.2B

DL m

F

Blocks

a. Chute blocks1. Height

1 2.9cH D m

2. Width and spacing

10.75

0.75 2.9 2.175 say 2.18

cW D

m m

No. of blocks2.18

52.18 2

Distance adjacent to wall 0.5 0.5 2.18

1.09

cW

m

Min. distance 13 3

2.9 1.087 1.09 O.K.8 8

D m

b. Floor blocks1. Position

183 63

BL m


10/10


10

2. Height1 2.9BH D m

3. Width and spacing1

0.75 0.75 2.9

2.175 say 2.18

D

m m

Top width 0.2

0.2 2.9 0.58say 0.6

BH

m

% Width of floor

4 2.18100 40% O.K. between (40-55)%

2.18

c. End sill20.07 0.07 7.3 0.51 say 0.6C D m m

Top thickness 0.7 0.7 0.6 0.42 say 0.5C m m

d. Side wall2 7.3 2.43 say 2.5

3 3

DZ m m

Stilling Basins

Documents

Transcript of Stilling Basins