32.3 Baffle Stilling Basin - nptel - Indian Institute of Technology Madras

Hydraulics Prof. B.S. Thandaveswara 

32.3 Baffle Stilling Basin 

The baffle sill basin involves a transverse sill of height 'h' with a minimum width. For a 

given approach flow depth y 1 and approach Froude number F 1 , various types of flows 

may occur, depending on the relative sill height S = h/ y 1 and the tail water level y t . 

Following types of jumps are identified: 

Indian Institute of Technology Madras 

• jump with end of roller above sill, (as practically no scour potential it is suitable 

for easily erodible beds). 

• B - jump with a lower tail water level, a surface boil on the sill and the roller 

extending in the tail water, with small erosion mainly along the side walls. 

• Minimum B-jump with a secondary roller, and plunging flow beyond the sill that 

does not reach the basin bottom, suitable for the channel with rocky beds in the 

downstream. 

• C-jump with plunging flow that causes inappropriate tailwater flow, and scour 

potential, 

• Wave type flow with supercritical flow over the sill and unacceptable energy 

dissipation. 

In case of sufficient tail water submergence type A and B-jumps are very effective for 

stilling basins. On the other hand, the type - c jump and the wave jump are 

unacceptable in view of the very poor dissipation of the energy. Figure 32.5 shows the 

significance of tail water submergence in the basin design. This is also an example 

representing the cases of a slight decrease of tailwater level below the sequent depth. 

The purpose of any baffle element should thus involve a length reduction. Figure shows 

the baffle sill basin and the hydraulic jump basin. The sill is defined with the relative 

height S = h/ y 1 and the relative sill location 

0 s 

X 

L 

srj 

s = . The sequent depth ratio required 

Lrj 

ysb = y −∆y 

includes the effect of normal hydraulic jump and the influence of the sill 

(Hager, 1992).


y1 

y1 

Lrj 


Lj 

h 

y2 

Classical hydraulic jump 

y2 

a) b) 

Adequate Tail waterproper 

formation of the jump 

and effective dissipation of 

energy. 

y1 

y1 

Lsrj 

Ljb 

h 

xs bs 

c) d) 

h 

Inadequate Tail waterhence 

Submergence is 

wanting. 

xs = Lrj 

Baffle sill basin 

y2 

h 

y2 

Lsrj ____ h 

S = __ 

, 

Figure 32.5 - Definition Sketch for Stilling basin with Sill 

( ) 2 

07 . 

∆ Y s = 07 . S + 3S 1−x 

s 

For any sill height h 1 , minimum approach Froude number F 1min is necessary for the 

formation of the hydraulic jump, and the corresponding maximum relative sill height S max 

for any approach flow Froude number is given by 

1 5/ 3 

S max = F1 

. 

6 

The relative sill height is normally limited to S max = 2 in practice. It may be noted that the 

sill should neither be too small nor too large. 

The optimum sill height S opt is 

1 25 . 

Sopt = 1+ F 1 . 

200 

y1


Depending mainly on the relative sill position X s three types of jump may form: 

I. A-jump X > 08 . ( to 1) 

II. B-jump 065 . < X > 08 . 

III. Minimum B-jump 055 . < X > 065 . 


s 

s 

s 

The length of the jump L jb from the toe to the end of the bottom roller relative to the 

Ljb 

1/ 3 

length of the classical jump L j is = 1−06 . S ( 1-Λ) 

. 

L 

j 

The length of the sill basin jump L jb is marginally less than the length of a classical jump 

L j for all three types of flows mentioned above. A sill basin improves the stabilization of 

a hydraulic jump under variable tailwater and is somewhat shorter than a classical 

hydraulic jump. 

Baffle Block Basin 

For optimum basin flow, the blocks must have an appropriate location and adequate 

height to overcome the ineffectiveness or overforcing of flow. Basco in 1971 defined the 

optimum height of the baffle as the ratio of 

given by, 

S 

h 

= 

1 

opt 

opt 

y 

2 

( ) 16 75 F1 − 

L jb / h = . + . 

S opt 

opt 

1 

= 1+ F1−2 40 

( ) 2 

and the optimum basin length is 

Figure 32.6 shows the basin with the standard USBR blocks, where spacing of the 

blocks sp is equal to the block width sp= wband 

sp 

h 

= 075 . .


y 


1 

WB 

Xs 

a) Longitudinal section 

s 

p 

WB 

b) Standard baffles 

Figure- 32.6 Typical Baffle block basin 

A coefficient for representing the force on the blocks P B is given by 

Φ P / ⎡ρg w y / 2⎤ 

2 

= B ⎣ b 2 ⎦ 

for optimum basin performance, the coefficient Φ is 

F1 

1 

Φ opt = + 

7 100 

1/ 2 

⎛ 2 ⎞ 

and the sequent depth ratio is Y b = ⎜ ⎟ F1−05 . 

⎝1+ Φ ⎠ 

The tail water reduction is above 10% when compared to the classical jump. Type II, 

Type III and Type IV basins are shown below. 

y 2 

h



2y1 min 

0.8 y2 

Chute blocks 

Lb = 4.3 y2 

Fractional space 

Space = 2.5 w 

Lb = 6.1 y2 

Baffle 

piers 

dentated sill 

(a) Type II basin F1 > 4.5 

v1 > 18.0 m/s Tail Water (TW) = 0.97 y2 

Chute 

blocks 

Fractional space 

h3 = y1(4+F1)/9 

End 

sill 

(b) Type III basin F1 > 4.5 

v1 < 18.0 m/s Tail Water (TW) = 0.83 y2 

(c) Type IV basin 2.5 F1 < 4.5 

Tail Water (TW) = y2 

Slope 2:1 

Sill optional 

h4 = y1(9+F1)/9 

h4 = y1(9+Fr1)/9

32.3 Baffle Stilling Basin - nptel - Indian Institute of Technology Madras

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