Contents preface (VII) introduction 1—37

Fig. 17.20 Different forms of hydraulic jump

an oscillating hydraulic jump (form B of Fig. 17.20). The oscillating flow causes objectionable surface waves. A stable and well-balanced jump forms when the Froude number of the incoming

SPILLWAYS

591

flow lies between 4.5 and 9.0 (form C of Fig. 17.20). Water surface downstrem of the jump is relatively smooth and the action of the turbulence is confined within the body of the jump. When the Froude number exceeds 9.0, the surface roller and the turbulence are very active resulting in a rough water surface with strong surface waves downstream of the jump (form D of Fig. 17.20).

Of the different types of USBR stilling basins, Type III (Fig. 17.21) is commonly used. This basin is suitable when the Froude number of the incoming flow exceeds 4.5, and the velocity of incoming flow does not exceed 15.0 m/s. The purpose of providing accessories, such as baffle blocks, chute blocks, and sill is to ensure the formation of the jump even in conditions of inadequate tail-water depth, and thus permit shortening of the basin length. Energy dissipation is due to the turbulence in the jump and also by the impact on blocks. Because of the large impact forces on the baffle blocks and owing to the possibility of cavitation along the

		4
	1
s
h	h
or	2
	1
b
h	h
		0
		3
III
	2
L	h
		2

4

Chute blocks	0.2hb	End sill
	0.5h1	Baffle blocks
		h1	0.375hb
	h	0.75hb
			1
h1				0.75hb
			2 : 1 slope
hb			1 : 1 slope
0.8h2		^LIII

(a) Type III basin dimensions

Baffle block height h^b /h¹

End sill height h_s /h₁

(b) Height of baffle blocks and end sill

6 8 10 12 14 16 18
Froude Number, F₁
(c) Length of stilling basin

Fig. 17.21. USBR stilling basin Type III

surface of the blocks and floor, the use of this basin is limited to such conditions in which the velocity of incoming flow does not exceed 15.0 m/s. For good hydraulic performance, the side walls of a stilling basin are kept vertical or as nearly vertical as is practicable. For known

592 IRRIGATION AND WATER RESOURCES ENGINEERING
conditions of incoming flow, the parameters of stilling basin can be determined from the curves of Fig. 17.21. A freeboard of 1.5 to 3.0 m should always be provided to allow for surging and wave action in the stilling basin.
When the velocity of the incoming flow exceeds 15 m/s, or when baffle blocks are not to be used, the USBR stilling basin, designated as Type II and shown in Fig. 17.22, should be adopted. Since the energy dissipation is accomplished mainly by hydraulic jump action, the basin length is bound to be longer than required for Type III basin. Also, the water depth in the basin should be about 5 per cent larger than the computed value of the post-jump conjugate depth.

L
^d2

				Dentated sill
Chute blocks				0.02 d2
^d1
2		s2 = 0.15 d2
s1 = d1
w1 = d1	w	2	= 0.15 d	2
h1 = d1	h2 = 0.2 d2	Slope = 2:1

L
(a) Type II basin dimensions

5


4

3

4 6 8 10 12 14 16 18

Froude number
(b) Length of jump



Fig. 17.22 USBR stilling basin Type II
Example 17.4 Design a stilling basin of USBR type III for an ogee spillway with the

following data:
Design discharge		= 13875 m^3/s
Tail-water level at the design discharge	= 180.30 m
Crest length of spillway		= 183 m
Tail-water depths (ht) and post-jump depths (h2) for different discharges are as follows:
Q (m^3/s)	0	3000	6000	9000	12000	13875
ht (m)	0	6.90	9.40	11.30	12.90	13.80
h2(m)	0	8.47	12.11	14.99	17.41	18.79

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593

Solution : For the design discharge of 13875 m^3/s, h

= 18.79 m

2

F = 13875 / (183 × 18.79) = 0.297

2

9.81

× 18.79

L

O

∴

h =

h2

1 + 8F

2

M

− 1

1

2

P

2

N

Q

=

18.79 L

2

O

M 1 + 8 (0.297)

− 1P = 2.875 m

2 N

Q

F = 13875 / (183 × 2.875) = 4.966

1

9.81

× 2.875

From Figs. 17.21 (b) and (c)

hb

= 1.49

h

1

∴

hb = 4.28 m

hs

= 1.30

h

1

∴

hs = 3.74 m

^LIII

= 2.32

h2

∴

LIII = 43.59 m

Other dimensions of the stilling basin as well as chute and baffle blocks can be determined using Fig. 17.21 (a).