Contents preface (VII) introduction 1—37

Fig. 17.9 (a and b). Variations of coefficient of dischrge for ogee spillway (Contd...)

576 IRRIGATION AND WATER RESOURCES ENGINEERING

Modified

C

C

1.00
	hO
0.90	hd
HO
	P	d
0.80

0.76	1.1	1.2	1.3	1.4	1.5	1.6	1.7	1.8
1.0

(h_o + d)/H_o
(c) Ratio of discharge coefficient due to apron effect

	1.0
	0.8
	0.6						hO			hd
/C						HO				d
S
C	0.4					P
	0.2
	0
	0	0.1	0.2	0.3	0.4		0.5			0.6	0.7	0.8
					hd /Ho
	(d) Ratio of discharge coefficient due to		tailwater effect
	1.1
	1.0
0										^ha
C/C						HO
0.9					H		e
						P
	0.8
	0	0.2	0.4	0.6	0.8	1.0	1.2	1.4	1.6
					He / Ho

(e) Coefficient of discharge for H_o other than the design head

Fig. 17.9 (c, d and e) Variations of coefficient of discharge for ogee spillway (4)
Example 17.3 Obtain USBR profile for an ogee spillway with the following data:
Design discharge = 13875 m³/s
Crest length of spillway = 183 m
Crest level of spillway = 203.34 m
River bed level = 166.50 m

SPILLWAYS					577
Solution: Assuming C = 2.20 in Eq. (17.5), one obtains
		L	13875	O	2 /3
	Ho = M		P	= 10.59 m
		N	183 × 2.20 Q
and	P = 203.34 – 166.50 = 36.84 m
∴	P
		= 3.48
Ho

From Fig. 17.9 (a), C = 2.18
With C = 2.18, Eq. (17.5) yields H₀ = 10.655 m
Design head for spillway profile = 0.75 × 10.655 = 7.99 m ~₋ 8.00 m (say)

∴			P	36.84
				=				= 4.605
	Ho	8.00
Approach velocity, va =				13875		= 1.69 m/s
183 (36.84		+ 8.00)
∴	ha = 0.15 m		and		ha/Ho = 0.019
From Figs. 17.8 (b) and (c), k = 0.504	and n = 1.861

Hence, the profile of the ogee spillway, [Fig. 17.8 (a)] is given by

y			L	X O1.861
	=	− 0504.	M		P
8.00
			N	8.00Q