Contents preface (VII) introduction 1—37

Fig. 17.16 Profile of the undernappe of the standard-crested shaft spillway (6)
17.4.6. Siphon Spillway

A siphon spillway (Fig. 17.17) is essentially a closed conduit system which uses the principle of siphonic action. The conduit system is the shape of an inverted U of unequal legs with its inlet

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end at or below normal reservoir storage level. When the reservoir water level rises above the normal level, the initial flow of water is similar to the flow over a weir. When the air in the bend has been exhausted, siphonic action starts and continuous flow is maintained until air enters the bend. The inlet end of the conduit is placed well below the normal reservoir water level to prevent ice and drift from entering the conduit. Therefore, once the siphonic action starts, the spillway continues to discharge even after the reservoir water level falls below the normal level. As such, a siphon-breaking air vent is always provided so that siphonic action can be broken once the reservoir water level has been drawn down to the normal level in the reservoir. Siphon spillways can be either constructed of concrete or formed of steel pipe. The thickness of the wall of the siphon structure should, however, be sufficiently strong to withstand the negative pressures which develop in the siphon. Pressure at the throat section (i.e., section 2) can be determined by the use of Bernoulli’s equation. Thus,

p	v	2				v	2
2	+	2		+ H	1	=	3		+ h
ρ g	2g		2g	L

where, h_L is the head loss between sections 2 and 3. Therefore,

	p	= − H1	F v ^2	−	v	2 I	(17.23)
	2	+ G	3	2	J ^+ ^hL
	ρ g		H	2 g		2 g K
Upper limb or	Air vent	Crown
upper leg				Hood or cowl
(Deprimer hood)
	1		2			Throat
				Siphon duct
Entrance lip							Lower limb or
							lower leg
of hood
			CREST				Exit lip of hood
					3

Inlet or mouth

Cup basin type water seal

Fig. 17.17 Siphon spillway
If the cross-sectional areas at sections 2 and 3 are the same, v₃ = v₂ and h_L < H₁, the pressure at the throat is always negative. Besides, the pressure distribution is non-uniform due to the curvature of streamlines and the pressure is lower at the crest and higher at the crown. Keeping these in mind, the total drop of siphon structure should be limited to about 6 m so that the negative pressures do not reach cavitation pressures.
To expedite the priming of siphon spillway, some kind of priming device is always used. The priming device could be a joggle (or step), a steel plate or some other suitable arrangement. A joggle, [Fig. 17.18 (a)] deflects the sheet of water flowing over the crest of the spillway to strike against the inner side of the hood thus forming a water seal which results in early priming of the spillway. The presence of the step, however, offers resistance to flow when the siphon ducts run full. A steel plate hinged at the spillway surface [Fig. 17.18 (b)] will also act as a priming device. Once the siphon duct starts running full, the plate is pressed downwards and is flush with the spillway surface so that there is no obstruction to the flow.

588			IRRIGATION AND WATER RESOURCES ENGINEERING
Full	Full
reservoir	reservoir
level			level

Flexible steel

plate
Joggle or tudel or step

(a) Siphon spillway with a joggle (b) Siphon spillway with a flexible steel plate
Fig. 17.18 Priming of siphon spillway
If the permissible negative head is h₀ and the radii of curvature of the crest and crown are r₀ and R₀, respectively, the unit discharge, q, through a siphon spillway, shown in Fig. 17.17, can be worked out on the assumption of free vortex conditions which are approximately obtained. If v is the velocity of flow at radius r, then in a free vortex flow

	vr = v0 r0
where, v0 is the velocity at radius r = r0 (i.e., the crest) and equals	2 gh0 .
∴	v = ^r0	2gh0
	r
Thus,	q = zr0^R^0 vdr
	R0			dr
	= zr0	^r0 ^2 ^gh0 r
or	q = r0	2 gh0	ln	R0	(17.24)
r0

The main advantages of siphon spillway are: (i) its automatic operation without any mechanical device, and (ii) its ability to pass higher discharges at relatively low surcharge head resulting in lower height of dam as well as less surrounding area to be acquired for reservoir submergence.
Besides being an expensive structure and of limited capacity, it has a serious disadvantage due to the occurrence of sudden surges and stoppages of outflow as a result of erratic siphonic action, thus causing severe fluctuations in the downstream river stage. A minor crack in the cover of the siphon would interfere with the siphon. Therefore, siphon spillway is usually constructed in batteries so that the entire spillway is not affected even if cracks have developed in one or few units. In addition, the structure and foundation have to be strong enough to resist vibration stresses. Further, there exists a possibility of clogging of the siphon due to debris and floating material. Like other types of closed conduit spillway, a siphon spillway too is incapable of handling flows appreciably greater than the designed capacity. As such, siphon spillway, whenever provided, is used as a service spillway in conjunction with an auxiliary or

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emergency spillway. In canyons of small width and small flood discharge, the suitability of a siphon spillway should always be examined.