• This spillway consists of one or a number of units of conduits formed by inverted U-tubes.
  • The initial discharge through the conduits is similar to that of a weir.
  • when air is drawn away from above the end over the crest, the discharge starts flowing by siphonic action.
  • The inside of the bend of inverted U-tube is at normal reservoir level.
  • When water level in the reservoir rises, the water first flows as for the weir spillway.
  • when air inlet is block by rising water, siphonic action starts and spillway starts discharging with full capacity.



Spillway continues to discharge with full capacity unless the reservoir level drops below the air inlet pipe, when air enters the conduit and breaks the siphonic action.

Siphon spillway can be classified into two categories.

(i) Saddle siphon spillway.

(ii) Volute siphon spillway.

Fig. 14.8. Saddle siphon spillway.


1. Saddle Siphon Spillway.

  • Saddle siphon spillway can be constructed in many designs but basic concept for all of them is the same.
  • One types of saddle siphons
  • The diagram consists of a U-shaped siphon pipe whose D/S end remains submerged in tail water.
  • An air inlet is provided near the top of the bend.
  • Crest of the inside of U-tube is maintaine at the normal reservoir level and air inlet slightly above this level.
  • At normal reservoir level no waterflows through the siphon.
  • During floods as the water level in the reservoir rises above the normal reservoir level, water will start flowing through the conduit of the spillway.
  • When rising water level blocks the air inlet, the flow through the conduit immediately starts under siphonic head.
  • A number


  • of such siphonic units may be install depending upon the diameter of the conduit of the siphon and the amount of flood water to be hand.
  • When water level in the reservoir drops below the air inlet, siphonic action is broken.
  • Closing of the air inlet by rising water is know priming and opening of the air inlet by dropping level of water is know depriming of the siphon spillway.
  • Discharging capacity of a saddle siphon spillway is obtaine by following formula

\[Q= CA\sqrt{2gH}\]


A = Area of cross-section at crown.

H = Operating head. It is taken as vertical height from reservoir level to the centre of the outlet if outlet is discharging in atmosphere and upto tail water level if outlet is submerg.

C = Coefficient of discharge whose average value is taken as 0.65. Siphon spillways can be further classifie as high head, medium head and low head siphons.


2. Volute Siphon Spillway.

  • It is special type of siphon spillway.
  • It comprises a vertical shaft which
  • ts top end consists of funnel shap lips in which a number of volutes are develop.

Fig. 14.9. Volute siphon spillway.

  • funnel shaped lip remains cover by a concrete dome support on number of pillars.
  • A deprimer is also install over the dome which connects the outer air with the funnel shap lip.
  • The outer end of the deprimer is kept just at the reservoir level.
  • When water level in the reservoir rises, it closes the deprimer
  • Entry of air is prevented.
  • Now water flowing over the volutes sucks off the entrapped air due to spiral flow of water, and siphonic action starts and siphon starts discharging at its full capacity.
  • When reservoir level falls below the air inlet, air enters and breaks the siphonic action.
  • The discharging capacity of this siphon is found out by following formula:

\[Q= A\sqrt{2g(H-H_{L})}= CA\sqrt{2gH}\]

A = area of the cross-section of the pipe.

C = coefficient of discharge.

H = Maximum operating head.

HL = Head loss through the siphon which is generally very small and may be ignored.

Advantages of Siphon Spillway

  1. Since its discharge does not depend upon the head above the crest, it starts discharging with full capacity once siphonic action starts.
  2. Since these spillways starts discharging with full capacity right from the beginning, the reservoir level will not rise much.
  3. This will cause less submergence of U/S areas during floods.
  4. River training works of low heights will have to be constructed.


  1. They do not allow additional waters to be store in the reservoir to augment the supplies at a later stage.
  2. They cause lot of vibrations when in action.
  3. This may cause cracks in the dam and may lead to their ultimate failure.
  4. They have to be maintaine regularly.