Definition of Spillway.

Spillway is a waterway provided to dispose of surplus flood water from a reservoir after it has been filled upto its maximum capacity. Spillways are provided for all the dams and these act as safety valves for the dam. A Spillway may be located within the body of the dam or at the end of the dam or completely away from the dam as an independent structure. It is essential to provide spillway of sufficient capacity. A spillway of insufficient capacity may however lead to over-topping of the dam resulting in serious and permanent damage to the dam or in the failure of the dam.

Design Criteria for Spillway.

• Spillway must be hydraulically and structurally adequate.
• The spillway must have sufficient capacity.
• It must be so located that it must provide the safe disposal of water.
• The surface of the spillway must be erosion resistant to withstand the high scouring velocity created by the drop from reservoir surface.
• Usually some device will be required for dissipation of energy on the down-stream side of the spillway.

Different Types of Spillway.

1. Straight Drop Spillway.
2. Ogee Spillway/ Overflow Spillway.
3. Side Channel Spillway.
4. Shaft Spillway.
5. Siphon Spillway.
6. Chute Spillway/ Through Spillway/ Open Channel Spillway.

• Straight Drop Spillway.

This is the simplest type of spillway and may be constructed on small bunds. It is a low weir and simple vertical fall type of structure. The down-stream face of the structure may be kept vertical or slightly inclined. The crest is sometimes extended in the form of an overhanging lip. So as to keep smaller discharge away from the face of overfall section. The waterfalls freely from the crest under the action of gravity. Since vaccume gets created in the underside portion of falling jet, sufficient ventilation of the nappe is required in order to avoid  pulsating and fluctuating effect of the jet.

• Ogee Spillway/ Overflow Spillway.

Its an improvement upon tree overfall spillway and used in cone, mason and buttress dam. The profile of this spillway is made in accordance with the shape lower nappe of free falling jet. The shape of free falling jet over sharp crested weir can be determined by principle of projectile. Now, if the space between lower nappe and sharp crested weir is filled with concrete or masonry, the weir so formed will have the profile similar to Ogee, hence it is called as Ogee weir or Ogee Spillway.

• Chute / Through / Open Channel Spillway.

For earthen and rock-fill dams a separate spillway is generally constructed in a flank or a saddle way from main valley. The chute spillway is the simplest type of spillway which can be easily provided independently and at low cost. It is lighter and adaptable to any type of foundation and hence provided easily on earth and rock-fill dams. Sometimes it is called as waste weir. Chute spillway is constructed in continuation to the dam at one end, it may be called flank weir. If it is constructed in a natural saddle in a bank of river separated from main dam by  a high ridge, it is called as saddle weir.

• Side Channel Spillway.

The side channel differs from the chute spillway in the sense that, in chute spillway water flows at right angle to the weir crest after spilling over it, where as in side channel it flows parallel to the weir crest. This type of spillway is provided in narrow valley where no side flanks of sufficient width to accomodate a chute spillway are available. The water entering the side channel spillway has no momentum in the direction in which it has to move. Therefore the slope of the side channel should be sufficient of overcome friction losses as well as to provide acceleration in the direction of flow against the mass of incoming water.

• Shaft Spillway.

Water from reservoir enters into a vertical shaft which conveys this water into a horizontal tunnel which finally discharges water into river down-stream. For small heights, shafts may be constructed entirely of metal or concrete but for large heights reinforced cement concrete may be used. No special design required foe inlet of small height but for large height, a flat inlet is often used. Shaft spillway is used when the possibility of overflow spillway and through spillway has been ruled out because of non-availability of space due to topography.

• Siphon Spillway.

  
  1. Tilted outlet type of Siphon Spillway.

When valley is very narrow and no space is available for constructing a separate spillway, siphon pipe can be installed within the dam body. An air vent is connected to the siphon pipe. While entry point kept still lower so as to prevent the entry of debris or floating matters in the siphon. As soon as the water level increases, the suction in the air vent pipe starts. Syphonic action will continue till the water level goes down to normal level.

2. Hooded type of Siphon Spillway.

In this case, a R.C.C hood is constructed over an overflow section. Inlet is kept submerged to prevent the entry of debris. A small deprimer hood is kept above the main hood and connected by air vent.

3. Volute Siphon Spillway.

It is consisting a vertical barrel which is bent at its discharge end and opened out in the form of funnel at the top. The lip of the funnel is kept at the permissible level of reservoir. Number of blades just like in centrifugal pump are fixed in the funnel for introducing spiral motion to the water passing along them. Concrete dam supported on number of pillars is kept over the funnel. Deprimer hood is provided over the main dome. When water level rises, it seals the entry of air and sheet of water starts flowing from all sides.