What is Spillway?

A spillway is the overflow portion of the dam that carries the surplus discharge flow from the reservoir to the downstream, that is, it carries the flood water safely downstream, which is enough to be stored in that reservoir. Therefore, it acts as a safety valve for the dam because many failures of the dam arise due to the incorrect design of the spillway.

Spillways can be classified into different types based on their utility and prominent features, such as their control structures, their components, or the discharge channel.

Main Spillway

It is the concrete structure that functions under the design of floods. It is constructed either within the body of a dam or at one of its ends.

Subsidiary Spillway

It is the concrete structure that functions when emergency conditions arise, that is, when a flood greater than the design flood occurs. A subsidiary spillway functions during the shutdown of the outlet works, or failure of spill gates.

Straight Drop Spillway

This is also called the free overfall spillway, which is commonly used for low earth dams or the earthen bunds. Straight drop spillways are used when the hydraulic drop is less than 6 meters. The crest is extended in the form of an overhanging lip to carry a small discharge away from the face of the overfall section. The control structure is at low height narrow crested weir for which the downstream face is either vertical or nearly vertical. These spillways are suitable for thin arch spillways and not suitable for high drops on yielding foundations.

Ogge Spillway

This is also called overfall spillway, which is commonly used for concrete, masonry, arch, and buttress dams. The overflowing water is guided smoothly over the crest and its profile to not to break its contact with the spillway surface. At the downstream face, a smooth gradual reverse curvature is provided which reverses the flow of the apron of a stilling basin or into the spillway discharge channel.

Ogge Spillways are classified under two categories based on the ratio of their height (P) measured from the river bed to the design head (Hd).

1. High Overfall Spillway: The ratio of their height (P) measured from the river bed to the design head (Hd) is greater than 1.33.
2. Low Overfall Spillway: The ratio of their height (P) measured from the river bed to the design head (Hd) is less than or equal to 1.33.

Design of the Ogge Spillway

• Head: The vertical distance from the water surface to the crest axis is called the head.
• Design Head: The value of the head for which the Ogge spillway is designed.
• Head due to the approach velocity: It is represented as ${\mathrm{H}}_{\mathrm{a}}$.
•  ${\mathrm{H}}_{\mathrm{a}}=\frac{v_a^2}{2g}$, where $v_a$ is the approach velocity.
• Total energy head: It is represented as ${\mathrm{H}}_{\mathrm{e}}$. This is the sum of the actual head (H) and the head due to the approach velocity. ${\mathrm{H}}_{\mathrm{e}}=\mathrm{H}+{\mathrm{H}}_{\mathrm{a}}$

Side channel Spillway

This is the spillway on which the flow is carried away from the channel running parallel to the crest after being passed over a weir or Ogge crest. The flow of the water is turned to 900 after spilling over the crest so that it flows parallel to the weir crest. These types of spillways are suitable for narrow valleys where there are no side flanks of sufficient width and where a long overflow crest is desired to limit the surplus head. The slopes of these spillways should be adequate to overcome the friction losses and provide acceleration in the direction of flow.

Chute Spillway

This is also called the open channel or the trough spillway. These are the simplest type of spillways that are easy to construct at low costs. They may have a chute-type discharge channel that is controlled by an overflow crest, a gated orifice, a side-channel crest, or other control devices. The discharge is carried from the reservoir to the downstream level through an open channel placed along the dam or through the saddle. The water flows at right angles with respect to the weir crest after spilling over it. They are suitable for earthen or rockfill dams.

Design of Chute Spillway

• Chute slope: The minimum slope of the chute is governed by the condition that the supercritical flow should be maintained. It should be adequate to fulfill the flow requirement from the crest without any filling.
• Side walls: The side walls should be a suitable height to avoid the spilling of water.
• Approach channel: The approach channel should be trapezoidal with the side slopes 1:1, which can be constructed to lead the reservoir water up to the control structure. 

Conduit Spillway

They are also called tunnel spillways. It consists of a closed channel that carries the discharge around or under the dam. This closed channel can be of the vertical or inclined shaft. These spillways are designed for the partly full flow to prevent siphonic actions and negative pressures. The ratio of flow area to the total tunnel area is limited to 75% to ensure free flow in the channel. The air vents are also provided at the critical points along the tunnel of the spillway to provide sufficient air supply.

Shaft Spillway

This is also called the drop inlet spillway or the morning glory spillway. The water enters from the reservoir into a vertical shaft that carries this water forward into a horizontal tunnel from where that water was discharged into the downstream channel. This spillway can be used where there is non-availability of space due to the topography and where there is a suitable rock spur available upstream of the reservoir. It consists of three elements: an overflow control weir, a vertical transition, and a closed discharge channel.

Siphon Spillway

This spillway consists of siphon units that discharge the surplus water downstream by utilizing the siphonic action, instead of allowing it to spill over the crest of the dam or weir. It is a closed conduit system in the shape of U that positions the inside of the load of the upper passageway at the normal storage level. It consists of the components- an inlet, an upper leg, a throat, a lower leg, and an outlet.

Types of  siphon spillway

Saddle Siphon Spillway

This spillway consists of reinforced concrete that is constructed over an overflow section of the dam. The inlet is kept submerged in water to avoid the entry of debris into the siphon. The lower leg is submerged in the tailwater to avoid the entry of air into the downstream end of the reservoir.

Discharge through Saddle Siphon Spillway

$Q=C·A\sqrt{2gH}$

Where,

• $C$= Coefficient of discharge, whose average value is 0.65
• $A$= Area of a cross-section at the crown
• $H$= Operating head

Volute Siphon Spillway

This is the special type of siphon spillway that is designed in India. There are vertical barrels at the discharge end and it is opened at the top in the form of a funnel, which is kept at the reservoir level. There are several volutes which are blades like structures, which are fixed to induce the spiral motion of water that flows across them.

Discharge through Volute Siphon Spillway

$$\begin{gathered} Q=A\sqrt{2g(H-H_L}) \\ =C·A\sqrt{2gH} \end{gathered}$$

Where,

• $A$= Area of a cross-section of the pipe
• $C$= Coefficient of discharge
• $H$= Maximum operating head
• $H_L$= Head loss through Siphon

Siphon Spillways are classified based on their heads as a medium head siphon, high head siphon, and low head siphon.

1. Medium head siphon: The outlet area is the same as the throat area.
2. High head siphon: The outlet area is made smaller as compared to the throat area by a nozzle or by tampering with the lower part of the siphon.
3. Low head siphon: The outlet tube is more divergent as compared to the throat area.

Context and Applications

Spillways are the safety valves of the dams. They should be hydraulically and structurally adequate to store the surplus flood water. They should be capable of disposing off the surplus water. Therefore, its required capacity can be determined by using flood routing methods.

• Bachelors of Technology in Civil Engineering
• Masters of Technology in Civil Engineering
• Bachelors of Technology in Mechanical Engineering
• Masters of Technology in Mechanical Engineering

Practice Problems

1. Which component is not used in the shaft spillway? 

1. Tunnel
2. Radial gates
3. Radial piers
4. Bridge

Answer– b 

Explanation: A shaft spillway does not consist of radial gates as its components. Its components are tunnel, bridge, and radial piers. 

2. Which is the effective location to place a shaft spillway?

1. On the side channels of the dam
2. Inside the upstream reservoir
3. Outside the downstream reservoir
4. Submerged in the reservoir

Answer- b

Explanation: A shaft spillway should be located inside the upstream reservoir. 

3. Which is also known as the morning glory spillway?

1. Drop Inlet Spillway
2. Ogge Spillway
3. Conduit Spillway
4. Siphon Spillway

Answer- a

Explanation: Morning Glory Spillway is the name for Drop Inlet Spillway. This is also called a Shaft Spillway. 

4. Which of the spillway is not suitable for the earthen dam?

1. Shaft Spillway
2. Chute Spillway
3. Ogge Spillway
4. Straight fall Spillway

Answer– c

Explanation: Ogge Spillway is not suitable for an earthen dam. It is suitable for concrete, masonry, arch, and buttress dams.

5. Which is the correct equation to calculate the discharge through Saddle Siphon Spillway?

1. $Q=A\sqrt{2g(H-H_L})$
2. $Q=C_d L_e{H}_e$
3. $Q=A\sqrt{2gH}$
4. $Q=V·A$

Answer– c

Explanation: The discharge through Saddle Siphon Spillway can be calculated using the equation- $Q=A\sqrt{2gH}$