Question

Write the types of spillway and explain each briefly.

Answer 1

A spillway is a structure constructed across a dam or levee to control the release of water to its downstream side. When there is excess water in the reservoir spillway ensures that water does not overflow over the dam. The different types of spillways are:

1. Freefall spillway or Straight drop spillway:

As the name suggests this type of spillway allows the water to drop straight down freely from the crest to the stream bed. As a result of this straight free fall there is a possibility of scouring the stream bed and finally results in the formation of a pool. This type of spillway is suitable for an arch dam.

2. Ogee spillway or Overflow spillway:

Ogee spillway is an improved form of straight drop spillway. In this spillway the downstream side is shaped in the form of english alphabet 'S'. Here water flows down smoothly and the scouring effect is less as compared to the straight drop spillway. This spillway is most commonly used with gravity dams, arch dams, buttress dams etc.

3. Chute spillway or Trough spillway:

In a chute spillway water flows through an open channel on a steep slope to the downstream side of the river. This spillway can handle flows with supercritical velocity. The main parts of this spillway are inlet, vertical curve section, steep-sloped channel and outlet. This spillway is used when site conditions are not suitable to construct a drop spillway.

4. Side channel spillway:

A side channel spillway is located on the upstream side of a dam and are constructed to the side of a dam. The side channel is constucted nearly parallel to the crest. The water flowing over the crest gets collected by the side channel and is transferred to the downstream side by a chute or a tunnel.

5. Shaft spillway or Morning glory or Bell mouth spillway

In the shaft spillway a vertical shaft is constructed on the upstream side of the dam and it is connected to a horizontal tunnel which conveys water to the downstream. The water enters the funnel shaped vertical shaft constructed at the upstream side and moves down to the downstream side through a horizontal tunnel at the bottom.

6. Siphon spillway:

A syphon spillway consists of a syphon pipe whose one end is connected to the reservoir on the upstream side and the other end discharges water to the downstream end.

7. Labyrinth spillway:

A labyrinth spillway is a specially designed overflow weir. The weir is folded in plan view to provide a longer total effective length for a given overall spillway length. The total length of the spillway is three to five times its width. Its capacity is about twice as that of a standard or overflow spillway.