What is Spillway?
Spillway is the hydraulic structure constructed in the dam to provide a safe way for diverting surplus water from the reservoir after it has been filled to its max level of capacity. Spillways function as safety valves for the dam and are provided for all types of dam.
Spillways are constructed entirely away from the dam as independent structure or located at the end of dam structure or within the dam body. It is important to give a spillway of enough capacity. A spillway of insufficient capacity may be responsible for overflowing of the dam, serious damage to the dam or failure of the dam structure.
Design Criteria for Spillway
- The spillway must be both structurally and hydraulically adequate.
- The spillway must have enough capacity.
- It must be located in such a way that it permits safe excess water disposal.
- The surface of the spillway must be erosion resistant to resist the high scouring velocity produced by the drop from the reservoir surface.
- Normally, some types of energy dissipation device will be required on the downstream side of the spillway.
Various Types of Spillway
- Straight Drop Spillway or Free Overfall Spillway
- Ogee Spillway or Overflow Spillway
- Chute Spillway or Open Channel Spillway
- Side Channel Spillway
- Shaft Spillway
- Siphon Spillway
Straight Drop Spillway
A straight drop spillway or free overfall spillway is a simple and vertical wall type low weir spillway. When a reservoir’s water level increases over the normal pool level, the surplus water falls from the crest of the weir freely and thus the name of spillway is straight drop spillway.
On the downstream side, an artificial pool with concrete base is constructed to prevent scouring of downstream beds by falling water jets. Straight drop spillways work best with earthen dams or thin arch dams.
Ogee Spillway
The name Ogee indicates the shape of the weir on the downstream side. The Ogee spillway is a better improvement of the straight drop spillway. It is constructed within the body of the gravity dam. In this case, the downstream face of the weir is constructed to match the lower nape of the free fall water jet (ogee shaped). The Ogee face of the downstream side is designed on the foundation of the principle of projectile.
Surplus water is freely discharged through its ogee-shaped crest, so it is called overflow spillway. Ogee spillway commonly used in gravity dam, buttress dam and arch dam.
Chute or Open Channel Spillway
A chute spillway is a type of spillway that diverts surplus water through steep sloped open channels from upstream to downstream.It is generally constructed at the end of the dam in a natural saddle on river bank.
A chute spillway is suitable for gravity dam, earthen dam or rockfill dam. Although, it is suggested when the valley of the river is very narrow.The water flows rive d/s through steep sloped open channels. The “chute” term refers to a steep sloped open channel spillway.
Side Channel Spillway
Side channel spillway is similar to the open channel spillway. The basic difference is that the crest of the side channel spillway is located on one of its sides and the crest of the open channel spillway is situated between sidewalls.
Shaft Spillway
Shaft spillways consist of a vertical shaft followed by a horizontal tunnel. The surplus water enters the vertical shaft, then enters the horizontal tunnel and finally discharges to the downstream.
The shaft can be either natural or artificial.Vertical shaft can be located at the upstream side of the dam and the horizontal tunnel either passes through the foundation of the dam or body of the dam. Shaft spillways are suggested when there is inadequate space to locate other types of spillways, such as straight drop spillway, ogee spillway etc.
Siphon Spillway
A siphon spillway is a type of spillway that diverts surplus water downstream through an inverted U-shaped conduit that is located inside the dam.
Some types of siphon spillway are given below.
- Tilt out type of siphon spillway
This type of siphon spillway is recommended when the valley is too narrow for building a separate spillway, siphon pipe can be set up within the body of the dam and air ventilation is joined to siphon pipe. While point of entry kept low so as to prevent entry of debris in the siphon. As soon as the level of water rises, the suction in the air ventilation pipe initiates. Siphonic action will continue till the level of water goes to normal pool level.
- Hooded type of siphon spillway
In this type of siphon spillway, a RCC hood is located over the overflow section. Inlet kept submerged to avoid entry of floating matters. A little hood is kept above the main hood and joined by air ventilation.
- Volute siphon spillway
This type of siphon spillway consists of bent at discharge end and opened out at the top in the form of a funnel. The lip of the funnel was kept at the permissible level of the reservoir. To start spiral motion, a number of blades are fixed in the funnel. When water level increases, it seals air entry and water sheet initiates flowing from all sides.
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