7 Different Types of Spillways

A spillway is a hydraulic structure built at a dam site for diverting the surplus water from a reservoir without it has been filled to its maximum capacity. Spillways are classified into variegated types on the understructure of the wattle of the tenancy structure, a conveyance waterworks and a terminal structure. In this article, we will discuss in unenduring all the variegated types of spillways with pictures.

Types of Spillways

Different types of spillways are as follows:

  1. Straight Drop Spillway
  2. Ogee Spillway
  3. Shaft Spillway
  4. Chute Spillway
  5. Side Channel Spillway
  6. Siphon Spillway
  7. Labyrinth Spillway

1. Straight Drop Spillway

A Straight drop spillway consists of low height weir wall having its downstream face roughly or perfectly vertical. When the water level in the reservoir rises above the normal pool level, the surplus water falls freely from the crest of the weir and hence it is known as Straight drop spillway or free overfall spillway.

Straight Drop Spillway
Fig 1: Straight Drop Spillway

To prevent the scouring of downstream bed from falling water jet, an artificial pool with a concrete apron and low secondary dam is constructed on the downstream side. Proper ventilation should be provided on the underside portion of a falling jet to prevent pulsating and fluctuating effects.

Sometimes, an overhanging projection is provided on the crest of the weir to prevent the entrance of small discharges onto the face of the weir wall. Straight drop spillways are most suitable for thin arch dams, earthen dams or bunds.

Straight Drop Spillway Components
Fig 2: Straight Drop Spillway Components

2. Ogee Spillway

Ogee spillway, as the name says, represents the shape of the downstream squatter of the weir. It is an improved form of a straight waif spillway. In this case, the downstream squatter of the weir is synthetic respective to the shape of lower nappe of freely falling water jet which is in ogee shape.

The ogee shape of the downstream squatter is designed on the understructure of the principle of a projectile. In general, the shape of lower nappe of the water jet is not unvarying for all water heads hence, the shape obtained for the maximum throne is taken into worth while designing ogee spillway.

Ogee Spillway of Walayar dam, India
Fig 3: Ogee Spillway of Walayar dam, India

Whenever there is surplus water, it will be freely disposed of through ogee spillway along its ogee shaped crest hence it can also be called as an overflow spillway. Ogee spillways are most commonly used in case of gravity dams, arch dams, buttress dams, etc. For gravity dams, it is generally located within the dam body.

3. Shaft Spillway

A Shaft spillway is a type of spillway which consists of a vertical shaft followed by a horizontal conduit. The surplus water enters into the vertical shaft and then to the horizontal conduit and finally reaches the downstream of the channel.

Shaft Spillway
Fig 4: Shaft Spillway

The shaft synthetic is either strained or natural. Excavation for the natural shaft is possible only when the nonflexible rocky layer is present on the upstream side. The horizontal conduit either passes through the dam soul or through the foundation of the dam.

In the specimen of large projects, the inlet slum of the vertical shaft is specially shaped which is tabbed as morning glory or glory slum of the spillway. Hence, shaft spillway is moreover tabbed as Morning glory spillway or Bell Mouth spillway. Shaft spillway is recommended when there is no space to provide for other types of spillways such as ogee spillway, straight waif spillway, etc.

Shaft Spillway, Monticello Dam, USA
Fig 5: Morning Glory Spillway, Monticello Dam, USA (Source : gfycat.com)

4. Chute Spillway

Chute spillway is a type of spillway in which surplus water from upstream is disposed to the downstream through a steeply sloped open channel. It is generally constructed at one end of the dam or separately away from the dam in a natural saddle in a bank of the river.

Chute spillway is suitable for gravity dams, earthen dams, rockfill dams, etc. But it is preferred when the width of the river valley is very narrow. The water flows along the steeply sloped chute or trough or open channel and reaches the downstream of the river. Chute spillway is also called as trough spillway or open channel spillway.

The slope of chute spillway is designed in such a way that the flow should be always in supercritical condition. To dissipate energy from the falling water, energy dissipators can be provided on the bed of chute spillway.

Chute Spillway, Tehri Dam, India
Fig 6: Chute Spillway, Tehri Dam, India

5. Side Channel Spillway

Side channel spillway is similar to chute spillway but the only difference is the crest of side channel spillway is located on one of its sides whereas crest of chute spillway is located between the side walls. In other words, the water spilling from the crest is turned to 90 degrees and flows parallel to the crest of side channel spillway unlike in chute spillway.

Side Channel Spillway
Fig 7: Side Channel Spillway

Side channel spillways are preferred over chute spillways when flanks of sufficient width are not available, usually to avoid heavy cutting. The angle of turn of water flow after passing weir crest can also be kept between 00 and 900.

Side Channel Spillway of Hoover Dam, USA
Fig 8: Side Channel Spillway of Hoover Dam, USA

6. Siphon Spillway

A siphon spillway is a type of spillway in which surplus water is disposed to downstream through an inverted U shaped conduit. It is generally arranged inside the body or over the crest of the dam.

In both types of siphon spillways, air vents are provided at the bent portion of the upper passageway to prevent the entrance of water when the water level is below the normal poll level. Whenever the level rises above normal pool level, water enters into the conduit and is discharged to the downstream of the channel by siphonic action.

Siphon Spillway
Fig 9: Siphon Spillway

7. Labyrinth Spillway

A labyrinth spillway is a type of spillway in which the weir wall is constructed in a zigzag manner in order to increase the effective length of the weir crest with respect to the channel width. This increase in effective length raises the discharge capacity of the weir and hence higher water flow at small heads can be conveyed to the downstream easily.

Labyrinth spillway of Ute Dam, Mexico
Fig 10: Labyrinth spillway of Ute Dam, Mexico
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