Types of Spillways - Classification of Spillways

          There are different types of spillways that can be provided depending on the compatibility of the site and other parameters. Typically a spillway consists of a control structure, a conveyance channel and a terminal structure, but the former two may be combined in the same for certain types. The more common types are briefly discussed below:

1. Straight Drop Spillway

2. Ogee Spillway

3. Siphon Spillway

4. Chute or Trough Spillway

5. Shaft Spillway

6. Side Channel Spillway

1. Straight Drop Spillway:

          In a Straight drop spillway, when the water level in the reservoir rises above the normal pool level, the over flowing water falls freely and almost vertically on the downstream side of the hydraulic structure, and hence it is referred as Straight drop spillway (or) free overfall spillway. This sort of spillway is suitable for weirs (or) low dams.

          The crest of the spillway is provided with nose so that the water jet might not strike the downstream base of the structure. To prevent the scouring of downstream bed from falling water jet, an artificial pool (which is known as water cushion) with a concrete apron (horizontal impervious apron) and low secondary dam is made on the downstream side. This cushion serves the purpose of energy dissipater.

          Proper ventilation should be provided on the bottom 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.

2. Ogee Spillway:

Ogee-spillway-of-Shahghasem-dam

          Ogee spillway represents the shape of the downstream face of the weir. In this case, the bottom face of the weir is constructed corresponding to the shape of lower nappe of freely falling water jet, which is in ogee shape. This is a modified form of straight drop spillway. The Ogee spillway is usually provided in rigid dams and forms a part of the main dam itself if sufficient length is available. 

          The shape of the lower nappe isn't same for all the head of water above the crest of the weir. It differs with the head of water.

          Whenever there is surplus water, it will be freely disposed of through ogee spillway along its ogee shaped crest, the overflowing water just follows the curved profile of the spillway and there is no gap between the water and the spillway surface and the discharge is maximum. Ogee spillways are most commonly preferable in case of gravity dams, arch dams, buttress dams, etc., For gravity dams, it is generally located within the dam body.

3. Siphon Spillway:

          A siphon spillway is a type of spillway where the surplus water is disposed to downstream through an inverted U-shaped conduit. It is generally constructed inside the body (or) over the crest of the dam. On siphon principle this spillway works. This spillways are having high discharge capability.

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

4. Chute Spillway:

          The Chute spillway is a type of spillway in which overflowing 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 faraway from the dam in a natural saddle in a bank of the river. The spillway can be provided close to the dam or at an appropriate saddle away from the dam where site conditions permit.

Oroville Dam Chute Spillway

          Chute spillway is preferred when the width of the river valley is very narrow. This spillway is mostly suitable for gravity dams, rockfill dams, earthen dams. These spillways are also termed as "Trough spillway" (or) "Open channel spillway".

          The slope of spillway is designed that it should maintain a supercritical flow. Energy dissipators can be provided on the bed of chute spillway to dissipate energy from the falling water.

5. Shaft Spillway:

          A Shaft spillway is a type of spillway which consists of a vertical shaft connected by a horizontal conduit. The overflow water in the reservoir enters into the vertical shaft and then flows to the horizontal conduit and eventually reaches the downstream of the channel.

          The shaft constructed is either artificial or natural. Only when the hard rock strata is present then it is suitable for natural shaft otherwise we should choose artificial shaft constructed with heavily reinforced concrete. The horizontal conduit either passes through the dam body or through the foundation of the dam.

Ladybower Reservoir East spillway shaft

          In the case of major projects, the inlet hole of the vertical shaft is specially shaped (a stepped circular rings shaped) which is called as "Glory hole of the spillway". Hence, shaft spillway is also referred as "Morning glory spillway" (or) "Bell mouth spillway". Shaft spillways are preferred when there is no space to provide for other sorts of spillways such as Ogee spillway, Straight drop spillway, etc., 

6. Side Channel Spillway:

          Side channel spillways are located just upstream and to the side of the dam. The water spilling from the crest is turned 90 degrees and flows parallel to the crest of side channel spillway. Sometimes a tunnel could also be used rather than a chute.

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