Even the largest dams, such as the Hoover Dam, contain spillways. Spillways can come in several different types, including those with a controlled gate and those without.
The first spillway was a siphon spillway. While siphons have been in use for more than 2,000 years, they were not used for spillways until approximately the 1840s. They made their way to Europe in the 1870s. The first spillways were not automatic like they are today. They had to have an ejector constructed within them to incite the water. Spillways with automatic priming tools appeared on the scene shortly afterwards in both Italy and Prussia. Although they weren’t the most efficient of spillways, their existence would prove pivotal for the construction of dams for the future to come.
Spillways appeared in France and made their way to America in the 1900s. The first siphon spillway to be built in the U.S. was constructed either by engineer G.H. Stickney in 1909 or another engineer by the name of W.P. Creager.
 How it Works
Spillways work in a similar manner to the overflow hole of a bathroom tub. When the water reaches the height of the spillway opening, it escapes through the hole, rather than overflowing the dam. Spillways are typically located at enough distance below the top of the dam to ensure that the structure does not reach dangerously high levels of flooding before the water is released through the spillway. The Hoover Dam spillway, for example, is 89 feet (27 m) below the dam's top. There is one spillway on either side of the dam.
When water gets into the spillway, it is transported through tunnels that can be as much as 50 feet (15 m) in diameter and 600 feet (183 m) long. The spillway tunnels are constructed with an inverted design and eventually meet with the other diversion tunnel.
Spillways are constructed to contain a large volume of water. The Hoover Dam spillways, for example, are each capable of 200,000 cubic feet (5,663 m3) per second of water.
Spillways comprise steel drum gates and work to contain water that can be stored in the reservoir. The Hoover Dam consists of four steel drum gates that are 100 feet (30 m) long, 16 feet (4.9 m) high and 5,000,000 pounds (2.3 million kg) each. They are automatically designed to raise and lower the gates as needed. The gate can be held in the raised position for as long as required by the water pressure.
The Hoover Dam spillway has only been used twice in the history of its existence. On one occasion, it was a for a test; the second occasion was for a flood that occurred in 1983.
Spillways can be designed in the shape of tunnels or as open-cut channels, the latter being the most popular. Tunnel designs typically do not have as much capacity as the open-cut channels.
When designing a spillway dam, engineers consider factors such as the capacity of floodwater that a spillway should be able to manage, and how much it could handle.
Controlled spillways are structures with built-in gates that can be opened and closed, regulating how much and for how long, water can escape through it. Uncontrolled spillways, on the other hand, do not consist of gates, and when water is released into the spillway hole, it is transported directly into a reservoir.
Another type of spillway is the inverted bell-shaped spillway. Also known as the morning glory, this type of spillway is uncontrolled and allows water to circle the perimeter of the bell before entering the spillway hole in the center.