We normally build a dam to hold water back and store it for use in water supply, irrigation, hydropower, or flood control. But sometimes we have to let some water go. Whether we need it downstream or the impounded water behind the dam is simply too full to store any more, nearly every dam needs a spillway to safely discharge water.

To understand spillways, we have to start with hydrology. More specifically, we need to understand the tremendous variability in inflows that can affect dams and reservoirs. Designing a dam would be simple if rainfall and snowmelt were consistent throughout the year. In fact, most dams wouldn’t even be necessary, since hydrologic variability is the reason why most dams exist in the first place - to provide storage of water and smooth out the ebbs and spikes of inflows to protect us from flooding or so that water can be used to meet our needs throughout the year. But, those spikes of inflow can be enormous. It’s not unusual for a watershed to generate the majority of its entire annual volume of water in a single storm event. Those inflows can reach a reservoir with very little warning, so dams need to always be ready to handle major storm events.

As far as infrastructure goes, dams are fairly risky. Depending on the size of the structure and what’s downstream, the failure of a dam can be catastrophic. In fact, some of the worst human-caused disasters in history have been failures of dams. For this reason, they’re often required to withstand the biggest storm that we could possibly conceive, called the Probable Maximum Flood. It’s too expensive to build a dam so tall that it can store the entirety of this flood. On the other hand, we can’t just let the flood overtop the dam, because flowing water can damage and destroy the structure. So in most cases, dams are designed with at least one spillway, a structure that can safely discharge floodwaters without causing injury or deterioration to the dam.

The water stored behind a dam is called its reservoir, and the term “spillway” usually is reserved for structures that release excess inflows, when the reservoir is already full (e.g. floods or heavy snowmelt). This distinguishes spillways from other structures that provide releases from reservoirs like intakes that serve pump stations and penstocks that serve hydro turbines. Because of the variability in inflows, many large dams have two or more spillways. The smaller one is called the principal or service spillway that passes normal inflows when the reservoir is full. And, the other is called the auxiliary or emergency spillway that only engages during extreme events. Depending on the design, the auxiliary spillway may only flow for a few scary moments in a dam’s entire lifetime. Because of that, they can be as simple as an excavated channel cut around the dam. It might not last very long, but it can protect the dam from failure in an extreme situation.

No matter how often it flows, a spillway has only three main jobs, and there is a wide variety of types of structures that can accomplish these objectives. But I think if you’re going to demonstrate a spillway on the internet, there’s only one obvious choice for the model: the morning glory. This is a type of drop shaft spillway that has enchanted the internet with crazy vortex photos, and I built a model of one in my shop so we can use it to discuss the basic functions of a spillway. And the first basic function is the most obvious: to manage the water level in a reservoir.

A morning glory spillway is in a class of spillways that we call uncontrolled. In general, they are set and forget. There are no gates or moving parts to manage. They regulate the reservoir level simply by existing. If it gets too high, water flows out and the pool goes down. If the pool is below the crest, no water is released, and the level goes up as precipitation makes its way into the reservoir. Most uncontrolled spillways are weirs, which I covered more in a previous video. A weir is simply a structure that allows water to pass over its crest. The morning glory acts like a circular weir at first, but as the water level goes up, the bell mouth chokes and the behavior changes. This type of spillway is used in narrow canyons where there isn’t much room for a more conventional overflow. Uncontrolled spillways normally need to be pretty big to handle even the largest storms that a reservoir might face without any moving parts. That can get expensive quick, so an alternative can be to use controlled spillways with different types of gates. The gates add complexity to a spillway, but they can also reduce its cost by providing flexibility in discharge capacity allowing for a smaller overall structure. The gates can be operated to match the any size of storm event, even if the spillway is relatively small.

The next job of a spillway is to safely convey the flow to the downstream side of the dam. In most spillways, including my model, the water has to get from the top of the reservoir to a natural watercourse downstream of the dam. That’s often a big drop in elevation, which means the water can pick up a lot of speed. This high velocity flow can cause major damage, so we need some way to contain it safely. Sometimes that’s a pipe or conduit like in my model drop shaft spillway. And for open-channel spillways, it’s called a chute. A chute also needs training walls on the sides to keep the flow contained. Both spillway conduits and chutes are often made of concrete too because it’s one of the only materials strong enough to resist the damaging forces of the high velocity flow. That leads me to the final objective of a spillway: energy dissipation.

I mentioned that all the water moving so quickly can cause serious erosion downstream of a dam. If not controlled, this erosion can progress upstream, eventually leading to failure of the dam. So, all spillways need a way to dissipate hydraulic energy and slow down the flow before releasing it into a natural watercourse. For large spillways, this is often accomplished in a structure called a stilling basin that forces a hydraulic jump to occur. This is another topic I covered in a previous video, so check that out if you want to learn more. For smaller spillways, the dissipation can be simpler like rock riprap or even just letting the flow plunge into a deep pool.. Once most of the hydraulic energy is lost, the water can safely travel downstream without causing damage.

Like most of my videos, I’m just scratching the surface of a gigantic topic. The spillway is a critical part of any dam and often the most complex component. Designing a spillway usually requires a team of engineers performing structural, geotechnical, electrical, mechanical, hydrologic, and hydraulic analysis to get it right. All so we can safely discharge water from a reservoir during high inflow events when there’s no more room to store it. Thanks for reading this blog and please let me know what you think!