How do different spillway gates work?

Types Of Spillway Gates

A spillway gate is also referred to as a stop gate. These are adjustable gates which can control the water flow of rivers, streams and reservoirs. Therefore, they act as a type of barrier as well for storage of additional water too. They help to pass water safely and in a controlled way around, over and through a dam when there is an excess of water. Let review different types of spillway gates and uses. 

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There are several types of spillway gates, each serving a specific purpose for dams. In this article, we will see what a spillway gate is and how they work.

How Do Spillway Gates Work?

Spillway gates are used to control the amount of water that enters into the reservoir or river. This is done by controlling the size of the opening between the gate and the reservoir. The smaller the opening, the more water will enter the reservoir. When the gate is closed, it creates a wall between the reservoir and the river. The larger the opening, the less water will enter the reservoir from the river.

With different types of spillway gates, each have its own function. There are three main ways that spillway gates work: gravity, hydraulics, and mechanical. Gravity-based spillway gates rely on the force of gravity to move them. Hydraulic-based spillway gates use hydraulic power to move them. Mechanical-based spillway gates are powered by motors.

Gravity-Based Spillway Gates

These types of gates are based on the principle of gravity. Gravity dams have a particular function. The weight of the gate causes it to fall. When the gate falls, it opens the gap between the upstream part and the downstream part. Once the gap is opened, water can flow freely between the upstream part and downstream part.

Hydraulic-Based Spillway Gate

These types of gates use hydraulic power to open and close them. The hydraulic system consists of two chambers separated by a diaphragm. One chamber contains air while the other contains water. When the valve is closed, the air fills the water chamber. This creates an air bubble which pushes down on the diaphragm. This forces the diaphragm to rise up and open the valve. When the valve is opened, the air bubbles escape out of the water chamber. This causes the diaphragm’s downward push to stop. This closes the valve again.

Mechanical-Based Spillway Gateway

These types of gates operate with motors. The motor moves the gate back and forth. When the gate is fully closed, the motor stops moving. When the gate is completely open, the motor starts moving again.

Different Types Of Spillway Gates

There are a range of different spillway gates that you may come across. Here at Gracon’s, we have vast experience with working with all of these different types of gates. 

Sluice Gate

The most common type of spillway gate is a sluice gate. It consists of two parts: an upstream part and a downstream part. Both parts are connected with each other via a hinge. The upstream part has a small hole on its top edge. Water flows through the hole and down into the downstream part. The downstream part has a large hole at the bottom edge. Water then flows out of the downstream part and into the reservoir. If you want to open the gate, you need to lift the upstream part so that it releases the water. Then you can lower the downstream part so that it closes the gap between the upstream and downstream part.

Slide Gate

Another kind of spillway gate is called a slide gate. It works similar to a sluice gate, but instead of having a hole on the top edge of the upstream part, it has a slot. You can see the difference below:

The advantage of a slide gate is that it does not require any lifting mechanism. All you need to do is slide the upstream part along the downstream part until it reaches the desired position.

Crest Gate

With a crest gate, they use a bottom hinge flap, which is used to control the water levels. This is a much more compact design, with the gate being built on a fixed axis. 

These are typically hydraulically operated, which makes the gates open when there are power issues or flooding. This type of gate will open and close with the hinges of this gate rotating. Making this gate the ideal option when you also need bees to pass other materials such as ice downstream. 

Radial Gate

This type of gate is positioned at the top of a dam, which increases the overall water and reservoir captivity of the dam. With this gate, the aim is to maintain the right water levels. The gate can be opened or closed with very little effort. As the pressure transfers from the curved face of the gate through to the support beams and into the curved arms, which provides much better resistance.

Miter Gate

The miter gate is also referred to as the canal lock or lock gates. As they are made with two leaves that offer closure at the one end of the lock which then forms an angle that is pointing up the stream. These are used to block the entrance and exit of the lock, which allows water to flow through. 

These gates help the water levels go up or down in the lock. They are still commonly used in waterways or canals today.

Drum Gate

A drum gate is a hollow structure which is seen floating on the water. It can rotate up or down and is hinged to the top of a dam in a horizontal cylinder. This cylinder can then be raised to allow more water to enter. 

These gates are perfect when you want to precisely control the water levels of your reservoir. 

What Are Spillway Gates Used For?

With different types of spillway gates, what are they used for? Spillway Gates are used for many purposes. Some examples include:

• To prevent flooding in reservoirs – flooding occurs when the level of water rises above the height of the dam. Dam operators use spillway gates to prevent this.

• Regulating water levels – when dams are built near cities, it is important to keep the water levels low enough so that people don’t get hurt if the dam breaks.

• Protect against erosion –  Erosion happens when the soil erodes away because of excessive water pressure. Dams are often built close to rivers and streams. As such, they may cause erosion. An example of this is the Grand Coulee Dam, located in Washington State.

• Store excess water – when dams are built, they usually create reservoirs. Reservoirs collect rainwater, snowmelt, and runoff from rivers and streams. However, sometimes these reservoirs regulate the amount of water coming into the city.

• Releasing water – when a dam is being repaired or rebuilt, the operator may decide to release some water stored behind the dam. 

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• Protects against erosion – sometimes dams are located in areas where there is a lot of soil movement. If the dam were to break, the soil would erode away. By using a spill gate, the operator can reduce the amount of soil movement.

• Control the flow of water – there might be times when the dam operator wants to change the direction of the water flow. He/She can do this by using a spillway gate. The gate allows him/her to direct the water towards it. In order to release some water, spillway gates are used.

Additionally, when dams are built near lakes, it’s important to make sure that the lake doesn’t overflow. This could damage the surrounding land. With spillway gates, the operator can regulate how much water goes where.

Why Is Maintenance Key For Spillway Operations?

Spillways play an important role in maintaining safe drinking water. They help prevent flooding and protect property. However, if spillways are not maintained properly, they could become ineffective. This would result in serious consequences. Below are some examples of how poorly maintained spillways can harm society. Different types of spillways gates can protect against these issues if maintained properly:

Flooding

Flooding occurs when the spillway doesn’t work correctly. If the spillway becomes blocked, then the water cannot drain away. This results in flooding. Flooding can damage houses and other structures. It can also kill animals and plants.

Property Damage

Damaging property is another problem caused by floodwaters. Damaged property can mean higher repair costs. It can also mean that insurance companies won’t cover your losses.

If you own a business, then you know how much money you spend every year on repairs. Damaging property can cost you more than just money. It can also hurt your reputation. Your customers will think less of you if you let them down.

Environmental Impact

Damaging property can lead to environmental problems. For example, if the spillway is leaking into rivers, this can pollute those areas. It can also contaminate soil and groundwater.

Health Issues

When the spillway leaks, it can release dangerous chemicals. These chemicals can enter our bodies through our skin or lungs. Some of these chemicals can cause cancer. Others can affect our nervous systems.

Safety Issues

Damaging property can make the spillway unsafe. A damaged spillway may fail at any time. This can put people in danger.

Types of Spillway Gates & Gracon Dam Services 

Here at Gracon’s, we have vast experience working with dam gate construction and replacements. As you can see, there are various spillway gates that can be used to control the water levels on a dam or reservoir. We have repaired and replaced over 100 gates. Therefore, you can trust us when it comes to your dam’s repairs.

We hope you have enjoyed this article, and now have a further understanding of spillway gates.

[Note that this article is a transcript of the video embedded above.]

In the heart of Minneapolis, Minnesota on the Mississippi River is the picturesque Upper Saint Anthony Falls Lock and Dam, which originally made it possible to travel upstream on the river past the falls starting in 1937. It’s a famous structure with a fascinating history, plus it has this striking overflow spillway with a stilling basin at the toe that protects the underlying sandstone from erosion. But there’s another dam just downstream, that is a little less-well-known and a little less scenic, aptly called the Lower Saint Anthony Falls Lock and Dam. Strangely, the spillway for the lower dam is less than half the width of the one above, even though they’re on the exact same stretch of the Mississippi River, subject to the same conditions and the same floods. That’s partly because, unlike its upstream cousin, the Lower Saint Anthony Falls dam is equipped with gates, providing greater control and capacity for the flow of water through the dam. In fact, dams all over the world use gates to control the flow of water through spillways.

If you ask me, there’s almost nothing on this blue earth more fascinating than water infrastructure. Plus I’ve always wanted to get a 3D printer for the shop. So, I’ve got the acrylic flume out, I put some sparkles in the water, and I printed a few types of gates so we can see them in action, talk about the engineering behind them, and compare their pros and cons. And I even made one type of gate that’s designed to raise and lower itself with almost no added force. But this particular type of gate was made famous in 2019, so we’ll talk about that too. I’m Grady, and this is Practical Engineering. On today’s episode, we’re talking about spillway gates.

Almost all dams need a way to release excess water when the reservoir is full. If you’ve ever tried to build an impoundment across a small stream or channel, you know how powerful even a small amount of flowing water can be. Modern spillways are often the most complex part of a dam because of the high velocities of flow. If not carefully managed, that quickly flowing water can quickly tear a dam apart. The incredible damage at Oroville Dam in 2017 is a striking example of this. Although many dams use uncontrolled spillways where the water naturally flows through once the reservoir rises to reach a certain level, gated spillways provide more control over the flow, and so can allow us to build smaller, more cost-effective structures. There are countless arrangements of mechanical devices that have been used across the world and throughout history to manage the flow of water. But, modern engineering has coalesced to variations on only a few different kinds of gates. One of the simplest is the crest gate that consists of a hinged leaf on top of a spillway.

A primary benefit of the crest gate is that ice and debris flow right over the top, since there’s nothing for the flow to get caught on. Another advantage of crest gates is that they provide a lot of control over the upstream level, since they act like a weir with an adjustable top. So, you’ll often see crest gates used on dams where the upstream water level needs to be kept within a narrow range. For example, here in San Antonio we have the RiverWalk downtown. If the water gets too low, it won’t be very attractive, and if it gets too high, it will overtop the sidewalks and flood all the restaurants. So, most of the dams that manage the flow of water in the San Antonio River downtown use steel crest gates like this one. Just up the road from me, Longhorn Dam holds back Ladybird Lake (formerly Town Lake) in downtown Austin. Longhorn Dam has vertical lift gates to pass major floods, but the central gates on the dam that handle everyday flows are crest gates. Finally, the dam that holds back Town Lake in Tempe, Arizona uses a series of crest gates that are lowered during floods.

Crest gates are attached to some kind of arm that raises or lowers the leaf as needed. Most use hydraulic cylinders like the one in Tempe Town Lake Dam. The ones here in San Antonio actually use a large nut on a long threaded rod like the emergency jack that comes in some cars. You might notice I’m using an intern with a metal hook to open and close the model crest gate, but most interns aren’t actually strong enough to hold up a crest gate at a real dam. In fact, one of the most significant disadvantages of crest gates is that the operators, whether hydraulic cylinders or something else, not only have to manage the weight of the gate itself but also the hydrostatic force of the water behind the gate, which can be enormous. Let’s do a little bit of quick recreational math to illustrate what I mean:

The gates at Tempe Town Lake are 32 meters or about 106 feet long and 6.4 meters or 21 feet tall. If the upstream water level is at the top of one of these gates, that means the average water pressure on the gate is around four-and-a-half pounds for every square inch or about 31,000 newtons for every square meter. Doesn’t sound like a lot, but when you add up all those square inches and square meters of such a large gate, you get a total force of nearly one-and-a-half million pounds or 660,000 kilograms. That’s the weight of almost two fully-loaded 747s, and by the way, Tempe Town Lake has eight of these gates. The hydraulic cylinders that hold them up have to withstand those enormous forces 24/7. That’s a lot to ask of a hydraulic or electromechanical system, especially because when the operation system fails on a crest gate, gravity and hydrostatic pressure tend to push the gate open, letting all the water out and potentially creating a dangerous condition downstream. The next kind of spillway gate solves some of these problems.

Radial crest gates, also known as Tainter gates, use a curved face connected to struts that converge downstream toward a hinge called a trunnion. A hoist lifts the gate using a set of chains or cables, and water flows underneath. My model being made from plastic means it kind of stays where it’s put due to friction, but full-scale radial gates are heavy enough to close under their own weight. That’s a good thing, because, unlike most crest gates, if the hoist breaks, the gate fails closed. The hoist is also mostly just lifting the weight of the gate itself, with the trunnion bearing the hydrostatic force of the water behind held back. These features make radial gates so reliable that they’re used in the vast majority of gated spillways at large dams around the world. If you go visit a dam or see a swooping aerial shot of a majestically flowing spillway, there’s a pretty good chance that the water is flowing under a radial gate.

The trunnion that holds back all that pressure while still allowing the gate to pivot is a pretty impressive piece of engineering. I mean, it’s a big metal pin, but the anchors that hold that pin to the rest of the dam are pretty impressive. Water pressure acts perpendicular to a surface, so the hydrostatic pressure on a radial gate acts directly through this pin. That keeps the force off the hoist, providing low-friction movement. But it’s not entirely friction-free. In fact, the design of many older radial gates neglected the force of friction within the trunnion and needed retrofits later on. I mentioned the story of California’s Folsom Dam in a prior video. That one wasn’t so lucky to get a structural retrofit before disaster struck in 1995. Operators were trying to raise one of the gates to make a release through the spillway when the struts buckled, releasing a wave of water downstream. Folsom Reservoir was half empty by the time they closed the opening created by the failed gate.

How did they do it? Stoplogs, another feature you’re likely to see on most large dams across the world. Just like all mechanical devices that could cause dangerous conditions and tremendous damage during a failure, spillway gates need to be regularly inspected and maintained. That’s hard to do when they’re submerged. The inspecting part is possible, but it’s hard to paint things underwater. In fact, it’s much simpler, safer, and more cost effective to do most types of maintenance in the dry. So we put gates on our gates. Usually these are simpler structures, just beams that fit into slots upstream of the main gate. Stoplogs usually can’t be installed in flowing water and are only used as a temporary measure to dewater the main gate for inspection or maintenance. I put some stoplog slots on my model so you can see how this works. I can drop the stoplogs into the slots one by one until they reach the reservoir level. Then I crack the gate open and the space is dewatered. You can see there’s still some leakage of the stoplogs, but that’s normal and those leaks can be diverted pretty easily. The main thing is that now the upstream face of the gate is dry so it can be inspected, cleaned, repaired, or repainted.

And if you look closely, it’s not just my model stoplogs that leak, but the gates too. In fact, all spillway gates leak at least a little bit. It’s usually not a big issue, but we can’t have them leaking too much. After all, there’s not much point in having a gate if it can’t hold back water. The steel components on spillway gates don’t just ride directly against the concrete surface of the spillway. Instead, they are equipped with gigantic rubber seals that slide on a steel plate embedded in the concrete. Even these seals have a lot of engineering in them. I won’t read you the entire Hydraulic Laboratory Report No. 323 - Tests for Seals on Radial Gates or the US Army Corps of Engineers manual on the Design of Spillway Tainter Gates, but suffice it to say, we’ve tried a lot of different ways to keep gates watertight over the years and have it mostly sealed up to a science now. Most gates use a j-bulb seal that’s oriented so that the water pressure from upstream pushes the seal against the embedded plate, making the gate more watertight. Different shapes of rubber seals can be used in different locations to allow all parts to move without letting water through where it’s not wanted.

In fact, there’s one more type of spillway gate I want to share where the seals are particularly important. Beartrap gates are like crest gates in that they have a leaf hinged at the bottom, but beartrap gates use two overlapping hinged leaves, and they open and close in an entirely different way. The theory behind a beartrap gate is that you can create a pressurized chamber between the two leaves. If you introduce water from upstream into this chamber, the resulting pressure will float the bottom leaf, pushing it upward. That, in turn, raises the upper leaf. The upstream water level rises as the gate goes up, increasing the pressure within the chamber between the gates. The two leaves are usually tied in a way that once fully open, they can be locked together. To lower the gates, the conduit to the upstream water is closed, and the water in the chamber is allowed to drain downstream, relieving the upward pressure on the lower leaf so it can slowly fall back to its resting position. It sounds simple in theory, but in practice this is pretty hard to get right.

I built a model of a bear trap gate that mostly works. If I open this valve on the upstream side, I subject the chamber to the upstream water pressure. In ideal conditions with no friction and watertight seals, this would create enough pressure to lift both leaves. In reality, it needs a little bit of help from the intern hook. But you can see that, as the water level upstream increases, the lower leaf floats upward as well. When the gates are fully opened, the leaves lock together to be self-supporting. Some old bear trap gates used air pressure in the chamber to give the gates a little bit of help going up. I tried that in my model and it worked like a charm. It took a few tries to figure out how much pressure to send, but eventually I got it down.

It’s not just my model bear trap gate that’s finicky, though. Despite the huge benefit of not needing any significant outside force to raise and lower the gates, this type of system has never been widely used. 

This chamber between the leaves is the perfect place for silt and sand to deposit. They were also quite difficult to inspect and maintain because you had to dewater the entire chamber and reroute flows. And because they weren’t widely used, there were never any off-the-shelf components, so anytime something needed to be fixed, it was a custom job. The world got to see a pretty dramatic example of the challenges associated with maintaining old bear trap gates in 2019 when one of the gates at Dunlap Dam near New Braunfels, Texas completely collapsed.

This dam was one of five on the Guadalupe River built in the 1930s to provide hydropower to the area. But over nearly a century that followed, power got a lot cheaper, and replacing old dams got a lot more expensive. Since the dam wasn’t built with maintenance in mind, it was nearly impossible to inspect the condition of the steel hinges of the gate. But that lack of surveillance caught up with the owner on the morning of May 14, 2019 when a security camera at the dam caught the dramatic failure of one of the gate’s hinges. The lake behind the dam quickly drained and kicked off a chain of legal battles, some of which are still going on today. Luckily, no one was hurt as a result of the failure. Eventually, the homeowners around the lake upstream banded together to tax themselves and rebuild the structure, a task that is nearly complete now more than three years later. Of course, there’s a lot more to this fascinating story, but it’s a great reminder of the importance of spillway gates in our lives and what can go wrong if we neglect our water infrastructure.

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