Dams are important structures that are constructed to store water. This water, generally from a channel, can later be used for work involving electricity and irrigation. Dams are also critical in flood control. What is a dam spillway and why are they important?
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Although the majority of dams are built using high tech designs, some experience an overflow of water. If this occurs, the whole structure and design of the dam can become damaged. Therefore, excess water needs to be discharged successfully to avoid any potential disasters.
What is a dam spillway? A spillway offers a way for the stored excess water to discharge. This is why most dams should be designed with at least one spillway. Whether water is required downstream or the stored water behind the dam has become overwhelming, just about every dam requires a spillway to safely discharge certain amounts of water.
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Designing dams would be more straightforward if environmental factors such as rainfall and snowmelt were consistent throughout the year. As a matter of fact, since most dams exist because of hydrologic variability, they wouldn’t be necessary if rainfall was consistent.
Dams exist to provide storage for water and to smooth out the varying nature of inflows. These structures protect us from flooding so the stored water can be used for different purposes during the year.
Those varying inflows are not always modest, however. Some can be gigantic with various watersheds generating the majority of their annual volume of water in just one storm event. In such a scenario, these inflows can race to a reservoir with little to no warning. Therefore, dams need to be ready to withstand and handle any major storms.
Because of each season’s variability in terms of weather, engineers can not predict the exact volume of floodwater due to melting snow and rainfall. Therefore, designing a dam for a specific discharge of water through its channel is nigh on impossible.
To combat this, spillways are included in most dams. If there is a sudden rise of water upstream of a dam, a spillway will allow the water to discharge through it and stop the dam from overflowing.
Spillways are made with the aid of a penstock and a large cross-sectional tube. A connection is made between the inlet of the spillway upstream and the outlet of the spillway downstream from the dam.
Spillways are classed into four distinct categories:
Each of these serves specific purposes for specific conditions. But, they all share the role of discharging water to inhibit any flooding from rising water levels. What is a dam spillway and different types? We’re explaining more here:
An overflow spillway is typically used with concrete gravity dams. These are usually gated or ungated and allow water to overflow if it builds up behind the dam.
A chute spillway is usually constructed for earth or rock-filled dams. Nevertheless, some concrete dams also use chute spillways. If this is the case, the dam would typically be located in a small canyon where there wouldn’t be enough space for an overflow spillway.
You will generally find a chute spillway through the abutment. This is adjacent to the dam itself. It can also be located in a saddle, found a little away from the dam.
Side-channel spillways are used in similar circumstances to chute spillways. Compared to other spillways, a side-channel has a more unique shape. This allows it to be built on a narrow dam abutment.
In general, these spillways are ungated but they can be designed with gates if required.
A limited service spillway is designed if the spillway will only be operated infrequently and, when this occurs, damage is possible. However, this damage can not be so bad that it causes a catastrophic release of water from the reservoir.
Other types of spillways include:
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A siphon spillway works as the name suggests, like a siphon. These are built with a hood over a standard spillway which forms a conduit. When the reservoir’s water level rises, it will start to flow over the crest. This is regarded as an ogee spillway.
This flowing water creates tiny air bubbles. Once the air in the crest is removed, the siphon action begins. This type of spillway has a larger discharging capacity meaning more water can flow.
This spillway uses a circular crest that is built into the surface of the reservoir. Here, the water flows over having been carried by a sloping or vertical tunnel that travels to a horizontal tunnel around the stream bed level. Eventually, this travels to the downstream side. Diversion tunnels constructed during the construction of the dam can later be used as the horizontal conduit in some cases.
Most large dams are constructed with two spillways with one called the service spillway and the other the emergency spillway. The emergency spillway, also known as an auxiliary spillway, is operated when there is an excess flow of water located upstream of the channel.
Ogee spillways are usually constructed in rigid dams. These form part of the main dam if there is enough length to do so.
This is a modified version of a drop spillway. In this case, the spillway’s downstream profile coincides with the lower nappe’s shape of the flowing, falling water jet from the crested weir.
What is a dam spillway? As explained, the main function of a spillway is to control the overflow of water that is stored upstream of a dam in a reservoir. It is simply constructed to maintain the level of water in the reservoir.
Spillways control this level of water without the need for moving parts or mechanical devices. If the level of water reaches above the spillway, then it will automatically fall through the pool of the spillway and escape through the outlet.
However, on occasions, spillways will require maintenance in order to withstand and allow the safe flow of water.
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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!
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