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. Hey, I’m Grady and this is Practical Engineering. On today’s episode, we’re talking about
spillways. This episode is sponsored by Skillshare. More on that later. 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 here 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 used 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 like I’m
doing here in my garage. 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 to Skillshare for sponsoring the video. As I record this, Practical Engineering is
about to cross the million subscriber mark, which is really exciting for me. Besides the awesome fans, I think a big part
of the channel’s success has been than I’m constantly trying to learn new skills and
improve the quality of these videos. One of they ways I do this is through Skillshare,
an online learning community for creators with more than 25,000 classes in design, business,
and more. Premium membership to Skillshare gives you
unlimited access to high quality classes from experts working in their fields to help you
gain new skills and live your best life. One I’m watching right now is by Simon Sinek
about how to make presentations more interesting and compelling. Getting tips and tricks from a successful
presenter like Simon is invaluable to me, and there’s nowhere else you can find content
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improve on an existing one, cut through the clutter of online tutorials and click the
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2 month free trial. Again, thank you for watching, and let me
know what you think!
Ive used some of his videos in meetings for completely non CE things since we deal with a lot of open channel type flow equipment
this dude is awesome.
Awesome info for those of us affected by flooding here in Oklahoma.