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Saint Anthony Falls Lock and Dam, which originally made it possible to travel upstream on the river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.
This episode is sponsored by Henson Shaving. More on them later.
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.
And here’s what can go wrong if you neglect to shave when you don’t look
good in a beard. My New Year’s Resolution is not to try and grow facial hair,
and I think it might be my wife’s resolution too. Last year Henson offered to sponsor an episode,
and I told them I wanted to try the razor first. So they sent me one, and I did try it. And I don’t
even know where my old razor is now because I never used it again after that. The Henson razor
was borne from a machine shop’s pivot away from aerospace parts during the pandemic slowdown,
and it shows. It’s just such a piece of craftsmanship. And here’s the thing:
Is a new razor going to change your life? Probably not. But, shaving’s a chore. A mundane chore for
most of us. And using a precision tool, at least to me, makes it feel less mundane, and instead, a
small part of my day that I actually look forward to. I had never used a safety razor before this,
but I’ve found that it’s easier to use because it doesn’t clog up, and the blade is supported across
the entire width, so it’s less likely to cause razor burn. I also thought $70 was a bit steep for
a razor until I did the math for how much I was spending replacing cartridge blades and realized,
for most people, it will pay for itself within a year. The blades for the Henson razor are so
cheap you could probably put a new one on for every shave and still save money. And in fact,
if you buy a razor, put the 100-pack of blades in your cart, and use my code PRACTICALENGINEERING at
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There’s no subscription service or a monthly fee, it’s just a cool razor that I really like
and I think you will too. Again, use my code PRACTICALENGINEERING at checkout to support
the channel and get 100-blades on me. Thank you for watching and let me know what you think.