The Most Dangerous Dams


Dams serve a wide variety of purposes from
hydropower to flood control to storage of water for municipal and industrials uses. But when a dam’s useful purpose fades away,
the structure itself still remains. Dams come in all shapes and sizes, but contrary
to what you might think, the most dangerous dams are often the smallest. Hey I’m Grady and this is Practical Engineering. Today we’re talking about the dangers of
low-head dams. This video is sponsored by Squarespace. Visit Squarespace.com/PracticalEngineering
to get a free trial, and use code PracticalEngineering to get an additional 10% off. More on that later. A low head dam, sometimes simply called a
weir, is a small structure that impounds a small amount of water and spans the width
of river or stream. Usually made from concrete, the purpose of
a low head dams is to raise the water level upstream on a river. This can assist with navigation of the channel
by boats, create a drop for generating hydropower, and make water available at intakes for water
supply and irrigation. Thousands of these structures have been constructed
over the years to take advantage of natural watercourses and rivers. The heyday of low head dam construction was
actually in the 1800s when mills and factories often relied on waterpower to drive grinding
wheels and other equipment. This was at a time when moving water was the
most consistent source of power available in large quantities before widespread adoption
of electricity. Most of these old mills and factories are
long gone, and the ones that still survive certainly don’t depend on water for power
anymore. That means many property owners are forced
to maintain these old structures that no longer have any practical use. Or more commonly and much worse, these dams
are abandoned by their owner and gradually fall into disrepair. In the U.S., dam safety regulations focus
primarily on the possibility of a dam breaching and causing a flood wave downstream. But, because low head dams are relatively
short, a breach poses minimal danger, so most states don’t keep track of these small structures. And, especially if they’ve been abandoned,
it can be difficult to enforce maintenance requirements on the owners. But, even though they pose little danger in
the event of a breach, low head dams create a public safety issue that has caused more
fatalities in the U.S. than all dam failures in the past 20 years. To understand why, we first need to know a
little bit about open channel hydraulics. If you haven’t seen my video about hydraulic
jumps, I’ll summarize it here. Go back and check out that video if you want
to learn more. Open channel flow – that’s flow not confined
within a pipe – has a very important property related to its velocity that governs its behavior. Slow, tranquil flowing water is called subcritical
because waves propagate faster than the flow velocity. Fast moving water is supercritical because
waves move slower than the flow velocity. Any time a supercritical flow encounters subcritical
flow, an interesting phenomenon called a hydraulic jump is formed. Low head dams almost always have subcritical
flow upstream. The flow is deep, slow, and tranquil as it
makes its way to the dam. But as the flow passes over the weir, it picks
up speed and becomes supercritical. When this supercritical flow transitions back
to subcritical flow in the slower moving water downstream, it creates a hydraulic jump as
you can see here in my model flume. It’s easy to see why these types of structures
could pose a threat to those using the waterway for recreation. Any location with fast moving water and high
turbulence can be dangerous to swimmers or kayakers, but the location of this hydraulic
jump can turn a manageable risk into an almost surefire way to drown. The depth of the flow downstream of a dam
is called the tailwater, and it controls the location of the hydraulic jump. In my model, I can adjust the elevation of
the tailwater by adding or removing these stoplogs. When tailwater is low, the hydraulic jump
forms away from the dam. This is a fully developed jump that follows
the traditional shape and flow patterns. If I send down this piece of wood as a kayaker
surrogate, it experiences some turbulence as it passes over the weir and through the
jump but, it doesn’t have much trouble escaping downstream. But, as the tailwater rises the jump moves
closer and closer to the dam. Eventually if the tailwater is high enough,
the hydraulic jump will reach the dam. This condition is called a submerged or drowned
jump. It may look fairly innocuous, but this is
when things get dangerous. Let’s send down our kayaker surrogate to
see why. A submerged hydraulic jump creates an area
of recirculation immediately downstream of the dam sometimes called a “keeper” for
obvious reasons. The jet of the hydraulic jump surfaces downstream
causing a boil point. Sometimes this is easy to see and sometimes
it’s not. Either way, objects or people can will only
be able to escape a submerged hydraulic jump if they are able to get beyond this boil point. And, any rescuers who approach a submerged
jump from downstream run the risk of being drawing into the hydraulic themselves. The recirculating currents that trap recreators
is dangerous enough on its own but there are other factors contributing to the danger at
low head dams. These currents also trap large debris between
the strong hydraulic forces and the hard concrete surface of the dam which can batter someone
trapped in the keeper. The water is often cold, increasing the potential
for hypothermia and further disorientation. The turbulence of the hydraulic jump entrains
a lot of air, reducing the buoyancy of a swimmer. And, low head dams often span the entire width
of the river, meaning there is no still water nearby that can be used as a safe haven. This is exactly why the low head dam is called
the perfect drowning machine. All these factors added together create a
situation that’s almost impossible to survive. There are a lot of ways to mitigate this issue. The simplest option is just to keep people
away from these structures. Some states require that exclusion zones be
established to make sure that kayakers safely portage dams instead of trying to run them. Good signage and buoys as warnings can sometimes
be enough to keep people safe. Another option is to modify the structure
to reduce the potential for recirculating currents. Researchers have proposed various retrofits
to existing dams to improve flow conditions when tailwater is high. Of course, the most obvious (but also most
expensive) way to address the issue is to remove these dams altogether. In many cases they are no longer serving an
important role, and removing dams can help restore ecosystems and improve connectivity
for aquatic species in addition to removing a hazard. If you’re swimming or paddling on a river
with a low head dam, don’t underestimate the danger of these powerful hydraulic forces. Different flow conditions on the river can
dramatically change the behaviour of the hydraulic jump, as we saw, so be careful. Thank you for watching and let me know what
you think! If you’re new to the channel, you might not
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