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Grady Hillhouse
Practical Engineering
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.
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.
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.
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!
Hey,
I’m Grady Hillhouse and this is Practical Engineering! I am a husband, a
professional civil engineer, and educational video producer in San Antonio,
Texas.
Randall
Munroe said, "You can look at practically any part of anything manmade
around you and think, 'Some engineer was frustrated while designing this.' It's
a little human connection." My goal for Practical Engineering is simple:
to increase exposure and interest in the field of engineering.
Of
course, as a civil engineer, much of my content is geared towards
infrastructure and the stories behind the humanmade world we live in. I like to
help people make a connection between themselves and their constructed
environment. In order for people to care about infrastructure, they need to be
interested in the engineering behind it and see people who are passionate about
finding innovative ways to meet humanity’s basic needs. I really believe this
and it’s important to me. I hope that my videos are helpful to you and
encourage you to take opportunities to be an advocate for civil engineering.
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