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What is a Weir?
Grady Hillhouse
Being able to control
the level of water in a river is beneficial in quite a few ways.
Historically, mills
relied on water power to drive saws, grinding wheels, and other equipment.
Raising the water
level in a river can also allow boats and ships to navigate areas that would
otherwise be inaccessible.
Finally, having
control of a river can help mitigate the damaging impacts of flooding. But how
we do get this type of control over the level in a body of water?
Hey I’m Grady and this
is Practical Engineering. On today’s episode, we’re talking about weirs.
A weir is a small dam
built across a river to control the upstream water level. Weirs have been used
for ages to control the flow of water in streams, rivers, and other water bodies.
Unlike large dams
which create reservoirs, the goal of building a weir across a river isn’t to
create storage, but only to gain some control over the water level.
Over time, the term
weir has taken on a more general definition in engineering to apply to any
hydraulic control structure that allows water to flow over its top, often
called its crest.
In fact, the
spillways of many large dams use weirs as control structures. So how do they
work?
If you watched my
previous video on the basics of open channel hydraulics, you’ll remember that
for subcritical flow, that is slow, tranquil flow seen in most rivers, the
depth is controlled by downstream conditions.
That means adding a
weir across a river will increase the water level upstream. But by how much
depends on the flow. This is the equation for flow over a weir.
We’re not going to do
any calculations here, but it’s important to know the factors that govern the
performance of our hydraulic structure.
This equation says
that the amount of flow that passes over the weir depends on three factors: the
length of the weir, the height of the water level above the crest of the weir,
and this coefficient which changes depending on the geometry of the weir.
The graph of a
hydraulic structure’s flow versus water level is called its rating curve, and
this is the rating curve for a typical weir.
In many cases, a weir
is a passive structure, meaning once it’s installed there’s no way to change
this rating curve. And that’s not always ideal.
Streams and river are
subject to tremendous variability in flow rate.
A hydraulic structure
may normally flow a small amount, but in flooding conditions be asked to pass
incredible volumes of water.
With a passive
structure and fixed rating curve, that variability in flow means tremendous
variability in the water level upstream.
During a flood, a
weir may back up the water badly enough to cause damage upstream.
If you’re using a
weir for the spillway on a dam, you might have to build your dam much higher
just to handle the water level that occurs during very rare but extreme cases,
increasing the overall costs of the structure.
Ideally, hydraulic
structures used to control water level would have a flat rating curve, meaning
over a wide range of flows, you only get small changes in level. So how could
we flatten this curve?
Going back to the
weir equation, there are only two other parameters available to increase the
flow for a given water surface.
We could improve the
geometry of the weir to increase its efficiency.
Different shapes of
weirs can pass flow more efficiently and thus have a higher discharge
coefficient, but this has a practical limit.
The most efficient
shape for a weir is to match the curve that the water would take off of a sharp
crest.
This part of the flow
is called the weir’s nappe, and the shape that matches it is called an ogee.
With ogee-crested
weirs, we can get discharge coefficients as high as around 4, but that’s pretty
much the limit.
The other parameter
we can change is the length of the weir, but in many locations, the available
footprint for the weir is a fixed size that can’t be increased.
Even if the footprint
isn’t fixed, increasing the length of the weir can add significant costs.
There are a wide
variety of types of controlled outlets used on hydraulic structures, including
crest gates that act like weirs that can be raised or lowered.
The benefit is that
the structure’s capacity can be increased while flows are high by opening
gates, and then decreased when flows return to normal.
Controlled structures
provide more flexibility in how water gets released or held back, essentially turning
a static rating curve into a family of curves which can be selected from to
meet the operational goals.
Of course, controlled
outlets come with a major disadvantage of increased complexity, and in many
cases, requiring an actual person be available 24/7 to operate the gates and
make releases based on inflows.
So, what if we could
get the benefit of a controlled outlet without the disadvantages of increased
complexity and operational obligation? Well, there’s one other trick that
hydraulic engineers have up their sleeves.
Remember when before
I said that you could only fit a certain length of weir within a fixed
footprint. That’s not completely true.
We can actually fold
a weir to get more length within a given space.
This is called a
non-linear weir and it’s used in situations where you want greater discharge
within a given footprint but without the need for actively controlled outlets.
To show how this
works, I’ve built this flume and some model weirs. This first weir just goes
directly across the flume with no bends.
I’ll mark the water
level in the flume first using this straight weir. Now, with the same flow
rate, I’ll replace the linear weir with the folded version.
This has just about
twice as much weir length in the same footprint.
You can see that,
even though the weir is passing the same amount of flow, the water level is
lower, almost half the distance to the crest from the original level.
We’ve flattened the
rating curve, allowing for greater discharge at a lower water level.
Non-linear weirs with
folded cycles like this are call labyrinth weirs and they’re becoming more
common as hydraulic control structures.
There are also
rectangular versions called piano key weirs.
It’s easy to see how
beneficial weirs can be, from generating power to improving navigation,
controlling floods, and even acting as the spillways for dams.
With all those
benefits, there are definitely some downsides as well. Impoundments across
rivers affect the aquatic environment.
Low head dams can
also pose a serious danger to swimmers and boaters, a topic I’d like to discuss
in the future.
In fact, many old
weirs that are no longer needed are being replaced or completely removed to
restore the river to its natural state.
But as long as we
need to control the flow of water in our constructed environment, weirs will
continue to be an important tool for a hydraulic engineer.
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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