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
wiers. This video is sponsored by Skillshare, the
best way to learn a new skill online. More on that later. 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. Of course, this challenge is easy to address
if we allow for structures with moving parts. Many dams and spillways have large gates or
valves to control flow. 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! Thanks to Skillshare for sponsoring this episode. One of my resolutions for 2019 is to do more
writing and improve my writing skills. You might not realize it, but before any Practical
Engineering video ever starts production, I spend hours and hours researching and writing. Of course, there’s no college classes for
engineering YouTube video production, but I don’t have any formal training in creative
writing of any kind, and I definitely have plenty of room for improvement. Skillshare allows you to learn new skills
from experts in their fields producing high quality classes, like this one by comedian
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have learned so many best practices and things that I can use to improve in my writing. There’s just not another platform where
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know what you think!
I found that way more interesting than I thought I would. Thanks for sharing!
Weir science
What is a weir? Oh, I know this one. It's a drowning machine.
Much like Wu Tang Clan, hydraulic vortices ain't nothin' to fuck with.
I love the practical engineering channel.
It's a guitar player for the Grateful Dead.
This is pretty cool. Weirs are used a lot for agriculture. We have weirs in our ditches to measure the water flow of the ditch. We have a headgate that we can open and close then about 50 ft down the ditch the us a weir that we use to calculate the cfs (cubic feet per second) flow that is in the ditch.
This is important because of water rights. Based on the water rights that are owned, you can only take so much water based on the level that the river is at.
What is it, Rodney?
Who is Bob??
Weir everywhere (~};}