What is a Weir?

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?

Writing/Editing/Production: Grady Hillhouse

Tonic and Energy by Elexive is licensed under a Creative Commons Attribution License

This video is sponsored by Skillshare.

[blipco]

I'm one who loves viewing dams and structures built to control waterways for our benefit. If I see a sign along the road that says "dam", I immediately turn off and investigate. This information will help me further bore my friends. Thank you for that.

[RealEngineering]

Found this really interesting. The weir in Galway, my hometown, creates a pretty unique cityscape. I have always been fascinated by it, but never really put much thought into it why it existed.

[SkoobySkeptic]

I had no interest in weirs but clicked on this all the same. Turns out I found your description and examples very interesting. Bravo sir.

[cwd]

Your videos should be shown in all schools to inspire young minds. I am a 69 year old retiree and I'm a perfect example of a why videos like yours are important! When I was in school I hated math and I was a mediocre student overall. I always say if that math could have been presented in a way that was interesting to me then I would have excelled in my studies. I did ok during my career, although when I look back at my life I would have enjoyed being a mechanical engineer because I have a natural inclination for it. If I had only known what was possible when I was a teenager in high school I would have taken a very different path!

[nicholasbrownlee]

You're the real ogee of engineering channels on Youtube. You provide valuable inflow on many different topics and although this video is a little bit weir, I still find it quite interesting.

[JohnGuastavino]

Me the next time I see a weir:
"It's actually called a weir."

[narcissistectomy]

So it’s grammatically correct to say that all rivers get “weird” after a weir is built.

[tomschmidt]

As a retired EE, enjoy your engineering videos. Here in New England many flow control structures use a low tech flow control method, flash boards. Wooden board are mounted at the top of the structure to increase static water level. In a high flow situation the boards break away. Crude but effective for small structures.


To your other point New England is littered with small dams used for water power in the 18th and 19th century. Many have been breached to improve aquatic conditions and fish migration. We had a lengthy discussion about breaching a couple in our town several years ago. Ultimately decided not to remove them as the impounded water is integral part of town. The other concern is release of contaminants held in sediment behind the dam, luckily in or case that proved not to be a severe problem.


Love your enthusiasm for engineering hope you have a long and successful career.

[flappy]

If a pier is an artificial outcropping over water for people, then it makes sense that a weir is an artificial outcropping over water for water!

[tam-kh]

youtube randomly recommends this video and I find it interesting.

[wafiywahidi]

*so the river just got "weir'd"?*

[vn-dchv]

3:10 "So how we can flatten this curve?"
Flattening curves before it was cool.

[seanwalter]

During the video you say, "Flatten the curve." I can't help but think that's why you just now showed up when I've been watching corona virus videos. The algorithm listens!

[gordonlawrence]

In the UK weirs were also installed to help clean up rivers. IE to introduce oxygen and remove carbon dioxide from the water in order for aerobic bacteria to get rid of (metabolise) certain classes of pollutants.

[marksanders]

Weirs have also been used, historically, for fishing. Some of the oldest weirs were built in a triangular fashion almost exactly as shown towards the end of this video, made of rocks placed across shallow rivers. This allowed water to flow over while trapping fish, behind, which could then easily be caught. There are some pretty obvious downsides to this approach - namely that it almost completely stops the ability of fish to move along the river's course - but old, historical examples can still be found, at least in pieces, in small rivers and streams.

[seabastard]

Youtube: Ever wondered about Weirs
Me: Nope "click"


And yet i watched it all, great vid.

[bradleysmith]

Just proves that even the simplest looking objects, turn out to be complex after investigating.

[ArztvomDienst]

Everything is interesting, if you go into it deeply enough.
~ Feynman

[justicewarrior]

FREAKING FINALLY YOUTUBE!!!
A super interesting video were I actually learned something I never heard before!

[icedlatt]

Grady speaks so well and explains so concisely and with brevity. He's able to find the perfect balance and give just the right amount of info without overwhelming the viewers, and make learning so engaging at the same time. Just wow, gj dude.