--- title: 'Why Are There No Short Arch Dams?' source: 'https://youtube.com/watch?v=_R0xPffklXQ' video_id: '_R0xPffklXQ' date: 2026-07-25 duration_sec: 1000 --- # Why Are There No Short Arch Dams? > Source: [Why Are There No Short Arch Dams?](https://youtube.com/watch?v=_R0xPffklXQ) ## Summary The video explains why arch dams are rare despite being iconic, focusing on their structural advantages for tall dams in narrow canyons and why they are not used for short dams. It contrasts gravity dams, which rely on weight, with arch dams, which transfer load to canyon walls via compression, and demonstrates key principles through physical models. ### Key Points - **Arch Dams Are Rare but Iconic** [00:01] — The US has about 92,000 dams, but only around 50 are arch dams (less than 0.1%). They are rare but appear archetypal because they are huge and often tourist destinations. - **Categories of Dams** [01:23] — Dams are categorized by how they resist water force: embankment dams (friction between particles), gravity dams (weight), and arch dams (geometry/compression). - **Gravity Dam Stability** [02:23] — Gravity dams resist sliding via friction, which depends on normal force (weight) and material coefficient. Stability analysis uses a cross-section slice. - **Failure Modes: Sliding and Overturning** [03:42] — Gravity dams have two failure modes: sliding (friction) and overturning (moment about downstream toe). Weight distribution affects overturning resistance. - **Self-Stabilizing Geometry** [05:36] — Extending the base upstream allows water weight to act as a stabilizer, but uplift from water seeping underneath can cancel this effect. - **Uplift Problem** [07:17] — Water seeping under the dam creates upward pressure (uplift), counteracting weight. This force increases linearly with depth, while lateral force increases with depth squared, demanding more material for taller dams. - **Arch Action as Solution** [09:09] — Arches transfer load to canyon walls via compression, allowing thinner structures. This works only in narrow canyons with strong rock abutments. - **Arch Dam Demonstration** [11:53] — A thin aluminum sheet formed into an arch holds water without deflection, showing the efficiency of arch action compared to a flat sheet. - **Drawbacks of Arch Dams** [12:08] — Arch dams require strong abutments to resist thrust, are limited to narrow canyons, involve complex 3D analysis, and are vulnerable to uplift without good drainage. - **Hybrid Designs and Tall Dams** [13:48] — Hoover Dam is a gravity-arch hybrid. Among the world's tallest 200 dams, about 40% incorporate arch action, highlighting their role in extreme heights. ### Conclusion Arch dams are a specialized solution for tall dams in narrow canyons, leveraging compression to reduce material while relying on strong abutments. Their rarity is due to site constraints and complexity, but they dominate the tallest dams where economically justified. ## Transcript Flaming Gorge Dam rises from the Green River in  northern Utah like a concrete wedge driven into   the canyon, anchored against the sheer rock  walls that flank it. It’s quintessential,   in a way. It’s what we picture when we think  about dams: a hulking, but also somehow graceful,   wall of concrete stretching across a  narrow rocky valley. But to dam engineers,   there’s nothing quintessential about it.  So-called arch dams are actually pretty rare.   For reference, the US has about 92,000  dams listed in the national inventory.   I estimate that we have maybe around 50 arch  dams - it’s less than a tenth of a percent. The only reason we think of arch dams as  archetypal is because they’re so huge.   I counted 11 in the US that have their own  visitor center. There just aren’t that many   works of infrastructure that double as tourist  destinations, and the reason for it is, I think,   kind of interesting. Because an arch dam isn’t  just an engineering solution to holding back   water, and it’s not just a solution to holding  back a lot of water. It’s all about height,   and I built a little demo to show you what I mean.  I’m Grady, and this is Practical Engineering. Engineers love categories, and dams are no  exception. You can group them in a lot of ways,   but mostly, we care about how they handle  the incredible force of water they hold back.   Embankment dams do it with earth or rock, relying  on friction between the individual particles that   make up the structure. Gravity dams do it  with weight. Let me show you an example. I have my tried and trusted acrylic flume with  a small plastic dam. Once this is all set up,   I can start filling up the reservoir. This  little dam is a little narrower than the   flume. It doesn’t touch the sides, so it  leaks a bit. The reason for that will be   clear in a moment. And hopefully you can  see what’s about to happen. This gravity   dam doesn’t have much gravity in it, so it  doesn’t take much water at all before you   get a failure. I’m counting failure as  the first sign of movement, by the way.   That’s when the stabilizing forces are overcome  by the destabilizing ones. And the little dam   by itself could hold until my reservoir  was about a quarter of the way to the top. Gravity dams get their stability against  sliding from… you guessed it… friction. Bet you thought I was going to say gravity.  And actually, it kind of is gravity,   since frictional resistance is a function of just  two variables: the normal force (in other words,   the weight of the structure) and a  coefficient that depends on the two   materials touching. Engineers analyze the  stability of gravity dams in cross-section,   essentially taking a small slice of the  structure. You want every slice to be able   to support itself. That’s why I didn’t want  the demo touching the sides of the flume;   it would add resistance that doesn’t actually  exist in a cross-section. The destabilizing   force is hydrostatic pressure from the reservoir,  which increases with depth. And the stabilizing   force is friction. There are some complexities  to this that we’ll get into, but very generally,   as long as you have more friction than pressure,  you’re good; you have a stable structure. So let’s add some normal force to  the demo and see what happens. You can see my little reservoir gets  a little higher before the dam fails,   And we can try  it again with more weight. But the result gets a little more interesting… Turns out gravity dams have two major failure  modes: sliding and overturning. Resistance to   sliding comes from friction, which really  doesn’t depend on how the weight of the dam   is distributed. That’s not true for overturning  failures. Let’s look back at our cross-section.   For a unit width of dam, the hydrostatic pressure  from the reservoir looks like this. Pressure   increases with depth. And the area under this line  is the total force pushing the dam downstream. We   can simplify that distribution and treat it  like it’s a single force, and it turns out   when you do that, the force acts a third of the  way up the total depth of water. Most dams want   to rotate about the downstream toe, so you have  a destabilizing force offset from the point of   rotation. In other words, you have a torque, also  called a moment. The dam has to create an opposite   moment around that point to remain stable. Moment  or torque is calculated as the force multiplied by   its perpendicular distance from the point of  rotation. So, the further the center of mass   is from the downstream toe, the more stable  the structure is, and the demo shows it too. Here’s where we left the weights the last  time, and let’s see it happen again. The reservoir makes it about two-thirds of the way  up the walls before the dam overturns. Let’s   make a simple shift. Just move the weights further  upstream and try again. The reservoir reaches about three-quarters  the way up before we see a sliding failure,   but shifting the weights did increase the  stability. And this is why a lot of gravity   dams have a fairly consistent shape, with most  of the weight concentrated on the upstream side,   and usually a sloped or stepped downstream face. itself in a way. Watch what happens  when I turn my little model around.   Now the hydrostatic pressure applies both  a destabilizing and stabilizing force,   so you get more resistance for a given depth.  A lot of deployable temporary storm barriers   and cofferdam systems take advantage of this  kind of configuration. You can imagine if I   extended the base even further, I could  create a structure that was self-stable   just from its geometry alone. The weight of  the water on the footing would overcome the   lateral pressure. But there’s a catch to  this. This is fully stable now, but watch   what happens when I give the dam just a bit of a tilt. This might seem kind of intuitive, but I think  it’s important to explain what’s actually going   on. Hydrostatic pressure from the reservoir  doesn’t only act on the face of a dam. With   smooth plastic on smooth plastic, you get a pretty  nice seal, but as soon as even a tiny gap opens,   water gets underneath. Now there’s upward  pressure on the bottom of the dam as well. If   you’re depending on the downward force of a dam  from its weight for stability, it’s easy to see   why an upward force is a bad thing. And it’s so  dramatic in the example with the upstream footing   specifically. In that case, the downward pressure  of the reservoir is acting as a stabilizing force,   but if water can get underneath that footing,  it basically cancels out. The pressure on the   bottom is the same as the pressure on the top.  But this isn’t only an issue in that case. The ground isn’t waterproof. In  fact, I’ve done a video all about   the topic. Soil and rock works more  like a sponge than a solid material,   and water can flow through them. That’s how  we get aquifers and wells and springs and   such. But it’s a problem for gravity dams,  because water can seep below the structure   and apply pressure to the bottom, essentially  counteracting its weight. We call it uplift. Looking back at the cross-section,  we can estimate this. Of course,   you have the triangular pressure distribution  along the upstream face. But at this point   you have the full hydrostatic pressure also  pushing upward. And at the downstream toe,   you have no pressure (it’s exposed to the  atmosphere). So, now you have a pressure   distribution below the dam that looks like  this. Of course, this part can get a lot more   complicated since most dams don’t sit flush  with the ground, and many are equipped with   drains and cutoff walls, so definitely go check  that other video out if you want to learn more. But let me show you the issue this causes with  some recreational math on our cross-sectional   slice of the dam. The taller the dam, the greater  the uplift force. That happens linearly. In other   words, the force is proportional to the depth  of the reservoir. But look at the lateral force.   Again, remember it’s the area under this  triangle. Maybe you remember that formula:   one-half times base times height. Well, the height  is the depth of the water. And the base is also   a function of the depth. More specifically, it’s  the unit weight of water times depth. Multiply it   together, and you see the challenge: the force  increases as a function of the depth squared.   So for every unit of additional height you want  out of a gravity dam, you need significantly more   weight to resist the forces, which means  more material and thus a lot more cost. Hopefully all this exposition is starting to  reveal a solution to this rapid divergence   of stability and loads as a reservoir  increases in height. Dams don’t actually   float in space like my demonstration and  graphics show. You know, by necessity,   they extend across the entire valley and usually  key into the abutments on either side. Naturally,   that connection at the sides is going to  offer some resistance to the forces dams   need to withstand. And if you can count on that  resistance, you can significantly lower the mass,   and thus the cost, of the structure. But, again,  this gets complicated. Let’s go back to the demo. Now I’m going to replace my gravity dam  with something much simpler. Just a sheet   of aluminum flashing, and, to simulate  that resistance provided by socketing the   structure into the earth, I’ve taped it to  the bottom and sides… with some difficulty, actually. When I fill up the reservoir with  water, it holds just fine. There’s a little   leaking past my subpar tape job, but this  is a fully stable structure. And I think   the comparison here is pretty stark. When you  can develop resistance from the sides you can   get away with a lot less dam. But it’s  harder than you might think to do that. For one, the natural soil or rock at a  dam site might not be all that strong.   The banks of rivers aren’t generally known for  their stability, so the prospect of transferring   enormous amounts of force into them rarely makes a  lot of engineering sense. But the other challenge   is in the dam itself. Take a look back at this  demo. See how my dam is bending behind the force   of the water. It’s holding there, but, you know,  we don’t actually build dams out of aluminum   flashing. Resisting loads in this way basically  treats the dam like a beam, like a sideways bridge   girder. Except, unlike girder bridges that  usually only span up to a few hundred feet,   dams are often much longer. Even the stiffest  modern materials, like prestressed concrete boxes,   would just deflect too much under load to  transfer all the hydrostatic pressure across   a valley into the abutments. Plus we usually  don’t like to rely on steel too much in dams   because of issues with corrosion and longevity.  So where a typical beam experiences both tensile   and compressive stress on opposite sides,  we really need to transfer all that load,   creating only compressive stress in the material.  I’m sure you see where I’m going with this. How have we been building bridges for ages  from materials like masonry where tensile   stress isn’t an option? It’s arches! The arch  is a special shape in engineering because you   can transfer loads by putting the material  in compression only, allowing for simpler,   cheaper, and longer-lasting materials like  masonry and concrete. You basically co-opt   the geology for support, reducing the need for  a massive structure. For completeness’s sake,   let me show you how it works in the demo.  I’ve formed a little arch from my thin sheet   of aluminum. Now when I fill up the reservoir,  there’s no deflection like the previous example.   And again, side by side, it’s easy to see the  benefits here. than you do with an earthen embankment dam or a gravity structure. Of course, there are some drawbacks here. For  one, arches create horizontal forces at the   supports called thrusts that have to be resisted.  Sites that use this design really require strong,   competent rock in the abutments to withstand  the enormous loads. And just like with bridges,   the span matters. The wider the valley,  the bigger the arch needs to be,   so these dams generally only make  sense in deep gorges and steep,   narrow canyons. The engineering is a lot more  complicated, too. You can’t use a simple 2D   cross-section to demonstrate stability.  The structural behavior is inherently   three-dimensional, which is tougher  to characterize, especially when you   consider unusual conditions like earthquakes and  temperature effects. And since they’re lighter,   arch dams don’t resist uplift forces very  well, making foundation drainage systems   more critical. All this means that it’s  really only a solution that makes economic   sense in a narrow range of circumstances,  one of the most important being height. For smaller dams, the additional complexity and  expense of designing and building an arch aren’t   justified by the structural efficiency. Gravity  and embankment dams are much more adaptable to   a wider range of site conditions. And  there are other types of dams, too,   that blend these ideas. Multiple-arch dams use a  series of smaller arches supported by buttresses,   dividing the span into more manageable components.  Even what is perhaps the most famous arch dam   in the world - Hoover Dam - isn’t a pure arch  structure. Technically, it’s a gravity-arch dam,   meaning it resists part of the water load through  mass while also distributing the forces into the   canyon through arch action. The proportions  are carefully balanced to take advantage   of the unique site conditions and relatively  wider canyon than most arch dams are built in. And so, when you look at the tallest dams on  Earth, one structural form dominates. By my   estimation, around 40 percent of the tallest 200  dams in the world incorporate an arch into their   design. There aren’t that many places where it  makes sense, but when you compare what it takes to   hold a reservoir back in a narrow canyon valley,  I think the case for arches is pretty clear. I think these models help a lot to explain  engineering principles, so I have built   quite a few of these acrylic demonstrations  in the garage to cover interesting topics.   One of my favorite creators, Neo, does the same  thing, but instead of demos, he uses beautiful 3D   graphics. His video about the construction of the  World Trade Center towers in New York City is a   fascinating look into how much effort, care, and  engineering went into these buildings before the   2001 attack brought them down. My favorite part  was the design of the slurry wall foundation,   and of course, the 3D animations. And  it was produced as a Nebula original. You’ve heard me talk about Nebula before. It’s  a streaming service built by and for independent   creators, including a lot of my favorites like  Neo, Wendover Productions, the Coding Train,   and Branch Education. I don’t know about  you, but independently-produced content   is most of what I watch these days. I just  like the authenticity and thoughtfulness of   videos that haven’t been through ten levels of  studio executives watering the information down   to capture the widest audience possible.  I just think passionate individuals and   small teams make the most compelling work,  and Nebula is the perfect place for it. Nebula’s totally ad-free, with tons of excellent  channels and lots of original series and specials   like Neo’s video on the Twin Towers. It’s also a  great gift, especially because a yearly membership   is 40% of the link in the description. At  thirty-six bucks for a year, that’s pretty   tough to beat. My videos go live on Nebula before  they come out on YouTube. If you’re with me that   independent creators are the future of great  video, I hope you’ll consider subscribing.   That’s go.nebula.tv/Practical-Engineering. Thank  you for watching, and let me know what you think!