[Note that this article is a transcript of the video embedded above.] Lewis and Clark Lake, on the border between Nebraska and South Dakota, might not be a lake for much longer. Together with the dam that holds it back, the reservoir provides hydropower, flood control, and supports a robust recreational economy through fishing, boating, camping, birdwatching, hunting, swimming, and biking. All of that faces an existential threat from a seemingly innocuous menace: dirt. Around 5 million tons of it flows down this stretch of the Missouri River every year until it reaches the lake, where it falls out of suspension. Since the 1950s, when the dam was built, the sand and silt have built up a massive delta where the river comes in. The reservoir has already lost about 30 percent of its storage capacity, and one study estimated that, by 2045, it will be half full of sediment. On the surface, this seems like a silly problem, almost elementary. It’s just dirt! But I want to show you why it’s a slow-moving catastrophe with implications that span the globe. And I want you to think of a few solutions to it off the top of your head, because I think you’ll be surprised to learn why none of the ones we’ve come up with so far are easy. I’m Grady, and this is Practical Engineering. I want to clarify that the impacts dams have on sediment movement happen on both sides. Downstream, the impacts are mostly environmental. We think of rivers as carriers of water; it’s right there in the definition. But if you’ve ever seen a river that looks like chocolate milk after a storm, you already know that they are also major movers of sediment. And the natural flow of sediment has important functions in a river system. It transports nutrients throughout the watershed. It creates habitat in riverbeds for fish, amphibians, mammals, reptiles, birds, and a whole host of invertebrates. It fertilizes floodplains, stabilizes river banks, and creates deltas and beaches on the coastline that buffer against waves and storms. Robbing the supply of sediment from a river can completely alter the ecosystem downstream from a dam. But if a river is more than just a water carrier, a reservoir is more than just a water collector. And, of course, I built a model to show how this works. This is my acrylic flume. If you’re familiar with the channel, you’ve probably seen it in action before. I have it tilted up so we get two types of flow. On the right, we have a stream of fast-moving water to simulate a river, and on the left, I’ve built up a little dam. These stoplogs raise the level of the water, slowing it down to a gentle crawl. And there’s some mica power in the water, so you can really see the difference in velocity. Now let’s add some sediment. I bought these bags of colored sand, and I’m just going to dump them in the sump where my pump is recirculating this water through the flume. And watch what happens in the time lapse. The swift flow of the river carries the sand downstream, but as soon as it transitions into the slow flow of the reservoir, it starts to fall out of suspension. It’s a messy process at first. The sand kind of goes all over the place. But slowly, you can see it start to form a delta right where the river meets the reservoir. Of course, the river speeds up as it climbs over the delta, so the next batch of sediment doesn’t fall out until it’s on the downstream end. And each batch of sand that I dump into the pump just adds to it. The mass of sediment just slowly fills the reservoir, marching toward the dam. This looks super cool. In fact, I thought it was such a nice representation that I worked with an illustrator to help me make a print of it. We’re only going to print a limited run of these, so there's a link to the store down below if you want to pick one up. But, even though it looks cool, I want to be clear that it’s not a good thing. Some dams are built intentionally to hold sediment back, but in the vast majority of cases, this is an unwanted side effect of impounding water within a river valley. For most reservoirs, the whole point is to store water - for controlling floods, generating electricity, drinking, irrigation, cooling power plants, etc. So, as sediment displaces more and more of the reservoir volume, the value that reservoir provides goes down. And that’s not the only problem it causes. Making reservoirs shallower limits their use for recreation by reducing the navigable areas and fostering more unwanted algal blooms. Silt and sand can clog up gates and outlets to the structure and damage equipment like turbines. Sediment can even add forces to a dam that might not have been anticipated during design. Dirt is heavier than water. Let me prove that to you real quick. It’s a hard enough job to build massive structures that can hold back water, and sediment only adds to the difficulty. But I think the biggest challenge of this issue is that it’s inevitable, right? There are no natural rivers or streams that don’t carry some sediments along with them. The magnitude does vary by location. The world’s a big place, and for better or worse, we’ve built a lot of dams across rivers. There are a lot of factors that affect how quickly this truly becomes an issue at a reservoir, mostly things that influence water-driven erosion on the land upstream. Soil type is a big one; sandy soils erode faster than silts and clays (that’s why I used sand in the model). Land use is another big one. Vegetated areas like forests and grasslands hold onto their soil better than agricultural land or areas affected by wildfires. But in nearly all cases, without intervention, every reservoir will eventually fill up. Of course, that’s not good, but I don’t think there’s a lot of appreciation outside of a small community of industry professionals and activists for just how bad it is. Dams are among the most capital-intensive projects that we humans build. We literally pour billions of dollars into them, sometimes just for individual projects. This is kind of its own can of worms, but I’m just speaking generally that society often accepts pretty significant downsides in addition to the monetary costs, like environmental impacts and the risk of failure to downstream people and property in return for the enormous benefits dams can provide. And sedimentation is one of those problems that happens over a lifetime, so it’s easy at the beginning of a project to push it off to the next generation to fix. Well, the heyday of dam construction was roughly the 1930s through the 70s. So here we are starting to reckon with it, while being more dependent than ever on those dams. And there aren’t a lot of easy answers. To some extent, we consider sediment during design. Modern dams are built to withstand the forces, and the reservoir usually has what’s called a “dead pool,” basically a volume that is set aside for sediment from the beginning. Low-level gates sit above the dead pool so they don’t get clogged. But that’s not so much a solution as a temporary accommodation since THIS kind of deadpool doesn’t live forever. I think for most, the simplest idea is this: if there’s dirt in the lake, just take it out. Dredging soil is really not that complicated. We’ve been doing it for basically all of human history. And in some cases, it really is the only feasible solution. You can put an excavator on a barge, or a crane with a clamshell bucket, and just dig. Suction dredgers do it like an enormous vacuum cleaner, pumping the slurry to a barge or onto shore. But that word feasible is the key. The whole secret of building a dam across a valley is that you only have to move and place a comparatively small amount of material to get a lot of storage. Depending on the topography and design, every unit of volume of earth or concrete that makes up the dam itself might result in hundreds up to tens of thousands of times that volume of storage in the reservoir. But for dredging, it’s one-to-one. For every cubic meter of storage you want back, you have to remove it as soil from the reservoir. At that point, it’s just hard for the benefits to outweigh the costs. There’s a reason we don’t usually dig enormous holes to store large volumes of water. I mean, there are a lot of reasons, but the biggest one is just cost. Those 5 million tons of sediment that flow into Lewis and Clark Reservoir would fill around 200,000 end-dump semi-trailers. That’s every year, and it’s assuming you dry it out first, which, by the way, is another challenge of dredging: the spoils aren’t like regular soil. For one, they’re wet. That water adds volume to the spoils, meaning you have more material to haul away or dispose of. It also makes the spoils difficult to handle and move around. There are a lot of ways to dry them out or “dewater” them as the pros say. One of the most common is to pump spoils into geotubes, large fabric bags that hold the soil inside while letting the water slowly flow out. But it’s still extra work. And for two, sometimes sediments can be contaminated with materials that have washed off the land upstream. In that case, they require special handling and disposal. Many countries have pretty strict environmental rules about dredging and disposal of spoils, so you can see how it really isn’t a simple solution to sedimentation, and for most cases, it often just isn’t worth the cost. Another option for getting rid of sediment is just letting it flow through the dam. This is ideal because, as I mentioned before, sediment serves a lot of important functions in a river system. If you can let it continue on its journey downstream, in many ways, you’ve solved two problems in one, and there are a lot of ways to do this. Some dams have a low-level outlet that consistently releases turbid water that reaches the dam. But if you remember back to the model, not all of it does. In fact, in most cases, the majority of sediment deposits furthest from the dam, and most of it doesn’t reach the dam until the reservoir is pretty much full. Of course, my model doesn’t tell the whole story; it’s basically a 2D example with only one type of soil. As with all sediment transport phenomena, things are always changing. In fact, I decided to leave the model running with a time-lapse just to see what would happen. You can really get a sense of how dynamic this process can be. Again, it’s a very cool demonstration. But in most cases, much of the sediment that deposits in a reservoir is pretty much going to stay where it falls or take years and years before it reaches the dam. So, another option is to flush the reservoir. Just set the gates to wide open to get the velocity of water fast enough to loosen and scour the sediment, resuspending it so it can move downstream. I tried this in the model, and it worked pretty well. But again, this is just a 2D representation. In a real reservoir that has width, flushing usually just creates a narrow channel, leaving most of the sediment in place. And, inevitably, this requires drawing down the reservoir, essentially wasting all the water. And more importantly than that, it sends a massive plume of sediment laden water downstream. I’ve harped on the fact that we want sediment downstream of dams and that’s where it naturally belongs, but you can overdo it. Sediment can be considered a pollutant, and in fact, it’s regulated in the US as one. That’s why you see silt fences around construction sites. So the challenge of releasing sediment from a dam is to match the rate and quantity to what it would be if the dam wasn’t there. And that’s a very tough thing to do because of how variable those rates can be, because sediment doesn’t flow the same in a reservoir as it would in a river, because of the constraints it puts on operations (like the need to draw reservoirs down) and because of the complicated regulatory environment surrounding the release of sediments into natural waterways. The third major option for dealing with the problem is just reducing the amount of sediment that makes it to a reservoir in the first place. There are some innovations in capturing sediment upstream, like bedload interceptors that sit in streams and remove sediment over time. You can fight fire with fire by building check dams to trap sediment, but then you’ve just solved reservoir sedimentation by creating reservoir sedimentation. As I mentioned, those sediment loads depend a lot not only on the soil types in the watershed, but also on the land use or cover. Soil conservation is a huge field, and has played a big role in how we manage land in the US since the Dust Bowl of the 1930s. We have a whole government agency dedicated to the problem and a litany of strategies that reduce erosion, and many other countries have similar resources. A lot of those strategies involve maintaining good vegetation, preventing wildfires, good agricultural practices, and reforestation. But you have to consider the scale. Watersheds for major reservoirs can be huge. Lewis and Clark Reservoir’s catchment is about 16,000 square miles (41,000 square kilometers). That’s larger than all of Maryland! Management of an area that size is a complicated endeavor, especially considering that you have to do it over a long duration. So in many cases, there’s only so much you can do to keep sediment at bay. And really, that’s just an overview. I use Lewis and Clark Reservoir as an example, but like I said, this problem extends to essentially every on-channel reservoir across the globe. And the scope of the problem has created a huge variety of solutions I could spend hours talking about. And I think that’s encouraging. Even though most of the solutions aren’t easy, it doesn’t mean we can’t have infrastructure that’s sustainable over the long term, and the engineering lessons learned from past shortsightedness have given us a lot of new tools to make the best use of our existing infrastructure in the future.

[Note that this article is a transcript of the video embedded above.] I’m Grady, and this is Practical Engineering. You know, every once in a while, all the science, technology, economic factors, and stylistic tastes converge into a singular, beautiful idea of absolute perfection. Am I being superfluous? I don’t think so. Destin’s got laminar flow. Grey thinks hexagons are the bestagons. Matt loves the number 3, for whatever reason. Vi prefers 6. Alec loves the refrigeration cycle. I am not going to mince words here; they’re just wrong. I’m not trying to say that cable-stayed bridges are the best kind of bridge. I’m saying they’re the best, period. So, on this day dedicated to the people and things we love, let me tell you why I adore cable-stayed bridges. Spanning a gap is a hard thing to do, in general - to provide support with nothing underneath. Even kids recognize there’s some inherent mystery and intrigue to the idea. Almost all bridges rely, to some extent, on girders - beams running along their length - to gather structural forces from the deck and move them to the supports. This action results in bending, known as moments to engineers, and those moments create internal stress. Too much stress and the material fails. You can increase the size of the beam to reduce the stress, but that creates more weight that creates a higher moment that results in more stress, and you’re back to where you started. For any material you choose as a girder, there is a practical limit in span because the self-weight of the beam grows faster than its ability to withstand the internal stress that weight causes. The easiest way to deal with a moment that might stress a beam too much is to simply support it from below; build another column or pier there. And in old-fashioned viaducts, this is precisely what you’ll see. But there are a lot of places we want to cross where it’s just not that simple. Putting piers in areas where the water is deep or the soil is crummy can be cost-prohibitive. And sometimes, we just don’t want more supports to ruin the view. Fortunately, “push” has an opposite. Cables can be used to pull a bridge upward toward tall towers, supporting the deck from above. There was a time when a suspension bridge was practically the only way to cross a long span. Huge main cables drape across the towers, and suspenders attach them to the deck below. You get that continuous support, reducing the demand on the girders and allowing for a much lighter, more efficient structure. But you get some other stuff too. All those forces transfer to the cables and to the tops of the towers. But the cables don’t just pull on the towers vertically. There’s some horizontal pulling too, and I’m sure you know what happens when you put a horizontal force at the top of something very tall. So the cables have to continue to the other side, balancing the lateral component. And that’s just kicking the force-can down the road; ultimately they have to go SOMEWHERE. In most suspension bridges, it’s the anchorage - a usually enormous concrete behemoth that attaches the main cables to the ground. The anchorages on the Golden Gate Bridge weigh 60,000 tons each. Compare that to a cable-stayed span. Get rid of the main cables and just run the suspenders - now called stays - diagonally straight to the tower. You have balanced horizontal forces on the tower without the need for a massive anchorage that can be expensive or, in places with poor soils, completely infeasible. Instead, those horizontal forces transfer into the bridge deck and girders, but because they’re balanced, there’s no net horizontal force on the deck either. Of course, with traffic and wind loads, you can get slight imbalances in forces, but those can be taken care of with the stiffness of the tower and the anchor piers at the end of each backspan, which are much simpler than massive anchorages. I should note that some suspension bridges do this too. So-called self-anchored suspension bridges also put the deck in compression in lieu of anchorages. In that case, the entire bridge deck has to withstand the full compression force from the main cables attached at its ends. In a cable-stayed bridge, the maximum compressive force in the deck is localized near the towers and diminishes as you get further from them, allowing you to be more efficient with materials. This tension management also means cable-stayed bridges work well in multi-span arrangements. Consider the Western side of the Bay Bridge, an admittedly impressive multi-span bridge connecting traffic from San Francisco to Oakland. This is two suspension spans connected to one another, but look what’s in between them. This manmade mountain of a concrete anchorage is an unavoidable cost of this kind of construction. Compare that to the sleek multi-span wonder of the French Millau(MEE-oh) Viaduct with eight spans, six of which are longer than a thousand feet or three hundred meters. While there certainly is a significant volume of concrete in the viaduct, it’s all in the deck and eight elegant pylons. No hulking anchorages to be seen; just gently curving spans above the French countryside. It also happens to be the tallest bridge in the world, with its tallest pylon surpassing the Eiffel tower! If that doesn’t make your heart flutter, nothing will. And speaking of flutter, suspension bridges have another downside. You’ve probably seen this video before. Gravity loads aren’t the only forces for long-span bridges to withstand. The lightness of a suspension bridge is actually a disadvantage when it comes to the wind. Because of the droopy, parabolic shape of the main cables, suspension bridges are susceptible to relatively small forces causing outsized deflections of the structure. This is true laterally. But it’s also true for vertical forces. Since the main cables reach very shallow angles, even horizontal in the center of the span, huge tensions are required just to withstand moderate vertical loads, and those tensions come with large deflections as the cables straighten. Put another way, it’s a lot easier to straighten a sagging cable than to stretch one that’s taut. For a cable-stayed bridge, they’re already straight. There’s very little sag in the stays, so any deflections require the actual steel to stretch along its length. That makes cable-stayed bridges generally much stiffer than suspension bridges, giving them aerodynamic stability and allowing the decks to be lighter. The thing about a bridge is that you can design pretty much anything on paper, or in CAD, but at some point, it has to be built. You have to get the structure into place above the area it spans, and that can be a tricky thing. Consider an arch bridge. That arch can’t do its arch thing until it’s a continuous structure member. Before that, forces have to be diverted through some other temporary structure or falsework, usually something underneath. For one, that requires engineers to design, essentially, several different versions of the same bridge, where (in some cases) the construction loads actually govern the size and shape members rather than the final configuration. For two, if building extra vertical supports was easy, then we would just design the bridge that way in the first place. Check out this timelapse of the construction of the I-11 bridge over the Colorado River downstream of the Hoover Dam. If you look carefully, you can see that before the arch is complete, it is supported by cable stays! And this is where you see the huge advantage that cable-stayed bridges have: constructability. The flow of forces during construction is the same as when the bridge is complete. But it’s not just that; the construction itself also is much simpler. Look at a conventionally anchored suspension bridge. You have to build the towers and anchorages first. Only when they’re complete can you hang the main cables. That’s a process in itself. Main cables are too heavy and unwieldy to be prefabricated and hoisted across the span, so they are generally built in place, wire by wire, in a process called spinning. Then you have to attach the suspenders, and only then can you start building the road deck. It’s an intricate process where each major step can’t start until the one before it is totally finished. Self-anchored suspension bridges are even more complicated, because you have to have the entire deck built before the cable can be anchored, but you have to have the cable to suspend the deck. It’s a chicken and egg problem that you have to solve with temporary supports. None of this is true with cable-stayed bridges. You can have your chicken and egg, and eat it too! You start with the pylons, and then as you build out the bridge deck, you add cable stays along the way, slowly cantilevering out from the towers. Since they’re usually symmetrical, the forces balance out the whole time. The loading is the same during construction and after, and there’s no need for falsework or temporary supports, dramatically lowering the cost to build them. Some bridges can even begin work on the deck before the tower is even finished, speeding up the construction timeline and reducing costs even more. This constructability also has a positive feedback loop when it comes to contractors and manufacturers as well. As the popularity of cable-stayed bridges has exploded since the second half of the twentieth century, more and more contractors have recent and relevant experience, and more and more manufacturers can produce the necessary materials, reducing the costs even further and making them more and more likely to be chosen for new projects. But once you put up a bridge, you also have to keep it up. Maintenance is another place cable-stayed bridges shine. Besides the stays themselves, most of their parts are easily accessible for inspection. Most structures don’t rely heavily on coatings to protect the steel, so you don’t have to contract with specialized, high-access professionals for maintenance. And just using more concrete instead of steel means fewer problems with corrosion. With more rigidity, you get less fatigue on materials. And they’re redundant. Suspension bridges rely on the two massive main cables for all their structural support. You can’t take one cable out of service for repair or replacement without very complicated structural retrofits. With cable-stayed bridges, it’s no problem. The stays are designed to be highly redundant, so if one breaks or you need to replace them, the remaining cables can still effectively support the bridge's load. And each cable can be tensioned individually, so the structure can be “tuned” to match the design requirements just like a piano, and adjusted later if needed. You might be looking at all these examples and thinking, this is kind of obvious. But there are a lot of reasons why cable-stayed bridges only started becoming popular in the last few decades. Part of that is in the field of engineering itself. Where the deck, tower, and main cables of a suspension bridge behave fairly independently, a cable-stayed structure is much more interdependent. Each stay is tensioned independently, meaning you have lots of different forces on the deck and towers that depend on each other, and they have to be calculated for each loading condition. Solving for all the forces in the bridge is a complicated task to do by hand, so it took the advent of modern structural analysis software before engineers could gain enough confidence in designs to push the envelope. And that brings me to a deeper point about structural elements resisting forces. Cable-stayed bridges just make such efficient use of materials, many of which have existed for centuries, but have been refined and improved over time. A lot of engineering sometimes feels like designing around the weaknesses of various materials, but cable-stayed bridges take full advantage of materials’ strengths. We put the towers and deck in compression and make them out of high-strength concrete, a material that loves compressive stress. We put the stays in tension and make them out of high-strength steel. They love tension. We’ve slowly gained confidence in the innovations that make these bridges possible, like parallel wire strands, concrete-to-cable anchoring systems, segmental construction, and prestressed concrete. And all these gradual improvements in various aspects of construction and material science added up to create the pinnacle of engineering technology. You want to know the other reason why cable-stayed bridges are becoming more popular? It’s taste. Bridges are highly visible structures. They are tremendous investments of public resources, and the public has a say in how they look. I hate to even say the word outloud, but oftentimes, there are architects involved in their design. The swooping shapes of suspension structures were in vogue during the heyday of long-span bridge design, but no more! One of the huge benefits of cable-stayed bridges is that they’re flexible. Not structurally flexible of course, but architecturally. Most bridges do have a few rules of thumb - the tower height is usually about a fifth of the main span length, and the side spans about two fifths of the main span. However the number of variations on the theme is practically endless. Let me show you some examples. For short spans, you’ll typically see single cable planes. Each of the masts of the Millau viaduct has a single cable plane, connecting the cables along a central line of the bridge deck. Go a little bigger and you’ll see double cable planes. This is the Russky Bridge in Russia, the current world record holder with a main span of 1,100 meters or 3,600 feet. The two cable planes give the structure extra stiffness. Double planes can be parallel like you see in the Øresund bridge in Denmark. Or, cable planes can be inclined towards one another, like in the Charilaos Trikoupis bridge in Greece. They can use the radial or “fan” style, where the stays originate from the pylons near a single point at the top, like the Pasco Kennewick bridge. Or they can use the harp style, where the stays are more or less parallel. Lots of structures use a style somewhere between the two. If the pylons get tall enough, they might get connected by a cross member, giving H pylons. Continuing in the alphabetical trend, another option is A-frames with inclined cable planes. If an A-frame gets too tall, though, you end up requiring two foundations per pylon, which can quickly get pricey or just too challenging to construct. In that case, tuck the legs back in towards each other, and you’ve got stunning diamond frames. You might see asymmetrical designs like Malaysia’s famous Seri Wawasan bridge or Spain’s Puente del Alamillo. You’ve got Sao Paolo’s Octávio Frias de Oliveira Bridge with its iconic X-shaped pylon holding two curved roadways, each with double cable planes inclined and crossing each other. Even my home state of Texas boasts some impressive cable-stayed bridges. Corpus Christi’s Harbor Bridge will be finished soon, now that they got the construction issues worked out. Houston has the double diamond-framed Fred Hartman bridge. And Dallas has the iconic Margaret Hunt Hill Bridge with its high arched single pylon gracefully twisting its single cable plane through the third dimension. You can see how these simple structural principles work together to allow architects to really get creative while still allowing the engineers and contractors to bring it into reality. I mean, just look at this. There’s nothing extraneous. Nothing extravagant. This is the highest form of utility meets beauty. Have you ever seen something like this? I hope you can see why we’re in the heyday of cable-stayed bridge construction. This is my opinion, and maybe I’m a little bit biased, but I don’t think there’s a better example in history where all the various factors of a technical problem converged into a singular solution in this way. Many consider the Strömsund Bridge in Sweden, completed in 1956, to be the first modern cable-stayed bridge. But it’s only been over the past three or four decades that things really took off. Now, there are more than 15 with spans greater than 800 meters or 2600 feet, not including the Gordie Howe Bridge, which will soon be the longest cable-stayed bridge in North America. Even the famously hard-hearted US Federal Highway Administration declared their affection for the design, stating, “Today, cable-stayed bridges have firmly established their unrivaled position as the most efficient and cost-effective structural form in the 150-m to 460-m span range.” And that range is only growing. We humans built a lot of long bridges in the 20th century, and a lot of them are reaching the end of their design lives. I can tell you what kind of bridge most of them are going to be replaced with. And I can tell you that any time a new bridge that needs a span less than 1000 meters or 3,300 feet goes into the alternatives analysis phase, it’s going to get harder and harder not to choose a cable-stayed structure. They’re structurally efficient, cost-effective, easy to build, easy to take care of, and easy to love. The very longest spans in the world are still suspension bridges, but I would argue: we don’t really need to connect such long distances anyway. Doctors don’t tell you this, but engineers don’t actually have heartstrings; they have pre-fabricated parallel wire heart strands, and nothing tugs on them quite like a cable-stayed bridge. Happy Valentine's Day!

[Note that this article is a transcript of the video embedded above.] For as straightforward as they are, there’s a lot of mystery to sewers. They’re mostly out of sight, out of mind, and ideally out of smell too. But there’s one familiar place you can get a hint of what’s happening below your feet, and that’s the manhole. Sanitary engineers know that there’s actually a lot of complexity in this humble hallmark of our least-glorified type of infrastructure. So, I set up a see through sewer system so you can see what’s happening inside. I’m Grady, and this is Practical Engineering. There are a lot of kinds of manholes. If it’s a utility of any kind and you put it underground, there’s a good chance you’ll need some access to it at some point in time. But I figure if you picture a manhole in your head, it probably leads to a sewer system: either a sanitary sewer that connects to your drains and toilets, a storm sewer that connects to storm drains, or a combined system that carries it all. Unlike what you see in movies, most sewer systems aren’t huge tunnels full of totally tubular turtles and their giant rodent mentor. They’re mostly just simple pipes sized according to the amount of flow expected during peak periods. Sewer networks have a tributary structure. Gravity carries waste along downward sloping pipes that converge and concentrate into larger and larger lines, like a big underground river system…but grosser. Terminology varies place to place, but in general, it goes like this. Pipes that service individual buildings are usually called laterals, and those servicing particular streets are branches. Larger pipes that collect wastewater from multiple branches are called mains or trunk sewers. And the most significant lines furthest downstream in the system are usually called interceptors. And connecting each one is a manhole. This is my model sewer system. I’m just pumping water into an upper manhole and letting it flow through the system by gravity. I chose to do this with nice blue water for anyone watching while having lunch. In real life, the color in a sewer isn’t quite this nice. Unlike regular plumbing, where you use “fittings” to connect lengths of pipes together, sewers lines are connected with manholes. Any change in size, direction, alignment, or elevation is a place where debris can get caught or turbulence can affect the flow. So instead of elbows or tees in the pipe, we just put a manhole instead. In fact, unlike many underground utilities, you can usually trace the paths of a sewer network pretty easily, because it’s all straight lines between manholes. They provide a controlled environment where the flow can change direction, and more importantly, a place where technicians can get inside to inspect the lines, remove clogs, or perform maintenance (hence the name). Unlike fresh water distribution systems that can usually go a long time without any intervention, sewers are a little… more hands-on (just make sure you wash your hands afterwards). There’s just no end to the type of things that can find their way into the pipes. Fibrous objects are particularly prone to causing clogs, which is why so many sewer utilities have campaigns encouraging people not to flush wipes, even if they say “flushable” on the package. Fats, oil, and grease (or FOG, in the industry) are also a major problem because they can congeal and harden into blockages sometimes not-so-lovingly known as “fatbergs”. Of course, a lot of people aren’t aware of what’s safe to flush or wash down the drain, and even for people who know, it’s easy to let something slide when it’s not your problem anymore. And in most cases, the rules aren’t very strictly enforced outside of large commercial and industrial users of the system. But if you use a sewage system, in a way, obstructions really ARE your problem because a portion of your taxes or fees that pay for the sewer system go toward sending people - not always men (despite the name) - into manholes to keep things flowing. And the more often things clog up, the higher the rates that everyone pays to cover the cost of maintenance. There’s quite a lot of sophistication in keeping sewers in service these days. It’s not unusual for a city or sewer district to regularly send cameras through the lines for inspection. Technology has made it a lot easier to be proactive. In fact, there’s a whole field of engineering called infrastructure asset management that just focuses on keeping track of physical assets like sewer lines, monitoring their condition, and planning ahead for repairs, maintenance, and replacement over the long term. A lot of the unclogging and cleaning these days is done by hydro jetting: basically a pressure washer scaled up. Rotating nozzles blast away debris and propel the hose down the line. In fact, one of the benefits of manholes is that, if a sewer line does need maintenance, it can be easily taken out of service. You can just run a bypass pump from one manhole to another and keep the system running. But maintenance isn’t the only thing a manhole helps with. You can see a few more things in this demo. For one, manholes provide ventilation. Along with the solids and liquids you expect, gases can end up flowing through sewer pipes too. You can see the bubbles moving through the system. Air bubbles can restrict the flow of fluid in a pipe, and air pressure can cause wacky problems like backflow. Along with regular air, toxic, corrosive, or even explosive gases can also build up in a sewer if there’s no source of fresh air, so ventilation from manholes is an important aspect of the system. Sometimes you’ll even notice condensed water vapor flowing up from a manhole cover. In a few cities, like New York, that might be related to an actual steam distribution system running underground. But it can also happen in sewers when the wastewater from sources like showers and dishwashers is warmer than the outside air, especially in the winter. I added a third manhole to my model so you can see how a junction might look. It just provides a nice way to confluence two streams into one pipe, which is an important job in a sewer system, since a “sewershed” all has to flow to one place. The manhole acts kind of like a buffer, smoothing out flows through the system. At normal flows, that’s not a super important job. It’s basically just a connection between two pipes. But the peak flows for most sewers, even if they’re not storm sewers, happen during storms. Drains may be improperly connected to sanitary sewers, plus surface water often finds a way in through manhole covers and other means. In fact, a lot of places require sealed and bolted covers if the top of the manhole is below the floodplain. That’s why you sometimes see these air vents sticking up out of the ground. Many older cities use combined systems where stormwater runs in the same pipes. So rainwater in sewers can be a major challenge. And you can see when you get a big surge of water, the manhole can store some of it, smoothing out the flow downstream. These storm flows are actually a pretty big problem of the constructed environment. You may have heard about the trouble with holding swimming events in the Seine River in Paris during the 2024 summer olympics. Same problem. Wastewater treatment plants can only handle so much flow, so many places have to divert wastewater during storms, often just discharging raw, if somewhat diluted, sewage directly into rivers or streams. In fact some of the most impressive feats of engineering in progress right now are ways to store excess wastewater during storms so it can be processed through a treatment plant at a more manageable rate. But overflows can also happen way upstream of a treatment plant if the pipes are too small. Sometimes that storage available through manholes isn’t enough. I can plug up the pipe in my demo to simulate this. If the sewer lines themselves can’t handle the flow, you can get wastewater flowing backwards in pipes, and if things get bad enough, you can get releases out of top of manholes. And of course, this doesn’t have to be the result of a storm. Even a blockage or clog in the line can cause wastewater to back up like this. Obviously, having raw sewage spilling to the surface is not optimal, and many cities in the US pay millions of dollars in fines and settlements to the EPA for the contamination caused by backups. Another thing this model shows is that not all pipes have to come in at the bottom. They call this a drop manhole when one of the inlets is a lot higher than the outlet. The slope of a sewer line is pretty important. I’ve covered that topic in another video. There’s a minimum slope to get good flow, but you don’t want too much slope either. Wastewater often carries rocks and grit, so if it gets going too quickly, it can wear away or otherwise damage the pipes. So if you’re running a line along a steep slope, sometimes it’s a better design to let some of that fall happen in a manhole, rather than along the pipe. It’s not normally done this way where my pipe just juts in. You usually don’t want a lot of splashing and turbulence in a manhole, again to avoid damage, but also to avoid smells. So most drop manholes use pipes or other structures to gently transition inlet flow down to the bottom. I hope it’s clear how useful manholes are by now. Doing it this way - by making the plumbing junctions into access points - just provides a lot of flexibility, while also kind of standardizing the system so anyone involved, whether its a contractor building one or a crew doing maintenance, kind of knows what to expect. In fact, if you live in a big city, there’s a good chance that the sewer authority has standardized drawings and details for manholes so they don’t have to be reinvented for each new project. In many cases, they’re just precast concrete cylinders placed into the bottom of an excavation. Those cylinders sit on temporary risers, and then concrete is used to place the bottom, often with rounded channels to smooth the transition into and out of the manhole. I did a video series on the construction of a sewage lift station and showed how a few of these are built if you want to check that out after this and learn more. Constructing manholes reminds of that famous interview riddle about why manhole covers are round. There’s a lot of good answers: a round object can’t fall down into the hole, it can be replaced in any orientation, it’s easy to roll so workers don’t have to lift the entire weight to move it out of the way. A professor of mine had an answer that I don’t think I’ve heard before. Manhole covers are round because manholes are round. It’s almost like asking why pringles lids are round. And manholes are round for a lot of good reasons: it’s the best shape for resisting horizontal soil loads. It’s easier to manufacture a round shape than a rectangular one. For those reasons, manholes are usually made of pipes, and pipes are round because it’s the most efficient hydraulic section. It’s one of those questions, like the airplane on a treadmill, that can spawn unending online debate. But I like pipes, so that’s my favorite answer.

[Note that this article is a transcript of the video embedded above.] I am on location in downtown San Antonio, Texas, where crews have just finished setting up this massive 650-ton crane. The counterweights are on. The outriggers are down. And the jib, an extension for the crane's telescoping boom, is being rigged up. This is the famous San Antonio River Walk, a city park below street level that winds around the downtown district. It’s one of the biggest tourist attractions in the state, connecting shops, restaurants, theaters, and Spanish missions (the most famous of them being the Alamo). Every year, millions of people come to see the sights, learn some history, and maybe even take a tour boat on the water. It’s easy to enjoy the scenery without considering how it all works. But, how many rivers do you know that stay at an ideal, constant level, just below the banks year-round? One of the critical structures that make it all possible is due for some new gates, and it’s going to be a pretty interesting challenge to replace them without draining the whole river in the process. I’ve partnered up with the City of San Antonio and the San Antonio River Authority to document the entire process so you can see behind the scenes of one of my favorite places. I’m Grady, and this is Practical Engineering. After a catastrophic flood in 1921 took more than 50 lives in San Antonio, the city took drastic measures to try and protect the downtown area from future storms. Back when my first book came out, I took a little tour of some of those measures, one historical - Olmos Dam - and one more modern - the flood diversion tunnel that runs below the city. But another of those projects eventually turned into one of San Antonio’s crown jewels. A major bend in the river, right in the heart of downtown, was cut off, creating a more direct path for floodwaters to drain out. But rather than fill in the old meander, the city decided to keep it, recognizing its value as a park. Gates were installed at both connections, allowing the bend to be isolated from the rest of the river. Later a dam was built downstream on the San Antonio River with two floodgates. During normal flows, these gates control the level upstream on the river, maintaining a constant elevation for the Great Bend and the cutoff. If a flood comes, these gates can be shut to maintain a constant level in the bend, and these gates can be opened to let the floodwaters pass downstream. Essentially, this pair of floodgates are pivotal parts of the San Antonio River Walk. They hold back flow during sunny weather to keep water levels up, and they lower to release water during storms to keep downtown from being flooded. They were installed way back in 1983 and already planned for replacement. Then this happened. One of the floodgates’ gearboxes had a nut with threads that had worn down, and eventually stripped out. It caused one side of the gate to drop, damaging several components and rendering the floodgate inoperable. The City of San Antonio immediately installed stop logs upstream of the gate to block the flow and prevent the water level in the River Walk from dropping. But the gate is still unable to lower in the event of a flood, halving the capacity of this important dam. So they sprung into action to design replacements for these old gates. It’s been a long road finding a modern solution that fits within this existing structure. But it’s finally time to remove the old gates and bring this dam into the 21st century. There’s a lot of work to do before the broken gate can come out. The first job is just to get the water out. This dam has a place for stoplogs, both upstream and downstream of each gate. Historically, they’d be wood, hence the name, but modern stoplogs are heavy steel beams that stack together to create a relatively watertight bulkhead on either side. Those stoplogs have been installed since the gate went out of service, and while they hold back a whole lot, they aren’t completely watertight. Inevitably, some water gets through to fill up the area between them, making it challenging to work in this area. The contractor has brought in a large diesel pump and perched it on the bank next to the broken gate. They get it running, and it’s not long at all before the area between the upstream and downstream stoplogs is dry enough to work. The first thing to go is the drive shaft between the two gate operator gearboxes. When these gates are functioning, this shaft delivers power to the opposite side of the gate and keeps both sides raising or lowering at the same rate. But now it’s just in the way and needs to come out. It is disconnected, and the crane lowers it to the ground. The next piece is the support beam between the two operators. Same as before: it is detached by the crew, rigged to the crane, and lifted away from the dam. It’s flown across the site to the staging area and set down. All this equipment will eventually be hauled away and recycled for scrap. It might be obvious, but even though it’s broken, this gate is still attached to the rest of the dam, at the bottom with hinges, and at the top, with the two stems that would raise and lower the leaf when it was working. Before the crew can detach the gate, it will need some additional support. The crane lowers its hook. And the crew wraps two massive chain slings around it. Then the crane cables up to provide support for the gate while it gets detached. It’s not easy doing big projects like this in the downtown core of a major city. The River Authority has had to lease the parking lot next door for a place to put the crane and other equipment. There are strict rules about when they can work to make sure the project doesn’t cause too much disturbance to all the neighbors. And, this is part of the River Walk, which means it's a heavily trafficked pedestrian route. The contractor has to set up barricades during work hours and then take them down at the end of each day. They also have safety spotters who make sure there are no wayward pedestrians or workers within the swing of the crane during heavy lifts. If you’ve worked on a device or turned a wrench, you’ve probably been faced with a stuck bolt before. But what do you do if the bolt is as big around as your arm? Pretty much the same thing you’d do at a smaller scale. Apply some penetrating oil… Beat it with a hammer… Use a cheater bar on the wrench… Bring out a hydraulic press… And then you just decide to cut the whole thing off. This gate’s being scrapped anyway so there’s no use treating it with kid gloves. The crew gets out the oxyacetylene torch to cut the ears off the top. First one. And then the other. Next come the hinge pins that connect the gate at the bottom. A few come out pretty easy. A few take a little extra effort. With a chain hoist pulling, the hydraulic toe jack pushing, and a little percussive persuasion, this crew eventually gets them all out. Just cutting and hammering and pushing and pulling all the connections this gate has to the dam is an entire day’s work. These are big, heavy items in awkward positions, so each time they move, disconnect, or lift something out of the work area, they have to do it thoughtfully and carefully to ensure it's done safely. By the end of the day, the gate is finally free, but the crew decides to set it down and wait until tomorrow for the critical operation of lifting it out. The next morning, it’s time for the big lift. The chain slings are re-secured around the gate, and the crane reaches over the trees and river to slowly remove it from the dam. It’s a big moment, so the whole crew gathers around to watch. Safety spotters coordinate with the crane operator to pull the gate free from the dam, then hoist it up and over. Safety personnel are making sure no one wanders into the area, but just in case, a horn sounds when the load is over the sidewalk. Eventually, the gate makes it to the staging area in the parking lot - on dry ground for the first time in 40 years. It did its job admirably, it was a great gate, but it’s easy to see from its condition that it was definitely time for retirement. With the gate out, a boom lift is lowered into the area to help remove some of the remaining pieces. Most of the day is spent cutting and removing pieces of the gate and attachment hardware. At this point, the area will mostly sit idle while the new gate is being fabricated. But there’s more work to do in the meantime. Another part of this project is the nearby pump room. The flows in the San Antonio River often drop to a mere trickle, and this is something the city designed for when these gates were installed back in the 80s. With these gates keeping the water up at a constant level, the River Walk works kind of like a bathtub; it takes a big volume of water to fill up the channel that snakes around downtown. But, if water leaves the River Walk faster than it can be replenished, that level will drop, kind of like trying to fill a bathtub without stopping up the drain. So this dam was designed with a pump to lift water from downstream into the channel above if needed. This is a screw pump, one of the oldest and simplest hydraulic machines, sometimes called an Archimedes Screw. A motor turns a steel cylinder with a screw inside. As the screw rotates, water is lifted upwards until it spills out at the top. In this case, it falls into a flume that flows out to the river above the dam. It’s ingenious in its simplicity, and apparently worked great when it was first installed. But, not long afterward, San Antonio built its landmark flood control tunnel that allows floodwaters to bypass downtown. It’s an incredible project of its own, and it included the means to recirculate water in the San Antonio River from downstream to up. That keeps the river flowing during dry times, maintaining the level in the River Walk downtown, and rendering the old screw pump obsolete. So it never got turned on again and has been sitting here unused for many years. This new project is going to repurpose the area to create a bypass for the two gates. It will add a bit more capacity, but more importantly, it will help create some circulation in the stagnant area downstream of the dam. Still water allows sediment to build up, collects debris, and grows algae and mosquitoes. With the screw pump not running, this area just doesn’t quite see enough water movement, so the bypass will allow it to be easily flushed out when needed. But first, the screw pump has to come out. This is the same story as the gate: oxyacetylene torches and hammers. Piece by piece, the pump is cut away and hauled off as scrap. With the pump out, the room gets some modifications. Some concrete is taken out… And new concrete is installed to create a chute for the water. And then it gets its own new gate to control the flow. Luckily this small pump room has an overhead crane, because getting this gate into place was a tight fit. Back outside, crews start working on the retrofits to the dam to get ready for the new gate. Unlike the electric motors used for the old gates, the new ones will use hydraulics. These piers that flank the gates have to be modified to fit the new system. The tops of the piers get some careful demolition to accommodate the hydraulic cylinders. And the hinges from the old gate still need to be removed. This area will also have some concrete modifications so the new gate fits perfectly in the old slot. Nearly a year after the old gate was cut out, the new gate finally arrives on site. It sounds like a long time, but this project was specifically scheduled around the fabrication of these gates. They aren’t just parts you can pick up at the local hardware store. A lot of design, construction, testing, and finishing touches went into each one. And they’re so big, they have to be delivered in two parts. Today’s job is to connect them into a single gate. The halves get a layer of sealant to prevent leaks, and then a whole bunch of bolts to attach them together. And finally, this gate is ready to install. You know I love crane day. And it’s even better when there’s a small crane to assemble the big crane. This 650-ton capacity monster is configured with a luffing jib to reach out over the trees and water. But the first step is to get the gate off the stands. It has to be lifted horizontally from these saw horses, but it will be installed vertically. So the gate is rigged for the first lift, moved to the ground, and then rerigged for the main event. I’m a sucker for heavy lifts so this was a pretty fun thing to see in person. It’s incredible how much work and setup went into a milestone that only took less than an hour to complete. It’s the civil engineering equivalent of a rocket launch. The crane swings the gate up and over the trees and down to the dam. As it gets closer, the movements are slower and more deliberate. Each time the crane moves, the crew waits for the massive gate to stabilize before calling for the next step. They carefully move it into position, and when everything is lined up just right, it sits down on the base plates, ready to be connected. While it’s held by the crane, the crews begin installing the bolts that attach the gate to the concrete. This is allowed by safety regulations, but only under a set of rigid guidelines, so safety is at the top of everyone’s mind. A detailed lift plan, a pre-work safety briefing, and several spotters make sure that there are no wrong moves. These bolts are torqued to the specifications one by one, on both the upstream and downstream side of the gate. And once it’s firmly attached, the crane lowers it to the ground. The next day, the beam across the top of the piers and the hydraulic cylinders are flown into place. These cylinders will lift and lower the gate, working against the immense water pressure pushing on the upstream face. They’ll attach to these beefy hinge points on the side of each gate. The cylinders are attached to a new hydraulic power unit installed in the pump room. This unit has the valves, pressure regulators, pump, and oil reservoir to make these gates operate more efficiently and reliably than the old electric motors did. Everything is operated from the City’s tower that overlooks the dam. From here, operators can control all the city’s flood infrastructure, including the dams and gates on the river and the flood bypass tunnels that run below ground. And I have to say, it’s a pretty nice view from the top. And in fact, some of the timelapse clips I’ve shown are from a camera mounted on top of this structure. This is run by the US Geological Survey, and I’ll put a link below where you can go check out the dam in real-time. Once everything is hooked up, it’s time to test this gate out. Unfortunately, you can’t schedule a flood. Since there are just ordinary flows at the moment, the crews have to be careful not to drain the entire River Walk while they do it. The gate gets lowered just a bit to make sure nothing is binding and that the hydraulic system is working. Of course, it’s a big day to see it all working for the first time, so everyone involved in the project is on-site to see it happen. And the test went flawlessly. But it’s not the end of the project. These stop logs were installed in early 2021, and it’s finally time to pull them out nearly four years later. You can see they grew some nice foliage during their service. This process requires a professional diver to rig each one for the crane. It’s just one of the many steps made much more complicated because this structure still has to serve its purpose during the entirety of the project, and more importantly, the River Walk can’t be drained. The stop logs get lifted out of the slots. Then they’re moved directly next door to get ready for the next gate. I didn’t document as much of the second gate, because it was pretty much identical to the first one, although it went a lot faster since the gate was already ready. The area was pumped out, the old gate removed, and the new one lifted into place. And pretty soon this old dam had two new gates, plus a bypass, ready to serve the city for the next several decades. If you visited the River Walk during construction, you wouldn’t have even known it was happening, and that was the entire goal of the project: revitalize a critical part of the city’s flood control infrastructure without causing any negative impacts on one of its crown jewels. And being on site to see it happen in real time was a lot of fun. I have to give a huge thanks to the City of San Antonio, the San Antonio River Authority, the engineer, Freese and Nichols, the general contractor, Guido, and all their subcontractors for inviting me to be a part of this project and document it for you. It was a pretty incredible experience, and I hope it gives you some new appreciation for all the thought, care, and engineering that goes into making our cities run.