[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.] This is the Veluwemeer (velOOwemeer) Aqueduct in Harderwijk (HAR-der-vehk), Netherlands. It solves a pretty simple problem. If you put a bridge for vehicles over a navigable waterway, you often have to make it either very high up with long approaches so the boats can pass underneath or make it moveable, which is both complicated and interrupts the flow of traffic, wet and dry. But if you put the cars below the water, both streams of traffic can flow uninterrupted with a fairly modest bridge. Elevated aqueducts aren’t that unusual, but this one is just so striking to see, I think, because it looks just like a regular highway bridge, except…the opposite. When I was a little kid, I read this book, The Hole in the Dike, about a Dutch boy who plugged a leak with his finger to save his town from a flood. And ever since then, as this little kid grew up into a civil engineer with a career working on dams and hydraulic structures, I’ve been kind of constantly exposed to this idea that the Netherlands is this magical country full of fascinating feats of civil engineering, like Willy Wonka’s chocolate factory but for infrastructure. I’m not necessarily proud to say this, but I think it’s true for a lot of people (especially here in the US) that my primary cultural touchpoint with the Netherlands is just that they’re really good at dealing with water. You know, you don’t have to browse the internet for very long to find viral (and sometimes dubious) posts about Dutch infrastructure projects. Sometimes, it feels like half of my comment section on YouTube is just people telling me that the Dutch do it better. I’m naturally skeptical of things that seem larger-than-life, especially when it comes to engineering. And without context, I think it’s hard to separate myth from facts (this TikTok video being a myth, by the way.) Here’s the actual scale of a cruise ship compared to the aqueduct. So let’s take a look at a few of these projects and find out if the Dutch really have the rest of the world outclassed when it comes to waterworks. And I’ll do my best to pronounce the Dutch words right too. Ik ben Grady, en dit is Practical Engineering. The first hint that the Dutch really do lead the world in water infrastructure is in the name of the country itself: The Netherlands translates literally to the lowlands, and that’s a pretty good description. A large portion of the country sits on the delta of three major rivers - the Rhine, the Meuse/Maas (MAHss), and the Scheldt (SHELLt) - that drain a big part of central Europe into the North Sea. Those rivers branch and meander through the delta, forming a maze of waterways, islands, inlets, and estuaries along the coast. About a quarter of the country sits below sea level, which creates a big challenge because it’s right next to the sea! As early as the Iron Age, settlers were involved in managing water. Large areas of marshland were drained with canals and ditches to convert them into land that could be used for agriculture. These plots of land, which, through human intervention, were hydrologically separated from the landscape, became known as polders. And the tradition of their engineering would continue for centuries to the present day. Unfortunately, that marshland, being full of organic material, decomposed over time. That, combined with the drainage of groundwater, caused the polders to sink and subside, increasing their vulnerability to floods. And that is kind of the heart of it. The Netherlands is a really valuable and strategic area for a lot of reasons: it’s flat; it has great access to the sea and major rivers providing for fishing and trade; it has prime conditions for farming and pastures, making it the second largest exporter of agricultural products in the world. The problem is that all those factors come with the downside of making the country extremely susceptible to floods, both from the North Sea and the major rivers that flow into it. So for basically all of its history, people were building dikes, embankments of compacted soil meant to keep water out of low-lying areas. Over the centuries, huge portions of the sinuous Dutch coastline became lined with dikes, and the individual polders were often ringed with dikes as well to keep the interior areas dry. Of course, you still get rain inside a polder, plus irrigation runoff and sometimes groundwater, so they have to be continuously pumped out. And before the widespread use of electric motors and combustion engines, the Dutch used the source of power they’re famous for: the wind. Windmills - or more accurately windpumps, since they weren’t milling anything in this context - could be used to turn paddle wheels or Archimedes screws to move water up and over dikes, keeping canals and ditches within the polders from overflowing. Over time, poldering dry-ish land, the Dutch realized they could use exactly the same technique to reclaim land from lakes. Typically land reclamation is done by using fill - soil and rock brought in from elsewhere to raise the area above the water. But it’s not the only way to do it, and it’s not that useful if you want to use that area for agriculture since the good soil is under the fill. Another option is to enclose an area below the water level, and then just get rid of the water. In this way, you can create arable land just for the cost of a dike and a pump. If you love cheese, you might be interested to learn that one of the first polders in the Netherlands reclaimed from a lake was Beemster. The soil of the ancient marsh provides a unique flavor of the famous Beemster cheese. One glaring issue with reclaiming land by drawing down the water instead of building up is that the low-lying polders are still vulnerable to floods. In 1916, a huge storm in the North Sea coincided with high flows in several rivers, flooding the Zuiderzee (ZIder-ZAY), a large, shallow bay between North Holland and Friesland (FREEZE-lahnd). The flood broke through several of the dikes, leading to catastrophic damage and casualties. Although the idea had been in discussion for years, the event provided the impetus for what would become one of the grandest hydraulic engineering projects in the world. One of the major issues with the Zuiderzee (ZIder-ZAY) flooding from a surge in the level of the North Sea is the sheer length of the coastline that has to be protected. Building adequately large and strong enough dikes to protect it all would be prohibitively expensive and just plain unrealistic. So Dutch engineers devised a deceptively simple solution: just shorten the coastline. If the effective coast of the Zuiderzee (ZIder-ZAY) could be substantially shorter, resources could go a lot further toward protecting the area against floods. So that’s just what they did. Between the late 1920s and early 1930s, a 20-mile (or 32-kilometer) dam and causeway called the Afsluitdijk (AWF-schlite-dike) was built across the Zuiderzee (ZIder-ZAY), cutting it off from the North Sea. Construction spread outward from four points, the coast on either side, and two small artificial islands built specifically for the project. The original dam was built from stones, sand, glacial till, stabilizing “mattresses” of brushwood, and thousands upon thousands of hand-laid cobblestones. Cutting off the Zuiderzee (ZIder-ZAY) from the ocean turned it into a large, and ultimately freshwater lake called the Ijsselmeer (ICE-el-meer), named for the river that empties into it. But that inflow is an engineering challenge. Without a way for it to reach the sea, the lake would just overflow. So, these sluices are like gigantic outflow valves that allow excess freshwater constantly building up in the Ijsselmeer (ICE-el-meer) to be discharged into the sea, as it would have been back when it was still the Zuiderzee (ZIder-ZAY). The sluices, which are titanic hydraulic engineering structures themselves, typically use gravity to drain water during low tide. When that passive discharge isn’t enough, new high-volume pumps can be used to make sure the level of the Ijsselmeer (ICE-el-meer) stays within the ideal range. Over the last few years, the Afsluitdijk (AWF-schlite-dike) has been undergoing a major facelift. With sea levels rising and the frequency of extreme weather events rising with it, the Dutch have completed a major overhaul, raising the crest of the dam by about 2 meters, adding thousands of huge concrete blocks to break waves and strengthen the structure. The larger blocks that are always in contact with the sea are truly gigantic, over 70,000 of them weighing six and a half metric tons EACH! The project also included upgrades to the lock complexes and sluices. And the highway that runs along the top is also getting upgrades (including, in true Dutch fashion, the bike lanes too). And human passage isn’t the only consideration for the project either. The Fish Migration River will allow fish to swim between the North Sea and the Ijsselmeer (ICE-el-meer) and river ecosystems upstream. The stark contrast between freshwater and saltwater is hazardous to fish, so the migration river spreads out the salinity gradient into something more manageable. It’s like a fish ladder, but on top of having an elevation gradient, it also is a ramp of saltiness. With the shallow Zuiderzee (ZIder-ZAY) protected from the North Sea, the Netherlands saw an opportunity to increase its food supply by creating new land. Over the middle decades of the 20th century, the Dutch built four gigantic polders in areas that were once the seafloor. These polders were built using the same principles as before, just with scaled-up 20th-century technology. There are even examples of our old friends, Archimedes screws being used, albeit with modern electric motors. Wieringermeer (veeRING-er-meer) and Noordoostpolder (NORD-OHST-polder) were built first, but the Dutch faced a problem. With such large areas of land dried up, the groundwater in adjacent areas flowed out and into the polders, causing subsidence and loss of freshwater needed for agriculture. The following polders, a pair of adjacent tracts called Eastern and Southern Flevoland (FLAYvo-lahnd), avoided this by retaining a small series of connected lakes. These bordering lakes keep the polders hydrologically isolated from the mainland, and this is also where you’ll find the Veluwemeer (velOOwemeer) aqueduct. The later three polders became Flevoland (FLAYvo-lahnd), a totally new province of dry land reclaimed from the sea. A succession of carefully selected crops were grown to rehabilitate the salty soil, making it fertile enough to farm. All you need to do to see how well it worked is look at these aerial photos of all the farmland in Flevoland (FLAYvo-lahnd)! There were plans for a fifth polder called the Markerwaard (MAHRKer-vahrd), and a huge dike was actually constructed for it. Hangups going as far back as the German Occupation of the Netherlands in the Second World War, to later environmental concerns, stopped the polder from being completed. The dike did create another freshwater reservoir, the Markermeer, and only recently, an artificial archipelago called the Marker Wadden (MAHRKer-vahdden) was built as a nature conservation project and host to migratory birds, fish, and ecotourists alike. Even as the Zuiderzee (ZIder-ZAY) Works protected parts of the Netherlands, many parts of the country were still facing threats from flooding. In the winter of 1953, an enormous storm in the North Sea raised a major storm surge, crashing into the delta, causing floodwaters to overwhelm much of the already existing and extensive flood control structures of the Netherlands. A staggering 9% of all of the farmland in the whole country was flooded, 187,000 farm animals drowned, nearly 50,000 buildings were damaged or destroyed, and over 1,800 people perished. It was one of the worst disasters in the history of the country. Just as with the Zuiderzee (ZIder-ZAY), the extraordinary length of the coastline of this area meant that adequately strengthening all the dikes in response to the storm wasn’t feasible. So, an incredibly intricate plan called the Deltawerken or Delta Works was put into motion to effectively shorten the coastline with a series of 14 major engineering projects, including dams, dikes, locks, sluices, and more. Unlike with the Zuiderzee (ZIder-ZAY) Works, fully enclosing the area and cutting off the sea wasn’t an option. Firstly, the Rhine and Meuse/Maas (MAHss) have gigantic flows. The Rhine is one of the largest rivers in Europe, and that can’t just be walled off. There are also concerns about environmental impacts and ensuring the easy movement of the huge amount of shipping that uses this waterway. So, many of these structures have to be functionally non-existent until they’re needed. The resulting projects, along with the Zuiderzee (ZIder-ZAY) works, have shortened the Dutch coast by more than half since the 19th century. These feats are so impressive they are on the American Society of Civil Engineering’s list of wonders of the modern world. And it’s easy to see why when you take a look. This is the Oosterscheldekering (OH-ster-SHELL-de-keering), the largest of all the Delta Works. It was initially designed to be a closed dam, similar in some ways to the Afsluitdijk (AWF-schlite-dike). If constructed as initially conceived, it would create another large freshwater lake. But, by the time it was under construction in the 1970s, environmental impacts were much more appreciated than they were in the 20s and 30s. So the dam was designed to include huge sluice gates to allow massive tidal flows during normal conditions while retaining the ability to fully close off the inland portion of the Delta from the sea during storm conditions. The Oosterscheldekering (OH-ster-SHELL-de-keering) comprises two artificial islands and three storm surge barrier dams connecting them. The larger of the islands also contains a lock, allowing for ships to pass through. The floodgates are staggering in scale; there are 62 steel doors, each 138 feet (or 42 meters) wide and weighing up to 480 metric tons! Even the piers between them were a monumental effort. They were built offsite, maneuvered into place with custom-built ships, then filled with sand and rock to sink them into place. Special ships also had to compact the seabed with vibration before placing the pillars. Another notable structure in the Delta Works is the Stormvloedkering Hollandse IJssel (storm-FLODE-keering--hoLAHNDse-ICE-el), a storm surge barrier protecting Europe’s largest seaport. The project has it all: a lock to allow for the passage of ships, a bridge for road traffic with a fixed truss and a moveable bascule portion crossing the lock, and two gigantic, moveable storm surge barriers crossing the main sluice. Each of these barriers is strengthened by a truss arch which makes them look like sideways bridges when viewed from above. And then, there’s the Maeslantkering. This is probably the most impressive storm surge barrier on the planet. Those tiktoks showing out-of-scale cruise ships crossing Veluwemeer (velOOwemeer) should have just shown actual gigantic ships cruising through the huge ship canal safeguarded by the Maeslantkering. It’s hard to communicate the scale of the two gates; they’re considered one of the largest moving structures on earth. And moving them is a process. The gates normally sit in dry docks. When it’s time to close them, the dry docks are flooded, and the hollow gates float in place. Then they’re pivoted around gigantic ball-and-socket joints at the ends of the truss arm. Each door is 690 feet (or 210 meters) wide, and once in place, they are flooded with water, so they sink to the bottom, completely blocking even the fiercest storm surge. In the event that the doors remain closed long enough for the flow of the Rhine to build up dangerously high on the inland side, they can be partially floated, allowing for excess river water to run out to sea. Since its completion in 1997, aside from annual testing, the Maeslantkering (mahs-LAHNT-keering) has only been closed twice: once in 2007 and again in 2023. And to me, that tells the story of Dutch waterworks more than anything else. It’s all a huge exercise in cost-benefit analysis. Look at two alternate realities: one where the Delta Works weren’t built and one where they were. And then just compare the costs. In one case, the costs are human lives, property damage, agriculture losses from saltwater, and all the disaster relief efforts associated with, so far at least, just two big storms. And in the other case, the costs are associated with designing, building, and maintaining an infrastructure program that rivals anything else on the globe. The question is simple: which one costs more? Look at many other places in the world, and the answer would probably be the Delta Works. Just the capital cost was around $13 billion dollars, and that doesn’t include the operation and maintenance, or environmental impacts of such massive projects. But in the Netherlands, where a quarter of the country sits below sea level, it’s a fraction of the cost of inaction. In the United States, most flood control projects are designed to protect up to the 1-in-100 probability storm. In other words, in a given year, there’s a 99% chance that a storm of that magnitude doesn’t happen. In the Netherlands, those levels of protection are much higher. River structures go from the 1-in-250 all the way to 1-in-1,250 and flood protection from the North Sea goes up to 1 in 10,000-year event. It only makes sense because practically the entire country is a floodplain; massive investment in protection from flooding is the only way to exist. And those projects come with other costs too. The Zuiderzee (ZIder-ZAY) Works cost the entire area’s fishing industry their livelihoods, and some consider converting such a large estuary into a freshwater lake one of the country’s greatest ecological disasters. So there are no easy answers, and the Netherland's battle against the sea will never really be over. Major waterworks are just the reality of the country, and they keep evolving their methods. One example is the Room for Rivers program which is restoring the natural floodplain along rivers in the delta. Another is the sand engine, an innovative beach nourishment project that relies on natural shoreline processes to distribute sand along the coast. The Dutch government expects the North Sea to rise 1 to 2 meters (or 3 to 7 feet) by the end of this century, meaning they’ll have to spend upwards of 150 billion dollars just to maintain the current level of protection. That sounds like a staggering cost, and it is, but consider this: that investment in protection for a major part of the country over three-quarters of a century is approximately equal to the economic impact of Hurricane Katrina, a single storm event in the US. Of course, the damage during Katrina was amplified by engineering errors, and we’re far from comparing apples-to-apples, but I think it’s helpful to look at the scale of things. Decisions of this magnitude are difficult to make, and even harder to execute, because we can’t visit those alternate realities to see how they play out. But what we can do is look at the past to see how decisions have played out historically, and there’s no place on Earth with a longer history of major public water projects than the Netherlands. In fact, the US Army Corps of Engineers and the Dutch government agency in charge of water, the Rijkswaterstaat (rikes-VAHter-stat), have had a memorandum of agreement since 2004 to share technical information and resources about water control projects. And in the aftermath of Hurricane Katrina, the Army Corps consulted with the Rijkswaterstaat (rikes-VAHter-stat) to help decide how to rebuild New Orleans’s flood defense system. In 2021, those systems were put to the test when the region was pummeled by Hurricane Ida. It was an extremely powerful storm, and the torrential rains and violent winds did enormous damage. But the storm surge was repelled by the levees, barriers, and floodgates built with the assistance of Dutch waterworks engineers. Many signs point to storms getting stronger and surges getting higher, which means that practically the whole world is in an uphill battle with floods. So we all benefit from that relatively small country with its low-lying delta lands, buttressed against the sea, and the expertise and knowledge gained by Dutch engineers through the centuries.

[Note that this article is a transcript of the video embedded above.] This is the Wallis Annenberg Wildlife Crossing under construction over the 101 just outside Los Angeles, California. When it’s finished in a few years, it will be the largest wildlife crossing (*of its kind) on the planet. The bridge is 210 feet (64 meters) long and 174 feet (53 meters) wide, roughly the same breadth as the ten-lane superhighway it crosses. Needless to say, a crossing like this isn’t cheap. The project is estimated to cost about $92 million dollars; it’s a major infrastructure project on par with similar investments in highway work. And it’s not the only example. The Federal Highway Administration recently set aside $350 million federal dollar to fund projects like this. The reasons we’re willing to invest so much into wildlife crossings aren’t as obvious as you might think, and there are some really interesting technical challenges when you’re designing infrastructure for animals. I’m Grady, and this is Practical Engineering. Roads fundamentally change the environments they cross through. And while on its face, it might seem that it’s always a disaster for wildlife, there are actually some winners amongst the losers. For vultures, crows, coyotes, raccoons, insects, and other decomposers, roads provide a buffet for nature’s scavengers. And they sometimes make for pretty good housing too, at least if you’re a swallow or a bat. In fact, cliff swallows are now so famous for nesting on the underside of highway overpasses that they’re often referred to as bridge swallows. The sides of highways have clear zones kept free from trees and similar obstacles for vehicle safety, but the lack of shade allows tender greens to thrive, creating a salad bar for species from monarch butterfly caterpillars to white-tailed deer. Of course, especially in the case of deer, this can attract animals into spending time eating dinner in danger. And the truth is that roads mostly range from a mild inconvenience to totally catastrophic for wildlife. In the battle between the two, wildlife usually loses, and in more ways than just getting squished. The ecological impacts of roads extend beyond the guardrails. Habitat loss and fragmentation, noise pollution, runoff, and of course, injecting humans into otherwise wild places are all elements of the environmental challenges caused by roads. It’s actually a pretty complicated subject, and there are even road ecologists whose entire careers are dedicated to the problem. And it’s not just wildlife that’s affected. According to the Federal Highway Administration, there are over 1,000,000 wildlife-vehicle collisions annually on US roadways. That results in tens of thousands of injuries, about 200 human fatalities, and over 8 billion dollars of damages per year. Even if you haven’t personally been involved in a collision like this, there’s a good chance that you know somebody who has. Along with the astronomical numbers reported by the FHA, it’s likely that a huge portion of wildlife collisions go unreported. There are lots cases that just don’t get counted, like if an animal is too small to notice, or if it survives the impact and escapes, or is collected by somebody practicing the dubious art of roadkill cuisine (yes, that’s a real thing and there are multiple cookbooks out there for it). There’s a wide range of consequences from animal collisions, from minor vehicle damage to human fatalities. When you average them out, researchers estimate that in 2021, the average cost of hitting a deer was $9,100. Of course, the bigger the animal, the bigger the economic loss. For a moose, that number is over $40,000 per collision. Regardless of how you might feel about environmental issues and wildlife, the economic impacts alone can justify the sometimes enormous costs required to let them safely cross our roadways. Luckily for the animal and human populations alike, there’s been increasing interest in reducing the negative impacts roads have on wildlife over the past few decades. I’m no stranger to infrastructure built for animals. It is fairly unusual for fish to get hit by cars, but they have their own manmade barriers to overcome, and I released a series of videos on fish passage facilities for dams you can check out after this if you want to learn more. Like aquatic species, there is a lot of engineering involved in getting terrestrial animals across a barrier. But fortunately, a lot of that research and guidance has been summarized in a detailed manual. I may not be a road ecologist, but I am an engineer, and I love a good Federal Highway Administration handbook! One of the most important decisions about building a wildlife crossing is where to put one. You might imagine that the busiest roads are where most of the collisions occur. And it’s true up to a point. As the number of cars on a road increases, the percentage of wildlife crossing attempts that end in a safe critter on the other side drops, and the fraction that are killed grows. But, if we keep increasing the daily traffic numbers, something unexpected happens: the number of “killed” animals declines! Eagle-eyed viewers may realize that so far, this graph is incomplete; these percentages don’t add up to 100%. That’s because there’s a third category: “repelled” animals. As highway traffic increases, you reach a point where the vehicles form a kind of moving fence, and all but the most brazen bucks will turn away. Road ecologists sometimes struggle to drum up support for wildlife crossings at high-traffic freeways (like the Annenberg crossing in LA) because of this effect. For some people, if they don’t see actual road kills on the shoulder, they struggle to accept the greater impact on wildlife populations. Habitat fragmentation caused by roads can be difficult for any species, but it’s especially hard-hitting for migratory species who HAVE to cross in order to survive and reproduce. For example, following the opening of I-84 in Idaho, biologists recorded the starvation of hundreds of mule deer mired in the snow, unable to cross to food sources. And it’s not quite as simple as the graph makes it seem. A study by Sandra Jacobsen breaks down animals into four categories of crossing style. Some animals, like frogs, are non-responders who cross roads as if they aren’t there at all. Their wild instincts compel these animals to cross without regard for their own safety, and they’re often too small for most motorists to notice. Next, you have the pausers, like turtles. These creatures, when spooked on the road or elsewhere, instinctively hunker down and stay put. While the shell of a box turtle might be impenetrable to a curious coyote, it is, sadly, no match for a box truck. Then you’ve got avoiders. This group often includes the most intelligent members of the local fauna. Grizzly bears, cougars, and other carnivores often fall into this category. For them, even low-traffic rural backroads can cause significant issues with habitat fragmentation, leading to poor genetic diversity. The small gene pool of a number of southern California cougars is one of the major drivers of the construction of the Annenberg bridge. Deer fall into the last category, speeders. As the name implies, these are fast, alert animals who, given the chance, will burst across a road to get to the other side. But even these categories have their exceptions. The poster-cat of the US-101 project, a cougar called P-22, famously crossed the 10-lane highway and took up residence in the shadow of the Hollywood sign. There just is no one-size-fits-all approach for getting animals across roads. Engineers and ecologists use a wide variety of mapping, including aerial photography, land cover, topography, habitat, plus ecological field data and even roadkill statistics to choose the most appropriate locations for new wildlife crossings. And in many cases, what works for one species may be completely ineffective for another. So most designs are made for a so-called “focal species,” with the hope that it works well for others too. But before you have a crossing, you have to get the animals to it. In most cases, that means fences, and even that is complicated. Do the focal species have a habit of digging under fences like badgers or bears? Well, then you’ll want to bury a few feet of fence to maintain its integrity. And where do they start and stop? Ideally, fences will terminate in areas that are intentionally hard to cross so animals don’t end up in a concentrated path across roadways. Sometimes boulders will be placed at the end of a wildlife fence to make it less likely that animals will choose to wander on the wrong side. But, inevitably, it happens. You don’t want to trap animals on the highway side of a fence, so many feature ramps or “jumpouts” that act almost like one-way valves for animals. There are even hinged doors for moderate-sized animals that allow wayward creatures to escape through fences. Once you’ve got a site selected, the next big choice is over or under. It turns out that going under a road is often the easiest option. In fact, in many cases, existing bridges and viaducts can naturally create opportunities for wildlife to get across our roadways. Sometimes it’s as simple as building fencing to funnel animals into existing underpasses. Another option for small animals is to use culverts as crossings. The engineering and materials for culverts are pretty well established since they’re used so much for getting drainage across roadways, so it’s not a big leap to do it with animals too. But it can be tricky getting them to use it. Since amphibians are also pretty lousy at walking long distances, it’s common to have many small tunnels installed near one another with special fencing to maximize survival. In some cases, they’re combined with buried collection buckets. During peak migration periods, the buckets are checked, and collected amphibians are manually transported across the road! Larger animals won’t fit in a culvert (or a bucket), but there are some special considerations to getting them to travel beneath a highway bridge. Many animals are hesitant about dark areas during the daytime, so it's important to get as much natural light in as possible. Lighting also affects the vegetation that grows under a bridge. More light means more natural-feeling areas, which means more animals will be willing to cross under. And of course, keeping people out is important too. Disturbance from the public can really affect animals' willingness to incorporate a new, unusual route into their routine. Many crossings are designed with cover objects like logs, rocks, and brush that can help encourage a wider variety of wildlife to take advantage of the intended path. But, for some species, underpasses just don’t work at all. You can’t FORCE a moose to do anything really, especially something like walking through a tunnel it doesn’t trust. In certain instances, the only effective way to allow safe passage across a road is over the top. For some particular focal species, an overpass might not need to be that grand. Canopy bridges just connect trees on either side of a road so primates and other tree-living creatures can get across. In Longview, Washington, there’s even a series of tiny bridges for squirrels, like the famous “Nutty Narrows” bridge. Of course, the most impressive, usually the most effective, and often the most expensive wildlife crossings are designed as overpass bridges. Examples include the famous ecoducts of the Netherlands, overpasses of the Canadian Rockies in Banff National Park, and American structures like the Wallis Annenberg Wildlife Crossing. I actually have one of these nearby. Opened in 2020, the Robert LB Tobin Land Bridge crosses the six-lane Wurzbach Parkway in San Antonio, Texas. These are full-on bridges designed specifically for the use of animals. Structures like these have all the same design issues as regular bridges for humans, plus their own engineering challenges as well. They have to hold up their own weight with a significant margin of safety, be designed to weather the elements for decades, and be inspected just like other bridges. They ALSO have to be engineered to be covered in thick layers of soil and vegetation (sometimes including trees), and be sized appropriately to accommodate focal species that might travel in huge herds or be wary of tight spaces. They have to be built to provide appropriate lines of sight for nervous crossers and often have walls that shield wildlife from the noise and light of the traffic below. One fun upside is that, at least in mountainous areas, the approaches can be a lot steeper than you might use for a vehicular bridge. An elk is pretty well suited for off-roading after all. As for the design of the bridges themselves, they’re built a lot like highway bridges, usually beam bridges or arches, just with dirt instead of concrete for the deck. While the distance across a highway is long for a wandering moose, it’s not generally enough to require a structure of more heroic engineering like cable-stayed or suspension bridges. Unlike vehicular bridges, the approaches often flare out when viewed from above, making it easier for animals to locate the bridge and for better sight lines across it. This, plus the fact that they are usually covered in native vegetation, means that wildlife overpasses are among the most striking bridges you can see. It also means that from the perspective of the wildlife crossing them, these bridges can blend into the scenery. Ideally, a herd of pronghorn wouldn’t even realize they’re on a bridge at all. It’s hard to think of any humanmade structures that have transformed the landscape more than modern roadways. They have an enormous impact on so many aspects of our lives, and it's easy to forget the impact they have on everything else that we share the landscape with. Sometimes when it comes to mitigating the negative impacts of roads on wildlife, the best thing is to just be more careful about where or IF we build a road at all. But for many of the roads we already have and the ones we might build in the future, it just makes sense - for safety, the economic benefits, and just being good stewards of the earth - to make sure that our engineering lets animals get around as easily as we can.

[Note that this article is a transcript of the video embedded above.] A lot of engineering focuses on structural members. How wide is this beam? How tall is this column? But some of the most important engineering decisions are in how to connect those members together. Take a column, for example. You can’t just set it directly on a foundation, at least not if you want it to stay up. It needs a way to physically attach to the foundation. This may seem self-evident, maybe even completely obvious to most. But in that humble connection that’s so ubiquitous you rarely even notice it, there is so much complexity. Baseplates are the structural shoreline of the built environment: where superstructure meets substructure. And even understanding just a little bit of the engineering behind them can tell you a lot of interesting things about the structures you see in your everyday life. I’m Grady, and this is Practical Engineering. Let me start us out with a little demonstration. If you’re a regular viewer, you know how much you can learn from our old friends: some concrete and a benchtop hydraulic press. I cast two cylinders of concrete about a week ago, and now it’s time to break them for science. These were cast from the exact same batch of concrete at the exact same time. For this first one, I’m pushing with a fairly narrow tool. I slowly ramp up the force until eventually… it breaks. I had a load cell below the cylinder, so we can see the force required to break this concrete. This scale isn’t calibrated, so let’s say it broke at 1400 arbitrary Practical Engineering units of force. Practicanewtons? KiloGradys? What would you call them? Now let’s do the same thing with a wider tool. At that same loading, this concrete cylinder is holding steady. In fact, it didn’t break until 3100 units. Here’s a trick question. Was the second cylinder stronger than the first one? Hopefully it’s obvious that the answer is no. Most materials don’t care about force. I mean, in the strictest sense, most materials don’t care about anything. But what I mean is that the performance of a material against a loading condition usually depends not on the total force, but how that force is distributed over an area. It’s pressure; force divided by area. Increase the area, lower the pressure. And pressure is what breaks stuff. So that’s what a lot of baseplates do. They transfer the vertical forces of a column to the foundation over a larger area, reducing the pressure to a level that the concrete can withstand. And that’s the first engineering decision when designing a baseplate. How big does it need to be? If you know the force in the column and the allowable pressure on the foundation, you can just divide them to get the minimum area of the plate. That’s the easy part. Because steel isn’t infinitely stiff. If I put this column on a sheet of paper, I think it’s clear that there’s no real load distribution happening here. The outside edges of the paper aren’t applying any of the column’s force into the table; I can just lift them. But this can be true for steel too. I filled up an acrylic box with layers of sand to make this clearer. If I use a thin base plate, the forces from my column don’t distribute evenly into the foundation. You can see that the baseplate flexes and the sand directly below the column displaces a lot more. I can try this with a thicker, more rigid baseplate, and the results are a lot different. Much more even distribution of pressure. So the second engineering decision when designing a baseplate is the stiffness of the plate, usually determined by the thickness of the steel, based on the loads you expect and how far the plate extends beyond the edges of the column. And in heavy-duty applications like steel bridge supports, vertical stiffeners can be included to make the connection even more rigid. So far, though, the baseplate isn’t really much of a connection. That’s the thing about compressive loads: gravity holds them together automatically. There are no bolts in the Great Pyramid of Giza. The blocks just sit on top of each other. And that could be true for some columns too. The main load they see is axial, along their length, pressing the plate to the ground. But we know there are other loading conditions too. A perfect example is a sign. Billboards and highway signs are essentially gigantic wind sails. They don’t actually weigh all that much, so the compressive force on their base isn’t a lot, but the horizontal forces from the wind can be significantly higher than that. Those horizontal forces can increase the compression force on one side of the base plate, so you have to account for that in the design. But they also can result in shear and tension forces between the baseplate and foundation, so you’ve got to have something in place to resist those forces too. That’s where anchors come in. There are a lot of ways to attach stuff to concrete. There are anchors that epoxy into holes, screw into place, or use wedges to expand into the hole. And of course, if you’re extra careful and precise, you can even embed anchor rods or bolts into the concrete while it’s still wet. There’s a huge variety of styles and materials that offer different advantages depending on your needs. Here’s just one manufacturer’s selection guide for the anchors and epoxies they provide. But like third year engineering students, all of those anchors can fail if they’re overloaded. And they can fail in a lot of different ways under tension or shear forces. The anchor rod itself can fracture or deform. It can lose its bond with the concrete and pull out. It can break out the surrounding concrete. Or if it’s too close to the edge, it can blow out the side. Calculating the strength of the anchor bolt and concrete connection against each of these failure modes is a lot more complicated than just dividing a force by a pressure to determine the baseplate area. So most engineers use software that can do the calculations automatically. But, there’s another challenge about baseplates I haven’t mentioned yet, and it has to do with tolerances. Concrete foundations can be pretty precise. As long as you set the forms accurately and make them strong enough to avoid deflection while the concrete is being placed, you can feel confident in the dimensions of the structure that comes out of them. But there’s usually one surface that isn’t formed: the top. Instead, we use screeds and trowels and floats to put a nice finish on the top surface of a concrete slab or pier. But it’s rarely perfect enough to put a column directly on top. That’s not to say it can’t be done. I’ve seen concrete finishing crews do amazing work. But it’s usually not worth the effort to get a concrete surface perfectly level at the exact elevation needed for every column, especially when you have the time pressure of concrete setting up. And those tolerances matter. Just one degree off of level will put a 16-foot or 5-meter column out of plumb by more than 3 inches or 80 millimeters. Unless you’re in certain parts of Tuscany, that’s not gonna work. It’s more than enough to misalign some bolt holes. And that only magnifies for taller columns like signpoles. So, we usually need some adjustability between the plate and the concrete. Sometimes that means shimming the baseplate to get it perfectly level. And the other primary option is to use leveling nuts underneath the plate. I welded up a custom-branded column and and baseplate that was laser-cut by my friends at Send-Cut-Send to show you how this works. These parts turned out so nice. By adjusting these nuts up or down, I can get the column to point in the exact direction required. And I can get it to the exact right elevation too. But maybe you see the problem here. All the work we did to make sure the baseplate distributes the vertical load even across its area is lost. Now the vertical loads are just being transferred through some shims or through the bolts directly into the anchors. So, in a lot of cases, we add grout between the plate and the concrete to bridge the gap. Grout is basically concrete without the large aggregate, mixed with a low viscosity so it flows more easily into gaps. And it often includes additives to prevent it from shrinking as it cures, making sure it doesn’t pull away from the surfaces above and below. When it hardens, the grout can transfer and distribute the loads into the foundation. So if you pay attention to baseplates you see out in the built environment, you’ll notice it’s pretty common that they sit on a little pedestal of grout and not directly on the concrete below. But even this comes with a few problems. First is load transfer. Even with the grout, some of the vertical loads are still going into the anchor bolts that might not have been designed for compression. So now we’ve added a few more new potential failure modes to the laundry list: punching through the bottom of a slab, and buckling of the rod itself. Sometimes contractors will use plastic leveling nuts that can hold the column during construction, but will yield after the column’s loaded so the grout supports all the weight. Second is fatigue. Especially for outdoor structures that see wind and vibrations, the grout under the baseplate might not hold up to repeated cycles of loading. Third is moisture. Grout can trap water, leading to problems with corrosion, especially for hollow columns like sign poles where condensation needs a way out. And the grout can hide that corrosion, making it difficult to inspect the structure. And fourth, adding grout below a baseplate is just an extra step. It’s kind of fiddly work to do it right, and it costs time and resources that might otherwise be spent somewhere else. In fact, there are a lot of cases where it’s an extra step worth skipping. You can design anchor bolts strong enough to withstand all the forces a column will apply, including the compressive forces downward. And you can design a base plate stiff enough that those forces don’t have to be distributed evenly across the entire area. And if you do, you have a standoff base plate. It just floats above the concrete with only the anchors in between. It looks like a counterintuitive design. We think of a baseplate as kind of a shoe, so it should be sitting on the ground. And a lot of them are designed that way. But for other structures, a baseplate is really just a way to connect a foundation to a column through an anchor. So if you pay attention, you’ll see these standoff baseplates everywhere. A lot of state highway departments have moved away from using grout to make signs and light poles easier to inspect. And they often install wire mesh to keep animals out from hollow masts. Clearly there’s a lot more to baseplates than meets the eye, and that means there’s also a few myths going around grout there. A common misconception is that standoff baseplates are meant to break away in the event of a collision. And I totally understand why. If an errant vehicle hits a signpost, a relatively minor deviation from the road can turn into a deadly crash. Smaller signs installed near roadways often do use breakaway hardware or features. You’ll often see holes drilled in wooden posts, bolts with narrow necks meant to snap easily, or slip bases like this one to make sure a sign gives way. But for larger structures like overhead signs and light poles, that’s generally not the case. Having one of these break away and fall across a highway could create an even bigger danger than having it stay upright. So, even though they might look similar, standoff baseplates are distinct from sign mounts designed to break loose in a collision. Instead, larger structures installed in the clear zones of highways are protected from crashes using a guardrail, barrier, or cushion. Baseplates are like bass parts in music, it’s easy to overlook them at first, but once you notice them, you can’t stop paying attention to how important a role they play. And just like bass lines, they might seem simple at first, but the deeper you dig, the more you realize how complex they really are.