[Note that this article is a transcript of the video embedded above.]

This is Waterloo Park in downtown Austin, Texas, just a couple of blocks away from the state capitol building. It’s got walking trails, an ampitheater, Waller Creek runs right through the center, and it has this strange semicircular structure right on the water. And this is Ladybird Lake, formerly Town Lake, about a mile away. Right where Waller Creek flows into the lake, there’s another strange structure. You saw the title of this video, so you know what I’m getting at here. It turns out these two peculiar projects are linked, not just by the creek that runs through downtown Austin, but also by a tunnel, a big tunnel. The Waller Creek Tunnel is about 26 feet (or 8 meters in diameter) and runs about 70 feet or 21 meters below downtown Austin. It’s not meant for cars or trains or bikes or buses or even high voltage oil filled cables, and it’s not even meant to carry fresh water or sewage. Its singular goal is to quickly get water out of this narrow downtown area during a flood. It’s designed with a peak flow rate of 8,500 cubic feet per second. That’s 240 cubic meters per second, or enough to fill a cubic olympic sized swimming pool in about 10 seconds. And the way it works is pretty fascinating.

Most major cities use underground pipes as drains to get rid of stormwater runoff so it doesn’t flood streets and inundate populated areas. But, a storm drain only has so much capacity, and a lot of places across the world have taken the idea a few steps further in scale. As I always say, the only thing cooler than a huge tunnel is a huge tunnel that carries lots of water and protects us from floods. And I built a model flood tunnel from acrylic, so you can see how these structures work and learn just a few of the engineering challenges that come with a project like this. I’m Grady, and this is Practical Engineering. In today’s episode, we’re talking about flood tunnels.

Floods are natural occurrences on earth, and in fact, in many places they are beneficial to the environment by creating habitat and carrying nutrient rich sediments into the floodplain, the area surrounding a creek or river that is most vulnerable to inundation. But, floods are not beneficial to cities. They are among the most disruptive and expensive natural disasters worldwide. If a flood swells a creek or river in a scattered residential neighborhood, it’s not ideal for the few homeowners who are impacted, but if a flood strikes the dense urban core of a major city, the consequences can be catastrophic with millions of dollars of damage and entire systems shutdown. What that means in practice is that we’re often willing to spend millions of dollars on flood infrastructure to protect densely populated areas, opening the door to more creative solutions. And heavily developed downtown areas demand resourceful thinking because they lack the space for traditional protection projects and they often predate modern urban drainage practices.

We can’t change the amount of water that falls during a flood, so we’re forced to develop ways to manage that water once it’s on the ground. The main way we mitigate flooding is just to avoid development within the floodplain. Don’t build in the areas of land most at risk of inundation during heavy storms. Seems simple, but it’s not an option for most downtown areas that have been developed since well before the advent of modern flood risk management. Another way we manage flooding is storing the water in large reservoirs behind dams, allowing it to be released slowly over time. Again, not an option in downtown areas where creating a reservoir could mean demolishing swaths of expensive property. A third flood management strategy is bypassing - sending the water around developed areas where it will cause fewer impacts. Once again, not an option in downtown areas where there is no alternative path for the water to go… unless you start thinking in the third dimension. Tunnels allow us to break free from the confines of the earth’s surface and utilize subterranean space to allow floodwaters past developed areas to be released further downstream. Let me show you how this works.

This is my model downtown business district. It’s got buildings, landscaping, and a beautiful river running right through the center. I have a flow meter and valve to control how much water is moving through that beautiful river, and here on the downstream side is a little dam to create some depth. Take a look at many major cities that have rivers running through them, and you’ll often see a weir or dam just like this to maintain some control over the upstream level, keeping water deep enough for boats or in some cases, just for beauty like the RiverWalk in downtown San Antonio. I put some blue dye and mica powder in the water to make it easier to visualize the flow.

I also have a big clear pipe with an inlet upstream of the developed area and an outlet just below the dam. Looking at this model, it might seem like a flood bypass tunnel is as simple as slapping a big pipe to where you want the flood waters to go, but here’s the thing about floods: most of the time, they’re not happening. In fact, almost all of the time, there isn’t a flood. And if you’re the owner of a flood bypass tunnel, that means almost all of the time you’re responsible for a gigantic pipe full of water below your city that has no real job except to wait. Watch what happens when I turn down the flow rate in my model to something you might see on a typical day. If we just leave the city like it is, all the flow goes into the tunnel, draining the channel like a bathtub and leaving the water along the downtown corridor to stagnate.

Standing water creates an environmental hazard. Without motion, the water doesn’t mix, and so it loses dissolved oxygen that is needed for fish and bacteria that eat organic material. Without dissolved oxygen, rivers become dead zones with little aquatic life and full of smelly, rotting organic material. Stagnant water also creates a breeding ground for mosquitoes, and is just unpleasant to be around. It’s not something you want in an urban core. The answer to this issue is gates, a topic I have a whole other video about. I can show how this works in my model. If you equip your gigantic flood bypass tunnel with gates on the inlet, you can control how much water goes into the tunnel versus what continues in the river. I just used this piece of foam to close off most of the tunnel entrance. I still have some water moving through there, but most continues in the river, keeping it from getting stagnant. This is why, on many flood bypass tunnels, you’ll see interesting structures at the inlets. Here’s the one in Austin again, and here’s the one just down the road in San Antonio. In addition to screening for trash and debris (and keeping people out) the main purpose of these structures is to regulate how much water goes into the tunnel.

But, some creeks and rivers don’t just have low flows during dry times, they have no flows. Intermittent streams only flow at certain times of the year and ephemeral streams only flow after it rains. Take a look at the stream gage for Waller Creek in Austin. Except for the days with rain, the flow in the creek is essentially zero. But, if you’re worried about stagnant water and lack of habitat on the surface, you want more water running in the river. You definitely don’t want to divert any of the scarce flows available into the tunnel. But you can’t just close the tunnel off completely, because then the water inside the tunnel will stagnate instead. You might think, “So what? It’s down there below the ground where we don’t have to worry about it.” Well, as soon as the next big flood comes and you open the gates to your tunnel, you’re going to push a massive slug of disgusting stagnant water out the other end, creating an environmental hazard downstream. So, in addition to gates on the upstream end, some flood tunnels, including the one in Austin, are equipped with pumps to recirculate water back upstream. I put a little pump in the model to show how this works. The pump pulls water from the river downstream and delivers it back upstream of the tunnel entrance. This allows you to double dip on benefits during low flows: you keep water moving in the tunnel so it doesn’t stagnate and you actually increase the flow in the river, improving its quality.

That’s 99 percent of managing a flood bypass tunnel: maintaining the infrastructure during normal flows. But of course, all that trouble is worth it the moment a big flood comes. Let’s turn the model all the way up and see how it performs. You can see the tunnel collecting flows, moving them downstream, and delivering them below the dam away from the developed area. The tunnel is adding capacity to the river, allowing a good proportion of the flood flows to completely bypass the downtown area. Of course, the river still rose during the flood, but it hasn’t overtopped the banks, so the city was protected. Let’s plug the tunnel and see what would happen without it. Turning up the model to full blast causes the stream to go over the bank and flood downtown. In this case, it’s not a huge difference, but even a few inches of floodwaters backing up into buildings is enough to create enormous damages and huge costs for repairs. Without any margin for increased flows, a big peak in rainfall can even wash buildings and cars away.

So, comparing flood levels between the two alternatives flowing at the same rate, it’s easy to see the benefits of a flood bypass tunnel. It resculpts the floodplain, lowering peak levels and pulling property and buildings out of the most vulnerable areas, making it possible to develop more densely in urban areas, not to mention creating habitat, improving water quality, and maintaining a constant flow in the river during dry times. Of course, a tunnel is an enormous project itself, and flood bypass tunnels are truly one of the most complicated and expensive ways to mitigate flood risks, but they’re also one of the only ways to manage flood risks in heavily populated areas.

I’ve been referencing projects in central Texas because that’s where I live, but despite their immense cost and complexity, flood bypass tunnels have been built across the world. One of the most famous is the Tokyo Metropolitan Area Outer Underground Discharge Channel that features this enormous cathedral of a subsurface tank. Unlike my model that works by gravity alone, the Tokyo tunnel needs huge pumps to get the water back out and into the Edogawa River. And some tunnels aren’t just for stormwater. Many older cities don’t have separated sewers for stormwater and wastewater, so everything flows to the treatment plants. That means when it rains, these plants see enormous influxes of water that must be treated before it can be released into rivers or the ocean. One of the largest civil engineering projects on earth has been in design and construction in Chicago since the 1970s and isn’t scheduled for completion until 2029. The Tunnel and Reservoir Plan (or TARP) includes four separate tunnel systems that combine with a number of storage reservoirs to keep Chicago’s sewers from overflowing into and polluting local waterways. And we keep finding value in tunnels where other projects wouldn’t be feasible. After record breaking floods from Hurricane Harvey in 2017, Houston started looking into the viability of using tunnels to reduce the impacts from future downpours. A 2.5-million-dollar engineering study was finished in 2022 suggesting that a system of tunnels might be a feasible solution to remove tens of thousands of structures from the floodplain. If they do move forward with any of the eight tunnels evaluated, that will complete the superfecta of major metropolitan areas in Texas with large flood bypass tunnels, but represent just one more of the many cities across the world that that have maximized the use of valuable land on earth’s surface by taking advantage of the space underneath.