Although it’s an entirely normal and natural process on Earth, flooding represents a huge problem for people, every year we collectively throw billions of dollars essentially into the trash because of flood damage to property, buildings and vehicles and equipment. But it’s not just private property that’s affected.
Nearly every part of the constructed environment is vulnerable in some way to heavy rainfall culverts, bridges, sewers, canals, dams and drainage infrastructure. They all have to be designed to withstand at least some amount of flooding. But how do we decide how much is enough and how do we estimate the magnitude of any particular storm event? Hame Grady. And this is practical engineering. On today’s episode, we’re talking about synthetic floods for designing infrastructure.
A big portion of the constructed environment has at least something to do with drainage if it’s exposed to the outdoors and almost all infrastructure is it’s going to get wet or deal with some water. Designers and engineers have to be thoughtful about how and where that water will go during a storm. This might seem self-evident, but someone had to decide how long to make this storm drain inland. How high above the river to build this bridge. How wide to make the spillway and how big to build this culvert. And these types of decisions aren’t arbitrary because infrastructure is expensive and it’s always built on a budget. You can’t waste dollars installing pipes that are two big bridges that are too high or spillways too wide, because then that money can’t be used to fund other projects or improvements. But how much is too much? After all, if you can imagine a flood that meets the capacity of a given structure, you can probably imagine a bigger one that exceeds it. On one hand, you have the structures cost and on the other you have its capacity, in other words, its ability to withstand flooding. Finding a balance point between the two is a really important job and it usually has to do with statistics, whether is sporadic, it’s noisy data, some days of rain, some days a dozen, some years. It rains nearly every day and some years not at all. But behind all that noise, there is a hidden beauty to weather data, which is the relationship between the storms magnitude and its probability. Small storms happen all the time, multiple times a year. Big storms happen rarely, only every few years. Massive floods occur only once every tens or hundreds of years. Their probability of occurring in a given year is low.
This is all relative, of course, especially depending on the location. But I hope you’re seeing why this matters, because if you know the probability of a particular storm will occur, you also know the average number of times that will happen over a given period of time. And why does that matter? Let’s use a simple case as an example. Say you have a roadway crossing a stream and you want to install a culvert. By the way, if you want to learn more about culverts, check out my video on that topic after this. Say you choose a tiny pipe for your cover to save some money. That’s fiscal responsibility, right? But every time even a small amount of rain comes along, the culverts capacity will be exceeded and the roadway will overtop and wash out. Your cheap pipe actually ends up being pretty expensive when you have to replace it every year. On the other hand, you can go for broke on a massive pipe that never gets full, even during huge rainstorms. You’ll never have to replace it, but you wasted money by building a much bigger structure than was necessary. That might not seem like a big deal for a single culvert, but if it’s your policy to do it, every time you have to cross a stream, you’ll run out of money in a hurry. We can’t just overbuilt all our infrastructure to avoid any exposure to flood risk. Usually the most cost effective solution is somewhere in the middle where you’re willing to accept some risk of being overwhelmed. Maybe on average it will happen once every ten years or once every 50 years to save the cost of overbuilding every single piece of drainage infrastructure. This works the same way as the floodplain, the area along coasts and rivers most likely to be impacted by flooding in the US at least, we arbitrarily decided to use one percent as a dividing line between at risk for flooding and not if the land has a one percent probability or greater of being inundated by a flood in a given year. It’s inside the quote unquote floodplain. And the storm that would completely flood this floodplain is colloquially called the hundred year flood. That’s a confusing name. And I made a video on that topic quite a while back, so I won’t rehash it here. This binary approach of drawing a line in the sand is also a little misleading because it implies this area is safe and this area is when the reality is there’s a continuum of flood risk. Those considerations aside, the concept of the floodplain is still really valuable, knowing our vulnerability to flooding helps us make good decisions about how to manage or mitigate. But actually figuring out that vulnerability is pretty challenging. The truth is that the only way we have to estimate how vulnerable different areas are to flooding is to look at how they flooded in the past. In the US, we do have a network of stream gauges dutifully recording the level of creeks and rivers, and some of them have been doing so for over a hundred years now. These instruments record the magnitude of floods through history so we can try to understand the relationship between the size of a flood and its frequency of recurring.
But the stream gauges are relatively expensive time-Consuming to maintain, and their data is only applicable to the watershed in which they’re installed, which means not every location you might want to build something has a historical flood record to review. However, there is a type of instrument that does exist practically everywhere with long duration historical records. Arrange rain gauges are simple and cheap and luckily in the US are. Government has seen fit to collect huge volumes of rainfall data, synthesize it and provide the information back to US citizens for our practical application or just for our curiosity. The latest version of this is called Atlas 14. And you can use the online web map to get statistical relationships between rainfall, volume, duration and probability for nearly everywhere in the US. But estimating the magnitude of a flood doesn’t stop with knowing how much rain is falling from the sky. It may not surprise you to know that the hundred year storm doesn’t really exist. It’s a synthetic storm of it invented by engineers and hydrologists. We fabricate it by taking that statistical amount of rain for a given watershed and use models to estimate how much flooding will result and where that flooding will occur within the landscape. These simulations allow us to understand flood risk so we know where not to build our buildings, how big to make our culverts, how tall to make our bridges, and how wide to make our drainage channels. But flooding doesn’t just cost money. It also affects public safety. In fact, some of the worst floods in history, like the Johnstown flood and Pennsylvania actually occurred because a storm overwhelmed a dam, causing it to fail and release a sudden wave of water downstream. In that case, over two thousand people lost their lives. With critical infrastructure like this, the calculus changes because it’s not just dollars on the other side of the balance. It’s also human lives. We are much less willing to accept the risk of overwhelming a dam if there are people who could be affected downstream. So how do we know how big spillways should be? Turns out there’s another type of synthetic flood in the toolbox, the probable maximum precipitation. This is the most extreme rainstorm that could ever occur given our knowledge of meteorology and atmospheric science. If all the factors perfectly aligned to carry and drop the maximum amount of rainfall in the shortest period of time, could our infrastructure withstand it in the case of dams? The answer is usually yes. That’s because they’re required to. We spent lots of time, money and effort researching storms to estimate this probable maximum precipitation across the US for this exact reason so we can build spillways big enough to safely discharge it without being overwhelmed. The field of engineering hydrology is huge, many engineers focus their entire careers on this one topic that we’ve just dipped our toes into flooding is one of the biggest challenges of building and developing the modern world. The ways we deal with it are constantly evolving, hopefully in a direction that puts greater emphasis on natural watershed processes and ecosystem services. But no matter how we deal with it, the first step will always be to understand our vulnerability to it.
I hope this gave you a little peek into the world of water resources engineering and how we make good decisions about infrastructure’s ability to handle flooding.