Cities, those dense congregations of people and buildings, have made possible economies and lifestyles our early ancestors could never have imagined. Whether you thrive in or despise the concrete jungle, there’s no denying its benefits. Putting all the people, houses, jobs, stores, offices, and diversions in one place gives us humans opportunities that wouldn’t be possible if we all lived agrarian lifestyles spread out across the countryside. But, there are some negative consequences that come from cramming so much into such a small area. At no time is this more clear than when it rains. Managing the flow of runoff through a city is an immensely complex challenge that affects us in so many ways from public safety to property rights, from the environment to the health and welfare of citizens. Hey, I’m Grady, and this is Practical Engineering. Today,, we’re talking about urban stormwater management.

The water cycle is one of the most basic science lessons we learn. So basic, in fact, that it’s easy to forget how relevant and important it is to our lives. Take a look out your window when it’s raining, even when it’s raining hard, and it doesn’t seem that significant. Some of the rain soaks into the ground, some gets taken up by plants, some gets caught in puddles, and some runs off downhill, usually into the street. One of the biggest challenges in a city is the proportions of all these different paths the water can take. All those streets, sidewalks, buildings, and parking lots cover the ground with impervious surfaces, which means that instead of water infiltrating, it runs off toward creeks and rivers, swelling them faster and higher and filling them with more pollution. One of the biggest impacts on the environment of building anything is its effect on how water moves above and below the ground during storms. Multiply that to the scale of a city and you can see how remarkably we modify our landscape. Instead of acting like a sponge to absorb rainwater as it falls, urban watersheds act like funnels, gathering and concentrating rainwater runoff. I want to walk you through some of the infrastructure cities use to manage this massive challenge and a few new ideas in stormwater management that are slowly taking hold in urban areas.

Like most of the biggest challenges of building and maintaining a civilization, the negative impacts from adding impervious cover don’t befall the property owner doing the adding, but rather the people downstream. Just like dumping pollution into the river carries away to the next guy, it’s easy to make bad drainage decisions into someone else’s problem. That’s why most large cities have rules about how to manage runoff and flooding when new buildings or neighborhoods get built. Drainage reviews are just a normal part of the process of obtaining a building permit these days. If you live in a major city, just do a search for your local drainage manual to see the kinds of things that are required. Increased runoff has been a problem since people started living in cities in the first place, and the first way we handled it was simply to get the water out and away as quickly as possible. That’s because runoff creates flooding, and flooding causes billions of dollars of property damage and many lives each year. This solution is in the name we still use for how cities manage storms: “drainage.” When it rains or when it pours, we try to give that runoff somewhere to go.

Most cities are organized so the streets serve as the first path of flow for rainfall. Individual lots are graded with a slope toward the street so that water flows away from buildings where it would otherwise cause problems. The standard city street has a crown in the center with gutters on either side for water to flow. This keeps the road mainly dry and safe for vehicle travel while providing a channel to convey runoff. But the streets aren’t the end of the line. Eventually, the road will reach a natural low point and start back uphill or will have collected so much runoff that it can’t hold it all in the gutter.

At this point, the water needs a dedicated system to carry it away. In the past, it was common to simply put all the runoff from the streets directly into the sewage system. It’s a well-developed network of pipes flowing by gravity out of the City… why not use it for stormwater too? Well, actually there’s a really good reason not to do that. At the end of each sanitary sewer system is a wastewater treatment plant that was almost certainly not designed to process a massive influx of combined sewage and stormwater runoff at the whims of mother nature. In the worst cases, these plants have to release untreated wastewater directly into waterways when it is too much to be stored or processed. That’s why most cities now use municipal separate storm sewer systems, usually abbreviated as MS4s. These are networks of ditches, curbs, gutters, sewer pipes, and outfalls solely dedicated to moving runoff from everywhere in the city to the natural waterways that eventually carry it away. These inlets aren’t just places for clowns to hang out, they usually represent a direct path between the street and the nearest creek or river. Just to be clear, there’s not usually any type of treatment happening along the way. These sewers are not for waste. Whatever you put into the storm sewer system goes directly into a waterway, so please don’t dump stuff in there.

It’s easy to see why cities try so hard to get stormwater out as fast as possible if you look at the floodplain. This is just the area most likely to be inundated during a major flood. Land is one of the most valuable things within a city, but its value goes way down if it is exposed to flood risk. No one wants to build something on land that could be flooded. That being said, humans are notoriously bad at assessing risk, and no matter where you look, you’re likely to find development near creeks and rivers. Getting the water out quickly reduces the depth of flooding and thus shrinks the floodplain. That’s a big reason why you see natural waterways in cities enlarged, straightened, and lined with concrete. You can see , for the same amount of flow, a channel with lots of vegetation moves water more slowly and thus at a higher depth. A channel with smooth sides gets the water moving faster, and thus reduces the depth of flooding. But, channelization isn’t all it’s cut out to be. It’s ugly for one. No one wants a big, dirty concrete channel as a part of their surroundings. But, channelization also worsens flooding downstream for the next guy and degrades the habitat of the original waterway. It didn’t take long for cities to realize you can’t just keep widening and lining channels to keep up with the increased runoff from more and more development.

That’s why most cities now require developers to take responsibility for their own increase in runoff. By and large, that means on-site storage for stormwater. Retention and detention ponds act like mini-sponges, absorbing all the rain that rushes off the buildings, streets, and parking lots and releasing it slowly back into waterways. This shaves off the peak of the runoff with the goal of reducing it back down to or less than it was before all those buildings and parking lots got built. They also help reduce pollution by slowing down the water so suspended particles can settle out.

Onsite storage is a pretty effective solution, and one you’ll see everywhere if you’re paying attention. But it still treats stormwater as a waste product, something to be gotten rid of. The reality is that rain is a resource, and natural watersheds do a lot more than just getting rid of it. They serve as habitat for wildlife, they naturally clean runoff with vegetation, they divert rain into the ground to recharge aquifers, and they reduce flooding by slowing down the water at the source rather than letting it quickly wash away and concentrate. That’s why many cities are moving toward ways to replicate and recreate natural watershed functions within developed areas. In the U.S., this is called low-impact development and it includes strategies like rain gardens, vegetated rooftops, rain barrels, and other ways to bring more harmony between the built environment and its original hydrologic and ecological functions. It can also include better management of the floodplain by using it for purposes less vulnerable to flooding like parks and trails. One low-impact strategy is permeable pavement, and I have a post just on that topic if you want to check it out after this one.

One thing I have to mention when talking about flooding is vehicle crossings. Any location where a waterway and a road cross paths, whether it’s a bridge, a culvert, or a low water crossing, there’s always a chance of flooding getting so bad that it overtops the road. If you ever see water over the top of a roadway, just turn around. Half of all flood-related deaths happen when someone tries to drive a car or truck through water over a road. If you can’t see the road you have no idea how deep the water is, and even if you can, it only takes a small amount of swift water to push a vehicle down into a river or creek. Water is heavy. Even when it’s flowing slowly, floodwaters can impart a massive force on a vehicle. Even if it didn’t, most cars will float once the water reaches the floorboard anyway. Some cities have warning systems to help block roads when they’re overtopped by floods, but it’s not something you should count on. It just isn’t worth the risk. Find another way. As they say: Turn around, don’t drown.

Just like cities represent a colossal alteration of the landscape and thus the natural water cycle, we’re also going through a colossal shift in how we think about rainfall and stormwater and how we value the processes of natural watersheds. Look carefully as you travel through your city and you’ll notice all the different pieces and parts of infrastructure that help manage water during storm events. You’ll see plenty of ways to get water out and away from buildings and streets, but you hopefully also notice elements of Low Impact Design - ways of harnessing and benefitting from stormwater on-site, treating it like the resource it truly is.