As much as I love infrastructure and the urban environment, it definitely has its downsides. Cities represent a remarkable transformation of the landscape from natural to human made. We change almost everything: cut down trees, level the ground, and slice and dice the land into individual plots. But one of the most significant changes to the landscape that comes with urbanization is impervious cover. I’m talking about anything that prevents rain from soaking into the subsurface: buildings, sidewalks, driveways, and the biggest culprits - streets and parking lots. Impervious cover is a big issue. When it rains, that water has to go somewhere. If it can’t soak into the ground, it washes off into creeks and rivers. That means increasing the magnitude of floods and the amount of pollution in waterways. It also means less water goes to recharge groundwater resources. When you pave paradise to put up a parking lot, you cause a pretty significant disruption to some really important natural processes in a watershed. But, not all cover has to be impervious. Today, we’re talking about permeable pavement.

Management of stormwater in urban areas is a vast field of study. Pretty much since humanity started building stuff, we also started building ways to keep that stuff dry. Traditional engineering had a single goal in mind - get stormwater off of the streets and property and into a creek, ditch, or river as quickly as possible. It’s not hard to see the problem with this strategy. Every new road and building means a higher volume of runoff in the waterways during a storm event. As cities grew, flooding problems became more severe and more frequent, streams were eroded, and receiving waterways were polluted. So, over time, municipalities adopted rules to try and curb these problems, focusing primarily on flooding. Now, in nearly every large city (at least in the U.S.), land developers are required in some way or another to make sure their projects won’t worsen downstream flooding. The traditional solution to this is control of flood peaks through onsite detention: essentially having a small pond to store runoff during a storm, allowing gradual release to mitigate flooding.

Detention and retention ponds have a lot of complexity and deserve their own separate video. They definitely help reduce flooding, but they don’t really replace the other functions of the natural landscape: the filtration and reduction of runoff volume that comes from water infiltrating into the ground. Also, these basins are usually pretty ugly and kind of gross, since they concentrate polluted runoff in one mucky area, and beauty is already in short supply in many urban areas. For all these reasons, cities are encouraging (and sometimes requiring) developers to take even greater responsibility for impacts on the natural landscape through a process called Low Impact Design, or just LID. LID practices are ways to integrate stormwater management as a part of land development and mimic natural hydrologic processes. There is a considerable variety of LID strategies that help manage urban stormwater, reduce erosion, minimize pollution, and help with flooding. These are things like rain gardens, green roofs, and vegetated filter strips. If you live in a big city, there’s a good chance your municipality has a manual describing the strategies that work best for your area. One of my favorites of these addresses the problem at its root: just make the cover less impervious.

Pavement serves a vital role in a city. A quick glance at the condition of dirt roads after a good rain is all you need to understand this. Pavement equals accessibility. In most places, the soil making up the ground isn’t a stable, durable surface for people to walk, roll, scoot, or drive. Particularly when the earth gets wet, it loses strength and turns to muck. You can see why we normally prefer pavement to be impermeable to water. Pavements protect against erosion and weakening of the soil. A poorly designed pervious pavement works about as well as if it wasn’t paved at all, since it doesn’t provide any protection against water. If you watched my previous video on potholes, you know the cruel fate of pavement that inadvertently lets water through. So, how is it possible to achieve the good parts without the bad, to allow water to infiltrate into the subsurface through a pavement without softening and weakening its foundation?

Luckily we have a pretty good example to help understand how this works. Some might even call it the OG permeable pavement. I’m talking about steel grating. You’ve almost certainly seen grating used on roads, sidewalks, or other surfaces to allow water in while keeping most everything else out. We can do precisely the same thing with traditional pavement as well. Concrete is a mix of cement, rocks, sand, and water. If you leave out the sand, you get something really cool: a material that behaves almost exactly like regular concrete, but that is full of voids and holes that can let water pass through.

This is a really cool effect that is almost an optical illusion. Our brains are so used to seeing water runoff a paved surface, they almost can’t make sense when it flows straight through. This has led to quite a few viral clips of water disappearing into parking lots or roadways. And this isn’t just possible with concrete. Asphalt can be made similarly porous, along with different kinds of pavers. The permeability of the pavement isn’t the end of the story, though. Going back to our permeable pavement proxy, steel grates don’t just sit directly on the ground. Look through, and you’ll see, the water passing through has to have somewhere to go. Soil usually can’t absorb 100 percent of the water when it rains. If it could, we’d never have any runoff and hardly ever any floods. That means, even if we can get rain to percolate through pavement, it needs somewhere to go after that.

The pavement itself gets all the glory, but the real workhorse of a permeable pavement system is the reservoir below. This is generally made from a layer of stones of uniform size to create voids that temporarily store water coming through the surface pavement. The design of the stone reservoir is just as crucial as the pavement above because it depends on how much water must be stored and how quickly that water can infiltrate into the ground. Both of these require careful engineering. For certain types of impermeable soils, like clay, it may not be feasible to try and get all that water to infiltrate, so some permeable pavements work like detention ponds, where the water is stored temporarily and released gradually over time through drains. Whether it soaks into the ground or is discharged into a waterway little by little, the permeable pavement has made a considerable improvement over the alternative of having rainwater wash right off the surface.

This is a really helpful strategy to address stormwater in urban areas, but it’s not without challenges. Most importantly, permeable pavement isn’t that strong. If you make concrete or asphalt with a bunch of holes and voids, it makes sense that it probably can’t hold up the weight of traditional mixes. That’s why we really don’t use these systems in areas with heavy traffic. Permeable pavements are mainly relegated to parking lots and road shoulders. But we also need to keep them away from buildings where you don’t really want a lot of water soaking into the foundation soils. And we can’t use them on slopes either, because the stored water would just flow along the slope through the reservoir and eventually back out, rather than staying in storage. The pavement itself can be clogged by dirt and leaves over time, so it has to be swept or washed regularly to remain permeable. Finally, although they help snow and ice melt faster naturally, using porous pavements in colder climates requires special consideration to avoid damage from freezing water and deicing salts. Even given its simplicity and use over the past few decades, permeable pavement is still a fairly new and innovative way to manage urban stormwater. There’s still a lot to learn about how to implement it effectively and efficiently. It’s a great example of using engineering to try and bring more harmony between constructed and natural environments..