What's Under Your Feet in New York City?

New York City is unlike any other city in the  United States. It’s the most densely populated area in the country by far, absolutely  packed with buildings, business, homes, and people. But for all that you can see  walking the streets or flying overhead,

there’s a whole other world of infrastructure  that makes the city possible below the surface. What if you could peel back the paving and  soil to see what one of my favorite authors, David Macaulay, called the city’s  “massive root system.” Let’s take

a tour underneath the Big Apple. I’m  Grady and this is Practical Engineering.

Let’s pick a generic Manhattan intersection  to start this journey. You’ve got cars, buses, bikes, buildings, sidewalks, traffic lights, fire hydrants, hotdog stands, manholes, and more. But open it up and there’s the whole other world below  the surface. We’ll start with the water lines.

If you think about it, water pipes could  run overhead like electrical wires. Digging trenches is a lot of work, after all.  And fixing underground pipes is pretty disruptive to streets. Of course, New York  isn’t unique in having them run underground.

It’s standard practice across most of the world  for a lot of simple reasons: water is heavy, so continuous support along the length of a pipe  is a structural convenience. Water also freezes, so putting pipes underground below the frost line  prevents them from freezing in the cold winters.

It also protects them from a whole host of natural  and human-caused hazards like car crashes and rogue parade balloons. That’s important because  a broken water main can be a big problem.

When you see a huge rooster-tail of water spraying  from the street, it’s easy to wonder why we need water mains to run at such high pressures. Of  course, pressure helps move water through pipes to all the individual places where it’s needed  across the city. But maybe more importantly,

that pressure pushing out keeps contamination  from getting in. You want any crack, hole, or break in a water main to be a one-way  street. If anything’s moving from one side of the pipe wall to the other, it’s pretty  important that it happens from in to out.

Like electricity, water lines typically run in  somewhat of a grid pattern. This adds redundancy, providing multiple paths for  water to reach a destination so that taking a line out of service  doesn’t disrupt the flow to residents.

It also makes sure that all the water in  the pipes is constantly flowing. If you build a water distribution system like the  branches of a tree, you end up with a lot of dead-ends where water can slow down or  even stagnate, making it unsafe to drink.

New York City’s water system is famously  gravity fed, with most of the source water coming from upstate at a higher elevation.  It also requires no filtration because the source watersheds are fiercely  protected to keep contamination out.