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We often think of the ocean’s tide as a simple rise and fall, connected to the motion of the Moon. But on any given shore, the reality is much more complex:.

The changing positions of the Sun and the Moon combine with the shape of the seafloor and coastline to create an intricate dance of rising and falling seas. And on top of that, oceanic scientists have realized recently that there’s another, more surprising factor in play: our ships. In particular, the channels we dig to get our ships to port are having a huge influence on the tides in port cities, and that effect is only getting bigger.

Thanks to the huge growth in the demand for stuff, the size of container ships has grown radically since they were first introduced in the 1950s. In just the last decade, the world’s largest ships have doubled in size, so to get them into ports, engineers have had to keep dredging deeper and wider channels. And that comes with consequences.

For example, let’s look at the port city of Wilmington, North Carolina, where the tides have been especially well studied. Since the late 1800s, engineers have more than doubled the depth of its natural river channel, from around 7 meters to more than 15. Meanwhile, the width has gone from 80 meters to 150.

And in that time, the difference in height between high and low tide has increased by 57 centimeters. As a result, Wilmington has seen a huge increase in flooding. One study found that in 2019, the city had 123 more days with floods than it did a century earlier.

Now, these are mostly nuisance floods, which don’t usually threaten life or limb, but they can still leak into basements, block roads, and mess with sewage systems. And a 2016 study provided clear evidence that this increased flooding is directly connected to the enlarged river channel. That paper modeled the river using computer simulations and proposed that, as water flowed in during high tide, the larger channel exerted less hydraulic drag.

That’s basically a fancy term for the friction water experiences as it flows against the bottom and sides of the river. In a larger channel, a smaller fraction of the water is in contact with the sides, so it experiences less friction. In particular, doubling the channel’s depth effectively cuts the friction in half.

That lets the tide travel farther upstream and raises the river’s level. But friction doesn’t just hold water back, it also suppresses a process called resonance. Resonance happens when the wavelength of the incoming waves matches up with the size of the channel.

As the waves bounce off the channel’s sides, every bounce gives them a boost, so you end up with larger waves and a higher tide. Friction helps break up this pattern, so reducing hydraulic drag is a double-whammy for raising tides. Today, this type of thing is happening in port cities around the world.

And the National Oceanic and Atmospheric Administration estimates that events like this could double or triple in the next decade alone. In fact, they suggest that flooding may just become synonymous with high tide in many places by the middle of the century. And that’s all before scientists even take into account the effects of climate change-driven sea level rise.

And as the oceans warm and the polar ice caps melt, seas are likely to rise an additional third of a meter or more by the end of the century. Nuisance flooding is nothing to shrug off. One 2017 study calculated that, in the near future, the property damage caused by nuisance flooding could be costlier than the damage done by hurricanes and other natural disasters.

So, taking action will be important. And that could come in many forms, from reducing the sizes of shipping channels to adding seawalls and pump systems to port cities. But no matter what path we take, it’s a reminder of how connected we are with our global environment.

Thanks for watching this episode of SciShow, and thanks as always to our patrons for helping to make it happen. If you’d like to help us make videos to make the whole internet a little smarter, you can check out patreon.com/scishow. [♪ OUTRO].