WEBVTT - How Do Tidal Bores Work?

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<v Speaker 1>Welcome to brain Stuff production of I Heart Radio, Hey

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<v Speaker 1>brain Stuff, Lauren vocal bomb. Here. The Alaskan coast is

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<v Speaker 1>as dramatic as it is vast, and it spans over

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<v Speaker 1>thirty three thousand miles or over fifty four thoud kilometers.

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<v Speaker 1>U s State owes its very name to this sweeping ocean. Border.

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<v Speaker 1>Alaska is an English language corruption of an Alouette term that,

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<v Speaker 1>according to one translation, means the shore where the sea

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<v Speaker 1>breaks its back. And if you drive out to a

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<v Speaker 1>place called turning in Arm in the Gulf of Alaska

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<v Speaker 1>at just the right time, you get to watch some

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<v Speaker 1>aquatic action that's pretty spectacular. The turnig In Arm Waterway

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<v Speaker 1>is a northern branch of the cook Inlet that cuts

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<v Speaker 1>into the Greater Anchorage area. Here, the water normally flows

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<v Speaker 1>out toward the inlet, but shortly after low tide waves

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<v Speaker 1>move in the opposite direction and travel upstream, and these

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<v Speaker 1>waves can be epic in scale. The biggest waves are

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<v Speaker 1>up to ten ft or three meters tall. This is

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<v Speaker 1>a well known example Boar tides also known as tidal

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<v Speaker 1>bores or just boars, and riding them has become a

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<v Speaker 1>rite of passage for many surfers in the Pacific Northwest.

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<v Speaker 1>While this phenomenon has been seen at numerous locales around

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<v Speaker 1>the world, it requires a very specific set of conditions.

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<v Speaker 1>But before we dive into those, it might be a

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<v Speaker 1>good idea to go over some basic tidal science. By

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<v Speaker 1>and large, tides are gravity's handiwork. Due to the gravitational

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<v Speaker 1>influence of our moon plus the inertia of our movement

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<v Speaker 1>through space, the ocean is always bulging out a bit

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<v Speaker 1>on opposite sides of the planet. While Earth rotates, it

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<v Speaker 1>passes through these tidal bulges. That means your favorite seaside

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<v Speaker 1>beach will experience high tide when it enters each bulge,

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<v Speaker 1>and low tide as it travels between them, So plan

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<v Speaker 1>your sandcastle contests accordingly. Because Earth finishes a new rotation

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<v Speaker 1>around its axis once every twenty four hours, most coastal

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<v Speaker 1>areas witness two high tides and two low tides per day,

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<v Speaker 1>but there are loads of exceptions. The layout of continents, islands,

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<v Speaker 1>and peninsulas impedes tides in certain places. Parts of the

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<v Speaker 1>Gulf Coast, for example, only see one daily set of

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<v Speaker 1>high and low tides. No matter where you are, though,

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<v Speaker 1>the tides will vary in strength from day to day

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<v Speaker 1>all over the world. During full moons and new moons,

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<v Speaker 1>when the Earth, moon and Sun are arranged in a

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<v Speaker 1>straight line, high tides are especially high and low tides

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<v Speaker 1>are really low. We call these extreme tides spring tides, which,

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<v Speaker 1>despite the name, occur all year round, not just in

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<v Speaker 1>the springtime. But let's turn back to turn Agin Arm.

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<v Speaker 1>Although it's shallow and narrow, the Turnigin feeds into a

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<v Speaker 1>broad bay, namely cook Inlet. On this wide expanse, the

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<v Speaker 1>difference between high and low tides, i e. The tidal

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<v Speaker 1>range can be stark. The cook inlets water level at

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<v Speaker 1>high tide maybe thirty five ft or ten and a

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<v Speaker 1>half meters higher than it was during the preceding low tide,

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<v Speaker 1>and these are exactly the ingredients needed to produce tidal bores.

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<v Speaker 1>After a low tide on cook Inlet, water funnels into

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<v Speaker 1>the herning In Arm, generating boar waves that barrel inland,

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<v Speaker 1>sometimes traveling at speeds up to twenty four miles per hour.

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<v Speaker 1>That's nearly thirty nine kilometers per hour. Since bores only

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<v Speaker 1>happen when the circumstances are just right, not all waterways

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<v Speaker 1>that touch the Ocean can get them. In total, there

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<v Speaker 1>are eighty or so rivers around the globe that undergo

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<v Speaker 1>tidal bores. The phenomenon tends to be at its most

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<v Speaker 1>dramatic during spring tides, whereas the turning in Arm has

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<v Speaker 1>twice daily bore tides. The Amazon River only receives bores

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<v Speaker 1>on the days of new and full moons. Once the

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<v Speaker 1>spring tides arrive, water from the Atlantic comes charging up

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<v Speaker 1>the Amazon River, temporarily reversing its natural flow. Waves generated

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<v Speaker 1>by this process have been known to journey about five

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<v Speaker 1>hundred miles or eight hundred kilometers inland, called porta rocca.

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<v Speaker 1>The Amazon bars attain considerable size, measuring upwards of thirteen

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<v Speaker 1>feet or four meters tall in some cases. The strongest

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<v Speaker 1>portor roca events of all take place every year on

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<v Speaker 1>the equinoxes. To prepare for these surges, local residents move

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<v Speaker 1>their boats and livestock safely away from the Amazon. Accidents

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<v Speaker 1>do still happen, though Jacques Cousteau once lost a boat

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<v Speaker 1>to pororoca waves while filming in South America. Title boars

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<v Speaker 1>impact ecosystems wherever they occur. Trees, rocks, and river bottom

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<v Speaker 1>sediment get pushed around by the waves, and those aren't

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<v Speaker 1>the only things they churn up. Down in the Amazon,

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<v Speaker 1>current reversals can leave stunned or dead animals floating in

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<v Speaker 1>the water, attracting hungry piranhas. Strong boars are also prone

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<v Speaker 1>to eject fish from rivers, stranding them on beaches or

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<v Speaker 1>launching them into the air. That's why Alaskan bald eagles,

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<v Speaker 1>Australian sharks, and Malaysian crocodiles like to scavenge in the

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<v Speaker 1>wakes of title boars. Big boars attract humans as well.

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<v Speaker 1>Sal Domingo Stolkapim, a community that borders the Amazon, has

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<v Speaker 1>been hosting the Brazilian National Poroca Surfing Championship since n

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<v Speaker 1>Alaskan wave chasers flocked the turning in arm shortly before

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<v Speaker 1>or after every spring tide when the boars are at

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<v Speaker 1>their largest. Oh and there's one last thing we should

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<v Speaker 1>mention about boar tides. They roar. The waves produce a

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<v Speaker 1>great deal of turbulence, wildsplacing sediment, scraping up shoals, and

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<v Speaker 1>unleashing loads of air bubbles. Add all those sources up

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<v Speaker 1>and you've got a recipe for thunderous low frequency booms

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<v Speaker 1>that can be heard over vast distances. Today's episode was

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<v Speaker 1>written by Mark Bancini and produced by Tyler. Playing brain

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<v Speaker 1>Stuff is a production of I Heart Radio's How Stuff Works.

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<v Speaker 1>For more on this and lots of other swell topics

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<v Speaker 1>you know, visit our home planet, how stuff Works dot com.

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