How do Tidal Bores Work?

[Music] most people are used to seeing waves break as they approach the coast but at a few locations around the planet there are special waves that break for miles and miles Inland they're called tidal bores and in this video we're going to explore the science behind why they happen and at the end we're going to hit the road and find one and spoiler alert we might try to Surf it what the hell am I doing foreign If you like these videos and want to support us to make more check out our Advocate apparel program at waterlust.com we make environmentally responsible UV protective swimwear that supports marine species and ecosystems that need our help every product ships with fun and fascinating educational materials that we create with leading Marine Science Educators that way when someone asks you about your whale shark leggings or sea turtle board shorts you'll be prepared and empowered to share what you've learned with others and drive positive change thank you so much for the support the ocean is filled with waves little capillary waves that Sailors look for when searching for puffs of wind steep choppy waves that can turn a boating day into a wet and wild ride and swells that travel long distances and sometimes break beautifully on reefs and coastlines the Delight of waiting Surfers while all of these waves have distinct characteristics they also have one thing in common they are disturbances in the surface of the water that are created by wind but there's another type of ocean wave that affects us every day that has nothing to do with wind it's called a tidal wave to understand how tidal waves work you first need to understand how the tides work we made a video that takes a deep dive into the physics of the tides and if you haven't seen it yet pause now and watch it before moving on we'll be referencing some of its Concepts in this video and seeing it first will make things easier to follow we'll put a link to it in the description below title Theory tells us that the gravitational fields created by nearby massive objects like the moon and the sun will produce a bulge of water on the sides of Earth closest and farthest away from them and low points of water in between as the Earth rotates these watery high and low points stay in the same place relative to the Sun and Moon creating daily monthly and yearly fluctuations in the ocean water levels that we call the tide the movement of the watery high and low points around the planet is called a tidal wave and it's similar to the ocean waves we've already mentioned except it has a much longer wavelength quick note people sometimes use the term tidal wave incorrectly to describe a tsunami a huge Rogue Wave created by an earthquake but they are not the same thing as the name suggests a tidal wave is a wave that is created by the tides because the Earth's tidal wave has such a long wavelength and small amplitude in most places you can't visually see it in the same way you can see an approaching ocean swell but there are exceptions in some parts of the world the tidal wave approaches as a visibly breaking wave called a tidal ore but why does this happen in some places and not others to explain this scientists model the coastline as a simple inclined plane as the tidal wave passes sea level rises and falls and the position of the water's edge moves in and out most of the time the speed at which this happens is very slow because most coastlines are relatively Steep and the amplitude of the tidal wave is low but if we were to increase the amplitude of the tidal wave while keeping the coast slope constant the incoming water would travel faster and farther up the coast or if the amplitude of the tidal wave were kept constant and the slope of the coast were reduced the incoming water would also travel faster and farther up the coast the key here is to identify two variables that can lead to incoming Tides traveling farther distance Inland in a short amount of time high tidal amplitudes and low Coastal slopes in our video about the tides we discussed why some parts of the world have higher tidal amplitudes than others places like the Bay of Fundy so we won't repeat those details here just keep in mind that some parts of the world have have larger tidal amplitudes than others and a high tidal amplitude is one of the two ingredients needed to create a tidal bore the second ingredient is a gently sloping Coastline but where on earth does that happen the answer is rivers that flow into the sea some Rivers have very steep slopes and water flows down them vigorously driven by Earth's gravity While others are gently sloped and water flows down them slower as luck would have it there are approximately 50 locations on Earth where large tidal ranges and gently sloping Rivers result in the creation of a tidal bore to visualize why this happens we can use the metaphor of an airplane creating a sonic boom if a plane is traveling slower than the speed of sound then the sound waves it creates will run ahead of it and no boom is created conversely if the plane is traveling faster than the speed of sound then the sound waves it creates will get left behind but a plane is traveling at exactly the speed of sound around 343 meters per second then the sound waves it creates travel along with it and build up over time into a large pressure wave we call a sonic boom the key here is to understand that there are two different variables playing a role in the creation of a big pressure wave the speed that the airplane flies through the air which is a function of how big its engines are and the speed that the sound waves the airplane creates move through the air which is a property of the air itself a tidal bore happens in a very similar way here instead of the speed of an airplane moving through air we consider the speed of incoming ocean water moving through river water which again is controlled by the tidal amplitude and the slope of the coast and instead of the speed of sound waves propagating through air we consider the speed of shallower water waves propagating through water which is a property of the water itself and just as a plane can move slower faster or at the same speed as the sound wave it creates the water from an incoming tide can travel slower faster or at the same speed as the water waves it creates if the speed of incoming water driven by The Tide is less than the speed of shallower water waves than any waves that get created will propagate ahead of the water and dissipate if the speed of the incoming water is faster than the speed of waves then any waves that get created get left behind but if the speed of the incoming water is equal to the propagation speed of waves then any waves that get created stay with the incoming front and a large wave can form whether or not a tidal bore is created and maintained is a delicate balance and waves can form or disappear along the same stretch of river depending on how its slope changes in a detailed study in 1928 of the tidal bore that forms in the Trent river in the United Kingdom scientists showed how no wave is created early on in the river where the slope is steepest marked here between positions A and B but that it does begin to form at position D when the river slope flattens out the wave reaches its largest size by position C and then gets smaller and smaller as the slope of the river reduces farther Upstream reading about these marvels of hydrodynamics is one thing but for this video we wanted to see one in person the closest title board to us happens in the Bay of Fundy home to the largest tidal range in the world if you want to understand why the tides are so big here check out our video about the tides we mentioned before we explain it in there the bore we set out to find travels up the petty Kodiak River from the Northern end of the Bay of Fundy to the city of Moncton the tides here are dominated by the principal semi-diurnal title constituent meaning that the boar happens every 12 hours or so so you can only see it once per day we arrived in the early morning just before the boar reached Moncton and like clockwork we could see it and hear it approaching watching the boar cruise by you can see the interplay between the phase speed of the waves and the speed of the front in some sections the waves begin to stack up and align with the front but just as quickly they become misaligned and dissipate as the front races ahead it's incredibly Dynamic and the presence and shape of the Waves changes constantly as the riverbank twists and turns in this section you can see the river getting more constricted the waves slowed down and get closer together but eventually lose touch with the leading front and dissipate entirely as the front surges ahead [Music] as we watch the footage after the boar passed I couldn't help but start mine surfing the Wave It's just something Surfers do even if it's only a six inch peeler you can't help but Wonder Could I ride that and as luck would have it I did have a board with me so why not stay one more day and try to surf the boar so that's what we did tidal wave associated with the M2 title constituent has a period of a little under 12 and a half hours so when we woke the next morning you'll notice the sun is a little bit higher in the sky than it was the day before I put on my thickest wetsuit grabbed some cameras and headed out to the section of the river where we had seen a wave form the day before at this stage I should mention that surfing a title bore is very dangerous especially in cold temperatures I don't recommend anybody try this at home and it is something that I will probably never do again and I'll explain why later thank you that was that was less than awesome I'm a chocolatey mess but I was warned about that what the hell am I doing it is eight o'clock in the morning and it's freezing cold it is I think the low freezing I can't feel my hands um and we've got about nine minutes until the title bar arrives these are the moments you start asking yourself the hell are you doing here uh the Water here is really really Brown so you can't see anything let's see it's uh it's pretty nerve-wracking doing this it's kind of weird knowing exactly when the wave is going to arrive that's a very odd feeling compared to like surfing when you just go and it shows up whenever and the other thing is that we only have one shot at this a day it's uh because it's late in the year and there's so little sunlight we can't get two low tides in daylight so if I fall or if I mess up then that means we gotta wait 24 hours or a little more than 24 hours until the next attempt so I'm a little bit nervous I don't want to fall um but I'm trying not to try not to think too much about it just do it just send it I see it I can start to see some of it coming around the bend it's pretty intimidating oh yeah it's on it's happening [Music] foreign [Music] [Applause] [Music] [Music] I was able to ride the boar for a minute and a half before the waves dissipated and I was unable to keep up with the front and it was awesome but I also want to be real with you it was also pretty scary when the ride ends is the part of tidal bore surfing that I think is particularly dangerous because after the front passes you suddenly find yourself in a river with very strong current flowing Upstream in the opposite direction if you've ever run Rapids on a river you know to always check out what is coming Downstream before you put in with the title board the float switches directions and it's incredibly disorienting that in climbing up Muddy Banks with quickly Rising Waters is not easy despite being covered in so much mud that I needed a pressure washer to clean my gear seeing and surfing a tidal born person was an amazing experience it made me appreciate the delicate balance between a tidal wave's amplitude and Coastal slope that is needed to create these unique waves how powerful it is to observe science and action in the real world

Channel: Waterlust

Views: 121,487

Keywords: Tidal Bore, tidal bore surf, tidal bore surfing, tidal wave, physics tide, physics tidal bore, moncton bore, nova scotia tidal bore, bay of fundy tidal bore, how tidal bore works?, waterlust, science tides

Id: F-EoL4Jf7ug

Length: 13min 24sec (804 seconds)

Published: Mon Feb 06 2023