What is driving this sudden dramatic wave and causing Britain’s longest river to flow uphill? In this film I explore tides, looking at how and where they form, presented from the banks of the tidal River Severn, where I capture the magnificent Severn Bore tidal wave travelling towards Gloucester.
#tides #tidalbore #SevernBore #tide #river #Severn #nature #documentary
CONTENTS:
Introduction
00:00 – Intro
00:34 – Watching the tidal bore
What are tides?
02:01 – What are tides?
03:20 – Night tidal bore
04:40 – What affects tides?
06:45 – Tidal surge overflowing the banks
How and where do tides form?
07:10 – How do tides work?
09:15 – Where do tides form?
10:20 – What causes tidal currents?
Outro
11:22 – Outro
12:01 – The Severn Bore passing Gloucester
Disclaimer:
As always, illustrations and animations featured in this film are not to scale and may not necessarily be scientifically accurate. Although lots of care has been taken to ensure the information in this film is correct, it shouldn’t be regarded as scientific fact, and some sections have been simplified for ease and entertainment purposes.
Tidal bore, tidal surge timelapse and drone footage:
– https://youtu.be/Mj8Dw0YiWCI?si=FE2UsBy6NFVW0lJR
My Website:
– https://oskarbrennan9.wixsite.com/filmandmedia
My Twitter:
– https://twitter.com/Oskar_Brennan
My Instagram:
– https://www.instagram.com/oskarbrennanphoto/
Purchase my images:
– https://fineartamerica.com/profiles/oskar-brennan
Copyright © Oskar Brennan, 2024
In just a few minutes, the silence along this river will be broken by a roaring wave. Driven by one of the largest tides of the year. In the hour that follows, the water level will rise many metres, Filling the river to the top of its banks
Before quietly turning and draining back out to sea. Today, I’m going to explore what creates tides, and how far this wave has actually travelled. Today, you join me alongside the River Severn, and that was the Severn Bore. Now, the river here is normally flowing in that direction, from the land to the sea.
But today you may be able to tell that it’s actually flowing in that direction. Uphill, from the sea into land. So what’s caused this wave and this reversal in the direction of flow in Britain’s longest river? Well the answer… is the tide.
In order to understand how tidal bores work, you first need to understand a bit about how the tides work. Right now I’m here on the Severn Estuary, just downstream from where we saw the bore. And this is actually the site of one of the largest tidal ranges in the world.
The tide can regularly rise by over 14 metres, And the land where I’m stood right now was actually under water just this morning. If you’ve ever been to the coast, you’ve probably experienced the tide. The regular rise and fall of the sea.
It normally takes around 6 hours and 12.5 minutes to go from low to high tide, And takes around 12 hours and 25 minutes to do a full tidal cycle, so from high to high.
And what that means is that in approximately 12 hours and 25 minutes, or 12 hrs and 25 mins since the wave came through, Another one should come through… And that is exactly what’s going to happen. I’ve just come up to the River Severn again, 12 hours and 25 minutes into the future.
And the wave has indeed turned up on time. I was going to try and do a few more pieces to camera, but as it was the highest tide of the year, despite the late hour, it was very busy and chaotic. There were hundreds of people crowding around on the bridge to watch,
Including the coastguard and a lifeboat making sure everyone stayed safe. And tonnes of people in the water as well on kayaks and surfboards in the darkness. We’ve just had an eel jump out the water. We think it’s an eel, we’re not sure, it was just a massive splash.
Hopefully that camera over there caught it on camera, we’re just filming the tidal surge coming in. Amazing, there’s like full trees in here! It’s just amazing how quick it’s coming in. There are lots of other things that can influence how the tides behave as well.
For example, some coastlines may have more than one high or low tide per cycle, some seas are too small or too disconnected to even have a tide, And also, the local weather conditions can have a big impact on how high, low, early or late the tides might come.
For example, low pressure tends to create higher tides, and high pressure tends to create lower tides. Here, if there is a south-westerly wind, then the tide will be higher, and if there’s a north-easterly wind, it’ll be lower.
And the bore is even more complex because it’s affected by things like the river channel as well as the weather conditions. So, for example if down in the estuary the river has carved out a very long windy channel,
Then the bore will take longer to get up here than if it was a very straight one. And if there’s too much water, or too much vegetation in the river, Then the bore can be slowed or even stopped completely. Tides come in different varieties depending on the moon phase.
Spring tides occur around the full and new moons when the gravitational attractions of the sun and moon align. And this means that in general the spring tides are a lot more pronounced, so the highs are higher, the lows are lower, and obviously there’s a much quicker rise and fall to go between them.
Neap tides are the opposite, and they occur when the sun and the moon are at right angles to each other, So their gravitational forces are working against each other. And that means that the tides are in general much less pronounced, so the lows are higher and the highs are lower.
The biggest spring tides tend to occur around the equinoxes, so in the spring and the autumn. And it’s these tides that the bore is most commonly associated with, So as a result, the bore is most commonly seen around the spring and the autumn as well. God, it’s getting really deep.
Look how close the water’s getting to the edge now. There was tree there a minute ago… There’s a bit of it, there it is! And of course sometimes, when the river’s high, The wave doesn’t properly come as a wave, the river just overflows in seconds, and spills out onto these fields
That can be quite dangerous if you’re not prepared. So how do tides actually work? Well the simplest explanation is this… The moon has a gravitational pull, on the surface of the Earth, And that pulls up on the oceans and creates a tidal bulge.
But that’s not quite the full story, because the moon doesn’t just pull on the oceans, it actually pulls on everything, including the Earth itself. The strength of this gravitational acceleration towards the moon is dependent on how close the moon is. So a particle on the moon-facing side of the Earth,
Will have a stronger attraction to the moon than the Earth itself. The net gravitational acceleration of a particle towards the moon is the particle’s gravitational acceleration minus the Earth’s gravitational acceleration. What that means for a particle on the moon-facing side of the Earth,
Is that the gravitational acceleration is up and away from the Earth’s surface, towards the moon. Where this force points away from the Earth’s surface, it’s not actually strong enough to create a tidal bulge, because it can’t overcome Earth’s gravity. But this force can come into effect on the sides of the planet,
Where Earth’s gravity is working perpendicular to it, so it’s not working against it. And so what happens is the ocean all around the sides of the planet is being pushed towards the moon, towards where that bulge forms.
And that is actually what forms the bulge, it’s more of a horizontal movement than a vertical one. What you may have heard is that there’s actually another tidal bulge on the opposite side of the Earth to the moon, And that is actually true.
Because the Earth is essentially more closely attracted to the moon than the objects on the far side of Earth’s surface. Once these tidal bulges have formed, the Earth then rotates, Moving through the tidal bulges and giving the impression of tides coming in and out.
But of course the tides here on Earth are even more complicated than that. Because the oceans are interrupted by large pieces of land And the only place where there is continuous ocean all the way around Earth’s axis, is just off the coast of Antarctica, so that is where the tides are generated.
The reason that there can be such a big tide here, very far from Antarctica, Is becuase that tidal bulge tries to flatten itself out, and in doing so creates a wave Which moves through the global oceans, wrapping around coastlines, moving into headlands and bays, and rising as it encounters shallower water.
And that’s why tides are different almost everywhere on the planet. The Severn Bore is a perfect visualisation of this tidal wave because, put simply, it is a tidal wave. The water you can just imagine moving through the Atlantic and moving up into the river here,
And only showing itself when the estuary was able to funnel it into a proper wave and the shallower water made it rise and start to break. It’s a really significant movement of water when the tide comes in
And pushes all that water up onto the land and up into the rivers and the estuaries. And that water has to move back out again, and when it does, it creates some strong tidal currents. So you often get in places like estuaries and places around headlands for example,
You can get some really strong tidal currents occurring in the water, and they can actually be really quite dangerous. There’s actually a perfect example of a tidal race going on just here,
So what we’ve got is a little basin there, and that water is now making its way back out to the main river channel, Because the river’s dropped a lot faster than the water could get out. And that’s a perfect example of one of these tidal currents and how that works.
And that’s happening across this entire estuary. And all the way up the river, all the way to where we saw the bore. All of that water has been pushed into the coast and it’s now got to get back out to sea,
And so it’s all being ripped down through the river here, through the estuary as a great big tidal current. So tides really are very complicated. And although it is caused by the influence of the moon, it’s actually more of a horizontal force than a vertical one which actually creates the tides.
And the bore that we saw here today, was actually created 12,000 kilometres away, in the southern hemisphere and has travelled up through the global oceans, into the Bristol Channel, into the Severn estuary, and 20 kilometres up the River Severn to break on this tree just here.
So next time you see a tidal wave, tidal race, or even just an incoming tide, think of the incredible journey it’s been on to get to where you are. It’s really quite a fascinating phenomenon if you think about it.
1 Comment
Fascinating!