Tidal bore in Arnside

Does that warning sign above (that I showed as a teaser in yesterday’s post on wave watching in and around Arnside) make you as curious as it made me?

Usually, water rises for approximately 6 hours until high tide is reached, and then falls again for another approximately 6 hours until low tide is reached. A tidal bore, however, is an incoming tide that behaves differently from what we typically see. In a tidal bore, rather than slowly rising, the leading edge of the tide comes as one wave shortly before high tide, so the tide raises extremely quickly. (SO OF COURSE I HAD TO SEE IT!!!!!!!!)

Because this can get very dangerous if you don’t expect this to happen, there are warning signs everywhere around Arnside, and a WWII air raid siren is sounded twice before the bore arrives (that’s what you will hear when you play the video below!)

There is a lot of conflicting information out there on when to expect the bore. Why is predicting of the arrival time of the bore so difficult? While tidal forcing by the moon, the sun, and tons of other things is known very accurately, a tidal bore more than a “normal” tide is influenced by other factors, especially the wind, and the shape of the estuary and thus changes in friction (which changes constantly, see yesterday’s post).

And not only don’t we know exactly when the bore will arrive relative to predicted high water times, there is a lot of conflicting information out there on when the siren in Arnside is sounded relative to the bore. To be safe, I had to take the most conservative approach to make sure we didn’t miss it. Luckily, Astrid and Felipe were willing to wait with me for a looong time to see the Arnside tidal bore, even though none of them was really keen on it! Always important to have patient friends ;-)

Anyway, once the tidal bore arrives, it is super spectacular (I think!). We weren’t there on one of the recommended days — high tide was only 8.4 meters and locals tell you to not bother for anything below 10 meters — but I still thought it was so impressive! The video below shows it at 8 times its actual speed to give you a quick first impression:


And here are the most interesting 3 minutes of the bore when it passes right in front of us. So cool how there is actually a wave trough right before the leading edge of the bore!

And once the front has passed, this is far from over! Once the front has passed, the calm waters of the estuary are replaced by a strong current running upstream relative to the river’s original direction of flow, now quickly raising water levels all throughout the estuary. And not only that: Also very interesting flow pattern, some of which are shown in the video below.

After the front had passed and we were walking back into town, we met a couple of very excited people who had absolutely no idea what just had happened and who were all “I HAVE NEVER SEEN ANYTHING LIKE THIS! WHAT WAS THIS???”. Remember, this is what the estuary looks like before the bore arrives: lots of mud, very little water. Hardly any waves. So having the bore come in — when you’ve been waiting for it for 90 minutes, but probably even more so when you aren’t expecting anything at all — is really really impressive. And it makes you realize that they aren’t kidding when they tell you about extreme danger due to fast rising tides… Imagine water filling up all the channels at that speed — you’d be cut off from shore extremely quickly, and then separated from the shore by really strong currents. So it’s an amazing spectacle to watch, but one that one should definitely not underestimate!

3 thoughts on “Tidal bore in Arnside

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