Author Topic: Boscastle Blowhole  (Read 712 times)

Offline NigelF

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Boscastle Blowhole
« on: December 15, 2019, 02:10:26 pm »
A question about Boscastle Blowhole (S entrance at SX 09370 91537 :   I have looked at some Youtube videos of the 'Blowhole'.  eg    
This appears to mostly  operate as the blow- back type in which an incoming swell seals the rather low entrance and compresses air inside the (usually dead end) tunnel. When the swell recedes a bit, the air comes back out taking spray with it more or less horizontally.
And then I looked at some dive videos which make it  clear that this is a through tunnel, more or less NS, right through the headland and, from gridreferencefinder.com , 63 m long.
Another blowhole video states that at low tide, waves from the N end produce spurts at the S end.
Both mechanisms may credibly apply at times, depending on tide and swell direction.

Has anyone any info on the ceiling profile along this tunnel ?
The diving videos, unsurprisingly, give no info on this.



Offline Pitlamp

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Re: Boscastle Blowhole
« Reply #1 on: December 15, 2019, 05:48:13 pm »
Interesting to watch!

I guess that, unless the cross sectional area is perfectly circular (unlikely) there will be domes in the roof and restrictions along the walls (however minor). The restrictions will cause water to back up briefly, causing a rise in pressure of any trapped air - and as soon as the entrance "water trap" is open, any pressurised air will force loads of spray out, even if it's a through tunnel.

There is a sort of analogy in Kingsdale in the Dales, which may partly help. The through dive from the KMC to Keld Head is wide open all the way. Yet in spate the water ponds up in the KMC against the sump, up to 6 m or 7 m deep. It does this because the passage shape changes frequently along the course of the long sump, with some areas acting as restrictions.

Is there a local BSAC branch, with members who might be able to give you details of the underwater topography? If you do get to the bottom of this one, it'd be great if you'd post the information on here.


Offline mrodoc

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Re: Boscastle Blowhole
« Reply #2 on: December 15, 2019, 10:48:42 pm »
The one under Asparagus Island at Kynance Cove (sees Farr's book) is a through cave with a blow hole feature.

Offline mikem

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Re: Boscastle Blowhole
« Reply #3 on: December 16, 2019, 11:13:15 am »
& the fact that swell can enter from both ends may increase the pressure on that airbell...

As the blowhole only appears at low water, the rest of the tunnel remains flooded.
« Last Edit: December 16, 2019, 11:40:41 am by mikem »

Offline rhychydwr1

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Re: Boscastle Blowhole
« Reply #4 on: December 16, 2019, 03:18:39 pm »
from Caves of Cornwall:

Boscastle Sea Cave
NGR SX 094 916  Explorer 111
“Boscastle, for example, has a notoriously dramatic blowhole associated with a 50m-long tunnel through a headland [called Penally Point], but this is rarely dived because of the fickle nature of the environment” from:
http://www.divernet.com/other_diving_topics/cave_diving/1906897/classic_darksite_diving.html
accessed 310713

Offline NigelF

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Re: Boscastle Blowhole
« Reply #5 on: December 17, 2019, 03:10:01 pm »
re #4
the current link to the ref to Boscastle blowhole is:
http://archive.divernet.com/cave-diving/p302441-classic-darksite-diving.html