Topic: Drafts in Caves (Read 4039 times)

menacer · « **on:** July 13, 2009, 10:40:28 am »

Ive been puzzling over Drafts in caves for a few weeks now and would really appreciate some thoughts/feedback/reference books on peoples individual experiences/knowledge.

I had been under some very mis guided notion that caves drew in air from the highest parts of the system or drew air in from active stream ways.
eg P40 to Guirs Mort or Cueto to coventosa
However 2 weeks in Montenegro has blown this completely away. (scuse pun)

PT4 blew from down hill to uphill even against the streamway!!!
Help!!
Could someone please explain.

A little bit of knowledge about the cave.
Its very likely to resurge in known dived passage in risan bay (under sea level)
Its entrance is within 10km of the coast.(havnt got exact distance)
The time of year end june beg july.
Outside conditions Storms/lightening / rain worst weather in Montenegro for 50 years !!!!
As the weather slightly improoved so did the draft.

Thanks In advance

Carm

graham · « **Reply #1 on:** July 13, 2009, 10:47:59 am »

Not a lot has been written on this subject, but there are a couple of articles in BSA Cave Science from about 1948 that might help you. Chap called A. Pill IIRC.

Check 'em out in the indexes on the BCRA website. If you can't find the actual papers I can probably scan 'em for you.

Alternatively buy a thicker set of thermal undies.

whitelackington · « **Reply #2 on:** July 13, 2009, 11:26:15 am »

Hello Menacer,
we had an unusual draft experience whilst doing our dig of Carcass Cave, Axbridge, Mendip.
That day there were three diggers, myself, Bill Chadwick and Martin Peck,
all digging veterans

We were digging in the largish choke (probably several hundred tons)
when a very forceful, coldish, outrush of air happened, it lasted for twenty minutes, then stopped.
It was never felt again.
Unfortunately when the younger digging party next turned up they rapidly ripped it apart,
we did find a sizable chamber underneath.
Our thinking was that we had opened up a sealed or virtually sealed chamber, once the air pressure was equalized, that was it.
Mick

langcliffe · « **Reply #3 on:** July 13, 2009, 11:38:26 am »

Quote from: menacer on July 13, 2009, 10:40:28 am

I had been under some very mis guided notion that caves drew in air from the highest parts of the system or drew air in from active stream ways.

As far as I know there are four primary causes for draughts in caves:

1. Cold air sinking / warm air rising
2. Streams drawing in the air
3. Re-establishment of the equilibrium of the atmospheric pressure inside and outside the cave
4. Radon fans (!).

The actual effect on a particular cave will very much depend on the number and location of the entrances, and whether there are alternative routes in the system.

cap 'n chris · « **Reply #4 on:** July 13, 2009, 12:15:45 pm »

Given the cave in question, only (3) would be relevant (and that is questionable since the cave was only blowing out, not drawing in, despite changeable weather); Menacer is a-wondrin' whether there may be some other influence though, like a tidal sump opening perhaps.

khakipuce · « **Reply #5 on:** July 13, 2009, 12:22:14 pm »

Couple of random thoughts: if it's a long system could atmospheric pressue have an effect (seems fairly unlikely) or could the wind blowing across an entrance create sufficient low pressure to pull air through the cave?

cap 'n chris · « **Reply #6 on:** July 13, 2009, 12:26:14 pm »

No wind on surface, apart from very cold air pouring out of cave.

Peter Burgess · « **Reply #7 on:** July 13, 2009, 12:35:30 pm »

Quote from: menacer on July 13, 2009, 10:40:28 am

A little bit of knowledge about the cave.
Its very likely to resurge in known dived passage in risan bay (under sea level)
Its entrance is within 10km of the coast.(havnt got exact distance)

Thanks In advance

Carm

Hi Carm

Even though the sea is the Med (isn't it?) what struck me was the connection with the sea, and I wondered if even a limited tidal effect might piston water in and out of the system, displacing and replacing air. However, you would have noticed a tidal cycle if that were the case.

menacer · « **Reply #8 on:** July 13, 2009, 03:07:56 pm »

The bay it possibley ressurges from (which has been dived to a depth of 90m,) has a tidal range of only 31cm, however if a large subterraen area/volume of water were dropping and rising it may account for such strong drafts.
This may be a possibility because we were only ever at the cave from about 10am to 10pm at night, over a 10 day period
I therefore couldnt catagorically state it did not draft the other way in between those times.
interesting

langcliffe · « **Reply #9 on:** July 13, 2009, 04:06:05 pm »

Quote from: menacer on July 13, 2009, 03:07:56 pm

The bay it possibley ressurges from (which has been dived to a depth of 90m,) has a tidal range of only 31cm, however if a large subterraen area/volume of water were dropping and rising it may account for such strong drafts.

I can't see that the significant factor is anything more than the surface area of the sump within the cave. So the volume of air being displaced / pulled in couldn't really account for a strong draught, considering that the 31 cm rise takes place over a period of a little more than six hours.

menacer · « **Reply #10 on:** July 13, 2009, 04:20:26 pm »

ok so that theory's out.
The karst landscape is very pitted with deep "shakeholes" and pits and no noticable evidence of surface streamways.
Surface prospecting was a nightmare as was locating the entrance with 3 gps.
It still took 1 hr to pin it down.
When it rains torrentially there is limited surface pooling, all the water seems to drain off straight away somewhere.
Only at a cave depth of about 100m did we start to see active inlets in to the cave, below this level they became more numerous the deeper you went.
There was some Otter sump style gurgles in wet weather with the first inlet.
The draft continues right up and out the cave entrance, it doesnt appear to be diverted anywhere else of significance and certainly doent disappear up the numerous encountered inlets.
Obviously the point it was most noticable was at the one single constricted below pitch one and at the top of pitch 2.
The wind chill and draft was very strong. (some footage was taken of paper fluttering in the draft)

The height gain from sea level to cave entrance was about 745m over a relatively short distance.

Les W · « **Reply #11 on:** July 13, 2009, 04:23:18 pm »

Is there the potential for a higher entrance, perhaps much higher and colder?

menacer · « **Reply #12 on:** July 13, 2009, 04:33:12 pm »

Not much higher and colder maybe 200m tops.
What are you thinking?? baro pressure??

Les W · « **Reply #13 on:** July 13, 2009, 04:46:43 pm »

No, just air at a higher entrance might be colder and more dense, thereby driving the draft.

Aubrey · « **Reply #14 on:** July 13, 2009, 04:49:02 pm »

The draught in Bath Swallet has always had us puzzeled.

I always understood that draughts were normally caused by the chimney effect of cold air in at the bottom level and warm air exiting from the higher level or entrance.

In the winter there is a very strong cold draught going down the Bath entrance and warm moist air coming out of Rods Pot.

Why should this be if there is no stream flowing in either cave?
The two cave entrances are at the same level (give or take the size of the entrances) but the draught is sometimes really strong and nearly always in the same direction.
For much of the winter there are dry walls in Bath caused by the dry air coming into the cave. We did find dry patches on the wall near the top of the Bear Pit in Rods where the draught was blowing into Rods and we know there are other voice or air links between the two caves.
The draught was there before the through trip was opened up.

mikem · « **Reply #15 on:** July 13, 2009, 08:22:42 pm »

Maybe air is entering through a lower route and the sea is warmer than the rest of the cave.

Mike

mikem · « **Reply #16 on:** July 13, 2009, 09:14:55 pm »

In a similar way maybe warmer air is rising through Rod's from unknown passages below, whilst Bath is only connected by the U tube of Purple Pot which will hold cold air (thus preventing warm air escaping through Bath), then this cold air will be warmed by the air in Rod's, dragging more cold air into Bath.

Mike

Aubrey · « **Reply #17 on:** July 13, 2009, 09:35:07 pm »

Quote from: mikem on July 13, 2009, 09:14:55 pm

In a similar way maybe warmer air is rising through Rod's from unknown passages below, whilst Bath is only connected by the U tube of Purple Pot which will hold cold air (thus preventing warm air escaping through Bath), then this cold air will be warmed by the air in Rod's, dragging more cold air into Bath.

Mike

1. How could the lower air be warmer?
2. As I said - there are several known air connections between the 2 caves, not just Purple Pot. The draught existed before Purple Pot was dug open.

Jackalpup · « **Reply #18 on:** July 14, 2009, 01:10:25 pm »

I am fairly sure there is only one "main" cause of draughts although there are 3rd party factors that affect "normality".

Draughts work on the principal of convection (ie. hot air rises and cold air sinks because it is denser) already mentioned above. The temperature of the cave is (all things being equal) generally constant but the external temperature can vary and fluctuate wildly.

For a draught to exist there must be at least two entrances/exits (or more). All things being equal, if the temperature outside is colder than inside the cave the draught will go from the upper entrance to the lower exit. If the temperature outside is warmer/hotter than inside then the draught will follow the opposite direction.

So far as I am aware, atmospheric pressure will have no effect. However, the flow of water can draw a draught with it and disturb a basic convection.

If you have a system with mulitple entrances/exits where some of those exits have a flow of water and some exits can become sumped, especially if you have temperature changes (summer Vs. winter) I would expect trying to understand the draught mechanics would be harder then trying to understand quantum physics $:-\$

I hope that helps a bit

Ian

Bob Smith · « **Reply #19 on:** July 14, 2009, 01:38:47 pm »

Quote from: Jackalpup on July 14, 2009, 01:10:25 pm

So far as I am aware, atmospheric pressure will have no effect. However, the flow of water can draw a draught with it and disturb a basic convection.

lechuguilla is a good example of a draft caused by differential pressure I would think; one entrance (AFAIK) and a howling gale at certain times.

I know it is a huge system, but saying atmospheric pressure would have no effect is a bit of a sweeping statement.

Jackalpup · « **Reply #20 on:** July 14, 2009, 02:01:45 pm »

Bob, you are right ... saying atmospheric pressure has no effect is too sweeping a statement. In fact, it does have an effect in that as the atmospheric pressure outside changes (the onset of a storm for instance), the pressure inside the cave has to be brought into equilibrium and that will, of course, create a draught but only until balance is restored.

However, the draft will (usually) be very slow (because the atmospheric pressure change is slow) in this circumstance alone and it would probably only be detectable in a narrow part of the system where the airflow was severly constricted.

It would be better to say that atmospheric pressure can have an effect (albeit nominal unless the external pressure changes suddenly and to an extreme).

It is also worth noting that a "chimney" effect (well, a venturi effect) can also create a draft (I know this was mentioned earlier in the thread) but, again, you would have to have a significant wind at one entrance without the same wind at the other (thus, the venturi is created).

.... I am off to find some aspirin now .....

Ian

Jackalpup · « **Reply #21 on:** July 14, 2009, 02:09:19 pm »

Having re-read Menacers original post again, I have paid more attention to the detail ~ especially the fact that it was June/July and there were storms. The draught in those circumstances was almost certainly caused by sudden and significant atmospheric pressure change as storms (usually) bring about sudden pressure changes and that would result in the anomalous draught effect she experienced.

Ian

Jackalpup · « **Reply #22 on:** July 14, 2009, 02:16:48 pm »

Actually, it's also worth mentioning a little more about waterflow in a cave too...... (brainstorming) .....

The water passing through a cave will either heat up or cool down the air in the cave (assuming the temperature of the water is different to that of the air in the cave ~ consider summer and winter for example). The heating or cooling of the air in the cave Vs. the temperature of the external air will also cause a convection

Ian

langcliffe · « **Reply #23 on:** July 14, 2009, 02:39:12 pm »

Quote from: Jackalpup on July 14, 2009, 01:10:25 pm

So far as I am aware, atmospheric pressure will have no effect.

You may be right, but this is my thinking:

Assume a linear cave system of 1,200 m with a passage size of 1 m across with one entrance. This gives the cave a total capacity of 1,200 cu. m.

Now assume a drop in outside atmospheric pressure of 5% in six hours. This will mean to equalise, 5% of the air will need to escape from the cave.

Thus 60 cu. m. of air will pass through the entrance in 6 hours, giving a flow rate of 10 cu. m. per hour. This will give a flow of 10 metres per hour at the entrance.

If my thinking is correct, I wouldn't call that insignificant.

However, my mathematical modelling is notoriously bad, so I am quite prepared to be howled down.

khakipuce · « **Reply #24 on:** July 14, 2009, 02:52:33 pm »

Quote from: Jackalpup on July 14, 2009, 01:10:25 pm

For a draught to exist there must be at least two entrances/exits (or more). All things being equal, if the temperature outside is colder than inside the cave the draught will go from the upper entrance to the lower exit. If the temperature outside is warmer/hotter than inside then the draught will follow the opposite direction.

But if the air outside is colder than the air in the cave, will the air in the cave not try to rise as cold air from the outside falls in? Consider a sloping entrance passage, would it not be the case that cold air would run down along the floor as warmer air from within the cave rises along the roof?

Author Topic: Drafts in Caves (Read 4039 times)