Author Topic: Passage size - Phreatic vs Vadose  (Read 10798 times)

Offline Rob

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Passage size - Phreatic vs Vadose
« on: January 22, 2012, 10:04:19 pm »
In all examples i can muster, when a vadose passage becomes phreatic it gets smaller.

Can anyone prove this theory wrong with an example, ideally in Derbyshire?

[Obviously assume no other water input, ever, and that those bits have been either vadose or phreatic for a good portion of their existance (yes, i know, very hard!)....]
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Offline Fulk

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Re: Passage size - Phreatic vs Vadose
« Reply #1 on: January 22, 2012, 10:13:36 pm »
Sorry, Rob, but I don't understand the nature of your query. Mostly, passages evolve from phreatic to vadose –so a vadose passage becoming phreatic is a relatively rare event, and could only happen – presumably – if the outlet for the water gets blocked and it backs up to inundate the vadose passage.

In addition, I don't see how a passage can become smaller; continuing abrasion/dissolution could only make it bigger (?). :confused: :confused:

Offline Rob

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Re: Passage size - Phreatic vs Vadose
« Reply #2 on: January 22, 2012, 10:26:41 pm »
Sorry, seemingly poorly worded.....

When a passage leads from an section of vadose formation into a section of phreatic formation. Basically, when a stream passage reaches a sump!
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Offline Pitlamp

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Re: Passage size - Phreatic vs Vadose
« Reply #3 on: January 23, 2012, 09:04:32 am »
Disagree - assuming all other variables are the same (which is unlikely anyway). The still phreatic section may be apparently smaller if lower flow velocity in the phreatic zone allows sediments transported from the vadose zone to be deposited and partially block the passage - but that's a different situation from what I think you're interested in.

Examples:

Peak Cavern: Buxton Water Sump is similar to the final section of the streamway preceding it.

Giants Hole: the final above water passage at the bottom is relatively small compared with the very large submerged section below water surface in East Canal.

Can you amplify on the reasons for asking about this?

Offline graham

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Re: Passage size - Phreatic vs Vadose
« Reply #4 on: January 23, 2012, 09:43:26 am »
There are really too many variables involved to state this as a rule. In some cases there will be an absolute change in size, for example where a roof tube which has been modified by vadose trenching will turn to an unmodified tube, but as Pitlamp sagely notes it frequently turns out that an apparent change in passage size is actually caused because much of the tubs is unseen, being masked by sediment.
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Offline SamT

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Re: Passage size - Phreatic vs Vadose
« Reply #5 on: January 23, 2012, 09:50:12 am »
I'm guessing rob is leaving sediment out of the equation.

Does it not stand to reason that a phreatic tube starts at size area = x - then has vadose action y i.e. new cross sectional area equals original phreatic size x plus vadose action y. 

Therefore

Anytime you transfer from a vadose section of cave to a phreatic section (of the same passage) it gets smaller.

I'm not sure I agree - far too many variables at play - water velocity etc - but I think this is the crux of what Rob is suggesting.

Offline Rob

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Re: Passage size - Phreatic vs Vadose
« Reply #6 on: January 23, 2012, 09:57:59 am »
Yes, i understand there are obviously a lot of variables, however definate examples at least proves that it can happen. And examples where sediment hide the definate truth are pointless.

Peak Cavern: Buxton Water Sump is similar to the final section of the streamway preceding it.
Hasn't the passage upstream mostly been formed under phreatic conditions? Either way, if BWS is the same size as the upstream passage i'm very impressed!

Giants Hole: the final above water passage at the bottom is relatively small compared with the very large submerged section below water surface in East Canal.
Yes, i was thinking about this as a possible counterexample. However the Filthy Five come in there, and also i'm unaware of the size of the actual passage downstream, rather than just the canal area itself. Is this as big as the vadose passage leading to it?!?

The reason for the enquiry is that i have a small vadose passage leading into a sump. I now know where this purched sump emerges. The passage downstream is phreatic and it seems much bigger. I hope something else joins in the sump...
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Offline gus horsley

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Re: Passage size - Phreatic vs Vadose
« Reply #7 on: January 23, 2012, 12:50:01 pm »
I agree with the posts above - there are too many variables.  However you do tend to get a change of passage shape from vadose to phreatic but I suspect everyone knows that already.

Offline Rob

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Re: Passage size - Phreatic vs Vadose
« Reply #8 on: January 23, 2012, 01:14:50 pm »
Too many variables to show an example?

I can think of tens of examples where the phreatic section of a passage is smaller than the vadose parts. None where it's the other way around....
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Offline langcliffe

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Re: Passage size - Phreatic vs Vadose
« Reply #9 on: January 23, 2012, 01:28:28 pm »
It's almost inevitable as a phreatic passage only needs to be as big as is needed to accommodate the water flow, whereas a vadose passage will continue to weather downwards towards a base level, hence soon exceeding the requirements for accommodating the water flow.
« Last Edit: January 23, 2012, 01:44:14 pm by langcliffe »

Offline Rob

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Re: Passage size - Phreatic vs Vadose
« Reply #10 on: January 23, 2012, 02:00:35 pm »
Yes, that makes sense.

However a counter arguement = phreatic should be bigger because more rock surface is in contact with the water at any one point, so it would dissolve/erode quicker...?!?
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Offline SamT

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Re: Passage size - Phreatic vs Vadose
« Reply #11 on: January 23, 2012, 02:05:54 pm »

However - the roof / wall / structure of a phreatic passage can be assumed to be supported by the pressure of the water.  Once vadose - the walls ceiling collapse - and the smaller blocks bits get erroded/washed away more quickly as they have a greater surface area.  (either that - or they cause sediments to become trapped between/behind them, thus your passage soon starts to fill up  :wall: )

Offline ian.p

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Re: Passage size - Phreatic vs Vadose
« Reply #12 on: January 23, 2012, 02:41:30 pm »
i think your looking at it the wrong way round Rob. The questions of why is this passage phreatic rather than vadose and why is it smaller than the previos passage? is probably rather more important.
then you can start asking questions like:
have you moved from a mature abandoned relic level into an immature phreatic undercatchment in which case there is a reasonable chance that the passsage will be smaller as it is younger and less well developed (not always the case as there may have been a greater flow for a longer period of time in the undercatchment than the relic level)
or is it part of the same level in which case why has this passage not enlarged and become vadose like the rest perhaps the rock is of a greater competency maybe you have moved from a fault controlled section of passage where heavily brecciated rock has been easily eroded and removed to a joint controlled section of passage where the rock has very few fractures to be exploited?

The mere fact that you go from one to the other has little to do with it. Think about the change in passage size going from the rift passage before sump 1 in swildons which is a classic vadose streamway to the passage between sumps 1 and 2 which is generaly bigger but more phreatic in character. Or another example the change from the upper series in wigmore swallet to the phreatic main drain im sure there are hundreds of other examples of small vadose passages leading to larger phreatic passages.

In short the fact that there are lots of examples of a large vadose passages leading to smaller phreatic passages is going to be a symptomatic effect rather than one caused by the change from vadose to phreatic regime itself.

Offline Rob

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Re: Passage size - Phreatic vs Vadose
« Reply #13 on: January 23, 2012, 10:39:48 pm »
...have you moved from a mature abandoned relic level into an immature phreatic undercatchment in which case there is a reasonable chance that the passsage will be smaller as it is younger and less well developed (not always the case as there may have been a greater flow for a longer period of time in the undercatchment than the relic level)...
I'm asuming, as the example is about one continuous passage with no other inputs, that both parts are of the same age.

...Think about the change in passage size going from the rift passage before sump 1 in swildons which is a classic vadose streamway to the passage between sumps 1 and 2 which is generaly bigger but more phreatic in character.
A good example, thanks. Although i don't know the area very well. Don't quite a few inlets come in between sumps 1 and 2?

In short the fact that there are lots of examples of a large vadose passages leading to smaller phreatic passages is going to be a symptomatic effect rather than one caused by the change from vadose to phreatic regime itself.
I'm sure you're explaining something useful here, but i'm sorry i'm struggling to understand. Do you mean that the reason the small phreatic bit is phreatic is because it's small? If so, I don't think that's true.

+500 views and less than a handful of potentially suitable examples so far. Can any more minds muster some up?
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Offline ian.p

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Re: Passage size - Phreatic vs Vadose
« Reply #14 on: January 24, 2012, 12:28:11 am »
sory no what i was saying was the fact that the phreatic passage is smaller is unlikley to be simply becasue its phreatic there will be physical reason why its smaller (younger age,reduction in flow, differences in rock competency etc)

Offline Rob

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Re: Passage size - Phreatic vs Vadose
« Reply #15 on: January 24, 2012, 07:17:11 am »
Yup ok. I understand that there's many variables, but would be more willing to agree fully if i knew more counter examples.

I hope people don't find me being overly defensive. I actually want to find no counter examples as it leaves better prospects for Gautries!
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Offline graham

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Re: Passage size - Phreatic vs Vadose
« Reply #16 on: January 24, 2012, 07:34:59 am »
OK

Faunarooska, Co. Clare.

Follow a narrow winding vadose canyon for quite some distance. The passage then changes character, with distinct phreatic passage forms & becomes much larger.
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Offline Rob

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Re: Passage size - Phreatic vs Vadose
« Reply #17 on: January 24, 2012, 08:25:48 am »
Faunarooska, Co. Clare.
Looking at the survey on p.10 of this: http://www.ubss.org.uk/resources/proceedings/vol8/UBSS_Proc_8_3_186-194.pdf that looks like a good example. Cross section P-P is certainly bigger than anything upstream of it.  :thumbsup:
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Offline graham

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Re: Passage size - Phreatic vs Vadose
« Reply #18 on: January 24, 2012, 09:57:41 am »
Faunarooska, Co. Clare.
Looking at the survey on p.10 of this: http://www.ubss.org.uk/resources/proceedings/vol8/UBSS_Proc_8_3_186-194.pdf that looks like a good example. Cross section P-P is certainly bigger than anything upstream of it.  :thumbsup:

And we've spent quite some time looking for an inlet that would explain the size change. If there is one it's hidden behind a lot of calcite.
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Offline Rob

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Re: Passage size - Phreatic vs Vadose
« Reply #19 on: January 24, 2012, 10:05:22 am »
I presume the passage is larger for a good length, and not just on that corner? Certainly downstream where the vadose incision begins the phreatic roof tube seems back to 'normal' dimensions.
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Offline graham

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Re: Passage size - Phreatic vs Vadose
« Reply #20 on: January 24, 2012, 10:28:29 am »
I presume the passage is larger for a good length, and not just on that corner? Certainly downstream where the vadose incision begins the phreatic roof tube seems back to 'normal' dimensions.

It's larger for a good length. The present stream cuts left into a steeply descending trench & after that teh size of the passage becomes more and more obscured by sediment, but remains larger than it was in the upstream vadose section.
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Offline Pitlamp

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Re: Passage size - Phreatic vs Vadose
« Reply #21 on: January 24, 2012, 01:21:26 pm »
Just one thing to bear in mind; the idea of equal solution in all directions under phreatic conditions was alluded to above - this simplistic model usually doesn't apply. Our problem is that most researchers (understandably) only have the opportunity to study DRAINED phreatic passages. Us sump floppers regularly come across layered water in active underwater caves; the layers probably have different chemistry (yet to be studied in the UK) and often different temperatures, resulting in sufficient density differences to cause the stable layering. Obviously this results in unequal dissolution rates.

I like Langcliffe's useful comment above but I'd also point out that many phreatic passages are actually far bigger than is needed to accomodate the flow even in extreme conditions, so perhaps his observation isn't quite as useful as you might at first think (but still useful).

Rob - I'm not convinced that you can draw any completely reliable conclusions from what you're trying to understand here - but perhaps you can recognise at least a shift in the balance of probablities? In case it helps though; I'm familiar with all the lower level passages in Giants and also with the underwater part of East Canal. I'd say (without being able to quote hard data) that the size of East Canal underwater is bigger than the combined development of the two approach passages. This in itself is actually less useful than it might appear due to the involvement of mineral vein cavitation in that area. There really is a large number of variables . . . . .

Offline Fulk

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Re: Passage size - Phreatic vs Vadose
« Reply #22 on: January 24, 2012, 01:35:03 pm »
Pitlamp: By langcliffe's comment above do you mean:

Quote
It's almost inevitable as a phreatic passage only needs to be as big as is needed to accommodate the water flow

I don't think that this is (necessarily) true. In the case of a Vauclusian rising the water cannot 'escape' as it were, except by overflowing, so it will continue to erode the passage for as long as it is 'trapped', and thereby could easily form a passage far bigger than that needed merely to accommodate  the water. You, indeed, have stated that the downstream phreas from Lancaster Hole sump is huge – there's no way that the stream in LH 'need's such a big passage.

Offline graham

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Re: Passage size - Phreatic vs Vadose
« Reply #23 on: January 24, 2012, 02:10:09 pm »
Assuming that the chemistry of the water doesn't change, surely passage size in the phreas is purely a function of discharge and time?
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Offline Fulk

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Re: Passage size - Phreatic vs Vadose
« Reply #24 on: January 24, 2012, 02:26:24 pm »
Quote
Assuming that the chemistry of the water doesn't change, surely passage size in the phreas is purely a function of discharge and time?

Of course, you are right; the point I was trying to make above concerns the length of time the water is trapped in the phreas. If it has to go uphill for some distance under pressure in order to resurge, then it may be trapped there for a long time, whereas if the uphill section is only short, then a surface channel could be eroded relatively quickly, which would make the phreatic phase of development much shorter.

Offline langcliffe

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Re: Passage size - Phreatic vs Vadose
« Reply #25 on: January 24, 2012, 03:06:23 pm »

Quote
It's almost inevitable as a phreatic passage only needs to be as big as is needed to accommodate the water flow

I don't think that this is (necessarily) true.

Actually, I think that it's a truism!

I was deliberately proposing a very simplistic model to make what I believe to be a valid point. Vadose passages will weather backwards from a base level, leading to an inevitably increase in size. Mechanical weathering is obviously an important factor in this process. Phreatic passages have the advantage, of course, that they continue to weather in low water conditions.

Offline Fulk

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Re: Passage size - Phreatic vs Vadose
« Reply #26 on: January 24, 2012, 03:15:27 pm »
OK – I concede that in theory a phreatic passage only needs to be big enough to accommodate the water flow available . . . but in practice they're often much bigger!

Offline Pitlamp

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Re: Passage size - Phreatic vs Vadose
« Reply #27 on: January 24, 2012, 04:43:22 pm »
I take your point Langcliffe.

Always remember folks that "mechanical weathering" is also an extremely important contributory factor in the development of phreatic passages as well as vadose ones. (I seem to remember we've been through this within another topic in the past.)

Offline danthecavingman

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Re: Passage size - Phreatic vs Vadose
« Reply #28 on: January 24, 2012, 08:12:00 pm »
Hello,

Going to jump in here with something maybe / maybe not relevant...

Vadose development is as a result of the lowering of the water table - for example the downcutting of the Hope Valley. Think about Jacob's Ladder just Upstream of Suprise View - this very immature canyon takes water from a relatively smaller phreatic tube (The Trenches) and has down-cut into the much bigger phreatic tube of Upper Gallery, which has been subsequently modified by the same stream to have a vadose canyon incised in it. This is a case of a vadose canyon forming a hydrological link between two very different levels of inception probably of very different ages geologically. I'm not sure there can be any direct comparison between passages we perceive to be one and the same where there is a transition between vadose and phreatic or vice versa and a lot of time has passed geologically.
The volume of water involved and the speed of downcutting must also have an effect. Slow erosion of a valley would would allow bigger vadose passage to form than say the unwatering of a valley in phreas by the breaching of a terminal morraine which would result in very rapid erosion due to the sudden drop in water levels. You must also consider in cases like this that small misfit streams may modify passages formed by much larger volumes of water.
I think what I'm trying to say is that there are an awful lot of factors to consider. Caves be where you find 'em.

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Offline Brains

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Re: Passage size - Phreatic vs Vadose
« Reply #29 on: January 24, 2012, 09:12:40 pm »
Probably not that helpful, but could a fossil example be where the Giants Crabwalk meets the relief passage? I am thinking of where the roof phrea descends to stream level before reascending in what would have been a sump. The vadose canyon virtually disappears in what would have been the sump before reappearing towards the junction to eating house...
There are inlets further upstream, but are they far enough away to be discounted?

Offline Rob

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Re: Passage size - Phreatic vs Vadose
« Reply #30 on: January 24, 2012, 11:26:48 pm »
...could a fossil example be where the Giants Crabwalk meets the relief passage?...
That sounds like a good example of vadose leading to phreatic, although not really proving much relating to my original puzzle because i wouldn't have said the cross sectional area of the Relief Passage is bigger than the cross section of the crabwalk (~30m high in places!).

Surely there's loads of prime examples of vadose leading to phreatic up in the Dales? None of these get bigger in the phreatic section???
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Offline danthecavingman

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Re: Passage size - Phreatic vs Vadose
« Reply #31 on: January 24, 2012, 11:37:44 pm »
What about Ireby Fell Cavern - Duke Street is way bigger than the streamway that now feeds into it....
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Offline Rob

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Re: Passage size - Phreatic vs Vadose
« Reply #32 on: January 24, 2012, 11:43:34 pm »
Isn't the main feed into Duke Street on the right as you enter, and now totally full of sediment? If so, I don't think the 'entrance series' has much to do with the formation of Duke Street.
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Offline cavermark

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Re: Passage size - Phreatic vs Vadose
« Reply #33 on: January 24, 2012, 11:52:38 pm »
Timescale is important too. Rob is defining his vadose passage as the currently dry one (presumably vadose in character). and the sump as Phreatic (but of unknown character). The whole lot could have been phreatic for the last thousands of years until a recent water table change, flow direction (or change due to sediment, earthquake slips, glaciers etc.)

Changes over short sections of the same passage could also be local variations in rock chemistry, structure, hardness etc.

Hard to make useful predictions

Offline Les W

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Re: Passage size - Phreatic vs Vadose
« Reply #34 on: January 24, 2012, 11:57:47 pm »
Is now the right time to chuck Paragenesis into the mix?  :-\
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Offline Rob

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Re: Passage size - Phreatic vs Vadose
« Reply #35 on: January 25, 2012, 07:33:03 am »
Is now the right time to chuck Paragenesis into the mix?  :-\
I wouldn't mind if it was done with the use of examples.

A lot of people have misinterpreted my querry as one that requires ooddles of science to be chucked at. I don't really care (for now) about all the different variables and methods possible. This is mainly because I understand that cave formation is extremely complex. But hard examples like Graham's at least show that it can happen, even if we don't understand all the methods at work.
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Offline graham

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Re: Passage size - Phreatic vs Vadose
« Reply #36 on: January 25, 2012, 07:53:36 am »
... hard examples like Graham's at least show that it can happen, even if we don't understand all the methods at work.

I'm still not convinced that something else wasn't happening in that cave at that point. It's very difficult to resolve a change in passage size to just one specific cause.
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Offline Pitlamp

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Re: Passage size - Phreatic vs Vadose
« Reply #37 on: January 25, 2012, 09:33:04 am »
Graham's right of course. You can't draw any meaningful conclusions without fully understanding all the factors that could potentially make such conclusions wrong.

Back to your vadose / phreatic size differences question; the streamway (vadose) in Ingleborough Cave below Giants Hall is significantly smaller in cross sectional area than the sump it feeds into (the 210 m long Lake Pluto to Beck Head Stream Cave sump). Downstream of this sump the vadose streamway in Beck Head Stream Passage is smaller again. (To keep Langliffe happy - as he knows Ingleborough Cave like the back of his hand - I'm talking about the bit of streamway immediately prior to Lake Avernus, thus removing the variable of the Shrimp Pool inlet.)

Offline ian.p

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Re: Passage size - Phreatic vs Vadose
« Reply #38 on: January 25, 2012, 10:33:40 am »
Right then third time lucky:

Just about anything is possable in terms of cave morphology theres all sorts of bizare things underground but youre not going to make head nor tail of them unless you look at WHY they are the way they are

If you have a length of passage that is subject to exactly the same conditions along its entire length then it will have exactly the same morphology along its entire length and it will either be completly vadose or compleatly phreatic and be exactly the same size.
A passage doesnt suddenly decide f@ck it vadose is getting a bit boring now maybe ill try phreatic for a bit there are underlying reasons for the change and these will be the drivers for deciding wether the passage is bigger or smaller than the precceding passage you wont ever have a hard and fast rule about what to expect from a passage just because youve reached a bit thats flooded every case will be different.

Offline graham

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Re: Passage size - Phreatic vs Vadose
« Reply #39 on: January 25, 2012, 11:13:27 am »
Right then third time lucky:

Just about anything is possable in terms of cave morphology theres all sorts of bizare things underground but youre not going to make head nor tail of them unless you look at WHY they are the way they are

If you have a length of passage that is subject to exactly the same conditions along its entire length then it will have exactly the same morphology along its entire length and it will either be completly vadose or compleatly phreatic and be exactly the same size.
A passage doesnt suddenly decide f@ck it vadose is getting a bit boring now maybe ill try phreatic for a bit there are underlying reasons for the change and these will be the drivers for deciding wether the passage is bigger or smaller than the precceding passage you wont ever have a hard and fast rule about what to expect from a passage just because youve reached a bit thats flooded every case will be different.

Wholeheartedly agree.

The only reason that I posted the example of Faunarooska was that Rob was not going to accept an argument that wasn't illustrated by actual cases, so I came up with one that countermanded his theory; even though I do not, personally, believe that this phase change was the reason for the size change.
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Offline Rob

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Re: Passage size - Phreatic vs Vadose
« Reply #40 on: February 21, 2012, 08:04:39 pm »
Digging this one back up again in case ayone has any more examples...

Pitlamp, thanks for the Ingleborough Cave example. Had a look at some surveys, seems feasible, although unfortunately quite a complex area, especially with Lake Pluto and Lake Avernos seemingly on a fault that may have created the enlargening, as with East Canal. Although i've never been there so i'm dependant upon you're knowledge of the system really.

ian.p, sorry i think i disagree again. A passage is only phreatic because the floor level somewhere downstream is higher than it. No other changes necessarily need to occur. Therefore there may well exist a relatively "clean" example that proves it can happen.
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Offline graham

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Re: Passage size - Phreatic vs Vadose
« Reply #41 on: February 21, 2012, 08:35:55 pm »
ian.p, sorry i think i disagree again. A passage is only phreatic because the floor level somewhere downstream is higher than it. No other changes necessarily need to occur. Therefore there may well exist a relatively "clean" example that proves it can happen.

You miss Ian's point which that there will be an external cause for the change in floor level that you describe.
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Offline Rob

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Re: Passage size - Phreatic vs Vadose
« Reply #42 on: February 21, 2012, 08:40:28 pm »
Downstream yes, but not necessarily in the zone around the change between vadose and phreatic development.
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Re: Passage size - Phreatic vs Vadose
« Reply #43 on: February 21, 2012, 08:46:29 pm »
Downstream yes, but not necessarily in the zone around the change between vadose and phreatic development.

A pint says that you cannot find a clear example of a vadose/phreatic phase change that does not have an external cause, usually geological or base-level.
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Offline Rob

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Re: Passage size - Phreatic vs Vadose
« Reply #44 on: February 21, 2012, 09:08:59 pm »
And 1700 views show to me that there's very few examples of phreatic sections being larger than their vadose counterpart.
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Offline Fulk

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Re: Passage size - Phreatic vs Vadose
« Reply #45 on: February 22, 2012, 08:25:21 am »
Rob, if I understand correctly what you mean by
Quote
And 1700 views show to me that there's very few examples of phreatic sections being larger than their vadose counterpart.
, here are a couple of samples; I'm sure there must be (many?) others – the 'classic' phreatic passages in Peak cavern and Ibbeth Peril Cave, where nearly-purely phreatic passage have hardly been modified at all by the vadose streams running on their floors.

Offline Rob

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Re: Passage size - Phreatic vs Vadose
« Reply #46 on: February 22, 2012, 08:45:18 am »
Unfortunately you've not understood. The first few posts explain better...
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Offline Fulk

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Re: Passage size - Phreatic vs Vadose
« Reply #47 on: February 22, 2012, 09:56:34 am »
 :-[ Right, I see, when a vadose stream enters a sump . . . well, I guess I'll have to leave it  to the divers.

Offline TheBitterEnd

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Re: Passage size - Phreatic vs Vadose
« Reply #48 on: February 22, 2012, 12:25:39 pm »
Rob, I expect that your assertion is correct for the vast majority of cases. The "Four state model" of cave development (Ford & Williams - Karst hydrogeology and geomorphology) would seem to lead to the situation you describe where lowering piezometric surface leads to vadose entrenchment of phreatic loops connected to sumped phreatic passage.

As others have pointed out and as one would expect with such a complex and chaotic process, there are exceptions.
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Offline Rob

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Re: Passage size - Phreatic vs Vadose
« Reply #49 on: February 22, 2012, 01:36:48 pm »
Oooo, looks like i've just found my new bedtime reading book. Thanks for the link :thumbsup:

The case studies i'm interested in don't require dewatering of the phreatic zone, as it's not the vadose entrenching that i'm looking at. Rather a fully vadose passage leading to a fully phreatic passage. That make sense?

Of course dewatering of the phreatic will be required if they going to be observed by non divers...
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