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!)....]

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 (?).

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!

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?

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.

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.

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.

Pitlamp said:

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

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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!

Pitlamp said:

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.

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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...

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.

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....

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.

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...?!?

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  )

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.

ian.p said:

...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)...

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I'm asuming, as the example is about one continuous passage with no other inputs, that both parts are of the same age.

ian.p said:

...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.

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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?

ian.p said:

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.

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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?

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)

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!

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.

graham said:

Faunarooska, Co. Clare.

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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. 

Rob said:

graham said:

Faunarooska, Co. Clare.

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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. 

Click to expand...

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.

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.