Vadose devlopment in chalk?

987056

New member
I read some were that chalk only forms caves wilst under preasure and when the floor of the passage is something impermiable. is this the case?
 

Les W

Active member
Vadose development is not dependent on lithology as it is an erosive process, not a solutional one. The only controlling factor on Chalk would be permeability/porosity. Chalk is very porous so you are right in the assumption that solutional caves form because the entire area is below the water table.
I would guess that if the water table was lowered then the solutional caves would de-water and the passages would dry up, as the flow could easily move through the pores of the rock.
Vadose development would rely on maintaining a flow in the solutional passage long enough for vadose development to occur. I guess that if the water table was lowered slowly enough then the easier route for flow would be preferred by the water and vadose development would happen but this would require the lowering of the water table to match the erosive power of the down cutting water.
It looks to me as there are quite a lot of factors that would need to be in some degree of equilibrium for it to happen so I would assume it is not common, however it will certainly occur somewhere.
There is plenty of vadose development in chalk caves in France but I think the rock there is more compact and less porous.
 

graham

New member
Les, both vadose and phreatic development are both erosional and solutional in all karst rocks. It's only the relative degree that varies.
 

Roger W

Well-known member
As Les says, you have to maintain a sufficient flow of water in the vadose passage for solution/erosion to occur.  If the passage is well above the water table, but is used for drainage when the overlying land is flooded by heavy rain, for example, you might well get some enlargement.
 

Les W

Active member
graham said:
Les, both vadose and phreatic development are both erosional and solutional in all karst rocks. It's only the relative degree that varies.

I know that but was looking at it more simplistically.
It is specifically the vadose development I was considering not the real mode of formation.
Vadose can only form if the passage is above the water table and in a porous rock the flow will disappear from the passage instead of down cutting occurring.

The only way to get vadose development is for flow to be maintained in the passage with a lowering of the water table. This will require the water table to match the erosive down  cutting or as Roger pointed out, seasonal raising of the water table (flooding), effectively rejuvenating the stream in the passage.
 

Peter Burgess

New member
Chalk is highly fissured, but is not really porous like a sandstone might be. Water flows through chalk along fissures and not through the body of the rock. Many a well or borehole has been sunk into chalk well below the local rest level of the groundwater, and has remained dry. Many wells required lateral tunnels to be driven before sensible flows of water were encountered when fissures in the chalk were cut, thus releasing the water flowing in them.
 

Pitlamp

Well-known member
. . . not forgetting that features which normally characterise vadose development can also be formed under phreatic conditions . . . .

Anyway, I once got harrangued by someone (I think it was John Gunn) for using the term "water table". Can anyone tell me what the difference is between the latter and a "piezometric surface"? (I'm genuinely interested; this is not pedantry or trolling or whatever they call it these days.) Thanks.
 

Peter Burgess

New member
Pitlamp said:
Anyway, I once got harrangued by someone (I think it was John Gunn) for using the term "water table". Can anyone tell me what the difference is between the latter and a "piezometric surface"? (I'm genuinely interested; this is not pedantry or trolling or whatever they call it these days.) Thanks.
I was waiting for someone to take me to task for using the term "rest level" which I think should only be used in reference to wells, and not as a natural horizon in the ground. I'll let someone else explain piezometric surface as I am sure to get it wrong and be "harrangued".
 

Ed W

Member
There are certainly vadoes, active, streamways in the chalk caves in Normandy.  The active streamway section intercepted by the Caumont quarries is a very pleasant walking sized vadoes streamway.

Ed
 
Swallet holes are documented in the Mole Vally, Surrey where water sinks into the chalk (Chelsea SS Records Vol 15 mentions some I believe). There are lots of caves caves documented in the Cliffs of Dover as well, but whether they were originally phreatic or vadose I don't know.
 

RobinGriffiths

Well-known member
Is there not some sort of transformation of the chalk going SW to NE from fissured to less so towards the North Sea oilfields. I remember watching an OU program which I think said something to that effect, but I can't remember the actual physical changes in the chalk or actually where the transition zone was.

Robin
 

TheBitterEnd

Well-known member
Pitlamp said:
Anyway, I once got harrangued by someone (I think it was John Gunn) for using the term "water table". Can anyone tell me what the difference is between the latter and a "piezometric surface"? (I'm genuinely interested; this is not pedantry or trolling or whatever they call it these days.) Thanks.

Alright, I'll have a go, in general (i.e. not Karst specific) the water table is the elevation in the ground where the water would be found, the piezometric surface is the level to which the water would rise if open to the atmosphere.

Often the two things are the same but in confined aquifers the water is under pressure - it is confined by an impermeable strata above. As an example, the fountian in Trafalgar Square was originally piezometric - a bore hole was drilled and the water flowed up above the ground level and fed the fountain.
 

graham

New member
Water Table

# Beneath the surface, many uneven layers of soil, rock and water interact with precipitation and surface water to change groundwater volumes. The uneven layers create underground flows that influence discharge to surface water and distribute and/or re-distribute minerals and contaminants. The water table is the surface at which water pressure is equal to atmospheric pressure. It is depicted on maps as a line across an aquifer.

Piezometric Surface

# A piezometer is a special tool that is used to take measurements within an aquifer. It is submerged within a well beneath the saturated zone, through less porous rock. Many piezometer wells are drilled within a confined aquifer at certain locations. The piezometric surface of water is the level of water within a piezometric well in a confined aquifer. It is depicted on maps as a line between the walls of a well. When several piezometric surface measurements are available, a hydrogeologist can determine recharge and discharge rates and most importantly groundwater-flow direction and rates.

Piezometric Surface vs. Water Table

# Piezometric surfaces can differ from the water table line. Calculations based upon piezometric surface levels in comparison to the water table levels show how groundwater volumes and qualities can change. Seasonal, contaminant or climate influences can be correlated to changes, providing important resource-management information.

Source.

That, as sirch2 also pointed out is not karst specific. One of the reasons why piezometric surface is preferred by karst hydrologists is that it does not imply a continuous body of water. Remember that karst drainage routes can cross without mixing.
 

gus horsley

New member
The mechanical properties of the rock might also determine whether vadose or phreatic development could occur.  If the chalk is mechanically weak (which is most of the chalk in England), then large passages are less likely to form which means that vadose development would be more limited as smaller passages tend to be phreatic.  In addition any passages in mechanically weak chalk will be more prone to collapse which would impede the flow of water, thus also creating conditions more condusive to phreatic development.
 
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