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Definition of perched water

Definition of perched water

(OP)
Gentlemen:

I'm looking for a textbook definition of 'perched water'. Any help out there?

Thanks in advance

RE: Definition of perched water

Water that is "perched" above an impermeable layer, or one of very low permeability, overlying one that is permeable.  In other words, without the impermeable layer, the water would not be there.

Mike McCann
MMC Engineering
http://mmcengineering.tripod.com
 

RE: Definition of perched water

A vertically isolated, laterally unconfined body of groundwater, separated from the primary groundwater by one or more layers of unsaturated soil or rock material.

RE: Definition of perched water

(OP)
Understand, but I'm looking for a 'textbook' definition. Particularly if it addresses extensiveness. Like if I provide a relief trench to an area that has some topographic relief and a year later it's still draining, is this beyond the meaning of 'perched water'?

Thank you

RE: Definition of perched water

No,  Perched waters are not limited in lateral extent & can be recharged, maybe continually recharged.
I would suggest ASTM list of definitions for Soils.  In the last 2 or 3 years I remember voting on some definitions similar for soils and environmental uses.

RE: Definition of perched water

a phreatic surface under which there's a vertical gradient of unity

f-d

¡papá gordo ain't no madre flaca!

RE: Definition of perched water

(OP)
"Perched waters are not limited in lateral extent & can be recharged, maybe continually recharged"

So there's no real difference beteen 'perched water' and 'ground water'?

Thank you

RE: Definition of perched water

Here's a simple explanation.  Sometimes there's a clay layer or some rock or some sort of barrier between the surface and way down where the groundwater is.  So when it rains, the water leaks down and pools on top of that barrier, instead of making it down to the groundwater, making a falsely high layer of groundwater.  Like a high, fake, water table.  That's perched water.

If you lay in a trench or something to drain the perched water away, but it continues to rain, then water can still get into the perched area to recharge it.  

Make sense?

Hydrology, Drainage Analysis, Flood Studies, and Complex Stormwater Litigation for Atlanta and the South East - http://www.campbellcivil.com

RE: Definition of perched water

From:
Groundwater Manual, US Department of the Interior, Bureau of Reclamation

page 7
(e) Perched Aquifers. - Beds of clay or silt, unfractured consolodated rock, or other material with relatively lower permeablility than the surrounding materials may be present in some areas above the regional water table. Downward percolating water may be intercepted and a saturated zone of limited areal extent formed. this results in a perched aquifer with a perched water table [4]. An unsaturated zone is present between the bottom of the perching bed and the regional water table. A perched aquifer is a special case of an unconfined aquifer. Depending on climatic conditions or overlying land use, a perched water table may be a permanent phenomenon or one which is seasonally intermittent (fig. 1-2)

see figure attached

RE: Definition of perched water

(OP)
Thanks CVG, I think that's helpful. M2, there's room for one wiseguy here and I'm it.

RE: Definition of perched water

the implication of the USBR definition's unsaturated zone is stating a vertical gradient of unity must exist.  For a phreatic surface to be present above such an unsaturated condition a contrast in permeability is needed (i.e., clay).  I'm going with my first post. . .

f-d

¡papá gordo ain't no madre flaca!

RE: Definition of perched water

3 definitions, all basically the same:
perched groundwater, n—in geohydrology/hydrogeology, a localized body of unconfined groundwater above and separated from the main body of groundwater by a groundwater barrier immediately below which lies unsaturated material.

DISCUSSION—There can be more than one perched groundwater zone in a specific subsurface area. Perched groundwater zones are frequently formed on aquitards or aquicludes.

perched water table—a water table usually of limited area maintained above the normal free water elevation by the presence of an intervening relatively impervious confining stratum.

perched water table—groundwater separated from an underlying body of groundwater by unsaturated soil or rock. Usually located at a higher elevation than the groundwater table. (ISRM)

all from D653-11 Standard Terminology Relating to Soil, Rock, and Contained Fluids

RE: Definition of perched water

(OP)
Thanks emm, I think that'll help.

RE: Definition of perched water

Perched water is groundwater when it affects earthwork.  

The only difference is that perched water usually requires less dewatering and once done.... tends to not return until after the grading is done.

RE: Definition of perched water

Textbook definition, from an actual textbook: Groundwater by R.A. Freeze and J.A. Cherry, Prentice-Hall, Inc., Englewood Cliffs, NJ, 1979.

"The existence of a low-permeability clay layer in a high-permeability  sand formation, for example, can lead to the formation of a discontinuous saturated lense, with unsaturated conditions existing both above and below."  Then, it refers to a figure labelling the top of the saturated zone the "perched water table," and the bottom of the saturated zone as the "inverted water table."  The key part is the existence of unsaturated soil or rock below the saturated soil or rock.

fattdad: Where does that gradient definition of "perched" come from?  I've never seen it before.

RE: Definition of perched water

dgillette:  I made it up.  Folks like darthsoilguy2, in construction, conclude the condition of "perched groundwater" when often it's just a reflection of the water table - not perched at all (and yes it's affecting construction).

As a geologist I can't count how many times I've seen a utility installed and the contractor's digging through sand gets to some depth and the water flows into the trench excavation (well the sand too).  Then the bucket hits a clay layer and everybody says, "Hey, it's a perched water table."  Well, it may or may not be perched.  You see to know whether it's perched, you'd need to know that there was no phreatic surface in the clay.  It's also likely that the clay and the sand share the same phreatic surface, but the flow from the clay is so slight it appears to be hydraulically disconnected.

If there is no phreatic surface in the clay, then you have a perched water table above the clay and zero pressure head in the clay (i.e., only elevation head).  If all there is is elevaton head, then that defines a vertical gradient of unity.

I'd say if there is a vertical component of flow gradient and some component of pressure head the aquitard (aquiclude) is not supporting a perched water condition.

Not to confuse. . .

f-d

¡papá gordo ain't no madre flaca!

RE: Definition of perched water

Another way to look at is, with the gradient of 1.0, the water flows through the clay, with a velocity of 10^-7 cm/sec.  Then, it hits the underlying sand or gravel, and switches over to unsaturated flow as it trickles down through the gravel.  

RE: Definition of perched water

eh, that would be the Darcy velocity - just divide by the effective porosity for the real velocity.

f-d

¡papá gordo ain't no madre flaca!

RE: Definition of perched water

Why did you assume that's the Darcy velocity?  If k is 10^-8 in a highly plastic clay, and effective porosity is ~0.1, you've got 10^-7 cm/s for the velocity .

I had deliberately not mentioned which it was, or permeability, because it wasn't important to the point I was trying to make, which is that there can indeed be a vertical flow velocity > 0 through the clay layer, and still have Freeze and Cherry's definition apply, because there is unsaturated material below.  

Curiously, the permeability of the clay and the gravel below it are the same, because of the very low degree of saturation in the gravel.  Same gradient of 1.0, same Darcy velocity, same permeability (q=ki).

RE: Definition of perched water

touche!

f-d

¡papá gordo ain't no madre flaca!

RE: Definition of perched water

wait-a-sec... i was saying "perched water is GROUNDWATER".  I'm talking contracts not science (unless the grader has got a really bad contract)

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