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Hi Steve & All, July 15, 2012
I wouldn't care to suggest that I have the physics correct; just a very
crude first approximation of the effect of head gradients over distance. But
way more correct than the one you advance.
Except for very simple artificial situations (assumptions that seldom
apply in practice) problems of liquid flow in porous media can not be solved
analytically and require computer modelling or field measurements. But flow
to any sink such as a well, tile drain or Gloosecap's buried garden hose,
from a phretic surface, will be at right angles to isopotential lines. I
recall this only in vague terms now but as distance from the sink increases
there is a dramatic increase in the length of flow paths and transit times.
I tried to find a sample representation of pumped well steady-states but
keep getting run time errors (started about a week ago with Internet
Explorer.
Yt, DW, Kentville
----- Original Message -----
From: "Stephen R. Shaw" <srshaw@Dal.Ca>
To: <naturens@chebucto.ns.ca>
Sent: Sunday, July 15, 2012 1:24 AM
Subject: Re: [NatureNS] Freshwater Shortages and Bay of Fundy Tides
> Quoting David & Alison Webster <dwebster@glinx.com>:
>
>> Hi Patricia & All, July 13, 2012
>> I don't know Greg Gerrits but I do know Elmridge Farm produce; top
>> quality and this implies unusually good management. So I would be
>> inclined to accept his observation of well replenishment rates as a
>> function of something to do with tides but it can not caused directly by
>> height of saltwater in the 'nearby' shore.
>>
>> Flow of any liquid in a porous medium is directly proportional to
>> hydraulic gradient (head z/lateral displacement y) and inversely
>> proportional to resistance. A head difference of 10m over lateral
>> displacement of 10 km (hydraulic gradient = 0.001) would induce
>> essentially zero flow Or approaching this from the other direction,
>> when water is pumped exhaustively from a well the water table (drawing
>> on memory) is seldom lowered beyond a radius of 50 metres.
>
> Dave, Patricia:
> I don't know if you have the physics of the above idea correct, Dave, but
> if you have, here is a possible suggestion/way out of your pessimistic
> assessment.
> Suppose that the water table out there is in part a continuous sheet or
> lake of (almost) incompressible water that extends all the way from the
> farm out to within, say, 1 meter of the Bay of Fundy. Your hydraulic
> gradient now rises to 10 instead of your 0.001, and the resulting forced
> flow, now feasible, would impress some seawater into the supposed
> continuous aquifer under pressure. The resulting pressure change would be
> felt very quickly even 10 km inland, explaining the increase in pressure
> in the well (as in water coming out of a long, full hose immediately
> after the tap (pressure source) is turned on -- no delay). This amounts
> to saying that there is zero resistance to flow over 10 km, which is not
> possible physically because of frictional wall effects (Poiseuille's law
> for tubes -- lower flow near the edges), but you can speculate that this
> might be a relatively small effect if the depth of water table is
> significantly large. A bit of the sea water would mix in at the edges
> but most would be removed again at the next low tide.
> I don't know if this is a physically reasonable model of what's down
> there, but if it is, it might make the farmer's observation feasible.
> Obviously it goes against your belief that the water table is extremely
> local. Do geophysicists conceptualize the water table around there as in
> part a continuous, shallow underground lake? Someone must have
> investigated this if you knew where to look or whom to ask.
> Steve (Halifax)
>
>
>> I would suspect that a tidal effect leads to a local artificial
>> hydraulic gradient. In effect, water running uphill to the well (and
>> well vicinity) during this peak inflow period.
>>
>> I ran into NatureNS by accident some years ago while trying to find
>> some understandable explanation of tidal effects on the internet. The
>> nearest I have come to an explanation was some oracular comment to the
>> effect that it is explained by math that almost no one understands. But
>> meanwhile the tides come in and go even though I don't understand how it
>> works.
>>
>> YT, Dave Webster, Kentville
>> ----- Original Message ----- From: "P.L. Chalmers"
>> <plchalmers@ns.sympatico.ca>
>> To: "NatureNS" <naturens@chebucto.ns.ca>
>> Sent: Friday, July 13, 2012 2:28 PM
>> Subject: [NatureNS] Freshwater Shortages and Bay of Fundy Tides
>>
>>
>>> For most of my life, my family home in Bedford was dependant on a
>>> drilled artesian well, as there was no municipal water supply in our
>>> neighbourhood. This is no longer the case, but I am still acutely
>>> aware of how valuable water is, and curious about hydrology. I was in
>>> the Annapolis Valley last week and saw how very dry the fields and
>>> orchards were there. So last Saturday, at the Halifax Seaport Farmers'
>>> Market, I was asking some of the farmers how they were getting on in
>>> the present drought.
>>>
>>> I had an interesting chat with Greg Gerrits of Elmridge Farm. Since
>>> there was so little snow last winter, and since it was such a dry
>>> spring, his normal water supply is down at least 40% from normal
>>> levels. He doesn't have enough water to irrigate more than a few acres
>>> at a time. However, he went on to explain the influence of the Bay of
>>> Fundy tides on his water supply. He said that the weight of incoming
>>> water in the Bay was sufficient to exert pressure on water deep below
>>> the ground, even where he farms (near Sheffield Mills, I think). The
>>> flow of fresh water into his well improves significantly when the Bay
>>> is full of salt water. So he has learned to schedule his irrigation so
>>> that it begins three hours before high tide, and stops three hours
>>> after high tide.
>>>
>>> There are so many kinds of knowledge that a successful farmer needs to
>>> have, but it hadn't occurred to me that a knowledge of the tides was
>>> one of them. I may not be representing this very well, but I found it
>>> intriguing; perhaps someone could say more?
>>>
>>> Patricia L. Chalmers
>>> Halifax
>
>
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