next message in archive
no next message in thread
previous message in archive
Index of Subjects
Hi Steve & All,
I tried Google Chrome and was able to read where Internet Explorer
failed namely--
http://www.hwe.org.ps/Education/Birzeit/GroundwaterEngineering/Chapter%203%20-%20Groundwater%20Flow%20to%20Wells.pdf
This is not light reading but examples 6.3 a&b & 6.4 provide samples of
effects at a distance from the well. In 6.4 e.g. water table was lowered 1m
at a distance of 500 m from the well after continuous pumping at 3888
M^3/day for 4.3 days. This reach is greater than I had remembered but this
rate is rather large.
Yt, DW
----- Original Message -----
From: "David & Alison Webster" <dwebster@glinx.com>
To: <naturens@chebucto.ns.ca>
Sent: Sunday, July 15, 2012 10:09 PM
Subject: Re: [NatureNS] Freshwater Shortages and Bay of Fundy Tides
> 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
>>
>>
>> -----
>> No virus found in this message.
>> Checked by AVG - www.avg.com
>> Version: 2012.0.2193 / Virus Database: 2437/5128 - Release Date: 07/12/12
>>
>
next message in archive
no next message in thread
previous message in archive
Index of Subjects