Armfield S12-MKII, Instruction Manual

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Armfield Instruction Manual
50
(1)
and the equation of continuity.
Q = Q
o
= constant, leads to Thiem's Formula
(2)
in which
Q = total rate of low in aquifer at radius r
v = water velocity in aquifer at radius r
H = thickness of aquifer
k = coefficient of permeability of aquifer
s = lowering of piezometric surface at radius r (from rest position)
Q
o
= steady discharge from well.
The coefficient of permeability has the dimensions of a velocity and typical values are
in the range 0.04 to 0.004 mm/s.
Procedure
Abstraction from a single well in a confined aquifer with radial symmetry
The well placed centrally in the catchment tank is used in this experiment. A shallow
depression is scooped in the sand until the top of the gauze well tube is exposed. A
flat sand surface is now prepared at this level, large enough to take the closed ring,
arranged with its centre opening over the well tube. The closed ring is now placed in
position with its transparent central standpipe (sight tube) in position, and a shallow
trench is excavated in the sand outside its periphery (see Figure G3).
Figure G3
Connect the flexible piping from the two French drains to the quick release connector
on the two flow meters- these will be used as inlet pipes not as drains for this
experiment. Maintaining equal flow through both flow meters, flood the sand until the
water level surrounding the closed ring is just below its rim or, if this level is not
possible, as near below as is possible. It will be necessary to weight the closed ring
to stop it floating when the bed is flooded, approximately 10 kg weight. Items used for
this must be suitable for use in water and must not damage the ring. The water level
in the sight tube must also be observable with the weights in place.