Armfield S12-MKII, Инструкция по эксплуатации

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59
Exercise J - Rainfall on a circular island with a central well
Theory
It is suggested that students read the theory of superposition described in Exercise I
before proceeding with this experiment.
Procedure
Connect the flexible tubing from the spray nozzles to the quick release connector on
the 5 l/min flow meter. The circular ended ring should be placed on the sand
concentric with the central well. The ring should then be pushed into the sand until
only half remains visible, and the inside volume should now be filled to the rim with
sand excavated to form a ditch just outside the ring (see Figure J1).
Figure J1
This ditch must connect with a second straight one linking it to the end outlet, which
will ensure a drainage route for water entering the ditch and so help to keep a
constant water level in it.
The spray nozzles are now turned on and, when the water table is steady, the
manometers can be read to enable a profile of the water table to be plotted. It is
important when plotting this profile to relate it accurately to the position of the ring
and other ground features. The water filled ditch surrounding the island serves to
isolate its ground water conditions from effects outside.
In the second part of this experiment, the spray nozzles are stopped and water is
withdrawn from the central well drain control valve.
A compensating flow must be introduced as a ground water supply to produce a
steady water table elevation. To do this, connect the flexible tubing from the 3 l/min
flow valve to the self-sealing connection on one of the French drains and slowly
introduce the water supply until steady conditions are attained. The well discharge
ought to be less than the rate of rain recharge falling on the central island in the first
part of the experiment, and the correct value can be selected only by trial-and-error.
When the water table profile has been determined and plotted, the sprays should be
turned on again to their previous setting and the combined effect of surface recharge
and central well abstraction determined. The water table under these combined
effects should reach a maximum height at some radius less than that of the "island".
This maximum marks the water divide for ground water flow, rain falling inside this