Physics for Geologists, Second edition

Gravity 3 7
Figure 3.5 Idealized cross-section across a river valley showing the paths of
ground-water flow from the hill to the river. The ground-water has
more energy under the hill than in the bed of the river. The aquifer
is assumed to be homogeneous and the re-charge uniform.
The flow lines are sketched from the following considerations assuming
constant recharge over the area from rainfall:
1 The energy of the water at the water table is equal to that of the water
at the same elevation of the water table above some arbitrary datum.
2 Flow is normal to the surfaces of equal energy - the equipotential
surfaces.
The equipotential lines and flow lines can be sketched as in the figure because,
at the water table, flow will be nearly along the water table (slightly down
on account of recharge), and the equipotential surface will be normal to the
flow, nearly normal to the water table.
Note that in the elevated parts of the aquifer the flow will be from low
pressure (atmospheric at the surface) to higher pressure.
Incidentally, in a swimming pool, the so-called skimmer box takes water
from all levels before it, not just from the surface. It skims floating material.
Flow to a water bore
The essence of liquid flow to a bore or well is that the water loses energy to
friction as it accelerates radially towards the bore, and so the free water sur-
face falls towards the bore. While this only applies strictly to an unconfined
aquifer, the principle can be applied to all (Figure 3.6).
Almost every borehole, artesian or not, has an initial production greater
than its stabilized production. There are two reasons for this.
Copyright 2002 by Richard E. Chapman
The initial energy of the aquifer at the borehole is large because no energy
has been lost to friction during production.