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FLOW NETS FOR HOMOGENEOUS ISOTROPIC SYSTEMS
A flow net is a graphical solution to the equations of steady groundwater flow. A flow net consists of
two sets of lines which must always be orthogonal (perpendicular to each other): flow lines, which show
the direction of groundwater flow, and equipotentials (lines of constant head), which show the
distribution of potential energy. Flow nets are usually constructed through trial-and-error sketching.
To construct a flow net:
1. make a two-dimensional scale drawing of the system under consideration (usually a profile, but
may be a map view.)
2. determine or specify the boundary conditions, i.e., indicate/label the position of the water table,
of any impermeable boundaries, of any points of known head or known pressure.
a. any surface of constant head (e.g., bottom of a flat-bottomed reservoir) is by definition an
equipotential, and flow lines must meet it at right angles.
b. since flow cannot cross impermeable boundaries, the flow at such a boundary must be
parallel to it, i.e., impermeable boundaries are flow lines, and equipotentials must meet them
at right angles.
c. the water table is, by definition, the surface where P = 0; it can thus be an equipotential only
if it is horizontal. At any point on the water table (no matter whether it is flat or sloping) h =
z, where z is the elevation of the water table above the datum.
If there is no seepage percolating down to the water table, it can be considered a flow line. In
the general case however (sloping water table, seepage across it), the water table is neither a
flow line nor an equipotential, and flow lines will intersect it at an angle.
3. Once you have defined the boundary conditions, start trial sketching of flow lines and
equipotentials, following the rules in step 2 above, and being sure that the flow lines and
equipotentials always intersect at right angles.
Try to make the flow net consist of curvilinear "squares", i.e., the boxes in the flow net may have
curving sides, but the midline lengths of the "square" should be approximately equal. (arrows
inside square in diagram below) This is especially important if the flow net is to be used for
calculations of groundwater discharge.
h¡ h™
q¡
q™
Keep sketching and refining until you have a good set of "squares" which satisfies the boundary
conditions.