Environmental Engineering Reference
In-Depth Information
Short Exercise 9: Simple Flow-Net Analysis
We do not need a sophisticated numerical model to give us a good first estimate of
groundwater flows to and from wetlands. Reasonable values for exchange between
groundwater and a wetland can be calculated with: (1) a map showing the locations
of a few monitoring wells and their hydraulic-head values, (2) a value for stage of
the wetland, and (3) estimates of hydraulic conductivity. In this brief exercise you
will make a flow-net analysis to determine flow between groundwater and a wetland
and also compare those values with values that were obtained with the segmented-
Darcy approach in short exercise SE 8.
The flow-net analysis is a graphical approach for determining 2-dimensional
groundwater flow. The Darcy equation is used to solve for flow through individual
“stream tubes” that are drawn based on contour lines drawn from head data. The
method assumes steady-state flow is two-dimensional. The flow net can be drawn in
plain view, as we did with SE 8, or in cross-sectional view. We will assume that the
aquifer is homogeneous and isotropic, although modifications can be made when
drawing the flow net if the aquifer is known to be anisotropic. A brief description of
how to draw a flow net follows. More detail can be found in Fetter Jr. (2001) and
Cedergren (1997).
A flow net consists of equipotential lines (contour lines of equal hydraulic head)
that are drawn perpendicular to flow lines that indicate the direction of groundwater
flow. The net is bounded by no-flow boundaries or constant-head boundaries. The
equipotential lines intersect no-flow boundaries at right angles and the flow lines
intersect constant-head boundaries, if present, also at approximately right angles. A
simple example is shown in Fig.
3.37
. Equipotential head drops consist of the area
Equipotential line
Flow line
No-flow boundary
Stream tube
Fig. 3.37 Diagram of a
simple rectangular flow net
showing boundary
conditions, equipotential
lines, and stream tubes
No-flow boundary
