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1.725 Fate and Transport
10/25/2002
Supplement on Flow Nets:
Flow nets are a graphical technique for predicting the magnitude of groundwater
flow ffom a given set of boundary conditions. They are not a rigorous determination of
flow, but they can give us a picture of what head looks like underground.
The original use of flow nets was for determining how much water was flowing
under a dam. If there is a substantial amount of flow underneath a dam, it can put a lot of
pressure on the sediments. Over time the groundwater can erode the sediments, and the
dam can collapse  it's flood timel This is a difficult problem to solve analytically, but it
turns out that flow nets can be used to get a graphical answer.
Flow nets are comprised of a set of equipotential lines (lines of constant head, h)
and a set of associated flow lines (lines along which groundwater moves). Two flow
lines define what is called a "stream tube." These lines are drawn within a certain set of
boundary conditions (e.g. surface water bodies, impenetrable rocks). Three potential
boundary conditions for a flow net that we may encounter are:
1. Noflow boundaries (e.g. a dam, low permeability bedrock, the Hancock Tower's
basement).
ah
Water cannot flow across a noflow boundary. When given a noflow
=
0
boundary, draw flow lines parallel to the boundary and head lines
perpendicular to the boundary. Remember that flow lines
cannot
touch the
boundary! Head lines should intersect the boundary.
I.
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This note was uploaded on 11/21/2009 for the course CEE 432 taught by Professor Ahmed during the Spring '09 term at Rowan.
 Spring '09
 Ahmed
 pH

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