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FlownetSupplementalMaterial - 1.725 Fate and Transport...

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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. No-flow boundaries (e.g. a dam, low permeability bedrock, the Hancock Tower's basement). ah Water cannot flow across a no-flow boundary. When given a no-flow = 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.
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