2.5 Drawing a Flow Net for an Unconfined System with a Water Table Boundary

Unconfined groundwater systems have a water table boundary which requires special consideration when drawing a flow net because the location of the water table boundary is not known until after the flow net construction is completed. Such systems may also have a seepage face, where groundwater seeps out along a sloping section of ground surface. The position of the water table and the length of the seepage face need to be adjusted along with the flow and equipotential lines while drawing the flow net. Because the water pressure is equal to the atmospheric pressure at the water table, the equipotential lines need to intersect the water table at the elevation equal to the value of the equipotential line label.

In the absence of recharge, the water table is itself a flow line. There is no flow across flow lines so a water table without recharge can be viewed as a no flow boundary of unknown position until after the flow net is drawn. When the water table is a flow line, equipotential lines meet the water table at right angles.

The procedure for constructing a graphical flow net does not accommodate boundaries with a defined flux other than zero. However, if there is recharge across the water table and the position of the water table is known, then values of head can be defined as equal to the elevation of the water table and a flow net can be drawn that will reflect inflow at the water table. The rate of inflow can be determined if the value of hydraulic conductivity is known. This is discussed in section 2.9 which addresses topographically driven flow.

An earth dam is used as an example for discussion of drawing flow nets in an unconfined system as illustrated in Figure 13 and described in Box 4 which can be accessed from the caption of Figure 13. In contrast to the previous section, material at the ground surface is impermeable, and earth material is brought in from nearby to construct a dam, so in this case water flows through the dam instead of under the dam. The dam surface is sealed to prevent infiltration of water into the dam structure. Consequently, the water table is a flow line.

Figure 13 – Click here to go to Box 4 which describes the procedure for drawing a flow net with a water table boundary.