For unconfined aquifers, surface topography and surface water bodies can interac

Question

For unconfined aquifers, surface topography and surface water bodies can interact with the water table. Lake surfaces are flat as is the water table beneath it, hence ground water contour lines must go around it (with some rare exceptions, such as a perched lake). Ground water contour lines for a V, pointing upstream when they cross a gaining stream. Ground water contour lines bend downsteam when they cross a losing stream. In general, the potentiometric surface of a confined aquifer is not influenced by surface topography and surface water features. The hydraulic gradient is steeper where contour lines are closed together. Ground water will flow in the general direction that the WT or potentiometric surface is sloping.

Describe the general flow pattern in the water table maps shown below.

Explanation / Answer

help: For unconfined aquifers, there’s also a water-table boundary. The water table is neither a flow line nor an equipotential line; rather it is a line where head is known. If there is recharge or discharge across the water table, flow lines will be at an oblique angle to the water table. If there is no recharge across the water table, flow lines can be parallel to it. A flow net is a family of equipotential lines with sufficient orthogonal flow lines drawn so that a pattern of “square” figures results. Except in cases of the most simple geometry, the figures will not truly be squares. The following steps are followed in the construction of a flow net.

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