CONSTANT HEAD EXAMPLE

Example of Potential Problems Which May Result From Misunderstanding / Misusing a Constant Head Boundary

If heads are fixed at the ground surface to represent a swampy area,

Figure 1

and an open pit mine is simulated by defining heads in the pit area, to the elevation of the pit bottom,

Figure 2

the use of constant heads to represent the swamp will substantially overestimate in-flow to the pit. This is because the heads are inappropriately held high, while in the physical setting, the swamp would dry up and heads would decline, therefore actual in-flow would be lower. The swampy area is caused by a high water table. It is not an infinite source of water.

Figure 3

Lesson: Monitor the in-flow at constant head boundaries and make calculations to assure yourself the flow rates are reasonable.