5.2 Flow in an Unconfined Aquifer
Understanding groundwater flow in unconfined aquifers is essential because this is the most common setting in which groundwater interacts with surface water. The figure below represents a cross-section of an unconfined aquifer that extends between two wells and a central river. The task is to identify the shape of the water table and how groundwater discharges toward the river.
Figure 26 – Cross-section of an unconfined aquifer between two wells and a central river (Cohen & Cherry, 2020).Example Problem 6
a) Draw the water table.
b) Draw the 70 m, 75 m, and 80 m equipotential contours and sketch several flow lines.
This problem demonstrates the relationship between hydraulic head and the geometry of the flow system. The equipotential contours are lines of constant hydraulic head, and flow lines follow the direction of decreasing head perpendicular to the equipotential lines.
Interactive Solution
This interactive exercise allows you to draw both the equipotential line and additional points over the unconfined aquifer cross-section. Select two points to define an equipotential line and observe the hydraulic head difference. Then, add additional points to trace the equipotential contours.
Guided interactive exercise
Exercise 6 – Flow in an unconfined aquifer
Loading image...
Select 2 points to create the equipotential line
Select 2 points to create the equipotential line on the unconfined aquifer diagram. You can also switch to coordinate mode.
Show solution
Problem 6 Solution
The key takeaways from this exercise are:
- The water table in an unconfined aquifer is a subdued replica of the topography, sloping toward the discharge point (the river in this case).
- Equipotential contours are lines of equal hydraulic head. In the interactive exercise, they are drawn by selecting pairs of points at the same elevation.
- Flow lines are perpendicular to equipotential lines and indicate the direction of groundwater flow — from higher hydraulic head to lower hydraulic head.
- In an unconfined aquifer near a river, groundwater flows toward the river, which acts as a discharge boundary. The equipotential lines curve as they approach the river, reflecting the geometry of the flow system.
- The hydraulic head decreases along a flow line from the recharge area toward the discharge point. The interactive exercise lets you measure this difference (Δh) between selected points.
Conceptual Connections
- Compare this exercise with Exercise 5, which discusses the representation of the water table as a planar surface with predominantly horizontal flow. Exercise 6 provides a concrete example where the water table interacts with rivers.
- Compare with Exercise 3, which introduces equipotential lines and flow direction in a simpler 1D column setup. Exercise 6 extends these concepts to a 2D cross-section with boundary conditions.
- The relationship between equipotential lines and flow lines demonstrated here is the foundation for understanding flow nets — a graphical technique used to solve groundwater flow problems (see Poeter & Hsieh, 2020).