Island3D

• Problem description
• Visualizations
• References

Problem description

A hypothetical, circular island in the sea undergoes a prolonged drought, during which the water table declines to sea level, and all ground water beneath the island becomes saline. Then, renewed freshwater recharge finally restores the island’s freshwater lens. This example simulates the post-drought recharge and restoration of the freshwater lens.

Following the drought, the water table is at sea level, and both the saturated aquifer below sea level and the unsaturated zone above sea level contain only seawater. Fresh rainwater recharge to the surface of the island begins and continues at a constant rate, raising the water table on the island, flushing out seawater, and eventually establishing a stable freshwater lens and a diffuse saltwater-freshwater interface. The aquifer on the island is unconfined, with both unsaturated and saturated zones, and the material properties are homogeneous, but permeability is anisotropic.

Although the solution is radially symmetric, the 3D simulation must arrive at this result using rectangular (x, y, z) (not radial) coordinates and a finite-element mesh that does not inherently favor a radially symmetric solution. To reduce the size of the simulation, the 3D mesh represents only one-fourth of the entire island, taking some advantage of radial symmetry of the 3D solution. The simulation involves transient, saturated-unsaturated, variable-density flow and transient solute transport.

This 3D example is described in detail in the SUTRA documentation [1]. It is intended for comparison with the 2D (radial) solution to the same problem, also described in the SUTRA documentation [1].

Visualizations

 The results shown below were generated using SutraGUI 2D3D.1, SUTRA 2.0 (2D3D.1), and Model Viewer 1.1.

Transition zone. Plot of the saltwater-freshwater transition zone after 20 years of recharge. Concentrations plotted range from 10% (yellow) to 90% (blue) of seawater concentration. Blue dots show recharge nodes on the surface of the island. One-quarter of island shown in oblique view with 4X vertical exaggeration. Animation shows evolution of transition zone over 20 years. Images generated using Model Viewer. Larger image (GIF, 52 KB) Animation (GIF, 1.93 MB) "3D" interactive image (VRML, 3.25 MB; VRML browser required)

Water table. Plot of the water table (blue curve, along which pressure is 0. and saturation is 1.) after 20 years of recharge. One-half of island shown in cross-section with 20X vertical exaggeration; axis of radial symmetry is along the left boundary. Water table intersects top surface of model at coastline, toward right end of image. Animation shows evolution of water table over 20 years. Images generated using Model Viewer. Larger image (GIF, 11 KB) Animation (GIF, 563 KB)

References

[1]  Voss, C. I., and Provost, A. M., 2002, SUTRA, A model for saturated-unsaturated variable-density ground-water flow with solute or energy transport, U.S. Geological Survey Water-Resources Investigations Report 02-4231, 250 p.