Impact of the 26-12-04 tsunami on groundwater systems and groundwater based water supplies

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The evolution of the fingering process is demonstrated in this concept. Here we use another geometrical scale (1m*0.5m) than in the freshwater lens concept (1000m*15m). The reason is that the fingers that are actually occurring in case of density inversion (saline groundwater above freshwater) can only be properly modelled in detail when numerical discretisation is very fine.

In the situation of figure 7, a saline groundwater water pocket descends in a fresh groundwater environment as a result of density differences. As the saline water pocket displaces (Figure 8), a mixing zone exists due to hydrodynamic dispersion. After a few minutes, fingers evolve at the bottom of the saline water pocket (animation). At the left side of the pocket, a strong rotating groundwater flow (vortex) exists. As a result, saline groundwater descends here faster than at the right side of the pocket. When the left side of the pocket reaches the bottom (see the animation after about 60 min), fresh groundwater underneath the pocket is entrapped. Still the saline groundwater will spread entirely over the bottom of the system due to density difference. With other words, during especially the first tens of minutes of the simulation, fresh groundwater will penetrate saline groundwater, and freshwater fingers are developed.

Figure 7: Example of a salt water pocket in a fresh groundwater environment: conceptual model

Figure 8: Evolution of the fingering process in a sand-box of 1m*0.5 m.

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