Some constructions of the groundwater reservoir are projected in order to obtain the irrigation water at oceanic islands in Okinawa Prefecture. This groundwater reservoir is darned up by a grouting subsurface barrier with practical restrictions on permeability and depth.
This report treats a usefulness of the semipermeable and partially intercepting barrier on defending the groundwater reservoir from intrusion of saline water. Two types of the barrier are considered, one is submerged type and the other is floating one. On each type, next technical problems are examined; 1) The possibility of the defense from intrusion of saline water by the barrier.2 ) The difference of the defense caused by the degree of an improvement on permeability.3 ) The comparison on the defensive effects between the two types.
On the above problems, the following results are derivered from the numerical calculations on two dimensional and two-phase'groundwater flow in steady condition. Possibility; the effective defense is confirmed by the semipermeable and partially intercepting barrier. Permeability; the barrier improved 1/4 times as much as the aquifer does not display defensive effects, the difference of the defensive effects is not evident between the barrier improved 1/400 times and one improved 1/4000 times. Comparison of the structural types; though the floating barrier has the hazard of saline water intrusion in downstream region, one is useful for the defense of the groundwater reservoir. It is also clarified that the defensive effect of the floating barrier works in proportion to the ratio of the intercepting height of the barrier to the height of the aquifer.

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The two-phase groundwater flow involving fluids of different density is analysed using the finite element method for groundwater flow equation and the characteristic finite element method for mass transport equation. Numerical solutions to the vertical, twodimensional, salt water-fresh water interface problem are compared to experimental results and an asymptotic analytical solution based on the Dupuit-Forchheimer assumption. Nonlinear effects during the initial stage of the interfacial movement pointed out by Liu et al. (1981) are observed in the numerical results as well, where locations and physical properties of marker particles are displayed. The distribution of velocity vector in the flow domain presents the same pattern as De Josselin de Jong's analytical results (1981) based on vortex theory.

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