Unsteady-state behavior of coastal groundwater: A sand tank experiment

Using a sand tank, the unsteady behavior of coastal groundwater was investigated in response to the sudden change in groundwater level at the upstream side and to the tide at the downstream side. The salt water wedge induced by the sudden change in groundwater level in the upstream region is faster when saltwater retreats than when it advances. In the advancing stage, the horizontal gradient of concentration near the bottom of the aquifer is smaller in the downstream region than in the upstream region, though such a phenomenon is not found in the retreating stage. The phase lags of wedge length oscillation, which follows the tide oscillation, are larger near the bottom than those of wedge height oscillation near the downstream end. The salt water wedge intrudes farthest inland at the mean sea level in the ebb tide stage, and the transition zone becomes widest at the same time. These experimental results agree with those of the numerical simulation.