Numerical study of airflow in the unsaturated zone induced by sea tides

The air pressures in coastal unsaturated zones fluctuate in response to tidal fluctuations. Two-phase air-water flow induced by sea tides in a coastal two-layered subsurface system is investigated through numerical simulations. The system consists of upper and lower layers with permeabilities of k(U) and k(L), respectively. A water table exists in the lower layer. The study demonstrates that airflow in the unsaturated zone is controlled by permeabilities of both layers and the fluctuation of water level. The tide-induced water level fluctuation may result in significant air pressure fluctuation in the unsaturated zone in an air-confined system; that is, the permeability of the upper layer is about 2 orders of magnitude lower than that of the lower layer. In such an air-confined system, horizontal airflow is dominant in the lower layer and vertical airflow is dominant in the upper layer. The simulation results show that the hydraulic head, rather than the sea tide, is the direct driving force of the air pressure fluctuation above the water table. The study on the relation between the air pressure amplitude and the landward distance demonstrates that the air pressure amplitudes increase to a maximum and then decrease with distance. The amplitude of air pressure fluctuation decreases progressively with kU in the lower layer. In the upper layer, however, the amplitude tends to increase with kU when kU is very low and decrease with kU when kU is relatively high. Overall, the air flux fluctuations across the atmosphere-soil interface attenuate landward gradually, and this attenuation becomes faster as the permeability of the upper layer increases.